How big, how often, and where from? This is almost a mantra for researchers trying to understand tsunami hazard and risk. What we do know is that events such as the 2004 Indian Ocean Tsunami (2004 IOT) caught scientists by surprise, largely because there was no “research memory” of past events for that region, and as such, there was no hazard awareness, no planning, no risk assessment, and no disaster risk reduction. Forewarned is forearmed, but to be in that position, we have to be able to understand the evidence left behind by past events—palaeootsunamis—and to have at least some inkling of what generated them.
While the 2004 IOT was a devastating wake-up call for science, we need to bear in mind that palaeotsunami research was still in its infancy at the time. What we now see is still a comparatively new discipline that is practiced worldwide, but as the “new kid on the block,” there are still many unknowns. What we do know is that in many cases, there is clear evidence of multiple palaeotsunamis generated by a variety of source mechanisms. There is a suite of proxy data—a toolbox, if you will—that can be used to identify a palaeotsunami deposit in the sedimentary record. Things are never quite as simple as they sound, though, and there are strong divisions within the research community as to whether one can really differentiate between a palaeotsunami and a palaeostorm deposit, and whether proxies as such are the way to go. As the discipline matures, though, many of these issues are being resolved, and indeed we have now arrived at a point where we have the potential to detect “invisible deposits” laid down by palaeotsunamis once they have run out of sediment to lay down as they move inland. As such, we are on the brink of being able to better understand the full extent of inundation by past events, a valuable tool in gauging the magnitude of palaeotsunamis.
Palaeotsunami research is multidisciplinary, and as such, it is a melting pot of different scientific perspectives, which leads to rapid innovations. Basically, whatever is associated with modern events may be reflected in prehistory. Also, palaeotsunamis are often part of a landscape response pushed beyond an environmental threshold from which it will never fully recover, but that leaves indelible markers for us to read. In some cases, we do not even need to find a palaeotsunami deposit to know that one happened.