The Fine-Scale Structure of Long Beach, California, and Its Impact on Ground Motion Acceleration

The metropolitan Los Angeles region represents a zone of high-seismic risk due to its proximity to several fault systems, including the San Andreas fault. Adding to this problem is the fact that Los Angeles and its surrounding cities are built on top of soft sediments that tend to trap and amplify seismic waves generated by earthquakes. In this study, we use three dense petroleum industry surveys deployed in a 16 x 16-km area at Long Beach, California, to produce a high-resolution model of the top kilometer of the crust and investigate the influence of its structural variations on the amplification of seismic waves. Our velocity estimates reveal substantial lateral contrasts and correlate remarkably well with the geological background of the area, illuminating features such as the Newport-Inglewood fault, the Silverado aquifer, and the San Gabriel river. We then use computational modeling to show that the presence of these small-scale structures have a clear impact on the intensity of the expected shaking, and can cause ground-motion motion acceleration to change by several factors over a subkilometer horizontal scale. These results shed light onto the scale of variations that can be expected in this type of tectonic settings and highlight the importance of resolution in modern-day seismic hazard estimates. Plain Language Summary With a population of almost 20 million, the metropolitan Los Angeles region represents a zone of high-seismic risk due to its proximity to several fault systems. This unique setting demands extensive seismic hazard analyses, which, in turn, depend on our knowledge of the elastic properties of the subsurface. With the use of three dense petroleum industry surveys, we image the shallow crust beneath Long Beach, CA, and construct a high-resolution velocity model of the region. We then use these estimates to investigate the amount of earthquake wave amplification that we can expect in the area. Our results reveal that small-scale structural heterogeneities have a significant impact on the propagation of seismic waves, and can even cause earthquake shaking to vary by several factors over a subkilometer horizontal scale.

Automated summary

The metropolitan Los Angeles region, with its high-seismic risk due to proximity to fault systems, has a complex interplay between structural variations and seismic wave amplification. By using petroleum industry surveys, researchers have been able to create a high-resolution model of the crust, revealing the significant impact of small-scale structures on earthquake wave propagation.