J. SE Asian Appl. Geol., Jul–Dec 2009, Vol. 1(2), pp. 32-36

ROAD TO EARTHQUAKE MITIGATION: LESSON LEARNT FROM THE EARTHQUAKE 2006

Subagyo Pramumijoyo∗1

1Department of Geological Engineering, Gadjah Mada University, Yogyakarta,

Abstract drilling until 60 m each, measuring seismic veloc- ity on bore hole, and magneto telluric measurement. At early in the morning of May 27, 2006, people We also have helped by Kyushu University in in- of Yogyakarta was stroke by earthquake and mostly stalling micro seismic net work. The research was heavily damaged building are in lowland or Yo- followed by either undergraduate and graduate stu- gyakarta depression where is occupied by the Young dents. Fortunately our research was financed by Merapi sediments. The magnitude of earthquake is AUN/Seed Net - JICA. Some of the results were pub- Mw = 6.2 and USGS rapid moment tensor shows lished in a book entitled The Yogyakarta Earthquake that this earthquake was due to strike-slip fault of May 27, 2006. Another outcome is the Maps movement. of Microzonation and Earthquake Hazard of Bantul Seismic history of Yogyakarta area shows that Yo- Area that dedicated to Bantul people. gyakarta was stroke by several earthquakes with dif- Based on aerial photograph observation and field ferent epicenter location. At least two earthquakes observation on Bantul , especially along the stroke the area, that is in 1876 and 1943. The dam- , and to Wonosari to the East, there was ages are similar to the damages of actual earthquake. no surface ruptures, so there is no fault on sur- Yogyakarta depression is mostly covered by Young face. Interpretation of aftershock data was showing Merapi sediments that consist of tuff, volcanic ash, the difference cluster. There is still open problem in breccias, agglomerate and lava with Quaternary in determining either epicenter or aftershock location. age. The thickness of this sediment is up to 100 m. The damage building was interpreted as due to its Our reactive work was to establish firstly zone of geologic setting, non engineered building, and close damage. For this purpose, we made aerial photo- to epicenter of earthquake. This heavily damaged graph along the most damaged area. In the same time building are located on the Young Merapi sediments one of our teams go to the field to measure the cracks, at Bantul Regency and lake deposits at Gantiwarno and the other teams to observe liquefaction, hydro and Bayat area where it can amplify the surface seis- geologic measurement, and observation on landslide mic wave. It implies that Peak Ground Acceleration induce by earthquake. Secondly, we must under- according to Indonesian National Standard should stand the soil properties and its thickness, because in be modified in Yogyakarta area. seismic history it was a similar damage on the same Keywords: Earthquake, seismic, epicenter, micro- area due to earthquakes however the earthquake epi- tremor, microzonation. centers were different. For this purpose we utilize the method of micro-tremors. We also made some 1 Introduction

∗Corresponding author: S. PRAMUMIJOYO, Depart- On Saturday morning of May 27, 2006, at ment of Geological Engineering, Faculty of Engineering, 5:53:59 AM Yogyakarta was stroke by earth- Gadjah Mada University, Jl. Grafika 2 Yogyakarta, 55281, Indonesia. E-mail: [email protected] quake According to United States Geological

