Information Note 05/2007 May 2007 EARTHQUAKE RISK IN HONG KONG Key Messages: a) The seismicity of Hong Kong is low to moderate. b) There is little evidence of significant recent fault activity in Hong Kong, either onshore or offshore. c) The possibility of significant earthquake damage to slopes, retaining walls and reclamations in Hong Kong is low. Introduction Every year, about two earth tremors are felt in Hong Kong. To put this in context, studies into earthquake risk in Hong Kong carried out by the Geotechnical Engineering Office (GEO) classified the seismicity of Hong Kong and the adjoining Guangdong Province as "low to moderate". The earthquake hazard in Hong Kong is therefore considered to be very much lower than in areas such as Japan, Taiwan and the western USA which lie close to the earth's more seismically active zones along crustal plate boundaries. However, earthquake risk in Hong Kong cannot be regarded as negligible. Causes of Earthquakes An earthquake is a complex series of ground vibrations caused by the release of stored energy in the ground, usually during a sudden, unpredictable movement along a geological fault. Movements are ordinarily resisted by friction and only occur when the stress across a fault has built up to a level at which it exceeds the frictional resistance. Other natural processes such as the movement of magma beneath volcanoes or magmatic movements beneath the earth’s crust can generate earthquakes. Earthquakes can also result from the activity of man, most obviously in relation to controlled or accidental large explosions, and to man-made changes such as the filling of newly constructed large reservoirs. Quantification of Earthquakes: Size and Effects Earthquakes are generally assessed in terms of their magnitude and intensity. (a) The magnitude of an earthquake is a measure of the amount of energy released at the origin of the earthquake, and it is most commonly quantified in terms of the Richter Scale. Earthquakes of magnitude less than 4 are considered minor and unlikely to cause damage, whereas those with magnitude 6 or greater are considered major events, capable of causing great damage in some cases. As magnitude increases by one unit, the energy released increases by about 30 times. (b) The intensity of an earthquake is an assessment of its surface effects, and in Hong Kong this is quantified by using the twelve-point Modified Mercalli Intensity Scale. Stationary people can feel an intensity III tremor; for intensity VI, many people are -2- frightened and run outdoors; some buildings suffer from cracking under an intensity VII event; tremors causing landslides are classed as intensity VIII; intensities of IX and above are significantly destructive events. Unlike the magnitude of an earthquake, which quantifies the energy released at the source within the earth, intensity reduces as the surface distance from the source of the earthquake increases. Hence, maps of intensity distribution related to a specific earthquake will generally show crude concentric zoning around the source of the earthquake. Global Distribution and Generation of Earthquakes The point on the earth's surface vertically above the source of an earthquake is its epicentre. The actual location of fault movements which generate most earthquakes felt at the earth's surface, however, is at variable depths of up to several hundred kilometres. Most major earthquakes occur in well established seismic zones which also contain very large fault systems. Most, but not all, of these zones are situated along the boundaries of the earth's crustal plates. The relative movement of the plates ultimately determines the frequency and magnitude of earthquakes occurring along individual faults. Although far fewer earthquakes occur along faults located far from plate boundaries, they can nevertheless still be of large magnitude. The Tectonic Setting of Hong Kong Hong Kong lies within the Eurasian Plate. It is located about 600 km from the nearest boundary which underlies Taiwan and trends south to the Philippines and northeast to Japan. This plate boundary is associated with both frequent and large magnitude earthquakes whose epicentres are concentrated in a zone up to 200 km wide along the eastern boundary of the Eurasian Plate. Active volcanoes also occur in this zone, as in Japan and the Philippines. Historical data indicate that the frequency of large magnitude earthquakes