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.
An earthquake occurs when a sudden release of mechanical energy takes place within the crust, at the hypocenter, where deformations have accumulated over years, decades or centuries. The generation of an earthquake can be described with the so-called seismic cycle (Fedotov, 1965). During one of the phases of this cycle (the interseismic phase), the accumulation of strain brings the system to a critical state that ultimately leads to the occurrence of the earthquake (coseismic phase). Although the origin of the stresses involved in the seismic cycle is mostly endogen (tectonic, volcanic, etc.), we could ask whether, in addition to the endogenous contribution, there could be other phenomena involved in triggering the seismic event. One of these phenomena is the tidal stress, built upon the gravitational interaction of the Earth with the Sun and the Moon. Despite being two or three orders of magnitude smaller than the typical stress drops, tidal strains are associated to stress rates even greater than the tectonic ones (Heaton, 1975;Emter, 1997).
For more than a hundred years, the idea of a relationship between tides and seismicity in the Earth has attracted the attention of scientists around the world. Most recent studies in this topic support a tidal contribution to seismic activity in different areas of the planet (Tanaka et al., 2002(Tanaka et al., , 2004;;Tanaka, 2010Tanaka, , 2012;;Cochran et al., 2004;Cadicheanu et al., 2007;Metivier et al., 2009;Chen et al., 2012;Tiwari and Chamoli, 2014;Vergos et al., 2015;Xie et al., 2015;Arabelos et al., 2016;Ide et al., 2016); the analysis of moonquakes also reveals a clear tidal contribution (Kolvankar et al., 2010). In some cases, results suggest strong correlations for the case of shallow events of reverse fault type (Tanaka et al., 2002) or shallow thrust earthquakes (Cochran et al., 2004); there is also evidence for tidal triggering of intermediate-depth earthquakes (Cadicheanu et al., 2007). However, there are cases in which results have been weak or contradictory, or do not reveal a link between lunisolar tides and seismicity (Heaton, 1982;Curchin and Pennington, 1987;Hatzell and Heaton, 1989;Vidale et al., 1998;Beeler and Lockner, 2003;Fischer et al., 2006;Ader and Avouac, 2013). Most of the studies up to date have focused on the direct effects of tides on tidal triggering, namely the effect of solid or body tides as measured at or around the hypocenter of the earthquake. Others, however, have also included the so-called indirect effects of the tides (see e.g. Tsuruoka et al., 1995;Vidale et al., 1998;Cochran et al. 2004;Tanaka et al., 2002, 2004, 2006, Tanaka 2010, 2012;Arabelos et al., 2016), namely, the effect that ocean tides could have in tidal triggering.
The indirect effect of the ocean tide is important because near ocean margins, it can be larger than the solid earth tides (e.g. Tanaka et al., 2002). In fact, the load effect in the induced stresses at oceanic basins is an order of magnitude larger than the induced stresses by the solid Earth (Cochran et al., 2004). Therefore, its role as a triggering mechanism should not be overlooked (e.g. Tsuruoka et al., 1995). More interesting is the fact that OTL may disturb the faults by a hold-and-release mechanism, arising from variations in water mass over the ocean basins (Cochran, 2004). This contribution of OTL may result in different effects than those expected by studying only the body tides (BT).
Modeling the OTL is more difficult than modeling solid Earth tides (Agnew, 2007). This fact has delayed the reliable study of these effects on tidal triggering with respect to what have been done with BT. However, recent technological tools and improved geographical models of the ocean basins worldwide, have contributed to improve the estimation of the effect.
In a previous work, we reported the discovery of correlation anomalies (anomalously low p-values of the Schuster Statistical Test) between intermediate depth seismicity in the Bucaramanga nest and Cauca cluster in Colombia (Moncayo et al., 2019) and the diurnal and monthly phase of the body tides. In that study, we neglected the indirect effect of OTL arguing that typical distances of the seismic clusters to the Caribbean and Pacific coasts were relatively large (200-500 km). Since the OTL decreases with distance to the coasts (e.g. Matsumoto et al., 2001;Wendt, 2004), we assumed that considering its effects was not mandatory.
In this work, we revisit the problem and study tidal triggering in the aforementioned seismic clusters, including now the indirect effect of the OTL. Moreover, we take a step further by including in our analysis other two areas of high seismicity rate, close to the West Coast of South America, namely the El Puyo (Ecuador) and Pucallpa (Peru) seismic swarms, where a noticeable concentration of intermediate-depth events has been observed (Zarifi and Havskov, 2003;Soles Valdivia, 2012;Taipe Acosta, 2013). In Moncayo et al. (201
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