Regional CODA magnitudes in central Asia and MB (LG) transportability.
- William Scott
- Hans E.
- Kevin M.
Local and near regional coda have been shown to provide accurate and precise estimates of source, path and site effects. Using empirical methods, we investigate tlic use of coda to determine moments and magnitudes using regional distance (to 2500 km) data from 21 stations in central Asia and China. We find source levels for bands between 33 s and 8 Hz from events recorded at Urumchi (WMQ) to be a factor of two more consistent for coda than for direct waves, for bands outside the microseism range. However, the anticipated path averaging of regional coda is insufficient io remove bias in all but the lowest frequency bands. We correct for path bias by spatially interpolating coda levels rclative to mb(PDE). For higher bands (1 Hz), the spatial correction patterns vary by an order of magnitude and are similar to patterns obtained using direct L,. For the lowest band (20-33 s) the maximum spatial variation shrinks to under a factor of 4 and changes sign, reflecting effects other than crustal Q. Thus, the low frequency coda could be useful for correcting for source effects in empirical or tomographic path studies, which is currently performed using mb. After rcmoving path bias from coda measurements, we find that amplitude measurement consistency between all 21 stations vanes considerably from pair to pair (a = 0.12 to 0.37), with low-Q surroundings and poor site conditions yielding the least stable measurements. CMT based moments (Mw) derived from 20-33 s WMQ coda are verified by comparing with moments derived from waveform fitting studies (a = 0.18). We continue investigations into the transportability of regional magnitudes using the m{sub b}(L{sub g}) scale devised by Nuttli. Previous work has shown that mb(L) is portable for earthquakes provided that L, attenuation is well calibrated for propagation paths. ln this study, our focus shifts to explosion sources, and the question of transportability of m{sub b}(L{sub g}) for different test sites. We revisit Nuttli's results, which were based on observations at far-regional and teleseismic distances, and depended critically upon accurate knowledge of the L, Qo. In this paper, measurements of m{sub b}(L{sub g}) are reported for stations within 1000 km, and as such, errors due to uncertainties in the path correction are smaller than was the case for Nuttli's measurements.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 975710
- Report Number(s):
- LA-UR-01-4837; TRN: US201018%%798
- Resource Relation:
- Conference: "Submitted to: 23rd Seismic Research Review, September, 2001, Jackson Hole, Wyoming"
- Country of Publication:
- United States
- Language:
- English
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