NORMAL FAULTING IN A BACK ARC BASIN - SEISMOLOGICAL CHARACTERISTICS OF THE MARCH 2, 1987, EDGECUMBE, NEW-ZEALAND, EARTHQUAKE

The Edgecumbe earthquake occurred beneath a coastal river plain on the southeastern margin of the Central Volcanic Region (CVR) of the North Island of New Zealand. The estimated hypocenter and fault plane solution are consistent with the observed surface faulting. Focal mechanisms of both mainshock and aftershocks were similar to focal mechanisms previously determined for events in the CVR and its offshore extension. Normal faulting mechanisms make up 75% of the events with the remainder strike slip (dextral assuming a northeast striking fault). The distribution of mechanisms is consistent with the regional strain field as previously determined from geodetic observations. The mainshock has been modelled as a complex event with a second subevent about 3 s after the first, with both episodes of moment release initiating at a depth of about 8 km. The most unusual feature of the Edgecumbe earthquake was the high level of foreshock activity in two separate clusters 35 km apart. This foreshock activity and the widespread nature of the aftershocks are attributed to a level of stress throughout the CVR that is permanently close to the critical level for shear failure. A mechanism that is unknown, but undoubtedly related to volcanic or plutonic processes and probably involving fluids, enables stress changes within the CVR to be rapidly transmitted over tens of kilometres. -from Authors