The Bengkulu SouthernSumatra, Earthquake of 4 June 200 (Mw = 7. 7): Another Warning to Remote Metropolitan Areas
Tso-Chien Pan, Kusnowidjaja Megawati, James M. W. Brownjohn, and Chin Long Lee Protective Technology Research Center, Nanyang Technological University, Singapore
INTRODUCTION were felt in Singapore and in Kuala Lumpur, Malaysia (Pan and Sun, 1996). The shaking of some buildings in Singapore It has been recognized that urban areas located rather dis- again caused panic, and some office workers rushed out of tantly from earthquake sources may, under some special cir- their high-rise offices. In both incidents, the buildings that cumstances, be affected by the earthquake tremors. A well responded to the remote earthquakes were located in the known example is the 1985 Michoac~in earthquake, in southeastern part of the island underlain by Quaternary which a large earthquake (M~ = 8.1) along the coast of deposits, namely the Kallang Formation (Pitts, 1984). Build- Mdxico caused destruction and loss of life in Mexico City, ings in other areas of Singapore had no apparent response. It 350 km away from the epicenter. Much of the destruction was clear that the Quaternary deposits amplified the incom- was due to significant amplification of the earthquake ing earthquake waves in both incidents. In October 1995, ground motions by thick sedimentary deposits in the down- even stronger and more extensive tremors were caused in town area of the city (Seed et al., 1987). This might be a Singapore by an Ms = 7.0 earthquake centered 450 km away. peculiar case, but obviously soft-soil amplification effects This earthquake also generated ground tremors in Kuala are, to some extent, present in many places. Lumpur and in the southern state of Johor in Malaysia. The The Malay Peninsula and Singapore are located in a recent Bengkulu earthquake of 4 June 2000, which had a M w low-seismicity region, where the closest known earthquake of 7.7 and an epicenter 700 km south-southwest of Sin- sources, the Sumatra Fault and the Sumatra subduction gapore, caused the strongest tremors felt in the city in the last zone, are located more than 350 km away. Earthquakes have 40 years. Many high-rise buildings scattered around the never posed any real problems in the region. The two great- whole island were reportedly shaken, regardless of the local est earthquakes on the subduction zone, M w = 8.8 in 1833 ground conditions. and M w = 8.4 in 1861 (Newcomb and McCann, 1987), It is therefore reasonable to postulate that larger and occurred during a time when there were practically no high- closer earthquakes in Sumatra might result in higher, or even rise structures. Although they were reportedly felt in Sin- damaging, ground motions on the Malay Peninsula. This gapore (Pan and Sun, 1996), these earthquakes did not cause problem is intensified when coupled with the fact that earth- any damage. In line with the rapid regional economic devel- quake-resistant design has yet to be specifically required in opment in recent decades, many high-rise buildings and current regional building codes. In this publication, the complex infrastructures have been constructed, some of Bengkulu earthquake, damage in the epicentral region, public which are on soft soils or reclaimed land. Long-period struc- awareness in remote metropolitan areas, and the earthquake tures, such as tall buildings, are known to be more suscepti- ground motions recorded in Singapore are reported. Discus- ble to distant earthquakes than shorter-period structures. sions on the local seismic hazard in this region are presented. Consequently, the number of felt earthquake tremors in the high-rise buildings of Singapore has been increasing in SEISMOTECTONICS OF SUMATRA recent years (Pan, 1997). In February 1994, some buildings in the densely popu- Sumatra is located adjacent to the Sunda trench (Figure 1), lated areas of Singapore responded to an earthquake of Ms = where the Indian-Australian Plate subducts beneath the Eur- 7.0 that occurred near Liwa in southern Sumatra (Figure 1), asian Plate at a rate of about 67 +_ 7 mm/year, N 11~ _+ 4 ~ more than 700 km away (Pan, 1995). Hundreds of people (Demets et al., 1990; Tregoning et al., 1994). The islands of were awakened and rushed out of their high-rise flats in Sumatra and Java lie on the overriding plate, a few hundred panic. In May 1994, tremors from an earthquake near kilometers from the trench. Convergence is nearly orthogo- Siberut Island, 570 km away, which was only magnitude 6.2, nal to the trench axis near Java, but it is highly oblique near
Seismological Research Letters Volume72, Number2 March/April2001 171 8 I I ~!iiii!i~ :' .[ii!iiiiiiiii!ii!iii!/iiiiiiiiiiiiiiiiiii[iiiiiiiii: I
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,i Figure 1. Tectonic setting of Sumatra, epicenters of the 4 June 2000 main shock and large aftershocks (as of July 16), and seismic intensities on the MMI scale for the major cities in the region.
