Volcanic eruptions in 2000–2019
As demonstrated in Fig. 2a-b, after the 2012 earthquakes, the number of volcanic eruptions effectively increased along the Central American Volcanic Arc (CAVA hereafter). Some volcanoes increased the number of eruptions and explosivity compared with previous years (Santa Marían and Fuego), while others began to erupt such as San Miguel, Momotombo, Rincón de la Vieja, Poás and Turrialba. The eruption rate increased by a factor of 2.88. Similar trend was also observed after the Mw=9.3 Andaman-Sumatra earthquake (December 24, 2004), with an increment of four times in the eruption rate in the same region20. However, after the 2012 Central America earthquakes, the volcanic eruptions after occurred diachronously: some shortly after (days), and some months to years after the earthquakes. Moreover, we cannot recognize a migration of the volcanic eruption based on the location and time.
The three large earthquakes occurred within ten weeks, almost equidistant from each other (420–450 km), but at different hypocenter depths (Fig. 3). The hypocenter depth of El Salvador earthquake (August 27, week − 1, Mw=7.3) was 11.8 km and had low high-frequency (HF) energy radiation and a long period, which are classical characteristics of “tsunamogenic earthquakes”29–33. Nine days later, the Costa Rica earthquake (September 5, week 0, Mw=7.6) struck with a moderate HF energy radiation and with a hypocenter depth of 15.8 km30,32,33. The Guatemala earthquake hypocenter depth (November 7; week 9; Mw=7.4) was 24 km, where conditionally stable areas surround small patchy in the slab that, at failure, produce moderate slip and high HF radiation30,32,33.
The spectra of the El Salvador and Costa Rica earthquakes range from 0.07 to 1.2 Hz, with a frequency domain between 0.07–0.1 Hz in broadband stations. It is important to consider both, the resonance frequency of the fluids34 (i.e. magma, gas, vapour, or liquid) as well of the volcanic edifice35, to evaluate if Fuego and San Cristóbal volcanoes could enter in resonance and squeeze out the magma after crack opening. We calculated a resonance frequency of a theoretical fluid (magma) filled conduit as a dyke (frd) with a hypothetical width of 100 m and 10 m is 0.09 and 0.28 Hz, respectively. The resonance frequency of the volcanic edifices calculated (frv) for Fuego volcano is around 0.16 Hz and for San Cristóbal volcano is 0.27 Hz (more details of these calculation in the supplementary material). These resonance frequencies of fluid filled and volcanic edifices of Fuego and San Cristóbal volcanoes are in the or close of the range of the frequency domain of the earthquakes, which means, that it is possible to considered this could be a mechanism that prompted these eruptions.
Dynamic stress is generally considered for time intervals of a few seconds. However, in the El Salvador earthquake case, Telica volcano experienced around of 120 seconds (vs the general 20–50 seconds12), of continuous frequency seismicity and, correspondingly, continuous dynamic stress; this can promote the sloshing mechanism in the hydrothermal and/or magmatic plumbing system34,36.
In addition, sloshing depends on the viscosity34,36, considering a higher viscosity for silicic magmas with respect to mafic magmas. For San Cristóbal and Fuego volcanoes, the predominant magmas in recent eruptions are basaltic-andesitic, which means that the overpressure needed to trigger an eruption is lower than for dacitic/rhyolitic magmas.
Volcanic eruptions shortly after the earthquakes
On September 8 and 13, the San Cristóbal (VEI = 2) and Fuego (VEI = 3) volcanoes had paroxysmal eruption, respectively. To evaluate if these eruption were possibly triggered by the earthquakes, we calculated a lithostatic pressure in the reservoir22, of 98 MPa (4000 m magma chamber depth) and 73.5 MPa (3000 m magma chamber depth), respectively (see supplementary material). The total change in the pressure by σD of the Costa Rica earthquake was < 0.26% for San Cristóbal, and 0.02% for Fuego of the lithostatic pressure. This estimate implies that the earthquake itself could not have triggered the eruptions. Nevertheless, the disturbance in the stress regime created by the earthquake could have favored other mechanisms (such as rectified diffusion, bubble growing, increment the gas dissolved, magma migration, etc5,14,23,37) to facilitate the eruptions. Regarding the role of static stress the respective alignment system could be crucial38. In the case of San Cristóbal, the σsdiff = 2.5 kPa, located around the zone of high and low rigidity, i.e. the contact between the country-rock and magma chamber boundary (3000 m depth), respectively (more details in the methodology and supplementary material table 3–4). The σsdiff is hence a potential parameter to predict crack opening and could have induced crack propagation and consequent fluid migration. Contrarily to San Cristóbal, the static stress of Fuego volcano was less than 1 kPa, arguably too low to create crack opening and fluid migration. Nevertheless, some studies suggest that a change in stress of a few kPa can trigger volcanic eruptions, such as for Etna and Stromboli (both in Italy)39,40, and Merapi (in Indonesia)41, three of the world's most frequently erupting volcanoes. In the case of the 2006 Merapi eruption, geological evidence shows that the preceding December 24, 2004 Andaman-Sumatra earthquake added xenoliths from the carbonate basement to the magma chamber, causing an internal pressure increase generated by CO2, before eventually culminating in an eruption42.
