EarthquakesSeismologists detail lessons learned from the 24 August 2014 Napa earthquake
In the recent issue of Seismological Research Letters, a journal published by the Seismological Society of America, scientists have detailed the lessons learned from the 24 August 2014 Napa earthquake. Several authors in the issue acknowledged that the Napa temblor has helped them develop a fast and more accurate mapping of fault systems, which will give municipalities and developers a better sense of where to safely rebuild after an earthquake.
Scientists have detailed the lessons learned from the 24 August 2014 Napa earthquake in the recent issue of Seismological Research Letters, a journal published by the Seismological Society of America. The Napa Valley Register reports that several authors in the issue acknowledged that the Napa temblor has helped them develop a fast and more accurate mapping of fault systems, which will give municipalities and developers a better sense of where to safely rebuild after an earthquake.
William Barnhart, author and assistant professor for earth and environmental sciences at the University of Iowa, in one article — “Geodetic Constraints on the 2014 M 6.0 South Napa Earthquake” — credits the combination of satellite-borne radar and ground-based tracking as instrumental to helping identify the path and depth of the West Napa Fault, which shook at magnitude 6.0, killing one person, injuring 200, and producing more than $400 million in damage.
Analyzing the fault data shortly after the quake allowed seismologists to disseminate information about the fault system before roads were rebuilt and buildings were repaired. “We were able to use a combination of satellite and ground-based GPS observation to produce a very detailed image of slippage during the quake, and we were able to do it on a short time frame — within 24 hours,” said Barnhart.
Using ground GPS measurements, scientists were able to develop an initial estimate of the fault shape the day after the earthquake, they then produced a more precise estimate a week later using data from satellite-based Synthetic aperture radar, a form of radar that uses microwaves sent from satellites to create 2D or 3D images of landscapes. Images of the fault shape in July were compared with data from three days after the earthquake, said Sang-Ho Yun, a Jet Propulsion Laboratory geophysicist and radar engineer who co-authored a paper on the mapping of the fault. “It gives a very spatially dense image of how the ground shifted, allowing us to precisely map where the fault is, how much slip there was, and create a three-dimensional map of the fault going down into the earth,” Barnhart said.
Annemarie Baltay, a United States Geological Survey (USGS) geophysicist at Menlo Park and co-author of another article, said on-the-ground data and patterns of damage confirmed the accuracy of equations used to forecast ground motion in a Bay Area quake. Those equations influence building codes, which according to Baltay, have for four decades set minimum standards for earthquake resistance. “The most important message to me is that overall, the equations we rely on for building codes performed well,” Baltay said. “This shows the Napa quake was not unusual in the ground motions observed.”
USGS regional coordinator for Northern California, Jack Boatwright, co-authored a report in the journal focusing on Napa buildings damaged by the quake. The report noted that 165 buildings were tagged as too dangerous to occupy without repairs, and 1,707 were tagged for restricted access while repairs were being made. Napa’s highest concentration of damaged buildings after the earthquake is in its downtown and southern districts. According to Boatwright, the majority of tagged buildings were built before 1950 and do not meet seismic safety codes first passed in 1975.