Sumatra, where strain is strongly partitioned between dip- believed to have caused a 500-km-long rupture along the slip on the subduction zone interface and right-lateral slip on interface extending from the southern island of Enggano to the Sumatra Fault along the western coast of the island Batu Island (Figure 1). Another major earthquake of M~ (Fitch, 1972; McCaffrey, 1991). Most earthquakes in between 8.3 and 8.5 occurred near Nias Island in 1861 Sumatra are of shallow to intermediate focal depth, while (Newcomb and McCann, 1987). Both earthquakes occurred deep events are very unusual. The earthquake focal mecha- in the subduction zone. Since then, there had been no large nisms and hypocentral distributions indicate that the sub- earthquake occurring in the subduction zone until the recent ducting plate in Sumatra dips less than 15 ~ beneath the outer Bengkulu earthquake. On the land side, a dextral strike-slip arc ridge and becomes steeper to about 50 ~ below the volca- fault, the great Sumatra Fault, constitutes yet another source nic arc (Newcomb and McCann, 1987; Fauzi et al., 1996). of numerous earthquakes (Katili and Hehuwat, 1967). The The relatively shallow dip angle gives a strong coupling fault is more than 1,500 km long and runs through the between the overriding and the subducting plates, and large entire length of Sumatra, coinciding with the Barisan Moun- earthquakes have been generated in the region. tain belt. The fault is about 350 km away from the major cit- A major earthquake of an estimated moment magnitude ies, such as Kuala Lumpur, Penang, Ipoh, and Melaka, along (M~) between 8.7 and 8.8 occurred in 1833, and it was the western coast of the Malay Peninsula.
172 Seismological Research Letters Volume72, Number2 March/April 2001 TABLE 1 Moment-tensor Solutions of the Main Event Institution USGS Harvard CMT ERI, University of Tokyo
Epicenter 4.773~ 102.050~ 4.99~ 101.73~ 4.56~ 102.56~ Depth Shallow 54.2 km 50 km Magnitude Ms= 8.0, M,,= 7.7 Ms= 7.9, %= 7.9, M,,= 7.8 M,,= 7.9 Best Double Couple 76 ~ , 44 ~ , 133 ~ 14 ~ , 53 ~ , 25 ~ 300 ~ ' 31 ~ , 90 ~ (strike, dip, slip) 205 ~, 59 ~, 57 ~ 269 ~' 71 ~, 140 ~ Source Duration 61 s Rupture Area 120 km x 60 km Averaged Dislocation 1.5m Averaged Stress Drop 3.2 MPa
THE BENGKULU EARTHQUAKEOF 4 JUNE 2000 which is also associated with the Mentawai Fault stretching from Nias Island in the north through Enggano Island in the Main Shock and Aftershocks south (Figure 1). This might suggest that the earthquake was A great earthquake with a magnitude of M~ = 8.0 (according not caused by subduction activities alone but also by interac- to the United States Geological Survey) occurred in southern tions between the subduction interface and the Mentawai Sumatra, Indonesia on 4 June 2000 at 16:28:25.8 UTC Fault. Further detailed studies have to be done to understand (23:28:25.8 local time or 5 June 2000, 00:28:25.8 Singapore the rupture mechanism fully. time). According to the USGS National Earthquake Infor- The earthquake was followed by more than 1,800 after- mation Center, the epicenter of the earthquake was located shocks as of 11 June, but only about 50 of them could be felt at 4.773~ 102.050~ under the Indian Ocean, about 110 by local residents. Aftershocks that had large magnitudes km off the west coast of Bengkulu Province on Sumatra were collected from the USGS National Earthquake Infor- Island. The epicenter was about 540 km west-northwest of mation Center and are summarized in Table 2. The locations Jakarta and 700 km south-southwest of Singapore, as shown of the epicenters of the aftershocks are shown in Figure 1. in Figure 1. The focal depth was reported to be shallow. The Two aftershocks, on 4 June at 16:39:46 UTC (mb = 6.6) and tremors from the earthquake were reportedly felt as far away on 7 June at 23:45:26 UTC (Ms = 6.7), were reportedly felt as Jakarta, Singapore, and Kuala Lumpur, 875 km north of in Jakarta and Singapore. the epicenter. Moment-tensor solutions of the great earthquake have Damage in Bengkulu and Enggano Island, Indonesia been reported by three institutions (USGS, Harvard CMT, Bengkulu is one of the eight provinces on Sumatra Island. It and Earthquake Research Institute of the University of is located on the western coast of southern Sumatra and has Tokyo), and they are summarized in Table 1. The ERI solu- a population of about 1.2 million. The capital of the prov- tion was derived from vertical components of P waves ince is Bengkulu City, with a population of about 260,000. recorded at IRIS (Incorporated Research Institutions for This small province was the worst hit area in the earthquake Seismology) stations. The waveforms suggested that the rup- due to its proximity to the epicenter. According to official ture process might have been very complicated. The fault reports on 11 June, 90 people were confirmed dead, 803 mechanism revealed a low-angle thrust with a northwest- people badly injured, and 1,782 people lightly injured. Most southeast strike, suggesting that the Bengkulu earthquake of the deaths and injuries were due to falling debris from col- was an interplate earthquake along the subduction zone in lapsed houses. At least 122,000 people were left homeless. the Sunda trench. However, there are some discrepancies According to an official assessment report of 11 June, among the solutions derived by the three institutions. 