<p>A number of natural gas fields in the North of the<br /> Netherlands show moderate seismicity induced by gas extraction.<br /> The gas reservoirs are located underneath a thick<br /> layer of Zechstein evaporites (salt). The presence of the salt<br /> has two important effects on the wave motions of induced<br /> events at the surface. The first effect is defocusing of seismic<br /> energy in upward direction with effects on amplitudes<br /> and radiation patterns. The second effect is relatively efficient<br /> conversion of P- to S-energy at the bottom of the salt<br /> leading to the presence of S-wave precursors. Failure to recognize<br /> these effects may lead to misinterpretation of source<br /> location and mechanism. Moreover, the S-wave precursors<br /> provide a handle to reduce uncertainty in depth estimation.</p>