Coda Q in Qeshm Island, south of Iran, using aftershocks of the Qeshm earthquake of November 27, 2005
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ORIGINAL PAPER
Coda Q in Qeshm Island, south of Iran, using aftershocks of the Qeshm earthquake of November 27, 2005 Habib Rahimi 1 & Abbas Gholamzadeh 2
Received: 10 October 2016 / Accepted: 14 May 2017 / Published online: 1 June 2017 # Saudi Society for Geosciences 2017
Abstract Coda wave attenuation is estimated for Qeshm Island which is located in the southeastern part of Zagros. For this purpose, the aftershocks of Qeshm earthquake in November 27, 2005, recorded within an epicentral distance less than 100 km, have been used. More than 829 earthquakes were recorded by a local temporary network consisting of 16 short period stations installed after a week after the main shock for ∼10 weeks. The coda quality factor, Qc, was estimated using the single-backscattering model in frequency bands of 0.5–24 Hz. In this research, lateral and vertical variations of coda Q in Qeshm Island are explored. In Qeshm Island, absence of significant lateral variation of coda Q is observed. To investigate the attenuation variation with depth, the coda Q value was calculated for coda time windows with different lengths (5, 10, 15, 20, 25, and 30 s). It is observed that coda Q increases with depth. However, in our study area, the rate of increase of coda Q with depth is not uniform. Beneath Qeshm Island, the rate of increase of coda Q is greater at depths less than ∼40 km compared with those of larger depths. This is indicating the existence of a low attenuation anomalous structure under the ∼40-km depth which may be correlated with the Moho depth in this region. The average frequency relation for this region is Qc = 36 ± 1.2f 0.94 ± 0.039 at a 5 s-lapse time window length and Qc = 110 ± 1.8f 0.88 ± 0.09 at a 30-s lapse time window length.
* Habib Rahimi [email protected]
1
Institute of Geophysics, University of Tehran, Tehran, Islamic Republic of Iran
2
Department of Physics, Faculty of Science, Hormozgan University, Bandar Abbas, Islamic Republic of Iran
Keywords Coda waves . Seismic attenuation . Qeshm Island . Zagros . Iran
Introduction For the first time, the average decay of the envelope of the tail portion (coda wave) of a seismogram is studied by Aki (1969). He postulated that numerous small-scale randomly distributed heterogeneities in the lithosphere are generated coda wave by the superposition of backscattered primary S waves. Two endmember models for the scattering medium are proposed by Aki (1969) and Aki and Chouet (1975): the single-scattering model where loss of energy from primary waves is neglected and the diffusion model where the coda forms as the result of the wave field encountering several scatterers. In many tectonically active or stable regions, the single-scattering model has proven very successful in estimating the coda quality factor (Qc) value, especially for local earthquakes with a magnitude less than about 6. For example, a strong correlation between coda wave attenuation and the degree of fracturing is observed in the lithosphere associated with seismicity (Roecker et al. 1982; Pulli 1984). Co
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