Vanuatu Earthquakes Revised

Sunday’s earthquakes near Vanuatu were a mainshock pair instead of a mainshock-aftershock pair. Following further analysis, the U.S. Geological Survey (USGS) has revised its magnitude estimates and location data for the events.

The revision brings the USGS data more in line with data supplied by Geoscience Australia.

The two seventh magnitude earthquakes struck 28 minutes apart on Sunday March 25th 2007. The first quake struck at 12:40 NZ Standard Time, 125 km south of Isangel, Tanna, Vanuatu at a depth of 35 km. Geoscience Australia report the quake’s magnitude as 7.3, with the USGS reporting magnitude 7.1.

The second quake of the pair is reported to have been magnitude 7.0 by Geoscience Australia, while the USGS now report magnitude 6.9, nearly a magnitude higher than they originally estimated. This event, which struck 135 km south of Isangel, Tanna, Vanuatu is thought to have been at a depth of 35 km by the USGS and 10 km by Geoscience Australia.

The revision of earthquake data is quite common, once seismologists have been able to analyse earthquake waveforms from more seismic instruments. Whilst some of this data is available in real-time, some is received in data batches which may take hours or days to arrive.

Geoscience Australia estimate that the first quake may have been felt within a radius of 1680 km of the epicentre, an area which includes the northern part of Northland. Damage is expected within about 130 km of the epicentre.

They estimate that the second quake would have been felt within 1250 km of the earthquake’s epicentre, with damage limited to a radius of 100 km.

Interestingly, New Zealand’s seismograph records show distinctly stronger traces for the first quake of the pair, supporting the view that the second event was shallower and therefore felt over a more restricted distance.

The determination of a mainshock pair tends to explain the lack of aftershock activity. Only two subsequent quakes of magnitude 5.1 and 5.5 have been reported in the area by the USGS which publishes data for earthquakes of magnitude 4+ outside the United States on its website.

[Compiled from data provided by Geoscience Australia, the US Geological Survey and Geonet and their contributing agencies.]

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