1,800 houses were totally destroyed, 10,196 houses were There was no tsunami reported following the earth- heavily damaged, and 18,378 houses were lightly damaged quake. The earthquake should have had a large potential to by the earthquake. Most of the houses were nonengineered, generate tsunamis if it were a pure subduction event. The traditional wood and masonry structures. Types of damage fact that the earthquake did not create tsunamis raises ques- observed in nonengineered structures included collapse and tions about the rupture mechanism, suggesting that the rup- cracking of masonry walls, collapse of wood flames that sup- ture did not have a large vertical component. The epicenter port the roof, and failure in beam-column joints. Many of the earthquake was located in the Sunda trench where the dilapidated houses were destroyed by the earthquake. Severe Indian-Australian Plate subducts below the Eurasian Plate, structural damage to engineered structures occurred to only
Seismological Research Letters Volume72, Number2 March/April 2001 173 TABLE 2 Large AftershocksFollowing the Main Event
Date (UTC) Time (UTC) Magnitude No. year/mm/dd hh:mm:ss Latitude (~ Longitude(~ Depth (km) m b M s 1 2000/6/4 16:39:46 4.672 S 102.140 - 33 N 6.6 2 2000/6/4 17:30:59" 5.665 S 103.070 - 33 N 5.3 3 2000/6/4 20:14:01" 4.559 S 101.904 - 33 N 5.0 4 2000/6/4 21:12:53" 4.798 S 102.047 - 33 N 4.8 5 2000/6/4 21:53:37" 4.728 S 101.847 - 33 N 5.0 6 2000/6/4 22:05:15 4.474 S 102.123 - 33 N 4.8 7 2000/6/4 22:30:50 4.851 S 102.150 -- 33 N 5.4 5.1 8 2000/6/4 23:10:47 5.334 S 102.632 - 33 N 4.8 9 2000/6/4 23:14:35" 5.042 S 102.232 - 33N 5.4 4.7 10 2000/6/4 23:27:59* 4.349 S 102.116 - 33 N 4.6 11 2000/6/5 0:47:43* 4.828 S 102.598 - 33 N 5.2 12 2000/6/5 2:22:58* 4.945 S 102.250 - 33 N 4.9 13 2000/6/5 2:.46:20 4.483 S 102.320 - 33 N 5.0 14 2000/6/5 3:00:26 5.695 S 102.957 - 33 N 5.6 5.5 15 2000/6/5 4:53:05 4.405 S 102.202 - 33 N 5.2 4.4 16 2000/6/5 6:34:12 4.958 S 102.701 - 33N 5.4 5.4 17 2000/6/5 7:15:20 4.418 S 102.074 - 33 N 4.8 18 2000/6/5 9:17:36" 4.371 S 102.193 - 33N 5.4 4.6 19 2000/6/5 10:53:53" 4.358 S 102.608 -- 33 N 4.8 20 2000/6/5 15:05:53" 4.943 S 102.055 - 33 N 4.6 21 2000/6/5 23:55:43* 4.093 S 101.900 - 33N 5.7 5.1 22 2000/6/6 2:37:01 4.347 S 102.153 - 33 N 5.5 4.9 23 2000/6/6 5:31:23 5.001 S 102.763 -_- 33N 5.4 5.0 24 2000/6/6 7:13:55" 5.617 S 102.611 - 33 N 4.6 25 2000/6/6 9:58:07 5.091 S 102.764 - 33 N 5.8 6.1 26 2000/6/6 17:31:04* 4.980 S 101.926 - 33 N 5.0 4.9 27 2000/6/6 19:46:34? 4.77 S 102.72 E 33 N 5.0 28 2000/6/7 4:07:06? 4.69 S 101.82 E 33 N 4.7 29 2000/6/7 16:24:48" 4.850 S 102.015 E lOG 4.8 30 2000/6/7 21:41:27? 4.63 S 101.91 E 33 N 4.4 31 2000/6/7 23:45:26 4.652 S 101.982 E 33 N 6.2 6.7 32 2000/6/8 0:08:56? 4.45 S 101.97 E 33 N 33 2000/6/8 0:11:09" 4.647 S 102.228 E 33 N 5.3 34 2000/6/8 1:40:36? 4.84 S 101.72 E 33 N 4.8 35 2000/6/8 4:34:51 4.437 S 101.851 E 33 N 4.9 4.9 36 2000/6/8 12:59:57 4.288 S 102.111 E 33 N 5.1 37 2000/6/8 14:07:41 4.585 S 101.874 E 33 N 4.8 4.7 38 2000/6/8 14:55:03" 4.470 S 101.905 E 33 N 4.6 39 2000/6/8 23:24:31? 4.50 S 101.97 E 33 N 4.6 UTC TIME: Origin time in Coordinated Universal Time. Symbols Following Origin Time: * Indicates a less reliable solution. In general, the geometric mean of the semimajor and semiminor axes of the horizontal 90% confidence ellipse is greater than 8.5 km and less than or equal to 16.0 km. ? Indicatesa poor solution, published for completeness of the catalog. In general, the geometric mean of the semimajor and semiminor axes of the horizon- tal 90% confidence ellipse is greater than 16.0 km. This includes a poor solution computed using data reported by a single network. Symbols Following Depth: N Indicatesdepth was restrained at 33 km for earthquakes whose character on seismograms indicate a shallow focus but whose depth is not satisfactorily determined by the data. D Indicatesdepth was constrained by the computer program based on 2 or more compatible pP phases and/or unidentified secondary arrivals used as pP. G Indicatesthe depth was constrained by a geophysicist.
