نوع مقاله : مقاله پژوهشی
1 دانشجوی کارشناسی ارشد، پژوهشکده زلزلهشناسی، پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، تهران، ایران
2 استادیار، پژوهشکده زلزلهشناسی، پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، تهران، ایران
3 دانشیار، پژوهشکده زلزلهشناسی، پژوهشگاه بینالمللی زلزلهشناسی و مهندسی زلزله، تهران، ایران
عنوان مقاله [English]
The Mw 6.0 Do-Ghaleh Fariman earthquake occurred at 10:39 local time (06:09 GMT) on 2017 April 5, in 46 km away from Fariman city of Khorasan Razavi province in northeast Iran (Figure 1). The mainshock had a maximum Mercalli intensity of VIII (Severe) (Ahmadzadeh et al., 2018), and was felt by many people a radius of 200 km in eastern part of Iran. Despite the low population density, the earthquake caused widespread destruction, killing 2 people and injuring a further 100 people. Although many historical and instrumental destructive earthquakes have occurred in Great Khorasan, no evidences from large earthquakes reported in Fariman region. Immediately, after Do-Ghaleh Fariman earthquake, International Institute of Earthquake Engineering and Seismology (IIEES) decided to design an intensive seismic network around epicenter for monitoring aftershocks and seismological aspects studies. The IIEES local seismic network contains 16 velocitymeter (Lenartz 20 Sec) and 3 accelerometers (CMG-5TD Guralp with ±2g sensitivity) that deployed in the region for 40 days (Figure 1). The sampling rate of waveform data have been chosen at 200Hz for all seismic stations. Data acquisition is leading to 1500 aftershocks with high quality waveforms in this area. The IIEES velocity model is used as initial velocity model in Lotus12 program for optimizing velocity model in Fariman region. The optimum derived velocity model (as shown in table 4) is used for relocation of aftershocks. Figure 3 shows the location map of relocated aftershocks and seismic stations. The cross sections of well relocated events show a NW-SE dip direction (Figure 3).
To relocate the mainshock and to derive the fault plane solution we have retrieved all waveforms from seismic stations, both Iran Seismic center (ISC) belong to Institute of Geophysics at University of Tehran (IGUT) and Iran National Center of Broadband of Seismic Network belong to IIEES. For fault plane solution the first P-wave polarity method (Snoke et al., 1984) is used. The result of our relocation and fault plane solution of the main shock is shown in figure 5 & table 5 in comparison with other seismic agencies reports. To estimate the fault plane solutions of well-relocated aftershocks, we extracted 120 aftershocks with azimuthal gap less than 160°. The results of our fault plane solutions of 38 aftershocks with high quality are shown in figure 6 that have azimuthal gap less than 120° and recorded at least in 16 seismic stations. Focal mechanisms of 15 aftershocks are reversed which is numbered from 1 to 15 as shown in figure 6 and table 7. However, the rest of fault plane solutions show reverse mechanisms with strike slip component. Generally, the total average trend of reactivated fault, show NNW-SSE direction based on our study that is in good agreement with the trend and focal mechanism of Mozdoran fault (figure 6). Therefore, reactivation of the Mozdoran fault can be considered as main source of Do-Ghaleh Fariman Mw6 earthquake on April 5 2017. It should be noted that in some technical reports (e.g. Naimi, 2017) and old geological maps the final section of the Mozdoran fault is termed in Chah-Mazar fault.