A burst of earthquake activity has been recorded off the coast of the Gisborne district in recent days.
Tuesday 13th April 2010
Two offshore earthquakes of fourth magnitude have been recorded off the coast near Gisborne today.
At 10:29 this morning a magnitude 4.3 quake struck 30 km south-east of Tolaga Bay (50 km east of Gisborne) at a depth of 33 km. The quake was felt in the Gisborne and Hawke’s Bay areas, strongly at Gisborne and Waimata.
A magnitude 4.5 quake, located 40 km south-east of Tolaga Bay (60 km east of Gisborne) also at a depth of 33 km, occurred at 1:39 this afternoon. This quake was felt in the Gisborne district, inland at Matawai, and at Opotiki and Taneatua in the Bay of Plenty.
On Saturday afternoon, another off-shore quake struck the area. This event was located 40 km south-east of Gisborne also at a depth of 33 km.
The East Coast in the vicinity of Gisborne has been experiencing what is known as a slow-slip earthquake since mid-January. During a slow-slip event, the two tectonic plates in the area slip past each other smoothly and slowly over a period of days or weeks instead of abruptly in a typical earthquake.
The slow-slip near Gisborne has been occurring in three distinct phases. The first occurred off the coast near Tolaga Bay from mid-January to mid-February. A second zone of slippage occurred to the south near Mahia Peninsula from late January to mid-February. The third zone began slipping off the coast near Bartletts, south of Poverty Bay, mid-March.
Saturday’s earthquake occurred within the third zone of slippage occurring off-shore near Poverty Bay. It was located within the underlying Pacific Plate.
Today’s earthquakes struck within the zone of slippage that was observed near Tolaga Bay in January and February. These too, were located within the underlying Pacific Plate.
In a media release issued yesterday, GNS Science noted that the Poverty Bay slow-slip had started to die away, but another pulse of activity had been observed.
A similar-sized earthquake was recorded near Oponae in the Waioeka Gorge inland from Gisborne last evening. The magnitude 4.0 quake at 7:19 p.m. was located 20 km north-west of Matawai, 30 km south of Opotiki, at a depth of 50 km.
[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST.]