M 7.3 - Flores Sea

Tectonic Summary

The December 14, 2021, M 7.3 earthquake north of Maumere, Indonesia, occurred as the result of strike-slip faulting within the Flores Sea. Focal mechanism solutions for the earthquake indicate that rupture occurred on either a left-lateral fault striking northeast-southwest, or a right-lateral fault striking approximately east west. The earthquake occurred within the broader Banda arc where the Australia plate subducts northward beneath the Sunda plate. The relatively shallow depth of the earthquake indicates that is occurred in the overriding Sunda plate. At the location of the earthquake, the Australia plate moves approximately northward with respect to the Sunda plate at a rate of 74 mm/yr. Although compressional reverse/thrust faulting is a more common faulting style in subduction zone environments, strike slip faulting can occur to accommodate plate boundary stresses that are not released by earthquakes on thrust faults.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip events of the size of the December 14, 2021 earthquake are typically about 90 km x 15 km in size (length x width).

Within the past century, six earthquakes of M7 or larger have occurred within 250 km of the December 14, 2021 earthquake. In 2015, an M7.0 strike-slip earthquake occurred approximately 40 km northeast of the December 14 earthquake; however, the 2015 earthquake occurred at a depth of 552 km within the subducted Australia slab. Similarly, an M7.9 normal faulting earthquake in June 1996 occurred approximately 50 km northeast but at a depth of 587 km within the subducted Australia slab. On December 12, 1992, an M7.8 thrust faulting earthquake occurred approximately 60 km south of the December 2021 earthquake. The 1992 earthquake was relatively shallow (28 km deep), had a faulting mechanism that favored the generation of tsunamis, and was located closer to population centers. That 1992 earthquake generated a tsunami and strong shaking on land, leading to at least 2,200 fatalities.