When the shaking happens the land can go up and down. Some places change from being high up before the shaking to being lower down after. Other places change from low down to higher up.
We're describing the vertical component of coseismic deformation resulting from a subduction zone earthquake. This is typically less than a few metres (either uplift or subsidence) for sites on the Chilean mainland. R. Grapenthin, Univ. Alaska, niftily visualises this phenomenon for the 2011 Japan earthquake.
We look for things that can tell us how and when the land has changed in the past and when waves have covered the land. To do this we look at tiny bits of rock and very small living things.
We're particularly interested in working out when past earthquakes and tsunamis have happened and how big they've been. As part 1 mentioned, historical records may be too short to provide a realistic assessment of seismic hazards. Instead, we turn to sediments (tiny bits of rock) from coastal environments. Certain low energy settings, like the tidal marshes north of Valdivia, preserve evidence for both tsunami inundation and coseismic deformation. The very small living things are diatoms, single-celled algae. More on how we use diatoms to look at land-level change in the next blog post.
We have found the biggest shaking and largest waves happened four times in the past 1000 years in the area we are working.
Ed's PhD, based on sites a few hundred kilometres south of this project, found sedimentary and diatom evidence for the 1960 and 1575 earthquakes, as well as two older earthquakes predating written records in Chile.