REMOTE TRIGGERING OF DISPLACEMENT EVENTS ALONG THE AFZ (N-CHILE) MONITORED WITH THE IPOC CREEPMETER ARRAY

The relationship between the megathrust seismic cycle and the activation of large trench parallel faults in the overriding plate is not well understood. The Atacama Fault System (AFS) is the dominant trench parallel fault in N-Chile with an along strike extent of > 1000 km. The well-exposed fault scarp evidences that long-term creep, as well as surface rupturing due to seismic events contribute to displacement accumulation through time. Paleoseismological studies show that at least 3 earthquakes with an estimated magnitude of up to M_w=7 occurred in the past 10 kyr. This implies that additionally to the megathrust, the AFS is capable in generating large shallow earthquakes and needs to be included in hazard assessment of the N-Chilean forearc. In order to characterize the mode of deformation accumulation and its spatio-temporal distribution, we are continuously monitoring active fault strands of the AFS with an array of 11 creepmeters with a resolution of 1 μm at a period of 30 sec.

Over the first two years the creepmeters recorded a creep signal equivalent to extensional displacement across the fault zone. The displacement rate ranges between 0.02 mm/yr and 0.1 mm/yr and is less than the geological long-term rate of +/- 0.3 mm/yr. This signal is superimposed by sudden displacement events temporally related to subduction zone earthquakes in the near and far field depending on the magnitude of the event. The most prominent event recorded at all creepmeters was the Mw=8.8, 2011 Maule Earthquake with an epicentral distance of 1800 km. Correlation with records of IPOC seismometers shows that the displacement events have been triggered at the time of passage of the surface waves. The displacement events lasted a few minutes; only one station recorded a creep event of 6 hours. Displacements ranged between 10 – 63 μm. This observation demonstrates that besides ground shaking on a nearby fault large events in a distance of thousands of kilometres can transiently increase the shear stress and/or reduce the friction on a shallow fault, permitting it to slip incrementally. Therefore remote triggering of faults contributes a large fraction to the total strain accumulation and depending on the preexisting stress level of the AFS needs to be taken into account in hazard assessment.