M 7.1 - 141 km NW of Ternate, Indonesia

Tectonic Summary

The November 14, 2019, M 7.1 earthquake northwest of Kota Ternate, Indonesia, occurred at a depth of ~45 km on or near the interface between the subducted Halmahera slab and the overlying Sunda plate, in the complex plate boundary region of eastern Indonesia, about 300 km to the west of the major plate boundary between the Sunda and Pacific plates. Faulting mechanism solutions for the event indicate that it activated a reverse faulting structure, with a moderate dip towards either the northwest or the southeast, in line with the general trend of earthquakes in the region. Though the earthquake occurred near the boundary between the Halmahera slab and the Sunda plate, this earthquake is not a traditional plate interface event such as those experienced in other subduction zones, since the Halmahera slab is completely subducted and has no surface expression. Slip on a fault aligned with either nodal plane is thus consistent with the tectonic setting of this event.

Tectonics in eastern Indonesia are extremely complex and are dominated by the mostly convergent interactions of the Pacific, Australia, Philippine Sea, and Sunda plates, with some authors labeling the most proximate edge of the Pacific plate here as a separate tectonic block called the Caroline plate. The edges of the Sunda and Australia plates are also often subdivided into smaller tectonic blocks, including the Molucca Sea and Birds Head microplates immediately to the south and east of the November 2019 earthquake, respectively. In this context, the November 2019 event most closely aligns with the boundary between the broader Sunda plate and the Birds Head microplate. At depth beneath this earthquake and the Molucca Sea in general, the inverted-U-shaped Halmahera plate, which has no surface expression, also plays a role in regional tectonics. At the location of the November 14th earthquake, the Sunda and Philippine Sea plates are converging in an east-west direction at a rate of approximately 109 mm/yr.

This area of the Molucca Sea frequently hosts moderate to large earthquakes; nearly 110 M 6+ events have occurred within 250 km of the November 14, 2019, earthquake over the past half century, seven of which were M 7+. The largest, a M 7.5 event in August 1986, struck along the same microplate boundary structure just 25 km to the northeast of the 2019 event. Another M 7.5 event in January 2007 occurred 60 km to the southwest of today’s event. Despite the large number of events in the region, few have been damaging because of their oceanic setting, though the 2007 event previously mentioned did result in 3 shaking related fatalities and minor damage on the nearby island of Sulawesi.