Evauluating earthquake fault lines with buried explosives
If you can fight fire with fire, can we gain an upper hand on earthquakes by purposely creating seismic waves in the ground? It won’t prevent future quakes exactly, but geologists are planning on detonating small explosives near the Hayward and Chabot Faults near Oakland, California, in the coming weeks to get a better idea what to expect during the next big seismic event. As it turns out, releasing a relatively tiny amount of energy below the ground is one of our best ways to measure the state of the fault, hopefully giving us insight into what to expect when things really start moving at some point in the future.
Explosives don’t do the job on their own, of course. The first step to these measurements is to install a battery 500 of seismographs over a 10 mile range, crossing over both fault lines. Once those are in place, 16 small explosives can be detonated in 30-foot-deep holes. While they’ll be detonating the explosives in the middle of the night, they likely won’t be waking up the neighbors, as only a small amount of the released energy will make it to the surface. However, the seismographs will be sensitive enough to pick up the explosion’s energy as it is transmitted through the ground.
Registering wave ratios
That energy will be transmitted in various types of waves that stretch and contort the ground as they move away from the disruption’s epicenter. Primary waves, also called “p” waves, are a bit like like sound waves moving through the air. The ground is compressed laterally, but quite often isn’t the most destructive part of an earthquake. P waves move very quickly, and may be what animals detect moments before the more violent shear waves of an earthquake make themselves evident.
Shear waves, also called secondary or “s” waves, radiate out from the source of disruption with a vertical movement. They cause the ground to oscillate up and down, and are able to violently toss things around on the surface. Critically, they are slower than p waves, and can only move through solid rock, which makes them useful for measuring the state of the soil 30 feet below the surface.
As these two types of waves hit the buried seismographs, their arrival time and intensity will be compared. If p waves seem to be oddly higher than s waves, it will mean that the rock underneath the Oakland area isn’t solid and continuous. Pockets of other materials indicate damaged, weakened structures, basically describing more dangerously faulted rock, ripe for the next big earthquake. If this is discovered, it will help reshape earthquake prediction maps, and show what structures need to be more aggressively retrofit for seismic safety.
Source: Hayward Fault: Tiny explosions to simulate earthquakes along dangerous East Bay fault by Lisa M. Krieger, The Mercury News