ANALISIS SEISMISITAS MALUKU DAN BARAT PAPUA BERDASARKAN DISTRIBUSI FREKUENSI-MAGNITUDO GEMPA

ANALYSIS OF SEISMICITY IN MALUKU AND WEST PAPUA BASED ON FREQUENCY-MAGNITUDE

DISTRIBUTION

Upaya pengurangan risiko bencana seismik di wilayah Maluku dan barat Papua dapat dilakukan melalui studi seismisita sdan stres batuan bawah permukaan dengan analisis parameter 𝑎-value dan 𝑏-value. Nilai 𝑎 merefleksikan level seismisitas wilayah penelitian dan nilai 𝑏 menunjukkan level stres tektonik struktur batuan bawah permukaan. Parameter tersebut merupakan hasil hitung kurva distribusi frekuensi-magnitudo (FMD) gempa. Perhitungan parameter seismik 𝑎-value dan 𝑏-value maupun 𝑀𝑐 dilakukan dengan membagi study area menjadi tiga zona seismik. Penelitian ini menerapkan hukum Gutenberg-Richter dengan menggunakan pendekatan maximum likelihood terhadap kejadian gempa dari katalog ISC-GEM tahun 1970-2023 untuk kejadian gempa 3,0 ≤ 𝑀𝑤 ≤ 8,0 serta kedalaman sumber dangkal (shallow sources) dan relatif dalam (intermediate sources). Tujuan penelitian adalah menganalisis seismisitas Maluku dan barat Papua berdasarkan kurva FMD serta peran variasi spasial 𝑏-value sebagai prekursor gempa tektonik. Berdasarkan distribusi frekuensi-magnitudo (FMD) gempa dengan menggunakan bantuan ZMAP7.0, hasil ukur nilai parameter 𝑏 ≈ 0,6−0,8 menunjukkan struktur batuan bawah permukaan Maluku dan barat Papua mengalami stres tektonik relatif tinggi. Melalui hasil ukur nilai 𝑎 yang ditentukan berdasarkan ketidakpastian nilai 𝑏 dan hasil ukur nilai 𝑏 ditentukan oleh ketidakpastian 𝑀𝑐, maka dapat diketahui bahwa seismisitas wilayah utara Maluku dipengaruhi oleh sumber relatif dalam sedangkan wilayah selatan Maluku dan barat Papua dipengaruhi oleh sumber dangkal. Seismisitas sumber relatif dalam di barat Papua (Zona 3) adalah yang terendah dibandingkan dua zona lain (Zona 1 dan Zona 2) pada kedalaman yang sama atau lebih dangkal. Analisis variasi spasial 𝑏-value menunjukkan gempa kuat dengan kedalaman tertentu (shallow atau intermediate) ditemukan di area dengan 𝑏-value rendah ditandai oleh kontur warna biru gelap atau biru terang. Distribusi frekuensi kejadian gempa tektonik wilayah Maluku dan barat Papua menunjukkan dominasi gempa dangkal (< 70 km) sebagai akibat dari struktur area bawah permukaanpermukaan (tapi dekat permukaan) yang cenderung labil.

Kata kunci: seismisitas, Maluku, barat Papua, 𝑎-value, 𝑏-value

Through analyzing 𝑎-value and 𝑏-value characteristics, seismicity and stress studies of subsurface rocks can be used to lower the risk of seismic disasters in the Maluku region and western Papua. The value of 𝑎 reflects the level of seismicity of the study area and the value of 𝑏 indicates the level of tectonic stress of subsurface rock structures. This parameter is the result of calculating the frequency-magnitude distribution curve (FMD) of the earthquake. The calculation of 𝑎-value and 𝑏-value seismic parameters as well as 𝑀𝐶 is carried out by dividing the study area into three seismic zones. This study applies the Gutenberg-Richter law by using the maximum likelihood approach to earthquake events from the ISC-GEM catalog for 1970-2023 for earthquake events of 3,0 ≤ 𝑀𝑤 ≤ 8,0 as well as shallow sources and intermediate sources. The purpose of the study was to analyze the seismicity of Maluku and western Papua based on the FMD curve and the role of spatial variation of 𝑏-value as a precursor to tectonic earthquakes. Based on the frequency distribution (FMD) of the earthquake using the help of ZMAP7.0, the measurement of parameter value 𝑏 ≈ 0.6−0.8 shows that the subsurface rock structure of Maluku and western Papua is experiencing relatively high tectonic stress. Through the measurement results of the value of 𝑎 which is determined based on the uncertainty of the value of b and the measurement of the value of 𝑏 is determined by the uncertainty of 𝑀𝐶, it can be seen that the seismicity of the northern region of Maluku is influenced by deep relative sources while the southern regions of Maluku and western Papua are influenced by shallow sources. The seismicity of the deep relative source in western Papua (Zone 3) is the lowest compared to the other two zones (Zone 1 and Zone 2) at the same depth or shallower. Analysis of 𝑏-value spatial variation shows strong earthquakes of particular depth (shallow or intermediate) found in areas with low 𝑏-values characterized by dark blue or light blue contours. The frequency distribution of tectonic earthquakes in the Maluku region and western Papua shows the dominance of shallow earthquakes (< 70 km) as a result of the structure of the subsurface area (but near the surface) which tends to be unstable.

Keywords: seismicity, Maluku, west Papua, 𝑎-value, 𝑏-value