Analyses of far-field coseismic crustal deformation observed by a new laser distance measurement system

We have developed a novel absolute distance meter (ADM) for determining coseismic strain steps and analysed far-field crustal deformation caused by earthquakes. By measuring the optical resonant frequency of a 100-m-long optical cavity in relation to an accurate time standard, the absolute distance can be determined with a resolution of 2 × (10−9–10−10); absolute measurements ensure the detection of distance changes before and after an earthquake, even when encountering large seismic waves that can often hinder relative distance measurements. The proposed system, consisting of the ADM in combination with a conventional laser strainmeter (LSM), a relative distance meter having a resolution of 1 × 10−10 in strain, can successfully observe coseismic strain steps ranging from 1 × 10−10 to 3.5 × 10−8. We analysed strain steps of 10 earthquakes (M 5.8–7.4 and depths of 8–374 km) that occurred around the central part of mainland of Japan at hypocentral distances of 100–530 km from the Kamioka underground observatory. The observed strain steps were compared with expected strains calculated using a variety of fault parameters inferred from existing observations (both seismic and geodetic), assuming two typical models: a homogeneous elastic half-space (Case 1) and a spherically symmetric earth model with a modified PREM structure (Case 2). After correcting the estimated rigidity around the source region for calculating a point dislocation from a seismic moment, the observations strongly supported Case 2 rather than Case 1 for deep earthquakes (hypocentral depth >20 km). On the basis of our analyses, we concluded that the distance measurement system could detect coseismic strains with a resolution of 1 × 10−10 and a wide detection range of at least 3.5 × 10−8, and could also precisely determine crustal deformation far from the source region. The observation constrains dislocation theories, especially for the deep region, and may geodetically constrain fault parameters on the basis of far-field deformation even for deep and ocean earthquakes whose surface deformations are difficult to observe with conventional geodetic methods.