GENERAL QUESTIONS Where can I find basic information about seismology on the Internet? What are some good books to read? Where can I find information on historical earthquakes? Where can I find information on current earthquakes? I just felt something … Where can I learn about earthquake safety & disaster preparedness? Where can I learn about retrofitting old buildings? How close are we to predicting earthquakes scientifically? . MEASURING EARTHQUAKES: How does a seismograph work? What information is contained in a seismogram? How does a seismic network locate earthquakes? What is the difference between epicenter, hypocenter & fault rupture? How do we measure the magnitude of an earthquake? Why are there several different magnitudes & which ones are the best? How small an earthquake can we measure? How big does it need to be for a person to feel it? How big does in need to be to cause damage? When & where was the world’s largest earthquake? What is earthquake intensity (Modified Mercalli)? How do we gather intensity information?	MYTHS & RUMORS: Are we falling into the ocean soon? Is the number of earthquakes increasing with time? Can animals, some people’s headaches, etc. help predict earthquakes? Do all the small earthquakes relieve the stress so we won’t have as many big ones? SLIGHTLY MORE TECHNICAL: How can we determine fault orientation & motion from the seismograms? Or what are those little beach balls on all the maps? What magnitude types does SCSN use? How many earthquakes has SCSN recorded since 1932? How many do we normally record now per day, week, month, etc.?
GENERAL QUESTIONS Where can I find basic information about seismology on the Internet? There are many, many good sites out there. A good place to start is http://www.geophys.washington.edu/seismosurfing.html , http://pasadena.wr.usgs.gov/info/seismolinks.html , and http://www.seismolab.caltech.edu Others with good general information are: http://neic.usgs.gov/ http://www.cisn.org/ http://www.seismo.berkeley.edu/seismo/Homepage.html Of particular note for non-technical people is the “virtual earthquake” site: http://www.sciencecourseware.com/VirtualEarthquake/ What are some good books to read? Technical: Richter, Charles F. (1958). Elementary Seismology, W. H. Freeman. Shearer, Peter (1999). Introduction to Seismology, Cambridge University Press. Not-so-technical: Bolt, Bruce A. (1999). Earthquakes, 4 th edition, W. H. Freeman. Hough, Susan E. (2002). Earthshaking Science: What we Know (and Don’t Know) about Earthquakes, Princeton University Press. Sieh, K. E. and Simon LeVay (1999). Earth in Turmoil: Earthquakes, Volcanoes and Their Impact on Humankind, W. H. Freeman. Yeats, Robert S. (2004). Living with Earthquakes in the Pacific Northwest; a Survivor’s Guide, 2 nd edition, Oregon State University Press. Yeats, Robert S. (2001). Living with Earthquakes in California; a Survivor’s Guide, Oregon State University Press. Where can I find information on historical earthquakes? For a comprehensive history of large world-wide earthquakes, in searchable form, see: http://neic.usgs.gov/Each regional seismic network has its own searchable catalog as well. For southern California, see http://www.data.scec.org/. For northern California, see http://www.seismo.berkeley.edu/seismo/Homepage.html. A link for each regional network can be found on the “seismo-surfing” page: http://www.geophys.washington.edu/seismosurfing.html. Where can I find information on current earthquakes? If you are looking for information on earthquakes local to California & Nevada that happened in the last week, start with http://quake.wr.usgs.gov/recenteqs/latestfault.htm. For world-wide earthquakes in the last week, try: http://earthquake.usgs.gov/recenteqsww For general earthquake information, go to: http://earthquake.usgs.gov I just felt something … The place to go to report feeling an earthquake or to learn about where it was felt, is http://pasadena.wr.usgs.gov/shake/ca/. This site displays intensity information (on the Modified Mercalli Intensity scale, or MMI. See the question below that covers intensity. Where can I learn about earthquake safety & disaster preparedness? A good basic place to start is http://www.earthquakecountry.info/index.html. Click on their “featured resource” “Putting Down Roots in Earthquake Country”, as well as the”What Should I Know?”, “Why should I care?”, etc. icons. Disaster preparedness information that can be applied to any disaster is available at many, many web sites. A good place to start would be http://www.redcross.org/services/disaster. Where can I learn about retrofitting old buildings? You can begin with the “Putting Down Roots in Earthquake Country” resource listed above. There are many earthquake retrofit web sites, most of them belonging to contractors who do that sort of work. I suggest that you use your favorite search engine to find “earthquake retrofit” & pick out someone in your geographic area. How close are we to predicting earthquakes scientifically? Scientific earthquake predictions are made currently, on an experimental basis. For example, in 1985, a M6 earthquake was predicted for the small California town of Parkfield before 1993 (see http://quake.wr.usgs.gov/research/parkfield/BakunLindh85.html), based on a semi-regular series of historic earthquakes of similar size. The predicted earthquake happened in 2004, more than 11 years late. Another recent example is a prediction of a M6.5+ earthquake in a wide swath of southern California desert between January 6, 2004 & September 5, 2004, by a team of seismologists at UCLA (see http://www.college.ucla.edu/keilisborok.htm). This predicted earthquake did not happen during the time window specified, either, although other earthquakes predicted by the same group have occurred. To be useful, an earthquake prediction must include a date/time window, a reasonably well-defined geographic area, a magnitude range, and hopefully a probability estimate (similar to that used by weather predictors). To be useful, the prediction must come from a method with a proven track record. Herein lies the problem. Scientific methods of predicting earthquakes are not as rare as one might think. Proven, reliable methods are much harder to come by. Some scientists make a distinction between a fairly specific earthquake “prediction” and a more general earthquake “forecast”, or a “probabilistic hazard assessment”. Such an assessment might say, for example: knowing all that we do about the surrounding faults, their characteristic earthquakes & recurrence intervals, a particular geographic location has a 50% chance of experiencing damaging earthquake shaking in the next 30 years. (See http://www.consrv.ca.gov/cgs/rghm/psha/ofr9608/). This approach may be more useful to land-use planners, engineers & others concerned with disaster planning than a possibly accurate short-term prediction. MYTHS & RUMORS Are we falling into the ocean soon? California is not falling into the ocean. We sit atop the plate boundary between the North American plate & the Pacific plate, which is a transform fault. As such, it moves mainly horizontally, as a strike-slip fault. Los Angeles, San Diego & all the areas west of the San Andreas fault are technically sitting on the Pacific plate & are being carried to the northwest toward San Francisco. Look at a map & see that this motion has pulled Baja off the coast of Mexico, a process which has taken about 5 million years. The relative motion between the plates is about 5 cm per year, or 2 inches per year. In case you are looking for something to visualize, this is about the rate that your fingernails grow. Is the number of earthquakes increasing with time? If you look at any raw earthquake catalog, you will find that by far most of the earthquakes listed took place in the last few