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 ::: :~:~ii~i~iii~}i~i~i~i}i!~!i}~iii~iiiiiiii~i~i~i~iiiiiiiii~iii~iiii!iiiiiiii~ii!i}~iiiiiiiiii ....~i~!!i~: i ~iii!ii~i!i~iiii::i~)~i~iiiii~i~i?iiiiiiiiiiiiiiii!iiiiii!i!iiiiiiiiili ~ ~ ......................... ................... ' ....~iiiiiiiii~!:~ iiii!iii)!iiiiiiiiiiiiiiiiiiiii!iiiiiiiiiiiill.... ilili " ::~ "i!i!iiiii!iii~ ........................... iii 9i ,iii~ "N \ .... 'i!iiiiiii!i!!iiiiilii ~!~ 2 m ~ : ::. ,i:i......... ~i'~i;i~l~i~@i'~iil!iiiii!i',iii!i!iii~:-.................. ! .................. , Nias .........%11~ !! Singapore (Ill-IV) ....~:~ ......... ~,. ~i ...... :4ii!i ~ ..... ...................................... _ -2 - ! 1 ..........67rem) i.......... '5 -4 ............................................................... i _ ! -6 ...................... i I ~ I I ' I................ .. ::~,s;~ o 500 km Jakarta Enggano .., ...................... , ..................... : ..................... ,..__ -8 ...................... !............................................. 0 aftershoCks (up to 07/16) - 9 'nainshock I I I 92 94 96 98 100 102 104 106 108 ,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.