Using a new application of old technology, BYU geology professor John McBride and his students discovered previously unmapped faults, re-evaluated earthquake hazards, and confirmed pre-existing geological theories.
While the area around Rock Canyon has been thoroughly studied by geologists, small faults associated with the Wasatch Fault zone near Provo were unknown because they had been covered by residential development. With housing developments already in place, faults that would otherwise be visible at the surface are hard to locate.
“We’re at the stage where the building has already happened, and thus all we have is a limited picture of what the faulting looks like at the earth’s surface,” McBride said.
Locating these hidden faults is important in evaluating an area for seismic (earthquake) potential and hazards, a task that can be difficult in a well-developed area like the neighborhoods around Rock Canyon. To map out hidden faults, geologists use sound waves to “see” breaks in rock formations under the ground.
In order to perform a “seismic survey”, geophysicists often drill holes and place dynamite in them as a sound source. However, it can be difficult or impossible to get permission to use explosives in a populated area like Provo, so when McBride was offered a day with a vibroseis vehicle and a professional survey crew from the United States Geological Survey, he gladly accepted.
“One of the things that we wanted to show is that this particular strategy of geophysical surveys can detect these faults,” McBride said.
A vibroseis vehicle is a large truck with a vibrating disk underneath that, when lowered to the ground, sends sound waves that bounce off the rock formations below. The sound waves are received by a set of geophones spread out over 1000 meters along the ground surface.
Originally used by oil companies to look for oil deep beneath the ground, McBride used the technology to look at the shallower rock formations broken up by the Wasatch Fault near Rock Canyon in Provo.
Using vibroseis, McBride and his students were able to accurately map fault lines in Provo without any explosives. They confirmed the theory that the Wasatch Fault is not just one thin line, but a wider zone of faulting.
During their study they also found evidence that in poorly consolidated horizontal sediments, like the beds of ancient Lake Bonneville where Provo sits today, the angle of faulting is almost vertical. This had been previously seen in geological studies near the surface, such as in trenches crossing the Wasatch Fault, but had not been observed deeper below the ground surface.
“We found a way to actually see those faults that people suspect are already there,” McBride said.
This new information also contributes to seismic hazard studies, McBride said. The angle of the fault determines what direction the ground will move if it breaks apart along pre-existing fault lines.
Using this technology, McBride has gained a better understanding of how faulting occurs. More detailed fault line maps will allow builders to better plan for and protect important structures.