Earthquake scientists are learning warning signs of 'The Big One.' When should they tell the public?

Japan issued its first -ever “megaquake” warning last week after a 7.1 -magnitude earthquake hit Kyushu .

Seismologists were concerned that the quake would create stress that could trigger a bomb ticking offshore.

Japan ’s Nankai trough, likely the country’s most dangerous fault, has potential to generate 100-foot-tall tsunami waves.

U.S. has a similarly dangerous fault: the Cascadia subduction zone, which runs along the West Coast .

If a major Cascadia quake were to hit, more than 100,000 people would be injured, projections say.

Tsunami waves would batter the coast for 10 hours , inland hillsides would liquify, take out roads and bridges.

Washington state advises residents that they would likely have to fend for themselves and against the elements for two weeks .

The U.S. lags on seafloor monitoring, but Schmidt and Tobin are part of a group that received $10.6 million in federal funding to add seismic sensors and seafloor pressure gauges to a fiber optic cable off the Oregon coast. The devices will help keep tabs on Cascadia . If the data can help researchers learn about what’s normal for the fault, they might also be able to determine when it’s time to worry. This article was originally published on NBCNews.com .