Denis Legrand

SO ₂ flux and gas composition were measured during the 2021 flank eruption of Pacaya Volcano (Guatemala). The SO ₂ flux, measured with Tropomi, gradually rose from ∼2 to ∼30 kg/s over a 5 month period before the eruption. Field UV camera data showed that >95% of the SO ₂ was emitted from the summit crater. The Multigas composition measurements evidenced a depletion in CO ₂ and a significant enrichment in H ₂ O over SO ₂ in the plume from the flank eruptive fissure compared to the central crater plume, and punctual amounts of H ₂ S. We explain these variations by a combination of (a) Fractional degassing between a vertical conduit and an almost horizontal dyke diverging at a calculated pressure of 10–20 MPa, (b) SO ₂ scrubbing by a local and temporary hydrothermal system, and (c) Inclusion of groundwater in the magma dyke, even if visually the activity looked purely magmatic (lava spattering and effusion).

SUMMARY: The triple junction between the North American, Caribbean and Cocos plates at the Guatemala–Mexico border is not well understood. It forms a broad region from around the active Tacaná volcano up to the Guatemala City graben. Tacaná is the westernmost active volcano of the Central American volcanic arc and is located at the intersection of four major active faults: the Polochic, Motagua, Jalpatagua and Tonalá faults. Using seismicity around the Tacaná volcano, we show that there is moderate to low tectonic seismic activity between the Guatemala City graben and the Tacaná volcano, possibly related to the ancient extremes of the Motagua and Jalpatagua faults. Therefore, we speculate that the triple junction would be located onshore, around the Tacaná volcano. We located earthquakes around the Tacaná volcano between January 2017 and October 2018, a period that includes the large Mw8.2 Tehuantepec (Chiapas) earthquake of 2017 September 8, located ∼190 km away. We identified four distinct types of seismicity, interpreted as having tectonic, hydrothermal, intermediate depth magmatic and deep magmatic origins. The tectonic seismicity occurred at depths between ∼5 and ∼30 km b.s.l., and may be associated with three faults around the Tacaná volcanic complex. These faults are oriented in NE–SW, aligned with the four Tacaná volcanic edifices; NW–SE, consistent with the Jalpatagua fault; and approximately EW, corresponding to the Motagua fault. The hydrothermal seismicity is observed at shallow depths, from the subsurface to about 2 km b.s.l., predominantly in the western sector of the Tacaná summit, and partially beneath the San Antonio volcano, an area known for intense hydrothermal activity. This seismicity is spatially related to the shallow portions of the same three faults described above. The intermediate depth magmatic seismicity is detected at depths between 5 and 12 km b.s.l. and is interpreted to be related to the presence of a shallow magma chamber beneath the Tacaná volcanic complex. Finally, the deep magmatic seismicity is located in the eastern part of the Tacaná, at depths ranging from 15 km to about 22 km b.s.l. This seismicity is interpreted to be due to a vertical dike intrusion that connects a deep magma reservoir located between 30 and 40 km depth, to the hypothesized shallower magma chamber associated with the intermediate depth seismicity.

El Chichón volcano is the most active volcano in the state of Chiapas, México, and experienced its last Plinian eruption (VEI = 5) in 1982. To better assess its volcanic hazard, we studied its readiness to erupt by estimating changes in its internal stress state. These stress changes are difficult to calculate accurately, for example in the absence of focal mechanisms, but their existence can be indirectly revealed by the presence of volcano-tectonic earthquakes, for example following a large tectonic earthquake. We show that the seismic rate recorded at El Chichón volcano increased slightly after the large M_w8.2 Tehuantepec earthquake of 8 September 2017, Chiapas. However, this rate quickly returned to its background level after only 2 months, without any external volcanic manifestations, suggesting that the volcano is not ready to erupt in the near future. Previous observations of slight increases in the volcanic seismicity rate following large earthquakes have been explained by the presence of active hydrothermal systems in the vicinity of the volcano. We propose a similar explanation for El Chichón volcano which is known for its large hydrothermal system. Furthermore, the characteristics of the 2017 seismicity (spatial and magnitude distributions), and the horizontal-to-vertical spectral ratio also confirm the presence of high amounts of water near the volcano. We show that the 2017 volcano-tectonic seismicity is of hydrothermal rather than magmatic origin, in agreement with recent independent geochemical and aeromagnetic studies.

