Horizontal rotation signals detected by "G-Pisa" ring laser for the Mw=9.0, March 2011, Japan earthquake

TL;DR

This study demonstrates the detection of ground rotations caused by the 2011 Japan earthquake using a ring laser gyroscope, establishing a relationship between rotations and accelerations and confirming wave velocities with theoretical models.

Contribution

It introduces the use of a ring laser gyroscope to measure earthquake-induced ground rotations and derives a theoretical link between rotations and accelerations for Rayleigh waves.

Findings

Excellent coupling of ring laser signals with vertical accelerations at periods >100s

Derived a theoretical relationship between rotation and acceleration for Rayleigh waves

Confirmed that apparent velocities are always ≥ true wave velocities due to mounting orientation

Abstract

We report the observation of the ground rotation induced by the Mw=9.0, 11th of March 2011, Japan earthquake. The rotation measurements have been conducted with a ring laser gyroscope operating in a vertical plane, thus detecting rotations around the horizontal axis. Comparison of ground rotations with vertical accelerations from a co-located force-balance accelerometer shows excellent ring laser coupling at periods longer than 100s. Under the plane wave assumption, we derive a theoretical relationship between horizontal rotation and vertical acceleration for Rayleigh waves. Due to the oblique mounting of the gyroscope with respect to the wave direction-of-arrival, apparent velocities derived from the acceleration / rotation rate ratio are expected to be always larger than, or equal to the true wave propagation velocity. This hypothesis is confirmed through comparison with fundamental-mode, Rayleigh wave phase velocities predicted for a standard Earth model.