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Various earthquake models predict that aseismic slip modulates the seismic rupture process but actual observations of such seismic‐aseismic interaction are scarce. We analyze seismic and aseismic processes during the 2014 Iquique earthquake sequence. High‐rate Global Positioning System displacements demonstrate that most of the early afterslip is located downdip of the M 8.1 mainshock and is accompanied by decaying aftershock activity. An intriguing secondary afterslip peak is located ∼120 km south of the mainshock epicenter. The area of this secondary afterslip peak likely acted as a barrier to the propagating mainshock rupture and delayed the M 7.6 largest aftershock, which occurred 27 hr later. Interevent seismicity in this secondary afterslip area ended with a M 6.1 near the largest aftershock epicenter, kicking the largest aftershock rupture in the same area. Hence, the interevent afterslip likely promoted the largest aftershock nucleation by destabilizing its source area, favoring a rate‐dependent cascade‐up model. Plain Language Summary Subduction zone faults host both fast (regular earthquakes, seismic) and slow (aseismic) slip. Simulation models predict that slow slip can affect fast slip processes. We explored such an interaction taking place during the 2014 Iquique earthquake offshore northern Chile using observation data of crustal deformation by Global Positioning System and earthquakes. We discovered that the fast mainshock slip was terminated by a slowly slipping fault zone, which prevented the simultaneous occurrence of the largest aftershock. Furthermore, afterslip, one type of slow slip following the mainshock, helped the occurrence of the largest aftershock 27 hr after the mainshock. Therefore, the sequential occurrence of large earthquakes can be controlled by slowly slipping faults. Key Points Global Positioning System captured crustal deformation during 27 hr between the 2014 Iquique mainshock and its largest aftershock The mainshock and the largest aftershock areas are separated by an aseismic area, likely preventing both from rupturing as a single event The largest aftershock nucleation is a mixture of seismicity and decelerating afterslip, favoring a rate‐dependent cascade‐up model

Subduction zones are home to the world’s largest and deepest earthquakes. Recently, large-scale interactions between shallow (0-60 km) and intermediate (80-150 km) seismicity have been evidenced during the interseismic period but also before and after megathrust earthquakes along with large-scale changes in surface motion. Large-scale deformation transients following major earthquakes have also been observed possibly due to a post-seismic change in slab pull or to a bending/unbending of the plates, which suggests the existence of interactions between the deep and shallow parts of the slab. In this study, we analyze the spatio-temporal variations of the declustered seismicity in Japan from 2000 to 2011/3/11 and from 2011/3/11 to 2013/3/11. We observe that the background rate of the intermediate to deep (150-450 km) seismicity underwent a deceleration of 55% south of the rupture zone and an acceleration of 30% north of it after the Tohoku-oki earthquake, consistent with the GPS surface displacements. This shows how a megathrust earthquake can affect the stress state of the slab over a 2500 km lateral range and a large depth range, demonstrating that earthquakes interact at a much greater scale than the surrounding rupture zone usually considered. In this study, the authors analyze the spatio-temporal variations of the seismicity in Japan due to the Tohoku-Oki earthquake. They show that a megathrust earthquake can affect the stress state of the slab over large lateral and depth ranges.

In Cascadia, the concomitance of slow slip events (SSE) and tremors during Episodic Tremor and Slip (ETS) episodes is well documented. Brittle tremor patches embedded in the ductile matrix deforming aseismically is the most common concept for the fault structure, but whether tremors and their patches impact the SSE initiation is under debate. This study focuses on 13 initiations of major Cascadia ETS. Limited observational constraints exist on the details of ETS initiation because spatiotemporal SSE inversions usually over‐smooth their temporal evolution. Scrutinizing tremors and SSE at the beginning of major ETS events gives us insights into their mechanical relationship. We directly retrieve the temporal evolution of the SSE moment by stacking sub‐daily Global Positioning System (GPS) time series at multiple sites, without slip inversions. Comparison of the GPS stack with tremor count demonstrates that SSE moment release accelerates drastically ∼${\\sim} $ 1 day after the onset of vigorous tremor activity. On the other hand, once the SSE moment release accelerates, the tremor area expands more rapidly, suggesting that the growth of the ETS occurs through a feedback mechanism between slip and tremor once the SSE is well developed. By combining these and previous observations, we propose a conceptual model of ETS initiation: heterogeneous interface strength limits the growth of SSE with unruptured tremor patches acting as relatively high‐strength pins contributing to this heterogeneity. In other words, major ETS emerges probably only when collective tremor patches are critically stressed. Plain Language Summary Slow slip events (SSE) and tremors are aseismic and seismic components of the slow earthquake family, respectively, which have considerably lower slip rates than regular fast earthquakes. In the Cascadia subduction zone, they occur at the down‐dip extension of the seismogenic zone along the subduction interface. Their interaction during the initiation of SSEs should offer insights into their mechanical connection, but it was so far unclear because of the technical limitations of conventional SSE analysis methods. By analyzing the temporal evolution of SSEs in Cascadia using a stacking method for geodetic time series at multiple observation sites, we found that tremor occurrence tends to precede the acceleration of SSE during the initiation stage. Enlightened by our knowledge from other seismological observations and computer simulations of SSE and tremors, the observed lag implies that unruptured tremor patches might represent a relatively strong location of the locked plate interface and that SSE can grow more efficiently after the rupture of these tremor patches, unpinning the interface. Key Points Multi‐site stacking of geodetic deformation time series efficiently resolves moment function of slow slip during its initiation Geodetic moment tends to dramatically accelerate ∼1 day after the tremor onset during the initiation of Episodic Tremor and Slip in Cascadia The delay between tremor and slow slip events (SSE) acceleration suggests that the growth of SSE is conditioned by the rupture of tremor patches

