Characterization of the near-surface shear wave attenuation in the Groningen gas field using borehole recording

The Groningen gas field located in the northern Netherlands is the largest gas field in Europe. Many induced earthquakes have occurred since the gas extraction commenced in the 1960s, especially in the last two decades, which have caused concerning social and economic problems. To better quantify the seismic hazards caused by the induced earthquakes, the near surface attenuation in the Groningen area needs to be characterized from actual earthquakes. The spectral ratio method based on multiwindow analysis has proven to be a powerful tool to estimate the S-wave attenuation parameters of shallow sediments. In this study, waveforms from earthquake events recorded by the dense network of 70 boreholes, each of which is equipped with four geophones are used to estimate the S-wave attenuation parameters of shallow sediments. There are strong lateral variations in the near-surface attenuation, which is structurally similar to the S-wave model from a previous study in the same area. Besides, the calculated values are inversely proportional to the frequency and depth. The averaged values for depths between 0-150 m and 0-200 m are close to those of the soft soil Sendai basin in Japan. The values from this study have been used to model a surface peak ground acceleration (PGA) map. As expected, both near surface values and source mechanism have a profound effect on the modelled PGA.