ANALISIS POTENSI BAHAYA BENCANA SEISMIK DAN KERENTANAN WILAYAH NTB DAN NTT

ANALYSIS OF SEISMIC HAZARD POTENTIAL AND VULNERABILITY OF THE NTB AND NTT REGIONS

Upaya pengurangan risiko bencana seismik bisa dilakukan sedini mungkin melalui analisis potensi bahaya dan kerentanan wilayah terhadap bencana seismik dengan menghitung parameter a dan b. Kedua parameter mendiskripsikan level seismisitas dan akumulasi stres mekanik yang disimpan oleh batuan geologi bawah permukaan. Studi kasus dalam penelitian ini adalah Provinsi NTB dan NTT dengan data gempa diperoleh dari katalog USGS antara 1970-2021 dan magnitudo 3,1 ≤ M_w ≤ 8,3 serta kedalaman sumber mencapai 640 km. Perhitungan kedua parameter diperoleh dari penerapan hukum Gutenberg-Richter dan pendekatan maximum likelihood dengan memanfaatkan distribui frekuensi-magnitudo (FMD) untuk 4 zona seismik yang merepresentasikan wilayah kedua provinsi dan mengakomodasi sumber potensial pemicu gempa, yaitu Flores Backarc Thrust di utara dan zona megathrust di selatan kedua provinsi. Hasil perhitungan dalam bentuk kurva FMD untuk 4 zona seismik menunjukkan bahwa nilai a berkisar antara 8-9 dengan a (annual) rata-rata 6,6 refleksi dari seismisitas dan seismicity rate yang tinggi sedangkan nilai b bervariasi antara 1,01-1,21 indikator dari mayoritas kejadian gempa di NTB dan NTT adalah events of moderate magnitudes (4,5 ≤ M_w ≤ 7,0). Meski gempa besar jarang terjadi, namun jalur sesar Flores Backarc Thrust berada di Laut Flores yang memanjang dari ujung barat sampai timur wilayah perairan utara NTB dan NTT sehingga potensi pembangkitan gelombang tsunami akibatt akktivitas Flores Backarc Thrust tetap ada seperti saat gempa M_w 7,8 memicu tsunami Flores 1992. Demikian pula, gempa kuat M_w 8,3 berpusat di perairan selatan NTB memicu tsunami Sumba 1977. Hhistogram gempa terhadap kedalaman menunjukkan bahwa mayoritas kejadian gempa merupakan shallow events dengan rata-rata kedalaman sumber 35-40 km. Dengan seismisitas tinggi, mayoritas sumber dangkal dan beberapa kali diguncang gempa berpusat bahwa laut baik di wilayah perairan utara maupun selatan, maka kerentanan wilayah NTB dan NTT terhadap bencana seismik perlu diwaspadai.

Kata Kunci: seismisitas, kerentanan wilayah, NTB dan NTT, a-value, b-value

Efforts to reduce the risk of seismic disasters can be carried out as early as possible by analyzing the hazard potential and vulnerability of the region to seismic disasters by calculating parameters a and b. Both parameters describe the level of seismicity and accumulated mechanical stress deposited by subsurface geological rocks. The case studies in this study are the Provinces of NTB and NTT with earthquake data obtained from the USGS catalog between 1970-2021 and a magnitude of 3.1 ≤ M_w ≤ 8.3 and a source depth of up to 640 km. The calculation of the two parameters is obtained from the application of the Gutenberg-Richter law and the maximum likelihood approach by utilizing the magnitude-frequency distribution (FMD) for 4 seismic zones that represent the two provinces and accommodate potential sources of earthquake triggers, namely the Flores Backarc Thrust in the north and the megathrust zone in the south. both provinces. The results of calculations in the form of FMD curves for the 4 seismic zones show that the a value ranges from 8-9 with a (annual) average 6.6 reflection of seismicity and a high seismicity rate while the value of b varies between 1.01-1.21 indicators of the majority of earthquake occurrences in NTB and NTT are events of moderate magnitudes (4.5 ≤ M_w ≤ 7.0). Although large earthquakes rarely occur, the Flores Backarc Thrust fault line is in the Flores Sea which extends from the west to the east of the northern waters area of ​​NTB and NTT so that the potential for generating tsunami waves due to the activities of the Flores Backarc Thrust remains as it was when the M_w 7.8 earthquake triggered a tsunami Flores 1992. Likewise, a strong M_w 8.3 earthquake centered in the southern waters of NTB triggered the 1977 Sumba tsunami. The earthquake histogram against depth shows that the majority of earthquake events are shallow events with an average source depth of 35-40 km. With high seismicity, the majority of sources are shallow and several times shaken by earthquakes centered on the sea in both the northern and southern waters, the vulnerability of the NTB and NTT regions to seismic disasters needs to be watched out for.

Keywords: seismicity, regional vulnerability, NTB and NTT, a-value, b-value