About
The objective of FIMOPTIC project is to improve the detection and mechanical characterization of the sources of the weak strain signals caused by transient instability of fault zones, which are sometimes precursory to large seismic ruptures.
For this, we will take advantage of the optical interferometry technology Fabry-Perot that our team (ESEO, IPGP and ENS) has developed in the last 10 years for high resolution optical instruments (long base tiltmeter, seismometer, and strainmeter), and will evaluate their advantages over commercial instruments.
Two very active seismogenic pilot sites are selected, on the science and monitoring of which the proponents have been much involved:
- The western rift of Corinth, focusing here on the pore pressure and slow slip instabilities of the fault system, a site monitored since 2000 by the Corinth Rift Laboratory (CRL), involving in particular IPGP and ENS;
- Inside a deep, active mine in Sweden, in Garpenberg (BOLIDEN mine), for reaching the neighborhood of repeating seismic asperities of metric dimension, frequently cycled (“repeaters”), around 1100 m in depth, a site monitored for the last 6 years by INERIS.
The optical long base tiltmeter and seismometer have been already qualified, the former at the top of the La Soufrière volcano in Guadeloupe, the latter at CERN.
The optical strainmeter remains to be developed, on the basis of a buried, sensitive decametric fiber, integrating the rock strain, more suitable on the field than classical measurements with Bragg grating systems, limited to decimetric integrations. First tests with the ESEO interrogator allows us to aim at a resolution better than 10 nanostrain, resolving the earth tide.
In Corinth, the long base tiltmeter will be installed in a tunnel, associated to an optical strainmeter, in the vicinity of the Psathopyrgos fault, the most threatening of the site. Another strainmeter will be cemented in a borehole. The optical seismometer will be installed in a 200 m deep borehole. During the 3 years of the project, dozens of seismic swarms are expected at less than 10 km, with possibly detectable episodes of transient deformation. The records will be jointly analyzed with those of CRL (GNSS, InSAR, borehole strainmeters, seismic array), using correlation and A.I., and the source of the strain signals will be modelled.
In Garpenberg, a first installation plans 500 m of optic cable in galleries close to the repeater families (less than 20 m), for an interrogation by commercial systems DAS and BOTDR, complemented by seismometers, for determining the precise location of the activated asperities. A second installation plans to focus on one or a few repeaters, to get very close to them (a few meters) thanks to small boreholes equipped with optical fibers for our strainmeter prototype and for the BOTDR. These instruments will be in operation until the end of the project. All these measures will be jointly analyzed, integrating the geological information from the drillings, and with A.I. tools. The rupture cycle of one asperity will be analyzed and modelled by friction laws on the fault. The rythm of rupture of these asperities (days-months), guarantees the recording of several complete cycles, possibly including phases of unstabilities, or even of rupture initiation or nucleation.
FIMOPTIC project will bring new knowledge on the mechanics of deformation and on the seismic hazard of the two sites. It will also bring generic knowledge on the seismic-aseismic coupling of fault systems. The validation of the capabilities of our instruments should favour their use for the geomonitoring in academic (observatories for seismicity, volcanoes, offhshore,…) as well as industrial sector (mines, georeservoirs, deep geothermy, …) of the seismic cycle of asperities and on the unstable seismic- aseismic coupling in fault zones.
As a complementary “product” of FIMOPTIC project, the new instruments will be demonstrated and qualified in the field, and advertised for being used in other contexts of geo-monitoring in academic or industrial environment.
