Rupture Pattern of the 2015 Alor Earthquake Sequence, Indonesia

A sequence of earthquakes occurred on Alor Island, Nusa Tenggara Timur, Indonesia, beginning in November 2015 with the mainshock ( M-w 6.2) on 4 November 2015. We calculate the centroid moment tensor (CMT) solutions for nine of the earthquakes with M-w >= 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 M w 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 M-w 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 M w > 3.9 at depth-3 -15km. We further determine the fault plane and rupture direction of the mainshock and the largest foreshock ( M-w 5.3) by relocating the hypocenter and examining its geometrical location with respect to the centroid. We find that the fault plane strikes 97 degrees +/- 9 degrees 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.