a) Earthquake magnitude versus distance for events detected by land stations during the HDAS–OBS overlap. The black line shows the best-fit logarithmic curve, and the red line indicates the sensitivity threshold. b) Magnitude versus distance for the 229 earthquakes examined for HDAS recordings. Colors represent waveform quality (see text). The red line shows the proposed logarithmic reference threshold law M*. In blue we show M*+1 in magnitude, which is close to the threshold for good-quality P-waves recorded by HDAS.