Tensile rock mass strength estimated using InSAR

The large‐scale strength of rock is known to be lower than the strength determined from small‐scale samples in the laboratory. However, it is not well known how strength scales with sample size. I estimate kilometer‐scale tensional rock mass strength by measuring offsets across new tensional fractures (joints), formed above a shallow magmatic dike intrusion in western Arabia in 2009. I use satellite radar observations to derive 3D ground displacements and by quantifying the extension accommodated by the joints and the maximum extension that did not result in a fracture, I put bounds on the joint initiation threshold of the surface rocks. The results indicate that the kilometer‐scale tensile strength of the granitic rock mass is 1–3 MPa, almost an order of magnitude lower than typical laboratory values.