HR: 12:05h
AN: S12A-07    [Abstracts]
TI: Evidence for Non-Constant Energy/Moment Scaling
AU: * Mayeda, K
EM: kmayeda@llnl.gov
AF: Lawrence Livermore National Laboratory, P.O.Box 808, L-205, Livermore, CA 94551 United States
AU: Gok, R
EM: gok1@llnl.gov
AF: Lawrence Livermore National Laboratory, P.O.Box 808, L-205, Livermore, CA 94551 United States
AU: Hofstetter, A
EM: rami@seis.mni.gov.il
AF: Geophysical Institute of Israel, P.O.Box 182, Lod, 71100 Israel
AU: Walter, W R
EM: bwalter@llnl.gov
AF: Lawrence Livermore National Laboratory, P.O.Box 808, L-205, Livermore, CA 94551 United States
AB: Unlike the well-established techniques of long-period waveform modeling for seismic moment (Mo), reliable measurements of radiated seismic energy (Er) have been elusive. In addition to correcting for source heterogeneity (e.g., directivity and source radiation pattern effects) significant broadband path corrections are required at scale lengths that are poorly resolved in the Earth. At frequencies above ~0.5 Hz, site response corrections become increasingly important, especially for small-to-moderate sized earthquakes. Comparing Er for events distributed over a broad region can make resolving scaling variations impossible. For example, if the region includes a mixture of source types (e.g., high and low stress drop events at a given magnitude) or if the path corrections are overly simplistic, then the study becomes an apples to oranges comparison. Instead, it may be more prudent to look for scaling within earthquake sequences or smaller geographical regions. To avoid the above mentioned problems we did the following: 1) We chose a wide range of event sizes from selected aftershock sequences that were recorded by multiple broadband stations; 2) We applied the coda methodology to estimate the source spectra to minimize any source heterogeneity; 3) We validated our spectra against independent moment estimates from regional moment tensor inversions. Using the large earthquakes sequences Mw 7.4 Izmit, Mw 7.2 Gulf of Aqaba, and Mw 7.2 Hector Mine our findings point to the scaled energy (e=Er/Mo) increasing with increasing moment for 3.7 < Mw < 7.4. As a validation, we posed the following question to ourselves. "What source spectral shape is needed to maintain good agreement with independent moments and have a constant scaled energy?" We tried to transform our spectra by adjusting the site correction terms such that the scaled energy would be constant (e.g., 5.0e-5, 5.5e-5 or 1.0e-4) but at the same time maintaining good agreement with our seismic moments and those derived from waveform modeling. All three cases resulted in very unusual, non-self similar source spectra. We believe that this study properly accounts for many of the problems that are commonly encountered in energy studies and that the use of the coda-derived source spectra provide unparalleled stability.
DE: 7203 Body wave propagation
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting