Activity, Unrest, and Hazard Evaluation at Stratovolcanoes and Calderas I Posters
Presiding: C R Kilburn, Benfield Hazard Research Center, University College London; C Troise, INGV-Osservatorio Vesuviano
V23A-01 1330h
Historical Earthquakes and Expected Seismic Damage at Ischia Island, Resurgent Caldera (Southern Italy)
Information on the seismicity of the Ischia island spans about eight centuries, starting from 1228. This is characterized by the occurence of earthquakes with low energy and high intensity. The most recent earthquake of 1883 caused 2333 deaths and the destruction of the historical and environmental heritage of some areas of the island, specially at Casamicciola town. This event (Imax = XI degree MCS), represents an important date in the prevention of natural disasters, in that it was after this earthquake that the first Seismic Safety Act in Italy was passed. After the 1883 earthquake there was a period of seismic quiescence except for some isolated events felt at beginning of the last century and the very occasional micro-earthquakes recorded in the last 20 years in the northern part of the island. The epicenter of all known earthquakes are on the northern slope of Mt. Epomeo (787 m a. s.l.) resurgent block, while analysis of the effects of earthquakes and the geological structures allows us to evaluate the stress fields that generate the earthquakes. The Mt. Epomeo is a resurgent structure in the central sector of the island, whose uplift is correlated to the caldera resurgence process, for the increase of pressure of shallow magma reservoir. The caldera was formed after a large explosive eruption that deposited the Mt. Epomeo Green Tuff, about 55 ka B.P. The uplift, which started about 30 ka B.P., was of about 900 meters. The resurgent structure is bordered by a system of faults and fractures, with NW-SE, NE-SW and N-S strike and along these faults, in the northern sector, the seismicity has been localized. In a volcanic area, interpretation of the mechanisms of release and propagation of seismic energy is made even more complex as the stress field that acts at a regional level is compounded by that generated from migration of magmatic masses towards the surface, as well as the rheologic properties of the rocks dependent on the high geothermic gradient. Such structural and dynamic conditions make the island of Ischia a seismic area of considerable interest. Thus a seismic hazard map of the island is proposed, according to a comparative analysis - using GIS - of various types of data: the geology, tectonics, historical seismicity, damage caused by the 28 July 1883 Casamicciola earthquake and the source model.
V23A-02 1330h
Preliminary Results of Continuous gas Geochemical Monitoring at Solfatara Volcano (Campi Flegrei) Southern Italy
While continuous geophysical monitoring have become widely accepted as major tools for volcanological forecasting, geochemical surveillance of volcanic activity is still less frequently applied. Investigations on the chemistry and mineralogy of fumaroles gas and sublimates as well as isotopic investigations of volcanic gas sampled at Solfatara volcano (Campi Flegrei southern Italy) were performed using various methodologies prevailing on monthly/weekly basis since the bradisesmic crisis 1983-84. This scattered gas chemical investigations gave important information on the behavior of the Solfatara volcano system but did not record systematic short term gas variation or interdependency of the gases with time. This makes difficult to correlate gas composition data with magma motion, eruptive events or geophysical parameters such as sesmicity, tilt, strain. For this reason, it was, therefore, necessary to develop a new technique for the continuous geochemical monitoring of the volcanic gas. The main objective of our investigations was to set up a continuously-operating gas monitoring station equipped with a diode-laser based apparatus (H2O-CO2 concentrations and and ä13C of CO2 composition) a quadrupole (amu 1-100) mass spectrometer coupled with an appropriate heating line to prevent any gas condensation before the injection into the analyzing chamber. These mass spectrometric analysis performed at the Solfatara volcano will be important for the evaluation of the gas oscillation phenomenon. Further the highly corrosive prohibitive condition and maintenance needed by these sophisticate instruments will be a key to develop in the near future more simple and less expensive devices based on various sensor technique and innovative material to be installed at the Solfatara volcano in order to allow local authorities and scientists to better understand how volcanoes work.
