How to read these data
Magnitude measures the energy released at the focus: each step is about 32 times more energy, so an M 9 earthquake releases about a thousand times more energy than an M 7. Its effects, however, also depend on focal depth, distance to cities, ground conditions and building quality – a shallow M 7 beneath a city can be deadlier than a deep M 8 under the ocean.
For earthquakes before the era of dense seismic networks, location, depth and magnitude are estimates from later analyses of records and accounts (e.g. the ISC-GEM catalogue).