decades. An earthquake catalog, by the way, is a list of historic earthquakes for a region or the world, made with the idea of having the most complete list possible giving the seismographic coverage, data processing methods, etc. For this apparent increase in activity, we can thank modern electronics & computers. Many more instruments are installed now than has been so in the past & the data can be analyzed much more easily. To take the Southern California catalog as an example, many of yesterday’s earthquakes were between magnitude 1.0 and 2.0. These quakes are not large enough for a person to feel. In the 1930’s & 40’s, when the network included less than 10 stations, we simply did not record most earthquakes in this size range, so they are not in the catalog. To find out if the number of earthquakes is increasing, we need to search the catalog for only earthquakes as large as the early network was able to detect reliably, which was about M3.3. The number of earthquakes of M3.3 and above per year, on the average, has been about constant since 1932. So, although we have had some pretty spectacular individual years (1933, 1952, 1992, 1994), the number of earthquakes overall is not increasing. Can animals, some people’s headaches, etc. help predict earthquakes? Throughout history, there have been many anecdotal reports of animals exhibiting unusual behavior prior to large earthquakes. There are a few documented cases, however, mostly in China (for example, the Haicheng earthquake in 1975). The earthquakes that were successfully predicted in China had many foreshocks. The animals described may have sensed the foreshocks. In any case, unusual animal behavior can be difficult to interpret. If animals are shown to somehow be able to sense earthquake precursors, instruments are likely to do a more reliable job of it. Do all the small earthquakes relieve the stress so we won’t have as many big ones? Yes, every earthquake relieves some stress in the Earth’s crust. However, due to the fact that earthquake magnitude is a logarithmic scale, the small earthquakes do not relieve enough stress to delay a big one by any significant amount. It turns out that a M7.0 releases about 30 times the amount of energy of a M6.0, which in turn releases 30 times the energy of a M5.0, which in turn releases 30 times the energy of a M4.0, and so forth. From this we can see that when we get down to the day-to-day magnitude M2’s & M3’s, the overall energy release is miniscule. We actually observe a relationship between the number of earthquakes & their magnitudes. This is known as the Gutenberg-Richter relation, after it’s discoverers. Approximately, every time we go down in magnitude by a whole point, there are 10 times as many earthquakes. So, over a time period where we have a thousand M3’s, a hundred M4’s & ten M5’s, we would also expect to see one M6 possibly damaging earthquake. Viewed in this way, the small quakes look more like reminders that we live in Earthquake Country than they do stress relief mechanisms! MEASURING EARTHQUAKES How does a seismograph work? The ideal seismograph would be anchored in a reference frame that travels with the Earth’s rotation, but does not participate in any of its vibration (maybe that elusive “sky hook”?). We could devise a way to record the relative motion between the Earth & the shy hook, giving us an exact record of the ground displacement. Lacking such a stable reference frame, we use instead a pendulum (or, for vertical motion) a heavy mass on springs. The pendulum does not actually stand still as the Earth moves, but it does lag behind the motion, so a recording, called a seismogram, can be produced. There have been many ingenious designs since the late 1800’s, many of the older ones using a mechanical recording stylus or a light beam to record on photographic paper. http://neic.usgs.gov/ has a very interesting section on historical seismographs. The modern “broadband” seismometers use an electronic feedback loop to keep the pendulum mass stationary, while outputting the voltage necessary to do this. The terms “seismograph” and “seismometer” are often used interchangeably, with some tendency to use the latter term for a well-calibrated instrument, or sometimes the pendulum apparatus without the recording system. Most “high magnification” (very sensitive) seismographs output the velocity that the ground is moving, rather than the displacement. Most “low magnification” or “strong motion” instruments output ground acceleration; hence they are also called accelerometers. What information is contained in a seismogram? The seismogram (or recording of an earthquake or several earthquakes) duplicate the ground motion at the station in magnified form. Generally, several different pulses of energy, representing different waves traveling through the Earth at different speeds & by different paths, appear for each quake. When an earthquake starts, the sudden shifting of rock along the fault causes two types of “body waves”, which propagate outward & downward from the hypocenter, starting at the same time. P (primary) waves & S (secondary) waves travel at different, for the most part known, speeds. (Think of throwing a rock into a pond, except instead of one type of wave, there are two.) In addition, “surface waves” may be generated, which travel along the surface of the Earth only. There are two basic types of surface waves: Rayleigh waves & Love waves. For earthquakes that are near the station (within about 120 km), we usually see only one P & one S wave, because the first waves travel a direct path through the crust to the station. At greater distances, however, waves that take multiple different paths, may be visible, such as a P wave that bounces follows the bottom of the crust (called the Moho), followed immediately by one that takes the direct path. Earthquakes at great distances from the station (more than 1000 km) are called teleseisms. Teleseisms generally have very complex seismograms: P & S waves can bounce off Earth’s surface or the core-mantle boundary & they may change from P to S or vice versa. We generally use a special chart called “travel time curves” to tell us when to expect each “phase” or wave, depending on the distance of the teleseism from the station & its depth below the surface. We generally measure the onset time of the P & S waves, to help with location (see next question). We also measure the amplitude, or the peak deflection, of at least the largest wave on the record. For some purposes, we measure the period (or how long it takes the wave to go through one cycle). How does a seismic network locate earthquakes? The first step is getting the data from our remote seismographic stations to our offices at Caltech. This process is called “telemetry” & may include land line paths, radio, microwave, frame relay (telephone data circuits), T1 & the Internet. We receive the data streams, usually one measurement every 100 th of a second, very close to real time. A regional seismic network locates an earthquake based on the arrival times of the P waves and S waves at the various stations. The waves arrive at the seismographic stations in order of their distance away from the earthquake’s hypocenter (where the first waves of the earthquake originate), and the time delay between P and S indicates the distance. The data streams coming in from the stations are fed into two real-time computers (a primary & a backup), which look for strong increases in amplitude. This process is called “P picking”, because we are looking for P waves (S waves, too). An associator program tries to fit all the “picks” together to make a rational earthquake location. A magnitude program also looks at the peak amplitudes & tries to assign a magnitude automatically. If the earthquake location & magnitude meet