V23A-03 1330h
On the Interplay Among Structure, Regional and Magmatic Stress at Calderas: a Detailed Study of Campi Flegrei Earthquakes
This work presents new improved seismicity maps for Campi Flegrei earthquakes, located in a new 3D model obtained by local earthquakes tomography as well as from direct laboratory measurements of seismic velocities on rock samples. Earthquake locations are obtained from a new Bayesian algorithm, which gives both accurate point locations and maps of earthquake/moment density. Coulomb stress changes are computed with a FEM model which takes into account the presence of caldera ring faults, regional stress and magma overpressure in a shallow chamber put in evidence by seismic studies. The model enlight the important role played by both structural and regional/local stress in local earthquake generation at calderas, and gives a detailed explanation for the occurrence of earthquakes at Campi Flegrei during unrest episodes
V23A-04 1330h
RTK DGPS Tecnique Applied to Campi Flegrei Caldera (Southern Italy) Monitoring
The Campi Flegrei District (Naples - Italy) includes the volcanic areas of the Campi Flegrei and the islands of Ischia and Procida. The Campi Flegrei are characterized from a caldera 35,000 years old (eruption Campanian Ignimbrite) by which numerous volcanic monogenic apparatus developed inside. The last eruption of this caldera rose again in 1538 and carried to the origin of Mount Nuovo. From the geological point of view, the caldera is mainly formed by volcanic rocks and subordinately by clastic sea sediments; from the structural point of view, the configuration of Campi Flegrei is the result of deformations related to the regional and volcano-tectonic events. The regional tectonic is the cause of direct faults with NE-SW and NW-SE direction and subordinately with NS direction. The magmatic chamber is located at low depth (about 4-5 Km). The dynamics of this volcanic field was characterized by slow and continuous vertical movements as well known as Bradyseism. During 1969-72 (maximum uplift 170 cm) and 1982-84 (maximum uplift 184 cm) this area has been interested by two intense episodes with strong uplift of the ground and moderate seismic energy activity. Both the episodes were followed from a phase of subsidence interrupted by modest phenomena of uplifts, the last of which pointed out during the period March-September 2000 (maximum uplift about 4 cm). Even though the two main uplift crises are not culminated into an eruption, it is fundamental considering that these events caused significant damages to the buildings and to the economy of the Campi Flegrei, which has 250,000 inhabitants. The OV-INGV ground deformations studies are also carried out by the application of GPS and precise levelling techniques: in Campi Flegrei area were installed a CGPS network, consisting of 8 permanent stations, and a precise levelling network, consisting of about 300 benchmarks distributed along a distance of 120 km, with a mean distance of about 400 m, on twelve loops. Several methods based on the use of GPS reference networks for real-time kinematic positioning have been proposed and tested in recent years. The use of such methods is advantageous to overcome some of the limitations of the standard single reference station differential carrier phase positioning method. Accuracies at the centimetre level are possible under ideal conditions. Thanks to a greater rapidity, the real-time kinematic (RTK) positioning can be useful for periodic survey and for quickly solving field problems in period of crisis compared to the time for carried out a levelling survey. In particular, as regards the Campi Flegrei volcanic district, considering the high density of permanent stations of the Osservatorio Vesuviano-INGV GPS surveillance network, it is of great interest to focus the analysis on the possibility to take advantage of using these stations as reference in order to operate with a single receiver. This paper presents the results of a test based on the use of an advanced post-mission RTK DGPS positioning method with a good agreement with levelling data.
V23A-05 1330h
Laboratory and Field Determination of Seismic Velocities at Campi Flegrei: a Joint Interpretation of Mechanical Features.
We present an interpretation of mechanical features of Campi Flegrei rocks as obtained by the joint consideration of laboratory measurements and of local earthquake tomography. We studied seismic velocities and permeabilities on Campi Flegrei caldera (Italy) tuffs. Measurements were made in a servo-controlled steady-state-flow permeameter at effective pressures from 5-80MPa, during both increasing and decreasing pressure cycles. Selected samples were thermally stressed at temperatures up to 600° C to induce thermal crack damage Acoustic emission output was recorded throughout each thermal stressing experiment. Campi Flegrei tuff is a strongly heterogeneous pyroclastic flow material, which includes cavities, pumice and crystals. It has an initial porosity of around 45%, and a P-wave velocities that increases from around 2 km/s to around 3 km/s over the pressure range 5 to 80 MPa. Significant velocity hysteresis was observed in the tuff, with only about 50% of the velocity change recovered during depressurization. The P and S velocities as measured in laboratory are very coherent with the results obtained in this work from local earthquake tomography. AE recorded during thermal treatment shows higher bursts of activity around 200-300° C and 500-600° C, further confirming that thermal cracking is a primary deformation mechanism too, acting by enhancing crack propagation and dehydration of the zeolite phases present. In order to model the deformation processes involving the Campi Flegrei area and further understand the relatiosnhips between stress and strain acting in the area, we measured elastic moduli and fracture parameters of Neapolitan Yellow and Mt. Epoemo Green Tuffs. Fracture parameters have been determined on the basis of the maximum compressional strength, obtained through stress-strain curves. Experimental work has been carried out with the facilites installed at the Laboratory of Rock Physics at OV-INGV.