When a frictional interface is subject to a localized shear load, it is often (experimentally) observed that local slip events propagate until they arrest naturally before reaching the edge of the interface. We develop a theoretical model based on linear elastic fracture mechanics to describe the propagation of such precursory slip. The model’s prediction of precursor lengths as a function of external load is in good quantitative agreement with laboratory experiments as well as with dynamic simulations, and provides thereby evidence to recognize frictional slip as a fracture phenomenon. We show that predicted precursor lengths depend, within given uncertainty ranges, mainly on the kinetic friction coefficient, and only weakly on other interface and material parameters. By simplifying the fracture mechanics model, we also reveal sources for the observed nonlinearity in the growth of precursor lengths as a function of the applied force. The discrete nature of precursors as well as the shear tractions caused by frustrated Poisson’s expansion is found to be the dominant factors. Finally, we apply our model to a different, symmetric setup and provide a prediction of the propagation distance of frictional slip for future experiments.

Repeated cross‐correlations of ambient seismic noise indicate a long‐term seismic velocity change associated with the 2006 M7.5 slow‐slip event (SSE) in the Guerrero region, Mexico. Because the SSE does not radiate seismic waves, the measured velocity change cannot be associated with the response of superficial soil layers to strong shaking as observed for regular earthquakes. The perturbation observed maximized at periods between 7 s and 17 s, which correspond to surface waves with sensitivity to the upper and middle crust. The amplitude of the relative velocity change (∼10−3) was much larger than the volumetric deformation (∼10−6) at the depths probed (∼5–20 km). Moreover, the time dependence of the velocity perturbation indicated that it was related to the strain rate rather than the strain itself. This suggests that during strong slow‐slip events, the deformation of the overlying crust shows significant nonlinear elastic behavior. Key Points Mexico 2006 Slow slip event produced nonlinear deformation at depth We observed a seismic velocity perturbation associated to deformation at depth Strong shaking cannot be invoked to explain seismic velocity changes during SSE

Slow slip events (SSEs) originate from a slow slippage on faults that lasts from a few days to years. A systematic and complete mapping of SSEs is key to characterizing the slip spectrum and understanding its link with coeval seismological signals. Yet, SSE catalogues are sparse and usually remain limited to the largest events, because the deformation transients are often concealed in the noise of the geodetic data. Here we present a multi-station deep learning SSE detector applied blindly to multiple raw (non-post-processed) geodetic time series. Its power lies in an ultra-realistic synthetic training set, and in the combination of convolutional and attention-based neural networks. Applied to real data in Cascadia over the period 2007–2022, it detects 78 SSEs, that compare well to existing independent benchmarks: 87.5% of previously catalogued SSEs are retrieved, each detection falling within a peak of tremor activity. Our method also provides useful proxies on the SSE duration and may help illuminate relationships between tremor chatter and the nucleation of the slow rupture. We find an average day-long time lag between the slow deformation and the tremor chatter both at a global- and local-temporal scale, suggesting that slow slip may drive the rupture of nearby small asperities.

