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ANALISIS PROBABILISTIK BAHAYA SEISMIK DI DENPASAR DAN SEKITARNYA BERDASARKAN PENDEKATAN PSHA Hielmy, Rayhan Irfan; Rajagukguk, Martha Lisauli
JOURNAL ONLINE OF PHYSICS Vol. 11 No. 1 (2025): JOP (Journal Online of Physics) Vol 11 No 1
Publisher : Prodi Fisika FST UNJA

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.22437/jop.v11i1.47825

Indonesia berada di wilayah dengan aktivitas seismik tinggi akibat pertemuan tiga lempeng tektonik utama, menjadikannya rawan gempa, termasuk Kota Denpasar yang berdekatan dengan Subduksi Sunda, Flores Back Arc Thrust, dan Sesar Lombok. Penelitian ini menganalisis bahaya seismik di Denpasar menggunakan metode Probabilistic Seismic Hazard Analysis (PSHA) dengan perangkat OpenQuake Engine dari Global Earthquake Model. Perhitungan menghasilkan nilai percepatan tanah maksimum (PGA) dan percepatan spektral (SA) pada periode 0,2 s dan 1 s, dengan periode ulang 50 tahun untuk Probability of Exceedance 2%, 5%, dan 10%. Hasil menunjukkan PGA untuk Probability of Exceedance 2%, 5%, dan 10% berturut-turut berkisar 0,474–0,483 gal, 0,344–0,349 gal, dan 0,265–0,269 gal. SA 0,2 s masing-masing 1,094–1,123 gal, 0,769–0,782 gal, dan 0,575–0,583 gal, sedangkan SA 1 s berkisar 0,278–0,285 gal, 0,197–0,200 gal, dan 0,146–0,148 gal. Nilai tertinggi terletak di Denpasar Selatan karena lokasinya yang dekat dengan zona subduksi, sedangkan terendah di Denpasar Timur, Utara, dan Barat. Hasil ini dapat menjadi acuan perencanaan tata ruang, desain bangunan tahan gempa, dan strategi mitigasi bencana.

Performance Evaluation of Automated and Manual Seismic Phase Picking for Rapid Earthquake Parameter Determination in the Indonesian BMKG Network Hielmy, Rayhan Irfan; Pranata, Bayu; Wijayanto; Daryono
Jurnal Meteorologi dan Geofisika Vol. 26 No. 2 (2025)
Publisher : Pusat Penelitian dan Pengembangan BMKG

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.31172/jmg.v26i2.1189

Indonesia is situated at the intersection of three major tectonic plates, resulting in high seismic activity and significant earthquake vulnerability.1 Rapidly determining initial earthquake parameters—including origin time, epicenter location, depth, and magnitude—is critical for effective early warning systems. This study evaluates the reliability of automated versus fast manual picking (<3 minutes, S-wave-based) by comparing their performance against final validated results. Utilizing data from the BMKG SeisComP system for the period of May 18, 2024, to May 17, 2025, the study analyzed 2,790 seismic events across Indonesia, including low-seismicity regions such as Kalimantan. Performance was assessed across six key parameters (depth, origin time, RMS, azimuth gap, magnitude, and epicenter) using a numerical scoring system (0–100) based on deviation from validated data. The results indicate that while automated picking processed a significantly higher volume of events (1,857 events; 66.6%) compared to manual picking (327 events; 11.7%) within the target timeframe, manual picking achieved a superior 'good' quality rating (score 75–100) at 96.9%, compared to 88.5% for automated methods. Nevertheless, automated picking remains the preferred method for rapid dissemination (<3 minutes) due to its operational speed. Furthermore, the study establishes regional thresholds for the minimum seismic phases required for reliable automated picking, ranging from 8 to 16 phases depending on the region, with a national average of 15 phases.

