Normal Mode Composition of Earthquake T Phases

understanding the nature of the coupling between the coupling between the underwater acoustic field and the land seismic field is important for evaluating the performance of the T-phase satations in the international monitoring system for the comprehensive nuclear-test-Ban Treaty. For upslope propagation in an ocean environment, the places where underwater , te places where underwater acoustic field energy couples into the land seismic field are determined to first approximation by the local water depth and the normal mode composition of the acoustic energy. therefore, hte use of earthquake-generated T pahses as natural probes of water-to-land coupling characteristics is aided by knowledge of their modal composition. Data collected by a 200-element, 3000-m-aperture vertical gydrophone array during a 1989 experiment in the deep northeast Pacific Ocean are used to determine the mode composition of T-phase arrivals from two mb 4.1 earthquakes near the west coast of te U.S., one occurring offshore and the other on land. Results from an eigenanalysis offshore and the conventional mode decomposition for the two wvents are consistent and show that at 5Hz, the offshore eventʹs arrivals have higher-ordetmode content compared to those from hte event on land.Single hydrophone recordings at Pt. Sur of two mb 4.4 Hawaiian events in 1996 and 1997, one occurring offshore and the second on land, display time-frequency arival sturctures that are explainable by the dispersion characteristics over the oceanic path. Although other effects de to complex source time functions and shear wave and dispersive propagation effects along the initial land path cannot be separated with these single elememt data, differences in these two eventsʹ arrival stuctures sggerst differences in normal mode content consistent with those seen in the pair of 1989 events.