174 Seismological Research Letters Volume72, Number2 March/April 2001 TABLE 2 (Continued) Large Aftershocks Following the Main Event Date (UTC) Time (UTC) Magnitude No. year/mm/dd hh:mm:ss Latitude (o) Longitude(o) Depth (km) m b M s 40 2000/6/8 23:49:43* 5.595 S 102.509 E 33 N 4.7 41 2000/6/9 4:21:02" 5.629 S 102.789 E 33 N 5.3 5.0 42 2000/6/9 5:35:50 5.364 S 102.800 E 33 N 5.4 5.1 43 2000/6/9 6:27:26* 5.372 S 102.735 E 33 N 5.4 5.2 44 2000/6/9 6:47:31" 4.991 S 102.611 E 33 N 5.0 45 2000/6/9 8:00:22 5.616 S 102.645 E 33 N 6.0 5.8 46 2000/6/9 17:27:52" 4.363 S 101.949 E 33 N 4.7 47 2000/6/9 22:07:05 4.501 S 102.032 E 33 N 5.2 4.7 48 2000/6/10 5:11:32? 5.20 S 102.37 E 33 N 4.6 49 2000/6/10 7:41:29 5.014 S 102.160 E 33 N 4.9 4.3 50 2000/6/10 8:13:59 4.407 S 101.998 E 33 N 5.0 51 2000/6/10 13:59:21 5.017 S 102.153 E 33 N 5.0 52 2000/6/10 23:24:06* 5 103 S 102 119 E 33 N 4 2 53 2000/6/11 9:25:56* 5.457 S 101.497 E 33 N 5.1 5.2 54 2000/6/11 22:17:29" 5.517 S 101.908 E 33 N 4.7 55 2000/6/11 22:29:04 4.971 S 102.156 E 33 N 5.4 4.9 56 2000/6/12 1:35:02? 5.80 S 103.58 E 33 N 4.3 57 2000/6/12 1:51:58? 5.62 S 102.56 E 33 N 4.7 58 2000/6/14 15:30:44" 4.386 S 102.196 E 33 N 4.9 59 2000/6/15 5:52:33* 4.753 S 102.783 E 33 N 4.9 4.1 60 2000/6/15 15:22:29" 4.946 S 102.448 E 33 N 5.3 61 2000/6/16 0:02:15" 4.587 S 101.917 E 33 N 4.7 62 2000/6/19 1:49:35 4.955 S 102.690 E 33 N 5.2 63 2000/6/20 6:54:26? 3.87 S 102.04 E 33 N 4.9 64 2000/6/20 12:54:20" 4.798 S 101.975 E 33 N 5.0 4.5 65 2000/6/21 4:45:54* 5.665 S 101.419 E 33 N 5.1 66 2000/6/22 12:27:15? 5.69 S 101.81 E 33 N 5.1 67 2000/6/22 15:54:22" 4.792 S 101.872 E 33 N 5.2 4.2 68 2000/6/24 14:11:11" 4.336 S 102.624 E 33 N 4.8 69 2000/6/26 1:45:21" 4.595 S 102.825 E 33 N 4.9 70 2000/6/26 16:55:50" 5.403 S 102.544 E 33 N 4.5 71 2000/6/28 5:12:04" 4.505 S 101.930 E 33 N 5.0 4.5 72 2000/7/1 11:51:15? 4.84 S 101.73 E 33 N 4.7 73 2000/7/5 0:10:00 4.640 S 101.999 E 33 N 5.4 4.9 74 2000/7/8 4:52:55 5.465 S 102.703 E 33 N 5.6 5.8 75 2000/7/8 5:14:32" 5.362 S 102.807 E 100 G 4.6 76 2000/7/10 10:39:39 4.472 S 103.800 E 103 D 5.8 77 2000/7/11 9:21:13" 5.752 S 102.645 E 33 N 4.6 78 2000/7/16 16:11:37 5.182 S 102.172 E 33 N 5.2 UTC TIME: Origin time in Coordinated Universal Time. Symbols Following Origin Time: * Indicates a less reliable solution. In general, the geometric mean of the semimajor and semiminor axes of the horizontal 90% confidence ellipse is greater than 8.5 km and less than or equal to 16.0 km. ? Indicatesa poor solution, published for completeness of the catalog. In general, the geometric mean of the semimajor and semiminor axes of the horizon- tal 90% confidence ellipse is greater than 16.0 km. This includes a poor solution computed using data reported by a single network. Symbols Foflowing Depth: N Indicatesdepth was restrained at 33 km for earthquakes whose character on seismograms indicate a shallow focus but whose depth is not satisfactorily determined by the data. D Indicatesdepth was constrained by the computer program based on 2 or more compatible pP phases and/or unidentified secondary arrivals used as pP. G Indicatesthe depth was constrained by a geophysicist.
