The Controls on Earthquake Ground Motion in Foreland-Basin Settings: The
Effects of Basin and Source Geometry
Abstract
Rapid urban growth has led to large population densities in foreland
basin regions, and therefore a rapid increase in the number of people
exposed to hazard from earthquakes in the adjacent mountain ranges. It
is well known that ground shaking is amplified in sedimentary basins.
However, questions remain regarding the main controls on this effect. It
is, therefore, crucial to identify the main controls on earthquake
shaking in foreland basins as a step towards mitigating the earthquake
risk posed to these regions. We model seismic-wave propagation (using
SW4 finite difference code) from range-front thrust-faulting earthquakes
in a foreland-basin setting. The basin geometry (depth and width) and
source characteristics (fault dip and source-to-basin distance) were
varied, and the resultant ground motion was calculated. We find that the
source depth determines the amount of near-source ground shaking and the
basin structure controls the propagation of this energy into the
foreland basin. Of particular importance is the relative length scales
of the basin depth and dominant seismic wavelength (controlled by the
source characteristics), as this controls the amount of dispersion of
surface-wave energy, and so the amplitude and duration of ground motion.
The maximum ground motions occur when the basin depth matches the
dominant wavelength set by the source. Basins that are shallow compared
with the dominant wavelength result in low-amplitude and long-duration
dispersed waveforms. However, the basin structure has a smaller effect
on the ground shaking than the source depth and geometry, highlighting
the need for understanding the depth distribution and dip angles of
earthquakes when assessing earthquake hazard in foreland-basin settings.