Near-synchronous and delayed initiation of long run-out submarine sediment flows from a record breaking river-flood, offshore Taiwan

Subsea fiber-optic telecommunication cables can break under fast sediment flows that travel 100s of kilometers through the deep ocean in response to earthquakes and submarine landslides. Similar flows are inferred to form from major river floods whose sediment-laden waters plunge and travel along the seabed. However, the complex initiation of flood-related flows and their hazard potential have not been observed until now. Here we use cable fault data from the Gaoping Canyon/Manila Trench off Taiwan to show that a major river flood, formed during Typhoon Morakot (2009), generated two, long run-out, destructive sediment flows; one during peak flood and the other 3 days later. The latter flow was more damaging with speeds and run-out similar to that of landslide-triggered turbidity currents formed in the same catchment. If the second flow was due to remobilized canyon sediment, it occurred during low earthquake (>Mw 2.0) activity, suggesting other triggering mechanisms.