Fig. 3

Ambient vibration survey data: normalized amplitude as a function of frequency (0.6–10 Hz) of Survey A (normal component, i.e. horizontal and perpendicular to the trace of the assumed fault). Each line represents a seismic station. At low frequencies (<2 Hz), the motion is about the same in all stations. For frequencies in the range between 2 and 7 Hz (yellow box), the signals are significantly different. For frequencies larger than 7 Hz, the signals are more or less consistent again