SUMMARY: In this study, we analyse the 2020 seismic swarm that lasted two months and occurred between the Tancítaro and the Paricutin volcanoes in the Michoacán Guanajuato Volcanic Field, Mexico. We developed a new method to automatically detect and locate about 100 000 earthquakes, enabling us to track the magma migration through narrow dykes. Additionally, we reveal the presence of two magma reservoirs from two seismic noise tomography results. The first reservoir is located from 8 to 20 km below sea level and beneath the Tancítaro volcano and probably corresponds to a complex network of dykes and sills. This crustal reservoir is fed by a mantle reservoir with a wide horizontal extension between 35 and 50 km below sea level. The seismic swarm initiated beneath the Tancítaro summit in the lower portion of the crustal magma reservoir. At this stage, the seismicity migration was mainly horizontal, which we interpret as its response to the higher normal stress caused by the gravitational load of Tancítaro. Once the magma was displaced laterally from beneath Tancítaro, magma migration became more vertical. The swarm reached the upper portion of the crustal magma reservoir but did not escape it. We also reveal the effect of a distant but strong tectonic earthquake on the seismic swarm. Before its occurrence, magma migration followed several paths; afterwards, it became more focused along a single path. Finally, after the swarm, we observed a second type of seismicity called post-swarm seismicity, with a lower earthquake rate but with higher magnitudes. The hypocentres were diffuse and horizontally centred on the previous swarm location. Furthermore, some earthquakes were aligned along shallow faults, generating a high seismic risk to the different Tancítaro nearby localities.

The birth of a new monogenetic volcano is difficult to forecast with precision, both in space and time. Nevertheless, seismic activity can alert of the imminence of such an eruption because it usually occurs as small-magnitude earthquake swarms that can last for a few weeks to months prior to an eruption. These swarms are usually related to magma that becomes stalled in the Earth’s crust for variable periods of time before its eventual eruption at the surface. For several reasons, volcanic seismic swarms have rarely been recorded with seismometers before the birth of a new monogenetic volcano. Over the past 25 years, six distinct seismic swarms (in 1997, 1999, 2000, 2006, 2020, and 2021) were detected between Tancítaro and Paricutin volcanoes, in the southwestern part of México’s Michoacán-Guanajuato volcanic field. They are believed to represent repeated attempts of magma to reach the surface hinting that in this region magma might become stalled for some time, so as to not reach the surface in a single ascent event from its source in the mantle. To better understand the magma’s migration path through the crust, we re-located with greater precision some of these seismic swarms by using the same methodology and velocity model to the entire data set. Our results show that these swarms originated within a small area beneath the NE flank of Tancítaro at depths of between 15 and 8 km below sea level (bsl). Apparently, magma is trying to reach the surface within the same conduit network at these crustal depths, but stalls when reaching a depth of ~ 8 km bsl. It is crucial to study these swarms because they might be precursors to a new eruption in this part of the Michoacán-Guanajuato volcanic field. This monogenetic field has been very active, producing several dozen eruptions during the Holocene, the last two Jorullo (1759–1774) and Paricutin (1943–1952). Furthermore, the Tancítaro area displays one of the highest densities of Holocene volcanoes within the entire field, making it a probable candidate location for the birth of a future monogenetic volcano. For these reasons, a permanent seismic network should be installed as soon as possible.

On 20 March 2012, a M_w 7.5 thrust earthquake started a series of seven large events (7.0 ≤ M_w ≤ 8.2) that struck central Mexico during a period of 9 years (2012–2021). Before this event, the Mexican subduction zone did not experience significant subduction earthquakes (M_w > 7.0) for at least 12 years. Five of the events occurred in the plate interface, resulting in a significantly larger interplate slip rate in the states of Oaxaca and Guerrero. In this study, we explore how an aseismic slip transient caused by the 2012 M_w 7.5 earthquake affected the Oaxaca region and whether this earthquake had a causal relationship with the M_w 7.2 Pinotepa Nacional event that took place six years later in a nerby zone. To this end, we identified and analyzed characteristic repeating earthquakes along the Mexican subduction zone for assessing the plate interface slip history and found a remarkable increase in the aseismic slip rate following the 2012 mainshock, which suggests a long‐standing slip perturbation near the trench in Oaxaca that continued until the Pinotepa Nacional earthquake of 2018.

On 20 March 2012, a Mw 7.5 thrust earthquake started a series of large events up to magnitude Mw 8.2 (2017) that struck central Mexico during a period of nine years. Before this event, the Mexican subduction zone did not experience subduction earthquakes (Mw > 7.0) for at least 12 years. Most of the events during this highly active period (2012-) occurred in the plate interface, resulting in a significantly larger interplate slip rate in the states of Oaxaca and Guerrero. In this study, we explore how the aseismic slip transient caused by the 2012 Mw 7.5 earthquake affected the region and whether this earthquake had a causal relationship with a nearby similar magnitude event that occurred in 2018 (Mw 7.2). To this end, we identified and analyzed characteristic repeating earthquakes along the Mexican subduction zone for assessing the plate interface slip history and found a notably increase in the aseismic slip rate inferred from repeating earthquake activity following the 2012 mainshock, which suggests a long-standing slip perturbation in Oaxaca near the trench that continued until the 2018 Mw 7.2 Pinotepa Nacional earthquake.