A powerful earthquake registered by the USGS as M7.9 hit near the coast of New Ireland, Papua New Guinea at 10:51 UTC on December 17, 2016. The agency is reporting a depth of 103.2 km (64 miles). Geoscience Australia is reporting M8.0 at a depth of 95 km (59 miles). Tsunami is possible along some coasts of Papua New Guinea. Powerful aftershocks are shaking the region.
According to the USGS, the epicenter was located 132.8 km (82.5 mi) E of Kokopo (population 26 273) and 302.0 km (187.7 mi) NW of Arawa (population 40 266), Papua New Guinea.
There are about 23 877 people living within 100 km (62 miles). About 6 200 people are living within 50 km (31 miles).
First witness reports mention shaking lasting up to 4 minutes.
Based on all available data, hazardous tsunami waves are forecast for some coasts, the PTWC said.
Tsunami waves reaching 1 to 3 meters (3.3 to 9.8 feet) above the tide level are possible along some coasts of Papua New Guinea. Tsunami waves are forecast to be less than 0.3 meters (1 feet) above the tide level for all other areas.
USGS issued a yellow alert for shaking-related fatalities and economic losses. Some casualties and damage are possible and the impact should be relatively localized. Past yellow alerts have required a local or regional level response.
Estimated economic losses are less than 1% of GDP of Papua New Guinea.
Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist. The predominant vulnerable building types are unreinforced brick masonry and informal (metal, timber, GI etc.) construction.
Recent earthquakes in this area have caused secondary hazards such as tsunamis and landslides that might have contributed to losses.
Estimated population exposure to earthquake shaking
At 11:27 UTC, a powerful aftershock measuring M6.3 hit the same region.
Aftershocks map, last updated at 12:20 UTC
The earthquake occurred as the result of reverse faulting at an intermediate depth. Focal mechanism solutions indicate rupture occurred on a structure striking either northwest or southeast, and dipping at a moderate angle. At the location of the earthquake, the Australia plate converges with and subducts beneath the Pacific plate at a rate of about 105 mm/yr towards the east-northeast. At the location of the earthquake, some researchers consider the edges of the Australia and Pacific plates to be divided into several microplates that take up the overall convergence between Australia and the Pacific, including the Solomon Sea and South Bismark microplates local to this event. In this context, the December 17th event occurred along the boundary between the Solomon Sea and South Bismark microplates. The Solomon Sea microplate moves slightly faster and more northeasterly with respect to the Pacific plate (and South Bismark microplate) than does the Australia plate due to sea-floor spreading in the Woodlark Basin several hundred kilometers to the southeast of the December 17th earthquake, facilitating the classic subduction evident beneath New Britain and New Ireland. The location, depth, and focal mechanism solutions of the December 17th event are consistent with its occurrence within the interior of the subducted Australia plate lithosphere, rather than on the shallow thrust interface between these two plates.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the December 17, 2016, M 7.9 earthquake are typically about 135x60 km (length x width).
Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed "intermediate-depth" earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters. "Deep-focus" earthquakes, those with focal depths greater than 300 km, also occur in the subducted Solomon Sea microplate to the north. Earthquakes have been reliably located to depths of about 500 km in this region.
The Papua New Guinea region frequently hosts large earthquakes. Over the preceding century, 33 other earthquakes with M 7+ occurred within 250 km of the December 17th event. 8 of these occurred at intermediate (70-300 km) or deep (300+ km) depths. The December 17th, 2016 earthquake is almost co-located with a M 7.6 event in September 2005, with a similar faulting mechanism. The 2005 event is not known to have caused damage or fatalities. One of the largest nearby historic events was a shallow M 8.0 earthquake in November 2000, about 140 km to the northwest, which resulted in at least 2 fatalities and left more than 5,000 people homeless.
Seismotectonics of the New Guinea region and vicinity
The Australia-Pacific plate boundary is over 4000 km long on the northern margin, from the Sunda (Java) trench in the west to the Solomon Islands in the east. The eastern section is over 2300 km long, extending west from northeast of the Australian continent and the Coral Sea until it intersects the east coast of Papua New Guinea. The boundary is dominated by the general northward subduction of the Australia plate.
