Tiltmeter

Abstract: The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio-soundings acquired in various latitude and climate conditions. It is shown that with such a model, given the 3300 meters topography of Etna, tropospheric variations can generate up to 4π phase rotations between the top and the bottom of the volcano. In 16 out of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991–93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14±1 km. During the second half of the eruption, the subsidence rate at the top of the volcano is roughly stable at 13±3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.

Abstract: For thousands of years man has marvelled at the gigantic structure that makes up Mt. Etna, the largest active volcano in Europe, and has lived side by side with the mountain, which despite its intense eruptive activity has always been considered a “friendly giant.” After the Second World War, with its frequent but non life-threatening eruptions, Mt. Etna represented an ideal location for volcanological research for the national and international scientific community. Numerous scientists from Belgium, Germany, France, the United Kingdom, and the United States of America have taken part in volcanological research aimed at understanding the volcano.

Abstract: Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.

Abstract: [1] A series of complex events at Kīlauea Volcano, Hawaii, 17 June to 19 June 2007, began with an intrusion in the upper east rift zone (ERZ) and culminated with a small eruption (1500 m3). Surface deformation due to the intrusion was recorded in unprecedented detail by Global Positioning System (GPS) and tilt networks as well as interferometric synthetic aperture radar (InSAR) data acquired by the ENVISAT and ALOS satellites. A joint nonlinear inversion of GPS, tilt, and InSAR data yields a deflationary source beneath the summit caldera and an ENE-striking uniform-opening dislocation with ∼2 m opening, a dip of ∼80° to the south, and extending from the surface to ∼2 km depth. This simple model reasonably fits the overall pattern of deformation but significantly misfits data near the western end of an inferred dike-like source. Three more complex dike models are tested that allow for distributed opening including (1) a dike that follows the surface trace of the active rift zone, (2) a dike that follows the symmetry axis of InSAR deformation, and (3) two en echelon dike segments beneath mapped surface cracks and newly formed steaming areas. The en echelon dike model best fits near-field GPS and tilt data. Maximum opening of 2.4 m occurred on the eastern segment beneath the eruptive vent. Although this model represents the best fit to the ERZ data, it still fails to explain data from a coastal tiltmeter and GPS sites on Kīlauea's southwestern flank. The southwest flank GPS sites and the coastal tiltmeter exhibit deformation consistent with observations of previous slow slip events beneath Kīlauea's south flank, but inconsistent with observations of previous intrusions. Slow slip events at Kīlauea and elsewhere are thought to occur in a transition zone between locked and stably sliding zones of a fault. An inversion including slip on a basal decollement improves fit to these data and suggests a maximum of ∼15 cm of seaward fault motion, comparable to previous slow-slip events.