Source parameters for small-moderate earthquakes in Marmara Region (Turkey)
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Geosciences Journal
GJ
Source parameters for small–moderate earthquakes in Marmara Region (Turkey) Tahir Serkan Irmak1*, Evrim Yavuz1, Hamdullah Livaoğlu1, Erman Şentürk2, and Emre Y. Sahin1 1
Seismology Section, Department of Geophysics, Kocaeli University, Kocaeli 41100, Turkey Department of Surveying Engineering, Kocaeli University, Kocaeli 41100, Turkey
2
ABSTRACT: The main aim of this study is to investigate the self-relation and self-similarity of earthquakes in and around the Marmara Sea (NW Turkey) by using these obtained source parameters. With this purpose, spectral source parameters for 77 small to moderate earthquake (3.5 ≤ ML ≤ 5.2) that occurred during 2004–2018 have been determined from P and S wave spectra according to Brune’s source model by using regional broadband seismograms. The average ratio of P/S wave corner frequency is found to be 1.3 that suggesting higher corner frequency for P wave. The static stress drops range from 0.1 and 136 MPa with a median value of 9.8 MPa (98 bars). The high stress drops for these events can indicate high frictional strength and low strain-rate of the faults. Similarly, the low values of the stress drop can indicate a general weakness of the faults in the study area. There is no clear dependence between the seismic moment and the static stress drop in the analyzed events but some events which have lower seismic moment value also have lower stress drop. Obtaining results indicated the corner frequency decreases with increasing of the seismic moment. Also, a slight depth dependence of the corner frequency has been observed for the analyzed events. Shallower events have larger corner frequency value than deeper events. Also, a clear depth dependence of the stress drop values has not been observed. However, the depth dependence of the seismic moment is seen more clearly. Our results indicated that the deeper events have larger seismic moment values in the study area. In spite of scattering in small events, a linear relationship between local magnitude (ML) and moment magnitude (MW) could be obtained as MW = 1.4261(± 0.31) + 0.6399(± 0.08)ML from P waves spectra and MW = 0.0136(± 0.21) + 0.9883(± 0.05)ML from S wave spectra and calculated MW values are consistent with waveform inversion (centroid moment tensor – CMT) results. These relationships may be useful for seismic hazard studies in the study area. Key words: Marmara, source parameters, stress drop, corner frequency, seismic moment Manuscript received November 30, 2018; Manuscript accepted November 10, 2019
1. INTRODUCTION Most earthquakes are inaccessible to direct observation, therefore it is not easy to determine the source parameters such as seismic moment, stress drop and source dimension by using field observation. The determination of source parameters is important for understanding the physics of earthquakes. However, due to the small number of large earthquakes, small and moderate earthquakes are important in the studies and understanding of the physics of earthquake source. The relations between
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