Analysis of ice-induced acoustic events in the Central Arctic

Ice-induced acoustic events in the Central Arctic are analyzed in this thesis, with the purpose to estimate parameters that characterize the motion of their generating mechanisms and ultimately to identify and distinguish the latter. Individual events, detected in ambient noise data from the SIMI experiment are analyzed in the frequency range 10-350 Hz. Parameters that pertain both to particle motion and source propagation are estimated. In particular, slip, rise time, particle velocity and the slip time function are first determined from corresponding time-domain event parameters. Due to the lack of independent measurements of ice-borne seismic waves (compressional and/or shear), for each event an appropriate fault model is assumed, based on the event radiation characteristics. The important physical conclusion from this analysis is that particle slip for ice processes is in the range O(10^-4)[-]O(10^{- 2}) m, at least 3 orders of magnitude lower than the corresponding values for earthquakes. Also, particle velocity is in the range ≃1-67 cm/s, and on average about 50% lower than particle velocity of a displacement discontinuity during an earthquake. Source parameters that pertain to source propagation, namely source speed, orientation (strike-angle) and dimensions are then estimated. On average, fracture speed estimates are in the range 200-1100 m/s, significantly lower than the previously assumed Rayleigh wave speed (1700 m/s). This result implies that the characteristic parameters for fracture propagation in Arctic ice vary significantly and deviate from theoretical limits, as is the case with rock. The wide range of speed estimates supports the hypothesis that either a multitude of event mechanisms or propagation features of a particular mechanism in the ice, are associated with the detected acoustic events. Source dimensions have also been determined from the displacement spectrum of the source- wave parameter, estimated from the acoustic event signal. Source length or range of deformation is in the range 0.7-100 m, and depth is 0.4-4 m. Both length-controlled and depth-controlled mechanisms are identified, based on the agreement of the data with predictions of existing fault models. Finally, the event radiation characteristics are estimated and modeled, with the purpose to identify the dominant types of ice-motion processes. Unloading motion and fracture, both shear and tensile are the most plausible mechanisms. Fracture dominates the mid-frequency range, although it is shown that some low-frequency events may be associated with secondary cracks, probably generated at an early stage in the fault formation. Secondary fracture features are shown to contribute to the field of sound radiation, particularly from a shear fracture. The contribution of sound radiation from secondary cracks to the acoustic pressure field due to a shear fault is an important physical conclusion, and shows that fractures in sea ice behave in a similar manner as fractures in rock. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253- 1690.) (Abstract shortened by UMI.)