Two more strong earthquakes and an associated tsunami struck the western Solomon Islands overnight.
Wednesday 6th January 2010
Earthquake activity has continued with two strong earthquakes, a tsunami and more 5th magnitude aftershocks striking the New Georgia Islands group in the western Solomon Islands overnight.
The largest event, a magnitude 6.9 earthquake which struck at 1:16 a.m. New Zealand time, was located 115 km south-east of Gizo in the New Georgia Islands at a depth of 35 km according to the U.S. Geological Survey.
Geoscience Australia also reports the quake as magnitude 6.9 but at the shallower depth of 26 km. It estimates that it would have caused damage within 90 km of the epicentre, which includes the islands of Rendova and Tetepare (Tetipari) and coastal parts of the main island of New Georgia. Geoscience Australia calculates that this quake would have been felt up to 1100 km away.
The Pacific Tsunami Warning Centre issued two bulletins for this event. The first bulletin, issued at 1:30 a.m. NZDT advised that a widespread tsunami threat was not thought to exist, but a local wave may have been generated. This was confirmed in a second bulletin, issued at 2:43 a.m. NZDT, which reported a 3 cm wave had been measured at Honiara, some 250 km from the earthquake epicentre.
A wave of 6 cm was measured at Honiara following Monday’s magnitude 7.2 earthquake and, on this occasion, the islands of Rendova and Tetepare experienced significant landslides and waves thought to be up to two metres in height. Early reports from the area indicate that a tsunami was experienced near this morning’s epicentre, and the Honiara wave gauge measurement suggests this latest event may have reached a metre in height at locations close to the earthquake.
A burst of increased earthquake activity followed this morning’s magnitude 6.9 event. Ten minutes later a magnitude 5.4 quake was recorded, with a magnitude 5.3 event eleven minutes after that. A magnitude 5.4 aftershock at 2:10 a.m. was swiftly followed by a quake of magnitude 6.0, two minutes later. The burst of activity appears to have ceased for the moment with a magnitude 5.3 earthquake recorded at 2:57 a.m. New Zealand time.
Initially, the sequence that began on Monday appeared to be a typical main quake, preceded by a substantial foreshock and followed by a series of aftershocks. With a magnitude 6.1 earthquake recorded yesterday at 28 minutes after midnight, the aftershock sequence seemed to be following a well-known trend with the largest aftershock being one order of magnitude lower than the main earthquake.
However, following this morning’s burst of activity, the nature of the swarm of earthquakes has changed. It now resembles a pair event which contains two main earthquakes of similar magnitude (7.2 and 6.9 in this case) followed by a series of aftershocks with at least one being about a magnitude lower than the main earthquakes. Such sequences have been observed in New Zealand, most notably in Hawke’s Bay in 1931 and Masterton in 1942.
In 1914 two quakes of about magnitude 6.5 struck East Cape, the first quake causing one death. In 1931, Hawke’s Bay experienced quakes of magnitude 7.8 and 7.3 which caused widespread damage and significant loss of life. The Wairarapa pair of 1942 were earthquakes of magnitude 7.2 and 7.0. In 1950 a pair of quakes with magnitudes around 7 struck the south of the South Island. Again, in 1976, a pair of quakes with magnitudes of 6.5 struck Milford Sound and the Puysegur Bank.
Returning to the Solomon Islands earthquakes, there has now been one quake of magnitude 7, with four events of magnitude 6 (including the foreshock) and eleven quakes of magnitude 5 in less than 48 hours.
A map of the New Georgia group of islands can be found here. The page includes information and damage maps relating to the deadly magnitude 8 earthquake and tsunami which struck the area in 2007.
[Compiled from data supplied by the U.S. Geological Survey, Geoscience Australia and the Pacific Tsunami Warning Centre and their contributing agencies.]