Abstract Database

ESTIMATION OF SOURCE, PATH AND SITE EFFECTS IN HANGAY REGION MONGOLIA
USING A DENSE BROADBAND SEISMIC ARRAY

MEE14602
Baigalimaa GANBAT
Supervisor: Toshiaki YOKOI, Takumi HAYASHIDA
Country: Mongolia
Abstract

Between July 2012 and July 2014, 75 temporal broadband seismic stations were installed in the Hangay region of central Mongolia. The database comprises a total of 1331 waveforms from 32 earthquakes (3.0 ≤ Ml ≤ 5.6), which occurred in the period between August 2012 and March 2014 including the 3 October, 2012 Bayanbulag earthquake and its aftershocks. We applied the spectral inversion method to the broadband data in order to estimate source, local site effect and path effect (quality factor). First, we relocated hypocenters using the detected P-wave arrival times. The averaged P and S-wave velocities of upper crust estimated from the relocated hypocenters are 6.1 km/s and 3.5 km/s, which corresponds well to the values of existing regional velocity structure model (CRUST 2.0). We also obtained seismic moment and focal mechanism of each earthquake using the time domain moment tensor inversion and polarities for the event. The result shows that strike-slip faults were dominant in the target region and estimated magnitude range was from Mw 2.9 to Mw 4.5. In the spectral inversion method, we applied two kinds of approaches, which were reference site and reference events for computational stability. We found that two different inversion approaches gave similar results for the site effects and quality factor in the frequencies range between 0.5 Hz and 10 Hz. For the constraint of reference site quality factor value follows Qs(f)=70f0.79 whereas quality factor value follows Qs(f)=80f0.84 for the constraint of reference event. The predominant frequencies of selected sites also correspond well to those obtained from horizontal-to-vertical (H/V) spectral ratios of S-wave portion. Using the estimated quality factor and site effects we determined source spectra of all events. Finally, we estimated seismic moment, corner frequency, crack radius and static stress drop for each event by using the displacement source spectra, quality factor and site effects obtained from inversions with reference events. Our results indicate that a large amount of dataset from the dense broadband seismic network provides fundamental and useful information for seismic hazard assessment in this region.