Investigation of the seismic damage caused to the Gunung Sitoli(Tögi-Ndrawa) cave by the 2005 Great Nias earthquake

2005 Büyük Nias depreminin Gunung Sitoli (Tögi-Ndrawa) ma¤aras›ndaneden oldu¤u sars›nt› hasar›n›n incelenmesi

Ömer AYDANTokai University, Department of Marine Civil Engineering, Orido 3-20-1, Shizuoka, 424-8610, JAPAN

Gelifl (received) : 14 Aral›k (December) 2007Kabul (accepted) : 03 Mart (March) 2008

ABSTRACT

The quantification of the seismic past of regions during a non-instrumental period is important for seismic designand disaster mitigation. The utilization of damage to the speleothems of caves as one of the tools of paleo-seis-mology has recently been receiving particular attention. The author investigated the Nias Island cave in relation tothe 2005 Great Nias earthquake that accurred in Indonesia in July 2007. In the first part of this article, a brief out-line of the seismo-tectonic and strong motion characteristics of the 2005 earthquake is given. Then the traces ofthe damage to speleothems by the 2005 Great Nias earthquake, as well as earlier large events, found in the caveof Gunung Sitoli in Nias Island during the investigation are presented and their implications are discussed. (Thereis no doubt that the utilization of damage to speleothems of caves is an important tool for the quantification of theseismic past.) Furthermore, the cave is also regarded as an underground rock structure and its stability is evalu-ated using available empirical and analytical methods. In addition, the susceptibility of seismic damage to stalac-tites and stalagmites is analytically evaluated using the seismic coefficient technique proposed and the implica-tions are discussed.

Keywords: Nias Earthquake, seismic damage, stalactite, Tögi Ndrawa Cave.

ÖZ

Aletsel dönem öncesi bölgelerin depremselli¤inin niceliksel olarak de¤erlendirilmesi oldukça önemlidir. Son y›llar-da karstik ma¤aralardaki sark›t ve dikitlerde oluflan hasarlar, aletsel dönem öncesi depremselli¤inde¤erlendirilmesinde kullan›lmaktad›r. 2005 y›l›nda meydana gelen Büyük Nias depreminde hasar gören NiasAdas› karstik ma¤aradaki 2007 y›l›n›n Temmuz ay›nda incelenmifltir. Bu makalede önce 2005 Nias Adas›’ndadepreminin sismo-tektonik ve kuvvetli yer hareket özellikleri k›saca sunulmufltur. Daha sonra Nias adas›ndaGunung Sitoli veya Tögi Ndrawa olarak adland›r›lan karstik ma¤aradaki sark›t ve dikitlerde meydana gelmifl hasar-lar sunulmufl ve 2005 Nias adas› depremi ile daha önceki depremler aras›ndaki iliflkiler tart›fl›lm›flt›r. Buçal›flmadan elde edilen en önemli sonuçlardan biri, hiç kuflkusuz, karstik ma¤aralardaki hasarlar›n geçmifltekidepremlerin belirlenmesinde oldukça kullan›labilir olmas›d›r. Bunun yan› s›ra, yeralt› kaya yap›s› olarak, karstik

Yerbilimleri, 29 (1), 1-15Hacettepe Üniversitesi Yerbilimleri Uygulama ve Araflt›rma Merkezi DergisiJournal of the Earth Sciences Application and Research Centre of Hacettepe University

Ö. AydanE-posta: aydan@scc.u-tokai.ac.jp

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ma¤aran›n durayl›l›¤› görgül ve analitik yöntemlerle de¤erlendirilmifltir. Sark›t ve dikitlerin deprem s›ras›ndakihasar görme olas›l›¤›n› incelemek üzere sismik katsay› yaklafl›m›na dayanan bir yöntem önerilmifl ve yap›lande¤erlendirmeler tart›fl›lm›flt›r.

Anahtar Kelimeler: Nias depremi, sars›nt› hasar›, sark›t, Tögi Ndrawa ma¤aras›.

