High-precision seismic sensors are key components in geophysical applications. Traditional seismic sensors apply levelling systems to avoid overload by the gravity projection, introducing additional volume, weight and cost. In this paper, we design, fabricate and experimentally demonstrate a levelling-free single-axis in-plane seismic sensor based on MEMS technology. Three kinds of capacitive sensors are integrated into one chip. Two encoder-like area-varying capacitive displacement sensors work alternatively to avoid the inflection points, enabling the seismic sensor to work linearly in arbitrary attitudes with a high sensitivity and an angle sensor selects the working array from the two arrays and measures the angle between the sensitive direction and gravity direction at the same time. In order to calibrate the scale factor, a gap-varying capacitive displacement sensor is integrated to measure the spacing. By the combination of these capacitive sensors, the micromachined seismic sensor is able to work in arbitrary attitudes with a resolution of better than 50 ng/√Hz at 1 atmosphere. Assembling three seismic sensors along axes orthogonal to each other, a three-axis seismic measurement system can be constructed. This system has successfully detected a MS7.0 earthquake at 2017-08-08 (UTC time), which happened in Sichuan Province (1000 km away), and another MS6.6 earthquake happened about 10 h later in Xinjiang Province (2700 km away). Being low cost and low power, this nano-g MEMS seismic sensor provides an alternative solution for geophysical applications.