Abstract. Both small-scale impact craters in the laboratory and less than 5 km in diameter bowlshaped craters on the Earth are strength (of rock) controlled. In the strength regime, crater volumes are nearly proportional to impactor kinetic energy. The depth of the cracked rock zone beneath such craters depends on both impactor energy and velocity. Thus determination of the maximum zone of cracking constrains impact velocity. We show this dependency for small-scale laboratory craters where the cracked zone is delineated via ultrasonic methods. The 1 km-deep cracked zone beneath Meteor Crater is found to be consistent with the crater scaling of Schmidt (1) and previous shock attenuation calculations. INTRODUCTIONImpact-induced fractures in rock have long been recognized beneath terrestrial impact craters. Simmons et al. (2) pointed out that the multiply impacted near-surface rocks on the Moon were fractured to depths of tens of kilometers and the fracture density controls the near-surf ace seismic velocity structure. Dvorak and Phillips (3) discovered cracks beneath young fresh craters that were unfilled on the Moon, or filled with air or water on the Earth gave rise to significant (negative) gravity anomalies. Ahrens and Rubin (4) demonstrated that the seismic velocity of rocks beneath small laboratory, strength-limited, craters increased with depth. This results from the near-surface zone of cracking beneath craters.The intensity of cracking decreases with increasing depth reflecting the decreasing dynamic tensile stresses with depth in the rock. The volume of strengthcontrolled impact craters has been demonstrated to be nearly proportional to impactor energy for soft and hard rocks (5).

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