Tectonic Summary

The M 7.7 (19:43 UTC) and M 7.4 (22:33 UTC) Papua, Indonesia earthquakes of January 3, 2009, 19:43 UTC and 22:33 UTC, respectively, occurred as a result of thrust faulting on a plate boundary along the northwest coast of the island of New Guinea. Eastern Indonesia is characterized by complex tectonics in which motions of numerous small plates are accommodating large-scale convergence among the Australia, Sunda, Pacific, and Philippine Sea plates. In broad-scale plate tectonic models that do not subdivide continent-sized plates into smaller plates, the locations of the January 3rd earthquakes would be on the boundary of the Pacific and Australia plates. The Pacific plate (located north and northeast of the epicenter) is moving southwest with respect to the Australia plate (located south of the epicenter) at a velocity of about 112 mm/yr at the epicenter of the earthquake, and the focal mechanism solutions of these earthquakes are broadly consistent with Pacific plate lithosphere being subducted beneath Australia plate lithosphere. The subduction zone along the northwest coast of New Guinea is characterized by an offshore oceanic trench, the New Guinea Trench, but teleseismically recorded earthquake hypocenters do not show a well-developed inclined seismic zone (a Wadati-Benioff zone) dipping south-southwest from the trench.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust-faulting events of the size of the January 3, 2009, M 7.7 earthquake are typically about 100x50 km (length x width); modeling of this earthquake implies similar dimensions, predominantly up-dip and to the west of the hypocenter.

The earthquake of January 3, 2009, 19:43 UTC, occurred about 470 km west of the M 8.2 earthquake of February 17, 1996. The 1996 earthquake produced a tsunami that was destructive on the island of Biak. At least 108 people were killed by the earthquake and associated tsunami.

The M 7.4 earthquake of January 3, 2009, 22:33 UTC, had the same focal mechanism solution as the M 7.7 event of the same date, 19:43 UTC, and was located about 70 km east of the earlier shock. The M 7.4 earthquake was almost certainly triggered by the earlier event. Seismologists sometimes refer to a pair of similarly sized shocks that occur at nearly the same time and location as an earthquake “doublet.”

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)