<p>CO₂ can be used as a fracturing and heat-exchange fluid in enhanced geothermal systems for volcanic rocks. Water-assisted CO₂ fracturing, in which water injection pressurizes the CO₂ in the borehole, has been experimentally shown to generate complex, wide-aperture fractures in volcanic rocks. However, injection-induced seismic hazards caused by pre-existing fractures or fault slips during CO₂ and water injection must be assessed to enable safe field applications. This study investigates the shear-slip behavior of pre-existing fractures inclined 45° from the sample axis in low- (~ 5%) and high-porosity (~ 20%) volcanic rocks during CO₂ and/or water injection at 200&#xa0;°C, initial axial stress of 80&#xa0;MPa, and confining pressure of 40&#xa0;MPa, for dry and water-saturated matrices. Across all tests, CO₂ injections generally produced slower fracture slips than water injections. In low-porosity rocks, higher fracture-slip velocities occurred during CO₂ injection into water-saturated matrices than into dry matrices. In high-porosity rocks, the fractures of dry samples exhibited an extended phase of slower slip, followed by an abrupt acceleration at high pressure in both water and CO₂ injections. During CO₂ and water injections, the fractures in water-saturated high-porosity rocks exhibited slip characteristics similar to those in water-saturated low-porosity rocks. Water-assisted CO₂ injection (simulating water-assisted CO₂ fracturing conditions) into water-saturated low-porosity rock fractures yielded a fracture slip behavior comparable to that with water injection into water-saturated fractured samples. Based on slip velocity as a proxy for induced-seismicity hazard, these results indicate that CO₂ injection does not pose a greater induced-seismicity hazard than water injection.</p>