Electrical resistivity variation connected to volcanic earthquake in the - PowerPoint Presentation

1. Electrical resistivity variation connected to volcanic earthquake in the Campi Flegrei, ItalyTarallo Daniela1, Cavuoto Giuseppe1, Di Fiore Vincenzo1, Pelosi Nicola1, Punzo Michele1, Milano Maurizio2, Contiero Massimo2, Iavarone Michele2, Iorio Marina21 ISPC - CNR – Institute of Cultural Heritage Science, Naples, Italy 2 ISMAR - CNR – Institute of Marine Science, Naples, Italy

2. TOPICIn this study we show an 2D Electrical Resistivity Tomography (ERT) survey acquired in Agnano site pre (Dec 5th, 2019) and post (Dec 12th, 2019) earthquake events occurred in Pisciarelli-Solfatara areas. This earthquake swarm consisted of sequence of 34 earthquakes with Magnitude (Md) -1.1≤Md≤2.8 at depths between 0.9 and 2.3 km. In particular, the earthquake of Dec 06th, 2019 at 00:17 UTC with Md = 2.8 (depth 2 km) was the maximum recorded event since bradyseismic crisis began in 2005.The hydrothermal system was identified by very low values of the electrical resistivity (<20 Ω m). Its downwards extension is clearly limited by the lava and pyroclastic fragments, which are relatively resistive (>100 Ω m). The resistivity values are increased after the main shock. The present observation suggests that the temporal variation of the resistivity values is related to the variation of the pore fluid pressure in the source area of the swarm, facilitated by earthquake and the subsequent fluid diffusion.

3. SummaryIntroductionStratigraphic dataAcquisition parameters and survey map locationPre-inversion compare resultsPost-inversion compare resultsResults interpretationConclusions

4. In this study we show an 2D Electrical Resistivity Tomography (ERT) survey acquired in Agnano (Campi Flegrei, Italy) site pre (Dec 5th, 2019) and post (Dec 12th, 2019) earthquake events occurred in Pisciarelli-Solfatara areas. Agnano area is located in the northeastern sector of the Campi Flegrei, Southern ItalyAstroni VolcanoSolfatara VolcanoPozzuoli cityEarthquake M=2.8This earthquake swarm consisted of sequence of 34 earthquakes with Magnitude (Md) -1.1≤Md≤2.8 at depths between 0.9 and 2.3 km. In particular, the earthquake of Dec 06th, 2019 at 00:17 UTC with Md = 2.8 (depth 2 km) was the maximum recorded event since bradyseismic crisis began in 2005. I t a l ySurvey areaINTRODUCTION

5. Geological backgroundCampi Flegrei is a volcanic field that occupies a large area (about 450 km2) that goes from the island of Procida to the western of the Naples city. They are mostly monogenic volcanoes, produced by a single explosive eruptive activity. The oldest rocks belonging to this volcanic field emerge right in the Neapolitan urban area and date back to about 78 thousand years ago. The eruptive history of the Campi Flegrei is certainly older but the previous volcanic products are altered, therefore unsuitable to be dated. Volcanological and geochronological considerations make it possible to subdivide the eruptive history of the Campi Flegrei into four periods (Scarpati et al., 2016): Paleoflegrei (>74/78 thousand years)I Period (from 39 thousand to 74 thousand years)II Period (from 15 thousand to 39 thousand years)III Period (present)INTRODUCTION

6. Agnano-Monte Spina eruption (Epoch III)It extruded a volume of magma of about 1.2 km3Petrological data show that the eruption was fed by two isotopically and chemically distinct magma batches that mechanically mixed during the eruption. Upper magma: alkali–trachytic Lower magma: trachyticStratigraphic sequence presents an erosional unconformity occurs in the central part of the sequence.Composite graphic log of the Agnano-Monte Spina eruption (redrawn from Dellino et al., 2020)(Scarpati et al., 2013)Distribution for all the samples A-MS analyzed related to the schematic stratigraphic (b), (Romano et al., 2020)STRATIGRAPHIC DATA

