STUDY ON TSUNAMIGENIC EARTHQUAKE CRITERIA FOR THE INDONESIAN TSUNAMI EARLY WARNING SYSTEM Nanang T. Puspito 1 1 Geophysics Research Group, Faculty of Mining and Petroleum Engineering, Institute of Technology Bandung (ITB), Jalan Ganeca 10, Bandung 40132, Indonesia E-mail: puspito@geoph.itb.ac.id ABSTRACT: The Indonesian Tsunami Early Warning System (Indonesian TEWS) consists of four main components, i.e. (a) earthquake monitoring system, (b) oceanographic monitoring system, (c) database of tsunami simulation, and (d) information dissemination system. The system is designed to issue a tsunami warning within minutes after the earthquake occurrence. Decision of a tsunami warning issuance is based on the examination of the determined earthquake parameters, i.e. epicenter, focal depth, and magnitude. Therefore, it is very important to explore the characteristics of tsunamigenic earthquakes in the Indonesian region. Tsunamigenic earthquakes in the Indonesian region occurred along subduction zones of Sunda Banda Caroline Philippine, Molucca Sea collision zone, Makassar Strait zone, and the Sunda back-arc thrusting zone. In term of their earthquake magnitude, most of tsunamis were generated by earthquakes that have moderate to great magnitude. The earthquake magnitude Ms varies from.8 to 9.0 and earthquake magnitude Mw varies from.8 to 9.3. About 94% of tsunamigenic earthquakes have earthquake magnitude Mw > 6., 79% have Mw > 7.0; 80% have Ms > 6., and % have Ms > 7.0. About 86% of tsunamigenic earthquakes are classified as shallow earthquakes that have focal depth < 60 km and about 7% of tsunamigenic earthquakes have focal mechanism of thrust fault type. 1. INTRODUCTION The Indonesian Tsunami Early Warning System (Indonesian TEWS) is expected to have capability to issue a tsunami warning in very short time after the earthquake occurrence. The decision of a tsunami warning issuance is mainly based on the examination of the determined earthquake parameters (RISTEK, 200). The determined earthquake parameters, i.e. epicenter, focal depth and earthquake magnitude, are examined and compared with the adopted criteria of tsunamigenic earthquake in the Indonesian TEWS. If the determined earthquake parameters meet with the adopted criteria of tsunamigenic earthquake, a tsunami warning should be issued. Therefore, the determination of earthquake parameters and the selection of tsunamigenic earthquake criteria are very important in the issuance of a tsunami warning. Tsunamigenic earthquake, i.e. the earthquake that has potential to generate tsunami, usually has special characteristics. Tsunamigenic earthquake usually is characterized by (a) the epicenter is located in the sea, (b) the focal depth is classified as shallow earthquake, (c) the earthquake magnitude (Ms) greater than 6.0, and (d) the earthquake focal mechanism are dip-slip type of thrust or normal fault (Iida, 1983). However, the characteristics of tsunamigenic earthquake sometime slightly different from region to region. For example, Puspito and Gunawan (200) have shown differences in the characteristics of tsunamigenic earthquake in the Sumatra region and in the Pacific region. Therefore, the criteria of tsunamigenic earthquake that will be adopted in the Indonesian TEWS should be selected properly. This paper attempts to study the proper tsunamigenic earthquake criteria for the Indonesian TEWS by examining the historical tsunami data. The proposed criteria of tsunamigenic earthquake will be based on the examination of the characteristics of tsunamigenic earthquake that occurred in the Indonesian region. Selected historical tsunami data from year 1600 to 2007 will be used to explore the characteristics of tsunamigenic earthquake in the Indonesian region. Back to Table of Contents 446
2. THE INDONESIAN TEWS The 26 December 2004 Aceh tsunami that killed more than 200,000 people in the countries around the Indian Ocean has been considered as the most catastrophic tsunami in history. The tsunami disaster has triggered countries around the Indian Ocean region to establish a tsunami early warning system for the Indian Ocean. Following the tsunami disaster several international meetings were held to deal with the need for the establishment of a regional warning system for the Indian Ocean. All of countries as well as international organizations that participated in the meetings have committed to collaborate in the establishment of a regional tsunami warning system for the Indian Ocean. It was also agreed that such a system should comprise a network of national arrangements, which are connected to a regional coordination center within a common platform, and linked to future global early warning arrangements. As a country that most suffered from the 2004 Aceh tsunami, the Government of Indonesia has committed to establish a tsunami early warning system for the Indonesian region. A grand scenario of the establishment of the Indonesian TEWS has been prepared by the Ministry of Research and Technology (RISTEK), the Government of Indonesia in 200 by coordinating several related national institutions (RISTEK, 200). According to the grand scenario, in the future the Indonesian TEWS will be strengthening to function as a regional tsunami warning system for the Indian Ocean region. The