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On Friday an 8.9 magnitude earthquake struck the coast of Japan, the largest earthquake on record to hit the country. To help make sense of the quake and its consequences, TIME spoke to Alice Walker, a seismologist at the British Geological Survey in Edinburgh, which monitors global seismic activity.(See pictures of the Japan earthquake.)

What type of earthquake was this?
It occurred in a ‘subduction zone‘ (where one plate is moving under another) and was what we call a ‘thrust’ earthquake. This means that the ground was actually thrust upwards, causing surface deformation of the seabed. The displacement at the surface caused by this deformation excites a big column of water — a tsunami. What’s happened in Japan is very similar to what caused the 2004 Indian Ocean tsunami; it’s the same process.(See pictures of the 2004 tsunami.)

Was the earthquake predictable?
You should always expect earthquakes in Japan. In fact any land that is close to a subduction zone, such as California or New Zealand, are at serious risk. There was a 7.2 magnitude earthquake in Japan two days ago, but when you get a 7.2 you pretty much think that’s it, you don’t expect an 8.9 to come two days later. We would normally only expect an 8.9 magnitude quake once every ten years, so it’s unusual.

How does this earthquake compare to others?
The ‘good’ thing about this quake was that it struck about 130km (81 miles) offshore, off the coast of Sendai. Although this means we’ve got a tsunami situation at least it wasn’t a direct hit on a city, which would have caused much more loss of life and destruction. To put it in perspective, the Christchurch earthquake [in New Zealand] last month was only a 6.5 magnitude, but because it was a direct hit on a city it caused considerably more damage.

So it’s the tsunami that is the real danger in this case?
Yes. Because the earthquake was relatively shallow, only 25km (15.5 miles) from the earth’s surface, it has caused a substantial tsunami. But the good thing about a tsunami, as opposed to an earthquake striking a city directly, is that you have more time to warn everyone. Earthquake shockwaves travel at about three miles per second; tsunamis travel at about 500 miles per hour, so you can prepare more for a tsunami, lessening the damage. It will take the tsunami about 15 hours to reach Chile from Japan, which leaves people enough time to get out of harm’s way. A pacific-wide tsunami warning has already been sent out, with coastal regions of countries like Chile, Peru and the U.S. all on high alert.

When was the last earthquake of this magnitude?
The last earthquake in the high-8 magnitude region occurred in Chile last year, which is also where the largest earthquake on record occurred, in 1960: it was a 9.5 magnitude quake. The most devastating earthquake to hit Japan was in 1923 and measured 7.9 on the Richter scale. It killed over 100,000 people and devastated the area in and around Tokyo. The reason it was so bad is because it was a direct hit on the city. So you could say we’re lucky this one was offshore; if an 8.9 magnitude had struck on land it could have been much worse.(See how Japan prepared for the earthquake.)

Is this it for now? Or will there be aftershocks?
Yes there will be aftershocks. For an earthquake of this size you tend to get a number of aftershocks, but they will be smaller than 8.9. We’d expect an order of magnitude down, so they could be big but will still be offshore so less severe.

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