Megathrust Earthquakes and Tsunamis: How Cascadia Snapped in 1700

0:00What causes the strongest earthquakes, and the huge sea waves called tsunamis?

0:06Earth's outer shell is split into tectonic plates. Here, an oceanic plate meets a continental plate.

0:14The denser oceanic plate sinks beneath the continental plate. This is a subduction zone.

0:21The sloping fault between the plates is the megathrust. Friction locks it, so they stick.

0:27For centuries, the sinking plate drags the upper plate's edge down, and the land behind bulges up.

0:36Suddenly the plates slip. The edge snaps up and the land behind drops: a megathrust earthquake.

0:44The sea floor lifts the water above, which spreads out as a tsunami, toward the coast and across the ocean.

0:52A tsunami is not one giant wave, but a series of waves that can keep coming for hours.

0:59A real example is the Cascadia subduction zone, off the Pacific Northwest coast.

1:04Its last great earthquake struck in January 1700. How do we know?

1:11Before 1700, forests grew on the raised coast. The earthquake dropped it, and sea water killed them: ghost forests.

1:20Trees grow one ring each year. Their last ring grew in 1699, so they died that winter.

1:29Across the Pacific, villages in Japan recorded a tsunami in January 1700, but felt no earthquake.

1:37Its timing gives January 26, 1700, and its height gives a magnitude, or size, of about 9.

1:46Plates stick and bend for centuries, then snap: a megathrust earthquake and a tsunami.