Implications of salt-related propagation and mode conversion effects on the analysis of induced seismicity

A number of natural gas fields in the north of the Netherlands show moderate seismicity induced by gas extraction. The gas reservoirs are located underneath a thick layer of Zechstein evaporites (salt). The presence of the salt has two important effects on the wave motions of induced events as recorded at the surface close to the epicenter. The first effect is the defocusing of seismic energy, with its implications on observed amplitudes and radiation patterns. The second effect is the relatively strong conversion from P- to S-energy at the bottom of the salt, leading to the presence of S-wave precursors. Failure to recognize these effects may lead to misinterpretation of source location and mechanism. Moreover, the S-wave precursors provide a handle to reduce uncertainty in depth estimation. We investigate the effects using moment tensor inversion and ray tracing for four recent events (M (L) = 2.6 - 3.5) in the Groningen field.