Published on May 23, 2013
The May 23, 2013 Mw 7.4 earthquake southwest of Vaini, Tonga, occurred as a result of normal faulting at a depth of approximately 170 km. At the location of this earthquake, the Pacific and Australia plates are converging at a rate of approximately 73 mm per yr in an east-west direction, resulting in the westward subduction of the Pacific plate beneath Tonga at the Tonga-Kermadec trench. The depth and faulting mechanism of the May 23rd earthquake indicate it ruptured a fault within the subducting Pacific lithosphere rather than on the shallower thrust interface between the two plates.
The Tonga-Kermadec arc has frequent moderate-to-large earthquakes, and has hosted over a dozen M6.5 plus earthquakes within 500 km of the May 23rd earthquake over the past 40 years. Most of these also occurred at intermediate depths; the largest was an Mw 7.7 earthquake in October of 1997, approximately 110 km to the north-northeast of the May 23 2013 event. None are known to have caused significant damage. Intermediate-depth (70-300 km) and deep-focus (depth less than 300 km) earthquakes are distinguished from shallow earthquakes (0-70 km) by the nature of their tectonic setting, and are in general less hazardous than their shallow counterparts, though they may be felt at great distances from their epicenters. The Tonga-Kermadec slab in the region of the May 23 2013 earthquake is seismically active to depths of over 650 km.
Seismotectonics of the Eastern Margin of the Australia Plate
The eastern margin of the Australia plate is one of the most sesimically 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 plus 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 plus earthquake along the Alpine Fault was 170 years ago; studies of the faults’ strain accumulation suggest that similar events are likely to occur again.
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 plus earthquakes have been recorded, mostly north of 30°S. However, it is unclear whether any of the few historic M8 plus 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.