The velocity and attenuation of ultrasonic stress waves in gabbroic rock samples (San Marcos, California) subjected to shock loading in the 2 GPa range were studied. From P wave velocity measurements we determined the damage parameter Dp and crack density &egr; of the samples and related these to the attenuation coefficient (quality factor) under dynamic strains of 2¿10-7 and at a frequency of 2 MHz using the ultrasonic pulse-echo method. A fit to the data yields the P wave spatial attenuation coefficient at a frequency of 2 MHz, αp(Dp)=1.1+28.2Dp (decibels per centimeter). From the relation between the attenuation coefficient and quality factor, the quality factor Q is given by Q-1=0.011(1+25.6Dp)(1-Dp)1/2. Using O'Connell-Budiansky theory relating crack density to velocity, the parameter in Walsh's theory was determined based on experimental data. An approximate method is also proposed to estimate the average half-length of cracks based on the attenuation measurements.¿ 1997 American Geophysical Union