7. Stratigraphic sequences of selected boreholes(De Vita et al., 1999)Recent sea level End of A-MS productsBeginning A-MS productsLegendSampling Map ERT survey STRATIGRAPHIC DATA

8. EWACQUISITION PARAMETERS AND MAP LOCATION

9. W mPseudosection before of the earthquakePseudosection after the earthquake(Dec 5th, 2019) (Dec 12th, 2019) Area of greatest increase of post-earthquake resistivityPRE-INVERSION COMPARE RESULTSEW

10. W mResidual post-pre earthquake pseudosectionRatio post-pre earthquake pseudosectionPRE-INVERSION COMPARE RESULTSEW

11. EWW m(Dec 5th, 2019) (Dec 12th, 2019) The ERT survey allow us to identify the main structural boundaries (and their associated fluid circulations) defining the shallow architecture of the Agnano volcano. The hydrothermal system is identified by very low values of the electrical resistivity (<20 Ω m). Its downwards extension is clearly limited by the lava and pyroclastic fragments, which are relatively resistive (>100 Ω m). Final ERT section before earthquakeFinal ERT section after earthquakePOST-INVERSION COMPARE RESULTS

12. W mResidual post-pre earthquake ERT sectionRatio post-pre earthquake ERT sectionPOST-INVERSION COMPARE RESULTSEW

13. Composite graphic log of the Agnano-Monte Spina eruption (redrawn from Dellino et al., 2020)Distribution for all the samples A-MS analyzed related to the schematic stratigraphic (b), (Romano et al., 2020)Structural map of the central sector of Campi Flegrei Caldera (Isaia et al., 2015)Fault ? RESULTS INTERPRETATION

14. The question is:Why does there be a variation of resistivity in subsoil?1. The earthquake produces an increase of gas injection in the system Gas injection allows an increase of resistivity or2. The earthqueke produces a variation of the internal porosity of the rock An instant increase in the rock porosity leads to a fluid content rarefaction resulting in increased resistivity3. Both hypotheses 1 and 2 occur RESULTS INTERPRETATION

15. - The resistivity values are increased after the earthquake main shock. Papadopoulou et al., 2018RESULTS INTERPRETATION- This increase in resistivity may have been caused by a change in the state of stress and a decrease in pore pressure (subsequent depressurization). Previously to the earthquake, an increase in pressurized fluids has been observed which have reduced the resistivity values. The present observation suggests that the temporal variation of the resistivity values is related to the variation of the pore fluid pressure in the source area of the swarm, facilitated by earthquake and the subsequent fluid diffusion. The combination of these qualitative results with structural analysis leads to a synthetic model of magmatic and hydrothermal fluids circulation inside the Agnano area, which may be useful for the assessment of potential hazards associated with a renewal of fluid pressurization, and a possibly associated partial flank-failure.The daily mean of the potential difference recorded at the time the source operated at the two channels of ISA. The green vertical line is drawn at the time of the drop in DV (in mV) on 21 November 2014. The yellow rectangles are placed at the occurrence time of the two earthquakes.By studying source recordings, authors found a decrease in the subsurface resistivity two months before a major earthquake in the area.

16. Post-earthquakeIf pores are not filled with water the CO 2 can diffuse to the surfacePre-earthquakeSome pores ‘dry up’ and thesoil is partly water saturated; measurement of moderate CO fluxes and increasing SP valuesPoints with greater flow of CO2 from the soil.Meanly, the resistivity is higher in these pointsPoints with greater flow of CO2 from the soil.Meanly, the SP are lower in these points(Sauer et al., 2014)RESULTS INTERPRETATION- The resistivity values are increased if CO2 flux increased.

17. The interpretations of geoelectric survey in volcanic areas also active seismically can be unreliableThe results obtained imply a possible volcanic monitoring integrate by SP, time-lapse ERT, meteorological parameters and soil-gas measurementsERT monitoring under certain conditions could be used as a precursor to seismic eventsCONCLUSION

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