Cumulative storage of magma in the Paricutin-Tancítaro region, Mexico, revealed by recurrent swarm seismicity and a high spatial density of morpho-chronometrically dated Holocene monogenetic cones.

A region located in the SW sector of the Michoacán-Guanajuato monogenetic field, in central Mexico displays a high spatial density of scoria cones, mostly around Tancítaro, a large central volcano active in the middle Pleistocene. This region became well known when in 1943 a new volcano, Paricutin, was formed in a cornfield at 11 km to the NW of the extinct stratovolcano. The birth of Paricutin was preceded by significant swarm-type seismicity. Afterward, new seismic swarms were reported in the area in 1997, 1999, 2000, 2006, 2020, 2021, 2022, and 2023, with a mean recurrence interval of only 4 yr, most of them (not all) showing the characteristics of a magmatic origin.  Aiming to shed some light on the relation between the high density of monogenetic cones and the recurrent seismicity, we have made a morpho-chronometric estimate of the relative ages of 170 scoria cones located in the Paricutin-Tancítaro region (PTR) within latitudes 19°N and 20°N and longitudes -102.0° E and -102.7° using the Average Erosion Index (AEI) which quantifies the degree of alluvial erosion of scoria cones from a Fourier analysis of their level contours. Monogenetic activity began in the PTR at about 1 Ma, and the AEI analysis shows that such activity increased after the end of the Tancítaro activity, around 232 ka, and further increased in the Holocene when about one-third of the scoria cones in the region were formed, with a mean interval between eruptions between 120 and 240 yr. On the other hand, a detailed study of two of the most energetic seismic swarms, recorded in 2020 and 2021 shows that the magma intrusion volume required to produce the measured cumulative seismic moment of both swarms amounts to about 140 million cubic meters, which is seemingly insufficient to produce a Paricutin-size eruption, which ejected about 1.3 cubic km of magma. We thus propose a possible conceptual explanation of the recurrent emplacement of monogenetic volcanoes and the frequent seismic swarm activity in terms of a persistent magma source under the crustal extension of the PTR producing numerous dike and sill forming intrusions. In some cases, such intrusions may have a cumulative effect forming temporary magma reservoirs capable of producing new monogenetic eruptions. Assuming that about 0.5 to 1 cubic kilometer of magma needs to accumulate to begin an eruption, about 7 to 14 sizable (similar to the 2020-2021) swarms may then represent a significant precursor.