An Evaluation of the SCSN Moment Tensor Solutions: Robustness of the M_w Magnitude Scale, Style of Faulting, and Automation of the Method

Abstract

We have generated moment tensor solutions and moment magnitudes (M_w) for >1700 earthquakes of local magnitude (M_L) >3.0 that occurred from September 1999 to November 2005 in southern California. The method is running as an automated real-time component of the Southern California Seismic Network (SCSN), with solutions available within 12 min of event nucleation. For local events, the method can reliably obtain good-quality solutions for M_w with M_L >3.5, and for the moment tensor for events with M_L >4.0. The method uses the 1D Time-Domain INVerse Code (TDMT_INVC) software package (Dreger, 2003). The Green's functions have been predetermined for various velocity profiles in southern California, which are used in the inversion of observed three-component broadband waveforms (10–100 sec), using data from at least four stations. Moment tensor solutions have an assigned quality factor dependent on the number of stations in the inversion, and the goodness of fit between synthetic and observed data. If a minimum quality factor is attained, the M_L or M_w is 5.0 or greater, and if the event is in the southern California reporting regions, the M_w will be the official SCSN/CISN magnitude. The M_w from the high-quality solutions determined from our method generally correlate very well with reviewed M_L, except in regions at the perimeter of the network. The M_w reported here indicates the SCSN M_L systematically underestimates the magnitude in the Mexicali region of Baja California, Mono Lakes area, Coso region, and the Brawley seismic zone, and overestimates the magnitude in the Coastal Ranges. Comparisons of the moment tensors determined using this model are made with Harvard Centroid Moment Tensors generated for larger earthquakes in the California region, and recent 3D models for events in the Los Angeles region, with excellent correlation. Most of the earthquakes with good-quality solutions exhibit strike-slip faulting, in particular, along the major late Quaternary strike-slip faults. Thrust faulting on east– west-striking planes is observed along the southern edge of the Transverse Ranges, while northwest-striking thrust faulting is observed in the Coastal Ranges. Normal faulting is most common in Baja California and southern Sierra Nevada including the western Basin and Range region. Poor-quality solutions with unreliable M_w are caused by excessive background noise in the waveforms. Small events (M_L < 4.0) can be affected by ambient noise or teleseisms, but larger events can also have unreliable solutions if they follow a recent large regional event.

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