Deterministic Seismic Hazard Analysis (DSHA) and Peak Ground Acceleration Mapping in West Java (2018–2024) Hielmy, Rayhan Irfan
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.12

West Java is a region with one of the most complex tectonic settings in Indonesia, influenced by the Sunda Megathrust subduction zone and an active inland fault system. This study aimed to estimate the Peak Ground Acceleration (PGA) values in the West Java region using a Deterministic Seismic Hazard Analysis (DSHA) approach with the Maximum of Maxima criteria. The data used is sourced from the BMKG earthquake catalog for the 2018–2024 period, which includes the most recent epicenter, hypocenter depth, and magnitude parameters. PGA calculations were performed using the Milne attenuation model for each grid point in the target area, followed by spatial interpolation using PyGMT software. The analysis results reveal a significant bimodal seismic hazard characteristic. An absolute maximum PGA value of 0.46 g was identified, triggered by a local shallow crustal earthquake with a magnitude of M 5.8 at a shallow depth within the inland region. Meanwhile, the southern coastal zone is dominated by the influence of a large Megathrust earthquake (M 8.7), although its shaking impact attenuates as it moves inland toward northern population centers. These findings confirm that while subduction sources possess greater energy potential, local active inland faults pose a more direct and destructive shaking threat to densely populated inland areas. The hazard map generated in this study is expected to serve as a crucial technical reference for resilient infrastructure planning and the updating of earthquake mitigation strategies in West Java Province.

Subsurface Structure of Palu-Koro Fault Zone Using TOPEX Satellite Gravity Data and Regional-Residual Anomaly Separation Hielmy, Rayhan Irfan
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.11

The Palu-Koro Fault in Central Sulawesi represents a highly active tectonic feature with significant seismic hazard potential, yet detailed subsurface mapping remains challenging due to the difficult terrain in the region. This study analyzed the subsurface structure of the fault zone by utilizing TOPEX satellite gravity data to overcome accessibility issues. The primary objective was to delineate the fault geometry and characterize subsurface lithological boundaries through density contrasts. The data processing stage initially determined the average surface rock density using the Parasnis method, which yielded a precise value of 2.45 grams per cubic centimeter. This density was subsequently applied to generate the Simple Bouguer Anomaly map. To distinguish between deep-seated regional trends and shallow local structures, the study employed two filtering approaches: the second-order polynomial method and the Moving Average method. The results demonstrated that both filtering techniques yielded consistent residual anomaly patterns. The main trace of the Palu-Koro Fault was clearly identified as a continuous low-anomaly zone, interpreted as a fracture system filled with low-density sedimentary deposits. Conversely, significant high-amplitude positive anomalies were detected adjacent to the fault trace, suggesting the existence of shallow high-density bodies such as igneous intrusions or uplifted basement blocks. This research concluded that the integration of satellite gravity data with regional-residual anomaly separation successfully mapped the structural complexity of the area, providing critical baseline data for updating seismic hazard models and enhancing disaster mitigation strategies in Sulawesi.

Subsurface Structural Identification and Seismicity Correlation in West Java Using EMAG2 Geomagnetic Data Hielmy, Rayhan Irfan
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.09

West Java represents one of Indonesia's most seismically active regions, characterized by complex interactions between the Indo-Australian subduction zone and active onshore fault systems. This study investigated subsurface geological configurations and their relationship to earthquake occurrences using Earth Magnetic Anomaly Grid (EMAG2) data with a 2-arc-minute resolution. The research methodology utilized Reduction to the Pole (RTP) to eliminate dipolar effects caused by the equatorial location, followed by spectral analysis to separate regional and residual anomalies. To precisely map structural lineaments, First Horizontal Derivative (FHD) and Second Vertical Derivative (SVD) techniques were applied to the residual data. The processed maps revealed distinct anomaly contrasts, with high magnetic intensities (up to 300 nT) associated with volcanic intrusions and low anomalies (approximately −100 nT) indicating sedimentary basins or hydrothermally altered zones. A critical analysis of the December 15, 2017, Tasikmalaya earthquake (Mw 6.5) identified a strong correlation between the epicenter and a significant low magnetic anomaly zone of −150 nT. This specific signature was interpreted as a fracture zone characterized by rock demagnetization resulting from tectonic stress accumulation. These findings confirm that integrating EMAG2 data with derivative filtering is a robust approach for delineating active tectonic structures, thereby contributing essential data for regional seismic hazard mitigation.

