LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATED WITH SUBDUCTION OF THE LOUISVILLE RIDGE

Abstract

Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.