Assessing seismic site response, how small should we go?

Abstract:

The degree of damage on buildings due to earthquakes is strongly dependent on the properties of the subsurface at that specific site. The shallow geology of the Netherlands consists of a very heterogeneous soft sediment cover, which has a strong effect on seismic wave propagation characteristics and in particular the amplitude of ground shaking. When the velocity decreases from deeper layers towards the top soil, the seismic wave amplifies towards the surface. By studying local amplitude variations from seismic waves, we obtain constraints on the seismic hazard. Since the top layer, consisting of Pleistocene and Holocene sediments, can vary in composition every tens of meters, small-scale site response studies are of interest.

Gas extraction in the Groningen field, in the northern part of the Netherlands, is regularly causing shallow (3 km), low magnitude (Mw max= 3.6), induced earthquakes. This region forms an excellent study area due to the permanent presence of a shallow (200m) borehole network containing 70 stations with approximately 4km interstation distance. In addition, flexible networks, consisting of around 400 surface geophones, with a spacing of 350-400 meter were deployed at several locations across Groningen for the period of one month. Detailed knowledge of the geology for the upper 50 metre of the subsurface is available to assess the influence of the local geology on the wave properties measured at several scales.

For each geophone in the multiple networks, horizontal-to-vertical spectral ratios from ambient noise measurements and local earthquakes are compared with the geological subsurface composition. Here we present relationships between the amplitude spectra and the corresponding geology to assess small-scale site response. Subsequently, we investigate the differences in amplification across the networks by constructing spatial correlation functions.