32 ROAD TO EARTHQUAKE MITIGATION: LESSON LEARNT FROM THE YOGYAKARTA EARTHQUAKE

Survey (USGS) this earthquake epicenter lo- ing since the emergency perceptive phase of cated at 25 km south-south east of Yogyakarta Disaster Cycle (Sutaryo, 2007) with Mw: 6.2, but according to Indonesian Me- teorological and Geophysical Agency (BMG) 2 Geology of Yogyakarta area in brief the epicenter is located at 40 km south of Yo- gyakarta with 5.9 Richter scale. USGS rapid Yogyakarta is located in a depression which is moment tensor shows that this earthquake was on the west limited by Kulon Progo Mountains due to strike-slip fault movement. and to the east bounded by Southern Moun- This earthquake is not great earthquake how- tains. ever it has resulted 5,048 victims, 19,401 serious Geology of Kulon Progo Mountains consist injuries, 8,437 minor injuries, 487 paralyzed and of Nanggulan Formation that consists of sand- 430,374 houses were damaged. The total cost of stone with lignite intercalation, sandy marls lose is Rp 29,1 trillion with currency: US$ 1.- = and claystone with limonite concretion and in- Rp. 9000.- (Yuswantana et al., 2007). In fact it tercalation with marls and limestone, sandstone is not the first time Yogyakarta was stroke by and tuff, with Eocene in age. This forma- earthquake. According to Newcomb and Mc- tion was covered unconformable by Old An- Cann (1987) there were some great earthquakes desite Formation that consists of andesitic brec- stroke Yogyakarta, there are: in January 4, 1840; cias, tuff, lapilli tuff, agglomerate and andesitic June 10, 1867; and March 28, 1875 and resulted lava intercalation. The age of this formation from multi sources. The 1840 event had very is Oligocene-Early Miocene. This formation localized tsunami area and in Yogyakarta may was covered unconformable by Jonggrangan be felt earthquake with MMI > VIII, possibly Formation that consists of conglomerate, tuffa- an epicenter on the sea or inland epicenter near ceous marl, calcareous sandstone with lignite coast line so that landslides occurred on steep intercalation, bedded limestone and coral lime- coast and resulted a small tsunami. The 1867 stone, with Middle Miocene in age. This for- event was no tsunami and indicate inland epi- mation is overlain by Sentolo Formation that center. This event heavily damaged building consists of limestone and marly sandstone. The and in Yogyakarta felt earthquake with MMI > Sentolo Formation spread along the lower hills VIII. The 1875 event was similar with the 1867 surrounding , east of the Mountains event, but Yogyakarta only stroke by earth- with age of Middle Miocene - Pliocene. quake with MMI = V-VII. No tsunami was re- To the east, the Southern Mountain consist ported in this event. According to Visser (1922 of: Semilir Formation that consists of tuffa- vide Husein et al., 2007) the 1867 event resulted ceous breccias, pumiceous breccias, dasitic tuff, 5 victims and 372 heavily damaged building in- andesitic tuff and tuffaceous claystone with cluding the water castle that the ruin is still ex- age of Oligo-Miocene; Nglanggran Formation ist until now. In 1943, it was earthquake that that consist of andesitic breccias, agglomer- resulted in Bantul regency, 31 victims, 564 in- ate, lava and tuff of Early Miocene; Sambip- juries, and 2.682 damaged building (van Bem- itu Formation that consists of tuff, shale, silt- melen, 1949). stone, sandstone and conglomerate of Middle The increase number of victims and damage Miocene; Wonosari Formation that consists of building is showing ignorance of people to the coral limestone, calcarenite and tuffaceous cal- impact of the earthquakes, it is also means that carenite with Early Miocene - Pliocene in age; people ignore the geology of the area and it ig- Kepek Formation that consists of marl and bed- nore the environment where they live in. Based ded limestone of Pliocene in age. on this fact the Geological Engineering Depart- Yogyakarta depression is mostly covered by ment, Faculty of Engineering, Gadjah Mada Young Merapi sediments that consist of tuff, University would like to mitigate earthquake volcanic ash, breccias, agglomerate and lava in order to minimize the victim together with with Quaternary in age. The thickness of this other disciplines (Karnawati et al., 2007a), start- sediment is up to 100 m (Hendrayana, 1993).

c 2009 Department of Geological Engineering, Gadjah Mada University 33 PRAMUMIJOYO

