Karakteristik Faktor Penguatan Gelombang Gempabumi oleh Lapisan Lunak Permukaan, Studi Kasus di Wilayah Perkotaan Cianjur dan Sekitarnya

Marjiyono Marjiyono — 2025-11-24

Disamping dekat dengan sumber gempabumi,
wilayah perkotaan Cianjur juga beresiko terhadap
penguatan gelombang gempabumi oleh lapisan lunak
litologi permukaan. Endapan soil yang cukup tebal di
kawasan ini berpotensi menguatkan gelombang ketika
terjadi gempabumi. Hasil pemodelan data mikrotremor
array di 18 titik ukur diperoleh kecepatan gelombang geser
soil berkisar antara 50 m/det. - 308 m/det. Endapan soil
ini menumpang di atas batuan vulkanik yang memiliki
kecepatan gelombang geser antara 246 m/det – 1415 m/
det. Adapun ketebalan soil berdasarkan pemodelan data
mikrotremor single station di 189 titik ukur berkisar antara
2 m – 62 m. Nilai faktor penguatan gelombang oleh lapisan
soil ini berkisar antara 1 - 2,4 kali dengan zona penguatan
tinggi berada di daerah Cugenang, Warungkondang dan
Cilaku. Berdasarkan catatan, ketiga wilayah tersebut
mengalami intensitas goncangan maksimum sebesar VII
VIII skala MMI.

Kata kunci: penguatan gelombang, lapisan lunak permukaan,
mikrotremor array, mikrotremor single station.

Abstract

The Cianjur urban area is exposed to a substantial
risk, particularly near the seismic source and due to site
amplification, resulting from a soft surface layer. The fairly
thick soil deposits potentially amplify seismic waves when
an earthquake occurs. According to the microtremor array
that was deployed on 18 locations, soil shear wave velocity
ranges from 50 m/sec to 308 m/sec. This soil deposit lies
above volcanic rock that has shear wave velocities ranging
from 246 m/s to 1415 m/s. Single-station microtremor data
modelling on 189 sites revealed that the soil thickness varies
between 2 m and 62 m. The calculated site amplification of
the research area ranges from 1 to 2.4 times, indicating that
the Cugenang, Warungkondang, and Cilaku regions have
significant amplification zones. Historical records showed
that the highest shaking intensity in the three regions ranged
from VII to VIII of the MMI scale.

Keywords: site amplification, soft surface layer, array
microtremor, single station microtremor

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

Yudhicara Yudhicara — 2025-11-24

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

Jurnal Geologi dan Sumberdaya Mineral

Karakteristik Faktor Penguatan Gelombang Gempabumi oleh Lapisan Lunak Permukaan, Studi Kasus di Wilayah Perkotaan Cianjur dan Sekitarnya

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