LARGE EARTHQUAKE DOUBLETS AND FAULT PLANE HETEROGENEITY IN THE NORTHERN SOLOMON-ISLANDS SUBDUCTION ZONE

In the Solomon Islands and New Britain subduction zones, the largest e arthquakes commonly occur as pairs with small separation in time, spac e and magnitude. This doublet behavior has been attributed to a patter n of fault plane heterogeneity consisting of closely spaced asperities such that the failure of one asperity triggers slip in adjacent asper ities. We analyzed body waves of the January 31, 1974, M(w) = 7.3, Feb ruary 1, 1974, M(w) = 7.4, July 20, 1975 (14:37), M(w) = 7.6 and July 20, 1975 (19:45), M(w) = 7.3 doublet events using an iterative, multip le station inversion technique to determine the spatio-temporal distri bution of seismic moment release associated with these events. Althoug h the 1974 doublet has smaller body wave moments than the 1975 events, their source histories are more complicated, lasting over 40 seconds and consisting of several subevents located near the epicentral region s. The second 1975 event is well modeled by a simple point source init iating at a depth of 15 km and rupturing an approximate 20 km region a bout the epicenter. The source history of the first 1975 event reveals a westerly propagating rupture, extending about 50 km from its hypoce nter at a depth of 25 km. The asperities of the 1975 events are of com parable size and do not overlap one another, consistent with the asper ity triggering hypothesis. The relatively large source areas and small seismic moments of the 1974 doublet events indicate failure of weaker portions of the fault plane in their epicentral regions. Variations i n the ''roughness'' of the bathymetry of the subducting plate, accompa nying subduction of the Woodlark Rise, may be responsible for changes in the mechanical properties of the plate interface. To understand how variations in fault plane coupling and strength affect the interplate seismicity pattern, we relocated 85 underthrusting earthquakes in the northern Solomon Islands Arc since 1964. Relatively few smaller magni tude underthrusting events overlap the Solomon Islands doublet asperit y regions, where fault coupling and strength are inferred to be the gr eatest. However, these asperity regions have been the sites of several previous earthquakes with M(s) greater-than-or-equal-to 7.0. The sour ce regions of the 1974 doublet events, which we infer to be mechanical ly weak, contain many smaller magnitude events but have not generated any other M(s) greater-than-or-equal-to 7.0 earthquakes in the histori c past. The central portion of the northern Solomon Islands Arc betwee n the two largest doublet events in 1971 (studied in detail by SCHWART Z et al., 1989a) and 1975 contains the greatest number of smaller magn itude underthrusting earthquakes. The location of this small region sa ndwiched between two strongly coupled portions of the plate interface suggest that it may be the site of the next large northern Solomon Isl ands earthquake. However, this region has experienced no known earthqu akes with M(s) greater-than-or-equal-to 7.0 and may represent a relati vely aseismic portion of the subduction zone.