article: Apr 14 2010 - A busy night of earthquakes for the Gisborne region

A number of moderate earthquakes off the coast of Tolaga Bay, north of Gisborne city, occurred overnight.

A map showing the recent earthquakes offshore of Tolaga Bay.

In the 24 hour period from 10 am on Tuesday, 13 April to Wednesday, 14 April six "felt" earthquakes with magnitudes between 4.0 and 4.5 occurred just off the coast of the Tolaga Bay area along with a number of smaller earthquakes. Earthquakes of this size in this area are not uncommon but it is unusual to have a large number of them in a short time span. These earthquakes have been occurring in the same area as a region of slow-slip which has been taking place in several episodes over the last couple of months. During slow-slip events an area on the interface between the Pacific and Australian tectonic plates that is normally locked is temporarily released, allowing the overlying crust to move back eastwards at rates of a few millimetres per day. It is likely that the earthquakes and the slow-slip event are related but exactly how is not yet clear.

Slow-slip events on the interface cause stress changes in the surrounding rocks. The stress changes have different effects at different locations. Offshore of the slow-slip event within the subducting Pacific plate (below the plate interface) the stress changes are extensional so there is a tendency for extensional (or normal-faulting) earthquakes to occur. On the plate interface offshore of the slow-slip event the stress changes are compressional so there is a tendency for compressional (or thrust) earthquakes to occur on the plate interface. Initial indications are that the recent events are of the normal-faulting variety within the subducting Pacific plate, but more work is needed to confirm this. If so, the earthquakes have similarities to the 2004-2005 Upper Hutt earthquakes that occurred near the top of the subducting plate following a slow-slip event beneath the Kapiti Coast in 2003-2004.