Reading the lines of a restless earth
The city of Matsumoto is in a basin sliced by one of Japan's most significant and active fault systems: the Itoigawa-Shizuoka Tectonic Line (ISTL). This series of faults is the western edge of a major geological rift called the Fossa Magna and accommodates the boundary between tectonic microplates. One segment, the Gofukuji Fault, has a slip rate of up to 9.5 millimeters per year, among the highest for any inland fault in the country. This constant motion builds stress that is released in periodic, large-magnitude earthquakes.
Before modern seismographs, the only records of these events were written down. Historical seismology in Japan benefits from a wealth of documents stretching back more than 1,400 years, including official chronicles, personal diaries, and temple records. These accounts of ground shaking, building collapse, and landslides allow scientists to reconstruct the location, magnitude, and date of earthquakes that occurred centuries ago. By cataloging these past ruptures, researchers can calculate the average time between major events—the recurrence interval—and assess the current hazard.
A history of shaking
Detailed study of historical documents and geological trenching reveals a complex seismic history in the Matsumoto Basin. On April 28, 1714, the M6.25 Shotoku-Otari earthquake struck north of the basin, triggering a massive landslide that dammed the Himekawa River for three days. Damage reports from this and other events give important data points. For instance, records show that during a powerful 2014 earthquake, stone lanterns at the famous Zenkoji Temple collapsed, and similar damage was seen in accounts from the 1714 event.
Paleoseismic trenching across the Gofukuji Fault has uncovered evidence of major ruptures long before written records were kept. Scientists date the three most recent major faulting events to between 445-1386 A.D., 150-334 A.D., and 839-189 B.C. Many researchers believe the most recent large rupture corresponds to a historic earthquake in 841 A.D. The average recurrence interval for the fault segments in the Matsumoto area is estimated to be between 515 and 840 years. With over 1,100 years having passed since the 841 A.D. event, geological evidence suggests the fault system has accumulated significant strain.