HR: 0800h
AN: P41A-0894    [Abstracts]
TI: Tomography Study of Shock-Induced Damage Beneath Craters by Normal and Oblique Impacts
AU: * Ai, H
EM: ahr@gps.caltech.edu
AF: Caltech, Caltech, 252-21, Pasadena, CA 91125 United States
AU: Ahrens, T
EM: tja@gps.caltech.edu
AF: Caltech, Caltech, 252-21, Pasadena, CA 91125 United States
AB: Comparisons of laboratory impact craters produced in rock and planetary-scale impact structures, indicate that the observed reductions in elastic wave velocities by shock-induced damage of rock beneath impact craters can be used to constrain the impact history. A series of small-scale normal and oblique impact experiments were conducted on 20x20x15 cm samples of San Marcos granite by a 1.2 km/s, 2 kJ impactor. The resulting largely circular (8 cm in diameter) crater dimensions agrees closely with previous data. By conducting a multiple source-receiver ultrasonic survey of the shocked rock beneath laboratory craters (sampled by 290 ray paths beneath the crater) we have tomographically mapped the in-situ P-wave velocity beneath craters and find measurable damage, as defined by $>$ 0.1 km/s velocity reduction, are induced to depths of 7 cm beneath the crater for normal impacts. However, oblique impacts produce shallower damage zone ($\sim$ 3 cm deep) that are asymmetric along the plane containing the impact trajectory. The downrange shows more damage than the uprange. Since the extent of the shock-damage region depends on impact velocity and impact energy, the extent of damage in our laboratory impact structures , and we presume also planetary scale impact structures, carries both impact velocity and direction of impact information not previously recognized or sought. Hence damage zone dimensions are expected to constrain planetary impacts parameters. Oblique impacts, where the tracjectory is $\geq$ 15$\deg$ relative to the impacted surface, yields approximately circular craters, can in principle, provide information on impactor trajectory. For planetary impacts, the damage profile, as measured by seismic velocity deficit, beneath craters allow some statistical constraint on impacts produced by low-inclination orbit objects (asteroids and Jupiter-family comets), versus, high-inclination orbit objects (long-period and new comets).
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5420 Impact phenomena (includes cratering)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting