Abstract
Since the 2004 Sumatra–Andaman earthquake (M w 9.2), the northwestern part of the Sumatran island has been a high seismicity region. To evaluate the seismic hazard along the Great Sumatran fault (GSF), we installed the Aceh GNSS network for the Sumatran fault system (AGNeSS) in March 2005. The AGNeSS observed co-seismic offsets due to the April 11, 2012 Indian Ocean earthquake (M w 8.6), which is the largest intraplate earthquake recorded in history. The largest offset at the AGNeSS site was approximately 14.9 cm. Two M w 6.1 earthquakes occurred within AGNeSS in 2013, one on January 21 and the other on July 2. We estimated the fault parameters of the two events using a Markov chain Monte Carlo method. The estimated fault parameter of the first event was a right-lateral strike-slip where the strike was oriented in approximately the same direction as the surface trace of the GSF. The estimated peak value of the probability density function for the static stress drop was approximately 0.7 MPa. On the other hand, the co-seismic displacement fields of the second event from nearby GNSS sites clearly showed a left-lateral motion on a northeast–southwest trending fault plane and supported the contention that the July 2 event broke at the conjugate fault of the GSF. We also calculated the Coulomb failure function ΔCFF caused by the first event to evaluate its effect on the second event. The results showed that the July 2 event was likely brought 0.1 MPa closer to failure by the January 21 event.
Highlights
Sumatra–Andaman earthquake (Mw 9.2), the northwestern part of the Sumatran island has been an area of high seismicity (Nalbant et al 2005) because there have been nine earthquakes of Mw ≥ 7.5 in the region, with four of them being responsible for triggering damaging tsunamis
We present the GNSS measurements of co-seismic offsets resulting from the 2012 Indian Ocean earthquake and two subsequent Mw 6.1 earthquakes that occurred in the northwestern part of the Sumatran islands
Indian Ocean earthquake and the January 21 and July 2, 2013, events detected by the AGNeSS in the northwestern part of the Sumatran islands
Summary
Since the December 26, 2004 Sumatra–Andaman earthquake (Mw 9.2), the northwestern part of the Sumatran island has been an area of high seismicity (Nalbant et al 2005) because there have been nine earthquakes of Mw ≥ 7.5 in the region, with four of them being responsible for triggering damaging tsunamis. The Great Sumatran fault (GSF) system is a major 1900-km trench-parallel strike-slip fault system with several segments. Cattin et al (2009) suggested that CFF increases by approximately 2 MPa on the northwestern part of the Sumatran fault, resulting in a greater seismic hazard potential for this region that may trigger a fault slip in the northwestern part on the Sumatran fault in the near future. Such earthquakes have yet to occur, and seismic activity along the GSF has not changed greatly.
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