Paul A. Korswagen1, Edwin Meulman2, Michele Longo2, and Jan G. Rots2
1)  Delft University of Technology
Stevinweg 1, 2628 CN Delft, NL
2)  Delft University of Technology
Stevinweg 1, 2628 CN Delft, NL

Keywords: Shallow earthquake; Induced seismicity; Unreinforced masonry structures; Crack analysis; Digital Image Correlation; Damage scale

Abstract. In recent years, the gas extraction in the northern part of the Netherlands is causing induced, shallow earthquakes. Besides safety, the prediction and evaluation of ‘light’ damage due to induced ground motions is important, being related to economic and serviceability losses, and societal unrest. Hence, the initiation and propagation of cracking (related to light damage) need to be investigated. An experimental and numerical campaign is
ongoing at the Delft University of Technology, aiming to improve the knowledge of the underlying physics of crack initiation and crack propagation in unreinforced masonry (URM) structures typical in the Netherlands. A damage scale and damage parameter with a look at cracking in the light damage stages are defined in order to objectively quantify the cracks in a URM structure.
Different causes, among which dynamic loads (earthquake vibrations), imposed deformations, and repeated loading are investigated. The cracking mechanisms are studied for URM walls and spandrels subjected to in-plane loading. Displacements, strains and loads under which cracking starts and propagates are evaluated and correlations are sought. The Digital Image Correlation (DIC) measuring system is used to accurately detect the crack formation and the evolution of the crack pattern. This is used also to validate and calibrate FEM models. The models are used to extrapolate to situations of combined loading, difficult to accomplish experimentally. These situations include walls pre-damaged by settlement or shrinkage stresses, which also exhibit comparatively more damage when then subjected to earthquake loads. With there results, (fragility-like) indicative resistivity curves are produced.