Analysis of waveform and catalog data of aftershocks for properties of earthquakes and faults.

摘要

We study the statistical relationship between aftershock productivity and background geophysical features using an earthquake catalog with &sim340,000 events that occurred between 1984 and 2004 in the Southern California region. We analyze properties of individual aftershock sequences and the average properties of stacked aftershock sequences in five regions. The results indicate that productivities of the aftershock sequences are inversely correlated with heat flow and existence of deep sedimentary covers, in agreement with the damage model predictions given by Ben-Zion and Lyakhovsky (2006). Using the observed ratios of aftershock productivities, along with simple expressions based on the damage model, we estimate the seismic coupling coefficients in the different regions. We estimate seismic strain-drops from a data set with &sim26,000 aftershocks that occurred along the Karadere-Duzce branch of the North Anatolian Fault after the 1999 Izmit mainshock. We use a method that is associated with separation of source, travel-time and station spectral terms, and stacking results at several stages to enhance the signal-to-noise ratio. The strain-drops are obtained by fitting iteratively the separated source spectral of 201 nearest neighboring events in different amplitude bins to the omega square source spectra model. We analyze the variations of the obtained strain-drops along the fault zone, with the depth extension. Furthermore, we analyze seismic source properties using both P and S waves on seismograms. We develop a procedure to fit for P/S corner frequency ratios together with relative strain-drop systematically. We consider five different schemes to analyze the S wave, and the obtained median values of frequency ratios from all schemes are larger than 1.5. We describe an algorithm to detect clipped waveforms in the same &sim26000 data set based on statistical characteristics of observed amplitude ranges, properties of neighboring points in seismograms. To the detected clipped waveforms, the intensity of waveform clipping increases with proximity to the fault and site effects. We compare two correcting methods (the Kriging method and similar waveform method) to several simple cases with 1-9 consecutive clipped points on artificially clipped waveforms.