Marino Protti

OBSERVATIONS AT COSTA RICAN VOLCANO OFFER CLUES TO CAUSES OF ERUPTIONS

Recent measurements of ground deformation at an active volcano show great promise for elucidating the processes that lead to volcanic eruptions. A long-term effort to continuously monitor ground deformation over a very wide bandwidth using state-of-the-art geodetic, seismic, and acoustic instruments at Arenal Volcano in Costa Rica is producing high-quality recordings of harmonic tremor and explosive events and offers an exciting opportunity to explore possible lunar periodicities in volcanic eruptivity.

A Tremor and Slip Event on the Cocos-Caribbean Subduction zone as measured by a GPS and Seismic Network on the Nicoya Peninsula, Costa Rica

In May 2007 a network of global positioning systems (GPS) and seismic stations on the Nicoya Peninsula, of northern Costa Rica, recorded a slow-slip event accompanied by seismic tremor. The close proximity of the Nicoya Peninsula to the seismogenic part of the Cocos-Caribbean subduction plate boundary makes it a good location to study such events. Several centimeters of southwest motion were recorded by the GPS stations over a period of several days to several weeks, and the seismic stations recorded three distinct episodes of tremor during the same time span. Inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25–30 km, downdip of the main seismogenic zone. Estimated temperatures here are ∼250°–300°C, lower than in other subduction zones where events of this nature have been previously identified. There may also be a shallower patch of slip at ∼6 km depth. These results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding, and that a critical temperature threshold is not required for slow slip. Tremor and low-frequency earthquake locations are more difficult to determine. Our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip, but not spatially colocated.