Stress Drop Variations in the Region of the 2014 MW8.1 Iquique
Earthquake, Northern Chile
Abstract
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.