3.3.1. Event I

The energy of event I in DF1 band (LPDF, 0.1–0.2 Hz) starts to appear in late hours of day 2014/325 (red line on day 325 in Figure 4b) and reaches a higher strength on days 2014/326 and 327 (Figures 2 and 4b). The ocean waves in frequency band of 0.05–0.1 Hz in the deep ocean close to the continental slope are uniformly higher on day 326 than day 327, and extend to the continental slope (Figure 4a). In addition, intersections of great circles concentrate at several small areas (purple circles) evenly distributed on the continental slope on day 326. All these observations might explain the occurrence of event I at the same time on day 326. The ocean wave energy decreases in the deep ocean close to the continental slope and increase slightly at the deep ocean area south of Georges Bank on day 327. The primary vibration directions at all stations cover a wider range, and the intersections of great circles are not as concentrated as on day 326. These changes can explain the decrease of PSD levels at most stations except for eight northernmost stations on day 327.
