Hydroacoustic events located at the intersection of the Atlantis (30°N) and Kane (23°40′N) Transform Faults with the Mid-Atlantic Ridge



We investigate the characteristics of T-phase events located at the ends of two segments of the Mid-Atlantic Ridge. Our motivations for the study were to understand whether T-phase locations represent earthquake epicenters (and thus whether accurate geological inferences can be made from their spatial patterns) and to understand further the relationship between T-phase event characteristics and earthquake properties. We examine the characteristics of 158 T-phase events with respect to both event location water depth and source to receiver distance. The propagation paths of the T-phases are also modeled to study the effects of encountering seafloor topography. We find that existing models for T-phase excitation and propagation cannot explain adequately all of our observations. The amplitudes (Received Levels) of T-phases at the hydrophones show no dependence on event water depths, in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth. The Received Levels are observed to decrease with increasing source to receiver distance, and events from the two study areas exhibit different trends in relative Received Levels between hydrophones, once attenuation is taken into account. Our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1-D topography between the event and each hydrophone, but the variances in both the observed data and model are high. We observe a pattern of short T-phase onset times for shallow water events and long onset times for deep water events, where onset time is defined as the time interval between the appearance of the T-phase envelope above the ambient noise and its first peak. This suggests that the onset time may be a function of several variables, including efficiency of energy conversion based on local topography, efficiency of propagation based on event water depth, and hypocentral depth in the crust. The results of this study underscore the complexity of T-phase excitation and propagation and argue that current models of T-phase excitation and propagation need to be improved to explain the observed characteristics of T-phase data.