An earthquake alarm system for the Maui A Offshore Platform, which is situated in the Tasman Sea off the coast of New Zealand for the production of natural gas, is described. The function of the alarm system is to give an immediate warning when accelerations equal to half the design level for the platform have been reached in order that safety procedures can be initiated. The alarm system was designed to monitor only the response of the lower modes of the platform, as these would make the major contribution to the stresses in the platform. Ambient vibration measurements were performed which showed the values of frequency for the six lowest modes to be in the range of 0.45 to 1.35 Hz. The alarm system was accordingly designed so that the signal from the transducers passed through a filter with a nominally flat frequency response over a bandwidth 0.3 to 2 Hz with a sharp roll-off at 24 dB per octave at higher frequencies. A radio link with similar detectors on shore was arranged to give confirmation that an earthquake had occurred in order to reduce the risk of false alarms.