Lapse time and frequency-dependent attenuation of coda waves in the Zagros continental collision zone in Southwestern Iran: Fid-Bau Portal

Lapse time and frequency-dependent attenuation of coda waves in the Zagros continental collision zone in Southwestern Iran

H Rahimi / H Hamzehloo

Abstract

The coda Q, Q_c, were estimated for the Zagros continental collision zone in southwestern Iran by analyzing the coda waves of 51 local earthquakes recorded on the three stations of the Iranian National Seismic Network (INSN) with magnitudes of between 3.1 and 4.9 recorded in the region during March and April 2006. Most of the analyzed events are foreshocks and aftershocks of the Darb-e-Astane earthquake which occurred on 31 March 2006 with a magnitude of 6.1 (IIEES). The earthquakes had an epicentral distance of between 120 and 200 km and a focal depth of about 18 km. The Q_c values were computed at nine central frequencies of 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 12.0, 16.0 and 20.0 Hz through eight lapse time windows from 25 to 60 s starting at double the time of the primary S-wave from the origin time using the time-domain coda-decay method of a single backscattering model. In this study the collected data were compared between the events that occurred before and after the main event (foreshocks and aftershocks). The analysis showed a significant variation in the value of coda, Q, for the study region in different lapse times and frequencies. The variation of the quality factor, Q_c, before and after the main event was estimated at different lapse time windows to observe its effect with depth. The estimated average frequency-dependent relation of Q_c for foreshocks varies from Q_c = (144 ± 24)f^{(0.42 ± 0.23)} at 25 s to Q_c = (85 ± 10)f^{(0.92 ± 0.11)} at 60 s lapse window time length, respectively. For aftershocks, it varies from Q_c = (121 ± 55)f^{(0.97 ± 0.26)} at 25 s to Q_c = (212 ± 59)f^{(0.82 ± 0.15)} at 60 s. The averages of Q_c in all stations and lapse times window are obtained as Q_c = 99f^0.84 and Q_c = 178f^0.86 for foreshocks and aftershocks, respectively. The Q frequency relationship for foreshocks is similar to that for the South Carolina, Koyna, western Anatolia and Aleutian earthquakes, whereas for aftershocks it is similar to the Kumaun, NW Himalaya and Parkfield events.