A tsunami warning was issued for Northern California Thursday morning, with great drama that reverberated across national news outlets, and then rescinded just over an hour later. But why was it issued in the first place?

A 7.0M earthquake is, to most of our minds, a big one, and news that there had been such a big quake under the ocean floor off the Northern California coast was met with some panic, and a blaring warning that was blasted out to all of our cellphones. SFist received at least one anecdotal report of someone evacuating from their Outer Sunset home with two cats in tow.

Should we all have had reason to be concerned? Maybe, but only really if you were a) camped on a beach, or b) on a dock in Emeryville or Santa Cruz. And chances are the waves or water inundation we'd see from an earthquake this size would still be fairly minimal.

Ultimately, around noon, just before the tsunami wave would have likely hit SF, the tsunami warning was lifted without much explanation, with the US Geological Survey simply saying that no tsunami waves were expected from this event.

Stanford geophysics lecturer Ross Stein, who is the CEO of earthquake information tech startup Temblor, spoke to ABC 7 Thursday afternoon in the wake of the warning being lifted — and after ABC 7 had sent their helicopter to hover over Ocean Beach for, as it turned out, no reason.

Stein explained that the initial report of a shallow, 6.6M quake at a particular spot near the Mendocino Fault Zone, caused particular concern because of the propensity for such shallow quakes to cause a great deal of vertical motion on the ocean floor — which tend to cause more tsunamis.

But, Stein says, within a short span of time, geologists learned that the quake had been about 8 miles deep and much larger — a 7.0M is about ten times the size of a 6.6M. And Stein tells ABC 7 this is "a very typical garden variety event on this northern extension of the San Andreas Fault... In that respect it doesn't move the sea floor up and down very much, and it would be very unlikely for this earthquake to produce a large tsunami."

By contrast, Stein says, a magnitude-8 quake in this same area could be more likely to send a tsunami our way, but we still would not likely see anything more dramatic than what we saw following the 2011 Tohoku earthquake and tsunami in Japan, which was produced by a 9.0M quake and the waves reached us about 12 hours later. A wave just a few feet high was able to do damage in Santa Cruz Harbor and in Crescent City, California, and some effects were seen in Emeryville as well. (Tsunami waves coming through the Golden Gate basically head toward Emeryville.)

"A magnitude-9 earthquake, even though it's so far away, is able to launch a much larger tsunami than even a magnitude-8 at our northern end of the state," Stein tells ABC 7. He adds, "Recognize that this isn't going to be the kind of giant wave that takes out the Golden Gate [a la Hollywood], but something modest but still worthwhile to get off the beach [for]."

Stein adds, when it comes to future tsunami warnings, "If you're off the beach, you're fine."

Top image: Tsunami waves following the Tongan volcano eruption in Jan. 2022. Photo via gmkl/Twitter