Volcanic eruptions long after the earthquakes
For the August 27, El Salvador earthquake, the static stress applied was low, in the order of ± 2 kPa in three volcanoes (San Miguel, San Cristóbal and Telica). In the case of the September 5, Costa Rica earthquake, the σsdiff was from 5 kPa for Turrialba to 55 kPa for Rincón de la Vieja. The σsdiff in the November 11, Guatemala earthquake was from 12 kPa for Pacaya to 0.1 MPa for Santa María. The static stress in Karymsky volcano (in Kamchatka, Russia), produced by a tectonic earthquake (Mw=7.1) promoting a dyke intrusion and triggering the 1996 eruption, was 0.2 MPa39, which means that it is difficult to explain with our results that only static stress could have opened cracks and generated a dyke intrusion. Quantifying the stress regime around each single volcano is a necessary constraint to determine whether the static stress reduces or increases the country-rock strength. An example of how the static stress changes according to the different alignment is provided by Rincón de la Vieja volcano; this stress regime had at least three directions (N-S, W-E and 45°). The largest σsdiff (55 kPa) occurred in the deepest part of the magma chamber with a W-E alignment. This differential in the pressure could cause magma rise towards the shallow reservoir, creating an overpressure, and superheating of the shallow magma chamber.
The possible response of volcanoes on the long term (months to years) to the earthquakes depend on the degree of the critical stage of each volcano, explaining why some volcanoes that received more stress (dynamic and static) than others responded later, or not at all. For example, Rincón de la Vieja and Poás volcanoes erupted in 2017, and received more stress change compared to Turrialba volcano that already erupted in 2014. Volcanic processes, such as magma migration from the mantle to the crust or magma mixing, can occur on various time scales, from months to years to even centuries43,44. In addition, the presence of a mush zone, of which part of it could be an eruptable melt at crustal depth, a seismic event or some other processes, like the addition of a mafic melt can trigger eruptions years after (e.g. the deadly phreatic eruption of Ontake volcano in 201445).
Another hypothesis in favor of the increase in the number of eruptions years after the earthquakes, is the post-seismic activity in Central America. The region is well known for the occurrence of post-seismic slow slip earthquakes, as also manifested for the El Salvador and Costa Rica 2012 earthquakes46,47. An example is provided by San Miguel volcano, which erupted in December 2013, after 37 years of quiescence. According to GPS data, the horizontal displacement by co-seismic slip at San Miguel was around 1.2 cm46. Nevertheless, almost one year after the three earthquakes, the horizontal displacement was 2 cm by post-seismic slip46.
Some of the 19 studied volcanoes were already erupting prior to the 2012 earthquakes, but the number of eruptions increased after the earthquakes. For Poás volcano, the number of eruptions and the magnitude of phreatic explosions increased after the January 8, 2009 Cinchona earthquake15–17, a Mw=6.2 tectonic event with an epicenter 10 km from Poás and also after 2012 earthquakes. On April 10, 2014, a Mw=6.1 tectonic earthquake hit near Momotombo volcano (Nicaragua), triggered seismic swarms48 and resumed explosive activity in December 2015. The most impressive change in the increase in the number of eruptions occurred at Fuego volcano. In three years (2015–2018), 50 paroxysmal eruptions occurred, including the deadly eruption of June 3, 2018, while between 1999 to 2012, Fuego volcano generated 21 paroxysmal eruptions27 (Note: the Fuego eruption is still ongoing, but the most recent data are not included in this study).
