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When an earthquakes' focus is below the sea bed, is the epicenter given as a point on the oceans' surface at mean sea level, or as a point on the sea bed?

There is some ambiguity in the term 'earths surface' here that is causing confusion in the English Language Learners sister site, which I'm hoping the experts here can clear up.

I'm using the definition of epicenter found here. Is the epicenter always directly above the hypocenter?

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Technically the depth is distance below the ocean floor, but bear in mind that unless there happens to be a cluster of seismometers close to the earthquake (unlikely in the ocean environment), the accuracy of depth estimates is only approximate. If you look at the table of earthquake depths in, for example, the Tonga Trench, you will see a few really deep earthquakes and many times more 'shallow earthquakes' at a nominal depth of 10 km - meaning 'shallow-ish'.

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