Previous smaller events could have triggered major quake
Professor Ian Main, professor of seismology and rock physics at the University of Edinburgh, explained that today's catastrophic quake could have been caused by a series of smaller earthquakes near the epicentre.
A magnitude 7.2 quake hit on March 9 approximately 25 miles away, and a further three earthquakes greater than magnitude 6 on the same day.
He said: "These events may have triggered the magnitude 8.9 event."
The expert said the damage from the tsunami will be greater than the earthquake, and "significant casualties are likely".
"The level of ground-shaking is expected to be strong to very strong across almost all of Japan, but the impact from the direct effects of ground-shaking is likely to be less than that of the tsunami," he said.
Prof Main explained the tectonic shifts that caused the disaster: "The earthquake occurred on the north-western edge of the Pacific plate, where dense oceanic lithosphere is slowly being recycled into the mantle under the Japanese islands by a process known as subduction.
"The earthquake mechanism is a shallow-angle thrust fault, consistent in orientation and slip direction with this tectonic process."
He also said the US Geological Survey anticipates economic losses and has issued a "red alert" because of the "extensive" and "widespread" damage, and warned the international community may need to help.
Prof Main said: "They estimate economic losses will be less than 1pc of GDP (gross domestic product) of Japan. Past events with this alert level have required a national or international level response."