Indonesian TEWS is operated by the Meteorological and Geophysical Agency of Indonesia (BMG). The development of the Indonesian TEWS is expected to be completed by the end of 2009. Design of the Indonesian TEWS basically is adopted from the Pacific Tsunami Warning System (ITSU, 1999) and the Japan Meteorological Agency (JMA, 2001). The Indonesian TEWS consist of four main components, i.e. (a) earthquake monitoring system, (b) oceanographic monitoring system, (c) database of tsunami simulation, and (d) information dissemination system. The Indonesian TEWS is also strengthening by two supported components, i.e. (a) crustal deformation monitoring system, and (b) community preparedness. Figure 1 shows main components of the Indonesian TEWS. Figure 1 Components of the Indonesian TEWS (Puspito et al., 2006). It was planned that the earthquake monitoring system will consist of 160 broad-band seismographs that will be distributed in most part of the Indonesian region, especially in the seismic active areas. The database of tsunami simulation will consists of thousands of hypothetical tsunami modeling. The source of hypothetical tsunamis, their tsunami heights and its corresponding travel times at Back to Table of Contents 447
several points along the coastal area will be stored in the database. The oceanographic monitoring system will consists of 120 tide gauges and 1 tsunami buoys. Most of tsunamis in the Indonesian region are classified as local tsunami type. The distance between tsunami sources and the nearest coastal areas are usually less than 200 kilometers. The travel times of tsunami wave from the source to the nearest coastal areas are usually less than 30 minutes. In some places the travel times even less than 10 minutes such as in the case of the 1996 Biak tsunami and the 2004 Aceh tsunami. Therefore, the tsunami warning in the Indonesian region should be issued in very short time just after the earthquake occurrence. The Indonesian TEWS is designed to issue a tsunami warning within minutes after the earthquake occurrence. According to the design of the Indonesian TEWS, the earthquake monitoring system should have capability to determine the earthquake parameters (epicenter, depth, and magnitude) of any earthquake occurring in any particular location in the Indonesian region within 3 minutes after the earthquake occurrence. The determined earthquake parameters are then compared with the adopted criteria of tsunamigenic earthquake in the Indonesian region. According to the grand scenario of the Indonesian TEWS, the adopted criteria of tsunamigenic earthquake in the Indonesian TEWS are described as (a) the epicenter is located in the sea, (b) the earthquake magnitude (Ms) 6., and (c) the focal depth is shallow less than 60 km. If the determined earthquake parameters meet with the adopted criteria of the tsunamigenic earthquake, a tsunami warning issuance should be prepared. The tsunami modeling of the predicted tsunami will be retrieved from the database of tsunami simulation. The database will give estimation of the predicted tsunami height and their arrival time at several locations along the coastal area. Within the first minutes after the earthquake occurrence, the tsunami warning should be issued and disseminated to the authorities, media, and communities by short message service, telephone, facsimile or email. Confirmation of the tsunami occurrence will be based on the observed data from the oceanographic monitoring system, i.e. from the tide-gauge and tsunami buoy data. If the observed data from tide-gauge and tsunami buoy confirmed the tsunami occurrence the tsunami warning dissemination will be continued. On the other hand, if the observed data does not confirm the tsunami occurrence, the cancellation of the tsunami warning should be issued. 3. TSUNAMIGENIC EARTHQUAKES Tsunami could be generated by three main sources, i.e. earthquake, volcanic eruption, and landslide that occurred in the sea. In the Indonesian region, the three sources could generate large tsunami such as the 2004 Aceh tsunami that generated by earthquake, the 1883 Krakatau tsunami that caused by volcanic eruption, and the 1979 Lomblen tsunami that caused by landslide. According to the historical data most of tsunamis that occurred in the Indonesian region were generated by earthquake. The historical tsunami data compilation done by Latief et al. (2000) pointed out that about 90 percent of tsunamis in the Indonesian region were generated by earthquake. The occurrence of the 26 December 2004 Aceh tsunami and recently the 17 July 2006 Java tsunami has confirmed that the Indonesian region is one of the most tsunami prone regions in the world. Based on the historical data compilation taken from several sources (Latief et al., 2000; Gusiakov, 2004) a total of 184 tsunamis caused by earthquake occurred in the Indonesian region for a period from year 1600 to 2007. Figure 2 shows some of tsunamis that occurred in the Indonesian region since 1600. Figure 2 shows that most of tsunami sources in the Indonesian region are located along the seismic active zones. The earthquakes that generated tsunamis in the Indonesian region are located along the Sunda subduction zone, the Sunda back-arc thrusting zone, the Banda subduction zone, the Molucca Sea collision zone, the Caroline of Pacific subduction zone, and the Philippine subduction zone. Back to Table of Contents 448