certain consistency requirements, the “event” is published, on the “Recent Earthquakes” web sites, by e-mail subscription & so forth. Currently, all automatic solutions for earthquakes in the SCSN are reviewed by an seismic analyst (who may or may not be offended if you call him a P picker). What is the difference between epicenter, hypocenter & fault rupture? When an earthquake is felt or detected by a seismograph, there was always a patch on some fault that broke and experienced an offset, to cause the seismic waves. If the quake was small, the area of the patch was small & the waves generated were weak. If the quake was large, a large area of fault broke, and so forth. This area of breakage is called the fault rupture. The fault rupture does not all break at once, however. It generally starts at one place and propagates along the fault. The place where the rupture starts is the place that generates the first P and S waves, which are measured by the P picker and used to locate the earthquake, even though it may not represent the whole rupture at all. It is called the hypocenter. The epicenter is the place on the map directly above the hypocenter. Note that, for a very large earthquake with a large fault rupture, many areas damaged due to being near the fault rupture, may actually be quite far from the epicenter. Also, if the fault dips, or in other words lies at an angle below the ground, the epicenter, on a map, may not be an the surface trace of the fault. How do we measure the magnitude of an earthquake? In the 1930’s, when Charles Richter and Beno Gutenburg first tried to produce a list of all the earthquakes detected (called an earthquake catalog) in southern California, they were faced with distinguishing in the list which ones were the large ones & which ones were not. They borrowed the concept of stellar magnitude from astronomy & established an arbitrary logarithmic scale based on the amplitude of the seismographic recording. Amplitude in this context is how far the pen swings off the center line during the earthquake. This original “local magnitude” or ML was & is truly arbitrary: if a quake is 100 km away from a standard Wood-Anderson seismograph & the amplitude is 1 millimeter, the quake is a 3.0. If the amplitude is 10 millimeters, it’s a 4.0, if it’s 100 millimeters, it’s a 5.0 and so forth. The used empirical observations to correct for distance to stations that did not happen to be at 100 km away. Why are there several different magnitudes & which ones are the best? ML works well for local earthquakes (within a few hundred km from stations) between about 2.0 and 5.0. If they are too small, the Wood-Anderson does not record them & if they are too large, the pen falls off the paper & gets stuck. We actually do still use ML for most of our day-to-day small quakes, except that, since we don’t use the Wood-Andersons any more, we use synthetic Wood-Anderson records generated in the computer from our digital, broadband seismometers. Various other magnitude scales have been developed over the years to cover small, larger, more distant & deeper earthquakes which are outside of ML’s useful range. Mb uses teleseismic (distant) body waves (P and S waves). MS uses teleseismic surface waves with a 20 sec period. Md (duration) & Mc (coda) magnitudes address the tiny earthquakes. The most reliable & physically sensible magnitude, which can be computed for earthquakes over about 4.0, is called moment magnitude Mw & is based on a physical quantity called the seismic moment. Seismic moment can be thought of as an indicator of how much rock moved how far during the quake. To compute seismic moment, we match the whole broadband seismogram at several stations to a computer model of the earthquake. The seismic moment is converted to Mw based on a formula that is intended to match ML, should one be available. Mw differs from the older MS and ML magnitudes significantly for very large earthquakes, since only Mw truly represents the prodigious long period energy these earthquakes produce. For example, the 1964 Alaska earthquake had a MS of 8.4, but a Mw of 9.2, making it the second largest earthquake ever recorded instrumentally. Note that all this sometimes results in multiple magnitudes for the same earthquake. Although the calibrations are such that these numbers should be roughly the same, they often do not agree because they are computed entirely different ways. It has become customary for the news media to select the largest one & use that as the “Richter scale” reading. Our practice is to use Mw when it appears to be reliable & ML or Md for smaller events. How small an earthquake can we measure? The smallest earthquake magnitude that we can reliably detect, process & list in our catalog is called the detection threshold & it varies from place to place, depending on the areal density of seismic stations, the type of instrumentation & the amount of cultural (man-made) or environmental seismic “noise” that interferes with the detection. In other words, we can locate smaller earthquakes if we have more & more sensitive instruments nearby. Traffic, railroads, trees & radio towers that move in the wind & other factors cause seismic noise. Geologic conditions also affect the sensitivity of a station, with soft sediments introducing the most noise onto the records. In most areas of the SCSN, the detection threshold from 1.2 to 1.5, well below the “feelable” threshold. How big does it need to be for a person to feel it? What an earthquake feels like depends on many factors: the magnitude, your distance from the hypocenter (considering the depth of the quake), the type of soil or rock you are on, the building you are in or if you are outdoors & what you are doing at the time. Under ideal conditions, you are lying or sitting still in an upper floor, right on top of a shallow earthquake, you might feel a 1.8 if you were paying attention. Usually, however, it takes at least a magnitude of 2.0 for multiple people to notice a quake & recognize it as a quake. A 4.0 usually gets a lot of public attention if it happens under a populated area. How big does in need to be to cause damage? The damage potential of an earthquake also depends on several factors: the magnitude, your distance from the fault rupture (not necessarily the epicenter), directivity effects, soil or rock type, type, age & quality of building construction & various geologic “special cases”. Directivity is a phenomenon related to the direction the fault rupture is propagating. If it is propagating toward you, the seismic waves are stronger & the damage potential greater than the case where it is propagating away from you, even at the same distance from the fault rupture. In general, soft sediment amplifies ground motion, so damage is likely to be worse in geographic basins, as opposed to on rocky mountains or hills at the same distance from the fault rupture. Besides magnitude, however, the most significant factor in damage is the construction of the building itself. Unreinforced masonry (URM) or adobe is by far the worst, since the materials are heavy & weak. Reinforced masonry or “retrofitted” URM buildings are somewhat better, but still may be dangerous. The best is properly built one or two story wood frame construction, anchored to the foundation & with plywood shear walls. Steel frame structures & reinforced concrete structures are usually also resistant to earthquake damage, with age & overall design being a factor. California has historically had problems with URM’s, the 1933 Long Beach earthquake being the most famous example, after which the building code disallowed that type of construction. There have also been problems (1971 San Fernando quake & 1994 Northridge quake) with more modern buildings that are basically boxes on legs. These could be