Volcanic unrest 2007–2012
The change in volcanic activity beyond background behavior to worrisome levels (i.e. volcanic unrest) sometimes escalates into volcanic eruptions or triggers other hazardous events49–52. This research classified the information available on volcanic activity in three different “degrees of unrest”, based on the energy release of volcanoes53 from lowest (unrest 1), to intermediate (unrest 2) to the highest (unrest 3) degree. Each degree of unrest means: Unrest 1 = increase in the seismicity of the volcanic system (green color in the Fig. 4); Unrest 2 = increase in the temperature, deformation, degassing, phreatic activity, or small explosions (yellow color in the Fig. 4); Unrest 3 = occurrence of large eruptions with considerable ashfall, explosions with ballistics and paroxysmal events (red color in the Fig. 4). Between September 2007 and September 2017, 19 volcanoes in the CAVA showed signs of unrest before and/or after the earthquakes of 2012 (Figs. 1 and 4). Before the 2012 earthquakes, 13 volcanoes were in a state of unrest (Santa María, Pacaya, Fuego, San Miguel, San Cristóbal, Telica, Momotombo, Masaya, Concepción, Rincón de la Vieja, Arenal, Poás and Turrialba; Fig. 4), and among these Santa María, Fuego, Pacaya, San Cristóbal, Telica, Masaya, Concepción, and Arenal were erupting. After the earthquakes and until 2017, Concepción, and Arenal ceased their eruptions. Concepción volcano had a phreatomagmatic event in May 201125,54. The magnitude of the explosions in Arenal volcano was in a constant decrease since 200725, and the last explosion occurred in October 2010. After the 2012 earthquakes, these volcanoes decreased their level of unrest to 1 or 2 (Fig. 4). A possible reason why these volcanoes did not erupt after the 2012 earthquakes could lie on the fact that the magma volume erupted in the previous eruptions already released its internal pressure and both volcanoes are close-conduit system. The other volcanoes in eruption prior to September 2012, Santa María, Fuego, Pacaya, San Cristóbal, Telica and Masaya, are very active open-conduit systems and/or are in permanent unrest, for which internal pressure constantly reaches the threshold to trigger eruptions.
Some of the other volcanoes experienced decades without magmatic eruptions, but they already had unrest degrees of 1 and 2 (San Miguel, Momotombo, Rincón de la Vieja, Poás and Turrialba; Figs. 1 and 4) prior to the 2012 earthquakes. From the 19 volcanoes in a state of unrest in the period 2007–2017, eleven volcanoes erupted after the 2012 earthquakes and these were already in unrest before the 2012 earthquakes. A question to pose is why the other eight did not erupt? Different answers can be suggested (Fig. 4): 1) two of them had already released their energy upon large eruptions or prolonged periods, as explained before (Concepción and Arenal). 2) five volcanoes that previously did not show any sign of unrest switched into unrest only after the Costa Rica earthquake occurred (Apoyeque, Miravalles, Tenorio, Platanar and Irazú; Fig. 4). Among the latter five, only one volcano had erupted in historical times (Irazú, 1963–1965), while the other four are far from the recurrence period for a potential new eruption. These five volcanoes only showed unrest degree 1 (increased seismicity) some hours or a few days after the Costa Rica earthquake. This response can be linked to the dynamic stress that triggered some seismic swarms in the fault systems around these volcanoes6,8,55. 3) One volcano (Cerro Negro) had large explosions in August 1999, and unrest degree 1 was reached on June 4, 2013. All nine volcanoes showed evidence that the earthquakes themselves were insufficient to trigger volcanic eruptions, despite the fact that the earthquakes caused an increase in the degree of unrest for some of them. However, the same seismic energy transmitted to other volcanoes that were already in an advanced state of unrest was able to trigger a new eruption. In consequence, we postulate, according to the data presented and the obtained results, that dormant volcanoes or volcanoes with low activity did not change significantly their state just because of the earthquake's shaking, or the change in the stress field regime. The earthquakes were not able by themselves to bring the volcanoes from equilibrium to eruption.
From our findings stemming from the fortunate occurrence of three subduction tectonic earthquakes in a time span of 10 weeks in an active volcanic arc at Central America, we conclude that the postulated cause/effect relationship between tectonic earthquakes and volcanic eruptions is only valid when volcanoes are already in a high state of unrest prior to the earthquake. The energy supplied by the seismic shock may constitute the additional energy contribution necessary to trigger an eruption in a high stage of pre-eruption volcanic activity. The fact that the volcano may react shortly, or long term after the seismic input, does not seem to depend on the magnitude of the earthquake itself, but on the processes that occurred inside the volcano (type of magma, gas content, viscosity, strength of the hots rock, etc). Other earthquake characteristics in addition to magnitude and location (i.e. energy radiated, frequency, duration, etc) could also play a role on how tectonic earthquakes may contribute to volcanic eruptions. Nevertheless, this external energy supply, regardless of the distance between the earthquake epicenter and the volcano, and the magnitude of the event, is not sufficient to raise the state of a volcano from quiescence directly into eruption. Our results confirm the need to monitor all active volcanoes to know their degree of unrest at any time and hence prior to the next large earthquake, and to establish future scenarios of possible increased volcanic activity, and eventually volcanic eruption on the short (days) or long term (years). This kind of surveillance can be a useful forecasting tool for future eruptions, and will help the Civil Protection offices and decision makers to timely adopt strategies to disaster risk reduction at the regional or local scales.