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Chen, T.H., 1981

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Bibliographic Reference

Chen, T.H., 1981, The seismicity of forearc marginal wedges (accretionary prisms) and seismotectonics of convergent margins: University of Texas, Dallas, Ph.D. dissertation, 111 p., illust., maps.


Three different types of seismic data from island arc regions have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge. These types of data are: (1) Teleseismically-recorded earthquakes that have been reported in the literature as occurring in major arc-trench regions. These events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge. These data include many tsunamigenic earthquakes; (2) Local earthquakes located by combined ocean bottom seismograph and land networks in the arc-trench region in the New Hebrides and the central and eastern Aleutian Trench. None of the more reliable of these hypocenters lies within the accreted wedge; (3) S-P intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge. These data do not show the existence of events occurring within the accreted wedge; e.g., from 18 ocean bottom seismograph stations with a cumulative operation time of about one year, the smallest S-P time is about 2.5 sec for events in the New Hebrides and about 4 sec for events in the Adak and Kodiak regions. We found no S-P time smaller than 2 sec from 6 years of seismograms recorded at Middleton Island, Alaska, and no S-P time smaller than 4 sec from 25 years of seismograms recorded on Barbados. All of these events could have occurred outside the forearc marginal wedges. The absence or scarcity of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically.

Although we have located no events with hypocenters which are situated unambiguously within the accreted wedges, we have located many events which occurred in the vicinity of the accreted wedge. Some occur in the upper portion of the Benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate. These locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs.

These data provide an excellent opportunity to study composite focal mechanisms of events occurring near the accreted wedge. The results indicate that: (1) primary normal faulting occurs in the top of Benioff zone underneath the active margin, (2) primary thrust faulting occurs in the active margin of overriding plate and the angle between the axis of the compressional stress and the downdipping slab is increased with depth, and (3) primary normal faulting is also found above the zone of thrust faulting in the shallowest part of the overriding plate in New Hebrides.

We interprete the significance of the composite focal mechanisms in terms of plate tectonics as follows: As the downgoing slab subducted underneath the overriding plate, the bending caused normal faulting which occurs in the top of subducting plate is initiated in the outer rise. It continues to occur in the top of Benioff zone to a depth at least about 40 km inner the trench. The interaction between the convergent plates induces the thrust faulting in the overriding plate. The variation of direction of compressional stress with focal depth reflects the nature of the stress distribution in the active margin of the overriding plate. The normal faulting of the shallowest part of the overriding plate may relate to either the bending or slumping occurring in the top of the active margin.

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