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Sains Malaysiana 53(7)(2024):
1533-1544
http://doi.org/10.17576/jsm-2024-5307-05
Microtremor
Analysis to Identify Fissure Vulnerable Zones in Demak, Central Java, Indonesia
(Analisis
Mikrotremor untuk Mengenal Pasti Zon Terdedah Rekahan di Demak, Jawa Tengah,
Indonesia)
RAHMADANY LEONITA1,
SISMANTO1,*, MARJIYONO2
1Department of Physics, Faculty of
Mathematics and Natural Science, Universitas Gadjah Mada, Kaliurang Street Km
3, Yogyakarta, Indonesia
2Bandung Geological Survey Center,
Diponegoro Street No. 57, Bandung 40122, Indonesia
Received:
20 December 2023/Accepted: 6 June 2024
Abstract
Demak
Regency, located in Central Java, Indonesia stands out among the regencies for
its rapid development and regional growth, leading to a significant reliance on
groundwater. This is thought to be caused by low precipitation and the
geological characteristics of Demak Regency, which is mostly located on
alluvial plains. To address these concerns, this study aimed to pinpoint
regions vulnerable to ground fractures resulting from excessive groundwater
extraction compounded by seismic activity. The research encompassed 191
locations, including 177 single station microtremor measurement points and 14
array microtremor measurement points. Single station microtremor measurements
utilized the Horizontal to Vertical Spectral Ratio (HVSR) method, while array
measurements employed the Spatial Autocorrelation (SPAC) method. Ground shear
strain values spanning from 32.12 × 10-6 - 96.39 × 10-6.
Notably, areas exhibiting heightened values are concentrated in Wonosalam
Subdistrict and the Southwest of Karangtengah Subdistrict. The morphology of
the bedrock within the study area tends to mirror the sedimentary layer's
thickness, reflecting relatively minimal surface height discrepancies at the
measurement points. Vulnerability to fissures resulting from excessive
groundwater extraction and seismic activity is concentrated on the north side
of Wonosalam District, the eastern sector of Demak District, and a small
section of Mijen District.
Keywords: Bedrock; Demak; fissures;
HVSR; microtremor; SPAC
Abstrak
Keregenan Demak yang terletak di
Jawa Tengah, Indonesia menyerlah dalam kalangan keregenan kerana pembangunan
pesat dan pertumbuhan wilayahnya yang membawa kepada pergantungan yang ketara
kepada air bawah tanah. Hal ini disebabkan oleh curahan hujan yang rendah dan
ciri geologi Keregenan Demak yang kebanyakannya terletak di dataran aluvium.
Untuk menangani kebimbangan ini, kajian ini bertujuan untuk menentukan kawasan
yang terdedah kepada keretakan tanah akibat daripada pengekstrakan air bawah
tanah yang berlebihan ditambah dengan aktiviti seismik. Penyelidikan itu
merangkumi 191 lokasi, termasuk 177 titik pengukuran mikrotremor stesen tunggal
dan 14 titik pengukuran mikrotremor tatasusunan. Pengukuran mikrotremor stesen
tunggal menggunakan kaedah Nisbah Spektrum Mendatar ke Menegak (HVSR), manakala
pengukuran tatasusunan menggunakan kaedah Autokorelasi Spatial (SPAC). Nilai
terikan ricih tanah menjangkau daripada 32.12 × 10-6 - 96.39 × 10-6.
Terutamanya, kawasan yang menunjukkan nilai tinggi tertumpu di Mukim Wonosalam
dan Barat Daya Mukim Karangtengah. Morfologi batuan dasar dalam kawasan kajian
cenderung untuk mencerminkan ketebalan lapisan sedimen, mencerminkan
percanggahan ketinggian permukaan yang agak minimum pada titik pengukuran.
Kerentanan kepada rekahan akibat daripada pengambilan air bawah tanah yang
berlebihan dan aktiviti seismik tertumpu di sebelah utara Daerah Wonosalam,
sektor timur Daerah Demak, dan sebahagian kecil Daerah Mijen.
Kata kunci: Batuan dasar; Demak;
HVSR; mikrotremor; rekahan; SPAC
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*Corresponding author; email: sismanto@ugm.ac.id
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