Fig. 4

Detided strain (blue, compression negative) and atmospheric pressure (red) variations observed at the arrival of the first direct Lamb wave a \({L}_{1}\), the second wave b \({L}_{2}\), and the third wave c \({L}_{3}\). Linear trends were removed from the detided strain shown in Fig. 3 and from the pressure data prior to applying a 0.01 Hz low-pass filter. The visibility of \({L}_{2}\) may be lower than others and insignificant relative to the background fluctuations; however, its arrival was confirmed by a similar waveform recorded at a distant observatory (see Appendix)