Short-Term Dynamic Processes at the Restless Nisyros Caldera, Greece: Insights from Joint and Simultaneous Potential Field, Geodetic and Seismic Records

Gravity and deformation time series data are employed both to quantify the long-term subsurface dynamics at restless calderas and for forecasting volcanic activity. Critical to the interpretation of residual gravity data in terms of magma dynamics is the assessment of signals stemming from phenomena such as, for instance, secular variations in the level of the ground water table and the mass/density changes in active hydrothermal systems. An earlier study at the restless Nisyros caldera in Greece revealed short-term (40-60 min) gravity variations with amplitudes similar to those observed during annual microgravimetric surveys. It was inferred that these short-term variations might be caused by the hydrothermal/magmatic degassing process itself (for instance, the generation, ascent and dissipation of steam pockets from the boiling hydrothermal reservoir along fracture zones or faults as well as transient pressure variations during steam/liquid interface propagation). To investigate the short-term dynamics in more detail, we ran a two-week field experiment in May 2006 in the hydrothermal areas of the collapse caldera, using one continuously recording gravimeter, two field gravimeters, three GPS receivers, one seismometer, one very-low-frequency (VLF) receiver and one audiomagnetotelluric (AMT) receiver. In addition, an AMT survey was conducted within the caldera in order to map the electrical structure of the caldera. Gravimetric and geodetic data were recorded at a frequency of 1 Hz (with the exception of the field gravimeters, operated at 0.0033 Hz), the 5 s period seismometer recorded at 125Hz and the electromagnetic data were collected at a frequency of 21.75 kHz. Preliminary results reveal residual gravity changes of up to 40 microGal (peak-to-peak amplitudes) which are not attributable to deformation effects. The power spectrum of the entire continuous gravity record reveals marked periods between 45 and 50 min consistent with earlier observations performed during discrete gravimetric readings. The same periodic signal was also recorded in VLF-EM inphase values. The correlation between the seismic and gravimetric record indicates coupling effects of seismogenic events with sub-surface mass migration. The preliminary evaluation of the continuous gravimetric and continuous VLF-EM outphase records indicates a negative correlation, whereby gravity decreases with increasing outphase values. The joint record lead us to the preliminary conclusion that the major part of the short-term geophysical signals at the caldera is related to dynamic processes in a shallow hydrothermal system at a depth of less than 500 m. Magmatic signals would need to exceed this hydrothermal signal in order to be seen.