Variation of coda wave attenuation in the Alborz region and central Iran

Abstract

SUMMARY More than 340 earthquakes recorded by the Institute of Geophysics, University of Tehran (IGUT) short period stations from 1996 to 2004 were analysed to estimate the S-coda attenuation in the Alborz region, the northern part of the Alpine–Himalayan orogen in western Asia, and in central Iran, which is the foreland of this orogen. The coda quality factor, Qc, was estimated using the single backscattering model in frequency bands of 1–25 Hz. In this research, lateral and depth variation of Qc in the Alborz region and central Iran are studied. It is observed that in the Alborz region there is absence of significant lateral variation in Qc. The average frequency relation for this region is Qc= 79 ± 2f1.07±0.08. Two anomalous high-attenuation areas in central Iran are recognized around the stations LAS and RAZ. The average frequency relation for central Iran excluding the values of these two stations is Qc= 94 ± 2f0.97±0.12. To investigate the attenuation variation with depth, Qc value was calculated for 14 lapse times (25, 30, 35, … 90s) for two data sets having epicentral distance range R < 100 km (data set 1) and 100 < R < 200 km (data set 2) in each area. It is observed that Qc increases with depth. However, the rate of increase of Qc with depth is not uniform in our study area. Beneath central Iran the rate of increase of Qc is greater at depths less than 100 km compared to that at larger depths indicating the existence of a high attenuation anomalous structure under the lithosphere of central Iran. In addition, below ∼180 km, the Qc value does not vary much with depth under both study areas, indicating the presence of a transparent mantle under them.