Optical Instruments for Detection and Characterization of Slow Slip Events

Abstract:

GPS networks have sufficient sensitivity to map the location of Slow Slip Events (SSEs) only when averaged over days or weeks; they often are not able to distinguish the timing of various sub-events because of the need for such averaging. We have developed sensors that rely on optical interferometry that can detect crustal deformations and tremors with very high precision in harsh environments, including the seafloor.

The first of these sensors is an optical fiber strainmeter, which consists of an optical fiber cable elastically stretched between two anchor points separated by 100 m or more, either in a borehole or in a shallow trench. The fiber’s length is recorded with laser interferometry, enabling detection of nanostrain crustal deformations. Optical fiber strainmeters have their best signal-to-noise ratio at shorter periods, complementing GPS. Because SSEs evolve in complex patterns indicative of propagating stress fronts, it is important to resolve, both in scale and in time, their deformation signals to understand more fully the evolution of the rupture plane. In conjunction with GPS, the availability of low-cost but highly sensitive and stable strainmeters will enable such characterizations.

A second sensor system uses laser interferometry to record the displacements of inertial mechanical suspensions – spring-mass for the vertical component and pendulums for the horizontal components – housed in a borehole sonde. In a single borehole package, this provides a broadband vertical seismometer/gravimeter and a broadband two-component horizontal seismometer/tiltmeter. The combined system is able to measure vertical and horizontal ground velocities, gravity, and tilt with sensitivities that compare favorably with any existing system over time scales from 10 Hz to many days; the downhole components are entirely passive, giving a long instrument lifetime and resistance to high downhole temperatures.

Several versions of the borehole inertial system have been deployed on land with excellent results, and a number of our optical fiber strainmeters have been deployed – both onshore and offshore – with results indicating high probability for eventual detection of SSEs.