Aseismic slip across the EQ cycle

Seismogeodetic exploration of the EQ Preparation Phase

The mechanisms leading to earthquakes (EQs) are still debated, notably between cascade and preslip models. In the cascade model, a sequence of foreshocks progressively triggers the mainshock through stress transfer, whereas the preslip model proposes that a slow aseismic slip accelerates and ultimately transitions into dynamic rupture. Throughout my Ph.D. and postdoctoral research, I have been particularly interested in using seismogeodesy to evaluate the role of aseismic slip in this debate.

By studying the M_w 8.1 Iquique earthquake in northern Chile, we detected a westward acceleration lasting about eight months prior to the mainshock, with nearly 80% of the deformation being aseismic when compared with seismological estimates. Two weeks before the mainshock, a M_w 6.7 earthquake occurred, accompanied by a large GNSS-detected transient, still dominated by aseismic slip (about 35%). Similar behavior was observed for the M_w 6.9 Valparaíso earthquake in central Chile. There, GNSS data revealed a westward motion starting three days before the mainshock. By jointly analyzing geodetic and seismological data and accounting for observational and modeling uncertainties, we estimated that about 51% of the measured displacement originated from aseismic slip on the megathrust. These observations consistently indicate that aseismic slip on the subduction interface plays a key role during the earthquake preparation phase.

Slab role in the EQ Triggering and Preparation Phase

Beyond shallow interface processes, recent studies suggest that intermediate-depth seismicity may be linked to the triggering of megathrust earthquakes. Although the magnitudes of these deeper events are too small to directly trigger large EQs, they point to broader slab deformation processes that may influence the megathrust. During my Ph.D. and early postdoctoral work, I investigated this possibility using seismogeodetic observations.

In 2005, the M_w 7.8 Tarapacá slab-pull earthquake occurred at the same latitude as the 2014 Iquique event. Following the Tarapacá earthquake, we observed a broad change in surface deformation and an increase in background seismicity and depth interactions. These observations suggest an aseismic decoupling of the subduction interface triggered by slab deformation, potentially preconditioning the megathrust for the Iquique earthquake years later.

On a broader scale, three major earthquakes occurred in the Chilean subduction zone between 2010 and 2015. Analysis of seismic catalogs indicates that the M_w 8.4 Illapel earthquake may have been triggered by the earlier Maule and Iquique events through a deep transient slip propagating within the slab. Together, these results point to a central role of the slab in earthquake triggering and preparation, with slip rates at depth evolving over time and redistributing deformation over large spatial and temporal scales not captured by classical earthquake cycle models.

The false quiescence of the Interseismic Phase

Interseismic coupling models are commonly used to infer fault locking between large earthquakes. Locked regions generally correlate with future rupture areas, whereas partially locked zones are often interpreted as creeping aseismically, either steadily or episodically. However, whether aseismic slip occurs as a steady process or in transient bursts remains an open question, with major implications for our understanding of fault mechanics.

Figure 1. Left: Aseismic slip events in the southern Peru–northern Chile subduction zone. Right: Aseismic slip in northwestern Colombia.

This question motivated the development and application of dedicated methods to detect aseismic slip events. I have applied geodetic template matching (GTM) and multivariate singular spectrum analysis (MSSA) to long GNSS time series in the southern Peru–northern Chile subduction zone, along the central section of the North Anatolian Fault, and more recently in northern Colombia. In all three regions, these analyses reveal frequent bursts of aseismic slip that would remain undetected without careful data processing. In southern Peru–northern Chile (Figure 1), we identified more than twenty aseismic events over 14 years, primarily occurring in transitional coupling zones and at depths consistent with fluid-rich environments. Along the North Anatolian Fault, GNSS and creepmeter data reveal a clear contrast between steady creep and episodic slip behavior, while in northern Colombia similar transient signals point to previously unrecognized aseismic slip activity (Figure 1).

Taken together, these results demonstrate that the apparent quiescence of the interseismic phase is often misleading. Aseismic slip is widespread, episodic, and plays a fundamental role throughout the earthquake cycle. Properly accounting for these processes is essential for constraining fault mechanics and for developing numerical models that realistically represent deformation across the full seismic cycle.

Underlying physics of aseismic slip

Figure 2. (a) Pressure–temperature (P–T) conditions at the locus of SSEs across nine subduction zones. (b) Boundaries of major mineral dehydration reactions and consequent release of mineral-bound water (wt.%) from a representative metamorphic MORB.

