http://www.earthobservatory.sg/medi...s-may-be-at-risk-in-huge-sumatran-quake-.html
Singapore, KL High-rises Atop Soft Soils May Be at Risk in Huge Sumatran Quake
If a giant earthquake struck off West Sumatra, some medium- and high-rise buildings in Singapore and Kuala Lumpur could be vulnerable to damage from ground shaking, according to a recent study. The findings indicate that tall buildings constructed above soft soils may be at high risk because ground motion from distant earthquakes can be amplified at such sites. In contrast, the risk to buildings sited above underlying rock is low.
The research was done in response to earlier findings indicating that a quake of magnitude 8.8 or greater can be expected within the next few decades in the vicinity of the Mentawai Islands. Scientists say such a rupture is inevitable because of strain that’s been building up in an undersea fault line, called the Sunda megathrust, where two tectonic plates interact at their boundaries.
EOS principal investigator Kusnowidjaja Megawati and his colleagues did simulation studies to determine how ground motion from such a huge Sumatran quake would affect buildings in nine major cities across the area: Bengkulu, Jakarta, Kuala Lumpur, Medan, Padang, Palembang, Pekanbaru, Penang and Singapore.
Those cities were considered because they lie within two fault lengths of the Mentawai megathrust segment. Fault length refers to the length of the segment that’s expected to break. “It’s a rough estimate that an earthquake may affect an area which is within two or three fault lengths of the rupture,” explains Prof Megawati, an assistant professor in the School of Civil and Environmental Engineering at Nanyang Technological University.
Because it’s impossible to know exactly where the predicted fault break will originate and how large the rupture or series of ruputres will be, the study projected ground motions under three possible scenarios. The scenarios were based on scientific knowledge of past earthquakes in the area, data on the amount of seismic strain along segments of the fault and the physical characteristics of those segments – factors that influence the propagation of seismic waves and the nature of ground motion in a major rupture.
As might be expected given their location, Padang, Bengkulu and other major cities in Sumatra were found to be most vulnerable to structural damage from a powerful Mentawai-area quake. According to the study, medium- and high-rise buildings – those about 8 to 30 stories high – are likely to survive only if built to withstand ground shaking that lasts 3-5 minutes.
The results, reported in the April 2009 issue of the Bulletin of the Seismological Society of America, also raised the question of whether buildings in Singapore and Kuala Lumpur are adequately resilient.
The heaviest damage from earthquakes usually occurs near the point of rupture. But ground shaking can affect even high-rise buildings far away because low-frequency seismic waves travel great distances. Such waves are amplified in areas where the soil is soft.
Whether building vibrations from earthquake shaking lead to structural damage depends on various factors besides ground motion, including the strength of the building and its ductility capacity (how much it can bend or be deformed before it breaks).
A concrete structure, for example, is inherently brittle and susceptible to breaking under heavy ground agitation. Steel bars or other reinforcements in the walls, roofs and floors can make it more ductile under stress.
“Building codes in Singapore and KL were largely adopted from the British standards, which do not have provisions for seismic-resistant design,” says Prof Megawati. Singapore is moving toward the adoption of more stringent Eurocode standards, he notes. But it’s likely that much of the cities’ existing building stock lacks the kind of construction details that increase seismic protection.
As such, tall buildings in the two capital cities are “relatively brittle,” and “it is crucial to investigate their performance” under the kind of sustained ground shaking that would occur in a powerful Sumatran quake, the researchers conclude in their published report.
They call for engineering analyses to determine the performance of medium- and high-rise buildings in Singapopre and Kuala Lumpur under the scenario conditions. Approaches include direct monitoring of individual buildings, lab tests of structural elements and computer-based structural modelling.
The findings can help cities plan mitigation strategies such as retrofitting buildings in vulnerable areas to make them more earthquake-resilient.
