[Estimation of The Threat of Earthquake and Tsunami in Pidie Jaya Regency to Support National Security]. Bencana merupakan suatu ancaman non-militer dan nyata yang dihadapi dunia saat ini. Bencana mengancam keselamatan masyarakat yang pada akhirnya mengancam keamanan nasional suatu negara, termasuk Indonesia. Ancaman nyata yang saat ini dirasakan oleh masyarakat Indonesia adalah peningkatan kejadian bencana yang dirasakan hampir diseluruh wilayah Indonesia. Kabupaten Pidie Jaya yang merupakan bagian dari Provinsi Aceh juga memiliki ancaman multi bencana seperti gempabumi dan tsunami. Gempabumi yang terjadi di Pidie Jaya disebabkan oleh aktivitas Sesar Pidie dengan arah bidang patahan mendatar atau strike-slipe. Selain itu, terdapat Zona Megathrust Sumatera dan Sesar Besar Sumatera yang berada di Provinsi Aceh dengan aktivitas kegempaan yang dapat berpotensi tsunami. Dalam penelitian ini dilakukan suatu simulasi tsunami dengan output wilayah-wilayah yang berpotensi tsunami berserta run up atau ketinggian air tsunami akibat gempa di Zona Megathrust Aceh-Andaman. Penelitian ini menggunakan metode kualitatif (deskriptif analisis) sebagai desain penelitian. Penelitian dilakukan pada 18 Maret 2021 di ruang operasional InaTEWS (Indonesia Tsunami Warning System) Badan Meteorologi, Klimatologi, dan Geofisika (BMKG) menggunakan bantuan software TOAST (Tsunami Observation and Simulation Terminal).  Dari seluruh rangkaian pengolahan dan analisis data didapatkan hasil bahwa Zona Megathrust Aceh-Andaman berpotensi tsunami dengan run up (ketinggian air tsunami) yang paling tertinggi sebesar 10,5 meter di Meulaboh, Kab. Aceh Barat, 5,5 meter di Kota Sabang, dan 3 meter di Kabupaten Pidie Jaya dengan waktu tiba tsunami (golden time) sekitar 24 menit 55 detik. Kajian atas estimasi gempabumi dan tsunami ini dapat dijadikan sebagai sumber informasi dalam penentuan kebijakan BPBD Pidie Jaya dalam upaya mitigasi bencana untuk melindungi masyarakat Pidie Jaya dan mewujudkan keamanan nasional.

The cause of the swarm earthquakes occurring from September until November 2020 remains unclear. The swarm activities are close to the Tripa active fault but also close to Mt. Bendahara. This study offers the analysis of seismic P and S wave arrivals and relocate the hypocenter of the swarm. We analysed the hypocentre distribution that recorded by Badan Meteorologi Klimatologi dan Geofisika (BMKG) from July 2020 to January 2021. The arrival of P and S-wave from 103 events was used obtain 1-D seismic velocity model. The earthquake used for this study consisted of at least 6 phases of seismic arrivals with the azimuthal gap angle less than 180 degree. The different velocities occurred in the upper and lower of the crustal thickness of 0 – 40 km. The Vp values are about 5.0 – 7.0 km/s and 3.0 – 5.0 km/s for Vs value while the Vp/Vs ratio are ranging from 1.5 to 1.7. Our preliminary results indicate that the potential swarm activities near to Mt. Bandahara maybe triggered by an unknown fault activity as no volcanic activity was reported.

The condition of stress and recent seismicity in the seismic-prone area can be statistically analyzed with the Gutenberg-Richter relation. We apply this relation to the hypocenter distribution for the period 1970-2020 with M ≥ 4 and depth ≤ 200 km  in the northern part of Sumatra. Spatially, The results obtained, a-values and b-values, figure a lateral heterogeneity and stress accumulation with dense structures at the interface zone in the subduction system and the northern segmentation of the Seulimeum fault. In time, both zones illustrate a slow time-to-failure cycle and seismic gap with high stress accumulation in the specific clusters with high seismic parameter values. The results of the spatial temporal analysis illustrate that each major earthquake event is usually preceded by a low statistical parameter value.

After Palu Earthquake that occurred on September 2018, another moderate earthquake on April 2019 with 6.8 Moment Magnitude (Mw) at a depth of 17 km occured in Banggai Islands. The earthquake occurred on the shallow depth that generated by an active fault with horizontal mechanism on the Banggai’s tectonic system. To explain the tectonic system in the Banggai Islands, we present a well-calculated 1-D velocity model by solving the coupled hypocentre-velocity inversion for 385 local earthquakes that recorded by BMKG regional network. The earthquakes was selected based on azimuthal gap, minimum number of stations and root mean square of travel‐time residuals. Technically, the fit solution are simultaneously inverted for total 81 inital models and will give an unique final model. The model is constructed by using Velest program that analyse the fit velocity model from body wave traveltimes (P and S wave), together with station corrections. The final 1-D velocity model will be very useful to conduct another high precision relative relocation and make a focal mechanism inversion.

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