Spatial distribution and mechanisms of earthquakes in the southern New Hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations

Abstract

In 1977 a wide‐aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southern New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subduction zone. Surprisingly, small differences (less than about 10 km) were found between the locations of shallow‐ and intermediate‐depth earthquakes as located by the temporary local network and by the International Seismological Centre using worldwide stations. Analysis of the P wave travel time data from the OBS station closest to the trench shows limited evidence for a high‐velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by the spatial distribution of small size events located in this study. Shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blocks that are differentially uplifted along mainly NW‐SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the volcanic arc. Well‐determined depths of these events are between 11 and 22 km. The well‐located intermediate‐depth events define a 20‐km‐thick Benioff zone that has a dip of about 70°. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short‐term sample of small events determined by the local network and the long‐term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate‐depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern New Hebrides arc where the trench begins to curve around to the east.

The relationship between plate curvature and elastic plate thickness: A study of the Peru‐Chile Trench

“…Aleutians : Engdahl, 1977;Barazangi and Isacks, 1979b;Frohlich et al, 1982 Kurils: Sykes, 1966 Barazangi, 1977 Bonin: Carr et al, 1973;Katsumata and Sykes, 1969; Barazangi, 1977 Marianas: Katsumata andIsacks and Barazangi, 1977 Manila: Cardwell et al, 1980 Philippines: Cardwell et al, 1980 New Hebrides: Dubois, 1971Isacks and Barazangi, 1977; Pascal et al, 1979;Coudert et al, 1981 Tonga: Sykes, 1966Sykes et al, 1969;Mitronovas et al, 1969;Mitronovas and Isacks, 1971; Barazangi, 1977 Kermadec: Sykes, 1966; Barazangi, 1977 Middle America: Dewey andAlgermissen, 1974;Carr, 1976; Burbach et al, 1984 South America: Barazangi andIsacks, 1976;Barazangi and Isacks, 1979a …”

Normal fault type events in the upper plane of the double-planed deep seismic zone beneath the Northeastern Japan arc.

“…In any case, if the descending oceanic plate is bent locally, it is expected that the focal mechanism is affected by this bending of the plate (STAUDER, 1968;COUDERT et al, 1981). Figure 6 shows the seismicity of the intermediate-depth earthquakes and the P-axes of the composite focal mechanisms of the events in the upper seismic plane projected on the vertical section.…”

Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

“…Latham et al (1978) describe these instruments in some detail, and Buskirk et al (1981) have described apparent non-seismic signals recorded by these instruments. Several successful earthquake investigations utilized these OBS in the past (e.g., Coudert et al, 1981;Frohlich et al, 1982). The digital OBS used in this study is a descendant of strong-motion digital OBS described by Steinmetz et al (1979;, and is mechanically identical to that described by Nakamura et al (1987).…”

“…The study area chosen for the test (Figures 1, 2 and 3) lies about 200 km south of the area where earthquakes can be located by the Vanuatu regional seismic network , and was the focus of OBS and land network investigation in 1977 (Coudert et al, 1981). The 1986 study differed from that of 1977 in that the mean station spacing was considerably less, about 15 km compared to about 35 km.…”