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Source Mechanism and Rupture Directivity of the 18 May 2009 M_W 4.6 Inglewood, California, Earthquake

Luo, Yan and Tan, Ying and Wei, Shengji and Helmberger, Don and Zhan, Zhongwen and Ni, Sidao and Hauksson, Egill and Chen, Yong (2010) Source Mechanism and Rupture Directivity of the 18 May 2009 M_W 4.6 Inglewood, California, Earthquake. Bulletin of the Seismological Society of America, 100 (6). pp. 3269-3277. ISSN 0037-1106. doi:10.1785/0120100087.

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On 18 May 2009, an M_w 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 must 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 method in the long-period band (>5 s). Because of the low velocity shallow sediments in the Los Angeles (LA) basin, we use two velocity models in the inversion for stations inside and outside the LA basin. However, little difference is observed in the resolved source mechanism (M_w 4.6, strike 246°/145°, dip 50°/77°, rake 17°/138°) and depth (7 to ~9 km), compared to an inversion using the standard southern Calfornia model. With the resolved source parameters, we calibrate the amplitude anomaly of the short-period (0.5–2 Hz) P waves with amplitude adjustment factors (AAF). These AAFs are used as corrections when retrieving source mechanisms of the smaller aftershocks using short-period P waves alone. Most of the aftershocks show similar source mechanisms as that of the mainshock, providing ideal empirical Green’s functions (EGFs) 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 toward 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 are consistent with the hypothesis of rupture on a mature fault.

Item Type:Article
Related URLs:
URLURL TypeDescription DOIArticle
Zhan, Zhongwen0000-0002-5586-2607
Ni, Sidao0000-0003-2988-4850
Hauksson, Egill0000-0002-6834-5051
Additional Information:© 2010 Seismological Society of America. Manuscript received 5 April 2010. Supported by the Chinese Academy of Science (CAS) Key Laboratory of Dynamic Geodesy CAS fund kzcx2-yw-116-01, the Institute of Earth Science funding 02092410, China Earthquake Administration funding 200808078, U.S. Geological Survey (USGS) Award No. G09AP00082, and Southern California Earthquake Center (SCEC) Award No. 119938. Data were obtained via the Seismic Transfer Program from the Southern California Earthquake Data Center. Discussion with Hiroo Kanamori helped improve the manuscript. This research was supported by the SCEC, which is funded by National Science Foundation Cooperative Agreement EAR-0529922 and USGS Cooperative Agreement 07HQAG0008. The SCEC contribution number for this paper is 1439; it is Contribution No. 10041, Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology.
Group:Seismological Laboratory
Funding AgencyGrant Number
Chinese Academy of Sciencekzcx2-yw-116-01
Institute of Earth Science02092410
China Earthquake Administration200808078
Southern California Earthquake Center (SCEC)119938
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center (SCEC) 1439
Caltech Seismological Laboratory10041
Issue or Number:6
Record Number:CaltechAUTHORS:20110104-114334345
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:21579
Deposited By: Tony Diaz
Deposited On:25 Jan 2011 18:30
Last Modified:09 Nov 2021 15:58

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