Estimation of total lateral displacement including strike-slip offset and broader drag deformation on an active fault: Tectonic geomorphic and paleomagnetic evidence on the Tanna fault zone in central Japan

摘要

When determining deformation along an active strike-slip fault, it is desirable to include the cumulative distributed deformation due to drag away from the fault trace in addition to offset on the fault. We achieve this by integrating geomorphologic, paleomagnetic, and geologic data for the active left-lateral strike-slip Tanna fault zone in central Japan, which previous studies suggest is slipping at about 2. mm/yr. We combine new detailed mapping of multiple Quaternary strands of the fault zone with paleomagnetic measurements from volcanic rocks extruded across the faults from the Taga Volcano at about 0.6. Ma. From these data we determine differential counterclockwise rotations, from which we infer cumulative strike-slip drag deformation, at distances of about 1-3. km from the faults. These results suggest that the total strike-slip displacement accompanying cumulative drag deformation along the fault zone from approximately 0.6. Ma to the present is more than 2.3-2.7. km (based on a kinematic model), or about 6.0. km (based on a power law rheology model), after adding 1. km of offset along the fault trace. Thus, depending on the model applied, the total rate of strike-slip deformation along the Tanna fault zone is either more than 3.8-4.5. mm/yr, or about 10. mm/yr, respectively. Both of these rates are considerably higher than a previous estimate of about 2. mm/yr based solely on strike-slip offset without accommodating the drag deformation over the broader zone on either side of the fault break. Furthermore, the deformation rate based on the power law rheology model is comparable to the rate detected by past GPS observations.