Slip model with supershear rupture velocity and stress drop for an M4.5 aftershock of the 2019 Ridgecrest, California, earthquake sequence

Abstract

SUMMARY We use records from 24 stations with epicentral distances under 50 km to invert for slip and rupture time of an M4.5 aftershock of the 2019 Ridgecrest, California, earthquake sequence. Slip was determined by inverting source time functions (STFs), which were obtained at each station by decomposing a main shock, in this case an M4.5 aftershock, into a series of subevents using an empirical Green's function (eGf) as a template. The technique is iterative in the time domain with a positivity constraint. The STFs are characterized by a single pulse with durations that were longer in the southeast direction (about 1.0 s) and shorter in the northwest direction (about 0.6 s) suggesting rupture directivity to the northwest. We inverted the STFs for slip using a tomographic algorithm from that uses a high-frequency approximation to ray theory for the forward problem. The tomographic algorithm solves for slip and rupture time on a grid of points, which provides a better strategy for elucidating directivity than a model that uses subfaults. We solve for the strike-slip intensity on a near-vertical plane striking northwest, similar to the M7.1. After nine iterations, the slip model reduces the chi-squared estimate of fit by 68 per cent. Most of the slip is to the northwest, with a narrow well-defined peak in slip about 0.3 km NW of the origin. The peak is fairly symmetrical and about 1.5 km across its base. The model shows significant directivity to the northwest as suggested by the azimuthal variation of the STF pulse widths. Rupture velocity increases from nucleation to the peak in slip, becomes supershear, and then slows to about 3.2 km s−1, but there is little slip beyond 0.8 s contour in the forward direction. It appears that the peak in slip contributed stress to accelerate the rupture to the northwest. Low-level, parabolic-shaped slip patterns are seen to the northwest, which could be where the rupture front is dying out. Similar, but fainter curves facing the southeast suggests some bilateral rupture but the rupture to the northwest was stronger. Static stress drop was calculated using a formula for an elastic half-space giving a maximum of 35 bars at the peak in slip, but smoothing controls the absolute value. The average over a presumed fault plane is about 1.4 bars, much less than the peak. Slip and stress drop are highly heterogeneous.