A Submarine Seismograph
Off the coast of Japan's Boso Peninsula, the seafloor of the Sagami Trough holds a natural archive of the region's most violent earthquakes. Here, the Philippine Sea plate subducts, or dives, beneath the Okhotsk microplate at a rate of about 27 millimeters per year. This relentless tectonic grinding builds immense strain that is periodically released in the form of powerful megathrust earthquakes, known as Kantō earthquakes, which have repeatedly struck the region that includes Tokyo.
Each time one of these magnitude 8-class earthquakes ruptures the seafloor, it triggers immense underwater landslides. Torrents of sediment, sand, and mud known as turbidity currents surge down submarine canyons. As these currents slow, they deposit their sediment load in distinct, widespread layers on the deep ocean floor. These layers, called turbidites, are a geological record of seismic events. Scientists from institutions like the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) use research vessels to drill deep into the seabed, extracting long sediment cores. These cores, sometimes over a kilometer in length, are a physical timeline of past cataclysms that extends back nearly 10,000 years, far beyond any written history.
Reading the Record
Analyzing the sediment cores reveals a sequence of dark mud laid down by normal, slow deposition, punctuated by thicker, sandy-colored turbidite layers. Each turbidite records a prehistoric megaquake. To date these events, scientists perform radiocarbon dating on the fossils of tiny marine organisms, called foraminifera, found within the layers. This technique has allowed researchers to match specific turbidite deposits to historically documented earthquakes, such as the 1703 Genroku earthquake (estimated M 8.1-8.5) and the 1923 Great Kantō earthquake (M 7.9).
The sediment record reveals a complex pattern of earthquake recurrence. Historically documented earthquakes in the last few centuries show intervals between 220 and 410 years. Geological evidence from tsunami deposits and marine terraces suggests a broader average recurrence interval for major Kantō earthquakes of 200 to 400 years. Paleoseismologists have identified two main types of ruptures: the "Taisho-type," like the 1923 event, and the larger "Genroku-type," similar to the 1703 quake, which ruptures a much longer segment of the trough extending offshore from the Boso Peninsula. These larger Genroku-type events appear to happen every 2,000 to 2,700 years, after several smaller quakes have occurred. By understanding this long-term pattern, scientists can better assess the current seismic hazard facing the Tokyo metropolitan area, where over 36 million live people.