Abstract
Understanding variations of slip distance along major thrust systems at convergent margins is an important issue for evaluation of near-trench slip and the potential generation of large tsunamis. We derived quantitative estimates of slip along ancient subduction fault systems by using the maturity of carbonaceous material (CM) of discrete slip zones as a proxy for temperature. We first obtained the Raman spectra of CM in ultracataclasite and pseudotachylyte layers in discrete slip zones at depths below the seafloor of 1–4 km and 2.5–5.5 km, respectively. By comparing the area-under-the-peak ratios of graphitic and disordered bands in those Raman spectra with spectra of experimentally heated CM from surrounding rocks, we determined that the ultracataclasite and pseudotachylyte layers had been heated to temperatures of up to 700 and 1300 °C, respectively. Numerical simulation of the thermal history of CM extracted from rocks near the two slip zones, taking into consideration these temperature constraints, indicated that slip distances in the ultracataclasite and pseudotachylyte layers were more than 3 and 7 m, respectively. Thus, potential distance of coseismic slip along the subduction-zone fault system could have regional variations even at shallow depth (≤ 5.5 km). The slip distances we determined probably represent minimum slips for subduction-zone thrusts and thus provide an important contribution to earthquake preparedness plans in coastal areas facing the Nankai and Sagami Troughs.
Highlights
In a worldwide context, most of the seismic energy released by large earthquakes occurs in plate-subduction zones (Scholz 2002)
Raman spectra of carbonaceous material (CM) extracted from in and around the slip zones of the Emi major thrust (Fig. 4a–d) show small shoulder peaks at 1200, 1500, and 1620 cm−1, probably corresponding to D4, D3, and D2 bands, respectively, for CM from host rock and cataclasite, but these are absent for ultracataclasite
Comparison of the AD/ AG ratios of these Raman spectra with those of experimentally heated CM from nearby host rocks indicated that CM in the ultracataclasite layer from the Emi major thrust had been heated to temperatures up to 700 °C and that CM in the pseudotachylyte layer from the Kure of-sequence thrusts (OST) had been heated up to 1300 °C
Summary
Most of the seismic energy released by large earthquakes occurs in plate-subduction zones (Scholz 2002). To investigate the mechanism of gigantic seismic slip along the plate interface during the 2011 Tohoku-oki earthquake, Hirono et al (2016) undertook quantitative laboratory analyses of the properties of plate-interface material retrieved during the Japan Trench Fast Drilling Project (JFAST) and ran simulations of fault weakening and rupture propagation. Their results indicated a large stress drop and slip of about 75 m near the trench during. In this study we focused on the slip parameters of the subduction fault system taking into consideration the regional heterogeneity of the plate-subduction faults
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