We detected 52 seismic events associated with landslides and submarine slumps on 8 August 2009 when Typhoon Morakot hit Taiwan. These events were neither felt nor reported because their seismic energy was primarily in the long period band (20–50 s). Most of these events were located in mountain areas where the accumulated rainfall was extremely high, though some were found offshore southern Taiwan. Among all events, the fatal landslide earthquake was located at Hsiaolin Village, where 474 people were buried. Inversion modelling of the seismic waveforms generated by the Hsiaolin landslide shows that the seismic source was represented by single force, consistent with downhill sliding. Some offshore seismic events indicate that submarine slumps probably occurred along submarine canyons and steep slopes. Our study shows that broadband seismic monitoring can be used to issue early warnings for mitigating disasters generated by inland and offshore landslides.