Figure 1. Illustration of damage to spleothem by earthquakes (modified from Gilli, 1999).fiekil 1. Depremlerin sark›t ve dikitlerde neden oldu¤u hasarlar›n gösterimi (Gilli (1999)’dan de¤ifltirilerek).

INTRODUCTION

An earthquake with a magnitude of 8.7 occurredon March 28, 2005 near Nias Island. This earth-quake caused extensive damage to buildings,transporation facilities such as roadways andbridges in Nias Island as well as many slopeand embankment failures. Furthermore, itinduced extensive ground liquefaction and later-al spreading in sandy ground along the coastalarea.

Karstic caves develop only in limestones alongfracture zones caused by faulting movements.The percolation of rain water causes dissolutionof limestone in the vicinity of fracture zones,resulting in huge caves. Earthquakes maycause damage to stalactites and stalagmites inkarstic caves (Figure 1). Ground shaking, per-manent fault movements or both may inducethe damage to stalactites and stalagmites. Thepossibility of damage to stalactites is muchhigher than that to stalagmites.

The induced ground shaking was estimated tobe greater than 0.3g in Nias Island. Therefore,the possibility of damage to stalactites in karsticcaves in Nias Island was expected to be quitehigh. The damage may be observed as the fallof stalactites from the roofs of the caves. TheGPS network operated by CalTech had record-ed 4 m displacements in Lahewa. Therefore,the permanent ground deformations could alsohave induced damage to the caves in NiasIsland.

Geological considerations estimated the possi-bility of karstic caves to be high in the vicinity ofGunung Sitoli. Furthermore, karstic caves mayalso be found near Lahewa, TelukDalam andSirombu in view of the geology of the island.The author found from an Internet explorationthat a cave exists near Gunnung Stoli, calledTögi Ndrawa (in Nias language this meansinside the cave). In this article this which will becalled the Gunung Sitoli cave.

This paper is concerned with the damage in theGunung Sitoli cave induced by the 2005 GreatNias earthquake and the observed damage andits characteristics are described. The investiga-tion was carried out on July 31, 2007 about 28months after the earthquake.

GEOGRAPHY AND GEOLOGY

Nias Island lies about 125 km west SumateraIsland in the Indian Ocean. It covers an area of4771 km2, which is mostly lowland area. Thehighest elevation is 886 m. It is the biggest in agroup of islands on this side of Sumatra, whichis a part of the province of Sumatera Utara. Niasis 140 km long and 50 km wide (Figure 2). Thepopulation in this area is about 639,675 people(including Malay, Batak, and Chinese. GunungSitoli is the capital city of Nias and it is the cen-ter of administration and business affairs of theregency.

The geological formations in Nias Island,according to Djamal et al. (1994), are named asalluvium, Gunung Sitoli formation, Gomo forma-tion and Lelematua formation and melange(Figure 3). The alluvium belongs to Holoceneand Quaternary, and it is encountered alongshores and rivers.

The Gunung Sitoli formation consists mainly oflimestone with intercalations of clay, weaklycemented sandstone and marl layers. Its geo-logical age is Plio-Pleistocene and it is slightlyfolded. It has deposited in a shallow marineenvironment and unconformably overlies Gomoformation and Lelematua formation. Its thick-ness is about 120 m and karstic caves arefound in this formation.

The Gomo and Lelematua formations includeold volcanic rocks and consolidated sedimenta-ry rocks. The thickness of the Gomo formationranges between 1250 and 2500 m, while thethickness of the Lelematua formation is 3000 min the eastern part and 2000 m in the middle ofNias Island. The Lelematua formation belongsto Miocene and unconformably overlies amelange complex. The melange complexstretches from northwest to southeast and wasformed during Oligocene to early Miocene. Itcontains igneous and metamorphic rocks suchas peridodite, serpentinite, basalt and schist.

Structural features such as faults, folds and lin-eaments generally trend northwest- southeast.

Anticlines and synclines are generally asym-metric and some of them plunge northeast orsoutheast. Thrust faults are generally parallel tofold axes and dip northeast with an inclination of30-40o and bound melange units with youngersedimentary deposits. Thrust faults and foldsare crossed by strike-slip faults and normalfaults. Lineaments found in Tertiary rocks trendnorthwest southeast. Tectonic activity and therelated thrusting process of the melange unitsstarted in Oligocene. In Pliocene andPleistocene, a tectonic phase caused faultingand uplifting of all units. This tectonic activity stillcontinues today.

