Discovery of active fault traces based on dip-slip distribution pattern of strike-slip faults : A case study on Kawakami and Okamura faults of the Median Tectonic Line active fault system in Shikoku

We discovered active fault traces on the extension of Kawakami and Okamura faults of the Median Tectonic Line active fault system in Shikoku based on new criterion that strike-slip faults are characterized by the pattern of dip-slip distribution; the upthrown side along strike-slip faults are, in general, located on the fault blocks in the direction of relative strike-slip motion. The northeast part of the Kawakami fault is recognized as a 13km-long NE-SW trending trace with low fault scarplets with the south side upthrown, and this does not agree with the pattern of dipslip distribution along a right-lateral strike-slip fault. Detailed interpretation of large-scale airphotographs taken by US army corps about 50 years ago, revealed ENE-WSW trending low fault scarplets across young alluvial plain, that is located on the northern extension of already mapped The scarplets are less than 1 m high up to the north. The ENE-WSW trending Okamura fault has been only recognized as an 18km-long distinctive fault scarplet known as "Nakahagi" cliff with the south side upthrown. We examined its eastern extension and found a low ENE-WSW trending fault scarplet with the north side upthrown in the south of Niihama city. The trace continues further east about 13km with obscure fault features. We trenched the newly found fault trace at two sites and obtained geological evidence of Holocene faulting. Thus, the geometric criterion we propose in this paper gives a quite different interpretation of fault distribution from those by the previous workers. We expect that this criterion may be tested together with the other geometric criteria on major active fault and compared with segmentation based on paleoseismological data