A new U.S. Geological Survey-led study suggests that earthquake-related deformation just below the Earth’s surface can be quite different from how it is expressed at the surface. Scientists using laser images of grapevine rows deformed by the 2014 South Napa earthquake have found that the amount of surface displacement caused by the earthquake could be significantly less than estimates of the actual slip across the fault plane. The laser images show the amount that the portion of a vine row on one side of the fault was shifted horizontally with respect to the portion on the other side.
The findings are important because they provide unprecedented details of the process of earthquake-related fault slip reaching near the Earth’s surface, the place where structures are built and where people reside.
The team used computer models to relate the measured surface distortion to fault slip at depth and verified their results with trenches cut across sections of the West Napa Fault that produced surface disruption associated with the 2014 earthquake.
“If fault slip at a couple of meters’ depth could be different than slip right at the surface, and we can infer that using these types of high-resolution laser images and slip models, then we can use this information to make better estimates of the rates at which faults slip over multiple earthquake cycles,” said lead author USGS geophysicist Ben Brooks.
This, in turn, would lead to more accurate seismic hazard assessments and improved methodologies for mitigating and monitoring the possible interruption of underground infrastructures such as pipelines by near-surface faulting.
The multi-institutional team, including researchers from the California Geological Survey and geological consultants, used the same type of LIDAR (Light Detection and Ranging) laser technology that is mounted on the roofs of self-driving vehicles for navigation.
“We’re using the same imaging and navigation technology mounted on the roofs of the test robotic vehicles that people see driving around these days,” said co-author professor Craig Glennie of the University of Houston’s National Center for Airborne Laser Mapping. “Only we are processing the data to higher precision and accuracy so that we can detect ground motions on the order of centimeters.”
Before the ground-based laser scanning technology employed by the authors became available, geologists had to rely on satellite-based measurements of ground deformation that could only be made with much less spatial resolution and less frequently — measurements which wouldn’t have revealed the detail necessary to make the study’s breakthrough.
“What’s so exciting about these new imaging technologies is that we can now learn how much earthquake slip happens very close to the surface, which is where all the people and infrastructure are located,” said USGS geophysicist and co-author Sarah Minson.