office or classroom buildings designed with an architecturally “open” first floor. They may also be apartment buildings or restored vintage home with parking areas partially underneath the residence. Parking structures also tend to be problematic because they have largely open walls for ventilation & hence no “shear wall” to resist horizontal forces. Elevated freeways have historically caused problems also. Note that in California the building codes & relatively recent construction of most buildings have kept casualties very low in our large earthquakes, the 1906 earthquake being a major exception. Earthquakes in the M5 to M6 range typically don’t cause much damage here, but in some areas of the world such a quake is a major disaster due to traditional dry stone construction or lack of building codes or enforcement of existing building codes. When & where was the world’s largest earthquake? Since earthquakes have been seismographically recorded & magnitudes computed, which is since the late 1800’s, the largest quake was the Mw9.5 Chile earthquake on May 22, 1960. It ruptured a 1,000 km long section of the Andean subduction zone, raised the coastline up to 30 feet in some locations & caused a tsunami that killed people in Hawaii & Japan. What is earthquake intensity (Modified Mercalli)? In contrast with magnitude, which represents the intrinsic size of an earthquake, intensity relates to information about public perception & damage. Since that varies with location, intensity is normally displayed on a map. The Modified Mercalli Scale has twelve levels, each with a description, generally displayed with Roman numerals to easily distinquish it from magnitude. MMI I means the earthquake was not felt; too small to feel at that location. MMI XII means total destruction (MMI XII is very rare & normally occurs right on top of a fault surface rupture or area of soil liquefaction). For the full list, see http://neic.usgs.gov/neis/general/mercalli.html. MMI takes into account the different types of building construction, so it represents the strength of the ground shaking at the location. How do we gather intensity information? Intensity information used to be gathered largely by mail or by survey teams of engineers inspecting the damage. Results were often available only months after the quake. Currently, felt reports are can be submitted through the Community Internet Intensity Map site http://pasadena.wr.usgs.gov/shake/ca/, where you can fill out a short questionnaire about what you observed. Intensity maps of recent & historical earthquakes can also be viewed on this site. SLIGHTLY MORE TECHNICAL How can we determine fault orientation & motion from the seismograms? Or what are those little beach balls on all the maps? A focal mechanism is a representation of the geographic orientation of the fault that caused an earthquake, based on information from the seismograms. Dip, strike & rake conveniently indicated on a diagram of the focal sphere, which is an imaginary sphere surrounding the hypocenter of the quake. There are two basic types of focal mechanisms available. In a first-motion solution, the direction of the first motion on the vertical seismogram is indicated on the focal sphere, at the location where the ray to that particular station left the hypocenter. “Up” or “compression” first motion indicates that the rock on that side of the hypocenter moved away from the hypocenter first. “Down” or “dilatation” first motion indicates the opposite. There are two possible fault planes, at right angles to each other, for each first motion solution. The preferred one is usually selected based on knowledge of the geology. Examples for each type of fault can be found at http://quake.wr.usgs.gov/recenteqs/beachball.html. The other type of focal mechanism is part of the moment-tensor solution, which is the best fit to the whole seismogram at several broadband stations. The moment-tensor solution includes the orientation of the fault & motion, as well as the seismic moment, or strength of the earthquake, from which moment magnitude Mw is computed. The same “beach ball” diagram is used to represent the moment tensor. What magnitude types does SCSN use? Mw: Our preferred magnitude type, for local earthquakes 4.0 or larger, is moment magnitude Mw. Moment tensor software runs automatically on the real-time system & Mw may be inserted automatically if it passes quality criteria, or it may be inserted later by a seismic analyst. Note that a new earthquake may come up on the Recent Earthquakes maps with its ML, which may be replaced in a few minutes by the Mw, sometimes changing the number slightly. For large historical earthquakes, the Mw inserted into the catalog is usually the Harvard moment tensor solution. M_L: Lacking Mw, we currently use local magnitude ML for our local earthquakes. We require several good amplitude readings on at least two stations ,however, so ML does not usually work well below about ML2.0. Prior to 1990, ML’s were generated only based on the actual Wood-Anderson seismometers, so they were generally limited to earthquakes above about ML2.5 due to the relatively small number of stations so instrumented. M_h: For current smaller events & for events where there is a problem with the ML & the Mw (such as, for example, multiple earthquakes very close in time), the seismic analyst inserts an “eyeball” magnitude called Mh. Most Mh assignments are based on visual estimates of coda duration. Some may, however, be based on amplitude ratios with other earthquakes at the same location. In the historical catalog (prior to 1992), Mh may have been computed from amplitude readings on helicorder (drum) recordings. M_c: Coda magnitudes were based on an algorithm which fit a decay curve to the S wave coda & estimated the magnitude based on the signal strength on this curve. Mc was used from 1977 through 2000 for earthquakes that lacked an ML or Mw. M_d: Mc is the standard U.S.G.S. duration magnitude. We used it experimentally in the 1980’s & should be starting to do so again soon. *Regional earthquakes:* For regional earthquakes (such as northern California) we use the location & magnitude provided by the local network (U.C. Berkeley, U.S.G.S. in Menlo Park, or U. of Nevada Reno). *Teleseisms:* For teleseismis, we use the location & magnitude from the National Earthquake Information Center (NEIC). How many earthquakes has SCSN recorded since 1932? As of Nov. 29, 2004, we have 416,165 local earthquakes in the SCSN catalog. Of these, 18,399 had a magnitude of 3.0 or greater (in other words, might easily have been felt). 231 of them had a magnitude of 5.0 or greater, a size that might cause minor damage if located near poorly built structures. In addition to the earthquakes, there are also over 29,000 explosions in the SCSN catalog. These are mostly blasting at local mines & rock quarries, but the total also includes underground nuclear shots at the Nevada Test Site between the late 1950’s & 1992. How many do we normally record now per day, week, month, etc.? If we take the four weeks from November 1, 2004 through November 28, 2004 as a very typical “quiet” period of seismicity, we find 672 earthquakes detected & located within the SCSN. Of these, 82 were magnitude 2.0 or larger, of which 10 were 3.0 or larger & one was a 4.2. In other words, at minimum, southern California has about 170 detected earthquakes per week, of which 2 or 3 are feelable if they are favorably located with respect to population. The average for the last 14 years, however, shows 360 earthquakes detected per week, the difference being due to prolific aftershock sequences from earthquakes in 1992 (Joshua Tree Mw6.1, Landers Mw7.3, Big BearMw 6.3 produce about 100,000 aftershocks in all) and 1994 (Northridge Mw6.7 produced almost 17,000 aftershocks).

