Broadband waveform modeling over a dense seismic network

Abstract

We developed a two-way calibration technique for studying clustered events, particularly their mechanisms and rupture directivities. First, we demonstrate that the magnitude 4 events with known source mechanisms can be used to calibrate the path effects on the short-period (0.5-2 sec) P waves, so that the corrected P waves can be modeled for determining focal mechanisms of the smaller events. The correction is formulated in terms of a station-specific Amplitude Amplification Factor (AAF), whose origin is mainly due to the site effect. Second, we show that the smaller events with radiation pattern corrections provide excellent empirical Green's functions (EGFs) for investigating the detailed rupture processes of the magnitude 4 events. In Chapter 2 of this thesis, we present the application of our methods to the 2003 Big Bear sequence. A new technique CAPloc to retrieve full source parameters of small seismic events from regional seismograms is developed, which include origin time, epicenter location, depth, focal mechanism, and moment magnitude. In particular, we tested whether our new method could produce satisfactory results with as few as two stations, so that we can improve source estimates of poorly monitored events with sparse waveform data. We conducted the test in the Tibetan plateau. The focal mechanisms and locations determined from only two stations agree well with those from a well-distributed PASSCAL array. We use 49 Tonga-Fiji events recorded at the broadband TriNet array, southern California to develop a pure path upper-mantle shear velocity model. At the epicentral distances of 70-95 degree, multi-bounce S waves up to S5, including the guided waves, are observed and modeled to constrain the radial velocity structure. Our preferred model PAC06 contains a fast lid (Vsh=4.78 km/sec, Vsv=4.58 km/sec) ~60 km thick, and a prominent low velocity zone (LVZ) with the lowest velocities Vsh=4.34 km/sec, and Vsv=4.22 km/sec. Besides the 406 km and 651 km discontinuities, PAC06 also has a small (~1%) velocity jump at ~516 km. We consider these main features of PAC06 to be well determined, since PAC06 explains a large data set from various events. Therefore, it is ideally suited for comparing with mineralogical models.