PRELUDE TO THE APRIL 1, 2014 PISAGUA, CHILE GREAT EARTHQUAKE: POTENTIAL LESSONS FOR CASCADIA
The foreshocks that culminated with the Pisagua earthquake began some 36 km to the SSW and two weeks before the main shock with a Mw6.7 foreshock. We relocated the foreshocks using local network data from the Integrated Plate boundary Observatory Chile processed using both NonLinLoc and HypoDD to develop a local catalog of 336 events that occurred between March 15th and April 4th. Our relocation of the Mw6.7 foreshock indicates that its epicenter has a longitude and latitude of -70.902°, -19.904°. Its depth, determined from NonLinLoc, HypoDD and waveform analysis is 15±4.6 km, 13±1 km, and 14 km, respectively, indicating that it may have been an upper plate event as the megathrust is 20 km deep here according to the SLAB 1.0 model and data from the ANCORP and CINCA projects. The Mw6.7 moment tensor yields nodal planes that strike WNW highly oblique to the margin and the slab: P-axes plunge ~30° towards 210° to 220°, deviating by 35° or more from the P-axes from events clearly on the megathrust. Thus, the Pisagua earthquake sequence was initiated by an earthquake on an upper plate reverse fault orthogonal to the plate boundary. Stress change analysis indicates that the megathrust activity was triggered by unclamping of the slab surface by the upper plate fault.
Both northern Chile and Cascadia have well known Quaternary EW-striking reverse faults in the onshore forearc. These upper plate faults appear capable of triggering great earthquakes on the plate interface, as well as being triggered by megathrust activity, and thus constitute both primary and secondary hazards.