The Science Behind Earthquakes

California State Route 178 southwest of Trona, following the magnitude 7.1 earthquake. (USGS)

If you pay attention to the news, then no doubt you are familiar with the fact that last Thursday and Friday (7/4 & 7/5/2019) Southern, CA was rocked by two large earthquakes (6.4 then 7.1) plus many small aftershocks. Having lived in California, I can attest that small earthquakes (<4.0) are common and usually go unnoticed by longtime residents. However, larger earthquakes reap havoc on even the most seasoned person.

The earthquakes occurred along a shallow strike-slip fault system near the town of Ridgecrest. A strike-slip, or lateral fault is one of three types of faults (the other two being normal and thrust), where the motion occurs in a side-to-side motion. This type of fault is the same as the famous San Andreas. Most notably, this was an extremely shallow earthquake with an epicenter at only 8 km (4.9 mi) below the surface. This is extremely shallow considering that earthquake epicenters can be locate >100 km below the surface.

Mainshock and aftershock locations shortly after M7.1 event, exhibiting a distinctive “T” pattern created by two perpendicular fault zones. The M6.4 event ruptured along the NE-SW trending fault, and the subsequent M7.1 event ruptured along the NW-SE trending fault. (USGS)

Southern CA is full of fault systems consisting of all types. As the Pacific plate subducts under the North American plate, pressure is built up along the plate boundary (San Andreas) and the residual faults in the ranges to the east. When the pressure reaches a critical level, the faults slips to release the stored up energy which is felt as the earthquake. In multi-fault systems, slip on one can trigger another event elsewhere in the system where pressure has been storing up over time.

While any damage is unfortunate as it impacts people's lives, this is no exception, it is fortunate that this magnitude earthquake occurred as far inland as it did. The ground is much more solid in this region and thus the surface waves were not as damaging as they would be had the epicenter been focused closer to the coast. The LA region sits primarily on sand which amplifies the movement of surface waves, causing significantly more damage to infrastructure.

To everyone affected by these earthquakes: stay safe, keep your spirits up. Its usually during the worst times that we see the best of our neighbors.