Evidence of an aseismic slip continuously driving the 2017 Valparaiso
earthquake sequence
Abstract
Following laboratory experiments and friction theory, slow slip events
and seismicity rate accelerations observed before mainshock are often
interpreted as evidence of a nucleation phase. However, such precursory
observations still remain scarce and are associated with different time
and length scales, raising doubts about their actual preparatory nature.
We study the 2017 Valparaiso Mw= 6.9 earthquake, which was preceded by
aseismic slip accompanied by an intense seismicity both suspected to
reflect its nucleation phase. We complement previous observations, which
have focused only on precursory activity, with a continuous
investigation of seismic and aseismic processes from the foreshock
sequence to the post-mainshock phase. By building a high-resolution
seismicity catalog and searching for anomalous seismicity rate increases
compared to aftershock triggering models, we highlight an
over-productive seismicity starting within the foreshock sequence and
persisting several days after the mainshock. Using repeating earthquakes
and high-rate GPS observations, we highlight a transient aseismic
perturbation starting just before the first foreshock and extending
continuously after the mainshock. The estimated slip rate is lightly
impacted by large magnitude earthquakes and does not accelerate towards
the mainshock. Therefore, the unusual seismic and aseismic activity
observed during the 2017 Valparaiso sequence might be interpreted as the
result of a slow slip event starting before the mainshock and extending
beyond it. Rather than pointing to a possible nucleation phase of the
2017 Valparaiso mainshock, the identified slow slip event acts as an
aseismic loading of nearby faults, increasing the seismic activity, and
thus the likelihood of a large rupture.