LASER absolute long base TILTmeter: An innovative instrument to measure a new class of slow earthquakes.

GNSS and seismological networks have greatly developed to accurately measure large-scale crustal deformations along faults. It is possible to observe transient deformations at different time scales over durations ranging from hours to several months. But events below Mw6.5 are rarely detected because they are at the limit of GNSS sensitivity (~1 mm). Observing these slow transients is a key element in improving the understanding of the processes that can trigger an earthquake. Here we present a new innovative geodesic tool to improve the detection capacity of these transients. A first mechanical silica version of a long base inclinometer (ILB) operating without optics was installed on the northern Chilean subduction. In 2014, two ILBs were used to observe slow deformations of very low amplitudes (50 nrad/month) preceding the magnitude 8.2 Iquique earthquake. These slow pre-seismic slips can occur frequently but are rarely observed because of their low magnitude (Mw5 to 6), at the limit of GNSS sensitivity. Characterizing these events (location, spatial extension) thus requires more precise geodesic networks. Our previous mechanical inclinometer sensors were fragile, complex and difficult to install, making network installation impossible. We are proposing a new ILB concept. The biggest change concerns the direct non-contact measurement of the liquid level in the vessels, which is carried out by optical fiber(s) coupled to a high precision Fabry-Perot (FP) type interferometer. This system has 2 EU & USA patents. A first O-LBT (150m long) has been operational since 2012 at LSBB, France. Through a semi-industrial project, a second prototype with a modular design has been running at CERN from 2016 to 2019 to validate its performance for particle accelerator alignment. These O-LBTs demonstrate 10-11 rad resolution and stability (drift) like to the best installations in the world, together with long-term reliability in the context of deep tunnel installation and authorize network installations. The first instrument  (41 m long) in a seismic zone was installed in the Gulf of Corinth in Greece and a second network installation is planned for the coming months.