Gambar di samping menunjukkan kondisi tektonik Papua. Sesar Sorong di Papua ini yang menyebabkan gempa pada tanggal 4 Januari 2009 yang lalu. Data BMG menyebutkan kekuatan gempa 7.9 SR (kedalaman 48 km) dan 7.6 SR (kedalaman 10 km). Kedua gempa ini berselang tidak lebih dari 3 jam.
Sedangkan data USGS menyebutkan 7.4 SR dengan kedalaman 38 km.
Berikut ini summary dari USGS
The magnitude 7.4 Papua, Indonesia earthquake of January 3, 2009, 22:33 UTC, 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 between the Australia, Pacific, and Eurasia plates. In broad-scale plate-tectonic models that do not subdivide continent sized plates into smaller plates, the location of today’s earthquake 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) with a velocity of about 112 mm/year at the epicenter of the earthquake, and the focal-mechanism of today’s earthquake is 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.
The earthquake of January 3, 2009, 22:33 UTC, occurred about 400 km west of the magnitude 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 1996 earthquake and associated tsunami.
The magnitude 7.4 earthquake of January 3, 2009, 22:33 UTC, had the same focal-mechanism as the magnitude 7.6 shock of the same date, 19:43 UTC, and was located about 70 km east of the earlier shock. It was almost certainly triggered by the earlier earthquake. Seismologists sometimes refer to a pair of similarly sized shocks that occur at nearly the same time and location as an earthquake “doublet.”
Berikut ini animasi pembentukan Sesar Sorong. Di bagian kanan atas terdapat angka yang menunjukkan umur dalam satuan ‘juta tahun yang lalu’. Dari animasi ini kita mengetahui bahwa daerah Papua memang memiliki kondisi tektonik yang rumit dengan sesar-sesar yang sangat aktif.
Sumber animasi dari blog Dongeng Geologi
Animasi tektonik ini dibuat Oleh Robert Hall tahun 2001. Professor Robert Hall adalah Director of the SE Asia Research Group, Degree Programme Co-ordinator MSci Environmental Geology di Royal Halloway, University of London, Inggris.