Many signals of interest in structural engineering, for example seismic activity, lie in the infrasonic range (frequency less than 20 Hz). This poses a significant challenge for developing batteryless sensors that are required not only to monitor rare infrasonic events but also to harvest the energy for sensing, computation and storage from the signal being monitored. In this paper, we show that a linear injection response of our previously reported piezo-floating-gate sensor is ideal for self-powered sensing and computation of infrasonic signals. Our experimental results demonstrate that the sensor fabricated in a 0.5μm CMOS technology can compute and record level crossing statistics of an input seismic event. Collected data are in good agreement with results obtained using a standard data acquisition system. Also, the sensor consumes less than 100nA of current, which makes its operation based on infrasonic power-harvesting feasible.