Earthquakes in Los Angeles

We have had a number of small earthquakes in the Los Angeles Basin in the past month. These three earthquakes appear aligned with the Newport Inglewood fault system, the fault that ruptured in 1933 for what is generally called the Long Beach Earthquake. This earthquake was deadly and led to building codes for Long Beach that were the most stringent in the Nation at the time. Since then, seismic design for building codes have expanded nationwide and globally.

Here are the USGS links for the three earthquakes that I plot below. I show the focal mechanisms and how I interpret them. I also show where the San Andreas fault (SAF) is and the sense of motion related to a generic focal mechanism. The Newport-Inglewood fault (NIF) is shown as a red line, which abuts the Hollywood fault (HF) system. The NIF is a right-lateral fault system (synthetic to the SAF) and the HF system is left-lateral. In the focal mechanism illustration, I show how focal mechanisms can be interpreted in two ways (the red and orange arrows). The focal mechanism in the legend can either be a left-lateral fault or a right-lateral fault. We need to have additional observations or a knowledge about the local tectonics to know which is the better of the two ways to interpret the correct fault orientation and sense of motion. Based on our knowledge that the NIF system is right-lateral (note the green arrow pair that I placed on the fault), I interpret these three earthquakes to have right-lateral slip (as shown with my purple arrow pairs on the focal mechanisms for each earthquake).
• 2015.04.13 M 3.3 in the Baldwin Hills
• 2015.04.30 M 3.4 in Carson
• 2015.05.03 M 3.8 in the Baldwin Hills

Here is a primer on focal mechanisms, which are graphical depictions of a geometrical estimate of fault slip for an earthquake based upon seismologic observations. One may read more about focal mechanisms on the USGS website here.


I recently posted about the earthquake in April here. I spend more time talking about the thrust faults in the LA region, some of which are shown in the figure below.
This is a 3-D illustration from UNAVCO that shows how the Newport-Inglewood fault system is configured in the LA Basin (as well as the big players, the Compton, Elysian Park, and Puente Hills thrust fault systems).