Seasonal movement of GPS stations is often attributed to hydrological loading and other environmental factors. For the first time we observe seasonal motion associated with slow-moving landslides. Eight of 26 continuous GPS (cGPS) sites in the Central Range of Taiwan show long-term landslide-induced motion at rates of ~3-15 mm/yr, ~20%-60% of their tectonic interseismic velocities. The directions of movements after heavy rains and in the wet season are consistent with the slope directions derived from a high-resolution elevation model constructed by airborne lidar. Long-term and seasonal interseismic motions are modulated by slow-moving landslides. Seasonal motions of landslides at Lushan show peak to peak amplitudes of ~3-19 mm. Estimates of interseismic crustal strain can be biased if surface processes are not taken into account. Preliminary analyses indicate that rainfall and topography play strong roles in the occurrence of landslides. Discrimination between surface processes and motion that has a tectonic origin is the key to natural hazard assessments.
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