Source Mechanism and Rupture Directivity of the Mw4.6 May 18, 2009 Inglewood, California Earthquake

Yan Luo, Ying Tan, Shengji Wei, Donald V. Helmberger, Sidao Ni, & Egill Hauksson

Published 2010, SCEC Contribution #1602

On May 18, 2009, a MW 4.6 earthquake occurred beneath Inglewood, California and was widely felt. Though source mechanism and its location suggest that the Newport-Inglewood Fault (NIF) may be involved in generating the earthquake, rupture directivity has to be modeled to establish the connection between the fault and the earthquake. We first invert for the event’s source mechanism and depth with the Cut And Paste (CAP) method in the long period band (>5s). Because of the lw velocity shallow sediments in the Los Angeles (LA) basin, we use two velocity models in the inversion for stations inside LA basin and those outside. However, little difference is observed in the resolved source mechanism (Mw4.6, strike 246°/145°, dip 50°/77°, rake 17°/138°) and depth (7~9 km), compared to an inversion using the standard Southern Calfornia (SC) model. With the resolved source parameters, we calibrate the amplitude anomaly of the short period (SP) (0.5-2Hz) P waves with Amplitude Adjustment Factors (AAF). These AAFs are used as corrections when retrieving source mechanisms of the smaller aftershocks using SP P waves alone. Most of the aftershocks show similar source mechanisms as that of the mainshock, providing ideal empirical Green’s functions (EGF) for studying its rupture process. We use a forward modeling approach to retrieve rupture directivity of the mainshock, consistent with movement on the NIF with rupture towards the southeast. Although we focus on P waves for analyzing rupture directivity, the resolved unilateral pattern is also confirmed with the azimuthal variation of the duration of SH waves observed in the basin. The high rupture velocity near the shear velocity and relatively low stress drop is consistent with the hypothesis of rupture on a mature fault.

Luo, Y., Tan, Y., Wei, S., Helmberger, D. V., Ni, S., & Hauksson, E. (2010). Source Mechanism and Rupture Directivity of the Mw4.6 May 18, 2009 Inglewood, California Earthquake. Bulletin of the Seismological Society of America, 100, 3269-3277.