The effects of large-scale geological characteristics on the average spatial pattern of earthquake-induced ground motions

This paper investigates the predominant effects of sub-surface geological characteristics on the earthquake-induced ground-motion properties relevant to the design of infrastructure systems in urban environments. By considering ensembles of different earthquake scenarios and conducting numerical simulations to generate surface ground motion realizations, the contributing factors of earthquake source and earth properties in shaping the spatial pattern of ground motion amplitudes are scrutinized. Physics-based ground-motion simulations are conducted for 28 earthquake scenarios with moment magnitudes of 5.0 and 6.0 triggered with different azimuthal and geometrical properties. The earth wave-propagation properties are defined by considering data and empirical relationships that represent a typical geological setting with depth crustal rock and soft sedimentary basin (including a river channel). The spatial pattern of ground motion intensity measures (defined as the geometrical mean of the two horizontal pseudo-spectral accelerations) is used to show the average spatial pattern of ground motion severity. The results demonstrate that, even though the spatial ground motion patterns for a specific scenario earthquake depend on both the sub-surface geology and the source properties, the sub-surface geological characteristics impose a deterministic impact on the average spatial pattern of ground motions regardless of the earthquake location, azimuthal and geometrical properties. This clearly indicates that the regional seismic hazard assessments should allocate further resources for determining the sub-surface earth properties as they can disproportionally alter urban designs in contrast to the conventional concern on determining the location of probable future earthquakes and their small-scale characteristics.