Brittle failure in the crust manifests as diverse geological phenomena such as localization of earthquakes, induced hydrofracturing of subsurface reservoirs for oil and gas, and geothermal energy production. Predicting the occurrence of earthquakes is an important question in Earth science. Designing safe and efficient protocols for the extraction of geo resources is a geotechnical challenge. These geological phenomena happening at a kilometer scale can be understood by considering the material failure properties of rocks. Brittle failure in rocks has microstructural origins. My study aims at exploring the microscale deformation behaviour of rocks subjected to brittle failure. I deformed a centimeter scale rock specimen using a triaxial deformation apparatus, HADES (built in-house) and image the microstructure of rock samples using 4D X-ray tomography, which means imaging 3D rock for a series of incremental loads on the specimen. A series of digital volumes of the rock microstructure is obtained until failure, which facilitates tracking the accumulation of damage, localization of strain and underlying microstructural mechanisms that govern failure in brittle rocks. Using a detailed analysis of microstructural evolution, this experimental study provides the first test of theoretical models of failure proposed for heterogeneous brittle rocks and identifies mechanisms of strain localization and earthquake nucleation in them.