Bettina P. Allmann

A High-Frequency Secondary Event During the 2004 Parkfield Earthquake

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.

Spectral Discrimination between Quarry Blasts and Earthquakes in Southern California

We compare P-wave spectra of quarry blasts and earthquakes recorded by the southern California seismic network (SCSN) between 2000 and 2005, with the goal of developing methods to discriminate between these events. We process the spectra using an iterative robust least-squares method to isolate source, receiver, and propagation path contributions. This corrects for first-order attenuation structure, as well as near-receiver site effects and any errors in the instrument response functions. Using the earthquake spectra and a simple {omega}^-2 source model, we compute an empirical Green’s function (EGF) to remove the trade-off between the source terms and other parameters in our model. A constant stress-drop model gives a good fit to the observed average earthquake spectra over a wide range of moment magnitude, but provides a mediocre fit to the average quarry blast spectra, which have a generally steeper fall-off at high frequencies than {omega}^-2. We also compare P- and S-wave amplitudes and find modestly smaller average S amplitudes for the explosions compared to the earthquakes. For southern California, the root-mean-square (rms) misfit of P-wave spectra to an {omega}^-2 source model is a more reliable explosion discriminant than the S-to-P amplitude ratio and works for about 90% of the events.