Multiscale Spatial and Temporal Analysis of the b-value in volcanic areas

Determining the b-value of the Gutenberg-Richter law is of great importance in Seismology. However, its estimate is strongly dependent upon selecting a proper temporal and spatial scale due to the multiscale nature of the seismicity. This characteristic is especially relevant in volcanoes where dense clusters of earthquakes often overlap the background seismicity and where this parameter displays a higher spatial and temporal variability.

For this reason, we devised a novel approach called MUST-B (MUltiscale Spatial and Temporal estimation of the B-value) which allows a consistent estimate of the b-value, avoiding subjective “a priori ” choices, by considering simultaneously different temporal or spatial scales. This approach also includes a consistent estimation of the completeness magnitude (Mc) and the uncertainties over both b and Mc. We applied this method to datasets in volcanic areas proving its effectiveness to analyze complex seismicity patterns and its utility in volcanic monitoring and geothermal exploration. Besides, it may provide a way to distinguish seismicity caused by tectonic faults and volcanic sources in zones where there is a mix of both of them.

We present MUST-B applications to three volcanic areas: Long Valley caldera (USA), Tenerife and El Hierro (Canary Islands). The spatial analysis of the b-value in Long Valley shows an impressive chimney-like volume characterized by high b-values which coincide with the main pathway of geothermal fluids inferred by independent studies. For Tenerife, we applied MUST-B to analyze both spatial and temporal variations. The spatial pattern shows an interesting variation between 2004-2005 and the period 2016-2020. In both cases, high b-values appear in an area that hosted increased seismicity because of seismo-volcanic crises. These high b-values are also evidenced by the temporal analysis, which shows an increase in correspondence between these two periods. For El Hierro, we analyzed the seismicity preceding the 2011 submarine eruption of Tagoro volcano using a joint spatio-temporal analysis. Results show high b-values in the area where the vent opened and a drop of this parameter just before the beginning of the eruption.