The Role of Geologist to Mitigate Geologic Hazard through Geospatial Information

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The Role of Geologist to Mitigate Geologic Hazard through Geospatial Information

Heryadi Rachmat
Agung Adhibroto **)
Mines and Energy Office Of West Nusa Tenggara Province
Jl. Majapahit No. 40 Mataram (83010) Lombok, NTB
Tlp. 0370-621356, Fax. 0370-625766
E-mail: hery_rachmat@yahoo.com

Abstract
Nusa Tenggara Archipelago are two interaction of two megaplates (Hindia - Australia and Eurasia Plates ) which collision each other. Which this boundary is very labile part, indicated by active volcanoes and very high earthquake rate which potential to overcome geologic hazards like volcanic eruption, earthquake, and lanslide followed by flood. There are 21 A type active volcanoes in this area. Many of them had erupted frequently and caused earthquakes with many victims and losses of material. Ones of the volcano is Mt.Tambora, located in Sumbawa Island, which erupted in 1815 and caused 92.000 people died. Big earthquake (followed by tsunami), which caused more than 2500 people died, was Flores earthquake in 1992. Active volcanoes and earthquake in Bali - Nusa Tenggara Rim are potentially hazardous, sometime followed by other geologic hazards named tsunami, landslide, and flood, caused many victims and losses of material. IAGI of Nusa Tenggara has been doing of mitigation, investigation, and giving recommendation when it happened. Geological hazards like volcanic eruptions, earthquake and tsunami are untreatable hazards even with latest technology. When those events will happen is unpredictable as yet. That's why the role of geologist to mitigate geological hazards through geospatial information is needed.

Sari
Kepulauan Nusa Tenggara adalah tempat dua Lempeng berukuran benua (lempeng Hindia - australia dan Eurasia) berinteraksi dan berbenturan satu dengan yang lain. Batas lempeng ini adalah bagian sangat labil dan ditandai oleh gunungapi yang aktif serta kegempaan yang tinggi, sehingga berpotensi untuk terjadinya bencana alam geologi berupa letusan gunungapi, gempabumi, gerakan tanah disertai banjir.Dalam deretan pulau Bali - Nusa Tenggara terdapat 21 gunungapi aktif tipe A dan sering terjadi letusan maupun gempabumi menimbulkan korban jiwa dan harta benda. Salah satu gunungapi yang menimbulkan korban jiwa cukup besar (92.000 orang) adalah letusan G. Tambora di Pulau Sumbawa tahun 1815. Sedangkan gempabumi yang telah menimbulkan kerusakan maupun korban jiwa lebih dari 2500 orang terjadi pada saat gempa Flores pada tahun 1992. Gunungapi aktif dan gempabumi di deretan Pulau Bali - Nusa Tenggara merupakan potensi bencana yang sewaktu-waktu dapat diikuti dengan bencana berasfek geologi lainnya antara lain tsunami, tanah longsor dan banjir, yang dapat menimbulkan korban jiwa serta kerugian harta benda. Akibat sering terjadinya bencana alam tersebut, ahli geologi yang tergabung dalam IAGI Pengda Nusa Tenggara telah melakukan berbagai upaya dengan melakukan mitigasi, pengawasan dan penyelidikan saat terjadi bencana.Bencana alam berupa letusan gunungapi, gempabumi dan tsunami merupakan bencana yang tidak dapat dicegah dengann ilmu pengetahuan serta teknologi yang ada sekarang dan sampai saat ini belum mampu memprediksi secara tepat kapan bencana tersebut dapat terjadi sehingga diperlukan upaya peningkatan kemampuan dalam peramalan dan mitigasi melalui informasi geospasial.

1. PREFACE
Natural disaster is event or series of events which caused by nature and caused many victims and human suffering, loss of properties, damage of environment, infrastructures and public pasilities, emerging interperence toward live systems and community livelihood. Whereas geologic hazards mean natural disaster caused by geologic processes such as earthquakes, volcanic eruptions, lindslides and floods.

