294 Physics ofthe Earth and Planetary Interiors, 56 (1989) 294—310 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands Source process of the great 1971 Solomon Islands doublet Susan Y. Schwartz ~, Thorne Lay and Larry J. Ruff Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1063 (USA.) (Received August 25, 1988; revision accepted September 28, 1988) Schwartz, S.Y., Lay, T. and Ruff, U., 1989. Source process of the great 1971 Solomon Islands doublet. Phys. Earth Planet Inter., 56: 294—310. Large underthrusting earthquakes in the Solomon Islands tend to occur as multiplets, separated by a few hours to several days in time and 30—100 km in space. The largest sequence, a doublet on July 14, 1971 (M~= 8.0) and July 26, 1971 (M~= 8.1), is particularly unusual in that it occured at the junction of two trenches, with the July 14 event in the Solomon Islands Trench preceding the July 26 rupture in the New Britain Trench. The location and large moment release of this doublet as well as an incompatibility of published body wave and surface wave focal mechanisms for the July 14 event, motivated a detailed study of the source process of the 1971 sequence. To satisfy both the surface wave and body wave data, we explored the possibility of a change in the faulting geometry during rupture for the July 14 event. A fault model that changes orientation from a northwest—southeast striking, shallow-dipping plane, similar to the mechanisms of neighboring underthrusting events in the Solomon Islands Trench, to a more north—south striking plane — 70 s after rupture initiation is consistent with the observed Rayleigh and Love wave amplitude patterns. This change in mechanism is suggested by systematic variations in the mechanisms of smaller earthquakes near the cusp in the Solomon Islands plate. A simultaneous inversion of WWSSN P wave seismograms for the time, location, seismic moment and focal mechanism of individual subevents gives results in agreement with the surface wave model, although the precise change in mechanism is not well resolved. The spatiotemporal distribution of subevents reveals two major pulses of moment release. The first occurred on the initial fault near the epicenter at the origin time of the earthquake. The second occurred on the north—south striking fault, 50—70 km northwest of the epicenter — 70 s after the origin time. The change in mechanism appears to reflect contortion of the slab near the trenchjunction. Inversion of WWSSN P wave seismograms for the rupture history of the July 26 event reveals a westerly propagating rupture extending — 60 km along the New Britain Trench. For both of the 1971 earthquakes, regions of high moment release are located in the vicinity of the Solomon Islands—New Britain Trench junction. Contortion of the subducting lithosphere as it bends around the arc—arc junction may have enhanced the degree of mechanical coupling between the subducting and overriding plates, concentrating stress in this region, and in turn, promoting efficient triggering across the trench junction. 1. Introduction heterogeneity consisting of small strong patches (asperities) spaced closely together such that the The largest underthrusting earthquakes in the failure of one patch has an unusually high prob- Solomon Islands tend to occur as multiplets sep- ably to trigger slip in immediately adjacent patches arated by a few hours to several days in time and (Lay and Kanamori, 1980). The largest earthquake 30—100 km in space (Fig. 1). This behavior has doublet in the Solomon Islands occurred on July been attributed to a specific pattern of fault plane 14, 1971 (M~= 8.0) and July 26, 1971 (M~= 8.1). This sequence differs from other large Solomon * Present address: Charles F. Richter Seismological Labora- Islands earthquake pairs in that, rather than tak- tory, University of California, Santa C~, CA 95064, U.S.A. ing place along a linear trench segment, it oc- 0031-9201/89/$03.50 © 1989 Elsevier Science Publishers B.V. 295 ONTONG-JAVA - PLATEAU ~ ~ ~ruM ~ ~ ‘~‘ ~ /~t’ 0”~ 0) S — EWGUINE ~~~4(7.I) 1/3)174(7. is~~ 144 148 152 15$ 160 164°E Fig. 1. Map of the Solomon Islands region showing the location of the recent earthquake doublets and relevant tectonic features. The aftershock areas of the largest doublet events on July 14 and July 26 are indicated by the shaded regions. The arrows represent the local relative plate convergence directions. All lower hemisphere focal mechanism solutions are from Lay and Kanamori (1980); the mechanisms shown for the 1974 and 1975 doublet events have been obtained by averaging the values from both doublet events. curred at the junction of two trenches. The July 14 source studies of the July 14 and July 26 Solomon event is located in the northwestern Solomon Is- Islands earthquakes in an attempt to provide a lands Trench, whereas the July 26 event ruptured better understanding of the physical properties of the northeastern New Britain Trench (Fig. 