Coring is not boring

Meanwhile, things have been busy and a lot of work has been done on Lake Hamana by the offshore team. About time for a new update on how everything went.

Geopulse raft strikes a pose...

Similar to our surveys on the Fuji lakes, reflection seismic profiling and lake bottom coring has been executed on Hamana-ko (ko = lake in Japanese). Due to the larger size of our survey boat, a slightly different coring setup had to be constructed, which consisted of the usual winch mechanism and, in this case, a higher elevated, over-the-side pulley. The advantage of this setup resides within the fact that heavy, full liners do not have to lifted out of the water by direct man/woman-power, saving a whole lot of efforts. 33 cores from 14 different locations were collected during 2 days of hard work, providing us with interesting study material for exploring the sedimentary infill of Lake Hamana.

Survey boat, with cabin for seismics in rainy weather and huge deck for easy coring!

New coring setup with high over-the-side pulley.

The reason why we are interested in the bottom sediments of this lake, is related to its capacity to collect deposits through time. Every year, a certain amount of sediment accumulates on the lake floor. Whenever an earthquake and/or tsunami occurs, this process of gradual sedimentation can be disrupted. Earthquakes are able to remobilise instable slope deposits, after which these move downslope rapidly in the shape of a density current (turbidity current), travelling and settling down into the deepest parts of the lake. Tsunami waves, in turn, are supposed to wash in a lot of coarser grained material (coastal dunes), organic fragments (trees and other plant), marine organisms (shells, plankton) etc. Hence, large earthquakes, often accompanied by tsunami, are supposed to have left a distinct imprint in the cored lake sediments. These event-deposits can be interpreted in terms of paleoseismics in order to improve the existing understanding of source mechanisms and recurrence patterns of earthquakes in the study area and along the Nankai Trough.

Our collection of Hamana lake cores, nicely lined up.

However, at this point of the project we can only guess what is in the liners... until they are brought back to Belgium where they will be opened. Meanwhile, our lake samples as well as the samples collected by the on-land team have left Hamana and will be shipped soon.

Seismic hazards

We've spent the last couple of weeks getting muddy coring the coastal lowlands surrounding Lake Hamana and the lake itself. Evelien will update on progress on the lake in the next blog post and you can see the on-land team in action with the vibrocorer here.

The coastal area that Lake Hamana lies within is adjacent to the Nankai subduction zone, a plate boundary that generates very large earthquakes, termed megathrust earthquakes. These quakes are often accompanied by damaging tsunamis, as well as by intense ground shaking and liquefaction. The south central coast of Japan in highly urbanised and industrialised. Major global businesses including Suzuki Motor Co. and the piano makers Yamaha are based in Hamamatsu, a city of three quarters of a million people to the east of the lake. The coastal plain also supports the Shinkansen - the iconic bullet train - and major roads connecting Tokyo to Nagoya and Osaka. All in all, a lot of people, buildings, industries and lives depend on the coastal lowlands in this part of Japan. 

In the wake of the devastating Tohoku earthquake in 2011, the Japanese Cabinet Office issued new guidelines for assessing earthquake and tsunami hazards in Japan. You can read their initial report, published in 2011 and freely available in English, here. From the point of view of geologists, historians and geographers studying past earthquakes, a particularly important passage reads:

"Up until now, the earthquakes considered to be impending from among the very largest earthquakes experienced in Japan over the past few hundred years have been used for replication of seismic intensities and tsunami heights recorded in the past using seismic source models, and these have been treated as the hazard assumptions for the next largest-scale earthquake to occur. As a result, if the seismic intensity or tsunami heights of an earthquake were not reproducible by the model, the earthquake was regarded as having a low probability of occurrence, even if such earthquake may have occurred in the past, and was disregarded from the hazard assumptions. With regard to this disaster, there is a need to deeply reflect on the fact that earthquakes that are considered to have occurred in the past, such as the Jogan Sanriku Earthquake of 869, the Keicho Sanriku Earthquake of 1611, and the Enpo Boso Earthquake of 1677, were all disregarded when developing the hazard assumption."

Essentially, despite the fact that there was limited geological and other data suggesting that the impact of these earthquakes was substantial, because the data were incomplete, these earthquakes were not included when assessing seismic hazards. 

The response from the Cabinet Office (2011):

"In order to select earthquakes and tsunamis to be used for hazard assumptions, it is necessary to accurately investigate the historic occurrence of the earthquakes and tsunamis going back as far as possible in time, and proceed with investigations based on scientific knowledge such as analysis of ancient documents and other historical material and surveys of tsunami deposits and coastal topography."

So that's where QuakeRecNankai comes in. Our work on the Fuji Five Lakes, Lake Hamana and the coastal lowlands surrounding the lake uses a range of geological and geographical techniques to try to elucidate the nature of past earthquakes along the Nankai Trough. This data will feed into the seismic hazard assessments that are in turn used to develop evacuation procedures, to site emergency facilities and to define the height of coastal defences, amongst many other preparations. 

Hotels very close to sea level at the entrance to Lake Hamana