Hong Kie Thio

Calibration of the regional crustal waveguide and the retrieval of source parameters using waveform modeling

Abstract — 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 difficult 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 Mw = 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 find that the error in source depth does not necessarily trade-off 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 fixed-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 Mw = 4.7. This Mwthreshold 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 Pnl 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 different 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.

Probabilistic Tsunami Hazard Analysis for Ports and Harbors

The December 2004 Sumatra-Andaman earthquake emphasized the need for a consistent and comprehensive assessment of tsunami hazard. The authors have developed a method for Probabilistic Tsunami Hazard Analysis (PTHA) based on the traditional Probabilistic Seismic Hazard Analysis (PSHA) and therefore completely consistent with standard seismic practice. In lieu of attenuation relations, it uses the summation of finite-difference Green’s functions that have been pre-computed for individual subfaults, which enables one to rapidly construct scenario tsunami waveforms from an aggregate of subfaults that comprise a single large event. For every fault system, it is then possible to integrate over sets of thousands of events within a certain magnitude range that represents a fully probabilistic distribution. Because of the enclosed nature of ports and harbors, effects of resonance need to be addressed as well, which is why this method has been extended to not only analyze exceedance levels of maximum wave height, but also of spectral amplitudes. As in PSHA, these spectral amplitudes can be matched with the spectral response of harbors, and thus allow a comprehensive probabilistic analysis of tsunami hazard in ports and harbors.