Seismological Research Letters Volume72, Number2 March/April 2001 175 a few buildings, including a public low-cost housing block, a The Indonesian Meteorological and Geophysical bank building, ten schools, five hospitals and clinics, and the Agency assigned the seismic intensity at Bengkulu as V and airport buildings. Types of failure in engineered structures VI on the Modified Mercalli Intensity (MMI) scale. MMI V observed were mainly due to insufficient shear reinforcement signifies events that are felt by nearly everyone and cause of columns, especially those in the first floor. However, there many to be awakened; some dishes, windows broken; unsta- was much properly constructed public housing that survived ble objects overturned; pendulum clocks may stop. MMI VI the earthquake shaking. Nonstructural failures were signifies shaking that is felt by all, many frightened; some observed in many places, with the most common phenome- heavy furniture moved; a few instances of fallen plaster; non being sliding failure of roof tiles due to insufficient damage slight. attachment to the roof flame. The local airport was closed for operations due to fail- Public Responsesin Singapore, Malay Peninsula, and ures in navigation systems and power generators. Several Jakarta roads built on swamp areas showed signs of subsidence and "Tremors Hit Several Parts of Singapore" was the headline in cracking at 26 locations. Subsidence of the approaches to several Singaporean newspapers following the earthquake. several bridges was also reported. Electricity and water sup- According to The Straits Times, a major English newspaper in plies in Bengkulu and the surrounding areas were knocked Singapore, Singaporeans were awakened from their sleep by out. Only 20% of electricity supply and 25% of water sup- strong tremors, which shook many parts of the island in the ply were in service after the earthquake. Widespread tele- early morning of 5 June. Those who lived in high-rise apart- phone line disruptions were reported. The Indonesian ments rushed downstairs, still dressed in their pajamas. The Antara news agency said the quake damaged some 3,000 of tremors started between 00:30 AM and 00:45 AM (local time) 25,000 telephone lines in the provincial capital and another in the east and spread to the north and west, hitting Pasir 1,800 lines in southern Bengkulu. Ris, Marine Parade, Siglap, Braddell, Yio Chu Kang, Toa Enggano Island, with a population of 1,686, is only a Payoh, Balestier, Orchard Road, Rochor, Bugis Street, few kilometers away from the epicenter. Ninety percent of Jurong East, and Jurong West. The areas where the tremors the homes there were reported to have been badly damaged, were felt are shown in Figure 2, which indicates that virtually but no deaths were reported. Most of the houses were simply the whole island was shaken. Almost all of those who per- constructed huts, which were incapable of killing the inhab- ceived the tremors lived in high-rise buildings. The tremors itants when they collapsed. felt were the main shock and one aftershock that occurred
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A Fioure 2. Locations of Singaporean buildings reportedly responding to the Bengkulu earthquake.
