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Abstract Giant earthquakes with magnitudes above 8.5 occur only in subduction zones. Despite the developments made in observing large subduction zone earthquakes with geophysical instruments, the factors controlling the maximum size of these earthquakes are still poorly understood. Previous studies have suggested the importance of slab shape, roughness of the plate interface contact, state of the strain in the upper plate, thickness of sediments filling the trenches, and subduction rate. Here, we present 2‐D cross‐scale numerical models of seismic cycles for subduction zones with various geometries, subduction channel friction configurations, and subduction rates. We found that low‐angle subduction and thick sediments in the subduction channel are the necessary conditions for generating giant earthquakes, while the subduction rate has a negligible effect. We suggest that these key parameters determine the maximum magnitude of a subduction earthquake by controlling the seismogenic zone width and smoothness of the subduction interface. This interpretation supports previous studies that are based upon observations and scaling laws. Our modeling results also suggest that low static friction in the sediment‐filled subduction channel results in neutral or moderate compressive deformation in the overriding plate for low‐angle subduction zones hosting giant earthquakes. These modeling results agree well with observations for the largest earthquakes. Based on our models we predict maximum magnitudes of subduction earthquakes worldwide, demonstrating the fit to magnitudes of all giant earthquakes of the 20th and 21st centuries and good agreement with the predictions based on statistical analyses of observations.

The acceleration response spectrum in Singapore from a potential giant earthquake, having an Mw of 8.8, in the Mentawai segment of the Sumatran megathrust is estimated using a recently derived attenuation relationship. Two sets of ground motions recorded at a rock-site station in Singapore are scaled to match the target μ + σ spectrum. The ground motions at two typical soft-soil sites in Singapore are subsequently generated to study the local site effects. Two generic models, namely a 15-story slab block and a 30-story point block, are constructed based on structural systems commonly used in Singapore. The results from dynamic response analysis of the models indicate that the 15-story model will not suffer damage due to the simulated ground motions on soft-soil sites, while the 30-story generic model will have fine cracks in the masonry walls on the upper floors. However, both models show that structural damage is less likely. The floor acceleration responses of the 15-story model imply that occupants on the higher floors would experience vibration corresponding to Modified Mercalli Intensity (MMI) scale of V, while those living on the higher floor of the 30-story building may be subjected to MMI scale of VI. This level of vibration may cause some damage to non-structural components and overturning of unstable objects. [ABSTRACT FROM AUTHOR]

Abstract: Very high resolution reflection seismic profiling (3.5 kHz) revealed nine Holocene mass-wasting events in Lake Puyehue (South-Central Chile). These events are made up of numerous coeval mass-wasting deposits and some homogenites, which are radiocarbon-dated. The two youngest mass-wasting events could be attributed to the giant AD 1960 and AD 1575 Valdivia earthquakes. The most extensive event took place around 1660 cal yr BP. Evaluation of all possible slope failure processes led us to infer that giant earthquakes, roughly comparable to the AD 1960 Valdivia earthquake (M w =9.5), are the most likely trigger mechanisms of all mass-wasting events in Lake Puyehue. These occurred with a mean recurrence rate of 1000 yr although relatively aperiodically (ranging between 500 and 2000 yr). Quantitative comparison of mass-wasting events related to historically reported earthquakes (AD 1960 and AD 1575) showed significant differences although these earthquakes are assumed to have had a comparable strength. A lowered background sedimentation rate could be responsible for this variable earthquake recording, which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis. [Copyright &y& Elsevier]