Small pressure drop triggered near a fault by small teleseismic waves

We describe a pressure transient triggered near an active fault by seismic waves generated more than 10 000 km away during the 2003 Mw = 7.8 Rat Island earthquake. In contrast with previous similar observations, the pressure drop occurs simultaneously with the arrival of S waves, and not during the passage of the Rayleigh waves that have larger amplitudes and smaller frequencies. This small 60 Pa drop is compatible with a small slip on the fault, which induced either dilatant damage or a transient disruption in the impermeable barrier the fault constitutes.