What is the Mentawai Gap-Tsunami Earthquake Risk Assessment -MEGATERA- expedition?
MEGATERA is a scientific expedition that aims to gather information on the most seismic area of the world, which is an area with a lot of earthquake/ earth-movement activity. This information will help create better disaster-mitigation plans. During the expedition scientists will focus on a specific area of the sea floor where the subduction plate lies, and identify if the faults are active or passive. Active faults are correlated to their depth and the amount of water above them, providing a way to calculate the kind of wave that would be generated if an earthquake happened in that location. You can learn more about this expedition here.
What is a subduction zone?
The geophysicists onboard of Falkor are researching the Sumatra subduction zone. Subduction is when one tectonic plate bends and moves under another, convergent tectonic plate. This process creates great friction. When the plates slip, an enormous amount of energy is released in the form of an earthquake. Subduction zones can cause very strong earthquakes. If an earthquake occurs in the sea floor and causes rapid deformation, then there is potential for a tsunami.
What is the Mentawai Gap?
If you look at the map below, you will see that on the western coast of Sumatra lie the Mentawai Islands. Between Sumatra and the Mentawai’s there is a patch of the subduction plate that is locked between two sections that have already ruptured and released energy in past seismic events. This locked patch, the Mentawai Gap, has been storing energy for many years and is due to release at some point. Scientists believe the next great earthquake could originate here.
How do you map the depths of the seafloor?
Bathymetry is the measurement of water depth in oceans, seas, or lakes. Using R/V Falkor’s high resolution multibeams the team will scan the bottom of the ocean and image its relief – similar to a medical scan of the ocean’s floor. The multibeams operate using specialized sonar system. A mutlibeam sonar sends out sound waves that bounce off the sea floor and, by measuring these returning waves, it allows the science team to make a map. You can learn more about bathymetry here.
What is seismic reflection profiling?
Yes, it sounds intimidating, but, it actually is a simple concept. Scientists want to understand the composition of the ocean floors layers in order to assess the possibility of an earthquake on a given area. In order to do this, scientists recreate seismic waves and follow how they bounce against the different sub layers of the ocean’s bottom and come back. Since the waves are made of sound, we can use hydrophones to measure when they come back. You can read more about seismic reflection profiling here.
How is it possible that a tsunami originating on the coast of Sumatra can hit so many countries?
This part of our planet is a huge basin. The seismic wave creates a ripple effect that travels through the entire basin, hitting countries around it with different levels of force.
Does a big earthquake in the ocean equal a big tsunami?
No. One of the most important things to consider when assessing the risk of tsunami is not the magnitude of the earthquake itself, but the amount of water above the generating fault. When an earthquake happens, layers of seafloor bend and break which causes movements in the water body, that is, waves. So if you have a big earthquake with little water, then no big waves are generated. However, you can have a smaller seismic event that can cause the ocean floor to rupture and impact a huge amount of water above it, therefore generating a tsunami. Earthquakes that generate in superficial faults have a greater possibility of generating a tsunami.
How did scientists calculate the height of the 2004 tsunami?
Recent earthquakes have created tsunamis of unexpected height. After an event such as the 2004 Indian Ocean earthquake, scientists calculated the height of the waves by measuring the marks left by water on standing buildings or trees. Using this method, they were able to assess how far back the water went on land and how high it got. Scientists also gathered a lot of anecdotes and interviewed the survivors, their stories helped clarify the dimension of the waves.
How can a wave get so big?
When the water coming in from the ocean approaches the coast, it enters shallower depths. This causes the water to advance slowly while at the same time, it is being pushed from behind by new waves causing the water to gain height. By producing high-resolution bathymetric data offshore of Padang and along the Mentawai subduction front, the scientists on the MEGATERA expedition will be able to model how waves would react to underwater relief and what its potential run-up might be.