GENERAL QUESTIONS

Where can I find basic information about seismology on the Internet?
What are some good books to read?
Where can I find information on historical earthquakes?
Where can I find information on current earthquakes?
I just felt something …
Where can I learn about earthquake safety & disaster preparedness?
Where can I learn about retrofitting old buildings?
How close are we to predicting earthquakes scientifically?

MEASURING EARTHQUAKES:

How does a seismograph work?
What information is contained in a seismogram?
How does a seismic network locate earthquakes?
What is the difference between epicenter, hypocenter & fault rupture?
How do we measure the magnitude of an earthquake?
Why are there several different magnitudes & which ones are the best?
How small an earthquake can we measure?
How big does it need to be for a person to feel it?
How big does in need to be to cause damage?
When & where was the world’s largest earthquake?
What is earthquake intensity (Modified Mercalli)?
How do we gather intensity information?

MYTHS & RUMORS:

Are we falling into the ocean soon?
Is the number of earthquakes increasing with time?
Can animals, some people’s headaches, etc. help predict earthquakes?
Do all the small earthquakes relieve the stress so we won’t have as many big ones?

SLIGHTLY MORE TECHNICAL:

How can we determine fault orientation & motion from the seismograms? Or what are those little beach balls on all the maps?
What magnitude types does SCSN use?
How many earthquakes has SCSN recorded since 1932?
How many do we normally record now per day, week, month, etc.?

GENERAL QUESTIONS

Where can I find basic information about seismology on the Internet?

There are many, many good sites out there. A good place to start is http://www.geophys.washington.edu/seismosurfing.html ,

http://pasadena.wr.usgs.gov/info/seismolinks.html , and http://www.seismolab.caltech.edu

Others with good general information are:

http://neic.usgs.gov/

http://www.cisn.org/

http://www.seismo.berkeley.edu/seismo/Homepage.html

Of particular note for non-technical people is the “virtual earthquake” site:

http://www.sciencecourseware.com/VirtualEarthquake/

What are some good books to read?

Technical:

Richter, Charles F. (1958). Elementary Seismology, W. H. Freeman.

Shearer, Peter (1999). Introduction to Seismology, Cambridge University Press.

Not-so-technical:

Bolt, Bruce A. (1999). Earthquakes, 4 th edition, W. H. Freeman.

Hough, Susan E. (2002). Earthshaking Science: What we Know (and Don’t Know) about Earthquakes, Princeton University Press.

Sieh, K. E. and Simon LeVay (1999). Earth in Turmoil: Earthquakes, Volcanoes and Their Impact on Humankind, W. H. Freeman.

Yeats, Robert S. (2004). Living with Earthquakes in the Pacific Northwest; a Survivor’s Guide, 2 nd edition, Oregon State University Press.

Yeats, Robert S. (2001). Living with Earthquakes in California; a Survivor’s Guide, Oregon State University Press.

Where can I find information on historical earthquakes?

For a comprehensive history of large world-wide earthquakes, in searchable form, see: http://neic.usgs.gov/Each regional seismic network has its own searchable catalog as well. For southern California, see http://www.data.scec.org/. For northern California, see http://www.seismo.berkeley.edu/seismo/Homepage.html. A link for each regional network can be found on the “seismo-surfing” page: http://www.geophys.washington.edu/seismosurfing.html.

Where can I find information on current earthquakes?

If you are looking for information on earthquakes local to California & Nevada that happened in the last week, start with http://quake.wr.usgs.gov/recenteqs/latestfault.htm.

For world-wide earthquakes in the last week, try: http://earthquake.usgs.gov/recenteqsww

For general earthquake information, go to: http://earthquake.usgs.gov

I just felt something …

The place to go to report feeling an earthquake or to learn about where it was felt, is http://pasadena.wr.usgs.gov/shake/ca/. This site displays intensity information (on the Modified Mercalli Intensity scale, or MMI. See the question below that covers intensity.

Where can I learn about earthquake safety & disaster preparedness?

A good basic place to start is http://www.earthquakecountry.info/index.html. Click on their “featured resource” “Putting Down Roots in Earthquake Country”, as well as the”What Should I Know?”, “Why should I care?”, etc. icons. Disaster preparedness information that can be applied to any disaster is available at many, many web sites. A good place to start would be http://www.redcross.org/services/disaster.

Where can I learn about retrofitting old buildings?

You can begin with the “Putting Down Roots in Earthquake Country” resource listed above. There are many earthquake retrofit web sites, most of them belonging to contractors who do that sort of work. I suggest that you use your favorite search engine to find “earthquake retrofit” & pick out someone in your geographic area.

How close are we to predicting earthquakes scientifically?

Scientific earthquake predictions are made currently, on an experimental basis. For example, in 1985, a M6 earthquake was predicted for the small California town of Parkfield before 1993 (see http://quake.wr.usgs.gov/research/parkfield/BakunLindh85.html), based on a semi-regular series of historic earthquakes of similar size. The predicted earthquake happened in 2004, more than 11 years late. Another recent example is a prediction of a M6.5+ earthquake in a wide swath of southern California desert between January 6, 2004 & September 5, 2004, by a team of seismologists at UCLA (see http://www.college.ucla.edu/keilisborok.htm). This predicted earthquake did not happen during the time window specified, either, although other earthquakes predicted by the same group have occurred.

To be useful, an earthquake prediction must include a date/time window, a reasonably well-defined geographic area, a magnitude range, and hopefully a probability estimate (similar to that used by weather predictors). To be useful, the prediction must come from a method with a proven track record. Herein lies the problem. Scientific methods of predicting earthquakes are not as rare as one might think. Proven, reliable methods are much harder to come by.

Some scientists make a distinction between a fairly specific earthquake “prediction” and a more general earthquake “forecast”, or a “probabilistic hazard assessment”. Such an assessment might say, for example: knowing all that we do about the surrounding faults, their characteristic earthquakes & recurrence intervals, a particular geographic location has a 50% chance of experiencing damaging earthquake shaking in the next 30 years. (See http://www.consrv.ca.gov/cgs/rghm/psha/ofr9608/). This approach may be more useful to land-use planners, engineers & others concerned with disaster planning than a possibly accurate short-term prediction.

MYTHS & RUMORS

Are we falling into the ocean soon?

California is not falling into the ocean. We sit atop the plate boundary between the North American plate & the Pacific plate, which is a transform fault. As such, it moves mainly horizontally, as a strike-slip fault. Los Angeles, San Diego & all the areas west of the San Andreas fault are technically sitting on the Pacific plate & are being carried to the northwest toward San Francisco. Look at a map & see that this motion has pulled Baja off the coast of Mexico, a process which has taken about 5 million years. The relative motion between the plates is about 5 cm per year, or 2 inches per year. In case you are looking for something to visualize, this is about the rate that your fingernails grow.

