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MW 7.2 Northern Baja Earthquake
A major earthquake has been felt throughout Southern California, Arizona, Nevada, and Baja California Norte Mexico. The Mw7.2 event occurred at 3:40PM PDT, 22:40 UTC, on Sunday, April 4th, 2010. The epicenter was located 16 miles (26 km) SW of Guadalupe Victoria, Baja California Norte, Mexico. The fault rupture appears to extend 75 km northwest, from the epicenter in Baja California through the US-Mexico border.
CISN/SCSC Executive Summary (Ken Hudnut, Egill Hauksson, Lucy Jones, Sue Hough, Kate Hutton, Doug Given, Anthony Guarino, Shinji Toda)
Version 3 - Posted at 11:51 AM PDT April 5, 2010
The ground shaking was moderate to heavy in the area near the fault rupture (the area near the dark gray line). Moderate to light shaking was recorded across the San Diego, Imperial, Los Angeles, Riverside, San Bernardino, and Ventura counties. The peak recorded instrumental intensity was 1.1g, recorded at the Salton Sea. 1.1g is a measured intensity in the range of IX, with violent shaking and heavy damage. Click on the above image for the link to the M7.2 ShakeMap.
A Mw 7.2 earthquake occurred at 3:40PM on Sunday, April 4th, 2010, located near the Laguna Salada fault in northern Baja California in Norte (NTE) B.C., Mexico. This area is a known tectonically active area, and is the principle plate boundary between the North American and Pacific Plates. The fault rupture appears to trend southeast-northwest on the Laguna Salada fault, with the epicenter locate at 32 deg. 7.7 min. N (32.128N), 115 deg. 18.2 min. W (115.303W), although field geologists will need to confirm that this is indeed the case.
Preliminary data show the aftershock sequence so far has been robust, and demonstrates a linear feature that trends northwest across a 75-km-long zone, terminating on the USA side of the USA-Mexico border. The aftershock sequence so far has produced several M3+ aftershocks and a few in the M4 and M5 range. The largest aftershock as of 8:00PM on 4/4/10 was a M5.4, preliminarily located at 32.122N 115.061W, 12 miles (19km) SSE of Guadalupe Victoria, B.C. NTE, Mexico.
Prior to this M7.2 event, there were several events to occur very near the area of the epicenter. This sequence began in late March, with several events occurring on March 22 and 23, 2010. There were several foreshocks immediately prior to the event.
For every earthquake in California (and northern Baja), there is approximately a 5% chance that that earthquake will be a foreshock for a larger earthquake, but this percentage quickly declines with time. During the first 24 hours, the chance decreases to 3%, and after 24 hours, it decreases to 1%. Since no larger event has occurred in nearly 24 hours, the likelihood is near 1% at the time of this report.
Field geologists were dispatched to the location of the event, and should provide more data as time passes, at which time an update to this report may occur (large earthquakes with surface rupture generally take into account the surface rupture to calculate the moment magnitude). With most shallow major earthquakes, liquefaction phenomenon is expected, such as sand volcanoes/sand blows and ground water table fluctuations. Eyewitness reports indicate significant ground water table fluctuations have been observed, but field geologists will need to confirm later today.
This is the largest earthquake to occur in the northern Baja California region since the 1940 M6.9 Imperial Valley earthquake, and the 1892 M7.2 earthquake on the Laguna Salada fault. Another Imperial Valley earthquake occurred in 1979, with a magnitude of M6.4. SCSN has written special reports concerning other earthquakes near Laguna Salada, and has written about the nearby Cerro Prieto geothermal field.
Click on the above image for a larger version. Hauksson-Shearer relocations for M7.2 and aftershocks. Update of map will occur at least daily for next three days.
Update:
As of 11:30 AM PDT, we have had 74,356 responses for the CIIM (Community Internet Intensity Map), also known as the "Did You Feel It?" map. This map is strictly subjective; it is based on only observational reports from people in the community as they fill out a form with felt information on it.
The largest reported intensity for the CIIM reports were generated closest to the areas near the fault rupture. As fewer reports were made in Mexico, the map is less complete for the international areas. All international CIIM Maps represent felt reports with a colored circle, rather than a polygon zip code.
For the cities in the United States, the maximum reported intensity was Calexico, El Centro, Herber, and Imperial. So far, 146 reports exceeding intensity of VII have been collected, with several more less than VII. Other reports may be delayed due to power loss, lack of internet, or other earthquake-related problems. In this case, the ShakeMap may be used to infer intense shaking, as it is an objective map based primarily on instrumental observation (see ShakeMap above).
The damage extended as far south as Colonia Vincente Guerrero in Baja California NTE, Mexico, and as far north as northern southern Orange County.