1). asperities and of the efficiency of earthquake trig- These two earthquakes represent subduction of the Solomon Sea Plate beneath the Pacific Plate N and the Bismark Sea Plate respectively (Lay and Kanamori, 1980). This unusual faulting geometry has generated the largest earthquake doublets in — the Solomon Islands in this century, including — — KB pairs in 1919—1920, 1945—1946, and 1971 (Lay and Kanamori, 1980), suggesting an association between the distribution of stress heterogeneity W \‘~ ~NIL + ‘I E (asperities) and the contortion of the subducting •~ sBA RA~ ~ Solomon Sea Plate. • •/ •/ The asperity model, which attributes the occur- ~• rence of the largest earthquakes to the failure of o v~.. — the strongest regions of the fault plane, provides a —. good conceptual framework for understanding the / — L & K rupture process of large earthquakes. The model S - - - K BF has been successfully invoked to explain many —-• PAS observed earthquake phenomena such as the ex- Fig. 2. Lower hemisphere equal area projection of fault plane istence of foreshocks and aftershocks (Kanamon, solutions for the July 14, 1971 earthquake determined from 1981) and variations of seismic and aseismic slip surface waves by Lay and Kanamon (1980), body waves by Kikuchi and Fukao (1987) and from first motions by Pascal along plate boundaries (Lay et al., 1?82). How- (1979). Open and closed circles represent dilatational and ever, the physical properties of asperities are, as compressional first motions picked by the authors, labelled yet, not well known. This paper presents detailed circles are stations used in our body wave inversions. 296 genng in the presence of geometric complications by Pascal (1979) as well as the body wave mecha- in the subducting plate. nism published by Kikuchi and Fukao (1987) (Fig. 2). Furthermore, several P wave first motions col- lected in this study (Fig. 2) are clearly violated by 2. July 14, 1971 earthquake the surface wave mechanism. Kikuchi and Fukao (1987) studied the long-period P waves from this 2.1. Focal mechanism event, and interpreted the source process as a simple unilateral rupture propagating to the The July 14, 1971 Solomon Islands earthquake northwest along the strike of their preferred fault occurred at the northwesternmost end of the plane. However, their analysis was somewhat in- Solomon Islands Trench. The epicenter (5.5°S, complete, as they failed to reconcile their model 153.9 °E,ISC) is located in the southeast quadrant with the surfacewave results of Lay and Kanamori of a quasi-circular aftershock area (Fig. 1). Lay (1980). The inconsistency of the surface wave and and Kanamori (1980) determined a focal mecha- body wave analyses, and the apparent incompati- msm (strike, t~ = 3450, dip, tS = 450, rake, A = bility of the surface wave solution with the local 620) and a corresponding static seismic moment relative plate convergence direction suggest that (M 0 = 12 x 1027 dyn cm) for this event by analysis the July 14 event changed faulting geometry dur- of Rayleigh and Love wave amplitudes. Their ing its rupture process. In this section, we will mechanism (solid lines in Fig. 2) differs substan- construct a model for the source process of this tially from the solutions they obtained for the earthquake that is consistent with the body wave 1974 and 1975 doublet events located just to the and long-period surface wave observations as well southeast (Fig. 1). as with the tectonic framework in which this The 1974 and 1975 earthquakes have shallow earthquake occurred. First, we constrain possible dipping, northwest—southeast striking thrust fault faulting geometries using the long-period surface planes, which are consistent with an interpretation wave observations. Then we examine long-period of underthrusting of the Solomon Sea Plate be- P wave observations to ensure their compatibility neath the Pacific Plate. From an analysis of many with our preferred model. earthquake P wave first-motion focal mechanisms, Johnson and Molner (1972) estimated the local 2.2. Surface wave observations relative plate motion between the Solomon Sea Plate and the Pacific Plate to be in the direction Lay and Kanamori (1980) analyzed the surface N34°E (Fig. 1). The convergence direction mdi- waves from all six earthquakes comprising the cated by the 1974 and 1975 doublet event mecha- 1971, 1974, and 1975 Solomon Islands doublets. nisms is N35°E, in close agreement with the local They obtained radiation patterns for these events relative plate motion estimate. However, the con- by equalizing all traces to a common propagation vergence direction indicated by the surface wave distance, filtering the signals using a Gaussian mechanism of Lay and0 E,Kanamorimuch more(1980)easterlyforthanthe andbandpassadjustingfilter withthe aamplitudesshort-periodtocut-offa standardat 40 s, Julythe local14 eventrelativeis Nibplate motion vector. This raises WWSSN (15-100) instrument with a gain of 1500.