Accurate source depths and focal mechanisms of shallow earthquakes in western South America and in the New Hebrides Island Arc

Abstract

Synthetic seismograms are matched to long‐period P waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms. Accurate depths are obtained for a large sample of moderate‐sized (6 < Ms < 7) events which produced relatively simple P waveforms and do not require intensive analysis. The estimated error in the determination of depth is about ±5 km or less for most of the events considered. The procedure is applied to two suites of events, one in South America (83 events) and one in the New Hebrides (61 events). In these two areas of contrasting styles of subduction, the accurate depths provide new information on the bending of descending plates near trenches, the seismicity and tectonics of the upper plate, and the geometry and structure of the zone of contact between the upper and descending plates. Depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with Chapple and Forsyth's (1979) preferred model of a bending elastic‐perfectly plastic plate. However, an unusually deep event occurred beneath the Fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late Miocene time. The calculated depth of the event, 48 km, places it below the lithosphere‐asthenosphere boundary derived from thermal models. Vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin (<10 km thick) curved zone of contact dipping arcward or landward from the trench axis. However, a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact. These events would be accommodated by an interplate zone 15 km thick. In both subduction zones, interplate events occur at depths between about 15 and 50 km. However, the downdip width of the South American interplate zone is 2 to 3 times larger than that in the New Hebrides. Most of the intraplate events near the interplate boundary are located in the descending plate. In the upper plate, seismicity is more pronounced in the “back arc” regions of the two subduction zones. Thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc. Depths of shallow earthquakes in the eastern Andes and sub‐Andean regions of South America range between about 9 and 36 km, indicating that both the upper and middle crust are seismically active; about 75% of the events occurred in the upper 25 km. Furthermore, the focal mechanisms indicate that from Ecuador to Argentina the crust of the eastern side of the Andean Cordilleran is in a state of east‐west compression oriented nearly parallel to the direction of convergence between the Nazca and South American plates. The depths, the orientations of the earthquake nodal planes, and the close relationships to Quaternary tectonics imply basement‐involved crustal shortening in much of the Eastern Andes and sub‐Andean zones of Peru and Argentina.