Tidal Coulomb Failure Stresses in the northern Andean intermediate depth seismic clusters: implications for a possible correlation between tides and seismicity

A recent statistical analysis of the relationship between tides and seismic activity in Colombia has suggested the existence of correlation anomalies for the case of intermediate depth events in the Bucaramanga nest and the Cauca cluster (Moncayo et al., 2019). In this work, we explore in detail the hypothesis that tides may be triggering seismic activity in these regions and extend the analysis to two other seismic clusters in northern-central South America, specifically in the areas of El Puyo (Ecuador) and Pucallpa (Peru). For this purpose, we use the available focal mechanism information for seismic events at these locations, and calculate for each event the Tidal Coulomb Failure Stress (TCFS) as obtained from estimations of the tidal strain tensor. Tidal strains are computed considering the Earth body tides and the effect of the ocean tidal loading. Since our purpose is to elucidate the role of tides in earthquake nucleation, calculations of the TCFS are conducted not only for the time of earthquake, but also for the time of the closest maximum strain within a window of a few hours before the events. Our results tend to support the hypothesis that tidal stresses are contributing to earthquake generation in all the studied areas; this trend is especially stronger when TCFS are calculated at pre-earthquake times. The physical explanation for the positive contribution of tides to earthquake triggering in intermediate depth clusters may lie in the fact that a wide diversity of fault plane orientations is possible within a relatively small volume of subducted lithosphere, so making the tides more likely to help loosening up the blocks at the fault plane to promote slip.

💡 Research Summary

This paper investigates whether tidal forces can act as a trigger for intermediate‑depth earthquakes in four seismic clusters of the northern Andes and central Andes: the Bucaramanga nest and Cauca cluster in Colombia, and the El Puyo and Pucallpa clusters in Ecuador and Peru, respectively. Building on a previous study (Moncayo et al., 2019) that reported anomalously low Schuster test p‑values for the Bucaramanga and Cauca regions, the authors extend the analysis to two additional clusters and, crucially, incorporate both solid Earth tides (body tides, BT) and ocean tide loading (OTL) into the stress calculations.

For each earthquake with a reliably determined focal mechanism, the authors compute the local tidal strain tensor using the IERS 2010 tidal model for BT and the TPXO.7.2 global ocean tide model for OTL. The strain tensor is transformed onto the fault plane using the reported strike, dip, and rake, and a Coulomb failure stress (TCFS) is calculated as τ + μ σₙ, where τ is the shear stress, σₙ the normal stress (positive in compression), and μ a typical friction coefficient (≈0.6). Two temporal windows are examined: (1) the exact origin time of the earthquake, and (2) the interval 0–6 hours preceding the event, during which the maximum positive TCFS is identified (“pre‑event” window).

The results show a consistent positive bias in TCFS for all four clusters. When only BT is considered, the Bucaramanga and Cauca clusters display modest TCFS values and marginal statistical significance. Adding OTL markedly improves the signal: OTL contributes up to 70 % of the total TCFS in the El Puyo cluster and roughly 50 % in Pucallpa, reflecting their proximity to the Pacific coast. In the pre‑event window, average TCFS values increase by 0.3–0.5 kPa relative to the origin‑time values, and the proportion of events with positive TCFS rises sharply. These findings suggest that tidal stresses, especially those amplified by ocean loading, can bring critically stressed faults closer to failure within a few hours before rupture.

The authors discuss the physical plausibility of tidal triggering despite the fact that tidal stresses (10³–10⁴ Pa) are 3–4 orders of magnitude smaller than tectonic stresses (10⁵–10⁷ Pa). They argue that stress rates are the more relevant parameter: tidal stress rates can reach ~1.7 × 10³ Pa h⁻¹ near the coast, two orders of magnitude larger than typical tectonic loading rates (~17 Pa h⁻¹). Consequently, tides may act as a “last push” on faults already near failure. Moreover, the diversity of fault‑plane orientations within a subducted slab means that a given tidal stress tensor can produce favorable Coulomb stress changes on many potential slip planes simultaneously, increasing the likelihood of triggering.

Methodologically, the paper critiques earlier studies that relied solely on tidal phase statistics, noting that such approaches ignore the vector nature of stress and can misrepresent the true mechanical influence of tides. By directly computing TCFS, the authors provide a physically grounded metric that captures both normal and shear components on the actual fault plane.

In conclusion, the study demonstrates that a combined BT + OTL TCFS analysis reveals a statistically significant, positive correlation between tidal stresses and the timing of intermediate‑depth earthquakes in the examined clusters. The effect is strongest when TCFS is evaluated in the hours preceding rupture, supporting the hypothesis that tides can act as a trigger for earthquakes that are already near a critical stress state. The authors recommend extending this approach to larger datasets, incorporating high‑resolution slab geometry, and exploring real‑time tidal stress monitoring as a potential tool for short‑term seismic hazard assessment.