Another 5th magnitude earthquake struck New Zealand’s North Island this afternoon, continuing the burst of earthquake activity that began yesterday.
Friday 19th December 2008
Much of the North Island felt the effects of a deep magnitude 5.1 earthquake that struck 20 km south-west of Rotorua at 1:21 p.m., according to GeoNet. The quake’s depth of 180 km meant that it was widely felt, attracting reports of shaking from the Bay of Plenty, Gisborne, Hawke’s Bay, Manawatu, Wairarapa and Wellington areas. Of the 300-odd reports filed within an hour of the event, a small number also came from Blenheim, Nelson and Picton at the northern end of the South Island. Slight damage was reported from Pahiatua.
Analysis by the U.S. Geological Survey (USGS) puts the earthquake’s epicentre close to Raglan (118 km south of Auckland, 133 km west of Rotorua) at a depth of 35 km. While the two organisations differ in location and depth, the estimated magnitude is in close agreement, with the USGS reporting magnitude 5.2.
Despite a dramatic expansion of the New Zealand seismic network, GeoNet can sometimes have difficulty in accurately pinpointing the focus of deep on-shore earthquakes as its instruments are spread out on the long, narrow landmass of New Zealand. On this occasion, it is not clear whether uncertainty exists in GeoNet’s calculation.
The USGS advises that its horizontal calculation is accurate to with +/- 8 km and the depth has been calculated by a location programme.
Other onshore quakes since Wild Land’s report on yesterday’s activity include a very shallow magnitude 2.9 quake located 10 km south of Opotiki at 11:26 last night, a magnitude 3.3 forty km-deep event 10 km north of Hastings at 9:30 last night, and a 40 km-deep magnitude 3.5 quake centred 10 km south-west of Porirua at 7 o’clock last evening.
[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST; and the US Geological Survey and its contributing agencies.]
See that the USGS have revised their location in the meantime and followed Geonet, which, given the shaking maps, seems much more plausible. I’ve often noted that the USGS seem to be a little off with NZ quakes.. perhaps there is some systematic error in their calculations for our part of the world?