Remote triggering in Iran: large peak dynamic stress is not the main driver of triggering

Abstract

SUMMARYThe study of the dynamic triggering of earthquakes and tremors during large earthquakes at faraway distances is an active area of research. This type of remote dynamic triggering is often found in subduction zones. The Iranian plateau is part of the Alpine–Himalayan orogenic system and hosts different collision styles of deformation and significant strike-slip faults. Using 13 yr (26 December 2004–8 September 2017) of continuous data of Iranian National Seismic Network (INSN) and some dense temporary networks, for the first time we carried a systematic study of dynamic triggering in Iran during 47 recent large earthquakes with magnitude and depth ranges of 6.4–9.1 and 8–90 km, respectively. We explored the local catalogue of 124 805 events with a magnitude of completeness (Mc) of 1.8 for the study of dynamic triggering but did not find any convincing evidence of dynamic triggering from the catalogue. The waveform data of 24 hr duration around the main events were analysed to find possible dynamic triggering through manual analysis of the waveform, STA/LTA, and beta statistics and found the triggering. We found dynamic triggering in Iran during Sumatra, 26 December 2004, Mw 9.1; Tohoku-Oki, 11 March 2011, Mw 9.1; Indian Ocean, 11 April 2012, Mw8.6 and Baluchistan, 24 September 2013 earthquakes and also possible triggering during Sumatra, 12 September 2007, Mw8.5. Only ∼10 per cent of the analysed earthquakes produced dynamic triggering. The triggering initiates during the passage of high amplitude Love waves and continues through the passage of the Rayleigh waves. We found north, central and eastern regions are more probable for triggering than Zagros and Makran regions. The instances of triggering were not restricted to only a small region, but instead, occurred at multiple locations. We find the onset of tremor correlates with very small stress changes, on the order of 1 kPA. However, the amplitude of the dynamic stresses is not a sufficient condition since some of the areas with considerably larger dynamic stresses are not triggered any seismicity in the region. The backazimuth angle of ∼50° and ∼120° seems to play an important role in the triggering. Teleseismic waves most probable for triggering local earthquakes within NW and central Iran include incoming surface waves with an incident angle of ∼60°–90° with respect to the local fault fabric.