Near-Field Ground Motions from the July 2019 Ridgecrest, California, Earthquake Sequence

Abstract

The 2019 Ridgecrest, California, earthquake sequence, including an Mw 6.4 event on 4 July and an Mw 7.1 approximately 34 hr later, was recorded by 15 instruments within 55 km nearest-fault distance. To characterize and explore near-field ground motions from the Mw 6.4 foreshock and Mw 7.1 mainshock, we augment these records with available macroseismic information, including conventional intensities and displaced rocks. We conclude that near-field shaking intensities were generally below modified Mercalli intensity 9, with concentrations of locally high values toward the northern and southern termini of the mainshock rupture. We further show that, relative to near-field ground motions at hard-rock sites, instrumental ground motions at alluvial near-field sites for both the Mw 6.4 foreshock and Mw 7.1 mainshock were depleted in energy at frequencies higher than 2–3 Hz, as expected from ground-motion models. Both the macroseismic and instrumental observations suggest that sediments in the Indian Wells Valley experienced a pervasively nonlinear response, which helps explain why shaking intensities and damage in the closest population center, Ridgecrest, were relatively modest given its proximity to the earthquakes.

EXCEPTIONAL GROUND MOTION DURING THE SHALLOW Mw 4.9 2019 LE TEIL EARTHQUAKE, FRANCE

“…ref. [50][51] ). All the evidences for upthrown and displaced rocks are located in the vicinity of the SW part of the surface fault trace (Figures 1a and S6), an area that remarkably coincides with the area where simulated vertical PGA exceeds gravity (Figure 3).…”

Evaluating the identification methods of permanent displacement derived from strong motion records: Case studies in the 2023 Turkey–Syria earthquake

“…6 Pulse-like ground motion is typically attributed to the directivity effect. 7 This effect occurs when the fault rupture propagates toward a strong motion station and the rupture velocity is closely consistent with the shear wave velocity of the medium. In such instances, the velocity pulse can often be associated with a permanent displacement caused by the fling-step effect.…”

“…However, the mechanisms and processes that produce the velocity pulse remain elusive 6 . Pulse‐like ground motion is typically attributed to the directivity effect 7 . This effect occurs when the fault rupture propagates toward a strong motion station and the rupture velocity is closely consistent with the shear wave velocity of the medium.…”