We compute stress drops from P and S phase spectra for 972 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 carefully the influence of all 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 empirical Green’s functions, the combination of P and S phase spectra and a distributed high quality station network leads to a substantial reduction of uncertainty and comparatively robust stress drop estimates. The resulting stress drop values show a well-defined log-normal distribution with a median value of 2.7 MPa, most values range between 0.1 MPa and 100 MPa. There is no evidence for systematic large scale lateral variations of stress drop. A detailed analysis reveals a slight increase of the median stress drop with distance to the interface, but no increase with depth. This suggests that fault regime and fault strength play a more important role for 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 normal values after a few weeks.
