ABSTRACT

The west part of Java sits at the transition from oblique subduction of the Australian plate under the Sunda block of the Eurasian plate along Sumatra to orthogonal convergence along central and eastern Java. This region has experienced several destructive earthquakes, the 17 July 2006 Mw 7.7 earthquake and tsunami off the coast of Pangandaran and the 2 September 2009 Mw 7 earthquake, located off the coast of Tasikmalaya. More recently, on 15 December 2017, an Mw 6.5 earthquake occurred off the coast near Pangandaran, and, on 23 January 2018, an Mw 5.9 earthquake occurred offshore Lebak, between Pelabuhan Ratu and Ujung Kulon. Ground shaking and damage occurred locally and in Jakarta on the northern coast of Java. In this study, we use the double‐difference technique to relocate both mainshocks and 10 months of seismicity (228 events) following the earthquakes. The relocation result improved the mainshock locations and depth distribution of earthquakes. Moment tensor of the December 2017 event located the hypocenter at 108  km depth within the subducting slab. The best‐fit relocation places the depth at 61 km, close to the slab interface. Aftershocks occur between 68 and 86 km depth and align along a steeper plane than slab geometry models. The January 2018 event is located at 46  km depth. Aftershocks form a near‐vertical, pipe‐like structure from the plate interface to 10  km depth. A burst of aftershocks immediately following the mainshock shows a shallowing upward trend at a rate of 2  km/hr, suggesting that a fluid pressure wave released from the oceanic crust is causing brittle failure in the overriding plate, followed by upward migration of fluids. Five months later, shallow (<25  km) seismicity collocates with background seismicity, suggesting the January 2018 event activated the Pelabuhan Ratu fault system close to the coast.

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