In 2011, miles of rock shifted deep underwater in the Japan Trench, causing an earthquake that deformed the seafloor and caused a devastating tsunami. Off the Eastern Coast of the Tōhoku region in Japan, a huge piece of the Earth’s crust carrying the Pacific Ocean, known as a tectonic plate, is forcing itself beneath Japan and into the Earth’s interior. Researchers think rock from this plate pushed against rock far beneath Japan, building pressure until it became too great. The lower plate slipped, spreading energy from the slip site and causing the earthquake.
Plate slip can cause 2 types of outcomes. The first occurs kilometers deep in the Earth’s crust, causing earthquakes too small to trigger tsunamis. The second type causes extreme slip events that begin deep in the crust, like the 2011 event that extended into the Japan Trench and warped the seafloor. The moving seafloor displaces seawater, causing a tsunami. These events are referred to as slip-to-the-trench style earthquakes.
Researchers think this phenomenon is not unique to 2011, as Japan has a long history of tsunami-causing earthquakes. Charlotte Pizer and colleagues investigated sediment from the Japan Trench to unearth the geologic history of slip-to-the-trench style earthquakes.
In between earthquakes, Japanese islands continuously shed sediment that fills the trench, forming horizontal layers. Slip-to-the-trench earthquakes bend and fold these layers. The team hypothesized that if they found these folds in the Japan Trench and dated them, they could catalogue the record of slip-to-the-trench style earthquakes in the area.
Earthquakes like these occur repeatedly, meaning that younger ones can erase folds caused by older ones. To account for this overprinting, the researchers chose a study site within the Japan Trench about 100 kilometers (60 miles) north of where the most severe earthquakes occur. This site is only mildly deformed by slip-to-the-trench earthquakes since it’s so far from their source, making it easier for the team to identify individual events.
Pizer and her group pieced together data collected from this site by previous researchers to reconstruct Japan’s earthquake history. They compiled 3 main types of data. Two of these were images of sediment layers in the trench constructed from reflections of vibrations sent into the ground, called seismic profiles.
The first seismic profile captured the full trench, over 1 kilometer deep (about 0.6 miles), at 5-meter (16-feet) resolution. Using this method, any layers smaller than 5 meters thick won’t appear in the image. The second seismic profile captured only the top 40 meters (130 feet) of sediment, but could detect layers as thin as 10 centimeters (4 inches).
The final type of data they used came from a 36-meter (120-foot) long cylinder of sediment extracted from the bottom of the trench, called a sediment core. Researchers in the past tied layers within this sediment core to historic earthquake events. They showed that it recorded 2 specific earthquakes in the area, the Kyotoku earthquake of 1454 CE, and the Jogan earthquake of 869 CE, both of which likely caused tsunamis.
Pizer’s team could observe the depth of the Kyotoku and Jogan earthquake layers taken from the sediment core in their high-resolution seismic profile of the trench. In these high-resolution data, they noticed that deformation occurred in the sediment up to the layer marking the Jogan earthquake, but not above it, meaning deformation of the trench happened during this 869 CE event.
The high-resolution data were only able to image sediments from recent earthquakes since 869 CE, but the low-resolution profile, which imaged the full trench, was able to capture the deeper, older record. When they looked deeper into the trench sediment, the researchers noticed that deformation from the 869 CE event had spread from where the tectonic plate is colliding with Japan. They saw at least 6 more similarly deformed layers, implying each was its own slip-to-the-trench event, although the dates of these events are unknown.
Pizer and colleagues concluded that slip-to-the-trench earthquakes are long-standing, common events in the Japan Trench. They hope future scientists will continue to research this site to better assess tsunami risks in Japan.
