On the Variability of Earthquake Ground Motion from the Sage Brush Flats High Density Array in Southern California

Debi Kilb, Christopher W. Johnson, Annemarie S. Baltay, & Frank L. Vernon

Published August 8, 2019, SCEC Contribution #9386, 2019 SCEC Annual Meeting Poster #061 (PDF)

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We explore the spatial variability in peak ground velocity (PGV) at the southern California Sage Brush Flats (SGB) study area, along the Clark branch of the San Jacinto Fault zone. The SGB dense network array spans a 0.6 km by 0.6 km footprint and includes 1,108 geophones deployed for a ~1-month period (07 May 2014 through 13 June 2014), with station spacings 10 m orthogonal to the fault and rows 30 m apart. We explore data from 38 M≥2 events within 200 km of the network centroid. The array footprint covers 3 branches of the fault and a small basin. These structures have a significant impact on PGV, with standard deviations ranging up to 20-37% of the mean PGV. These findings highlight that a single earthquake can produce large PGV variability within the small foot-print of our study area, which should be taken into account when spatially extrapolating ground motion values within heterogeneous regions. Comparing to a co-located borehole station (B946, depth of 148 m), the surface PGV measurements exceed the borehole PGVs by factors of 3-10, indicating the local fault structure, basins, topography and near surface amplification from soft sediments alters the seismic waves, producing higher amplitudes within the local basin structure regardless of the azimuthal trajectory of the seismic waves across the array. We do find repeatable PGV values (within the uncertainties; 0.0171±0.0045 and 0.0148±0.0039 mm/sec) from a co-located pair of M2.6 events on May 14th, which also show similarities in the mapped PGV spatial patterns. We conclude that similar earthquakes produce repeatable ground motions, but that these PGV values are variable across the array, suggesting ground motion prediction methods should potentially increase PGV uncertainties in regions of known faults and basins.

Key Words
High density array, San Jacinto Fault, PGV, GMPE, Southern California, Clark Fault

Kilb, D., Johnson, C. W., Baltay, A. S., & Vernon, F. L. (2019, 08). On the Variability of Earthquake Ground Motion from the Sage Brush Flats High Density Array in Southern California . Poster Presentation at 2019 SCEC Annual Meeting.

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