Dr. Lorraine Wolf discusses the recent earthquakes in California

Published: July 10, 2019
Updated: July 15, 2019
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Dr. Lorraine Wolf, geosciences professor and director of undergraduate research in the College of Sciences and Mathematics, discusses the recent earthquakes in California.

These earthquakes left a crack so big that it can be seen from space. What exactly does this mean? Is that area extremely unstable now?

The 7.1 earthquake is estimated to have originated at a depth of about 8 kilometers, which for seismologists, is considered shallow. This, in addition to the size of the quake, led to a lot of shaking and ground failure (i.e., cracks, some landslides) that you have seen in the media. In the earlier July 4 quake, the earthquake nucleated a bit deeper, but then traveled towards the surface (according to one model I have seen). Although earthquakes are plotted as a dot on most maps, faults are really big structures.

After the two quakes, experts discussed the percentage likelihood of a third one. How long should people be worried about another quake in the aftermath of these?

The aftershock duration typically varies with the size of the main shock. Aftershocks are smaller in size (by definition) than main shocks or foreshocks (earthquakes that precede the main shock). The July 4 quake was preceded by smaller shocks and the aftershock interval between the July 4 quake and the July 6 quake included approximately 250 magnitude 2.5+ earthquakes. People should expect aftershocks to taper off with time, but in the near term, they could feel shaking for weeks to months, or maybe years.

Are these large quakes expected (based on historical patterns and what we know about seismic activity) or was this a surprise to those who research and study earthquakes?

This series of earthquakes occurred in what is known as the Eastern California shear zone. A shear zone (as the name implies) is an area of the Earth’s crust that is fractured in a series of subparallel faults that accommodate shear movement along a plate boundary. In southern California, the Pacific plate is moving northwest relative to the North America plate that is moving southeast. The stresses associated with this plate boundary (most people think only of the San Andreas fault as marking the boundary) is distributed over several fault systems. The Eastern California shear zone is just one of these—so no, conceptually, the earthquake is not surprising. However, no one expects one on any particular day, so that is always a surprise.

Is there anything new we can learn about quakes after the ones that took place this weekend?

First, this area (southern California)—although known to be seismically active—has been relatively quiet for some time. People often get complacent and stop thinking that a big earthquake may strike. However, we can learn that being prepared for large earthquakes and their aftermaths (such as unstable slopes, landslides, fires from burst gas lines and soil liquefaction) minimizes the impact on the communities affected by the quake. This community was pretty well prepared and it is sparsely populated. A similar event in Los Angeles would have had a much greater impact.

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