Static Laboratory Earthquake Measurements with the Digital Image Correlation Method

Abstract

Mapping full-field displacement and strain changes on the Earth’s surface following an earthquake is of paramount importance to enhance our understanding of earthquake mechanics. Currently, aerial and satellite images taken pre- and post-earthquake can be processed with sub-pixel correlation algorithms to infer the co-seismic ground deformations (e.g., [1, 2]). However, the interpretation of this data is not straightforward due to the inherent complexity of natural faults and deformation fields. To gain understanding into rupture mechanics and to help interpret complex rupture features occurring in nature, we develop a laboratory earthquake setup capable of reproducing displacement and strain maps similar to those obtained in the field, while maintaining enough simplicity so that clear conclusions can be drawn. Earthquakes are mimicked in the laboratory by dynamic rupture propagating along an inclined frictional interface formed by two Homalite plates under compression (e.g., [3]). In our study, the interface is partially glued, in order to confine the rupture before it reaches the ends of the specimen. The specimens are painted with a speckle pattern to provide the surface with characteristic features for image matching. Images of the specimens are taken before and after dynamic rupture with a 4 Megapixels resolution CCD camera. The digital images are analyzed with two software packages for sub-pixel correlation: VIC-2D (Correlated Solutions Inc.) and COSI-Corr. Both VIC-2D and COSI-Corr are able to characterize the full-field static displacement of the experimentally produced dynamic shear ruptures. The correlation analysis performed with either software clearly shows (i) the relative displacement (slip) along the frictional interface, (ii) the rupture arrest on the glued boundaries, and (iii) the presence of wing cracks. The obtained displacement measurements are converted to strains, using non-local de-noising techniques; stresses are obtained by introducing Homalite’s constitutive properties. This study is a first step towards using the digital image correlation method in combination with high-speed photography to capture the highly transient phenomena involved in dynamic rupture.