The Lower Tagus Valley Fault Zone is a set of northeast-southwest faults running through the sediment-filled Tagus basin near Lisbon in central Portugal. It lies within the diffuse boundary between the African and Eurasian plates, which converge slowly at a few millimetres per year across western Iberia.
The faults combine reverse and strike-slip motion and are partly buried beneath thick river alluvium, which amplifies ground shaking and complicates their mapping. Slow slip rates mean long recurrence intervals, yet the soft valley sediments make even moderate earthquakes locally destructive.
The zone is blamed for the damaging 1531 and 1909 Benavente earthquakes (the latter magnitude ~6.0), which struck the Tagus lowlands directly. It is also a candidate source for part of the shaking felt in Lisbon during the great 1755 event, and remains a key hazard for Portugal's densely populated capital region.