Near Surface Damage Caused by the Strong Ground Motion of the M6.9 Loma Prieta and M5.4 Chittenden Earthquakes

We use a catalog of 57 repeating earthquake sequences to study the damage to near-surface materials, manifest as changes in seismic wave velocity, caused by strong ground motion. We believe that near surface damage (cracking) is the most likely cause for velocity reductions that we observe immediately following both the M6.9 Loma Prieta and M5.4 Chittenden earthquakes. The strong ground motion during both of these events was strong enough to open cracks near the Earth's surface, the presence of which reduces seismic velocities. The velocity reductions heal with time, following Loma Prieta and Chittenden in a manner similar to the "slow dynamic" healing behavior observed in laboratory studies [TenCate, et al., 2000]. Since the damage left by Loma Prieta had not completely healed by the time Chittenden occurred, it is probable that the local rocks were more susceptible to further damage, allowing the much weaker motions of the Chittenden Earthquake to cause damage comparable in magnitude as that of the Loma Prieta Earthquake. We have identified the above conditions by studying repeating earthquakes (multiplets) on the San Andreas Fault. Using a moving window cross correlation technique to identify changes in the nearly identical waveforms of a repeating earthquake sequence, we can observe late-arriving phases, after both the Loma Prieta and Chittenden earthquakes. We attribute these delays to near surface velocity reductions localized to a damage zone close to the Loma Prieta rupture zone. We observe a similar phenomenon in the cross correlation coefficient (CCC) data. Immediately following the Loma Prieta and Chittenden Earthquakes, the CCC drops sharply and heals in time in a manner similar to the healing of the velocity reductions. This is not surprising because the changes in CCC reductions should scale linearly with the magnitude of the velocity perturbation. The drops in CCC don't always parallel velocity changes; however, they can also measure more general changes in waveform character. A combination of the two measurements not only allows us to identify parts of the seismogram where an arrival disappears or a new one appears, but it also allows us to further constrain the nature of the variation. TenCate, J.N., D.E. Smith, and R. Guyer, Universal Slow Dynamics in Granular Solids, Physical Review Letters, 85, 1020-1023, 2000.