Earthquakes preceding large events are commonly referred to as foreshocks. They are often considered as precursory phenomena reflecting the nucleation process of the main rupture. Such foreshock sequences may also be explained by cascades of triggered events. Recent advances in earthquake detection motivates a reevaluation of seismicity variations prior to mainshocks. Based on a highly complete earthquake catalog, previous studies suggested that mainshocks in Southern California are often preceded by anomalously elevated seismicity. In this study, we test the same catalog against the Epidemic Type Aftershock Sequence model that accounts for temporal clustering due to earthquake interactions. We find that 10/53 mainshocks are preceded by a significantly elevated seismic activity compared with our model. This shows that anomalous foreshock activity is relatively uncommon when tested against a model of earthquake interactions. Accounting for the recurrence of anomalies over time, only 3/10 mainshocks present a mainshock‐specific anomaly with a high predictive power.

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• We reevaluate previous reports of significantly elevated seismic activity prior to large earthquakes in southern California • Accounting for temporal clustering of earthquakes, we find that less than 10% mainshocks are preceded by anomalous foreshock sequences. • The other sequences are explained by background seismicity, cascades of foreshocks or by recurrent fluctuations in seismicity rate.

<p>Both laboratory experiments and friction theory predicts that earthquake ruptures do not begin abruptly but are preceded by an aseismic slip acceleration over a finite nucleation zone. Such a nucleation phase may also trigger precursory ruptures known as foreshocks. Therefore, the scalability of the nucleation phase and its detectability before actual earthquakes is an important question with direct implications for earthquake prediction and seismic hazard assessment. Both Slow Slip Events (SSEs) and seismicity rate increase have already been identified before a few large earthquakes and are often interpreted as evidence of their nucleation process. However, such observations still remain scarce and are associated with different characteristic lengths that raise doubt on the actual preparatory nature of these signals. Here, we further study the case of the 2017 Valparaiso Mw= 6.9 earthquake that was preceded both by an SSE and an intense seismicity suspected to reflect the nucleation phase. We further investigate seismic and aseismic interplay over the complete earthquake sequence, from foreshock up to post-mainshock times, to search for a possible connection with the mainshock occurrence. For that, we build a high-resolution catalog (Mc=2) of the region using cutting edge picking tools, reporting more than 100 000 events from 2015 to 2021 (compared with the ~8000 events reported by the Centro Sismológico Nacional over the same time-period). First, we search for anomalous seismicity rate increases in the vicinity of the mainshock compared to usual earthquake to earthquake triggering models. Using a modified Epidemic Type Aftershock Sequences model that accounts for short-term incompleteness (Hainzl 2021) and MISD declustering (Marsan and Lengliné 2008), we highlight a significant over-productive earthquake rate starting within the foreshock sequences and persisting continuously after the mainshock for several days. Then, thanks to repeating earthquakes, we show that the slow slip event is continuously decelerating from the foreshock sequences up to months after the mainshock. The estimated slip rate is lightly impacted by large magnitude occurrences and does not accelerate toward the mainshock or any large magnitude earthquake. The slip estimate from repeaters is also compared with original high-rate GPS observations during the complete 2017 sequence, further supporting the continuity of the slow slip from the foreshock up to post-mainshock times. The joint observation of an SSE and a transient background seismicity continuously from the foreshock up to post mainshock suggests a close connection between the SSE and the seismicity. Results suggest that the unusual seismic and aseismic activity observed do not reflect the nucleation phase accelerating to the mainshock dynamic rupture. The SSE would rather underlie the complete 2017 earthquake sequence, mediating a part of the seismicity, possibly by stress transfer. The resulting seismicity is then further enhanced with usual earthquake to earthquake triggering, building up the sequence. This suggests that high resolution analysis of seismic and aseismic processes over the complete earthquake sequence is needed to properly assess the significance of signals preceding mainshocks.</p>

Following laboratory experiments and friction theory, slow slip events and seismicity rate accelerations observed before mainshock are often interpreted as evidence of a nucleation phase. However, such precursory observations still remain scarce and are associated with different time and length scales, raising doubts about their actual preparatory nature. We study the 2017 Valparaiso Mw= 6.9 earthquake, which was preceded by aseismic slip accompanied by an intense seismicity both suspected to reflect its nucleation phase. We complement previous observations, which have focused only on precursory activity, with a continuous investigation of seismic and aseismic processes from the foreshock sequence to the post-mainshock phase. By building a high-resolution seismicity catalog and searching for anomalous seismicity rate increases compared to aftershock triggering models, we highlight an over-productive seismicity starting within the foreshock sequence and persisting several days after the mainshock. Using repeating earthquakes and high-rate GPS observations, we highlight a transient aseismic perturbation starting just before the first foreshock and extending continuously after the mainshock. The estimated slip rate is lightly impacted by large magnitude earthquakes and does not accelerate towards the mainshock. Therefore, the unusual seismic and aseismic activity observed during the 2017 Valparaiso sequence might be interpreted as the result of a slow slip event starting before the mainshock and extending beyond it. Rather than pointing to a possible nucleation phase of the 2017 Valparaiso mainshock, the identified slow slip event acts as an aseismic loading of nearby faults, increasing the seismic activity, and thus the likelihood of a large rupture.