We investigate the triggering of seismic tremor and slow slip event in Guerrero (Mexico) by the February 27, 2010 Maule earthquake (Mw 8.8). Triggered tremors start with the arrival of S wave generated by the Maule earthquake, and keep occurring during the passing of ScS, SS, Love and Rayleigh waves. The Rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor, indicating a strong modulation of the source of tremors by the passing surface wave. This correlation and modulation by the passing waves is progressively lost with time over a few hours. The tremor activity continues during the weeks/months after the earthquake. GPS time series suggest that the second sub‐event of the 2009–2010 SSE in Guerrero is actually triggered by the Maule earthquake. The southward displacement of the GPS stations starts coincidently with the earthquake and tremors. The long duration of tremors indicate a continuing deformation process at depth, which we propose to be the second sub‐event of the 2009–2010 SSE. We show a quasi‐systematic correlation between surface displacement rate measured by GPS and tremor activity, suggesting that the NVT are controlled by the variations in the slip history of the SSE. This study shows that two types of tremors emerge: (1) Those directly triggered by the passing waves and (2) those triggered by the stress variations associated with slow slip. This indicates the prominent role of aseismic creep in the Mexican subduction zone response to a large teleseismic earthquake, possibly leading to large‐scale stress redistribution. Key Points Triggering of slow slip event and tremors Surface wave dispersion footprints the envelope of NVT Correlation between slip rate changes and NVT

Geodetic positioning is the geophysical record of reference for slow slip events, but typical daily solutions limit studies of the evolution of slow slip to its long‐term dynamics. Accompanying seismic low‐frequency earthquakes located precisely in time and space provide an opportunity to image slow slip dynamics at subdaily time scales. Here we show that a high‐resolution time history of low‐frequency earthquake fault slip alone can reproduce the geodetic record of slow slip that we observe to be dominated by subdaily fault slip dynamics. However, a simple linear model cannot accommodate the complex dynamics present throughout the slow slip cycle, and an analysis of different phases of the slow slip cycle shows that the ratio of geodetic to seismic fault slip varies as a function of time. This suggests that the low‐frequency earthquake source region saturates as slow slip grows in moment and area. We propose that rheological heterogeneities at the plate boundary associated with low‐frequency earthquakes do not play a significant role in the slow slip rupture process, thus implying that their activity is incidental to the driving aseismic slip. Plain Language Summary Slow slip events can be observed in many subduction zones where they play an important role in the earthquake cycle. Decades after their discovery, slow slip events are now captured routinely in geodetic datasets with slip dynamics occurring over a broad range of time scales. Using high‐time resolution seismological observations together with the geodetic record allows us to go beyond the coarse daily GNSS sampling rate to image slow slip dynamics at short time scales. Here we use the temporal evolution of seismic slip produced by low‐frequency earthquakes to study the subdaily dynamics of a slow slip event cycle, reproducing the geodetic record of slow slip using only seismological observations. We develop a simple model where long‐term loading is in competition with the intermittent release of stress tied to the seismic slip of low‐frequency earthquakes. We show the full slow slip cycle is driven by bursts of slip at subdaily time scales that low‐frequency earthquake events witness only in their immediate source region. This result implies that the low‐frequency earthquake rupture process is incidental to slow fault slip and does not play a major role in the slow slip cycle. Key Points Low‐frequency earthquake activity can reproduce the geodetic record of the slow slip cycle Slow slip is the competition between subdaily dynamics and a fully coupled fault Low‐frequency earthquakes are incidental to slow slip and do not play a role in mediating aseismic slip

Slow, aseismic fault slip has emerged as a significant contributor to the seismic cycle. However, whether slow and fast slip arise from similar physical processes remains unresolved, due to detection biases affecting noisy surface measurements and the analysis of the source properties of slow slip. Using daily geodetic time series denoised with a deep learning model, we invert for 15 years of slow slip evolution on the Cascadia subduction with unprecedented temporal resolution. Our observations show that an upper bound for slow‐slip moment rates exists, and that scaling laws are strongly influenced by the chosen detection threshold and the signal‐to‐noise ratio. Moment rate functions evolve with magnitude: slow slip nucleates as a two‐dimensional expanding crack, propagating laterally when encountering the along‐dip limits of the transition zone. Our findings highlight a continuum of slow slip events of various sizes controlled by subduction interface geometrical constraints.

We study rapidly accelerating rupture fronts at the onset of frictional motion by performing high-temporal-resolution measurements of both the real contact area and the strain fields surrounding the propagating rupture tip. We observe large-amplitude and localized shear stress peaks that precede rupture fronts and propagate at the shear-wave speed. These localized stress waves, which retain a well-defined form, are initiated during the rapid rupture acceleration phase. They transport considerable energy and are capable of nucleating a secondary supershear rupture. The amplitude of these localized waves roughly scales with the dynamic stress drop and does not decrease as long as the rupture front driving it continues to propagate. Only upon rupture arrest does decay initiate, although the stress wave both continues to propagate and retains its characteristic form. These experimental results are qualitatively described by a self-similar model: a simplified analytical solution of a suddenly expanding shear crack. Quantitative agreement with experiment is provided by realistic finite-element simulations that demonstrate that the radiated stress waves are strongly focused in the direction of the rupture front propagation and describe both their amplitude growth and spatial scaling. Our results demonstrate the extensive applicability of brittle fracture theory to fundamental understanding of friction. Implications for earthquake dynamics are discussed.