Segmentation and Large-Scale Nucleation of the 2014 Pisagua Earthquake Sequence
Abstract:The 2014 Mw 8.1 Pisagua, Chile earthquake featured a remarkable foreshock sequence. Foreshock migration at speed similar to 2011 Tohoku foreshocks and slow slip events has been interpreted as driven by aseismic slip, instead of a cascade (possibly mediated by aseismic afterslip). However, current analyses of geodetic and tiltmeter data illustrate the challenge to discriminate between pre-slip and cascade models through in-land deformation observations. Here we attempt to distinguish these models via seismic observations only, identify evidence of a large nucleation process and discuss its implications for fault friction and earthquake predictability.
We analyze patterns of foreshock activity, including repeating earthquake sequences identified via waveform similarity on the nearest IPOC stations. We find that repeaters accelerate close in time and space to large foreshocks and their recurrence times decay following Omori’s law. This favors the cascade model, in which repeaters are aftershocks of foreshocks. Limitations of current catalogs impede more quantitative tests.
Direct evidence of large-scale nucleation is a separate foreshock cluster north of the mainshock hypocenter whose activity is independent of the dominant southern swarm. Over almost one year, swarms occurred near the southern and northern tips of the mainshock rupture and near the southern end of the largest aftershock. These clusters are located near boundaries of relatively high seismic coupling inferred from geodesy.
A picture of a segmented megathrust emerges where regions of aseismic deformation mark the boundaries between fault segments and produce earthquake swarms. Earthquake cycle models indicate aseismic slip slowly penetrates from segment boundaries into locked regions, until a critical distance is reached and unstable rupture (seismic or aseismic) follows. This view is further supported by temporal changes of foreshock swarm migration speed. The early swarm migration distance provides constraints on friction parameters and effective normal stress.
Such large-scale nucleation process, while appealing for earthquake predictability, is rare at a global scale. The predictability of final rupture size based on foreshock swarms is also limited by poor quantification of the likelihood of multi-segment rupture.