Is the number of earthquakes increasing with time?

If you look at any raw earthquake catalog, you will find that by far most of the earthquakes listed took place in the last few decades. An earthquake catalog, by the way, is a list of historic earthquakes for a region or the world, made with the idea of having the most complete list possible giving the seismographic coverage, data processing methods, etc. For this apparent increase in activity, we can thank modern electronics & computers. Many more instruments are installed now than has been so in the past & the data can be analyzed much more easily. To take the Southern California catalog as an example, many of yesterday’s earthquakes were between magnitude 1.0 and 2.0. These quakes are not large enough for a person to feel. In the 1930’s & 40’s, when the network included less than 10 stations, we simply did not record most earthquakes in this size range, so they are not in the catalog. To find out if the number of earthquakes is increasing, we need to search the catalog for only earthquakes as large as the early network was able to detect reliably, which was about M3.3. The number of earthquakes of M3.3 and above per year, on the average, has been about constant since 1932. So, although we have had some pretty spectacular individual years (1933, 1952, 1992, 1994), the number of earthquakes overall is not increasing.

Can animals, some people’s headaches, etc. help predict earthquakes?

Throughout history, there have been many anecdotal reports of animals exhibiting unusual behavior prior to large earthquakes. There are a few documented cases, however, mostly in China (for example, the Haicheng earthquake in 1975). The earthquakes that were successfully predicted in China had many foreshocks. The animals described may have sensed the foreshocks. In any case, unusual animal behavior can be difficult to interpret. If animals are shown to somehow be able to sense earthquake precursors, instruments are likely to do a more reliable job of it.

Do all the small earthquakes relieve the stress so we won’t have as many big ones?

Yes, every earthquake relieves some stress in the Earth’s crust. However, due to the fact that earthquake magnitude is a logarithmic scale, the small earthquakes do not relieve enough stress to delay a big one by any significant amount. It turns out that a M7.0 releases about 30 times the amount of energy of a M6.0, which in turn releases 30 times the energy of a M5.0, which in turn releases 30 times the energy of a M4.0, and so forth. From this we can see that when we get down to the day-to-day magnitude M2’s & M3’s, the overall energy release is miniscule.

We actually observe a relationship between the number of earthquakes & their magnitudes. This is known as the Gutenberg-Richter relation, after it’s discoverers. Approximately, every time we go down in magnitude by a whole point, there are 10 times as many earthquakes. So, over a time period where we have a thousand M3’s, a hundred M4’s & ten M5’s, we would also expect to see one M6 possibly damaging earthquake. Viewed in this way, the small quakes look more like reminders that we live in Earthquake Country than they do stress relief mechanisms!

MEASURING EARTHQUAKES

How does a seismograph work?

The ideal seismograph would be anchored in a reference frame that travels with the Earth’s rotation, but does not participate in any of its vibration (maybe that elusive “sky hook”?). We could devise a way to record the relative motion between the Earth & the shy hook, giving us an exact record of the ground displacement. Lacking such a stable reference frame, we use instead a pendulum (or, for vertical motion) a heavy mass on springs. The pendulum does not actually stand still as the Earth moves, but it does lag behind the motion, so a recording, called a seismogram, can be produced. There have been many ingenious designs since the late 1800’s, many of the older ones using a mechanical recording stylus or a light beam to record on photographic paper. http://neic.usgs.gov/ has a very interesting section on historical seismographs. The modern “broadband” seismometers use an electronic feedback loop to keep the pendulum mass stationary, while outputting the voltage necessary to do this.

The terms “seismograph” and “seismometer” are often used interchangeably, with some tendency to use the latter term for a well-calibrated instrument, or sometimes the pendulum apparatus without the recording system.

Most “high magnification” (very sensitive) seismographs output the velocity that the ground is moving, rather than the displacement. Most “low magnification” or “strong motion” instruments output ground acceleration; hence they are also called accelerometers.

What information is contained in a seismogram?

The seismogram (or recording of an earthquake or several earthquakes) duplicate the ground motion at the station in magnified form. Generally, several different pulses of energy, representing different waves traveling through the Earth at different speeds & by different paths, appear for each quake. When an earthquake starts, the sudden shifting of rock along the fault causes two types of “body waves”, which propagate outward & downward from the hypocenter, starting at the same time. P (primary) waves & S (secondary) waves travel at different, for the most part known, speeds. (Think of throwing a rock into a pond, except instead of one type of wave, there are two.) In addition, “surface waves” may be generated, which travel along the surface of the Earth only. There are two basic types of surface waves: Rayleigh waves & Love waves. For earthquakes that are near the station (within about 120 km), we usually see only one P & one S wave, because the first waves travel a direct path through the crust to the station. At greater distances, however, waves that take multiple different paths, may be visible, such as a P wave that bounces follows the bottom of the crust (called the Moho), followed immediately by one that takes the direct path.

Earthquakes at great distances from the station (more than 1000 km) are called teleseisms. Teleseisms generally have very complex seismograms: P & S waves can bounce off Earth’s surface or the core-mantle boundary & they may change from P to S or vice versa. We generally use a special chart called “travel time curves” to tell us when to expect each “phase” or wave, depending on the distance of the teleseism from the station & its depth below the surface.

We generally measure the onset time of the P & S waves, to help with location (see next question). We also measure the amplitude, or the peak deflection, of at least the largest wave on the record. For some purposes, we measure the period (or how long it takes the wave to go through one cycle).

How does a seismic network locate earthquakes?

The first step is getting the data from our remote seismographic stations to our offices at Caltech. This process is called “telemetry” & may include land line paths, radio, microwave, frame relay (telephone data circuits), T1 & the Internet. We receive the data streams, usually one measurement every 100 th of a second, very close to real time. A regional seismic network locates an earthquake based on the arrival times of the P waves and S waves at the various stations. The waves arrive at the seismographic stations in order of their distance away from the earthquake’s hypocenter (where the first waves of the earthquake originate), and the time delay between P and S indicates the distance.

The data streams coming in from the stations are fed into two real-time computers (a primary & a backup), which look for strong increases in amplitude. This process is called “P picking”, because we are looking for P waves (S waves, too). An associator program tries to fit all the “picks” together to make a rational earthquake location. A magnitude program also looks at the peak amplitudes & tries to assign a magnitude automatically. If the earthquake location & magnitude meet certain consistency requirements, the “event” is published, on the “Recent Earthquakes” web sites, by e-mail subscription & so forth. Currently, all automatic solutions for earthquakes in the SCSN are reviewed by an seismic analyst (who may or may not be offended if you call him a P picker).

