Revisiting the site of the 2010 M8.8 Chilean earthquake
Map of the Chile subduction zone showing the 2010 Maule earthquake location, known faults, and rupture areas of historical earthquakes (left), deployment strategy for ocean-bottom seismometers on this cruise (top right), and a cross-section of crustal structure (bottom right).
R/V Melville to study why rupture did not extend to the seafloor during the February 27, 2010 M8.8 megathrust earthquake beneath central Chile
The scientific party, which includes scientists and students from the US, Chile and China*, will image seafloor and sub-seafloor faults and folds and leave ocean bottom seismometers, flow meters and pressure gauges to monitor deformation as the outer accretionary complex adjusts to the change in regional stress caused by the earthquake.
Based on seismological, geodetic and bathymetric data, it appears that the February 27, 2010 M8.8 subduction zone earthquake beneath central Chile did not rupture to the seafloor during the earthquake. This contrasts with the March 11, 2011 Tohoku earthquake offshore northeast Japan, which clearly did rupture to the seafloor, resulting in a devastating tsunami. Although the 2010 earthquake generated considerable damage both through ground shaking and because of locally high tsunami run-ups, tsunami damage was not as great as it could have been if the earthquake had ruptured to the seafloor.
Expedition MV1206 of the R/V Melville is designed to study how outer sedimentary accretionary wedge is adjusting to the change in stress caused by slip down dip during the earthquake. Several different kinds of data will be acquired. The primary experiment entails deployment of 10 broadband ocean bottom seismometers (BB-OBS) with integrated flow meters to measure fluid flow in and out of the seafloor in response to deformation; half of the BB-OBSs will also be equipped with absolute pressure gauges (APG) to detect possible seafloor uplift. In addition, we will acquire new high-resolution multi-channel seismic reflection data to define the distribution of faults in the vicinity of the OBSs, which may affect deformation of accretionary prism and potential field data to better constrain structure of subducting plate. We will also acquire swath bathymetric data and will carefully repeat a transect that has already been acquired twice - once in 2008 before the earthquake and once in 2011.
By comparing our new data to data being obtained by others offshore NE Japan, we should develop new insights into why some subduction zone megathrust earthquakes extend to the seafloor and why slip for some is arrested at shallow depth. This knowledge should lead to more precise forecasts of where large tsunamis are likely to be generated around the globe.
An important aspect of MV1206 is education. The six students will be processing seismic and bathymetric data on board as well as standing watch, and shipboard discussions will include informal lectures about the principles underlying the procedures.
* The expedition is led by Anne Trehu from the College of Earth, Ocean and Atmospheric Sciences of Oregon State University and Mike Tryon of the Scripps Institution of Oceanography, funded by National Science Foundation award OCE-1130013.