Diffuse CO2 degassing from Brava Volcanic System, Cape Verde: a valuable tool to forecast volcanic unrest episodes
Abstract
Brava (67 km2) is the southwestern most and the smallest inhabited island of the Cape Verde archipelago. This oceanic active volcanic island is located 18 km west of Fogo Island and rises 976 m from the sea level. Volcanic hazard awareness among the Brava population and the authorities is very low because since no historical eruptions are recorded; therefore, its volcano monitoring program is scarce. A collaborative research on diffuse degassing and deformation monitoring at Brava among different Cape Verde and European institutions has been established. With the aim of providing a multidisciplinary volcano monitoring program for Brava, diffuse CO2 emission surveys have been carried out since 2010; approximately every 2 years. To quantify the diffuse CO2 emission rate, 100 sequential Gaussian simulations (sGs) were performed as interpolation method to construct soil CO2 emission contour maps. An increase trend of diffuse CO2 emission rate from 42 to 681 t d-1 at Brava was observed one year before the 2014-2015 Pico do Fogo eruption (Fogo Island) and almost three years before the seismic-volcanic crisis on August 2016 with more than 1000 seismic events registered by the Cape Verde INMG on August 1st, 2016 (Bruno Faria, personal communication). Due to this seismic-volcanic crisis, diffuse CO2 emission surveys at Brava were performed from August 2 to 10, and from October 22 to November 6, 2016. The estimated degassing rate yielded a value about 72 t d-1 (typical background values) and 1.700 t d-1 (maximum degassing rate measured) for the August and October-November surveys, repectively. These observed changes on diffuse CO2 emission are geochemical evidences which seemed to support a volcanic unrest for the recent anomalous seismic activity registered at Brava (García-Merino et al., 2017). In October-November 2018 a new survey was carried out at Brava with 258 sampling sites homogenously distributed along the island always depending on logistic reasons. Diffuse CO2 emission ranged between non detectable and 25.3 Kg m-2 d-1, with the highest values being measured at Baleia and Vinagre villages. The 2018 diffuse CO2 emission rate was estimated in 224 t d-1, a value similar than the measured in a pre-unrest survey (2012). Our results demonstrate that periodic surveys of diffuse CO2 emission are extremely important in the volcanic surveillance tools of Brava to improve the detection of early warning signals of future volcanic unrest episodes. García-Merino et al., 2017. Geophysical Research Abstracts EGU2017-8905.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2019
- Bibcode:
- 2019EGUGA..21.1403A