Fogo Volcano, Cape Verde Islands: seismicity-derived constraints on the mechanism of the 1995 eruption

Seismic data recorded in the vicinity of the active vent of the 1995 Fogo eruption [Global Volcanism Network Bulletin, 1995, 20(3) 2–4.] is used to constrain the associated stress field and deformation. Using the frequency content of the seismograms to distinguish between brittle fracture of cold host rock and deformation in the vicinity of the intruding magma, a sub-vertical dyke with 060° strike is identified as the feeder of the eruption, and delineated down to a depth of about 4 km. The local stress field during the eruption is estimated from composite focal mechanisms. Besides the expected σ3 direction normal to the dyke, a group of focal solutions point to a stress field with nearly dyke-parallel σ3, which is interpreted as a re-adjustment of the edifice following the eruption, maybe partially controlled by gravity. This effect may have increased the instability of the steep eastern flank of the island, since the deformation detected during the magma drainback is compatible with predominantly dip-slip motion on east-dipping surfaces.