HIGH RESOLUTION DISPLACEMENT MONITORING AT UPPER PLATE FAULTS IN THE N-CHILEAN CONVERGENT PLATE BOUNDARY

The Chilean convergent plate boundary belongs to the tectonically most active regions on earth and is a source area for large megathrust earthquakes. Historical data suggest a seismic gap in the northern part of the Chilean plate boundary which has not ruptured since 1877 (Iquique segment). In this study we explore the time series of displacement recorded with an array of 11 creepmeters since over two years. A main question is how strain is accumulated at upper plate faults during the megathrust seismic cycle and which processes are responsible for strain accumulation.

The creepmeter array has been installed at 4 upper plate faults identified as active fault strands of the Atacama fault system. The monitored faults include dip-slip (Mejillones Fault - MF, Salar del Carmen Fault - SCF) as well as strike-slip faults (Cerro Fortuna Fault - CFF, Chomache Fault - CF). The SCF is located in a forearc segment that ruptured in the 1995 Mw 8.0 Antofagasta earthquake. The other faults are situated in the Iquique segment partially overlying the rupture area of the 2007 Mw 7.8 Tocopilla earthquake. This allows the direct comparison of upper plate faults located in different segments of the forearc undergoing different stages of the megathrust seismic cycle.

The total displacement rates range from 4 µm/year (MF) up to 75-80 µm/year (SCF). Total displacement is composed of steady state creep, creep events and sudden displacement events (SDEs). The percentage of SDEs is over 90 % of the cumulative displacement for the CFF. Data from SCF show that 50-60 % of the cumulative displacement results from SDEs. This implies a clear variation in strain accumulation pattern of the studied fault zones, potentially depending on the location in the forearc.

Combined for all installations, 52 % of the SDEs appear to be triggered during the passage of seismic surface waves. On the other hand, 48 % of the SDEs occur without temporal correlation to an earthquake. They are potentially silent slip events, if other effects e.g. site effects or seasonal effects can be ruled out. This work in progress will concentrate on the discrimination of parameters controlling the strain accumulation pattern at upper plate faults related to seismic activity in the forearc system.