Patterns of seismicity associated with asperities in the Central New Hebrides Island Arc

Abstract

In the central part of the New Hebrides Island Arc (Efate ‐ Malekula region, 16°S–18.6°S) a sequence of moderate sized earthquakes, their aftershock sequences, and other clusters of small earthquakes together form an intricate but coherent time‐space pattern that probably reveals a major asperity complex along the interplate boundary of the subduction zone. This pattern is determined by study of data from a local network. The sequences, including one event with magnitude Mw 7.1, eight events with magnitudes Mw between 5.8 and 6.3, and nearly 13,000 smaller events, occurred during the six year period 1978–1984. The seismicity is very unevenly distributed in space: a sharply defined east‐west line near 17.2°S separates the very active Efate region to the south, where nearly 10,500 earthquakes occurred, from the less active Malekula region to the north, where less than 2,500 earthquakes occurred during the six year period. In the Efate region several spatial patterns are highlighted. First, the seismic regimes of the updip and the downdip part of the interplate boundary are different. The updip part is characterized by a low background activity and main shocks with large aftershock zones, while the downdip part is characterized by a very high level of background activity and main shocks with much smaller aftershock zones or no aftershocks. This difference in seismic regimes suggests that asperities are located in the updip part of the interplate boundary, while the downdip part may slip predominantly by creep. A creep episode may have been responsible for the tilt signal observed by periodic relevelings of a 1‐km‐aperture network of benchmarks on Efate Island. Second, specific locations along the interplate boundary are identified either as sites of very intense and repeatedly activated concentrations of hypocenters or as sharp boundaries limiting the spatial development of aftershock zones. Two areas of concentrated activity particularly stand out; one located in the updip part of the interplate boundary and the other in the downdip part. Most of the clustered activity that occurred during the six years of observation is concentrated in these two areas. Four boundaries limiting the spatial development of aftershock zones are located in the updip part of the interplate boundary. One of these boundaries coincides with one of the areas of intense activity. Epicenters of moderately large events that occurred in the region since 1960 are also concentrated along the boundaries. The zones of concentrated activity and the boundaries limiting the development of aftershock zones can be interpreted either as locations of asperities or as edges of asperities. An alternative interpretation is that a giant asperity is located in the Malekula region, with the different spatial patterns observed in the Efate region being interpreted as the southern edge of that asperity, a transition zone between a locked part to the north and normal subduction to the south. Study of the temporal patterns of the seismicity shows that the main shocks in the downdip part of the interplate boundary are not preceded by obvious long‐term (months to year) nor short‐term (days to week) precursory activity. In the updip part of the interplate boundary, short‐term foreshocks preceded each main shock, but only one of these foreshock sequences is outstanding. Although no clear cyclic pattern related to the main shocks is evident, clustered activity that occurred in the two zones of concentrated activity and two other zones located in the back‐arc region are correlated in time with the occurrence of the major sequences located in the updip part of the interplate boundary. Activation of these four zones always follows the sequences in the updip part of the interplate boundary with varying time intervals (days to months). These four zones are also sometimes activated before sequences in the updip part of the interplate boundary. In particular, one of the zones of concentrated activity shows possibly repeated, long‐term (few months) precursory activity. This phenomenon could be an indicator of increasing probability of occurrence of main events in the updip part of the interplate boundary.