Evolution of the Regional Coda Methodology
For the past decade Lawrence Livermore National Laboratory (LLNL) and Los Alamos National Laboratory (LANL), have been developing and testing a stable, regional coda magnitude methodology for the determination of magnitude and yield. The motivation behind this research was to take advantage of the averaging nature of coda waves in support of monitoring small seismic events from a sparse regional seismic network (e.g., International Monitoring System (IMS) network). The methodology as described in Mayeda et al., (2003) has been successfully applied in a variety of tectonic settings where the assumption of a 1-D, radially symmetric path correction was sufficient. In general, this resulted in inter-station amplitude scatter that was 3-to-4 times smaller than the traditional approach using direct S, Lg and surface waves (0.02< f <8.0-Hz). However, in more laterally complex regions there is a need to extend this approach to account for smaller scale 2-D variations in structure, especially at frequencies above {approx}1-Hz. Recently, Phillips et al., (2003) have applied a 2-D approach to data in central Asia by assuming that the coda envelope amplitude could be idealized as if it were a direct wave. They performed a tomography to invert for Q along the path and through the choice of damping parameter and geometrical spreading, they in effect distributed the attenuation over an area, not unlike what has been previously assumed in the single-scattering model of Aki (1969) where the attenuation is attributed to an ellipsoidal volume. As a test of the two methods, we chose over 100 well-located earthquakes (3.5 < Mw < 6.5) distributed throughout the northern California region recorded on high quality broadband stations of the Berkeley Digital Seismic Network (BDSN). The complicated tectonics of the region coupled with this high quality data, provides for an ideal situation to test the two approaches. We found that source spectra derived from the 2-D approach were essentially identical to those from the 1-D approach for frequencies less than {approx}0.7-Hz, however for the high frequencies (0.7 < f < 8.0-Hz), the 2-D approach resulted in inter-station scatter that was generally 30% smaller. Since our main objective is to improve moment-magnitude estimation for small regional events, the 30% improvement is significant in helping to meet the United States (U.S.) monitoring goals.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15014475
- Report Number(s):
- UCRL-CONF-205440; TRN: US200802%%1242
- Resource Relation:
- Conference: Presented at: Seismic Research Review, Orlando, FL, United States, Sep 21 - Sep 23, 2004
- Country of Publication:
- United States
- Language:
- English
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