Using waveform simulations to help constrain kinematics of small earthquakes at the Groningen gas field

Synthetic earthquake waveform modeling using finite difference is a good tool to strengthen our understanding of the wave-field as recorded on the surface as constrained by the source model and local geology. It also provides a strong framework for which models and methodologies can be tested and compared to real world observations for validity. Here we use waveform modeling to test the applicability of using back-projection methods to constrain the kinematics of the fault rupture for small (<M3.5) earthquakes. Of particular interest is how network array design and resolution of subsurface geology effect the precision and accuracy of such methods. The modeling incorporates the local geology using a high resolution detailed 3D velocity model from well data and 3D seismic to accurately reproduce source to receiver path effects. The source is modeled as a 2D grid of discretized source points, double couple moment tensor, that represent a finite fault surface. The kinematics of the fault rupture is controlled by the timing of each source point along the fault surface. It's our conclusion for back-projection to be successful both: 1) a strong understanding of the sub-surface is needed to accurately reproduce a 3-D velocity model and 2) a very dense network of receivers is needed to accurately image the kinematics.

The motivation behind this work is a means of better constraining the regional seismic hazard from induced earthquakes at the Groningen gas field. The Groningen gas field is one of the largest producing fields in Europe. Over the past decade the pore pressure decrease in the reservoir has led to an increase in low magnitude seismicity. To date the earthquakes have been isolated to normal faults within the relatively shallow reservoir interval. In assessing the hazard posed by this increase in seismicity it's important to understand the extent to which these faults can rupture beyond the reservoir into deeper sections of the basin and potentially into the basement. Understanding the kinematics of fault rupturing in the Groningen gas field can be used to help constrain whether and to what extent faults can rupture into the sub-reservoir section.