Analysis of Near-Source Ground Motion from the 2019 Ridgecrest Earthquake Sequence

ABSTRACT The Ridgecrest seismic sequence began on 4 July 2019 in California, on a hitherto relatively unmapped orthogonal cross-faulting system, causing mainly nonstructural or liquefaction-related damage to buildings in the vicinity of Ridgecrest and Trona, and also causing substantial surface rupture. The present study considers the near-source ground-acceleration recordings collected during the two principal events of the sequence—the 4 July moment-magnitude M 6.4 foreshock and the 6 July M 7.1 mainshock—to identify pulse-like ground motions, which may have arisen due to forward rupture directivity. Pulse-like seismic input is of particular interest to earthquake engineering due to its peculiar spectral shape and possibly increased damaging potential, and expanding the strong-motion databases with such records is a topical issue. In this context, a pulse identification methodology is implemented, partially based on computer-aided signal processing, but also involving manual classification. Nine ground-motion records were classified as pulse-like by this procedure. Further investigation led to the conclusion that, for some of these records, the impulsive characteristics could most likely be attributable to forward rupture directivity, whereas for others fling step may have also been an issue. Finally, clear signs of directionality were observed in these ground motions at periods near the pulse duration, manifesting as a polarization of the spectral ordinates toward the orientation of the impulsive component.

Performance of Seismic-Isolated Bridges in Relation to Near-Fault Ground-Motion and Isolator Characteristics

This paper investigates the performance of seismic-isolated bridges (SIBs) subjected to near-fault (NF) earthquakes with forward rupture directivity effect (FRDE) in relation to the isolator, substructure, and NF earthquake properties, and examines some critical design clauses in AASHTO's Guide Specifications for Seismic Isolation Design. It is found that the SIB response is a function of the number of velocity pulses, magnitude of the NF ground motion, and distance from the fault. Particularly, a reasonable estimation of the expected magnitude of the NF ground motion according to the characteristics of the bridge site is crucial for a correct design of the SIB. It is also found that the characteristic strength and post-elastic stiffness of the isolator may be chosen based on the characteristics of the NF earthquake. Furthermore, some of the AASHTO clauses are found to be not applicable to SIBs subjected to NF ground motions with FRDE.