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[HDS06-09] Tsunami hazards around Banda Sea and Molucca Sea, Indonesia
Keywords:Tsunami Hazards, Indonesia, Tsunami Numerical Simulation
Banda Sea and Molucca Sea regions in Indonesia is defined as one of the most complicated tectonic regions in the world. The Banda Sea is underlain the striking subduction zones, with a concave westward arc bending 180° in a tight 300 km radius of curvature. Large earthquakes and tsunamis that devastated the Banda Islands in the historical past should be attributed to a megathrust along this Banda outer arc. However, since the Banda Arc is a zone of arc–continent collision, it no longer features an oceanic trench and, therefore, no megathrust. Recently, Cummins et al (2020) suggested that those historical large earthquakes with a normal fault type were occurred along the Banda detachment where exists in the west of the Banda outer arc as an extensional setting. On the other hand, the Molucca Sea is underlain the striking double subduction zone, where the Molucca slab is subducting to the west and the east. The most of large earthquakes in this area occurred along a large splay fault system on top of the double subduction zone. The depths of those earthquakes are less than 50 km.
The most populated areas in both Banda Sea and Molucca Sea are Ambon Inland and Ternate and Tidore Islands, respectively. In this study, tsunami hazards in Ambon City and the populated areas in Ternate and Tidore Islands are estimated using the possible largest earthquakes in both Banda Sea and Molucca Sea regions by using tsunami numerical simulations.
In Banda Sea, we first computed tsunami waveform at Ambon Island by solving the linear longwave equations using the various normal faulting earthquake of Mw 8.5 along the Banda detachment, the thrust faulting earthquake of Mw 8.5 along the Banda outer arc subduction zone, and the high angle thrust faulting earthquake of Mw8.2 off Ambon Island. We found that one in the Banda detachment (Mw8.5) and one off Ambon Island (Mw8.2) generated large tsunamis to cause damages at the Ambon Island. Then, we numerically computed tsunami inundation in the Ambon city by solving the non-linear shallow water equations with the moving boundary conditions using those two large earthquakes. The numbers of damaged buildings were estimated from computed flaw depth and the fragility curve previously obtained for the 2004 Sumatra earthquakes. The number of damaged buildings from the high angle fault off Ambon Island was estimated to be about 900 which is much larger than that estimated from the normal faulting in Banda detachment. This result clearly shows that the significant tsunami damages are expected at the Ambon city by the future large earthquake.
In Molucca Sea, we estimated a magnitude of the largest expected earthquake using the seismic activity during 50 years because previous historical studies are not available in this area. The earthquake catalog of USGS (M > 4) is used to determine a b-value in this area. The b-value was estimated to be 0.909, and the largest magnitude of the earthquake during 50 years was estimated to be 8.5. Tsunami waveforms at Ternate and Tidore Islands were computed by solving linear long-wave equations using high angle fault models along the splay fault on top of the double subduction. The splay fault near Ternate and Tidore Islands generated significantly large tsunami. Then, the tsunami inundation was computed using this fault model. The number of damaged buildings calculated from computed flow depth and the fragility curve was about 1300. In addition to this fault model, we also computed tsunami inundation using a characterized fault model including one large slip area at the center. Form this fault model, the number of damaged buildings became about 2000. The significant tsunami damages are also expected at the populated areas in Ternate and Tidore Islands.
Finally, we discuss the tsunami early warning system for Ambon city and tsunami evacuation procedure for populated areas in Ternate and Tidore Islands
The most populated areas in both Banda Sea and Molucca Sea are Ambon Inland and Ternate and Tidore Islands, respectively. In this study, tsunami hazards in Ambon City and the populated areas in Ternate and Tidore Islands are estimated using the possible largest earthquakes in both Banda Sea and Molucca Sea regions by using tsunami numerical simulations.
In Banda Sea, we first computed tsunami waveform at Ambon Island by solving the linear longwave equations using the various normal faulting earthquake of Mw 8.5 along the Banda detachment, the thrust faulting earthquake of Mw 8.5 along the Banda outer arc subduction zone, and the high angle thrust faulting earthquake of Mw8.2 off Ambon Island. We found that one in the Banda detachment (Mw8.5) and one off Ambon Island (Mw8.2) generated large tsunamis to cause damages at the Ambon Island. Then, we numerically computed tsunami inundation in the Ambon city by solving the non-linear shallow water equations with the moving boundary conditions using those two large earthquakes. The numbers of damaged buildings were estimated from computed flaw depth and the fragility curve previously obtained for the 2004 Sumatra earthquakes. The number of damaged buildings from the high angle fault off Ambon Island was estimated to be about 900 which is much larger than that estimated from the normal faulting in Banda detachment. This result clearly shows that the significant tsunami damages are expected at the Ambon city by the future large earthquake.
In Molucca Sea, we estimated a magnitude of the largest expected earthquake using the seismic activity during 50 years because previous historical studies are not available in this area. The earthquake catalog of USGS (M > 4) is used to determine a b-value in this area. The b-value was estimated to be 0.909, and the largest magnitude of the earthquake during 50 years was estimated to be 8.5. Tsunami waveforms at Ternate and Tidore Islands were computed by solving linear long-wave equations using high angle fault models along the splay fault on top of the double subduction. The splay fault near Ternate and Tidore Islands generated significantly large tsunami. Then, the tsunami inundation was computed using this fault model. The number of damaged buildings calculated from computed flow depth and the fragility curve was about 1300. In addition to this fault model, we also computed tsunami inundation using a characterized fault model including one large slip area at the center. Form this fault model, the number of damaged buildings became about 2000. The significant tsunami damages are also expected at the populated areas in Ternate and Tidore Islands.
Finally, we discuss the tsunami early warning system for Ambon city and tsunami evacuation procedure for populated areas in Ternate and Tidore Islands