ADVANCED WAVEFORM SIMULATION FOR SEISMIC MONITORING EVENTS; REPORTING PERIOD NOVEMBER 1, 2006 - JANUARY 31, 2007

Abstract

The amplitude fluctuations of seismic waves from point source radiation patterns are among the direct results of rupture propagation. However, these amplitude signals are seldom used for small earthquakes due to the large uncertainties in the focal mechanisms of the events or the empirical Green’s functions (EGFs). Moreover, it has been controversial if radiation patterns still exist at high frequencies, e.g., 10 Hz. We present an evidence for high frequency radiation patterns by showing that with accurately determined focal mechanism solutions, the broadband P wave records from a thrust event (M 3.0) can be turned into those from a distinctly different strike slip event of similar magnitude with simple radiation pattern corrections. Moreover, we observe systematic azimuthal variations in the P and S wave amplitude ratios between the records from unilateral magnitude 4 events and the EGF events, which suggest that these simple amplitude ratios can provide fast and useful indications of the rupture directivity. For further estimates of the fault dimension and rupture speed, we developed a forward modeling approach to retrieve apparent source time functions (ASTFs) of small earthquakes, where we assume simple Haskell model and take a grid search approach to find the ASTFs for each recordingmore » station. Compared to the usually used deconvolution approach, this method fully utilizes the information of both duration as well as amplitude. We applied this approach to the 2003 Big Bear sequence, and successfully obtained the complex rupture propagation patterns of the few large events (3.5 < M < 4.25), where both uni-lateral and bi-lateral events, fast and slow rupture speeds are observed.« less