declines rapidly at distances greater than about 200 km from the plate boundary. Historic Earthquakes in the Vicinity of Hong Kong Historical records indicate that Hong Kong does not experience frequent, large magnitude earthquakes, as a result of its favourable location far removed from plate boundaries (see below). However, occasional earthquakes are recorded at locations almost throughout the Eurasian Plate. The biggest earthquake vibrations recorded in the last 100 years or so in the vicinity of Hong Kong occurred 300 km away in the Shantou area of neighbouring Guangdong Province in 1918. This earthquake was classed as Magnitude 7.3, but in spite of its classified VI to VII intensity in Hong Kong, the damage caused was minor. Other notable earthquakes have occurred near Hong Kong at Macau in 1905 (Magnitude 5.5), at Heyuan in 1962 (Magnitude 5.8), and at Yangjiang in 1969 (Magnitude 6.2). No damage was reported in Hong Kong due to these earthquakes. Every year, about two earth tremors are felt in Hong Kong. Most of these tremors are small and about 25% of them are due to earthquakes with epicentres near or at -3- Taiwan, for example, the Magnitude 7.7 earthquake at Taiwan in September 1999. The intensity of this earthquake measured in Hong Kong was classed IV. Bigger tremors are occasionally experienced when earthquake epicentres are nearer to Hong Kong; for example, the Magnitude 7.3 earthquake which occurred on 16 September 1994 and aroused some public concern in Hong Kong. The epicentre of this earthquake was located offshore near Shantou and its intensity in Hong Kong was classed as V to VI. An earthquake of magnitude 3.5 around Dangan Island in 2006 also aroused some public concern. Information on locally felt earth tremors can be found in the Hong Kong Observatory’s website: http://www.hko.gov.hk. To put the local earthquake risk in its context, it should be noted that small earthquakes are commonplace throughout the world, but the vast majority are of magnitudes so small that they are only detectable by very sensitive equipment, and their intensities are below the threshold felt by people. Of most concern to the public are the strong earthquakes that can cause damage to property and injury to people. Although the earthquake hazard in Hong Kong is considered to be very much less than in areas such as Japan, Taiwan and the western USA which are located along highly active plate boundaries, this does not mean that the risk is negligible. Earthquakes can occur at locations far from plate boundaries. Research and Development The GEO have been carrying out earthquake studies since 1988 and the study results are published locally. Researchers in local universities and practitioners have also carried out studies related to earthquake engineering. The results of these studies enable engineers in Hong Kong to make better provisions for earthquakes in their design. A summary of the recent earthquake studies in Hong Kong is given in Appendix A. Earthquake Provisions in Hong Kong The Buildings Department conducted a review on the seismic effects on buildings in Hong Kong. The GEO has published a guidance document on the design of retaining walls (GEO, 1993) which provides for earthquake loading. For man-made slopes, earthquake provision in the design is not considered necessary as severe rainfall is a more severe design condition. The current design practice for buildings in Hong Kong is such that provision for earthquake loading is not made routinely. Nevertheless, some large buildings and civil engineering projects have been designed against earthquakes, at the discretion of the owners. For example, the dam of the Plover Cove Reservoir has been designed to withstand earthquakes up to Intensity VIII. Prepared by Standards and Testing Division Geotechnical Engineering Office Civil Engineering and Development Department May 2007 -4- Appendix A Earthquake Studies in Hong Kong In Hong Kong, earthquake studies have in the past concentrated on data from two main sources: (a) Collecting historic records from Hong Kong and the neighbouring region - The GEO, in collaboration with the Observatory and seismologists in the United Kingdom and Guangdong Province, have reviewed information on earthquakes within a distance of about 350 km of Hong Kong. The information has been published in GCO Publication No.1/91 (GCO, 1991). This review revealed that, although earthquakes have occurred