Tectonic Geomorphology of the Hanging Wall Blocks of the Cimandiri Fault Zone, West Java, Indonesia

Abstract:

In areas where regional strain is accommodated by broad zones of short and low slip-rate faults, geomorphic and paleoseismic characterization of faults is difficult because of poor surface expression and long earthquake recurrence intervals. In humid areas, faults can be buried by thick sediments and undetectable until the next earthquake. In Java, despite the frequency of damaging shallow earthquakes, active faults are diffuse and their characterization is challenging. Among them is the ENE-trending Cimandiri fault. Cumulative displacement along the fault produces prominent ENE-oriented ranges with the east side moving relatively upward and to the north. Along its length, the few hundred meter wide fault zone is expressed in the bedrock by numerous NE, E and NW-trending thrust- and strike slip faults and folds. However, it is unclear which of these structures are active, as the diffuse nature of the fault zone has so far stymied conventional paleoseismic study. To address this, we performed a tectonic geomorphology analysis of the fault zone. We used the 30-m resolution SRTM-DEM to construct longitudinal profiles of 601 bedrock rivers along the ranges and calculated the normalized channel steepness index (k_sn). Our preliminary results rely on the assumption that k_sn is a reasonable proxy for relative rock uplift rate in a region, assuming variations in rock type and climate are insignificant. While the active traces of the Cimandiri fault are obscured, the spatial variation in k_sn allows us to delineate 4 discontinuous hanging wall blocks that vary between E and NE striking along the zone. The largest k_sn values are along the central-western block (Cibeber area). The longest block is in the central eastern portion of the fault zone and comprises 45 km of the 100 km long fault zone. The fault bifurcates at its eastern termination and steps into the Lembang fault. The distribution of k_sn suggests that reverse motion is more dominant than lateral because of a lack of kinematically consistent deformation associated with the steps along the fault zone. This geometric characterization of the active Cimandiri fault implies a maximum magnitude along its 100 km length of M7.5 but its geometric complexity suggests greater segmentation and smaller events—consistent with the limited historical record of upper plate earthquakes in Java.