Trenches dug across faults expose evidence of ancient earthquakes. Radiocarbon dating charcoal layers reveals the fault ruptures roughly every 500 years.
USGS Earthquakes, Public domain, via Wikimedia Commons
Reading earthquake history
In the mountains of Yunnan, geologists act as detectives, digging into the ground to read the history of earthquakes. This practice is called paleoseismology. Researchers excavate deep trenches directly across the Lijiang Fault to expose the layers of sediment and rock beneath the surface. These walls of earth, known as soil profiles, show a timeline of geological activity.
When an earthquake ruptures the surface, it slices through existing layers, displacing them. Sediments that accumulate after the event will then form new, unbroken layers on top. In a trench wall, this appears as a clean break in a sequence of strata, with younger, undisturbed layers covering the fault line. By identifying these offsets, scientists can pinpoint individual prehistoric seismic events. To determine when these ancient earthquakes happened, they use radiocarbon dating on organic materials like charcoal or plant fragments found within the displaced layers. This technique allows them to build a detailed chronology of the fault's past behavior, stretching back thousands of years.
A fault's recurring cycle
The Lijiang Fault is a right-lateral strike-slip fault, part of the larger Lijiang-Xiaojinhe Fault Zone that accommodates the southeastward movement of the Tibetan Plateau. Studies of the fault reveal a recurring pattern of large earthquakes. Paleoseismic trenching suggests a recurrence interval of roughly 500 years for major surface-rupturing events.
The most recent significant event was the Lijiang earthquake on February 3, 1996. It registered a magnitude of 6.6 to 7.0 and resulted in over 300 fatalities and more than 17,000 injuries. The earthquake caused widespread destruction, destroying approximately 358,000 buildings and leaving 320,000 people without homes. The economic losses were estimated at over 500 million US dollars. This event provided a modern example of the fault's destructive potential, a potential that has been building and releasing for millennia. The fault's slip rate—the speed at which the two sides are moving past each other—is relatively slow, but the accumulated strain over centuries is what fuels these powerful, periodic ruptures.
💡Fun Facts
The 1996 earthquake caused severe damage to Lijiang's Old Town, a UNESCO World Heritage site known for its traditional Naxi wooden architecture.
Paleoseismology is a young science, developing rapidly since the 1970s to extend earthquake records far beyond written history.
The immense pressure from the collision of the Indian and Eurasian tectonic plates drives the movement of the Lijiang Fault and others across the region.
The 1996 earthquake triggered over 200 landslides across a 12,000 square kilometer area, with more occurring in the following months due to monsoon rains.