Estimation of Jayapura 2023 Aftershock Decay Time Using Python-Based Secant Algorithm Hielmy, Rayhan Irfan; Sandy Tri Gustono
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.07

Accurately determining the termination time of aftershocks is crucial for disaster mitigation and establishing safe periods for community recovery. This study aimed to estimate the decay time of the January 2023 Jayapura aftershock sequence (M 5.4) using a numerical computational approach. The Mogi II decay model was selected due to its high compatibility with local seismicity. To resolve its complex non-linear exponential equations without analytical derivatives, the Secant Method was implemented using Python. The algorithm was initialized with starting guess values of x0=0 and x1=1, and an error tolerance of 0.0001. To validate algorithmic robustness and efficiency, a sensitivity test was conducted, and the method was benchmarked against the Bisection method. Results demonstrated that the Secant algorithm achieved superior computational efficiency, converging in exactly 10 iterations (~0.000115 seconds) compared to Bisection's 18 iterations, while remaining highly stable under arbitrary extreme initial guesses. The numerical solution predicted the decay termination at day 12.765, subsequently rounded to 13 days following the mainshock. This finding showed exact agreement with manual observational data, successfully extrapolating the decay trajectory beyond the 10-day BMKG recording window. The study concluded that the Python-based Secant algorithm is effective, rapid, robust, and precise in solving the Mogi II equation, demonstrating significant potential as an automated analytical tool to enhance disaster mitigation decision-making.

Revealing the Subsurface Geometry of the 2023 Sumedang Earthquake Sequence Using Double-Difference Relocation and Cross-Section Analysis Hielmy, Rayhan Irfan
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.06

The Sumedang Regency experienced a significant tectonic earthquake sequence beginning on December 31, 2023, which raised concerns regarding active fault structures in the region. Identifying the precise causative fault geometry was essential for seismic hazard mitigation but remained challenging due to the complex local geology and potential location errors in preliminary data. This study investigated the source mechanism and subsurface geometry of the aftershocks recorded between December 31, 2023, and January 8, 2024. The Double-Difference (HypoDD) method was applied to relocate earthquake hypocenters by minimizing travel-time residuals, utilizing a 1-D velocity model with a variable Vp/Vs ratio. Subsequently, vertical cross-section analysis was conducted to interpret the dip patterns and fault orientation perpendicular to the seismicity trend. The results revealed a significant transformation in the spatial distribution pattern; while initial data exhibited a linear north-south trend, the relocated hypocenters formed a distinct curved cluster extending from the south toward the west. The seismic activity was concentrated at shallow depths ranging from 3 km to 17 km. Cross-section interpretations suggested two potential fault geometries: a near-vertical alignment indicating a strike-slip mechanism potentially associated with the Subang Segment of the Baribis Fault, or an inclined, slab-like structure indicative of a thrust fault system. These findings provided critical constraints for future moment tensor inversion studies.

Estimasi Waktu Berakhirnya Gempa Susulan Menggunakan Model Peluruhan Statistik: Studi Kasus Gempa Bumi Bawean 22 Maret 2024 Rajagukguk, Martha; Hielmy, Rayhan Irfan
Journal of Computation Physics and Earth Science (JoCPES) Vol 5 No 2 (2025): Journal of Computation Physics and Earth Science
Publisher : Yayasan Kita Menulis - JoCPES

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.63581/JoCPES.v5i2.10

An earthquake struck Bawean Island on March 22, 2024 with a magnitude of 6.5 followed by a series of aftershocks. In this study, the decay of aftershocks in the Bawean region was analyzed from March 22 to March 31, 2024 using the Omori Method, Mogi I Method, Mogi II Method, and Utsu Method. The purpose of this research is to determine the duration of aftershock decay in Bawean and to identify the most suitable statistical method for predicting aftershock decay time in the region. The analyzed data were obtained from the BMKG earthquake repository. By applying the four methods, results were obtained in the form of aftershock decay duration and correlation coefficients for each method. The analysis shows that the most appropriate statistical methods for calculating aftershock decay time in Bawean are the Mogi I Method and the Utsu Method both producing a correlation coefficient of approximately -0.806 and an aftershock decay duration of 24 days consistent with the BMKG earthquake catalog data.

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