Multiple Slow-Slip Events Leading up to the 2014 Iquique, Chile Mw 8.1 Earthquake

The 1 April 2014 Iquique, Chile Mw 8.1 earthquake occurred at 23:46(UTC) along a mega-thrust fault off northernmost Chile, where the Nazca plate is subducting beneath the South America plate at a convergence rate of ~8 cm/year. Northernmost Chile has been quiescent for 136 years since the 1877 M 8.6 earthquake, resulting in a current "North Chile seismic gap" stretching for ~450 km length along the strike of the plate boundary. According to the US Geological Survey (USGS) catalog, the mainshock was preceded by intensive foreshock sequence lasting around 15 days. To obtain a precise record of the foreshock sequence before the 2014 Iquique, Chile Mw 8.1 earthquake, we applied a matched-filter technique to continuous seismograms recorded near the source region. We newly detected about 10 times the number of seismic events listed in the routinely constructed earthquake catalog, and identified multiple sequences of earthquake migrations at speeds of 2-10 km/day, both along-strike and down-dip on the fault plane, up-dip of the mainshock area. In addition, we found out repeating earthquakes from the newly detected events, likely indicating aseismic slip along the plate boundary fault during the foreshock sequence. These observations suggest the occurrence of multiple slow-slip events up-dip of the mainshock area. The final slow-slip event, which had the fastest migration speed, migrated toward the mainshock nucleation point. We interpret that several parts of the plate boundary fault perhaps experienced slow slip, causing stress loading on the prospective largest slip patch of the mainshock rupture (Kato and Obara, 2014).