CHARACTERISTICS OF THE 2005 NIASEARTHQUAKE AND STRONG MOTIONS

The USGS estimation of the magnitude (Mw) ofthe earthquake was 8.7 while HARVARD esti-mated that the moment magnitude (Mw) of theearthquake was 8.6 (Table 1). The epicentersdetermined by USGS (2005) and HARVARD(2005) differ from each other. While USGS(2005) estimated the hypocenter just beneathBanyak Island, HARVARD’s epicenter was fur-ther SW and near Nias Island. Since the dam-age was much heavier in Nias Island, it seemsthat the estimation by HARVARD may be muchcloser to the actual epicenter. The faultingmechanism of the earthquake was also esti-

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Figure 2. Location of Nias Island (OCHA, 2005).fiekil 2. Nias adas›n›n yeri (OCHA, 2005).

mated by the two institutes. The dominant fault-ing mechanism was inferred to be thrust-typeby HARVARD, while USGS inferred the domi-nant faulting mechanism to be sinistral strike-slip. However, the fault plane is very gentlyinclined and its inclination ranges between 4o-7o. Yagi (2005) of BRI (presently TsukubaUniversity) inferred the slip propagation andestimated the relative slip at the hypocenter tobe about 10 m (Table 2). Konca et al. (2006)recently re-analyzed seismic, geodetic andcoral uplift observations (Figure 4). Their

results are given together with those estimatedby Yagi (2005) and Yamanaka (2005). Theseresults indicate that the fault propagation pro-ceeded beneath Nias Island. Konca et al.(2006) reported that the horizontal offset andvertical uplift at Lahewa GPS station were 4.5m and 3 m, respectively.

The areas hit by tsunami were Singkil andSibolga in Sumatra Island, Simeulue Island,Banyak Islands and Nias Island. The height ofthe tsunami was 4 m at Singkil and SimeulueIslands, and more than 1 m at Sibolga. In Nias

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Figure 3. Geology of Nias Island (from Aydan et al., 2007 based on the map by Djamal et al., 1994).fiekil 3. Nias adas›n›n jeolojisi (Djamal vd. (1994)’n›n haritas›n› esas alan Aydan vd. (2007)’den).

Table 1. Main characteristics of the earthquake.Çizelge 1. Depremin bafll›ca özellikleri.

Institute M Mw LAT LON DEP NP1 NP2(N) (E) (km) strike/dip/rake strike/dip/rake

USGS 8.7 8.7 2.09 97.016 21.0 251/4/29 132/88/93HARVARD - 8.6 1.64 96.980 24.9 329/7/109 130/83/88

Table 2. Rupture and slip characteristics of the earthquake fault.Çizelge 2. Deprem fay›n›n k›r›lma ve at›m özellikleri.

Parameters Yagi Yamanaka Konca et al. Borges et al. (2005) (2005) (2006) (2005)

Strike/Dip/rake 329/14/115 320/12/104 325/10/110 330/10/106Moment tensor scale (Nm) 1.6 x1022 1.3 x 1022 1.0 x 1022 0.82 x 1022

Rupture duration time (s) 150 s 120 s 160 s 110Rise time (s) 10-20 7.8Rupture velocity (km/s) 2 km/s 3.3Depth (km) 28 27 30 28Rupture area (km2) 150 x 470 120 x 250 400 x 60 400 x 125Slip (m) 10 m 12 9 15

Island, the effects of the tsunami were observedby the JSCE team at Tuhemberua in the northand Sorake beach in the south, where woodenhouses and two stories RC buildings collapsedor were heavily damaged (Aydan et al., 2005,2007). According to the residents of these loca-tions, the height of the tsunami was 4 to 5 m and6 to 7 m, respectively. It was reported that thetsunami was up to 2 m high, and settlement ofground was observed in Banyak Islands. Therewere also reports of tsunami in other countriesaround the Indian Ocean, which were less thanseveral tens of centimetres. The tsunamiinduced by this earthquake was much smallerthan that of the 2004 event.

