Anthony Guarino, Egill Hauksson, Lucy Jones, Kate Hutton, Doug Given, Nick Scheckel) Version 1.0
ShakeMap generated by the M4.4 Fontana earthquake. ShakeMap shows the event was felt widely throughout Southern California, with maximum intensity of V.
Update – Wednesday, January 15, 2014 – 10:22 AM
A M4.4 earthquake occurred 5 km (3 mi) N of Fontana, CA this morning at 1:35AM. The event was particularly shallow, only 3.5 km, which contributed to it being felt sharply at the epicenter. The moment tensor and focal mechanism shows strike-slip motion (a small vertical component is present), with nodal planes at 33° and 302°.
The mainshock was preceded by five foreshocks, ranging in size from M0.5 to M1.0, with the sequence beginning on 1/8/14 with a M0.9. The mainshock was also followed by 8 aftershocks, ranging in size from M0.6 to M2.3. The aftershocks will likely continue for several more hours, becoming less frequent as time passes. A M4.4 generally produces at least one aftershock in the M3.4 range, and several more smaller events. This aftershock sequence has been fairly robust, as we have had 8 aftershocks in the last 9 hours.
The felt reports were contributed by the public to generate the map below. So far, more than 5,300 felt reports have been generated. To fill out a felt report, click here. The maximum average intensity reported by the public per zip code was V, near the epicenter in Fontana. The event was felt as far away as Thousand Oaks to the northwest, Baker to the north, and San Diego to the south.
Several events have occurred in this area historically. Since record keeping began in this area in the 1930s, we have had four other M4.0 or larger events within 5 km of this mainshock. The list below shows all of the events recorded within the last 80 years:
| EVID | MAG | DATE TIME | LAT/LONG | DEPTH | LOCATION |
|---|---|---|---|---|---|
| 3350653 | 4.1 | 1960/06/28 13:00:47 | 34 7.6 N 117 28.6 W | 1.3 | 4 km (3 mi) WNW of Fontana, CA |
| 3330616 | 4.6 | 1965/04/15 12:08:33 | 34 8.1 N 117 26.2 W | 6.0 | 3 km (2 mi) N of Fontana, CA |
| 503897 | 4.0 | 1983/01/07 23:19:30 | 34 8.0 N 117 27.2 W | 7.8 | 3 km (2 mi) NNW of Fontana, CA |
| 14116972 | 4.4 | 2005/01/06 06:35:27 | 34 7.5 N 117 26.3 W | 4.2 | 2 km (1 mi) N of Fontana, CA |
| 11413954 | 4.4 | 2014/01/15 01:35:18 | 34 8.6 N 117 26.5 W | 3.5 | 4 km (3 mi) N of Fontana, CA |
A M4.4 earthquake occurred in the same area back on January 6, 2005. Several events occur in this region, known as the Fontana Seismic Trend, an area that has a trending pattern of smaller events. The geology of this area is rather complex, as several larger fault systems all converge in this area. The San Jacinto fault, San Andreas fault, and Sierra Madre fault are all within a few miles of this event. The termination of both the San Jacinto fault zone (the most active fault in Southern California) and the Sierra Madre fault are within two miles. Due to the very complex nature of the geology, the small size of the earthquake that occurred, and the lack of surface rupture, it is difficult to identify which is the causative fault for this event. A larger earthquake may have produced surface rupture, which helps identify the fault in question. Smaller earthquakes at depth do not create surface rupture, therefore it is more challenging to identify the fault that ruptured with any certainty.
Earthquake Early Warning (EEW) is a prototype software that detects ground motion from an event and can send a warning in real-time before the shaking reaches areas at distance. It only warns that an earthquake has occurred, and the shaking is on its way. ShakeAlert, the program that sends the alerts, worked well for this event, and generated an alert for Pasadena with several seconds warning before the shaking actually began. The program is in beta test mode, and is being tested by first responders throughout CA, from the train operators to Emergency Operations Centers (EOC) like the County of Los Angeles EOC.
For additional information check USGS