The CIIM Map ("Did You Feel It?" Map) above shows nearly 75,000 responses within the first 20 hours after the event. Most of these were reported within the first hour, and nearly 65,000 had responded within the first two hours. Click on the above image to link to the CIIM Map and the "Did You Feel It?" form.
Tectonic Summary
Tectonic Summary A magnitude 7.2 earthquake occurred at 3:40:40 p.m. (local time at the epicenter) on Sunday, 4 April 2010 in Baja California, approximately 75 km south of the Mexico-USA border. The earthquake occurred at shallow depth (approximately 10 km) along the boundary zone between the North American and Pacific plates. Since earthquakes have been recorded instrumentally, only two similar sized earthquakes have been recorded in the area. The first was the 1892 earthquake estimated at magnitude 7.21 along the Laguna Salada fault just south of the USA-Mexico border. The second was the 1940 Imperial Valley magnitude 6.9 earthquake which occurred in southernmost California. Today's event is located nearly in line with these earthquakes along the plate boundary, but is situated farther to the south. There are several active faults in the vicinity of today's earthquake, and the particular fault that generated this quake has not yet been determined. Faulting is complex in this region, because the plate boundary is transitional between the ridge-transform system in the Gulf of California and the continental transform system in the Salton Trough. Most of the major active faults are right-lateral strike-slip faults with a northwest-southeast orientation, similar in style to the San Andreas fault to the north. Other faults in the vicinity with the same orientation include the Cerro Prieto fault and the Laguna Salada fault.
The Coulomb Stress model calculation for the M7.2 Laguna Salada Earthquake, created by Shinji Toda2.
Foreshocks, Mainshocks, and Aftershocks
There have been foreshocks indicated for this event, the closest to the mainshock was a M3.9, which occurred at 3:09PM, 31 minutes before the M7.2 mainshock. Prior to that time, there were a number of foreshocks due to the ongoing activity for a sequence in that area. The sequence that occurred prior to the mainshock was not out of the ordinary for the area.
To date, there have been more than 500 aftershocks associated with this event; most have been between M2.0 and M4.0, with a few exceeding M4.0. The list of aftershocks will be disseminated later today, as locations are reviewed by seismologists.
We will likely expect aftershocks for years with a magnitude 7+ event in Southern California. With most earthquakes in Southern California and northern Baja California, there is a significant likelihood that an aftershock one whole magnitude less will occur within the first week (i.e.in this case a ~M6-M6.2). The likelihood also decays as time passes. For most large earthquakes, a normal aftershock sequence that would follow a logarithmic relationship, where we would expect 10 magnitude 5 range events, 100 magnitude 4 range events, and 1000 magnitude 3 range events. As stated, the more time that passes, larger aftershocks are less likely.
Historical Seismicity
Large historical seismic events in the vicinity (both involved primarily right-lateral strike-slip faulting with significant surface rupture). The area has been very seismically active in the past, with many swarms and sequences near the location of this mainshock and aftershock zone. The following large events have occurred in the area:
1979 MW6.4 on the Imperial/Brawley/Rico faults: http://www.data.scec.org/chrono_index/imperi79.html
1940 MW 6.9 on the Imperial fault: http://www.data.scec.org/chrono_index/imperi40.html
1892 M 7.2 on the Laguna Salada fault: http://www.data.scec.org/chrono_index/lagsalad.html
Within a smaller 50 km search radius of the epicenter, this is the largest earthquake since a M6.1 in 1980
YEAR MO DA ORIG TIME LAT LONG DEP MAGNITUDE DIST (in km)
1978 05 05 210315.80 32.22 -115.32 5 5.5 MLPAS 15
1980 06 09 032818.90 32.22 -114.99 5 6.1 MLPAS 27
2001 12 08 233610.66 32.04 -114.91 10 5.8 MwGS 32
2002 02 22 193241.75 32.32 -115.32 7 5.7 MwGS 26
2009 12 30 184857.70 32.44 -115.17 9 5.9 MwGCMT 38
Step map indicating probability of experiencing MMI VI or higher based on statistics obtained from current and past seismicity.
Pager Population Exposure
More than one million people were exposed to shaking meeting or exceeding VI intensity. This is strong shaking with light damage to resistant structures (reinforced concrete, wood-framed homes, etc), and moderate damage or greater to vulnerable buildings (non-reinforced masonry, brick, cinder-block without reinforcement, etc). Nearly three million people were exposed to moderate shaking, with intensity of V, but little or no damage would be expected to reisistant buildings with intensity nearing V. Some mild to moderate damage may occur for buildings that are built vulnerable.