There was no acceleration record in any ofSimeulue, Nias and Banyak Islands, or on the

west coast of Sumatra Island. Therefore, it isalmost impossible to know the exact groundmotions induced by this earthquake. The onlyway is to infer the strong motions from the col-lapsed or heavily damaged structures such asreinforced concrete buildings, masonry orwooden houses and walls. The author inferredthe MM intensity as IX from observations of thecollapsed buildings. Furthermore, the maximumground accelerations were estimated to rangebetween 300 and 900 gals depending uponground conditions, using the approach pro-posed by Aydan (2002) (Figure 5). Figure 5 alsoshows the inferred maximum ground accelera-tions and maximum ground velocities from thetoppled simple structures observed in GunungSitoli and Teluk Dalam and their close vicinityusing the earlier suggestions by Aydan (Aydan2002, 2006; Aydan and Ohta, 2006). If the risetime and slip of an asperity are known, the max-imum ground acceleration and velocity at thesource area by the sliding on the asperity maybe given in the following form, by assuming thatthe resulting acceleration can be representedby a sinusoidal function:

(1)

If the estimated parameters used by Borges etal. (2005) are considered, the maximum groundacceleration and ground velocity at the sourcearea are estimated to be 155 gal and 384 kine.On the other hand, if the estimations of Koncaet al. (2006) are used, the estimated maximumground acceleration and ground velocity at thesource area would be 56 gal and 180 kine,respectively. Konca et al. (2006) stated that themaximum ground velocity could not be greaterthan 45 kine at the source area. However, the

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Figure 4. Comparison of estimated slip by Konca etal.(2006) with GPS observations.

fiekil 4. GPS gözlemleri ile Konca vd. (2006)’n›n tah-min etti¤i at›mlar›n karfl›laflt›r›lmas›.

maximum ground velocity estimated from a top-pled transformer is about 100 kine in GunungSitoli (78 km from hypocenter).

Aydan (2006, 2007) and Aydan and Ohta(2006) proposed some empirical relations toestimate the possible ground motions such asmaximum ground acceleration and maximum

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ground velocity by taking into account the posi-tion of observation points with respect to faultorientations. This approach is adopted byassuming that the moment magnitude of theearthquake is 8.6 and the shear wave velocityof ground is 1000 m/s. This ground property willbe equivalent to that of bedrock. However, itshould be noted that these values would begenerally amplified three to five times in softground. Figures 6 and 7 show the contours ofmaximum ground acceleration and velocityaround the epicentral area of the earthquake.When the maximum ground accelerations areconsidered, the maximum value is 1500 gal atthe epicentral area and gradually decreaseswith the distance from the hypocenter. Theground accelerations in Nias Island are expect-ed to range between 380 gals to 1300 gals. Thelargest value is expected to be in the town ofLahewa. The ground acceleration will begreater than 650 gals in Gunung Sitoli and 380gals in Teluk Dalam. Since there were manymulti-story buildings with poor earthquake resis-tance in Gunung Sitoli and Teluk Dalam, it is nosurprise that the ground motions were highenough to cause the collapse of such multi-story reinforced concrete structures, as alsonoted from Figure 5.

Similarly, the contours of the maximum groundvelocity at the bedrock in Nias Island will rangebetween 25 to 80 kines. Again these values canbe amplified three to five times in soft ground,as observed in Gunung Sitoli, Teluk Dalam,Lahewa and Sirombu. Recent investigations bythe author showed that bridge decks were dis-placed by 25 to 35 cm in Gunung Sitoli andIdano O’ou in spite of some restraints from theirsurroundings. The expected maximum groundvelocities would be greater than 20 kines forthese restrained structures.

Since the main purpose of this study is to esti-mate the ground motions near the Gunung Sitolicave, it may be concluded from this discussionthat the maximum ground acceleration is likelyto be greater than 600 gal at the site.

