Sub‐Kilometer Correlation Between Near‐Surface Structure and Ground Motion Measured With Distributed Acoustic Sensing.

Earthquake ground motion depends strongly on near‐surface structure, which is challenging to image in urban areas at high resolution. Distributed acoustic sensing (DAS) is an emerging technique that provides a scalable solution by converting preexisting fiber‐optic cables into dense seismic arrays. After the July 2019 M7.1 Ridgecrest earthquake, we converted an underground dark fiber across the city of Ridgecrest, CA, into a DAS array. The recorded aftershocks show substantial lateral variability in site amplification over only 8‐km in distance. To understand the cause of such variability, we used three months of continuous data, dominated by traffic‐generated seismic noise, to image near‐surface structure along the fiber path. We find that the lateral variations of earthquake shaking correlate well with the shallow shear velocity model at sub‐kilometer scales, in particular micro‐basins filled with soft sediments. These results highlight the great potential of DAS for high‐resolution seismic hazard mapping in urban areas. Plain Language Summary: Earthquake shaking is often highly variable in space. Yet, the physics behind this variability is not entirely clear, partially because high‐resolution instrumental observations are rare. Distributed acoustic sensing (DAS) is an emerging technique that can turn existing fiber‐optic cables into cost‐effective seismic arrays with meter‐scale spacing. After the Ridgecrest M7.1 earthquake in July 2019, we repurposed a 10‐km unused telecom fiber in the city of Ridgecrest, CA, as a 1250‐sensor DAS array and recorded thousands of aftershocks within three months. The aftershock data reveal significant variation of ground shaking over short distances. We used seismic noise generated by traffic along the same cable to image the near‐surface structure, which correlates well with the earthquake shaking intensity. Our findings show that DAS is a valuable tool for seismic hazard mapping in urban areas. Key Points: We established a DAS array to capture the aftershocks of the 2019 M7.1 Ridgecrest earthquake and continuous traffic noiseEarthquakes recorded by the DAS array show notable lateral variation of ground acceleration, after conversions from strainThe Vs30 profile imaged by DAS ambient noise interferometry correlates well with the site terms at sub‐kilometer scales [ABSTRACT FROM AUTHOR]