For last 40 years, geologic have been strike down in Bali - Nusa Tenggara area, causes more than 10.000 people died and loss of properties.

Indonesia located at Pacific Rim which three mega plates (Hindia- Australia, Pacific and Eurasia plate) are get interacted, collision each other. The movement of these plates could be divergence and shear. The edges of these plates are very labile, indicated by occurrance of active volcanoes and high earthquakes rates (Fig.1). At this part of plate accumulate of giant energy, which can explode anytime as earthquakes and/or volcano eruptions, and other geophysics symptoms like anomaly of gravitation, magnetism and so on.

Figure 1. Model of the meeting of two big plates (plate tectonic)
producting earthquake and volcano (Adjat Sudradjat, 1988).

Dealing with disaster means "all efforts and activities include mitigation, supervision, rescuing, rehabilitation, and reconstruction before, while and after disasters occur" . Bali Nusa Tenggara area comprises of 4 provinces, named Bali, West Nusa Tenggara, East Nusa Tenggara and West Timor, and have width of 5.632 km2, 20.153 km2, 47.389 km2 and 14.615 km2 each. Population in 1997 are 2.896.000, 3.646.000, 3.577.000 and 840.000 each. Geologic hazards sometime followed by secondary disasters that less dangerous. Earthquakes, for example could followed by landslide in rainy season followed by flash floods.

II. GEOLOGIC HAZARD IN BALI NUSA TENGGARA AREA
Bali-Nusa Tenggara are part of Java Sumatera Volcanic Rim and located the divergence of two mega plates (Hindia-Australia and Eurasia plates), indicated by 21 active volcanoes and high rates of earthquakes (Fig.2).

Each of these plates has different characteristic, for example Eurasia is 'Magmatic' while Hindia-Australia is 'basaltic'. These differences cause the explosiveness of Indonesia volcanoes, difficult to detect and control.

From overview of some geologic hazards in Bali-Nusa Tenggara area, both from direct overview and history records, earthquake usually followed by landslide and floods,

Figure 2. Map of Geological structure and volvanoes in Indonesia (Katili, 1994).

meanwhile volcanic eruption followed by floods, sometimes also followed by landslides. Some of geologic hazards in Bali-Nusa Tenggara area are discussed here.

2.1. Earthquakes, Tsunami, Landslide and Floods
The earthquakes in Bali-Nusa Tenggara area has cause many victims and losses of properties, especially if followed by tsunami, landslides and floods. Some of these are earthquakes :

Earthquakes followed by Tsunami in Lunyuk Sumbawa on August 19th, 1997, epicentrum on 11,10 south latitude, 119,00 east longitude magnitude 7 at Richter Scale, 33 km depth caused 107 people died, 54 missing, 440 houses destroyed, 467 ships/boats missing/ destroyed, 10 school and teacher's houses collapse. (Fig.3).
Earthquakes followed by Tsunami, Landslide and Floods in Flores, 1992. Epicentrum on 8,4800 south longitude 6,5 latitude, 121,8960 east longitude, magnitude 6,5 at Richter Scale, 28 km depth, caused tsunami with height 20 m and 300 m landward wipe off the north coast of Flores and surrounding small islands, 2500 people died, 500 people wounded
Earthquakes followed by Landslide in East Flores, on November 26th, 1987. Epicentrum on 8,40 south latitude,1240 east longitude, magnitude 5,8 at Richter Scale, 28 km depth, 237 houses collapse, 44 people died, 65 seriously injured, 42 people slightly wounded, magnitude MMI III recorded in Dili.
Earthquakes followed by Tsunami, in Alor, July 4th , 1991. Epicentrum on 7,3 south latitude, 126,36 east longitude, magnitude 6,7 at Richter Scale, 33 km depth, 22 people died, 181 people wounded, 1177 buildings damaged , 10 ships missing, 1080 people become homeless.
Earthquakes followed by Tsunami in Dili, May 14th , 1995. Epicentrum on 8,470 south latitude, 124,970 east longitude, magnitude 6,9 at Richter Scale, 47 km depth, 9 people missing, 33 ships and 5 5 houses washed away, 168 fisheries-net damaged, phisical damaged of Dili harbour and land slumps of Comoro river mouth.
Earthquakes followed by Tsunami and Landslide in Waiteba, July 18th , 1979. Epicentrum predicted on Sawu sea on July 17 th , 1979, 128 people died, 386 people missing, more than