River and coastal sediments consist of gravel, tion. Liquefaction can be found at Berbah and sand, silt, and clay with age of Holocene (Ra- (Eastern Bantul Regency) and was re- hardjo et al., 1995). ported also at Gantiwarno. Dry digging well is The Quaternary sediment is loose sediments also found, due to liquefaction. comparing to the “bed rock”, sediment up to The earthquake and its aftershock can make Pliocene age. In theory, when surface seismic a rock fall along the escarpment west and north wave pass trough a loose sediment its velocity of Southern Mountain. At Sengir, Prambanan, it should be decrease but its amplitude should be can be found subsidence along 150 m and 25 m increase so that the building on the loose sedi- wide (Karnawati, 2007). At Cermai cave, beside ment should be more shaking than on the base- toppling on its escarpment is also cracking in ment. side the cave (Yudistira , 2007). On the geological map of Yogyakarta quad- Based on BMG announcement on local news- rangle it is a fault system at the border east paper, aftershocks distribution clustered along of depression, that is striking NNE-SSW Kali the Opak River, but according to the result of Opak sub surface fault system. This fault was Kyushu University, Japan, the aftershocks was interpreted from its gravity pattern (Untung et spreading along NNE-SSW line, approximately al., 1973). On the seimotectonic map (Kerta- 10 km to the east of Opak River (Fukuoka et al., pati et al., 1992), this fault is an active fault in prep). but without its kinematics. The fault system On the Southern Mountain, at Ngalang River that was interpreted by Rahardjo et al. (1995), there is ground water level drops that made the was completed by Sudarno (1997). On the de- digging well become dried. This phenomenon pression of Yogyakarta, McDonald (1985, vide is different from those of liquefaction. We be- Hendrayana, 1993) by its bore hole and geo- lieve that there is also change of ground water electricity, interpreted some subsurface faults content, but unfortunately after earthquake the striking N-S and E-W. With additional bore- data is not finished yet. hole data on the site of damaged it was re- interpreted existing sub-surface faults of South 4 Whatwehavedone Yogyakarta depression (Karnawati et al., 2007b). It is not only N-S and E-W directions but also Firstly, we should know the damage that not all NW-SE and NE-SW directions. of building was damaged. In order to under- stand the distribution of housing damage and 3 Impacts of earthquake to look at the major crack or surface rupture we made aerial photograph along the most dam- Heavily damage area spreads along Pundong aged area in collaboration with Geodetic En- and Imogiri that striking NE-SW coincide with gineering Department, Faculty of Engineering, line of Opak river (Eastern Bantul Regency), University of Gadjah Mada. In the same time and at Gantiwarno and Bayat, that located one of our teams went to the field to measure along E-W line, north of Southern Mountains. the cracks, and the other teams either to the The damaged area is on scattered area in east field to observe the landslides induced by earth- Bantul Regency. quake or to observe liquefaction and its hydro- The cracks that have found are mostly on the geology. During this period we must explain asphalted streets with various directions and some phenomena of earthquake to panicking kinematics, however statistically showing the people. For this purpose we have filled an on NE-SW direction is dominant (Jayanto, 2007). air interactive program either on the radio or The lengthofthe cracks is up to8 m butif build- television and published an earthquake poster. ing on the tips of the cracks should be dam- Facing and explaining to panicking people was aged (Pramumijoyo and Sudarno, 2007). There really another problem for us and we consult is also crack on the ground either on the differ- to Faculty of Psychology, University of Gadjah ent elevations due to gravity or due to liquefac- Mada. And the mobile phones of our faculty

