$M_{w}$

The REDI system has routinely produced automatic estimates of moment magnitude ($M_{w}$) for many years. However, these estimates were not routinely used as the ``official" magnitude until after the 05/14/2002 Gilroy earthquake ($M_{w}$ 4.9, $M_{L}$ 5.1), motivated by the complications created by the publication of multiple magnitudes.

In last year's annual report, we discussed the issue of when to report $M_{w}$. As currently implemented, solutions that meet a minimum quality criterion are automatically reported (a variance reduction of 40% or higher). This criterion appears to work very well and screens out events contaminated by teleseisms. Over the last few years, nearly all events over 4.5 have met this criterion, as have a number of events in the M3.5-4.5 range.

As part of the effort to establish a statewide magnitude reporting hierarchy, we have looked more closely at the estimates of $M_{w}$ (Gee et al., 2003b; 2004b) and the comparison between $M_{w}$ and $M_{L}$.

Two methods of determining regional moment tensor (RMT) solutions have been automated as part of the REDI system - the complete waveform modeling technique (CW) of Dreger and Romanowicz (1994) and the surface wave inversion (SW) of Romanowicz et al. (1993). Comparison between the CW method and other regional moment tensor studies in northern California and the western United States show excellent agreement in the estimate of seismic moment and $M_{w}$. Over 128 events, the average difference in $M_{w}$ is 0.002 magnitude units.

Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2005, The Regents of the University of California