What is the difference between epicenter, hypocenter & fault rupture?

When an earthquake is felt or detected by a seismograph, there was always a patch on some fault that broke and experienced an offset, to cause the seismic waves. If the quake was small, the area of the patch was small & the waves generated were weak. If the quake was large, a large area of fault broke, and so forth. This area of breakage is called the fault rupture. The fault rupture does not all break at once, however. It generally starts at one place and propagates along the fault. The place where the rupture starts is the place that generates the first P and S waves, which are measured by the P picker and used to locate the earthquake, even though it may not represent the whole rupture at all. It is called the hypocenter. The epicenter is the place on the map directly above the hypocenter.

Note that, for a very large earthquake with a large fault rupture, many areas damaged due to being near the fault rupture, may actually be quite far from the epicenter. Also, if the fault dips, or in other words lies at an angle below the ground, the epicenter, on a map, may not be an the surface trace of the fault.

How do we measure the magnitude of an earthquake?

In the 1930’s, when Charles Richter and Beno Gutenburg first tried to produce a list of all the earthquakes detected (called an earthquake catalog) in southern California, they were faced with distinguishing in the list which ones were the large ones & which ones were not. They borrowed the concept of stellar magnitude from astronomy & established an arbitrary logarithmic scale based on the amplitude of the seismographic recording. Amplitude in this context is how far the pen swings off the center line during the earthquake. This original “local magnitude” or ML was & is truly arbitrary: if a quake is 100 km away from a standard Wood-Anderson seismograph & the amplitude is 1 millimeter, the quake is a 3.0. If the amplitude is 10 millimeters, it’s a 4.0, if it’s 100 millimeters, it’s a 5.0 and so forth. The used empirical observations to correct for distance to stations that did not happen to be at 100 km away.

Why are there several different magnitudes & which ones are the best?

ML works well for local earthquakes (within a few hundred km from stations) between about 2.0 and 5.0. If they are too small, the Wood-Anderson does not record them & if they are too large, the pen falls off the paper & gets stuck. We actually do still use ML for most of our day-to-day small quakes, except that, since we don’t use the Wood-Andersons any more, we use synthetic Wood-Anderson records generated in the computer from our digital, broadband seismometers.

Various other magnitude scales have been developed over the years to cover small, larger, more distant & deeper earthquakes which are outside of ML’s useful range. Mb uses teleseismic (distant) body waves (P and S waves). MS uses teleseismic surface waves with a 20 sec period. Md (duration) & Mc (coda) magnitudes address the tiny earthquakes. The most reliable & physically sensible magnitude, which can be computed for earthquakes over about 4.0, is called moment magnitude Mw & is based on a physical quantity called the seismic moment. Seismic moment can be thought of as an indicator of how much rock moved how far during the quake. To compute seismic moment, we match the whole broadband seismogram at several stations to a computer model of the earthquake. The seismic moment is converted to Mw based on a formula that is intended to match ML, should one be available. Mw differs from the older MS and ML magnitudes significantly for very large earthquakes, since only Mw truly represents the prodigious long period energy these earthquakes produce. For example, the 1964 Alaska earthquake had a MS of 8.4, but a Mw of 9.2, making it the second largest earthquake ever recorded instrumentally.

Note that all this sometimes results in multiple magnitudes for the same earthquake. Although the calibrations are such that these numbers should be roughly the same, they often do not agree because they are computed entirely different ways. It has become customary for the news media to select the largest one & use that as the “Richter scale” reading. Our practice is to use Mw when it appears to be reliable & ML or Md for smaller events.

How small an earthquake can we measure?

The smallest earthquake magnitude that we can reliably detect, process & list in our catalog is called the detection threshold & it varies from place to place, depending on the areal density of seismic stations, the type of instrumentation & the amount of cultural (man-made) or environmental seismic “noise” that interferes with the detection. In other words, we can locate smaller earthquakes if we have more & more sensitive instruments nearby. Traffic, railroads, trees & radio towers that move in the wind & other factors cause seismic noise. Geologic conditions also affect the sensitivity of a station, with soft sediments introducing the most noise onto the records.

In most areas of the SCSN, the detection threshold from 1.2 to 1.5, well below the “feelable” threshold.

How big does it need to be for a person to feel it?

What an earthquake feels like depends on many factors: the magnitude, your distance from the hypocenter (considering the depth of the quake), the type of soil or rock you are on, the building you are in or if you are outdoors & what you are doing at the time. Under ideal conditions, you are lying or sitting still in an upper floor, right on top of a shallow earthquake, you might feel a 1.8 if you were paying attention. Usually, however, it takes at least a magnitude of 2.0 for multiple people to notice a quake & recognize it as a quake. A 4.0 usually gets a lot of public attention if it happens under a populated area.

How big does in need to be to cause damage?

The damage potential of an earthquake also depends on several factors: the magnitude, your distance from the fault rupture (not necessarily the epicenter), directivity effects, soil or rock type, type, age & quality of building construction & various geologic “special cases”.

Directivity is a phenomenon related to the direction the fault rupture is propagating. If it is propagating toward you, the seismic waves are stronger & the damage potential greater than the case where it is propagating away from you, even at the same distance from the fault rupture.

In general, soft sediment amplifies ground motion, so damage is likely to be worse in geographic basins, as opposed to on rocky mountains or hills at the same distance from the fault rupture.

Besides magnitude, however, the most significant factor in damage is the construction of the building itself. Unreinforced masonry (URM) or adobe is by far the worst, since the materials are heavy & weak. Reinforced masonry or “retrofitted” URM buildings are somewhat better, but still may be dangerous. The best is properly built one or two story wood frame construction, anchored to the foundation & with plywood shear walls. Steel frame structures & reinforced concrete structures are usually also resistant to earthquake damage, with age & overall design being a factor.

California has historically had problems with URM’s, the 1933 Long Beach earthquake being the most famous example, after which the building code disallowed that type of construction. There have also been problems (1971 San Fernando quake & 1994 Northridge quake) with more modern buildings that are basically boxes on legs. These could be office or classroom buildings designed with an architecturally “open” first floor. They may also be apartment buildings or restored vintage home with parking areas partially underneath the residence. Parking structures also tend to be problematic because they have largely open walls for ventilation & hence no “shear wall” to resist horizontal forces. Elevated freeways have historically caused problems also.

