Observation of rock-tunnel response to earthquake motions is compared with calculated peak surface motions for 71 cases to determine damage modes and indices. Damage, ranging from cracking to closure occurred in 41 of the observations. Peak surface motions serve as the principal variables in the study since shaking affects the greatest tunnel mileage, is nonsite specific and has the greatest likelihood of multiple occurrence. The peak motions, calculated from the Richter Magnitude and distance from tunnel to causative fault, and associated damage are campared internally and externally. The internal comparison of earthquake observations yields threshold values of peak motions associated with specific modes of tunnel damage. These thresholds are then externally compared with damaging motions from explosion-test studies to determine the relative conservatism of the threshold values. The results of the comparisons indicate that the damage thresholds are conservative and that tunnels are safer than above-ground structures for a given intensity of shaking.