Blind-sided us. All we knew (from Kerry Sieh's work) is that the part of the subduction zone further south might have had an M9 200 years ago (which wouldn't have send such a strong tsunami west and north). A history of some 8+ events in the 1800's. I've seen nothing credible that the seismicity was acting up, either. An infrequent event, so infrequent we didn't have the evidence from the last one.