Along the South Solomon trench, the Australia plate converges with the Pacific plate at a rate of approximately 95 mm/yr towards the east-northeast. Seismicity along the trench is dominantly related to subduction tectonics and large earthquakes are common: there have been 13 M7.5+ earthquakes recorded since 1900. On April 1, 2007, a M8.1 interplate megathrust earthquake occurred at the western end of the trench, generating a tsunami and killing at least 40 people. This was the third M8.1 megathrust event associated with this subduction zone in the past century; the other two occurred in 1939 and 1977.
Further east at the New Britain trench, the relative motions of several microplates surrounding the Australia-Pacific boundary, including north-south oriented seafloor spreading in the Woodlark Basin south of the Solomon Islands, maintain the general northward subduction of Australia-affiliated lithosphere beneath Pacific-affiliated lithosphere. Most of the large and great earthquakes east of New Guinea are related to this subduction; such earthquakes are particularly concentrated at the cusp of the trench south of New Ireland. 33 M7.5+ earthquakes have been recorded since 1900, including three shallow thrust fault M8.1 events in 1906, 1919, and 2007.
The western end of the Australia-Pacific plate boundary is perhaps the most complex portion of this boundary, extending 2000 km from Indonesia and the Banda Sea to eastern New Guinea. The boundary is dominantly convergent along an arc-continent collision segment spanning the width of New Guinea, but the regions near the edges of the impinging Australia continental margin also include relatively short segments of extensional, strike-slip and convergent deformation. The dominant convergence is accommodated by shortening and uplift across a 250-350 km-wide band of northern New Guinea, as well as by slow southward-verging subduction of the Pacific plate north of New Guinea at the New Guinea trench. Here, the Australia-Pacific plate relative velocity is approximately 110 mm/yr towards the northeast, leading to the 2-8 mm/yr uplift of the New Guinea Highlands.
Whereas the northern band of deformation is relatively diffuse east of the Indonesia-Papua New Guinea border, in western New Guinea there are at least two small (<100,000 km²) blocks of relatively undeformed lithosphere. The westernmost of these is the Birds Head Peninsula microplate in Indonesia's West Papua province, bounded on the south by the Seram trench. The Seram trench was originally interpreted as an extreme bend in the Sunda subduction zone, but is now thought to represent a southward-verging subduction zone between Birds Head and the Banda Sea.
There have been 22 M7.5+ earthquakes recorded in the New Guinea region since 1900. The dominant earthquake mechanisms are thrust and strike slip, associated with the arc-continent collision and the relative motions between numerous local microplates. The largest earthquake in the region was a M8.2 shallow thrust fault event in the northern Papua province of Indonesia that killed 166 people in 1996.
The western portion of the northern Australia plate boundary extends approximately 4800 km from New Guinea to Sumatra and primarily separates Australia from the Eurasia plate, including the Sunda block. This portion is dominantly convergent and includes subduction at the Sunda (Java) trench, and a young arc-continent collision.
In the east, this boundary extends from the Kai Islands to Sumba along the Timor trough, offset from the Sunda trench by 250 km south of Sumba. Contrary to earlier tectonic models in which this trough was interpreted as a subduction feature continuous with the Sunda subduction zone, it is now thought to represent a subsiding deformational feature related to the collision of the Australia plate continental margin and the volcanic arc of the Eurasia plate, initiating in the last 5-8 Myr. Before collision began, the Sunda subduction zone extended eastward to at least the Kai Islands, evidenced by the presence of a northward-dipping zone of seismicity beneath Timor Leste. A more detailed examination of the seismic zone along it's eastern segment reveals a gap in intermediate depth seismicity under Timor and seismic mechanisms that indicate an eastward propagating tear in the descending slab as the negatively buoyant oceanic lithosphere detaches from positively buoyant continental lithosphere. On the surface, GPS measurements indicate that the region around Timor is currently no longer connected to the Eurasia plate, but instead is moving at nearly the same velocity as the Australia plate, another consequence of collision.
Large earthquakes in eastern Indonesia occur frequently but interplate megathrust events related to subduction are rare; this is likely due to the disconnection of the descending oceanic slab from the continental margin. There have been 9 M7.5+ earthquakes recorded from the Kai Islands to Sumba since 1900. The largest was the great Banda Sea earthquake of 1938 (M8.5) an intermediate depth thrust faulting event that did not cause significant loss of life. (USGS) More information on regional seismicity and tectonics
Featured image credit: USGS