Figure 2 Tsunami sources and the tectonic zones. The tsunamigenic earthquake magnitude Ms varies from.8 to 9.0 and Mw varies from.8 to 9.3. Figure 3 shows the tsunamigenic earthquake magnitude distribution, where the black blocks indicate Mw and the striped blocks indicate Ms. The figure shows that 94 % of tsunamis were generated by earthquakes with magnitude Mw > 6. and 79% have magnitude Mw > 7.0. About 80% of tsunamis were generated by earthquakes with magnitude Ms > 6. and % have magnitude Ms > 7.0. The figure indicates that most of the tsunamis in the Indonesian region were generated by earthquakes of moderate to great magnitude. 40 3 34 Percentage (%) 30 2 20 1 10 8 3 3 12 1 2 26 27 1 13 12 2 0.6-6.0 6.1-6. 6.6-7.0 7.1-7. 7.6-8.0 8.1-8. 8.6 Earthquake Magnitude (Mw and Ms) Figure 3 The tsunamigenic earthquake magnitude distribution. The depth of tsunamigenic earthquakes in the Indonesian region varies from 10 to 130 km. Figure 4 shows the tsunamigenic earthquake depth distribution. The figure shows that about 86% of tsunamis in the Indonesian region were generated by earthquakes that have focal depth less than 60 km, where about 24% have focal depth = 0 ~ 20 km, about 46% have focal depth = 21 ~ 40 km, about 16% have focal depth = 41 ~ 60 km, about 9% have focal depth = 61 ~ 80 km, and about % have focal depth = 81 ~ 100 km. This suggests that most of the tsunamis in the Indonesian region were generated by shallow earthquakes as same as suggested by Iida (1983). Back to Table of Contents 449
0 4 46 40 Percentage (%) 3 30 2 20 1 10 24 16 9 0 0-20 21-40 41-60 61-80 81-100 Earthquake Depth (km) Figure 4 The tsunamigenic earthquake depth distribution. Iida (1983) suggested that tsunami could be generated by shallow earthquakes with focal mechanism of thrust or normal fault type. Figure shows the CMT (centroid moment tensor) solutions of several earthquakes that generated tsunamis in the Indonesian region. A total of 20 CMT solutions of earthquakes that generated tsunamis were compiled. The data were taken from Harvard University CMT catalogue (Harvard University, 2007) for a period from year 1976 to 2007. The figure shows that about 7% of the earthquakes have focal mechanism of thrust fault, 20% shows strike-slip fault, and % shows normal fault. Figure CMT solutions of earthquakes that generated tsunami. 4. DISCUSSION AND CONCLUSION The selection of tsunamigenic earthquake criteria for the Indonesian TEWS is very important for the issuance of tsunami warning. The criteria should be determined properly based on the characteristics of tsunamigenic earthquakes that occurred in the Indonesian region. The characteristics of tsunamigenic earthquakes described above could be used as a basic consideration in the selection of tsunamigenic earthquake criteria for the Indonesian TEWS. Characteristics of tsunamigenic earthquakes in the Indonesian region could be summarized in the following: Back to Table of Contents 40
International International Conference Conference Earthquake on Earthquake Engineering Engineering and Disaster and Mitigation, Disaster Mitigation Jakarta, April 200814-1, 2008 1. The tsunamigenic earthquakes located along the seismic active zones, i.e. the Sunda subduction zone, the Sunda back-arc thrusting zone, the Banda subduction zone, the Molucca Sea collision zone, the Caroline of Pacific subduction zone, and the Philippine subduction zone. 2. In term of earthquake magnitude the tsunamigenic earthquakes are classified as moderate to great earthquakes. About 94% have magnitude Mw > 6., 79% have Mw > 7.0, 80% have Ms > 6., and % have Ms > 7.0. 3. Most of the tsunamigenic earthquakes are classified as shallow earthquakes where about 80% of them have focal depth less than 60 km. 4. About 80% of the tsunamigenic earthquakes have focal mechanism of thrust fault type.. REFERENCES Gusiakov, V.K. (2004). Historical Tsunami Database for the Pacific, 47 B.C 2004 A.D., Tsunami Laboratory, ICMMG SD RAS, Novosibirsk, Russia. (CD-Rom). Harvard University (2007). Harvard University CMT catalogue, http://www.globalcmt.org/ Iida, K. (1983). Some remarks on the occurrence of tsunamigenic earthquakes around the Pacific, In Tsunamis: Their Science and Engineering; edited by K. Iida and T. Iwasaki, Terra Publisher. Tokyo. ITSU (1999). Master plan for tsunami warning system in the Pacific, IOC - UNESCO. JMA (2001). Handbook for Tsunami Forecast in the Japan Sea, Japan Meteorological Agency. Latief, H., Puspito, N.T. and Imamura, F. (2000). Tsunami catalog and zones in Indonesia, Journal of Natural Disaster Science, Vol. 22, 1, 2-43 Puspito, N.T. and Gunawan, I. (200). Tsunami sources in the Sumatra region, Indonesia and simulation of the 26 December 2004 Aceh tsunami, ISET Journal of Earthquake Technology, Vol. 42, 4, 111-12 Puspito, N.T., Yudistira, T., Gunawan, I., Septiana, T., Robiana, R. and Gunawan, A. (2006). Database of tsunami simulation for the Indonesia TEWS, Jurnal Geofisika, 2006/2, 2 10 (in Bahasa Indonesia with an English abstract). RISTEK (200). Grand design of the Indonesia tsunami early warning system, the Ministry of Research and Technology (RISTEK), the Republic of Indonesia. Back to Table of Contents 41