V23A-06 1330h
Is hot fluid migration the source of deformation in the Campi Flegrei Caldera?
Campi Flegrei is an active volcanic caldera, 12-14 km in diameter, immediately west of Naples, Southern Italy. Geological and historical records indicate that Campi Flegrei has been the site of intense uplift and subsidence phenomena. A historical period of uplift has occurred in Campi Flegrei from mid-1982 to 1984. Peak surface deformation reached about 1.8 m and the caldera experienced significant seismic activity. Since January 1985, the caldera floor is slowly sinking. Several mechanisms have been proposed to explain the caldera unrest: hot fluid migration, intense magma degassing, or the intrusion of a magma body followed by fluid removal. Given the significant density difference between silicate melts (~ 2500 kg/m3) and hydrothermal fluids (~ 1000 kg/m3) we can use density estimates from the joint inversion of gravity and geodetic data to distinguish between these two possible sources of caldera unrest. In this work, we determine first the location and geometry of the inflation source inverting leveling and trilateration measurements collected between 1980 and 1983. Then we determine the density of the intrusion by inverting leveling and gravity data gathered between 1982 and 1984. Finally, we use bootstrap to compute 95% bounds on the source parameters. The source that best fit the geodetic and gravity data is a penny shape crack, 1.25 to 4.25 km deep beneath the town of Pozzuoli, radius between 1.25 and 5.25 km, volume 0.035 to 0.075 km3, mass 0.025 to 0.075 MU, and density 600 to 1100 kg/m3. We estimate the location and geometry of the deflation source inverting leveling and trilateration measurements collected between 1990 and 1995. The source that best fit the geodetic data is now a vertical prolate ellipsoid, 2.3 km beneath Pozzuoli and with an aspect ratio of 0.49. These preliminary results support the hypothesis that hot fluid migration is the source of deformation in the Campi Flegrei caldera.
V23A-07 1330h
A Ground Deformation Monitoring Approach to Understanding Magma Chamber Systems and Eruptive Cycles of Mount Cameroon
Mount Cameroon is a 13,400ft basanite volcano on the passive margin of West Africa. It has erupted seven times in the past century making it one of the most active volcanoes in Africa. Most recently Mount Cameroon erupted in 1999 and 2000 first issuing strombolian explosions from vents near the summit, and later erupting effusively from a fissure running southwest from the summit (Suh et al., 2003). Prior to 2004, the only monitoring equipment on Mount Cameroon was a small seismometer network installed following the 1982 eruption. By 1999 only a single seismometer in the network was functional. Seismic activity did not rise above background levels until the few days immediately preceding the eruption. In an effort to raise awareness of the volcano's condition and provide a more efficient warning of impending eruptions we have begun constructing a ground deformation network on Mount Cameroon. The new network currently consists of two Applied Geomechanics 711-2A(4X) biaxial tiltmeters capable of resolving 0.1 microradians of tilt. One station is located approximately 500 m from the 2000 summit vent, and the other is approximately 1km away from the central fissure approximately 5km southwest of the 2000 summit vent. Three primary processes could precede eruptions at Mt. Cameroon, offering the opportunity for detection and prediction by our network. These processes are magma chamber pressurization, magma ascent via a central conduit, and/or propagation of magma along the central fissure. Magma chamber location, if a significant chamber exists, is poorly constrained, however, previous petrologic