Active Fault and Paleoearthquake Researches No. 5 (2005)

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Trenching survey across the surface rupture associated with the 2004 Mid Niigata Prefecture earthquake

The 2004 Mid Niigata Prefecture earthquake sequence produced a nearly 1-km-long, N-S to NNW-SSE-striking, west-side-up surface faulting at Obiro, Uonuma City, eastern margin of the epicentral region. To understand the past behavior of the fault, we excavated in 2005 a trench across the surface rupture. A well-defined west-dipping reverse fault displacing the Plio-lower Pleistocene Uonuma Formation and overlying upper Pleistocene-Holocene deposits is exposed on the trench walls. This reverse fault is connected with the 10-cm-high surface rupture. From deformational characteristics of the deposits, past surface rupturing events might be reconstructed.