34 c 2009 Department of Geological Engineering, Gadjah Mada University ROAD TO EARTHQUAKE MITIGATION: LESSON LEARNT FROM THE YOGYAKARTA EARTHQUAKE staffs were opened to answer question from the surface only. Interpretation of aftershock data people. was showing the difference cluster between Secondly, we must understand the soil prop- Wagner et al. (in prep) that located at 15 km east erties and its thickness, because in seismic his- of Opak River, Fukuoka et al. (in prep.) that lo- tory it was a similar damage on the same area cated at 10 km east of Opak River and BMG in due to earthquakes however the earthquake Kedaulatan Rakyat (2006) located along Opak epicenters were different. There is scattered River. There is still open problem in determin- soil in Yogyakarta depression. To know the ing either epicenter or aftershock location. dimension of soil we need a tool to map the The damage building was spotted but clus- soil. We already know that earthquake surface tered at Imogiri – Pundong – Sewon Districts, wave will be amplified if its pass through loose Bantul Regency, and at Gantiwarno – Bayat Dis- soil. For this purpose we utilize the method tricts, Klaten Regency. It was interpreted as due of micro-tremors that offered by BPPTK, Yo- to its geologic setting, non engineered building, gyakarta. To check the thickness of subsurface and close to epicenter of earthquake. This heav- data of micro-tremor and to add soil data we ily damaged building are located on the Young also made some drilling until 60 m each. We Merapi sediments at Bantul Regency and lake have done in collaboration with Civil and Envi- deposits at Gantiwarno and Bayat area where ronmental Engineering, Faculty of Engineering, it can amplify the surface seismic wave, so that University of Gadjah Mada. We are also mea- the waves can damage non engineered build- suring seismic velocity on bore hole in collabo- ings. ration with ESDM and ITB. Beside these meth- It implies that Peak Ground Acceleration ods, we need also magneto telluric method to according to Indonesian National Standard recognize the present of fault in collaboration should be modified in Yogyakarta area. Mr. with Physical Department, UGM. We also have Tun Naing, doctoral student from Myanmar, helped by Kyushu University in installing mi- is studying the problem of soil properties in cro seismic net work. Fortunately our research relation with amplification of surface seismic was financed by AUN/Seed Net - JICA. The re- waves and the result should be modified the search was followed by either undergraduate Peek Ground Acceleration according to In- and graduate students. Two of AUN/Seed-Net donesian National Standard into greater scale. doctoral students are still working in the earth- Mr. Myo Tanth, another doctoral student from quake topics that are Mr. Myo Tanth and Mr. Myanmar, wills approaching the seismic haz- Tun Naing from Myanmar. ard of Yogyakarta area with Probabilistic Seis- Based on multidiscipline approach and due mic Hazard Assessment. to hard work of Robert Anderson from Califor- For the following years, the open problem of nia Seismic Safety Commission we can publish Yogyakarta earthquake attracts some scientists in 2008 a book entitle The Yogyakarta Earth- to cooperate with us, such as from University quake of May 27, 2006, and we also can pub- of East Anglia, UK, under direction Dr. Paul lished the Maps of Microzonation and Earth- W. Burton with financial support from British quake Hazard of Bantul Area that dedicated to Council, from San Diego State University, USA, Bantul people. under direction of Dr. Eric Frost with sup- port of California Seismic Safety Commission, 5 Follow up action and of course from Japan Supported Universi- ties that is: Kyushu University (coordinator), Based on aerial photograph observation and Kyoto University, and Hokaido University, and field observation on Bantul Regency, especially also from MI AUN/Seed-Net that supported by along the Opak river, and to Wonosari to the JICA. East, there was no surface ruptures, so there is no fault on surface The interpretation is that earthquake was not tearing the surface but sub

c 2009 Department of Geological Engineering, Gadjah Mada University 35 PRAMUMIJOYO

