Earthquake source processes and subduction regime in the Santa Cruz Islands region

Abstract

The source process of two large earthquakes that occurred in the Santa Cruz Islands subduction zone has been studied, one with a surface wave magnitude MS = 7.9 in 1966 and the other with MS = 7.7 in 1980. The seismic moment of the 1980 event estimated from long-period surface waves of Global Digital Seismograph Network (GDSN) and International Deployment of Accelerographs (IDA) records is 5.6 x 1027 dyn cm. Both of these events occurred at a shallow depth (i.e. between 20 and 40 km) with a similar thrust type focal mechanism with a strike (~ 347[deg]) parallel to the local trench axis, and they are located within 100 km in distance.The 1966 event is a break of a single asperity which ruptured unilaterally to the north along the local trench strike and was truncated sharply at the northern boundary. This rupture pattern and the aftershock area expansion to the north indicate that there was a distinct barrier on the south of the 1966 source area. The major moment release of the 1980 event represents the rupture of this barrier. However, inversion of P-waves with an assumption of a constant focal mechanism failed to constrain the source process thereafter. In particular, the deconvolved source-time functions of the 1980 event do not show such a clear truncation as is observed in the 1966 source process, indicating that some unresolved features followed the peak of the moment release. Nevertheless, the 1-day aftershock area suggests that the rupture propagated to the north into the source area of the 1966 event.We conclude that both earthquakes are subduction events between the Pacific and Indo-Australian plates; the 1966 event is a break of a single asperity and the 1980 event is a break of the remnant asperity which acted as a barrier for the 1966 source rupture. The subduction segmentation inferred from the intermediate-depth seismicity seems to control the mechanical condition of the subduction interface at shallow depths where the two large events took place. This interface created a barrier between the two source areas.