Normal faulting in a back arc basin: Seismological characteristics of the March 2, 1987, Edgecumbe, New Zealand, Earthquake

Abstract

The Edgecumbe earthquake (March 2, 1987, 0142 UT, 37.92°S, 176.76°E) occurred beneath a coastal river plain a the southeastern margin of the Central Volcanic Region (CVR) of the North Island of New Zealand, a back arc basin that is widening at a geodetically determined rate of about 12 mm/yr. Its situation enabled a wide range of geological and geophysical measurements to be made of the preseismic, coseismic and postseismic processes. The estimated hypocenter and fault plane solution are consistent with the observed surface faulting. Various estimates of the seismic moment of the mainshock range from 4.3×1018 N m (from long‐period P wave modelling of the first 5 s) to 10×1018 N m (from dislocation modelling of geodetic data). The variation in the values can be reasonably explained in terms of the methods used to determine them. 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 Edgecumbe earthquake was preceded by a large number of foreshocks, some near the mainshock, but most in a tight cluster 35 km away to the northwest (i.e., off‐strike). After the first half hour following the mainshock, swarms of aftershocks began occurring up to 50 km from the mainshock rupture, mostly along the strike of the faulting. Main rupture aftershocks were mostly located in the footwall of the main fault. A notable gap in the aftershock distribution is coincident with a geothermal field along strike of the main rupture. Swarms are common in the CVR and the whole foreshock ‐ main shock‐aftershock sequence has been interpreted as the contemporaneous occurrence of a number of swarms and a “standard” foreshock‐mainshock‐aftershock sequence associated with the mainshock rupture. The b values change from a low value prior to the mainshock to a very low value immediately afterwards, increasing to almost the long‐term, preearthquake value during the next few days. The temporal pattern of postseismic changes in b value was mirrored by the postseismic creep on one of the fault segments, which closely followed a Jeffreys‐Lomnitz law, suggesting that both phenomena were responses to a viscoelastic relaxation of the regional stress. Comparisons with similarly sized normal faulting events elsewhere show that 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.