Fred, Alison and I have been in the Western Province of the Solomon Islands for three weeks now (and the Solomon Islands for five). The Western Province is unique as it is one of the only pieces of land in the world that lies so close to a subduction zone, where oceanic lithosphere is devoured. However, this process of subduction is not smooth. At times, the down-going plate gets locked with the upper plate and when the stress is too much, the result is a rupture: an earthquake. Close to the trench where this subduction occurs, the upper plate gets uplifted during an earthquake (coseismic uplift) and further out, past the hingeline, the land subsides (coseismic subsidence). The last big earthquake in this region was a magnitude 8.1 in 2007. This was associated with a tsunami which reached heights up to 15m in certain locations. There were around 60 causalities. What about before 2007? Nobody knows. There are no known large earthquakes in the instrumental record, which spans a hundred odd years (the Brits were keeping record). 2007 was the first of its sort. Now, this is a scary thought! How frequently do earthquakes occur here? How large can they get? Has 2007 been the biggest, baddest one ever? One must look to the past to obtain perspective for the future. Corals that grow near shallow water are fantastic tools that give us clues to these questions.
The Solomon Islands is located in the Western Pacific Warm Pool where the climate is amiable for corals and coral reefs. They need warm water to thrive. Corals are found in deep and shallow water. They start from a point on suitable ground and grow outward, leaving behind secreted calcium carbonate. The colored outer portion of the coral is the only ‘level’ where they are alive (note: the color comes from the symbiotic zooxanthella algae and not the coral itself) – everything inside is aragonitic rock. If bioerosion is not too brutal on the corals, the corals can build microatolls and live for many centuries, becoming bigger and bigger from that point source. However, depending on the tides, there is a particular highest level of survival (HLS, as christened by Fred) for the corals. They can only grow in water so shallow. If the water is too shallow and the tide is too low during the day, the corals can die from being exposed under the sun. Therefore, sea-level changes (for example those associated with the El Niño Southern Oscillation) can kill corals. When they are killed there is no outer ‘level’ which is alive – CaCO3 rock is left behind. This too disappears through erosion depending on the environment (so, hunting for well-preserved ones is tough!) However, there is another way corals can die – earthquakes. With the latter, they are lifted out of the water, above their HLS and soon die out. Nevertheless, they can continue to stay alive and grow on the sides and bottom ends of the coral mound if they are still underwater i.e. the uplift wasn’t enough to thwart the entire coral head outside the HLS. There are many complications apart from the two-dimensional sea-level/seismicity issue such as repeated rapid subsidence and subsequent coseismic uplift, global sea-level rise, local effects and so on. In any case, by seeking out microatolls at various places we can piece together the clues they offer and learn more about earthquakes and eventually, subduction related processes.
Our main purpose in the Western Province is two-fold. One is to find coral microatolls that uplifted or subsided during the 2007 earthquake and understand how permanent deformation is retained (how much is coseismic, aseismic etc.) This mainly consists of offshore field work: we haul our 40HP outboard motor with Alison’s ≈3m canoe and go around looking for coral heads (be it via snorkeling or paddling) with the tropical sun beating down on us. We benefit from this as we can measure net post- (or pre-) seismic subsidence, understand ‘permanent’ anelastic uplift and so on. Our other intention is to find paleo-uplifted corals or corals uplifted by large earthquakes in the geologically recent past (≈2000 yrs ago) which (due to uplift) become part of the land itself. This gives us insights into the inner workings of the earthquake cycle through its frequency and magnitude. As you can imagine, this entails field work on land: hiking through thick, pristine, coastal rainforest in all its glory (bugs, thorns and topography included), we search (and search) for suitable, intact and preferentially in-situ paleo-coral reefs. By dating these (preferably with precise uranium/thorium dating) intact corals, we can glean information about paleo-earthquakes. Over the last three weeks, gradually, we’re working our way through this project: lagoon by lagoon, forest by forest, reef by reef and island by island, one day at a time!