Subspace identification techniques are a popular parametric time-domain method for extracting modal parameters of a structure under ambient condition. These techniques can simultaneously process a large quantity of data and potentially extract the most information from the measurement. Most of the developed subspace identification techniques, however, assume white-noise input, and hence cannot be used for structures under earthquake excitation. In this study, an input-output recursive combined subspace identification technique is proposed for tracking modal parameters of a time-varying structure under nonstationary earthquake excitation. The technique incorporates an orthogonal projection and an instrumental variable approach to eliminate the effect of earthquake input and measurement noise, respectively. A bi-iteration subspace tracker is then applied to extract step by step the structural modal parameters. The proposed technique is validated numerically on a four-degree-of-freedom structure and experimentally on a three-degree-of-freedom building model. Both numerical and experimental results show that the proposed technique can track structural modal parameters under nonstationary earthquake excitation quite well.