Transient deformations associated with foreshocks activity has been observed before large earthquakes, suggesting the occurrence of a detectable pre-seismic slow slip during the initiation phase. In this respect, a critical issue consists in discriminating the relative contributions from seismic and aseismic fault slip during the preparation phase of large earthquakes. We focus on the April-May 2017 Valparaíso earthquake sequence, which involved a Mw=6.9 earthquake preceded by an intense foreshock activity. To assess the relative contribution of seismic and aseismic slip, we compare surface displacements predicted from foreshock source models to the transient motion measured prior to the mainshock. The comparison between observed and predicted displacements shows that only half of the total displacement can be explained by the contribution of foreshocks. This result suggests the presence of aseismic preslip during an initiation phase preceding the mainshock.

The subduction zone of the Cocos Plate beneath Southern Mexico has major variations in the megathrust geometry and behavior. The subduction segment beneath the Oaxaca state of Mexico has relatively frequent large earthquakes on the shallow part of the megathrust and within the subducting slab, and it also has large aseismic slow-slip events. The slab geometry under Oaxaca includes part of the subhorizontal “flat-slab” zone extending far from the trench beneath southern Mexico and the beginning of its transition to more regular subduction geometry to the southeast. We study the rupture of the 16 February 2018 Mw 7.2 Pinotepa earthquake near Pinotepa Nacional in Oaxaca that was a thrust event on the subduction interface. The Pinotepa earthquake was about 350 km away from the 8 September 2017 Mw 8.2 Tehuantepec earthquake in the subducting slab offshore Oaxaca and Chiapas; it was in an area of Coulomb stress decrease from the M8.2 quake, so it seems unlikely to be a regular aftershock and was not triggered by the static stress change. Geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and time-series analysis of GPS station data constrain finite-fault slip models for the M7.2 Pinotepa earthquake. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite. Our Bayesian (AlTar) static slip model for the Pinotepa earthquake shows all of the slip confined to a very small (10-20 km diameter) rupture, similar to some early seismic waveform fits. The Pinotepa earthquake ruptured a portion of the Cocos megathrust that has been previously mapped as partially coupled and shows that at least small asperities in that zone of the subduction interface are fully coupled and fail in high-stress drop earthquakes. The previous 2012 Mw 7.4 Ometepec earthquake is another example of asperity in the partially coupled zone but was not imaged by InSAR so the rupture extent is not so well constrained. The preliminary NEIC epicenter for the Pinotepa earthquake was about 40 km away (NE) from the rupture imaged by InSAR, but the NEIC updated epicenter and Mexican SSN location are closer. Preliminary analysis of GPS data after the Pinotepa earthquake indicates rapid afterslip on the megathrust in the region of coseismic slip. Atmospheric noise masks the postseismic signal on early InSAR data.

One of the key questions in fault mechanics is how do earthquakes begin ? This is central to our understanding of earthquakes, including the long controversial issue of their predictability. Earthquakes preceding large events are commonly referred to as foreshocks. They are often considered as precursory phenomena reflecting a nucleation process of the main rupture potentially driven by an underlying slow pre-slip. On the other hand, some studies suggest that foreshock sequences may only be explained by cascades of triggered events. In this work, we choose to test the cascading hypothesis against the foreshock seismicity observed during a previously reported slow slip event preceding the 2017 Mw=6.9 Valparaiso earthquake. We build a very complete 5 years long earthquake catalog of the sequence using refine detection and location algorithms. We test the detected seismicity against the Epidemic Type Aftershock Sequences model. We identify time windows with anomalously high seismic activity compared to what is expected by the typical earthquake interactions. We analyze statistically these anomalies over 5 years to understand if the Mw=6.9 foreshock sequence presents a specific anomalous activity. In addition, using a hierarchical clustering method, we identify earthquakes with similar waveforms to evidence any repeating ruptures. We analyze the time distribution of these clusters over 5 years to understand if unusual rates of repeating events emerge during foreshock time ranges. The conjoint analysis of seismicity rate anomalies and repeating events along with the results of previous pre-slip studies allows to accurately describe the nucleation process and evolution of the 2017 Valparaiso earthquake sequence.