176 Seismological Research Letters Volume72, Number2 March/April 2001 eleven minutes afterward. No people were injured due to the the buildings. However, no injury or building collapse was tremors, but police officers were dispatched to various hous- reported. On the western coast of the Malay Peninsula, most ing complexes to reassure concerned residents. high-rise buildings are concentrated in Klang Valley (Kuala A day after the earthquake, the Singapore Police assured Lumpur), Johor Bahru, and Penang. However, Penang and people that preliminary assessment by the Building Control Perak, which are located further to the north on the Malay Authority (BCA) and the Housing and Development Board Peninsula (see Figure 1), were not shaken by the tremors. (HDB) showed that buildings were safe and that there was According to The Jakarta Post, a major English newspa- no cause for concern. BCA and HDB checked about 140 per in Indonesia, residents in Greater Jakarta also felt the "affected" private apartments and public flats around the main shock tremor, which lasted for several minutes, causing island and found them to be structurally sound. They panic among people living in high-rise buildings. The main assured the public that low-level vibrations and tremors shock tremors reportedly lasted for several minutes at 11:30 would not cause structural damage to buildings in Singapore VM (local time), followed by an aftershock, which lasted for because the inherent design requirements provided the two minutes. No damage or injury was reported. Some buildings with sufficient built-in structural resistance and panic-stricken people rushed out of their high-rise homes. robustness against the minor tremors generated by distant According to the Meteorological and Geophysical Agency of earthquakes. In a joint press briefing conducted by BCA, Indonesia, the seismic intensity in Jakarta ranged from II to HDB, the Meteorological Service of Singapore (MSS), the III on the Modified Mercalli Intensity scale. MMI II signifies Singapore Police, and the Singapore Civil Defence Force events that are felt only by a few persons at rest, especially on (SCDF) three days after the earthquake, it was stated that it upper floors of buildings. MMI III indicates that the event would take a force ten times stronger than that produced by was felt quite noticeably by persons indoors, especially on the tremors to cause any building damage. The BCA said the upper floors of buildings; many people might not recog- that buildings on reclaimed land were also safe, as they sit on nize it as an earthquake; standing motor cars might rock steel or concrete piles and not on the ground. Under the cur- slightly and the vibrations were similar to the passing of a rent building and construction codes, buildings are designed truck. to withstand a nominal horizontal force (1.5% of the charac- teristic weight of a building) and a wind speed of up to Seismic Instrumentation and Ground Motions Recorded 30 m/s. in Singapore MSS reported that over the last decade, felt tremors have In 1996, MSS installed a network of seismic stations, which been reported in Singapore every two years on average, but consists of two down-hole arrays (BES and KAP) of strong- the present quake was the worst felt in Singapore in the last motion stations and five teleseismic stations (BTDF, FTC, forty years. So far, all the tremors reported have been caused NTU, PTK, and SJA). The locations of the stations are by earthquakes in Sumatra. depicted by triangles in Figure 3. The two down-hole arrays Three days after the earthquake (8 June), the police and are located on the Kallang Formation of Quaternary depos- SCDF announced that a quake alert in the form of public its. The main station, located in the Bukit Timah nature warnings and information dissemination would go out faster reserve and denoted as BTDF, is a Global Seismic Network the next time a similar incident happens. They would work (GSN) station, which is equipped with a comprehensive set with radio stations and other media to speed up the trans- of sensors to record ground tremors continuously. BTDF is mission of announcements. When the main shock tremors located on a rock outcrop site. The recorded ground-motion hit Singapore between 00:30 AM and 00:45 AM on 5 June, signals are transmitted to MSS headquarters in Changi to be many people panicked and rushed out of their high-rise processed and analyzed. homes. Several people waited for radio announcements on Nanyang Technological University (NTU) has also what to do and whether it was safe to return to their homes. installed two additional seismic stations, one inside the cam- But the information was sent to the media only at 2:30 AM, pus (denoted as NYC) and the other in the basement of a about two hours after the tremors were felt. high-rise office building in the city center (denoted as RP). Aftershock tremors hit Singapore at 7:48 AM of 8 June The NYC station is located on a firm residual soil site, and but caused no alarm. This aftershock occurred in southern the building in which the RP station is located sits on rigid Sumatra and measured M s = 6.6. The tremors were felt caissons. The two NTU and seven MSS stations form an mainly in the Chinatown, Marine Parade, Meyer Road, array called the Singapore Array for Earthquake Response North Bridge Road, Crawford Lane, and Bishan. No injuries (SAFER). were reported. The two NTU stations and one of the MSS stations The tremors from the main shock and the following (BTDF) were triggered during the Bengkulu earthquake. aftershock were reportedly felt in several areas of Malaysia. Ground motions from the main shock (2000/06/04 They were felt in Johor Bahru, Pontian, Klang Valley, and 16:28:25 UTC) and the two major aftershocks (2000/06/04 Melaka. The affected buildings were all high-rise apart- 16:39:44 UTC and 2000/06/07 23:45:26 UTC) were suc- ments, public housings, and hotels. Hundreds of buildings cessfully recorded. Figures 4(A), 4(B), and 4(C) show the were shaken, causing thousands of the residents to flee from baseline-corrected accelerations, velocities, and displace-
Seismological ResearchLetters Volume72, Number2 March/April2001 177 ,~ ~SJA
~ o 5 10 p Kilometers
A Fioure 3. Seismic stations in Singapore (SAFERArray).
BTDFBHE I i.~ ~.:~.c~ _.9.6.~:~.~" BTDFBHN BTDFBHZ I '-~'~"~,-~T'~'~;~';--~'TI'~'~.,. . 3.8
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I 1 I I I I I 100 150 200 250 300 350 400 450 500 Time (s) ,A Fi0ure 4(C). Baseline-corrected displacements of the Bengkulu main shock (2000/06/04 16:28:25 UTC), recorded in Singapore.