Figure 3. Third floors building in Maumere collapse after ground shaking and liquefaction on Flores earthquake, 1992.

tens people wounded, 3000 m length and 300 m wide of land slump from 500 m height, 200m coastline wide gone, about 20 million m3 land rocks slumping into the sea, probably caused by submarine slumping.

Figure 4. Symptom of landslide on the steeply hill slope after Flores earthquake, 1992

2.2. Volcanic Eruptions and Floods
A-type active volcanoes in Bali - Nusatenggara area consist of 21 volcanoes that included on Sunda Group, spread along series of island that start from Bali, Lombok, Sumbawa, sangeang, Flores, Adonara, Lomblen, and pantai Island. Some of these volcanoes have special characteristic, which are :

Mt. Batur is the volcano that lacated in largest crater in Indonesia, even in the world, with 10 km in diameter.
Mt. Rinjani (3726m) is the second highest volcano in Indonesia after Mt.kerinci (3800m) in Sumatera. Mt.Rinjani has a beatiful crater lake named Segara Anak.
Mt.Tambora is the most claim casualties volcano in Indonesia, even in the world (92.000 people died) after erupted on 1815 and formed a crater with 7 km in diameter and 1 km depth.
Mt. Anak Ranakah (175 m over the former floor) is the youngest emerging volcano in Indonesia (emerging on 1989), while Mt.Hobal was emerged from sea level on July 10th, 1974.
Mt.Kelimutu is one of eight miracles in the world, which have 3 crater lake with 3 different colours and very acid pH (between 0,5 and 3,5)
Mt. Hobal is an active submarine volcano, which turns up to sea level for the first time on July 10th, 1974. At that time, Mt. Hobal have two peaks, 30 m and 70 m height each. These peaks disappeared by wave erosion on December 31st, 1974. Furthermore, it's erupted at a stretches on March 8th, 1976, 1981, 1982, and September 16th, 1993. Followed here are some of volcanic eruptions which claimed casulaties or less of properties in Bali-Nusa Tenggara area:
Mt. Agung eruption on 1963, which caused more than 10.000 people died by glowing avalanche and lahars. The temperature of glowing avalanche that reach over 2000C caused dehydration and sudden death.
Sangeangapi eruptions on 1953, 1985 and 1997 has produced lava flow, glowing avalanche, pyroclastic falls and lahar floods, and force all of villagers in Island Sangeangapi to avacuate to sangeangapi Land. (Fig.5).
Mt. Barujari eruptions on 1944,1966 and June 1994 produced lava flow, pyroclastic sediment and lahar floods. Ash sediment after the last eruption have caused lahar floods, which claim casualties of 37 people died, damaged of building/houses, rice fields, and silting up of irrigation tunnels. Apart from that, tens of international flight schedules from and to Denpasar were delayed because the height of ash from Mt. Barujari eruption is high enough (Fig.6)

Figure 5. Glowing avalance and lahar flows filled up Isu River after
eruption of Mt. Sangeangapi (1997).

Figure 6. The Eruption of Mt. Barujari (Mt. Rinjani Complex), 1994, which
produce lava flows and pyroclastic fall about 9 millions m3 and
lahar flows caused 37 people died.

