Earthquake Relocation in the Northeastern Side of MMT and Its Implication on the Seismogenic Structure

Abstract

The present study aimed to relocate earthquakes and identify seismogenic structure in the eastern side of Main Mantle Thrust in the NW Himalayas, Pakistan, based on double difference earthquake relocation, HypoDD, algorithm. The current study used 15561 earthquakes recorded over 25 stations from year 2010 to 2021 were acquired from Micro Seismic Monitoring System (MSMS) WAPDA. In total, 130043 P phases and 56107 S phases are used to relocate the earthquakes. The HypoDD algorithm relocates 6426 earthquakes. The results of relocated seismicity, Free Air Gravity Model (EGM2008), and Focal Mechanism Solutions (FMS) are used to deduce the seismogenic structure of‎ the‎ region.‎ The‎ study’s results showed that the HypoDD method significantly reduced the travel time residual 66 ms compared to the initial travel time 154 ms. After earthquake relocations, it is observed that mostly earthquakes are clustered at an average depth of 5 to 30 km and a few earthquakes are observed around depth of 50 km. Additionally, the seismicity distribution pattern shows low dip angle and NE trend which was consistent with focal mechanism solutions that indicates the presence of thrust faults with minor component of strike slip in the vicinity of MMT. The northeast-trending faults were inferred from the present study, specifically, in the Indus Kohistan Seismic Zone (IKSZ) which is considered as tectonically most active region. Furthermore, finding also showed a positive association among sharp gradient in free air gravity and dense seismicity. Thus, the current study can aid in hazard analysis, improving velocity models and tomographic studies.