Jurnal Geologi dan Sumberdaya Mineral

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

2025-10-13T05:38:55+00:00

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

2026-01-02T05:53:24+00:00

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

Provenance dan Karakteristik Batupasir Formasi Pulau Balang dan Formasi Balikpapan Daerah Loa Janan Ilir, Kalimantan Timur

2025-12-11T05:05:21+00:00

Formasi Pulau Balang dan Formasi Balikpapan yang merupakan bagian dari Cekungan Kutai dicirikan oleh keberadaan batupasir sebagai salah satu litologi dominan. Analisis komposisi butiran penyusunnya dapat memberikan informasi mengenai tatanan tektonik serta karakteristik batuan asal. Penelitian ini bertujuan untuk mengidentifikasi batuan asal serta memahami tatanan tektonik yang memengaruhi pembentukan batupasir di wilayah studi. Metode yang digunakan meliputi analisis petrografi guna menentukan asal batuan, kondisi iklim purba saat sedimentasi berlangsung, serta lingkungan tektonik daerah penelitian. Hasil penelitian menunjukkan bahwa wilayah studi berada dalam tatanan tektonik recycled orogenic dengan subzona berkisar antara transitional recycled hingga lithic recycled yang dipengaruhi oleh aktivitas subduksi dan kolisi yang membentuk sabuk lipatan Antiklinorium Samarinda. Rekonstruksi kondisi iklim purba saat pembentukan batupasir menunjukkan adanya variasi dari iklim basah hingga kering dengan batuan asal berupa batuan metamorf seperti gneis, sekis, dan sabak, serta granit yang mengalami metamorfisme tingkat rendah.

Kata kunci: asal batuan, batupasir, iklim purba, tatanan
tektonik

Abstract

The Pulau Balang Formation and Balikpapan Formation which are part of the Kutai Basin are characterized by the presence of sandstone as one of the dominant lithologies. Analyzing the composition of its constituent grains can provide insights into the tectonic setting and the characteristics of the source rocks. This study aims to identify the provenance of the sandstone
and understand the tectonic framework that influenced its formation in the study area. The methods used include petrographic analysis to determine the origin of the rocks, the paleoclimatic conditions during sedimentation, and the tectonic environment of the region. The results indicate that the study area is part of a recycled orogenic tectonic setting, with subzones ranging from transitional recycled to lithic recycled, influenced by subduction and collision activities that formed the Samarinda Anticlinorium fold belt. The reconstruction of paleoclimatic conditions during sandstone formation suggests variations from humid to arid climates, with the source rocks consisting of metamorphic rock such as gneis, schist, and slate, as well as granite that underwent low grade metamorphism.

Keywords: provenance, sandstone, paleoclimate, tectonic
setting

Geoheritage Potential of Non-Volcanic Hot Springs in Bangka Island: Implications for Geotourism Activities

2026-01-07T05:30:13+00:00

Natural hot springs are important for various domestic purposes, health treatments, recreational bathing as well as settings for socio-cultural and traditional functions for thousands of years. However, most people are not aware and familiar with the key geological processes of these hot springs. There are three natural hot springs were discovered in the Bangka Island, while the origin and source of these hot springs are still poorly understood. This study provides the water characteristics of those springs and discusses of their origin by considering geological setting, as well as identified these hot springs as geoheritage sites potential. The surface temperatures of thermal springs range from 46 - 49° C, and the pH values ranges from 5.0 to 6.0. The hydro geochemical characteristic of hot springs is divided into two types, mature water and peripheral water. The anion composition (Cl-SO4-HCO3) obtained from water samples of the Permis Hot Spring contains high Cl, indicates as mature water type, and unrelated to recent volcanism. On the other hand, a water sample from the Nyelanding Hot Spring is relatively high HCO3
which indicate as pheripheral water type, and far from the primary source. The Na-K-Mg ternary diagram reveals that water samples of the Permis hot spring is partial equilibrium water type with high Na and Cl compositions, while the Nyelanding one is immature water type characterised by high Mg content. Radiogenic granitic host rock represents the main heat source for both non volcanic hot springs in Bangka Islands. The heat is derived from deep-shallow circulation of meteoric water through fractures that cut into the granitic plutons. These hot springs are unique and show special features that play an important role in understanding of the dynamic of the Earth. These hot springs also have some significant geoheritage values, such as scientific, aesthetic, and recreational. Therefore, these hot springs have the potential to be properly conserved, managed and developed geoheritage sites and utilised for a sustainable geotourism development in Bangka Island.

Keyword: Non-volcanic hot springs, geoheritage, geotourism, Bangka Island