M 5.1 earthquake in La Habra

All Right!
A good sized shaker (nice and small) just happened near La Habra in southern CA. Here is the USGS web site for this earthquake. Here is some general information about earthquakes in southern CA. This is a really shallow earthquake at 1.9 km!
Based on reports from my family that live a few miles from the epicenter, they are shaken but not stirred.
Here is an early moment tensor. It looks like this is an oblique strike-slip/reverse earthquake. It probably shook pretty strongly there.

Here is a shaking intensity map based on a model of the earth and the magnitude of the earthquake. The colors represent shaking intensity using the Modified Mercalli Shaking Intensity Scale.


Here is a map showing some aftershocks. It appears that this may be a rupture on a northeast striking fault. A combination of a strike-slip earthquake and a thrust (compressional) earthquake. The hills north of Brea and La Habra are formed along the Whittier fault in the Elsinore fault zone, which is a sister fault system to the San Andreas fault system (a plate boundary transform fault system). If you click on the map, it will open an updated map that shows the main shock epicenter actually did change.


Based on Danielle Verdugo Madugo, Egill Hauksson thinks it is may be on the Puente Hills fault. Here is a map from Tucker and Dolan (2001). The Puente Hills blind thrust fault appears to be responsible for the uplift associated with the Coyote Hills east of La Mirada. The Puente Hills fault ruptured in 1987 with a M 6.0 earthquake. Here is a great paper by Leon et al. 2007 that describes the tectonics of the Puente Hills fault as it relates to seismic hazard.
Here is map and cross section showing the location of the 1987 M 6.0 “Whittier Narrows” earthquake. (A) Map showing Brea and downtown LA as they relate to the epicenter. The lighter shaded polygons are their estimates of the location of thrust fault planes as they dip to the north. The fault tips are the thick black lines. (B) The cross sectional view of the crust in this region. The cross section locations are located in (A) by the blue lines from B-B1-B2. There is a break in the cross section at B1.


The 1987 Whittier Narrows earthquake (i remember that one) is located at a depth of ~13 km. Tonight’s earthquake is also shallow, at about 8 km as i post this. The Whittier fault is marked by the “WF.” The 1987 EQ was north of the WF. Tonight’s earthquake would be up-dip of the 1987 earthquake, and further east. There are lots of faults in this region, so it is pretty complicated. Here is a map from Shaw et al. (2002) showing the different segments of the Puente Hills fault system.


Here is another cross section, from the Shaw paper, that shows where Santa Fe Springs and Montebello are. This cross section is west of today’s M 5.1 earthquake.


These are the seismic data Shaw et al. (2002) used to interpret their cross section above.


Based on the USGS web page:
“There have been 23 aftershocks as of 10:00PM on March 28, the largest of which was a M3.6 at 9:30PM, and was felt locally near the epicenter. The aftershock sequence may continue for several days to weeks, but will likely decay in frequency and magnitude as time goes by.
Here is a cool video from SCEC

The maximum observed instrumental intensity was VII, recorded in the LA Habra and Brea areas, although the ShakeMap shows a wide area of maximum intensity of VI. The maximum reported intensity for the Did You Feel It? map was reported at VI in the epicentral area.”


Here is an early “Did You Feel It?” map. I will update this later.


Here is a later DYFI map with more results:


Here is a plot showing how the intensity attenuates (is reduced) with distance from the epicenter. Distance in km is on the horizontal axis. Shaking intensity is on the vertical axis. The lines are the models (what the shaking intensity map is based on, above). The dots are the data from people who called in (like all of YOU)


This does a little to explain what the moment tensor represents (though this is a model of the focal mechanism plots, the concept is the same).