Soil Amplification Factor of Seismic Ground Motions in Laho (PMO), Kumang (SKO) and Sumandak (SBO) in Malaysian Offshore

Distant earthquakes in Sumatra and Sulawesi might possess hazard to the offshore
platforms in Malaysia underlain by soft soils as the soils might amplify the seismic
waves that reach the bedrock of Malaysia. A study of the soil amplification factors in
PMO, SKO and SBO is necessary to reduce the downtime loses and onsite casualties
of the offshore platforms. This research is one of the pioneers for Malaysian
offshores. Thus, Laho in PMO, Kumang in SKO and Sumandak in SBO are selected
as three representative case studies for this research. The shear wave velocities of the
soils at the selected sites are estimated using published cone penetration test
correlation equation. The input motion is based on the strongest earthquake recorded
in MMD station near to the selected sites between year 2004 and 2007. A second
analysis is conducted by scaling the earthquake recorded to 0.06g to simulate the
shaking caused by an earthquake with a return period of 475 years in Malaysia. Onedimensional
equivalent linear site response analysis is selected and performed by
DEEPSOIL v5.1. The unscaled earthquakes (PGA= 0.0015g) generate high
amplification factors but they are associated with very low levels of earthquake
shaking which hardly harm the structure on site. The scaled earthquakes (0.06g)
generate relatively lower amplification factors which are caused by the nonlinear
behaviour of soils. The scaled earthquake also generates a peak spectral
amplification at a longer period. However, the lower amplification factors of 1.6
(period 0.4s-2.0s) and 3.5 (period 3.5s) in Sumandak should be taken into account in
the design of structure because they are associated with PGA of 0.06g which is
significant enough to damage the platforms on site, especially if the seismic waves
are amplified and the structure has a natural period close to the amplified periods.
This research also shows that the impedance contrast of shear wave velocities at the
boundary of two soil layers amplifies the seismic waves. Besides, the soil
amplification factors depend on the intensity of shaking