The study hit an early roadblock when the quarry declined to share information about its blasting operations. To further complicate matters, seismograph monitoring data was difficult to find, since the study was to take place in an unpopulated area.
It’s not often that earthquakes are helpful. However, in this instance, Anderson and his team believed that the solution to their problem rested with data collected from earthquake activity.
“California is earthquake country, and there are hundreds of seismographs deployed to monitor earthquake activity,” Anderson said. “Drawing on my previous experience working for the U.S. Geological Survey Earthquake Research Center, I was able to make some good contacts and discuss a way to use the earthquake monitoring data for our purposes.”
The Southern California Earthquake Data Center at the California Institute of Technology (CalTech) in Pasadena has an extensive network of seismographs, and provided the research team access to its data, along with instructions on how to extract relevant events.
“As it turned out, one of the seismographs on a ranch near the quarry was located almost directly within one of the proposed alignments,” Anderson said.
Since quarry blasts are basically “noise” for earthquake studies, blast events recorded on the seismic network are filtered out of the earthquake data. Algorithms are used to determine the depth of an event, and shallow or surface events are usually labeled as quarry blasts and removed from the earthquake studies. Fortunately, quarry blast data is still recorded and kept in separate records.
“Since the ranch is about 16,000 feet from the quarry, the data was adjusted using industry-standard constants to provide an estimate of the vibration level at the proposed alignment,” Anderson said.
Collaborating with Xiaomin You of the St. Louis office, who provided computer modelling of blast effects, Anderson was eventually able to determine the range of vibration levels for about 100 events recorded at the ranch.
“Based on my experience with the explosives industry, I suggested some likely blast practices to Xiaomin as boundary conditions for her calculations,” Anderson added.