Large seismic events beneath or in the vicinityof Nias Island are reported to had occurred. Theevents of 1843 (M7.2), 1861 (M8.5), 1907(M7.6) and 2005 (M8.6) were just beneath theIsland while the events of 1797 (M8.6), 1833(M8.8), 1935 (M7.7) and 2004 (M9.3) took placeto the north or south of Nias Island.

Figure 5. Comparison of estimated strong motion pa-rameters with observations: (a) attenuationof amax, (b) estimated maximum ground ve-locity.

fiekil 5. Gözlemlerle tahmin edilen kuvvetli yer hare-ket parametrelerinin karfl›laflt›r›lmas›: (a)amax’›n azal›m› ve (b) tahmin edilen en bü-yük yerh›z›.

THE GUNUNG SITOLI CAVE (TÖG‹-NDRAWA) AND ITS UNDERGROUND CLIMATE

The formation of Gunung Sitoli cave (Tögi-Ndrawa) is associated with the NE dippingthrust fault. Such faulting always induces vari-ous fractures within the fracture zone of thefault. These fractures are known to be R-R’(Riedel) fractures, T-fracture, S-fracture and P-fracture. Particularly R and T fractures would becaused by tensile stresses, and they are likelyto create open spaces. If the seepage velocityof groundwater is high, these fractures are like-ly to be potential locations for the formation ofkarstic caves. The author investigated somekarstic caves in Padang and Banda Aceh andthis mechanism is confirmed by the site-investi-gations. One example is shown in Figure 8. Theauthor feels that the same mechanism was theprincipal mechanism for the formation of thekarstic Gunung Sitoli cave in Nias Island.

Figures 9 and 10 show the close up geology ofthe Gunung Sitoli cave and its location. Thecave was formed in the Gunung Sitoli formationand several fold axes are recognised near thecave site. Figure 11 shows drawings of the planand cross-sectional views of the cave. The caveis about 200 m long, 9-10 m wide and 8-12 mhigh with an overburden of 6-10 m. Theentrance to the cave is one of the collapsedsections (CS1, CS2, CS3). Collapsed sectionsnumbered CS2 and CS3 were observed and thecollapse of the roof rock progressed to theground surface forming sinkholes. Cross sec-tions of the cave are shaped by the main fault

dipping NE and secondary conjugate faults. Anadditional fault set, whose strike is perpendicu-lar to the longitudinal cave axis, is also foundand appears in the cave at regular intervals.The roof of the cave becomes higher at suchlocations and many stalactites and stalagmitegrowths occur near such faults.

Underground ventilation is natural and air flowtakes place through the head differencebetween the entrance and the collapsed sec-tions CS2 and CS3. The quality of air betweenCS1 and CS3, is generally good. Nevertheless,the air quality and air flow along the sectionbetween CS3 and the NW end of the cave grad-ually decreases. This part of the cave is dampand most of the cave inhabitants (bats) arefound in this section. The air temperature andhumidity range between 25-30 oC and 60-80%in the section between CS1 and CS3. However,the humidity increased to 90-95% near the NWend of the cave (L4) while the temperaturechange was quite small.

ROCK CLASSIFICATIONS AND STATICSTABILITY ASSESSMENTS

The rock surrounding the cave is limestone andits uniaxial compressive strength is expected torange between 20-40 MPa. The cave devel-oped at a fault zone and at least 3 to 4 disconti-nuity sets exist in the rock mass. The beddingspace is wide and is generally greater than 60 cm.The groundwater conditions can be considereddamp to wet. Of course, there is no boring atthe site. Nevertheless, the expected RQD val-ues would range between 50-90%. Since the

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Figure 6. Maximum ground acceleration contours forthe Nias Earthquake.

fiekil 6. Nias depremi için en büyük yer ivmesieflde¤er e¤rileri.

Figure 7. Maximum ground velocity contours for theNias Earthquake.

fiekil 7. Nias depremi için en büyük yer h›z›eflde¤er e¤rileri.

type of rock is limestone, the discontinuity sur-face with slickensides becomes either healed orrough. With these observations, the rock classi-fication according to RMR classification(Bieniawski, 1974, 1989) may be estimated asgiven in Table 3.

