Slow slip events (SSEs) at subduction zone plate boundaries sometimes trigger earthquake swarms and megathrust earthquakes. The causal relationship between SSEs and seismicity has been studied worldwide, but the epidemic-type aftershock-sequence (ETAS) model, which is a standard statistical model of seismicity, does not explicitly consider the seismicity-triggering effect of SSEs. Therefore, if an SSE occurs at a plate boundary, probabilistic earthquake forecasts based on the ETAS model fail to predict observed seismicity. Here, we constructed a statistical model named the SSE-modulated ETAS model by incorporating SSE moment rates estimated from observation data from the global navigation satellite system into the original ETAS model. Our model assumes a linear or power-law relationship between the SSE moment rates and seismicity rates and estimates its proportionality constant as a new ETAS parameter. We applied this new model to three SSEs and M 2.5 or greater earthquakes in the shallow part of the Hikurangi Trench, New Zealand. The results show that it is better than the original ETAS model, giving a significant reduction in the Akaike information criterion. In addition, we examined the functional forms (e.g., lag time and power exponent) of the equation relating the moment rate of the SSEs to the seismicity rate. The results imply that, in addition to SSE-induced stress changes, crustal fluid migration may be related to SSE-induced seismicity. We also examine the influence of SSEs on aftershock productivity. Our model can improve short-term forecasts of seismicity associated with SSEs and is useful for quantifying characteristics of the seismicity.

Understanding the fault behavior through geodetic data has an important impact in our assessment of the seismic hazard. To shed light on the aseismic evolution of a fault, we developed a new slip inversion strategy, the ELADIN (ELastostatic ADjoint INversion) method, that uses the adjoint elastostatic equations to efficiently compute the gradient of the cost function. ELADIN is a 2-steps inversion algorithm to better handle the slip constraints. In the first step, it finds the slip that better explain the data without any constraints and the second step refines the solution imposing the slip constraints through a Gradient Projection Method. In order to get a physical plausible slip distribution and to overcome the poor fault illumination due to scarce data, ELADIN reduces the solution space by means of a von Karman autocorrelation function that controls the wavenumber content of the solution. To estimate the resolution, we propose a mobile checkerboard analysis which allows to measure a lower bound resolution over the fault for an expected slip patch size and an specific stations deployment. We test ELADIN with synthetic examples and use it to invert the 2006 Guerrero Slow Slip Event (SSE). The later is one of the most studied Mexican SSE that unfortunately was recorded with only 15 stations, so a strong regularization is required. We compared our slip solution with two published slip models and found that our solution preserves the general characteristics observed by the other models such as an updip penetration of the SSE in the Guerrero seismic Gap. Despite this similarity, our resolution analysis indicates that this updip aseismic slip penetration might not be a reliable feature of the 2006 SSE.

The triggering of large earthquakes on a fault hosting aseismic slip or, conversely, the triggering of slow slip events (SSE) by passing seismic waves involves seismological questions with important hazard implications. Just a few observations plausibly suggest that such interactions actually happen in nature. In this study we show that three recent devastating earthquakes in Mexico are likely related to SSEs, describing a cascade of events interacting with each other on a regional scale via quasi-static and/or dynamic perturbations. Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stressing produced by the seismic waves of the great 2017 (Mw8.2) Tehuantepec earthquake, which strongly disturbed the aseismic slip beating over a 650 km long segment of the subduction plate interface. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable.

Slow slip events (SSEs) along subduction zones play an important role in accommodating relative plate motion. SSEs interplay with large megathrust earthquakes and other slow earthquakes, including low frequency and very low frequency earthquakes. The Kanto and Tokai regions of central Japan host frequent slow and large earthquakes, with significant differences in slip behavior along the subduction zones in the Suruga Trough, Sagami Trough, and Japan Trench. In this study, we conducted a systematic search to estimate the fault models and durations of short-term SSEs using continuous Global Navigation Satellite System (GNSS) data collected from 1994 to 2020. We detected 179 potential SSEs with moment magnitudes of 5.3–7.0 and durations of 0–80 days from the time series. Along the Sagami Trough, two shallow regions at a depth of 10–20 km host Mw ≥ 6.5 SSEs off of the Boso Peninsula and accommodate most of the relative plate motion aseismically. Some SSEs also occur on the deep plate interface down to ~50 km without low frequency tremors (LFTs). Along the Japan Trench, the cumulative slip of the SSEs exhibits a bi-modal depth distribution to avoid the large slip areas of past megathrust earthquakes at 30–40 km depth. The shallow SSEs are in the same depth range (10–30 km) as LFTs, but are spatially separate from LFTs along the trench. The detected SSEs have limited temporal correlations with other slow earthquakes and earthquake swarms, which suggests that many factors control the genesis of slow and regular earthquakes.