Spatial distribution of the b value under the Popocatepetl volcano and its relation with the structure of the magma chamber

Abstract:

The spatial distribution of the b value under volcanoes and other tectonic environments is not uniform but it rather shows pockets of anomalies related to different phenomena. We analyzed the frequency-magnitude distribution of the 2191 best located volcano-tectonic events with M_D ≥ 2.1 under the Popocatepetl volcano and performed 2D and 3D maps using a griding technique. For the 3D mapping we used samples of 100 events within spherical volumes with radii ≤ 3 km. Also, only volumes with σ ≤ 0.3 were taken into account. Three main anomalies with b ≥ 2.2 were detected within a normal crust (b ≤ 1.6). The first anomaly is observed north of the crater summit at depths 4-8 km and comprises a volume of approximately 15 km³. The second lies slightly to the E and SE of the crater at depths between 2 and 5 km, and the third and latter anomaly is located SE of the summit at depths of 6-12 km and spans over a volume of 40 km³ (or greater). In any case, we interpret our high b values anomalies as regions were high pore pressures (low effective stresses), high thermal gradients and/or high heterogeneity take place due to the presence of a magmatic body nearby. Whilst we cannot know with certainty if the source of the anomaly located N of the crater is a dyke complex or a shallow magma chamber, we infer that the shallowest volume mapped is related to Popocatepetl’s plumbing system. The greatest anomalous volume located at the SE is thought to represent the main shallow magma chamber, since it is located at depths similar to those found in other volcanoes in the world and correlates with previous geophysical studies carried out at Popocatepetl.