Clinton, John F. and Hauksson, Egill and Solanki, Kalpesh
An Evaluation of the SCSN Moment Tensor Solutions: Robustness of the M_w Magnitude Scale, Style of Faulting, and Automation of the Method.
Bulletin of the Seismological Society of America, 96
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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.
|Additional Information:||© 2006 Bulletin of the Seismological Society of America.
Manuscript received 2 December 2005.
Comments from the Associate Editor Jeanne Hardebeck, and an anonymous reviewer, greatly improved the manuscript. We thank Douglas Dreger, Lind Gee, Peggy Hellweg, and Pete Lombard from the Berkeley Seismological Laboratory for their discussions and advice. Georgia Cua also provided much feedback. Moment Tensors are computed using the mtpackagev1.1 package developed by Douglas Dreger of the Berkeley Seismological Laboratory, and Green’s functions were computed using the FKRPROG software developed by Chandan Saikia of URS. Many of the figures were created with GMT (Wessel and Smith, 1998). Much of the manuscript was prepared while J.F.C. was at the University of Puerto Rico at Mayagüez (UPRM). The support of the Puerto Rico Strong Motion Program (PRSMP) in the Civil Engineering Department at UPRM is gratefully appreciated. The SCSN is supported by NEHRP/USGS and ANSS through Cooperative Agreement 04HQAG0010, and California Office of Emergency Services through Contract 6028-2, and IRIS/USArray and through contacts 473 and 310; Contribution number 9147, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena. |
|Funding Agency||Grant Number|
|California Office of Emergency Services||6028-2|
|Other Numbering System:|
|Other Numbering System Name||Other Numbering System ID|
|Caltech Division of Geological and Planetary Sciences||9147|
|Official Citation:||John F. Clinton,
and Kalpesh Solanki
An Evaluation of the scsn Moment Tensor Solutions: Robustness of the Mw Magnitude Scale, Style of Faulting, and Automation of the Method
Bulletin of the Seismological Society of America, October 2006, v. 96, p. 1689-1705, doi:10.1785/0120050241|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited On:||06 Mar 2013 19:04|
|Last Modified:||24 Jan 2016 20:03|
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