Seismological Research Letters Volume72, Number2 March/April 2001 179 TABLE 3 Summary of Peak Values of the Bengkulu Earthquake Recorded in Singapore Event Date and Time Station Component PGA (gal) PGV (cm/s) PGD (cm) 2000/06/0416:28:25.8 UTC BTDF EW 0.38 0.47 2.32 NS 0.23 0.23 0.68 UD 0.38 0.40 1.11 NYC EW 1.48 0.73 3.23 NS 1.95 0.33 1.00 UD 1.10 0.43 1.23 RP B1A 0.74 0.42 1.07 B1B 0.58 0.68 3.10 2000/06/0416:39:45.6 UTC BTDF EW 0.46 0.36 0.89 NS 0.25 0.35 0.94 UD 0.67 0.63 1.64 NYC EW 0.58 0.47 1.30 NS 0.52 0.52 1.23 UD 0.81 0.64 1.81 RP B1A 0.55 0.63 1.59 B1B 0.42 0.36 0.92 2000/06/07 23:45:26.3 UTC BTDF EW 0.09 0.06 0.10 NS 0.06 0.05 0.07 UD 0.08 0.05 0.10 NYC EW 0.15 0.08 0.14 NS 0.19 0.05 0.09 UD 0.10 0.05 0.11 ments of the main shock, respectively. In each figure, the top reported that only high-rise buildings in the central and three traces are the E-W, N-S, and U-D components southeastern parts of Singapore were shaken in these events. recorded at BTDE The following three traces are the E-W, The locations of the buildings that responded to the earth- N-S, and U-D components at NYC, while the bottom two quakes are shown in Figure 7. There may be two reasons why traces are the horizontal components of the RP station ori- only tall buildings in these specific areas were affected. First, ented according to the two principal axes of the building. the central and southeastern parts of Singapore are underlain Figures 5(A), 5(B), and 5(C) present the baseline-cor- by a Quaternary marine clay deposit, namely the Kallang rected accelerations, velocities, and displacements recorded Formation, and part of the coastal area is reclaimed land during the first aftershock. The data for the second after- (Pitts, 1984). Soft sedimentary deposits might have ampli- shock are shown in Figures 6(A), 6(B) and 6(C). Note that fied the weak bedrock motions to a level that caused the the RP site instrument was not triggered during the second buildings to vibrate perceptibly. Second, tall buildings aftershock. Table 3 summarizes the peak ground acceleration respond more easily to amplified motions at lower frequency (PGA), peak ground velocity (PGV), and peak ground dis- from distant strong earthquakes because of the long natural placement (PGD) of each component recorded at each periods of these buildings. Therefore, the tall buildings were station. shaken more strongly than other types of structures in the soft soil area and than the structures in all other areas. DISCUSSION However, the Bengkulu earthquake shook many high- rise buildings scattered around the whole island of Sin- Building responses to earthquakes are dependent upon the gapore. The areas where these buildings are located are building type and the ground condition where the building shown in Figure 2. The fact that the locations of the build- stands. From 1971 to the time prior to the Bengkulu earth- ings are widely distributed throughout the island indicates quake, there had been ten earthquakes in Sumatra that that the earthquake, which is the largest in recent decades, caused tremors felt in Singapore (Pan and Sun, 1996). It was generated substantial ground motions capable of causing
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1 O0 150 200 250 300 350 400 450 500 Time (s) ,J, Figure 6(B). Baseline-correctedvelocities of the second Bengkulu aftershock (2000/06/07 23:45:26 UTC), recorded in Singapore.
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P Kilometers A Figure 7. Locations of buildings reportedly responding to Sumatra earthquakes prior to the Bengkulu earthquake. perceptible levels of vibration regardless of the local soil con- the giant 1833 Sumatra subduction earthquake might have dition. Therefore, in addition to the central and southeastern had a magnitude ranging from 8.8 to 9.2. Therefore, the parts, the other areas received strong enough tremors to effects of very large earthquakes in the subduction zone or on shake buildings perceptibly. the Sumatra Fault on high-rise buildings in Singapore and Current building design codes for structures in Sin- the Malay Peninsula need to be investigated further. gapore and Malaysia have been developed largely based on the BS8110 Code (BSI, 1985), which does not have any pro- CONCLUDING REMARKS vision for seismic loading. It requires, however, that all build- ings should be capable of resisting a notional (nominal) Resulting from rapid economic growth and development, ultimate horizontal design load applied at each floor level many high-rise buildings and complex infrastructure systems simultaneously for structural robustness. The horizontal load have been constructed in Southeast Asia. Some of them are is equal to 1.5% of the characteristic dead weight of the built on less favorable sites, such as soft soils or reclaimed structure. The design wind load should not be less than this lands. Low-seismicity regions, in which the local earthquake value. Given the moderate design wind speed of 30 m/s in hazard is low, may