While the presence of aseismic slip is now well established, the physical mechanisms controlling it remain debated. My recent work has focused on evaluating three main candidates: heterogeneities in fault constitutive properties, stress interactions induced by geometrical complexities such as damage zones, and the role of fluids. By combining global slow slip event catalogs, thermal modeling, petrological constraints, and machine learning approaches, we find growing evidence that fluids released by metamorphic dehydration reactions play a key role in promoting aseismic slip by increasing pore pressure and reducing effective normal stress (Figure 2).

These results suggest that aseismic slip is not an isolated phenomenon, but rather an intrinsic component of fault behavior, tightly linked to slab geometry, thermal structure, and fluid circulation. Understanding these interactions is essential for bridging short-term fault processes with long-term tectonic evolution and for improving time-dependent assessments of seismic hazard.

Relevant publications

An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust
Socquet, A., Valdes, J. P., Jara, J., Cotton, F., Walpersdorf, A., Cotte, N., Specht, S., Ortega-Culaciati, F., Carrizo, D., & Norabuena, E. GRL, 2017 44(9), 4046–4053. doi:10.1002/2017GL073023.

Long-Term Interactions Between Intermediate Depth and Shallow Seismicity in North Chile Subduction Zone.
Jara, J., Socquet, A., Marsan, D., & Bouchon, M. GRL. 2017, 44(18), 9283–9292. doi:10.1002/2017GL075029

Revisiting Slow Slip Events Occurrence in Boso Peninsula, Japan, Combining GPS Data and Repeating Earthquakes Analysis
Gardonio, B., Marsan, D., Socquet, A., Bouchon, M., Jara, J., Sun, Q., Cotte, N., & Campillo, M. JGR: Solid Earth. 2018, 123(2), 1502–1515. doi:10.1002/2017JB014469

Suspected Deep Interaction and Triggering Between Giant Earthquakes in the Chilean Subduction Zone
Bouchon, M., Marsan, D., Jara, J., Socquet, A., Campillo, M., & Perfettini, H. GRL. 2018, 45(11), 5454–5460. doi:10.1029/2018GL078350

Inferring Interseismic Coupling Along the Lesser Antilles Arc: A Bayesian Approach
van Rijsingen, E. M., Calais, E., Jolivet, R., de Chabalier, J. ‐B., Jara, J., Symithe, S., Robertson, R., & Ryan, G. A. JGR: Solid Earth. 2021, 126(2), 1–21. doi:10.1029/2020JB020677

Seismic and Aseismic Fault Slip During the Initiation Phase of the 2017 Mw = 6.9 Valparaíso Earthquake.
Caballero, E., Chounet, A., Duputel, Z., Jara, J., Twardzik, C., & Jolivet, R. GRL, 2021, 48(6), 1–11. doi:10.1029/2020GL091916

Observation of a Synchronicity between Shallow and Deep Seismic Activities during the Foreshock Crisis Preceding the Iquique Megathrust Earthquake
Bouchon, M., Guillot, S., Marsan, D., Socquet, A., Jara, J., & Renard, F. Seismica. 2023, 2(2). doi:10.26443/seismica.v2i2.849.

Daily to centennial behavior of aseismic slip along the central section of the North Anatolian Fault
Jolivet, R., Jara, J., Dalaison, M., Rouet-Leduc, B., Özdemir, A., Doğan, U., Çakir, Z. & Ergintav, S. JGR. 2023, 128(7), 1-35. doi:10.1029/2022JB026018.

Detection of slow slip events along the southern Peru - northern Chile subduction zone
Jara, J., Jolivet, R., Socquet, A., Comte, D. & Norabuena, E. Seismica. 2024, 3(1), 1-21. doi:10.26443/seismica.v3i1.980.

Detecting millimetric slow slip events along the North Anatolian Fault with GNSS
Özdemir, A., Jara, J., Doğan, U., Jolivet, R., Çakir, Z., Nocquet, J.-M., Ergintav, S. & Bilham, R. GRL. 2025, 52, e2024GL111428. doi:10.1029/2024GL111428.

Subduction parameters controlling the occurrence of shallow and deep slow-slip events revealed by machine learning
Arroyo-Solórzano, M., Crisosto, L., Jara, J., González, Á. & Cotton, F. ESSOAr. 2025, preprint. doi:10.22541/essoar.176296793.37188670/v1.

Metamorphic dehydration reactions trigger slow slip events in subduction zones
Jara, J., Soret, M., Jolivet, R., Cubas, N., Maksymowicz, A. & Cotton, F. ESSOAr. 2025, preprint. doi:10.22541/essoar.176337015.59445929/v1.