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Japan's most powerful earthquake since records began has struck the north-east coast, triggering a massive tsunami. Tonight our Science editor Susan Watts will give her assessment of what has happened and why, and ask how well the warning systems worked . Mishal Husain will be speaking to the EU Commissioner for Humanitarian Aid Kristalina Georgieva about what help we are offering. Meanwhile in Saudi Arabia, hundreds of police were deployed in Riyadh, on the day anti-government rallies were planned. Our correspondent Sue Lloyd Roberts is there and sends us a compelling report on the day's events. Security forces blocked roads and set up checkpoints in an attempt to thwart protests . Protests are illegal in Saudi Arabia, which has had an absolute monarchy since its unification in the 1930s. Read more from Sue in Riyadh by clicking here. And Iain Watston is at the Liberal Democrat spring conference - which is getting under way in Nick Clegg's constituency, Sheffield, just a short distance from Barnsley where the party suffered the humiliation of coming sixth in the recent by-election. We'll be talking to the Liberal Democrat MP Chris Huhne about the state of morale in the party.