Investigation of Slip Behavior Variation in the Shallow Part of Subduction Zone on the Basis of Vitrinite Reflectance.

Abstract:

Enormous earthquakes repeatedly occur in subduction zones, and the slips along megathrusts, in particular those propagating to the toe of the forearc wedge, generate ruinous tsunamis. Quantitative evaluation of slip parameters (i.e., slip velocity, rise time and slip distance) of past slip events for the shallow, tsunamigenic part of the fault is critical to characterize such earthquakes. Here we attempt to quantify these parameters for slip events that occurred along the shallow part of a megasplay fault and a plate boundary décollement in the Nankai Trough, off southwest Japan, and two fossilized analogue faults, hosted in the Miura-Boso accretionary complex (Shirako and Emi fault: East of Japan). We applied a kinetic approach to profiles of vitrinite reflectance data intersected the slip planes of the thrusts. This approach consists of calculation of heat generation and numerical analysis of vitrinite reflectance data. For the purpose of obtaining optimal slip parameters, residue calculation for fitting is implemented. As the result, the measured distributions of vitrinite reflectance around Nankai megathrust are fitted when heat generation rate (Q) and slip duration (t_r) are 16,600 J/s/m² and 6,250 s for the megasplay, and 23,200 J/s/m² and 2,350 s for the frontal décollement, implying slow and long-term slips. The results of numerical analyses on the measured data from the Shirako fault also indicate remarkable slow slips: slip velocity and slip distance of 0.14 cm/s and 5.17 m, respectively, under the optimal parameters set of Q = 14,500 J/s/m² and t_r = 3,600 s. For the Emi fault, however, the increasing in reflectance is limited only inside of the slip zone, and the calculate results shows the fast and short-time slip (~ 1 m/s and ~5m) has had occurred on this fault. This estimation is comparable to other chemical approach based on trace-element mobilization. These results show large variation of slip parameters in shallow part of subduction zone. Especially, slow slip velocity, long-term risetime and large displacement are recognized in the three fault zones (the megasplay, the frontal décollement and the Shirako fault). Estimated parameters are longer and slower than typical coseismic slip, but are rather consistent with rapid afterslip.