2.3. Landslides
In the part 25 yaers, in Bali-Nusatenggara has occurs several landslide which claimed casualties and loss of properties usually connecting with earthquakes. Some of the bigger landslides are :

Landslides of larantuka, occurson 1908,1938.1978, 99 people died because rock falls from hill slopes, on April 1979 secondary rock debris occurs and swept Larantuka city away trough river flows and dry gullies.
Landslides after Flores Earthquakes on 1992, particularly on regency of Ende, Sikka and East Flores. Some of variation of landslides are rock falls, subsidence of seashore in Harboure Maumere and Tanjung bunga in east flores, road cracks and damage of bridges along main road of Ende and Larantuka lane, rock falls and land slumps on steeply slope of maountain side.

III. STEP BY STEP ACTIONS TO DEALING WITH NATURAL DISASTERS
To anticipate those geologic hazards, geologist in Bali-Nusa Tenggara who joined in Indonesian association of Geologist Nusa tenggara have been doing some preventive actions such as mitigations, alertness, and sosializations before it's happen (Fig.7). Meanwhile investigation after disasters sometimes followed by sosialization. The geologist and other sciences recommended a step by step action to dealing with natural disasters. The axplanation of each steps are :

Mitigations, all efforts and activities to eliminate parts or whole of disaster that happen, among them are preparing a volcanoes, earthquakes and landslides dangerous zone map. And constuction of volcanoes, earthquakes and landslides observation posts.
Alertness, all efforts and activitiesto minimaze and reduce the impact of disasters, among them are observations, socialitazions, and training, build, first warning sign systems, and disseminating information.
Recommendation, after geologic investigation on the disaster locations, including of recommendation of the trigger of disasters, predicting the possibilities of secondary disasters, and the actions that have to be done.
Socialization and Training, all activities that can be done on locations right after disasters or periodically on the areas which have potential to affected by geologic hazards. The aim of these activities is to improve the ability/skill of local people and local government agencies on dealing with disasters (Fig.8)

Figure 7. Geologist from IAGI Pengda Nusra, BMG's and earthquake's
experts from Jakarta were discussed to prepare
recommendation for disasters prevention

Figure 8. Geologist from IAGI Pengda Nusra was given socialization to the
villagers on flood with Satlak PB, witnessed by Regent, sub
district head, villagers headman, and others.

Some of geologic hazards, such as earthquake and volcanic eruption, usually followed by other hazards like tsunami, landslide and floods. The recommendation to handling these hazards must comprehensive and fitting the local potential of geologic hazards.

Here are some general efforts that have to do to dealing with geologic hazards :

Dealing With Earthquakes Disaster
Preparing the earthquake dangerous zone map, arrange rules and regulations for building, monotoring the dangerous earthquake zone, ivestigation on disaster locations, and prepared the villagers by socialization and training.
Dealing With Volcanic Eruptions
Preparing the volcanic dengerous zone map, volcanoes observation/monitoring visually, seismic, chemical, petrochemical, magnetism, land deformation, resistance and statisties, investigation on eruption location, socialization and training for villagers.
Dealing With Landslides Disaster
Preparing the landslides dangerous zone map, observation/ monitoring the landslide potential areas, investigation on disaster locations, socialization and trainings.
Dealing With Floods Disaster Preparing the floods dangerous zone map, observation/ monotoring flood locations, construction of embankments, investigation on flood locations, socializations and training.

IV. CONCLUSIONS

Bali-Nusa Tenggara area located on convergence of two mega plates, Hindia-Australia and Eurasia plates. This area is very labile and potential to overcome geologic hazards such as earthquakes, volcanic eruptions, landslides and floods. This labile area marked by 21 active volcanoes and high earthquake rates.
Geologic hazards are unavoidable natural symptoms and usally related with threat to human lives, loss of properties, and environment decay. To anticipate all of impacts, geologists who are joined in Indonesian Association of geologists of Nusa Tenggara have been done various efforts to deal such as mitigation, alertness, investigation, socialization and training. Investigations are done after events to understand the disaster causal factors and the possibilities of secondary disaster that could be occur. The products of there investigations will be recommended to related agencies.
Plate tectonic process is the causal factor of volcanic eruptions, earthquakes, and other geologic hazards. Understanding of these process support us on efforts to dealing with these disasters.