in Guangdong Province, seismicity in the region is "low to moderate" and is lower in Hong Kong than elsewhere in the region. (b) Geological mapping by the Hong Kong Geological Survey includes locating faults and shear zones, and examining them for evidence of recent activity - onland faults have been recorded during detailed field geological surveying. Offshore faults have been located mainly using seismic and magnetic reflection surveys. There is little evidence to suggest significant recent activity on the faults which have been mapped either onshore or offshore. In parallel with the general review of seismicity and geological structure, a number of specific studies on the effects of earthquakes on different types of engineering structures in Hong Kong have been completed: (a) In 1994, the Government upgraded the local seismic monitoring network that is maintained and operated by the Hong Kong Observatory, and additional seismological stations have been established. It is now possible to record even very small-scale earthquakes in the vicinity of Hong Kong and locate the earthquake sources more accurately. (b) The GEO have looked at the effects of seismic loading on slopes, retaining walls and reclamations. Regarding slopes and retaining walls, there are no records of earthquake-induced landslides or wall collapses in Hong Kong. GEO's studies also indicate that seismic loading is generally not critical for man-made slopes and walls designed to current geotechnical safety standards. Similar findings for slopes were reported by Pappin & Bowden (1997). As for reclamations, the probability of earthquake-induced liquefaction has been shown to be low in Hong Kong. Sites with a long fundamental period (i.e. low natural frequency) have not been found. (c) The GEO engaged the University of Hong Kong to carry out a detailed study of the seismic hazard in Hong Kong in 1995. The University collaborated with seismologists from Guangdong and took into account the wealth of information gathered by Mainland seismologists in recent years. The results of the study (Lee et al, 1996) are consistent with those from earlier studies in the early 1990s. The return period for a major earthquake of Intensity VII to occur in Hong Kong is estimated to be 350 to 400 years, and Intensity VIII could be once in 2,500 years. (d) Although there are no known cases of landslides caused by earthquakes in Hong Kong, -5- the GEO has carried out a study to compare the risk of failure of man-made slopes induced by earthquakes to those induced by rainfall (Wong & Ho, 2000). The results of the study show that the risk of failure of man-made slopes due to earthquakes is much smaller than that due to heavy rainfall. (e) Research is currently continuing at local universities on a range of topics related to earthquake engineering, e.g. Chandler (2000), Chandler & Su (2000) and Chau et al (2002). References Chandler, A.M. (2000). Review of Hong Kong seismic parameters and determination of design level earthquake events. Transactions, vol. 7, no. 1. Hong Kong Institution of Engineers, pp 1-12. Chandler, A.M. & Su, R.K.L. (2000). Dynamic soil properties of Hong Kong reclamation sites for seismic applications. Transactions, vol. 7, no. 1. Hong Kong Institution of Engineers, pp 13-27. Chau, K.T., Lam, E.S.S., Xu, Y.L. & Wong, Y.L. (2002). Research and development of the earthquake engineering in Hong Kong. Proceedings of the One Day Seminar on Recent Developments in Earthquake Engineering. Hong Kong Institution of Engineers, Hong Kong, pp 16-33. GCO (1991). Review of Earthquake Data for the Hong Kong Region (GCO Publication No. 1/91). Geotechnical Control Office, 115 p. GEO (1993). Guide to Retaining Wall Design (Geoguide 1). (Second edition). Geotechnical Engineering Office, 268 p. Lee, C.F., Ding, Y.Z., Huang, R.H., Yu, Y.B., Guo, G.A., Chen, P.L. & Huang, X.H. (1996). Seismic Hazard Analysis of the Hong Kong Region (GEO Report No. 56). Geotechnical Engineering Office, 145 p. Pappin, J.W. & Bowden, A.J.H. (1997). The likelihood of earthquake induced landslides in Hong Kong. Proceedings of the Seminar on Slope Engineering in Hong Kong, Annual Seminar of the Geotechnical Division, Hong Kong Institution of Engineers, pp 177 -184. Wong, H.N. & Ho K.K.S. (2000). Preliminary Risk Assessment of Earthquake-induced Landslides at Man-made Slopes in Hong Kong (GEO Report No. 98). Geotechnical Engineering Office, Hong Kong, 69 p.