Rupture Pattern of the 2015 Alor Earthquake Sequence, Indonesia

Abstract

A sequence of earthquakes occurred on Alor Island, Nusa Tenggara Timur, Indonesia, beginning in November 2015 with the mainshock (Mw 6.2) on 4 November 2015. We calculate the centroid moment tensor (CMT) solutions for nine of the earthquakes with Mw≥3.9, which occurred between November 2015 and March 2016 using records from a temporary array of 30 broadband instruments in eastern Indonesia and Timor Leste (YS network). Our CMT results reveal an interesting pattern of ruptures in this order: (a) three foreshocks of Mw 4–5.3 all with strike-slip mechanisms that occurred with a centroid depth of ∼13 km in the three days prior to the mainshock, (b) the mainshock on 4 November 2015, with Mw 6.2 that occurred with a deeper centroid (∼25 km) and a strike-slip mechanism similar to the foreshocks, (c) followed by five aftershocks with Mw>3.9 at depth ∼3–15km. We further determine the fault plane and rupture direction of the mainshock and the largest foreshock (Mw 5.3) by relocating the hypocenter and examining its geometrical location with respect to the centroid. We find that the fault plane strikes 97°±9° from north and that the fault ruptures westward. We propose that the rupture of this sequence of events initiated at depth ∼10 km, propagating westward and triggering the mainshock to rupture at a deeper depth (within lower crust) on a similar faulting system. The aftershocks migrate back to shallower depths and occur mainly at depth <10 km.