Last night, we had another earthquake on the Newport Inglewood fault (NIF) system. This fault ruptured in 1933, which led to Long Beach adopting the most strict building codes in the nation. Later, these codes were adopted by the state, the nation, and the world. (Of course, they have been updated since then.) The 2015 Seismological Society of America (SSA) meeting was held in Pasadena. I attended a field trip where we reviewed the latest research on some of the faults in the region. The guidebook is available on the SSA 2015 Annual Meeting website here.
Here is a map showing the location of today’s M 3.4 strike-slip earthquake (slightly oblique, as evidenced by the focal mechanism). I also plot the moment tensor for the 1994 Northridge earthquake (a compressional earthquake). Focal mechanisms and moment tensors are two ways to use seismologic observations to learn about two possible fault plane solutions for an earthquake. These two calculations are performed differently, but their graphical depictions are the same. Take a looksie at the USGS websites to learn more about focal mechanisms and moment tensors.
In the above map, I also include the “Did You Feel It” map showing how broadly this earthquake was felt across the southland. I plot the Modified Mercalli Intensity (MMI) contours, along with the MMI legend. The MMI is a scale that relates ground shaking observations to a qualitative shaking intensity scale (ranging from I to XII). There is more on the MMI scale here.
The NIF system has been active lately. In April and May of this year (2015) we had two swarms of seismic activity. I discuss the 1994 Northridge earthquake more on the April NIF page. I summarize the seismic activity from April and May on the May NIF page. Don’t forget about the March 2014 La Habra earthquake. That was larger in magnitude (M 5.1) and had a more widespread effect.
Here is a great illustration from UNAVCO showing an interpretation of the fault configuration in the LA basin.
Los Angeles: Newport Inglewood Earthquake!
