Observed and Predicted Travel Times of Pn and P Phases Recorded at NORSAR From Regional Events

The principal objective of this study is to estimate the absolute travel-time path anomalies of regional seismic phases observed at the Norwegian Seismic Array (NORSAR) with respect to the Earth model AK135 and to determine their relationship to the Earth's three-dimensional structure. At least with respect to the large-scale features of the correction surfaces, the agreement between the empirical path anomalies and the model-predicted travel times is quite good. The observed and predicted regional anomalies show strong differences in the P and Pn anomalies observed between explosion source areas in northern and southern Novaya Zemlya, apparently produced by a sharp structural boundary that strongly affects ray paths to NORSAR from these sources. The separation distance between these source areas is on the order of only 300 km, yet the P and Pn median travel time anomalies observed at NORSAR at a distance of about 20 degrees differ by more than 5s. These differences between the two Novaya Zemlya sources are also reflected in typical waveforms observed at NORSAR from these test areas.