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 e ruption [Global Volcanism Network Bulletin, 1995, 20(3) 2-4.] is used to co nstrain the associated stress field and deformation. Using the frequency co ntent of the seismograms to distinguish between brittle fracture of cold ho st rock and deformation in the vicinity of the intruding magma, a sub-verti cal dyke with 060 degrees strike is identified as the feeder of the eruptio n, and delineated down to a depth of about 4 km. The local stress field dur ing the eruption is estimated from composite focal mechanisms. Besides the expected sigma(3) direction normal to the dyke, a group of focal solutions point to a stress field with nearly dyke-parallel sigma(3), which is interp reted as a re-adjustment of the edifice following the eruption, maybe parti ally controlled by gravity. This effect may have increased the instability of the steep eastern flank of the island, since the deformation detected du ring the magma drainback is compatible with predominantly dip-slip motion o n east-dipping surfaces. (C) 1999 Elsevier Science B.V. All rights reserved .