If the Q-system (Barton et al., 1974) is used, theQ values obtained range between 6.25 and 15.If some interrelations between RMR and Q-val-ues are used, the results would be quite similarto those computed above. There are two sec-tions within the cave where roof collapse tookplace and migrated up to ground surface. Atthese two locations, the width of the caveranges between 16 to 20 m. The stability of thecave can be analysed using empirical and ana-lytical techniques. Barton et al. (1974) suggest-ed an empirical line for the non-supported spanof the underground openings as shown inFigure 12. Barton (1976) also suggested the fol-

lowing formula between the unsupported span(the unit is m) and the Q-value

(2)

Lang (1994) drew two empirical bounding linesfor the span between stable and unstable open-ings on the basis of observations in mines, asshown in Figure 12.

The critical limiting span of the undergroundopening based upon the arching theory can begiven in the following form (i.e. Aydan, 1989;1990; Aydan et al. 2007):

(3)

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Figure 8. Formation by faulting of karstic caves in alimestone quarry near Padang.

fiekil 8. Padang yak›nlar›nda faylanmaya ba¤l› ola-rak bir kireçtafl› oca¤›nda karstik ma¤araoluflumu.

Figure 9. Location map of Tögi Ndrawa cave in NiasIsland.

fiekil 9. Nias adas›nda Tögi Ndrawa ma¤aras›n›nyerbulduru haritas›.

Table 3. Rating of rock mass according to RMR classification.Çizelge 3. RMR s›n›flamas›na göre kaya kütlesi de¤erlendirilmesi.

Property Description RatingUniaxial compressive strength 20-40 MPa 2-4RQD 50-90% 13-17Discontinuity spacing 0.6 m or greater 15-20Discontinuity condition Unweathered, hard filling, 20-25

rough, 0.1-1.0 mmaperture, persistence 10-20 m

Groundwater Damp to wet 7-10Basic RMR 57-76

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Where σcm, γ and H are uniaxial compressivestrength and unit weight of rock mass and over-burden height, respectively. The constant ξ isrelated to the stress distribution within the arch.If a triangular distribution for arching stress isassumed throughout the roof, it has a value of4/3. However, if a certain length of vertical crackis assumed in the center of the roof and abut-ments, the value of constant ξ would beobtained from the minimization procedure as3/2. The uniaxial compressive strength of rockmass can be obtained from the following empir-ical relation proposed by Aydan and Dalg›ç(1998) and Aydan and Kawamoto (2000)

(4)

Where β and σci are empirical coefficient anduniaxial compressive strength of intact rock,respectively. Aydan and Dalg›ç (1998) suggest-ed that the value of β could be taken as 6 on thebasis of experimental data from constructionsites in Japan. The estimations of a limiting spanby this arching theory approach are plotted inFigure 12 for an overburden height of 5 and 10m by assuming the intact uniaxial strength ofintact rock as 25 MPa on the basis of similar rockin Ryukyu Island (Aydan and Tokashiki, 2007).For the given conditions of Nias Cave, theempirical estimations given by the arching theo-

Figure 11. (a) Plan, and (b) longitudinal and crosssections of the Gunung Sitoli cave (not-to-scale).

fiekil 11. Gunung Sitoli ma¤aras›n›n (a) plan› ve (b)boyuna ve enine kesitleri (ölçeksiz).

Figure 10. Geology in the close vicinity of the cave (modifield from the map by Djamal et al., 1994).fiekil 10. Ma¤ara yak›nlar›n›n jeolojisi (Djamal vd. (1994)’den de¤ifltirilerek).

ry and the lower bound line of Lang (1994) are ingood agreement with observations while theestimations by the empirical relations proposedby Barton et al. (1974) and Barton (1976) are notcompatible with observations.

