We compute stress drops from P and S phase spectra for 534 earthquakes in the source region of the 2014 MW 8.1 Iquique megathrust earthquake in the northern Chilean subduction zone. An empirical Green’s function based method is applied to suitable event pairs selected by template matching of eight years of continuous waveform data. We evaluate the parameters involved in the stress drop estimation, consider the effect of the local velocity structure and apply an empirical linear relation between P and S phase related geometry factors (k values). Data redundancy produced by multiple EGFs and the combination of P and S phase spectra leads to a substantial reduction of uncertainty and robust stress drop estimates. The resulting stress drop values show a well-defined log-normal distribution with a median value of 4.36 MPa; most values range between 0.1-100 MPa. There is no evidence for systematic large scale lateral variations of stress drop. A detailed analysis reveals several regions of increased median stress drop, an increase with distance to the interface, but no consistent increase with depth. This suggests that fault regime and fault strength have a stronger impact on the stress drop behavior than absolute stresses. Interestingly, we find a weak time-dependence of the median stress drop, with an increase immediately before the April 1, 2014 MW 8.1 Iquique mainshock, a continuous reduction thereafter and a subsequent recovery to average values. Additionally, the data set indicates a relatively strong dependence of stress drop on magnitude which extends over the entire analyzed magnitude range.
A significant stress drop characterizes sometimes earthquakes induced by injection or extraction of fluids in rocks. Moreover, long-term fluid operations in underground reservoirs can impact a seismogenic reaction of the rocks per a unit volume of the involved fluid. The seismogenic index is a quantitative characteristic of such a reaction. We derive a relationship between the seismogenic index and the stress drop. We propose a simple and rather general phenomenological model of the stress drop of induced events in various faulting regimes. Our results suggest that high stress drops of some earthquakes induced by long-term underground fluid operations may be controlled by drops of cohesion of more cohesive faults getting seismically activated due to gradually increasing with time differential stresses. On the one hand, this effect can result in an increase of seismogenic index with production time. On the other hand, a production-caused depleting of the pore pressure can also cause a systematic increase of the stress drop. This provides an additional contribution to the growth of seismogenic index with production time at such reservoirs.