studies on Mount Cameroon (Suh et al., 2003; Fitton et al., 1983) suggest Mount Cameroon magmas originate at a depth less than 40km. Published seismic data (Ambeh, 1989) contains evidence of magmatic activity and possible chambers at depths ranging from 10km to 70km. Preliminary calculations using a simple Mogi model suggest deformation caused by pressurization of a large (greater than 2km radius) chamber near 40km deep could be detectable with proper instrument placement. Similar models involving small (500m-1km radius) chambers at 10km depth indicate the current network will be effective at detecting pressurization to 10 MPa or more. Our calculations also suggest the current network is capable of detecting magma ascending a 50m radius conduit at depths of 500 m or shallower. Using mean extrusion rates calculated during the 1999 and 2000 eruptions to estimate maximum magma ascent time (Suh et al., 2003), our current network could provide up to a week of warning prior to eruption. These preliminary calculations will be used in conjunction with future finite difference modeling and tiltmeter data from existing stations to place additional instruments in the network. References: Suh, C.E., Sparks, R.S.J., Fitton, J.G., Ayonghe, S.N., Annen, C., Nana, R., and Luckman, A. The 1999 and 2000 eruptions of Mount Cameroon: eruption behaviour and petrochemistry of lava (2003). Bulletin of Volcanology. Fitton, J.G., Kilburn, C.R.J., Thirlwall, M.F., and Hughes, D.J. 1982 eruption of Mount Cameroon, West Africa (1983). Nature Vol. 306 p. 327-332. Ambeh, William Bah. Seismicity and Seismological Studies of Mount Cameroon, Cameroon, West Africa (1989). PhD Dissertation Department of Earth Sciences, The University of Leeds. Leeds, LS 2 9JT. July 1989.
V23A-08 1330h
Stress State in Volcanic Areas due to Magmatic Intrusions in the Presence of Fluids
Magma intrusions commonly affect the seismic pattern in volcanic areas. We propose a model describing the stress change due to a magma intrusion represented by a tensile dislocation in a poro-elastic space. The presence of fluids in the porous medium is taken into account. Their diffusion involves a time dependent stress tensor, in turn the variation in pore pressure causes an elastic deformation in the medium. In this way areas where the seismicity is favored by a magmatic intrusion can change as a function of time. The results are compared with experimental data.
V23A-09 1330h
Interdisciplinary study on the effects of explosive eruptions of Somma-Vesuvius on people, animals and human settlements: a tool for risk assessment.
An interdisciplinary study of structures and human and animal victims of the Somma-Vesuvius explosive eruptions, dated from prehistory to modern age and based on the analysis of bioanthropological and geoarchaeological features, provides new results on the mechanical and thermal effects of the pyroclastic density currents (PDCs) and fallout. These studies allow us to define the dynamical overpressure associated with pyroclastic currents and to discriminate the role of mechanical impact, suffocation and thermal shock in causing death. Evidence of dynamical overpressure in the order of some tens of kPa have been recognized within a range of 10 km from the vent. Critical tephra accumulation above the roof collapse limit have been detected in archaeological sites NE to SE of the volcano, up to a distance of about 40 km. Evidence of effects induced by secondary pyroclastic density currents (lahars, flows, debris-flows) have been ricognized in all directions up to ca 50 km from the vent. Possible condition for lethalities due to suffucation have been assessed up to a distance of ca 25 km from the volcano. Our results provide new elements for emergency plans at Vesuvius.
