Seismic waveform attributes before and after the Loma Prieta earthquake: Scattering change near the earthquake and temporal recovery

During 1987-1995 several clusters of nearly identical seismic events (multiplets) occurred near the Loma Prieta source region. These multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 Loma Prieta main shock. For seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency. Post-Loma Prieta events have reduced coherencies with pre-Loma Prieta events in a spatially limited region close to the Loma Prieta hypocenter, while other paths remain nearly unaffected. These changes gradually recover within a time interval of 5 years after the Loma Prieta earthquake. A possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the Loma Prieta event. Postseismic relaxation processes such as crack healing, fluid diffusion, or after deformations lead to progressive closure of these cracks with time after the main shock. Thus the scattering properties of the local crust approach the pre-main shock state.