A shallow shear velocity model for Groningen

Abstract

Depletion of the Groningen gas field in the Netherlands causes induced earthquakes. To monitor the seismicity in the area, a large network of borehole seismometers was installed early 2015. Each bore- hole consists of an accelerometer at the surface and four equally spaced geophones up to a depth of 200 meters. To estimate the probabilistic seismic hazard, a Ground Motion Prediction Equation (GMPE) was developed in a previous study. GMPEs are empirical equations linking geophysical properties of the crust to surface ground motions. A very important element in the GMPE is the shallow shear velocity structure. For the current model, different methods are used to estimate the shear velocities at different depth intervals. In this study, vertical compres- sional and shear wave velocity profiles are calculated for each of the boreholes using passive seismic interferometry. This method makes use of data recordings of local seismicity. The results are seismic velocity profiles for each of the borehole stations in the network. For the four different depth levels, the station results are interpolated to make contour maps. These maps correlate well to results from previous work. An important part of this study was to develop a method to estimate the horizontal orientations of the borehole geophones, which are required to derive the shear wave velocities using seismic interferometry. This method is based on cross-correlations of the geophone and the co-located surface accelerometer. In addition, by analysing data from the new network on a level that had not previously been done, several installation errors could be found and resolved.