Slow slip events precursory to the 2014 Iquique Earthquake, revisited with long-base tilt and GPS records

Abstract

The M=8.1, April 1st, 2014 Iquique earthquake, which broke part of the northern Chile seismic gap, was preceded by a strong foreshock sequence starting early January 2014.The reported analysis of the continuous records of the nearby GPS stations from the IPOC North Chili array lead to contradictory results concerning the existence and location of slow slip events (SSE) on the interplate contact. Resolving this controversy is an important issue, as although many SSEs are reported in subduction zones, only a few were found to be precursory to large earthquakes. Here we show that the records of a long base tiltmeter installed near Iquique, when corrected for coseismic steps, long term drift, tidal signals, and oceanic and atmospheric loading, show significant residual signals. These 2 F. Boudin et al.can be modelled with a sequence of four SSEs located close to Iquique. Their signature was already reported on some GPS stations, but their source was then characterized with a very low resolution in time and space, leading to contradicting models. With the tilt records, we can rule out the previously proposed models with a single large SSE closer to the mainshock. Combining tilt with GPS records greatly improves the resolution of GPS alone, and one could locate their sources 100 to 180 km south-southeast to the mainshock epicenter, with moment magnitudes between 5.8 and 6.2, at the edge of the main aftershock asperities. These moderate SSEs thus did not directly trigger the mainshock, but contributed to trigger the main foreshock and the main aftershock. Only the sensitivity and resolution of the tiltmeter, added to the GPS records, allowed us to describe with unprecedented accuracy this precursory process as a cascade of cross-triggered, short term aseismic slip events and earthquakes on the interplate contact. This three months of precursory activation appears to be the final acceleration burst of a weaker, longer term SSE which started mid-2013, already reported, with a moment release history which we could quantify. From the methodological point of view, our study takes advantage of an interesting complementarity of tilt and GPS measurements, due to their different dependence in distance to the source of strain, which turns out to be very efficient for resolving location and moment of strain sources, even when both instruments are close to each other.It finally demonstrates the efficient removal of sequences of small or even undetected coseismic steps from high resolution tilt record signal in order to retrieve the purely aseismic signal, a presently impossible task for high time resolution GPS records due to low signal to noise.

A 20 year-long GNSS solution across South-America with focus in Chile

“…These can have variable duration and potentially be recurrent, as in the region of Copiapo (Klein et al, 2018). In some cases, they can precede earthquakes and contribute to the initiation of the main shock rupture, as for the 2014 Iquique earthquake (Ruiz et al, 2014, Socquet et al, 2017, Boudin et al, 2021 or the 2017 Valparaiso earthquakes (Ruiz et al, 2017). Some observed transient signal remain hard to relate to any known sources.…”

Bayesian inference on the initiation phase of the 2014 Iquique, Chile, earthquake

“…However, Boudin et al (2019) argue that adding to the GNSS data a tilt records from Santa Rose, Chile (∼8 km north-east of the station IQQE) changes the spatio-temporal resolution in such a way that it rules out the occurrence of a slow-slip event with magnitude larger than 6.6 near the epicenter of the future mainshock. Instead, adding the tilt records suggests that the slow-slip activity occurs more to the south (around 20.5 • S) compared to what we have identified in this study (around 20.0 • S ).…”

“…To test the sensitivity of the tilt records with respect to the slow-slip event from this study, we have computed the amount of tilt generated by the family of models for our slow-slip event (see Supplementary Materials S3). The main difference with Boudin et al (2019) is that the computation of the tilt is done using a layered velocity model for the region, the same that we have used to obtain our models. Our findings suggest that the final amplitude of the tilt generated by the slow-slip event from this study barely exceeds the noise level reported by Boudin et al (2019) -48 and 42 nrad for the north and east components, respectively.…”

“…The main difference with Boudin et al (2019) is that the computation of the tilt is done using a layered velocity model for the region, the same that we have used to obtain our models. Our findings suggest that the final amplitude of the tilt generated by the slow-slip event from this study barely exceeds the noise level reported by Boudin et al (2019) -48 and 42 nrad for the north and east components, respectively. Thus, it is possible that adding the tilt records bias the resolution towards the region near the tilt meter, allowing the identification of additional slow-slip events, but missing the one near the epicenter of the Iquique earthquake because of the small signal it generates at the tilt meter when compared to the noise level.…”

“…Some authors suggested that the primary cause of the pre-mainshock geodetic motion was seismic slip induced by the numerous foreshocks recorded over the couple of weeks preceding the Iquique earthquake (Schurr et al, 2014), and that any potential aseismic slip would correspond to the post-seismic phase following these foreshocks (Bedford et al, 2015). Using tilt records along the GNSS data corrected from the contribution of the foreshocks, Boudin et al (2019) also reached the conclusion that it was unlikely that a large slow-slip occurred near the epicentral region of the future Iquique earthquake, and that if so, it should have a magnitude of less than 6.6. From that angle, the initiation phase was more likely to be in line with the cascade model.…”