SEISMIC DAMAGE AND ITS ASSOCIATIONSWITH THE 2005 GREAT NIAS EARTHQUAKEAND PREVIOUS EVENTS

Seismic damage to the Gunung Sitoli cave maybe divided according to old and new seismicevents and they can be categorized as follows:

a) Stalactite fall

b) Stalagmite fracturing and sometimes toppling

c) Vertical and horizontal off-setting of frac-tured stalactite and stalagmites.

d) Ductile bending of stalactite and stalagmites

e) Growth of stalagmites over fallen stalactites

The new events can be directly related to the2005 Great Nias earthquake. The old eventsmay be related to previous events and they maybe identified through the height of stalagmiteson the fallen stalactites. However, the growthrate of stalactites and stalagmites is necessaryfor dating such events. The data from Japanesecaves in tropical zones and mainland indicatesthat the growth rate would range between 0.1 to0.143 mm/year (Aydan and Tokashiki 2007).

Location 1: This is just the entrance to the caveand it faces SE outside. Three large stalactiteswere fallen down. The length and average

diameter of the stalactites were 150 cm - 45 cm,130 cm - 60 cm and 160 cm - 65 cm, respec-tively (Figure 13). In addition, there was a rock-fall from the roof, which was about 4 m insidethe entrance.

Location 2: At this location various forms ofdamage were observed (Figure 14). A columnwas ruptured with a separation of about 20-25mm and 40 mm horizontal offset and the frac-tured part was healed. The same column wasnewly ruptured at 38 cm above the previouscrack location. Two stalagmites with a height of16-17 cm were found on the floor of the cave.Furthermore, a slab was separated by 40cmfrom the NE wall and there was a stalagmitegrowth with a height of 50 mm at the back of theseparated block.

Location 3: A large roof collapse had occurredand the diameter of the cave is about 20 m atthis location. On the NE wall of the cave, thesurface of the NE dipping fault can be observed(Figure 15a) Although the solution of the sur-face of the fault made it difficult to observe thestriations, it is possible to recognise a sense ofthe movement of the fault (Figure 15b).

Location 4: This location is very close to theNW end of the cave. This part of the cave hasmany fallen rockblocks and stalactites from theroof (Figure 16). The height of stalagmatitesranges between 5 to 16 cm. At this location, acolumn, which was broken by much earlier seis-mic events, was newly ruptured and was later-ally displaced, as seen in Figure 17a. Anotherinteresting observation was the re-rupturing of

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Figure 12. Comparison of Gunung Sitoli cave datawith empirical criteria.

fiekil 12. Gunung Sitoli ma¤aras› verilerinin görgül öl-çütlerle karfl›laflt›r›lmas›.

Figure 13. Fallen stalactites and rock blocks near theentrance (Location 1).

fiekil 13. Ma¤ara girifli yak›nlar›nda düflmüfl sark›tlarve bloklar (lokasyon 1).

previously ruptured sections. There was a newstalactite growth of about 25 mm in length and itwas ruptured, probably 2005 by the Great Niasearthquake.

DISCUSSIONS AND CONCLUSIONS

Ground shaking and/or permanent groundmovements may induce damage tospeleothems during earthquakes. Dependingupon the frequency characteristics of earth-quake waves, some speleothems may be moreprone to heavier shaking. Table 4 summarizesthe natural frequency characteristics ofspeleothems (Aydan and Tokashiki, 2007).Depending upon the damping characteristics ofspeleothems, the amplification of ground accel-eration would occur. For a velocity proportionaldamping of 10%, the amplification of groundacceleration would be limited to a rangebetween 4 to 6.

Stalactites are much more slender than stalag-mites. Furthermore, the axial stress acting onstalactites would be tensile, while it would becompressive for stalagmites under static condi-tions. However, it may be compressive whenstalactites and stalagmites grow to constitute asingle column. Speleothems can be consideredto be cylindrical cantilever beams. If a seismic

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Figure 14. Various forms of damage at Location 2.fiekil 14. Lokasyon 2’de de¤iflik hasar flekilleri.

Figure 15. Views of the cave and fault at location 3(CS3).

fiekil 15. Lokasyon 3’te (CS3) ma¤aran›n ve fay›n gö-rünümleri.

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Figure 16. Stalagmite growths on fallen stalactites.fiekil 16. Düflen sark›tlar üzerinde dikitlerin büyümesi.

