Tirua

We were woken yesterday morning by the house shaking… According to the USGS (link) a magnitude 4.7 earthquake, with an epicentre 20km east of Lebu was responsible. The excitement continued with the purchase of our third and fourth spades of the trip. If you’re thinking of digging holes in South America, we’d suggest avoiding Brazilian spades.

We’ve spent a couple of days investigating the area around Tirua. Locals told us that the February tsunami travelled 3 or 4 kilometres up the river valley, carrying several houses inland. The tsunami deposited dark grey sand in a layer up to 25cm thick (right). We found further sand layers in exposures along the side of the river that might be evidence for older tsunamis.

Concepcion and Tubul

In Concepcion we met with a group of academics led by Professor Marco Cisternas of the Universidad Católica de Valparaíso. Together with Lisa Ely, Rob Wesson, James Goff, Catherine Chague Goff and Zamara Fuentes we visited the coastal plain between Talcahuano and Penco, where 2010 tsunami deposits overlay evidence for older events. Marco suggests that the area lies on the hingeline – with the land to the east subsiding and the ocean to the west uplifting during the February earthquake.

From Concepcion we continued south to Tubul. Our accommodation appeared slightly precariously balanced, with several key supports removed by the tsunami, however we were assured it was safe. The Arauco Peninsula, on which Tubul lies, has been uplifted significantly by numerous major earthquakes. We cored a number of areas of the wetland system that lies to the west of the town, struggling though torrential rain and impenetrably stiff clay.

Lipimavida to Rio Mataquito

From Llico we travelled south to Lipimavida, Iloca and the Rio Mataquito. Post-earthquake imagery has been added to Google Earth for the area. Try using the historical imagery tool for the Chilean coastline around 35˚S - the changes are surprisingly evident.

The mouth of the Rio Mataquito is now a kilometre south of its former location, probably as a result of either the tsunami or subsidence. Fishermen at Duao told us that the tides are higher than before the earthquake, supporting coseismic subsidence. We took blocks of sediment from close to the new mouth of the river and will try to quantify the amount of subsidence by analysing the diatoms (single celled algae) they contain.

Bucalemu, Boyeruca and Llico

We’ve started the trip towards the northern end of the 2010 rupture zone, driving though the vineyards of the Colchagua Valley from Santiago to our first base at Laguna Torca, close to the village of Llico. The coastal villages in this area have been badly affected by the tsunami, with about half the population living in government issue temporary housing.

We were hoping that water levels in the estuaries at Bucalemu, Boyeruca (left) and Llico would respond to relative sea-level change, however all three have been separated from the sea by sand bars. Water levels in the lagoons behind changed with the breaching of the bars by the tsunami and also change when the bars are artificially cut (sometimes with dynamite) when winter river floods inundate farmland. A recent Science paper does, however, suggest the area may have coseismically subsided in February.