The Hydrothermal Source of the Md 4.0, 2017, Ischia Earthquake (Italy): a Multidisciplinary Approach
Abstract
On 21st August 2017, a M d 4.0, earthquake occurred at Ischia volcanic island (Italy). This event may be considered as representative of the seismic behavior of the island, at least regarding location, damage, magnitude, and shallow depth. The aim of our study is to decipher the mechanisms responsible for the 21 August 2017, M d 4.0 earthquake and its triggering factors by integrating the results of the analyses of (a) seismic records, (b) geodetic (Global Navigation Satellite Systems [GNSS] and differential interferometric synthetic aperture radar [DInSAR]) signals, (c) geodetic data inversions, and (d) available geological and geochemical information. Our results show that the Ischia earthquake is characterized by a low-frequency content (2 Hz), low value of stress drop (0.01 MPa), and a low S/P spectral ratio. These evidences suggest the involvement of fluids in the source mechanism. The determined focal solution indicates a mixed shear-tensile rupture with the P first arrivals showing up movements in the nearest stations. Ground deformations describe an E-W elongated coseismic subsidence overlapping a WSW-ENE fault bounding the hydrothermal reservoir at depth. The modeled deformation field is coherent with a double source constituted by a WSW-ESE striking, north-dipping normal fault, and by a closing sub-horizontal crack. This closure immediately follows an initial opening related to a fluid pressurization event responsible for the earthquake. We show that moderate magnitude earthquakes in active hydrothermal areas may be associated with the pressurization/depressurization cycles of a hydrothermal reservoir due to self-sealing processes and not to the arrival of new fluids from depth. Events like that recorded at Ischia highlight the role of the hydrothermal fluids, pre-existing faults, and self-sealing processes in triggering seismicity. Other events like that recorded on Ischia, which also affected the island in historical times, are not associated to 'volcanic unrests' episodes and testify the occurrence of a fault-valve mechanisms. Therefore, the dynamics of hydrothermal systems must be taken into account in the seismic hazard evaluation of volcanic areas.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S51E0456C
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 7290 Computational seismology;
- SEISMOLOGY