V23A-10 1330h
Locations of Long-Period Seismic Events Beneath the Soufriere Hills Volcano, Montserrat, W.I., Inferred from a Waveform Semblance Method
Analysis of long-period (LP) seismic events provides information about the internal state of a volcano because LP events are attributed mainly to fluid dynamics between magma and hydrothermal reservoirs in its volcano (e.g., Chouet, 1992). We analyzed LP events recorded by three borehole seismic stations (AIRS, OLVN, and TRNT) at Soufriere Hills Volcano (SHV), Montserrat, W.I., during the period from March to June 2003. Borehole stations were deployed by the Caribbean Andesite Lava Island Precision Seismo-geodetic Observatory project (e.g., Shalev et al., 2003; Mattioli et al., 2004) and equipped with three-component short-period velocity seismometers with a sampling rate of 200 Hz. We selected 61 LP events with high signal-to-noise ratios. Almost all of the selected LP events are characterized by dominant periods in a range of 0.3 to 2.0 sec and durations of about 30 sec. Several LP events appear to be generated by a single source, based on the strong similarity in their waveforms. We first identified a family of LP events based on the dimensionless cross-correlation coefficient (CCC) of their spectral amplitudes of a period in a range of 0.2 to 2.0 sec, under the assumption of a fluid-driven crack model (Chouet, 1986). Seven LP events are identified as a family of LP events with high CCCs, particularly CCCs at AIRS in the vertical component greater than 0.88 in each event. This result suggested that these LP events are probably due to a repeated excitation of an identical source mechanism. We next attempted to estimate the locations of the identified a family of LP events by a waveform semblance method (Kawakatsu et al., 2000; Almendros and Chouet, 2003). To apply the above method, we searched the seismic phases with a rectilinear polarization from LP events, by performing a complex polarization analysis (Vidale, 1986). These phases are identified as averaged particle motion ellipticities of all stations in a time window less than 0.50. Incident angles of the detected phases are rather shallow and range from about 30 to 70 degree. These particle motions point approximately to a shallow source located beneath the SHV lava dome. Assuming the detected phases to be P-wave motions, we conducted the waveform semblance method for 0.3 to 2.0 sec bandpassed seismograms containing these phases. Waveform semblances are calculated in a 1.2 sec time window with sliding increments of 0.4 sec, assuming a constant P-wave speed (3.56 km/sec) appropriate for the SHV (Rowe, Thurber, and White, 2004). Our model space consists of 14 x 20 x 10 nodes with node spacing of 500 m, extending from -5 to 2 km in the east-west direction, from -5 to 5 km in the north-south direction, and from -1 to 4 km in depth. The coordinated center is fixed at the dome (16.712N, 62.176 W) and at sea level. The source location is defined as the grid where the waveform semblance reaches its maximum. Note that location uncertainties are about 1.0 km in all directions. As a result, we found that all the analyzed sources of LP events are located about 2.0 km north of the dome and 3.0 km deep.
V23A-11 1330h
A Preliminary Study of Real-Time Seismic Event Amplitudes From the CALIPSO Borehole Dataset, Montserrat, 2002-2003
The Caribbean Andesite Lava Island Precise-Geodetic Seismic Observatory (CALIPSO) project installed instruments at four sites around the Soufriere Hills Volcano beginning in December 2002. The instruments include Sacks-Evertson strainmeter, borehole tilt meter, borehole seismometers and continuous GPS with the borehole instruments installed at a depth of ~200m. Two seismometers and three strainmeters were operating during the July 12-13, 2003 lava dome collapse and subsequent vulcanian explosive events. The seismometers recorded hybrid swarms beginning one week prior to the eruption and intensifying through July 11th. By the morning of July 12th, the discrete events had merged into a continuous tremor event. Real-time Seismic Amplitude Measure (RSAM) is an analysis method that computes the average amplitude of the seismic signals to give a measure of the total seismic energy in the volcanic system over a set period of time. This method is particularly effective during volcanic events and other periods of intense seismic activity because it is not limited by the requirement of separating the events. RSAM time plots will be created from the archived data starting with the July 12-13, 2003 event and continuing backward through time until no significant seismic energy is observed. Periods of high energy on the RSAM plots will be examined closely to identify event types and determine hypocenter locations. These data will be compared with other data from CALIPSO and the Montserrat Volcano Observatory (e.g. surface deformation) to examine precursory phenomena prior to the major collapse event.
V23A-12 1330h
Lanslide Disaster in Wabane Cameroon
The landslide disaster that occurred in Wabane, Cameroon, on 20th July 2003 killed 21 people. The area where the landslide occurred (Magha) is a caldera, with hot springs. The landslide was accompanied by earth fractures and people smelled gas characteristic of gun powder immediately following the incidence. The evidence above seem to suggest that there are some other forces acting underground. Renewed volcanic activity may be associated with the frequent landslide activity experienced in this part of Cameroon. If this is true, it could mean that magma is up welling in the caldera. Therefore there may be a greater disaster in the making and this is what the local Wabane population is not very aware of.