Active Faults in the Volcanic Environment of Pasirwangi Region, Garut Regency

Abstract

Garut Regency is surrounded by many volcanoes, both active and inactive. This area has also several active faults, which historically have been the source of earthquakes, including the Rakutak Fault Segment as part of Garsela Fault and several local faults. Some of earthquakes has magnitude less than M5.0 but causing damages, such as lately occured on September 18 and December 7, 2024.This study aims to identify the root causes of the major earthquakes that frequently occur in the Garut area, particularly those felt locally in Pasirwangi Sub-District and its surroundings, which causing infrastructure damages. Theresults are expected to contribute to disaster risk reductionand geological hazard-based spatial planning in Garut Regency, particularly in the study area. Active faults mapping was carried out using remote sensing,
and field verification using drone and visual observations to obtain data that characterizes the presence of faults, joints measurements, rock offsets, river diversion and morphology lineaments. Site class identification was carried out by microtremor measurement using seismograph. Indications of active faults are found in the form of morphological and river lineaments, rock offsets, river diversion and the emergence of springs as indications of fractures along the fault line. Several sagponds were found including in Cihurang which is connected to Lake Ciharuswhich forms the Rakutak fault segment lineament whichis relatively southwest-northeast (SW - NE) oriented.Others were found at Lake Cipondok which is connectedto the river lineament in the Cibongkor area which has awest southwest-east northeast (WSW-ENE) direction. The
down-dip faults were observed at Padaawas and has trend
relatively south southwest - north northeast (SSW - NNE). Microtremor measurements obtain the dominant frequency
values of the research area is in the range of 1 - 4 Hz, Periods
of 0,25 – 1 second, the earthquake amplification factors of
3 – 7 times of earthquake shaking, Vs30 of 233 - 545 m/sec.
The sediment thickness ranges from 25 - 64 m, and the land
type is Class C (Hard Soil) and Class D (Moderate Soil). The Pasirwangi Sub-District and its surroundings, is located
in a volcanic environment and is traversed by active faults,
which are relatively southwest – northeast (SW-NE),
west southwest - east northeast (WSW-ENE), and south
southwest – north northeast (SSW-NNE). The Rakutak
Segment of Garsela Fault, Padaawas and Cibongkor Faults
are active faults that cross the research area, which is in the
future could become the source of earthquakes, so that the
community needs to be aware, especially those located close
to fault lines. The potential for geological disasters in the
Pasirwangi Sub-District and its surroundings, needs special
attention from various parties, related to disaster mitigation
efforts, both structural and non-structural.

Keywords: Active Faults, Sagponds, Morphological
Lineaments, Rocks Offset, Site Class