The IPOC Creepmeter Array in N-Chile: Monitoring Slip Accumulation Triggered By Local or Remote Earthquakes

The Atacama Fault System (AFS) is an active trench-parallel fault, located above the down-dip end of coupling of the north Chilean subduction zone. About 3 M=7 Earthquakes in the past 10 ky have been documented in the paleoseismological record, demonstrating the potential of large events in the future. To investigate the current surface creep rate and to deduce the mode of strain accumulation, we deployed an array of 11 creepmeters along four branches of the AFS. This array monitors the interaction of earthquake activity on the subduction zone and a trench-parallel fault in the overriding forearc. The displacement across the fault is continuously monitored with 2 samples/min with a resolution of 1μm. Collocated seismometers record the seismicity at two of the creepmeters, whereas control of the regional seismicity is provided by the IPOC Seismological Networks.

Continuous time series of the creepmeter stations since 2009 show that the shallow segments of the fault do not creep permanently. Instead the accumulation of permanent deformation occurs by triggered slip recorded as well-defined steps caused by local or remote earthquakes. The 2014 M_w=8.2 Pisagua Earthquake, located close to the creepmeter array, triggered large displacement events on all stations. Another event recorded on all stations was the 2010 M_w=8.8 Maule earthquake located 1500km south of the array. All of the stations showed a triggered displacement event 6-8 min after origin time of the main shock, at the same time as the arrival of the surface waves recorded at nearby IPOC stations. This points to a dynamic triggering process caused by transient stresses during passage of the surface wave. Investigation of seismic events with Magnitudes <6 show displacement events triggered during P and S wave passage, pointing to static as well as dynamic stress changes for proximal events.

Analyzing the causative earthquakes we find that the most effective way to trigger displacement events on the AFS are deep (>100km) earthquakes on the subduction zone interface up to 300 km east of the array. Earthquakes located to the west of the AFS on the locked part of the subduction zone interface rarely trigger displacement events on the AFS. Only if such events are as large as the Pisagua Earthquake or its Mw= 7,6 aftershock, they trigger large displacement events.