Abstract. The catastrophic Zhouqu debris flows, which were induced by heavy rainfall, occurred at approximately midnight of 7 August 2010 (Beijing time, UTC + 8) and claimed 1765 lives. Broadband seismic signals recorded by the Zhouqu seismic station nearby are acquired and analyzed in this paper. The seismic signals are divided into two separate parts for the first time using the crucial time of 23:33:10 (Bejing time, UTC +8), with distinctly different frequency characteristics on time-by-time normalized spectrograms and amplitude increasing patterns on smoothed envelopes. They are considered to be generated by the development stage and the maturity stage of the Sanyanyu debris flow respectively. Seismic signals corresponding to the development stage have a broader main frequency band of approximately 0–15 Hz than that of the maturity stage, which is around 1–10 Hz. The N–S component can detect the development stage of the debris flow about 3 min earlier than other components due to its southward flow direction. Two sub-stages within the maturity stage are recognized from best-fitted amplitude increasing velocities and the satellite image of the Sanyanyu flow path and the mean movement velocities of the Sanyanyu debris flow during these two sub-stages are estimated to be 9.2 and 9.7 m s⁻¹ respectively.