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Tectonic Summary

The December 15, 2019 M 6.8 earthquake south of Magsaysay, on the Philippine island of Mindanao, occurred as the result of strike-slip faulting in the crust of the Sunda plate. At the location of the earthquake, the Philippine Sea plate converges with the Sunda plate at a velocity of approximately 105 mm/yr towards the west-northwest, subducting into the mantle beneath Mindanao at the Philippine Trench, about 220 km to the east of this earthquake. Preliminary focal mechanism solutions for the event indicate slip on either a steep left-lateral fault striking to the northwest, or on a more moderately dipping right-lateral fault striking to the northeast. Aftershocks most clearly align with the left-lateral plane, though complexity in faulting is evident in both these aftershocks and also in foreshocks – this event is part of an on-going sequence that began in October 2019.

Beginning around October 11, 2019, when a M 5.1 earthquake occurred approximately 17 km to the west of the December 15th event, this region has experienced a pronounced earthquake sequence involving over 150 M4.5+ events and four M6+ earthquakes – a M 6.4 event on October 16th, 20 km to the west; a M 6.6 event on October 29th, 23 km to the west-northwest; a M 6.5 event on October 31st, 22 km to the north; and finally this M 6.8 event on December 15th. These earthquakes have collectively resulted in at least a dozen shaking-related fatalities, over 150 injuries, and significant damage in the region. The sequence is geometrically complex, and suggests the activation of both left-lateral (e.g., between the earliest two M6+ events and today’s M6.8 earthquake) and right-lateral (e.g., between the earliest two M6+ events and the M 6.5 event) structures.

Northeastern Indonesia and southern Philippines are characterized by complex tectonics in which motions of numerous small microplates accommodate the large-scale convergence among the Philippine Sea, Sunda, Pacific, and Australia plates. The tectonic setting of the Philippines is unusual in that it is characterized by opposite-facing subduction systems on its east and west sides. To the east, the aforementioned Philippines Trench marks subduction of the Philippine Sea plate towards the west beneath the island, while to the southwest of Mindanao, the Cotabato system also hosts active subduction towards the northeast. Further north, Sunda subducts to the east beneath the Philippine Sea Plate at the Manila Trench. These complex tectonics result in frequent earthquakes, and the region within 250 km of today’s event has experienced 34 other M 6.5+ events over the 50 years preceding the latest sequence. Very few of those have occurred at shallow depth within the interior of the island of Mindanao.

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Tectonic Summary

The November 26, 2019, M 6.4 Albania earthquake occurred as the result of thrust faulting near the convergent boundary of the Africa and Eurasia plates. Focal mechanism solutions indicate reverse slip on a shallow or steeply dipping fault. Northwest-Southeast striking reverse faulting is consistent with the tectonics of the region. At the location of this event, the Africa plate converges with the Eurasia plate at a rate of 73 mm/year.

Tectonics of the Mediterranian Sea, in the convergent boundary region between Africa and Eurasia, are complex, and involve the motions of numerous microplates and regional-scale structures. In the context of the November 26, 2019 earthquake, reverse faulting in Albania on the eastern shores of the Adriatic is consistent with the closing of that sea, and shortening across the mountain belts stretching from Croatia to Greece.

Large earthquakes are common in this region; seven M6 and larger events have occurred within 150 km of this November 26 earthquake over the past 100 years. The largest was a M6.9 earthquake on April 15, 1979 that occured 70 km to the north-northeast of the November 26th earthquake, killing 100 people in Montenegro, 35 in Albania and leaving 100,000 people homeless. A M6.7 earthquake on November 30, 1967, 80 km to the east of today’s event, resulted in 19 fatalities and significant damage in the surrounding region.

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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.