Measurements of seismic energy from scattered coda waves indicate that apparent stress increases with increasing seismic moment, a consequential result for several reasons. For one, it may constrain possible forms of fault weakening with increasing slip. Moreover, if larger earthquakes more efficiently generate energy than their smaller counterparts, strong ground motion from large events would be more intense than anticipated from the extrapolation of smaller events. The relatively sparse strong motion data set does not appear to support this conclusion, leading us to reexamine seismic energy estimates. Coda envelope measurements of the source amplitude are much more stable than measurements made using direct waves because the coda averages over both path and source variability. We follow Mayeda et al.  by fitting narrowband coda envelopes at 15 frequency bands for events at a single station to an analytic expression. A(f,t,r)=P(r,f)t- γ(r)eb(r)t describes the time decay of the coda envelope, A(f,t,r), at each frequency band, distance and station, where γ(r) and b(r) are the distance dependent parameters being fit and P(r,f) describes the source amplitude, site response, path effects, scattering, spreading and attenuation. We make empirical distance corrections to tie the observed amplitudes to absolute moment-rate spectra. From this, we estimate the radiated seismic energy. Our analysis differs somewhat from that used previously in that we fit parameters simultaneously, rather than sequentially, and place soft bounds on them due to physical considerations. We analyze broadband data from the western United States and Japan to revisit the scaling of seismic energy using coda waves.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7200 SEISMOLOGY;
- 7209 Earthquake dynamics (1242);
- 7212 Earthquake ground motions and engineering seismology;
- 7215 Earthquake source observations (1240)