Figure 17. Re-rupturing of speleothems (Location 4):(a) re-rupturing of a speleothem, (b) re-rup-turing of stalactite (the separation is about25-30 mm).

fiekil 17. Sark›t ve dikitlerin yeniden k›r›lmas› (Lokas-yon 4): (a) birleflik sark›t ve dikitin yenidenk›r›lmas› ve (b) sark›t›n yeniden k›r›lmas›(ayr›lma yaklafl›k 25-30 mm).

Table 4. Natural frequency characteristics of speleothems (from Aydan and Tokashiki, 2007).Çizelge 4. Sark›t ve dikitlerin do¤al sal›n›m özellikleri (Aydan ve Tokashiki, (2007)’den).

Vibration mode Natural frequency

Longitudinal , n=1,2,3,

Transverse , n=1,2,3

Cantilever beam , first mode

Built-in beam , first mode

L: length; Vp: Longitudinal wave velocity; Vs: Transverse wave velocity; E: Elastic modulus; m: mass; I: inertiamoment of area.

coefficient approach is adopted, the fiber stressat the base of a speleothem can be written in thefollowing form (i.e. Aydan and Kawamoto, 1992)

(5)

where,

W: weight of speleothems,

A: base area of speleothems,

h–: distance of the center of gravity ofspeleothem from the base,

t: width or diameter of speleothem,

l: Second inertia moment of base area ofspeleothem,

η: seismic coefficient.

It should be noted that if a crack were initiatedduring shaking, it would end in the fall of stalac-tites (see discussion by Aydan and Kawamoto,1992). Therefore, the crack initiation will direct-ly correspond to the maximum ground accelera-tion acting on a stalactite. Figure 18 shows asimple computation for assessing the stability ofspeleothems by using the seismic coefficientapproach.

If the earthquake does not affect the overall sta-bility of caves, stalactites are more prone to bedamaged by the earthquake when comparedwith stalagmites. Speleothems are generallymade of calcite crystals and their tensilestrength is generally greater than 2 MPa.

However, if they have impurities such as clayeymaterial, their tensile strength may be drastical-ly reduced. The unit weight of stalactites andstalagmatites generally ranges between 21-25kN/m3.

The expected ground motions in the vicinity ofthe Gunung Sitoli cave is more than 600 gals.The slenderness ratio (h/a) of the fallen stalac-tites ranges between 4.5 and 6. Therefore, theexpected tensile (bonding) strength of the sta-lactites may range between 120-180 kPa (seeFigure 18). Under static conditions, 5-8 m longstalactites may be sustained by such tensilestrengths.

The healing of fractures with a separation of 20-25 mm at locations 2 and 4, implies a large pre-vious seismic event. If the growth rate of0.143mm/year is adopted, such an event couldhave been taken place 140 to 175 years beforethe present. This roughly corresponds to the1861 event (M8.6) just beneath Nias Island or tothe 1833 event (M8.8) to the south of NiasIsland. The new fractures shown in Figures 14and 17 are definitely associated with the 2005Great Nias earthquake. Stalagmite growths onfallen stalactites at locations 2 and 4 indicatethat there were very large seismic events 350-500 years and 1100-1600 years ago.

As pointed out by Forti (1998), Gilli (1999) andGilli et al. (1999), the damage to spleothems inkarstic caves may be induced by earthquakesand they may be used for dating unkown eventsin the seismological past of their regions. Thisstudy is a further contribution to such studiesand provides a specific example from NiasIsland, which experienced the M8.6 Great Niasearthquake. This study is probably the first of itskind in Indonesia to associate the damage tospeleothems in the caves of Nias Island. Theauthor expects that similar damage could existin caves in other seismically active parts ofIndonesia.

ACKNOWLEDGEMENTS

The author particularly thanks to Mr. F.Nakajima of CTI Engineering International,Tokyo, and Mr. T. Suzuki of the IndonesiaBranch of Tobishima Company as well as localengineers of the Public Works Department inNias Island for their help and guidance duringthe site investigation.

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Figure 18. Estimated tensile strength of speleothemsfrom the seismic coefficient method.

fiekil 18. Sars›nt› katsay›s› yöntemi ile tahmin edilensark›t ve dikitlerin çekme dayan›m›.

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BOfi SAYFA