Ground deformation monitoring of a potential landslide at La Palma, Canary Islands

The southern part of the island of La Palms comprises a north-south-oriented volcano known as the Cumbre Vieja. The steep gradient and high aspect ratio of the volcano, coupled with the prospect of future episodes of magma intrusion, highlight the potential for large-scale lateral collapse of the volcano, as recorded earlier in the history of La Palma and elsewhere in the Canary Islands. Historic eruptions of the Cumbre Vieja have occurred high up on the western flank of the volcano and along a single rift zone along the crest of the volcano. The geometry Of the recent activity and faulting which occurred during an eruption in 1949 indicates that a discontinuity may be present beneath the western flank of the volcano, along which a future collapse may occur. To identify any displacement of the western flank overlying the discontinuity, a ground deformation network has recently been established on the volcano. The initial network spanned the fault system that developed on the upper flanks of the Cumbre Vieja during the 1949 eruption. This small network was measured in 1994, 1996 and 1997 using infrared Electronic Distance Measurement. In;1997, the network was enlarged and re-occupied, using the Global Positioning System, to incorporate the west flank and the southern pan of the island. Although the results show that apparent displacements recorded during this period are within the error-margins of the techniques employed, the apparent movement vectors do suggest a coherent westward displacement of stations to the west of the 1949 fault system. Additional occupations of the network over the next few years will, however, be required to determine the reality or otherwise of this apparent coherent movement, and thus to decide whether or not the western flank of the Cumbre Vieja is currently sliding seaward. If not, it can be inferred that the volcano is stable during inter-eruptive periods. (C) 1999 Elsevier Science B.V. All rights reserved.