We developed a lower-power data logger for earthquake observations, because the lowest-power loggers available now still need heavy lead-acid batteries to operate continuously for longer than two weeks. Two ideas have been made as ways of reducing the power consumption of the data logger presented in this study, keeping its performance high enough for the earthquake activity researches. First, high and low frequency clocks are used for lowering power consumption. Second, a temperature sensing crystal is used for frequency modulation of a crystal clock to keep time. As a result, the data logger can continue 100Hz sampling with 6 alkaline D size 1.5 V batteries for more than 2 weeks.