have to face the fact that huge but infre- Singapore, the notional horizontal load is generally greater quent distant earthquakes might pose a real problem when than the wind loading for moderate-height buildings. Thus, they occur. Singapore and the Malay Peninsula are two clas- the notional load is usually the governing lateral load for sic examples of postulated areas with low seismic hazard but design. The largest peak horizontal ground acceleration high exposure. During the past 30 years, the high-rise build- recorded in Singapore during the Bengkulu earthquake was ings in the major metropolitan areas of the region have been 2 gals (0.2% g) at the firm soil site (NYC station). The esti- affected by many medium to large earthquakes in Sumatra. mated peak ground motions at the soft soil sites might have Although there has not been any seismic damage observed so been several times larger due to soil amplification effects. far in the region, the seismic performance of buildings in The base shear-force responses of buildings founded on soft giant Sumatra earthquakes, such as the one in 1833, has yet soils might therefore be comparable to that required by the to be investigated. The Bengkulu earthquake of 4 June 2000 notional horizontal load. has given a fresh reminder of the potential vulnerability of It should be noted, however, that the Bengkulu earth- the buildings and infrastructures in the region, where earth- quake was not really the maximum credible earthquake that quake-resistant design is not specifically required. The may occur in Sumatra. Sun and Pan (1995) found that the ground motions of the main shock and two aftershocks moment magnitude of an earthquake with a 10% probabil- recorded in Singapore during the Bengkulu earthquake ity of being exceeded in 50 years in Sumatra region was 8.5. could possibly give new insight into the level of seismic haz- Zachariasen et al. (1999) suggested that the magnitude of ard of the region. El
184 Seismological Research Letters Volume 72, Number2 March/April 2001 ACKNOWLEDGMENTS Pan, T.-C. (1995). When the doorbell rings: A case of building response to a long distance earthquake, Earthq. Eng. and Struc- tural Dynamics 24, 1,343-1,353. The authors would like to thank the Meteorological Service Pan, T.-C. (1997). Site-dependent building response in Singapore to of Singapore for the release of the records at BTDF station. long-distance Sumatra earthquakes, Earthq. Spectra 13, 475--488. Mr. Teddy Boen, Mr. Suhardjono (Indonesia Meteorological Pan, T.-C. and J. Sun (1996). Historical earthquakes felt in Singapore, and Geophysical Agency), and Dr. N. T. Puspito (Bandung Bull. Seism. Soc. Am. 86, 1,173-1,178. Institute of Technology) kindly shared their information on Pitts, J. (1984). A review of geology and engineering geology of Sin- gapore, QuarterlyJ. Eng. Geology 17, 93-101. the damage in Bengkulu and Enggano. Seed, H. B., M. P. Romo, J. P. Sun, A. Jaime, and J. Lysmer (1987). Relationships between soil conditions and earthquake ground REFERENCES motions in Mexico City in the earthquake of September 19, 1985, UCB/EERC-87/15, U. of California, Berkeley, California. British Standards Institution (1985). Structural Use of Concrete, Part 1. Sun, J. and T.-C. Pan (1995). The probability of very large earthquakes Code of Practice for Design and Construction, BS 8110: Part 1: in Sumatra, Bull. Seism. Soc. Am. 85, 1,226-1,231. 1985, British Standard Institution, London. Tregoning, P., E K. Brunner, Y. Bock, S. S. O. Puntodewo, R. McCaf- Demets, C., R. G. Gordon, D. E Argus, and S. Stein (1990). Current frey, J. E Genrich, E. Calais, J. Rais, and C. Subarya (1994). First plate motions, Geophys.J. Int. 101,425-478. geodetic measurement of convergence across the Java Trench, Fauzi, R. McCaffrey, D. Wark, Sunaryo, and P. Y. P. Harydi (1996). Geophys. Res. Lett. 21, 2,135-2,138. Lateral variation in slab orientation beneath Toba Caldera, north- Zachariasen, J., K. Sieh, E W. Taylor, R. L. Edwards, and W. S. Han- ern Sumatra, Geophys. Res. Lett. 23, 443-446. toro (1999). Submergence and uplift associated with the giant Fitch, T. J. (1972). Plate convergence, transcurrent faults, and internal 1833 Sumatran subduction earthquake: Evidence from coral deformation adjacent to Southeast Asia and the western Pacific,J. microatolls, J. Geophys. Res. 104, 895-919. Geophys. Res. 77, 4,432-4,460. Katili, J. A. and E Hehuwat (1967). On the occurrence of large tran- Protective Technology Research Center scurrent faults in Sumatra, Indonesia, J. Geoscience Osaka City School of Civil and Structural Engineering Univ. 10, 5-17. Nanyang Technological University McCaffrey, R. (1991). Slip vectors and stretching of the Sumatran fore arc, Geology 19, 881-884. Block N1, Nanyang Avenue Newcomb, K. R. and W. R. McCann (1987). Seismic history and seis- Singapore 639798 motectonics of the Sunda Arc, J. Geophys. Res. 92, 421-439. Republic of Singapore [email protected]
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