M 7.1 Earthquake Strikes Near New Caledonia in the South Pacific 8/29/18

August 29, 2018, M 7.1 earthquake about 300 km to the east of the island of New Caledonia in the southwest Pacific Ocean occurred as the result of thrust faulting on or near the plate boundary interface between the Australia and Pacific plates. 
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Focal mechanism solutions indicate the earthquake occurred on either a shallow fault striking northwest or on a steep fault striking southeast. At the location of this earthquake, the Australia plate moves towards the east-northeast with respect to the Pacific at a rate of approximately 78 mm/yr. At the South New Hebrides Trench, Australia lithosphere converges with and sinks beneath the Pacific plate, descending into the mantle and forming the New Hebrides/Vanuatu subduction zone, stretching from New Caledonia in the south to the Santa Cruz Islands in the north, a distance of about 1,600 km. The August 29, 2018 earthquake occurred approximately 50 km to the east of this trench, and its location, depth, and focal mechanism solution are all consistent with it occurring on the subduction zone interface between the two plates.
The Loyalty Islands region is very active seismically, and the region within 250 km of the August 29, 2018 earthquake has hosted 20 other M 7+ earthquakes over the preceding century. The largest was a M 7.7 earthquake in May 1995, which was located about 100 km to the south of today’s event, and to the southwest of the oceanic trench in the region commonly referred to as the outer rise. One of the most proximate of these historic earthquakes was a M 7.3 thrust faulting event in December 2003, about 30 km to the west of today’s earthquake. The 2003 earthquake was part of an active sequence of about 250 M 4+ events, beginning in December 2003 and continuing into February 2004. That sequence included both interplate thrust faulting earthquakes (the largest event in the sequence was the M 7.3 earthquake) and normal faulting earthquakes to the west of the oceanic trench (the largest being a M 7.1 earthquake in January 2004). A similar sequence of interplate thrust and outer rise normal faulting earthquakes occurred in October-December 2017, just to the north of the 2003-04 sequence. The 2017 sequence began with a M 6.7 thrust faulting earthquake on October 31, 2017. The largest event in the sequence was a M 7.1 outer rise earthquake on November 19, 2017. Between the M 6.7 earthquake and December 25, 2017, about 350 M 4+ events occurred as part of that sequence. Because of their remote locations far from land, these earthquakes do not cause significant shaking in populated areas and are not known to have resulted in damage or fatalities.
The eastern margin of the Australia plate is one of the most seismically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand.
Since 1900 there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these and the four largest occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.
North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear and includes two segments where old (>120 Myr) Pacific oceanic lithosphere rapidly subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench, the boundary curves sharply westward and changes along a 700 km-long segment from trench-normal subduction, to oblique subduction, to a left lateral transform-like structure.
Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back arc extension (or equivalently, slab rollback) causes the consumption rate of subducting Pacific lithosphere to be much faster. The spreading rate in the Havre Trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The southern tip of this spreading center is propagating into the North Island of New Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr. The overall subduction velocity of the Pacific plate is the vector sum of Australia-Pacific velocity and back-arc spreading velocity: thus it increases northward along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150 to 240 mm/yr.
The Kermadec-Tonga subduction zone generates many large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8+ events that have occurred close to the plate boundary were underthrusting events on the plate interface, or were intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise) earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench, generating a tsunami that killed at least 180 people.
Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the southern end of this trench, east of the Loyalty Islands, the plate boundary curves east into an oceanic transform-like structure analogous to the one north of Tonga.
Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the North New Hebrides trench, but the Australia plate consumption rate is increased by extension in the back arc and in the North Fiji Basin. Back-arc spreading occurs at a rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the D'Entrecasteaux ridge intersects the trench and causes localized compression of 50 mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern end of the trench.
Large earthquakes are common along the North New Hebrides trench and have mechanisms associated with subduction tectonics, though occasional strike-slip earthquakes occur near the subduction of the D'Entrecasteaux ridge. Within the subduction zone, 34 M7.5+ earthquakes have been recorded since 1900. On October 7, 2009, a large interplate thrust fault earthquake (M7.6) in the northern North New Hebrides subduction zone was followed 15 minutes later by an even larger interplate event (M7.8) 60 km to the north. It is likely that the first event triggered the second of the so-called earthquake "doublet".


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