Acknowledgement Kertapati, E. K., Soehaemi, A. dan Djuhanda, A., 1992, Peta Seismotektonik Indonesia. Pusat The author would like to thanks to all partici- Penelitian dan Pengembangan Geologi, Ban- pants who were taking the data on the field. dung. Newcomb, K. R. and McCann, W. R., 1987, Seismic History and Seismotectonics of the Sunda Arc. References Journal of Geophysical Research, vol. 92, no. B1, pp 421-439. American Geophysical Union. Fukuoka, K., Ehara, S., Fujimitsu, Y., Udi, H., Pramumijoyo, S. and Sudarno, Ign., 2007, Surface Setyawan, A., Setyadji, L. D., Harijoko, A., Cracking due to Yogyakarta Earthquake 2006. In Pramumijoyo, S., Setiadi, Y., and Wahyudi, in The Yogyakarta Earthquake 2006, Ed. by Kar- prep. 2008, Interpretation of the 27 May 2006 Yo- nawati, D., Pramumijoyo, S., and Husein, S., gyakarta Earthquake Hypocenter and Subsurface published by the Geological Engineering Depart- StructureDeduced from the Aftershock and Grav- ment, Faculty of Engineering, University of Gad- ity data. In The Yogyakarta Earthquake of May jah Mada. 27, 2006. Ed. by Karnawati, D., Pramumijoyo, Rahardjo W., Sukandarrumidi dan D. Rosidi, H. S., Anderson, R., and Husein, S. Star Publishing M., 1995, Peta Geologi Lembar Yogyakarta, Jawa. Company, Inc., Belmont, USA. Pusat Penelitian dan Pengembangan Geologi, Hendrayana H., 1993, Hydrogeologie und Bandung. Grundwasssergewinnung im Yogyakarta – Sudarno, Ign., 1997, Kendali Tektonik Terhadap Becken, Indonesien. Doktors der Naturwis- Pembentukan Struktur pada Batuan Paleogen senschaften genehmigte Dissertation, Technis- dan Neogen di Pegunungan Selatan, Daerah Is- chen Hochschule Aaachen, 117 p. timewa Yogyakarta dan Sekitarnya. Thesis Mag- Husein, S., Pramumijoyo, S., Naing, T., Tanth, M., ister Program Studi Geologi, Institut Teknologi and Murjaya, J., 2007, A Short Note on the Seis- Bandung, 167 p. mic History of Yogyakarta Prior to the May 27, Sutaryo, 2007, Health Isssues Relating to National 2006 Earthquake. In The Yogyakarta Earthquake Disaster. In The Yogyakarta Earthquake 2006, Ed. 2006, Ed. by Karnawati, D., Pramumijoyo, S., and by Karnawati, D., Pramumijoyo, S., and Husein, Husein, S., published by the Geological Engineer- S., published by the Geological Engineering De- ing Department, Faculty of Engineering, Univer- partment, Faculty of Engineering, University of sity of Gadjah Mada. Gadjah Mada. Karnawati, D., 2007, Eartquake Induced Landslide Untung, M., Ujang, K. dan Ruswandi, E., 1973, at Sengir, Prambanan District, Yogyakarta. In The Penyelidikan gaya berat di daerah Yogyakarta- Yogyakarta Earthquake 2006, Ed. by Karnawati, Wonosari, Jawa Tengah. Publikasi Teknik Seri Ge- D., Pramumijoyo, S., and Husein, S., published by ofisika, no. 3, Direktorat Geologi Bandung the Geological Engineering Department, Faculty Van Bemmelen, R.W.,1949, The Geology of Indone- of Engineering, University of Gadjah Mada. sia vol. I A. Martinus Nijhoff, The Haque. Karnawati, D., Pramumimoyo, S., and Husein, S., Wagner, D., Rabbel, W., Luehr, B.-G., Wasserman, J., (ed.), 2007a, The Yogyakarta Earthquake 2006. Walter, T. R., Kopp, H., Koulakov, I., Wittwer, A., Published by the Geological Engineering Depart- Bohm, M., Asch, G., and MERAMEX Scientists, ment, Faculty of Engineering, University of Gad- in prep. 2008, Seismic Structure of Central . jah Mada. In The Yogyakarta Earthquake of May 27, 2006. Karnawati, D., Husein, S., Pramumijoyo, S., Rat- Ed. by Karnawati, D., Pramumijoyo, S., Ander- domopurbo, A., Watanabe, K., and Anderson, son, R., and Husein, S. Star Publishing Company, R., 2007b, Earthquake Microzonation and Haz- Inc., Belmont, USA. ard Maps on Bantul Area, Yogyakarta, Indone- Yudistira, A.S.A. , 2007, Analisa Rekahan dan sia. In The Yogyakarta Earthquake 2006, Ed. by Gelombang Permukaan Gempabumi Yogyakarta Karnawati, D., Pramumijoyo, S., and Husein, S., 27 Mei 2006. Skripsi di Jurusan Teknik Geologi published by the Geological Engineering Depart- FT UGM. ment, Faculty of Engineering, University of Gad- Yuswantana, B., Harwani, A., Prijambodo, Sulistyo, jah Mada. and Wahyudi, S., 2007, Saiyeg Saeka Kapti, Re- Jayanto, G.D., 2007, Genesa Rekahan pada Per- fleksi Satu Tahun Gempa Yogyakarta. Pemerintah mukaan Tanah Akibat Gempabumi 27 Mei di Provinsi DIY, Kepatihan, Danurejan, Yogyakarta. Daerah Bantul. Skripsi di Jurusan Teknik Geologi Http//bmg.go.id FT UGM. Http// USGS. Gov./ NIEC

36 c 2009 Department of Geological Engineering, Gadjah Mada University