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

 

Diserahkan: 20 Disember 2023/Diterima: 6 Jun 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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*Pengarang untuk surat-menyurat; email: sismanto@ugm.ac.id

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

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