California has spent years preparing for “the big one”—the inevitable earthquake that will undoubtedly unleash all kinds of havoc along the famous San Andreas fault. But what if the fault that runs along the Pacific Northwest delivers a gigantic earthquake of its own? If the people of the Cascadia region have anything to do with it, reports Rachel la Corte for the Associated Press, they won’t be caught unawares.

The region is engaged in a multi-day earthquake and tsunami drill involving around 20,000 people, la Corte reports. The Cascadia Rising drill gives area residents and emergency responders a chance to practice what to do in case of a 9.0 magnitude earthquake and tsunami along one of the nation’s dangerous—and underestimated—faults.

The Cascadia Subduction Zone is big enough to compete with San Andreas (it’s been called the most dangerous fault in America), but it’s much lesser known than its California cousin. Nearly 700 miles long, the subduction zone is located at the intersection between the Juan de Fuca Plate and the North American Plate off the coast of Pacific British Columbia, Washington, Oregon and Northern California.

Cascadia is what’s known as a “megathrust” fault. Megathrusts are created in subduction zones—plate tectonic boundaries where two plates converge. In the areas where one plate is beneath another, stress builds up over time. During a megathrust event, all of that stress releases and some of the world’s most powerful earthquakes occur. Remember the 2004 9.1 earthquake and tsunami in the Indian Ocean off of Sumatra? It was caused by a megathrust event as the India plate moved beneath the Burma micro-plate.

The last time a major earthquake occurred along the Cascadia fault was in 1700, so officials worry that another event could occur at any time. To prevent that event from becoming a catastrophe, first responders will join members of the public in rehearsals that involve communication, evacuation, search and rescue, and other scenarios.

Thousands of deaths and other casualties are expected if a 9.0 earthquake were to occur, writes la Corte. First, the earthquake would rumble through metro areas including Seattle and Portland. This could trigger a tsunami that would do a number along the coast. Not all casualties can necessarily be prevented—but by coordinating across local, state, and even national borders, officials hope that the worst-case scenario can be averted. On the exercise’s website, FEMA officials explain that the report they prepare during this rehearsal will inform disaster management for years to come.

For hundreds of thousands of Cascadia residents, the big one isn’t a question of if, only when. And it’s never to early to get ready for the inevitable.