While volcanic eruptions or underwater landslides can cause tsunamis, approximately 80% to 90% are triggered by seismic activity. For a tsunami to occur, the earthquake must be of high magnitude (usually above 7.0) and must involve vertical movement of the crust. Horizontal (strike-slip) earthquakes, like those on the San Andreas Fault, rarely cause tsunamis because they don’t displace the water upwards. The sudden “rebound” of the overriding plate at a subduction trench acts like a giant paddle, pushing the ocean and creating a wave that carries immense energy across thousands of miles.