Note that in California the building codes & relatively recent construction of most buildings have kept casualties very low in our large earthquakes, the 1906 earthquake being a major exception. Earthquakes in the M5 to M6 range typically don’t cause much damage here, but in some areas of the world such a quake is a major disaster due to traditional dry stone construction or lack of building codes or enforcement of existing building codes.

When & where was the world’s largest earthquake?

Since earthquakes have been seismographically recorded & magnitudes computed, which is since the late 1800’s, the largest quake was the Mw9.5 Chile earthquake on May 22, 1960. It ruptured a 1,000 km long section of the Andean subduction zone, raised the coastline up to 30 feet in some locations & caused a tsunami that killed people in Hawaii & Japan.

What is earthquake intensity (Modified Mercalli)?

In contrast with magnitude, which represents the intrinsic size of an earthquake, intensity relates to information about public perception & damage. Since that varies with location, intensity is normally displayed on a map. The Modified Mercalli Scale has twelve levels, each with a description, generally displayed with Roman numerals to easily distinquish it from magnitude. MMI I means the earthquake was not felt; too small to feel at that location. MMI XII means total destruction (MMI XII is very rare & normally occurs right on top of a fault surface rupture or area of soil liquefaction). For the full list, see http://neic.usgs.gov/neis/general/mercalli.html. MMI takes into account the different types of building construction, so it represents the strength of the ground shaking at the location.

How do we gather intensity information?

Intensity information used to be gathered largely by mail or by survey teams of engineers inspecting the damage. Results were often available only months after the quake. Currently, felt reports are can be submitted through the Community Internet Intensity Map site http://pasadena.wr.usgs.gov/shake/ca/, where you can fill out a short questionnaire about what you observed. Intensity maps of recent & historical earthquakes can also be viewed on this site.

SLIGHTLY MORE TECHNICAL

How can we determine fault orientation & motion from the seismograms? Or what are those little beach balls on all the maps?

A focal mechanism is a representation of the geographic orientation of the fault that caused an earthquake, based on information from the seismograms. Dip, strike & rake conveniently indicated on a diagram of the focal sphere, which is an imaginary sphere surrounding the hypocenter of the quake. There are two basic types of focal mechanisms available. In a first-motion solution, the direction of the first motion on the vertical seismogram is indicated on the focal sphere, at the location where the ray to that particular station left the hypocenter. “Up” or “compression” first motion indicates that the rock on that side of the hypocenter moved away from the hypocenter first. “Down” or “dilatation” first motion indicates the opposite. There are two possible fault planes, at right angles to each other, for each first motion solution. The preferred one is usually selected based on knowledge of the geology. Examples for each type of fault can be found at http://quake.wr.usgs.gov/recenteqs/beachball.html.

The other type of focal mechanism is part of the moment-tensor solution, which is the best fit to the whole seismogram at several broadband stations. The moment-tensor solution includes the orientation of the fault & motion, as well as the seismic moment, or strength of the earthquake, from which moment magnitude Mw is computed. The same “beach ball” diagram is used to represent the moment tensor.

What magnitude types does SCSN use?

Mw: Our preferred magnitude type, for local earthquakes 4.0 or larger, is moment magnitude Mw. Moment tensor software runs automatically on the real-time system & Mw may be inserted automatically if it passes quality criteria, or it may be inserted later by a seismic analyst. Note that a new earthquake may come up on the Recent Earthquakes maps with its ML, which may be replaced in a few minutes by the Mw, sometimes changing the number slightly. For large historical earthquakes, the Mw inserted into the catalog is usually the Harvard moment tensor solution.

M_L: Lacking Mw, we currently use local magnitude ML for our local earthquakes. We require several good amplitude readings on at least two stations ,however, so ML does not usually work well below about ML2.0. Prior to 1990, ML’s were generated only based on the actual Wood-Anderson seismometers, so they were generally limited to earthquakes above about ML2.5 due to the relatively small number of stations so instrumented.

M_h: For current smaller events & for events where there is a problem with the ML & the Mw (such as, for example, multiple earthquakes very close in time), the seismic analyst inserts an “eyeball” magnitude called Mh. Most Mh assignments are based on visual estimates of coda duration. Some may, however, be based on amplitude ratios with other earthquakes at the same location. In the historical catalog (prior to 1992), Mh may have been computed from amplitude readings on helicorder (drum) recordings.

M_c: Coda magnitudes were based on an algorithm which fit a decay curve to the S wave coda & estimated the magnitude based on the signal strength on this curve. Mc was used from 1977 through 2000 for earthquakes that lacked an ML or Mw.

M_d: Mc is the standard U.S.G.S. duration magnitude. We used it experimentally in the 1980’s & should be starting to do so again soon.

Regional earthquakes: For regional earthquakes (such as northern California) we use the location & magnitude provided by the local network (U.C. Berkeley, U.S.G.S. in Menlo Park, or U. of Nevada Reno).

Teleseisms: For teleseismis, we use the location & magnitude from the National Earthquake Information Center (NEIC).

How many earthquakes has SCSN recorded since 1932?

As of Nov. 29, 2004, we have 416,165 local earthquakes in the SCSN catalog. Of these, 18,399 had a magnitude of 3.0 or greater (in other words, might easily have been felt). 231 of them had a magnitude of 5.0 or greater, a size that might cause minor damage if located near poorly built structures.

In addition to the earthquakes, there are also over 29,000 explosions in the SCSN catalog. These are mostly blasting at local mines & rock quarries, but the total also includes underground nuclear shots at the Nevada Test Site between the late 1950’s & 1992.

How many do we normally record now per day, week, month, etc.?

If we take the four weeks from November 1, 2004 through November 28, 2004 as a very typical “quiet” period of seismicity, we find 672 earthquakes detected & located within the SCSN. Of these, 82 were magnitude 2.0 or larger, of which 10 were 3.0 or larger & one was a 4.2. In other words, at minimum, southern California has about 170 detected earthquakes per week, of which 2 or 3 are feelable if they are favorably located with respect to population.

The average for the last 14 years, however, shows 360 earthquakes detected per week, the difference being due to prolific aftershock sequences from earthquakes in 1992 (Joshua Tree Mw6.1, Landers Mw7.3, Big BearMw 6.3 produce about 100,000 aftershocks in all) and 1994 (Northridge Mw6.7 produced almost 17,000 aftershocks).


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