Calibration of the Regional Crustal Waveguide and the Retrieval of Source Parameters Using Waveform Modeling

Abstract

Ð Regional crustal waveguide calibration is essential to the retrieval of source parameters and the location of smaller (M < 4.8) seismic events. This path calibration of regional seismic phases is strongly dependent on the accuracy of hypocentral locations of calibration (or master) events. This information can be dicult to obtain, especially for smaller events. Generally, explosion or quarry blast generated travel-time data with known locations and origin times are useful for developing the path calibration parameters, but in many regions such data sets are scanty or do not exist. We present a method which is useful for regional path calibration independent of such data, i.e. with earthquakes, which is applicable for events down to M w 4 and which has successfully been applied in India, central Asia, western Mediterranean, North Africa, Tibet and the former Soviet Union. These studies suggest that reliably determining depth is essential to establishing accurate epicentral location and origin time for events. We ®nd that the error in source depth does not necessarily trade-o only with the origin time for events with poor azimuthal coverage, but with the horizontal location as well, thus resulting in poor epicentral locations. For example, hypocenters for some events in central Asia were found to move from their ®xed-depth locations by about 20 km. Such errors in location and depth will propagate into path calibration parameters, particularly with respect to travel times. The modeling of teleseismic depth phases (pP, sP) yields accurate depths for earthquakes down to magnitude M w 4.7. This M w threshold can be lowered to four if regional seismograms are used in conjunction with a calibrated velocity structure model to determine depth, with the relative amplitude of the P nl waves to the surface waves and the interaction of regional sPmP and pPmP phases being good indicators of event depths. We also found that for deep events a seismic phase which follows an S-wave path to the surface and becomes critical, developing a head wave by S to P conversion is also indicative of depth. The detailed characteristic of this phase is controlled by the crustal waveguide. The key to calibrating regionalized crustal velocity structure is to determine depths for a set of master events by applying the above methods and then by modeling characteristic features that are recorded on the regional waveforms. The regionalization scheme can also incorporate mixed-path crustal waveguide models for cases in which seismic waves traverse two or more distinctly dierent crustal structures. We also demonstrate that once depths are established, we need only two-stations travel-time data to obtain reliable epicentral locations using a new adaptive grid-search technique which yields locations similar to those determined using travel-time data from local seismic networks with better azimuthal coverage.

Combining double-difference relocation with regional depth-phase modelling to improve hypocentre accuracy

“…1) are often detectable at regional distances and have been studied in various contexts (King 1979; Helmberger & Engen 1980; Zonno & Kind 1984; Langston 1987, 1996; Mulder & Lamontagne 1990; Zhao & Helmberger 1991; Bock 1993; Bock et al . 1996; Zhu & Helmberger 1997; Saikia 2000; Saikia et al . 2001; Langston et al 2003; Savage et al .…”

“…Depth phases such as sPg, sPmP and sPn (Fig. 1) are often detectable at regional distances and have been studied in various contexts (King 1979;Helmberger & Engen 1980;Zonno & Kind 1984;Langston 1987Langston , 1996Mulder & Lamontagne 1990;Zhao & Helmberger 1991;Bock 1993;Bock et al 1996;Zhu & Helmberger 1997;Saikia 2000;Saikia et al 2001;Langston et al 2003;Savage et al 2003;Uski et al 2003;Dineva et al 2007). These phases can be used to estimate focal depth if they, and their reference phases (Pg, PmP and Pn), can be correctly identified.…”

Source Parameters of Moderate and Strong Earthquakes in the Broader Area of Zakynthos Island (W. Greece) From Regional and Teleseismic Digital Recordings

“…The magnitude threshold for source parameters determination using teleseismic body-wave modeling is approximately M w =5.7, as teleseismic phases become too weak for smaller events and thus regional data must be used, which are generally much more complicated by strong interaction with the heterogeneous crust and upper mantle structures (Papadimitriou P., 1988). However at regional distances the above mentioned threshold is lowered to approximately M w =3.8, and frequency-magnitude relations have lead to the obvious conclusion that regional-distance modeling has allowed researchers to increase the number of studied events, especially in areas with regional broadband instrumentation (Saikia et al, 2001;Agalos et al, 2008).…”

Waveform modelling of local earthquakes in southwest Japan

“…At high frequencies, the effects of scattering in the crust become so intense that only the statistical properties of waveform are meaningful (Saikia et al, 2001). However, the high frequency P nl phases have some stable features which can be deterministically modelled to characterize the regional waveguide.…”