All together, nearly four million people may have been in the area that could have felt the earthquake with intensity of IV or larger, but felt reports indicate that some people may not have felt shaking in these areas.
Moment Tensor Solution
Centroid Moment Tensor 10/04/04 22:40:46.15 BAJA CALIFORNIA, MEXICO
Epicenter: 32.237 -115.083 MW 7.3 USGS CENTROID MOMENT TENSOR
10/04/04 22:41:23.89 Centroid: 33.000 -114.921 Depth 14
No. of sta:133 Moment Tensor; Scale 10**19 Nm Mrr=-2.77 Mtt=-5.63 Mpp= 8.40 Mrt= 3.15
Mrp=-5.70 Mtp=-1.00 Principal axes: T Val= 11.08 Plg=23 Azm= 81 N -3.02 41
329 P -8.06 39 193 Best Double Couple:Mo=9.6*10**19
NP1:Strike=220 Dip=43 Slip= -13 NP2: 321 80 -131
W Phase Moment tensor 10/04/04 22:40:45 BAJA CALIFORNIA, MEXICO
Epicenter: 32.192 -115.095 MW 7.2 USGS/WPHASE CENTROID MOMENT TENSOR
10/04/04 22:40:45.00 Centroid: 32.192 -115.095 Depth 15 No. of sta: 29
Moment Tensor; Scale 10**18 Nm Mrr=-1.72 Mtt=-4.90 Mpp= 6.61 Mrt= 2.90 Mrp=-1.71
Mtp= 0.14 Principal axes: T Val= 6.96 Plg=11 Azm= 87 N -0.28 55 339 P
-6.69 31 185 Best Double Couple:Mo=6.8*10**19
NP1:Strike=222 Dip=59 Slip= -14
NP2: 319 77 -147 USGS/NEIC Summary Regarding the Mw7.2 Laguna Salada Earthquake3:
The magnitude 7.2 northern Baja California earthquake of Sunday April 4th 2010, occurred approximately 40 miles south of the Mexico-USA border at shallow depth along the principal plate boundary between the North American and Pacific plates. This is an area with a high level of historical seismicity, and also it has recently been seismically active, though this is the largest event to strike in this area since 1892. Today's earthquake appears to have been larger than the M 6.9 earthquake in 1940 or any of the early 20th century events (e.g., 1915 and 1934) in this region of northern Baja California. At the latitude of the earthquake, the Pacific plate moves northwest with respect to the North America plate at about 45 mm/y. The principal plate boundary in northern Baja California consists of a series of northwest-trending strike-slip (transform) faults that are separated by pull-apart basins. The faults are distinct from, but parallel to, strands of the San Andreas fault system. The April 4 main-shock occurred along a strike-slip segment of the plate boundary that coincides with the southeastern part of the Laguna Salada fault. Although the location and focal-mechanism of the earthquake are consistent with the shock having occurred on this fault, we do not yet have surface rupture or other confirmation. Aftershocks appear to extend in both directions along this fault system from the epicenter of today's event. The aftershock zone extends from the northern tip of the Gulf of California to the Mexico-USA border. Earthquakes having magnitudes as high as 7 have been historically recorded from the section of the Pacific/North American plate boundary on which the 4 April 2010 earthquake occurred. The 1892 earthquake occurred along the Laguna Salada fault system, but significantly farther northwest than today's event epicenter. The 1940 Imperial Valley earthquake approached magnitude 7, though it occurred farther to the north and on the Imperial fault. Both the 1892 and 1940 earthquakes were associated with extensive surface faulting. An event of M 7.0 or 7.1 occurred in this region in 1915, and then a M 7.0 to 7.2 in 1934 broke the Cerro Prieto fault with up to several meters of surface slip. In the vicinity of the 4 April 2010 earthquake, there are several active faults and it has not yet been determined specifically which fault the earthquake occurred on. Within the transition from the ridge-transform boundary in the Gulf of California to the continental transform boundary in the Salton Trough, faulting is complex. Most of the major active faults are northwest-southeast oriented right-lateral strike-slip faults that are common in mechanism to the San Andreas fault and parallel Elsinore and San Jacinto faults, that run north of the Mexico-USA border.
1.) Revisiting the 23 February 1892 Laguna Salada Earthquake, Susan Hough and Austin Elliot
2.) Preliminary Coulomb Stress Model, Rectangular uniform slip source with seismic moment of 7.5e+26 dyne cm (Mw = 7.18) - Shinji Toda, Kyoto University, Japan
3.) USGS Summary Report on M7.2 http://earthquake.usgs.gov/earthquakes/recenteqsus/Quakes/ci14607652.php#summary