Day Three: Installation Continues, and First Earthquake Recorded!

Overview

June 17, 2005

Flush with the successful installation of the first PBO borehole strainmeter and seismometer, crews continued to finish work at the Hoko Falls station. The next steps included pouring layers of cement and sand into the borehole. First, 20-foot sections of 1-inch PVC were inserted into the borehole to a depth of 530 feet. After the field crew tested the seismometer and strainmeter instruments one last time for peace of mind, they lined up 94-pound cement bags and large buckets of water beside the mixer. Because of the size of the hopper, several batches of cement had to be mixed, making the timing of the operation and the water and cement proportions critical. As the hopper filled with cement, the cement was pumped under high pressure through a line that attaches directly to the top piece of PVC. Cement spilled out the bottom of the 530 feet of PVC and began to fill the borehole. After mixing and pumping over 11 bags of cement, the crewmembers removed the PVC sections from the borehole and thoroughly cleaned all of the equipment to remove any remaining cement that would dry and could possibly cause damage to the mixer or plug up the PVC sections.

While the grout cured for the next couple of hours, crewmembers started on assembling the VSAT and connecting AC power from the landowner’s fish hatchery building to the borehole site via a trench built under a gravel driveway.

After enough time passed, approximately 420-feet of 1.5-inch PVC was inserted into the borehole to transfer the next layer: approximately 33 feet of sand to allow pore pressure fluids to flow relatively freely through the borehole for hydrological measurements. Using water to flush the sand down the PVC, the crew poured eleven 50-pound bags of sand into the PVC with a large funnel and water hose.

Finally, the 1.5-inch PVC was removed from the hole and 400 feet of the 1-inch PVC pipe was reinserted in its place so one more layer of cement could be pumped on top of the sand layer. Another batch of 1400 pounds of cement was mixed up and pumped through the PVC.

This was avery time consuming process, which crews finished with the last cement layer at 7 PM. In the end, more than a ton of cement was mixed with water and poured into the borehole. After a good night’s rest, the crew will continue working on connecting AC power from the landowner’s building and digging a hole to bury the extra strainmeter data cable.

News Flash: An early start to the new era of EarthScope borehole strainmeters!

Only five hours after the first PBO borehole strainmeter was installed, a magnitude 7.2 earthquake occurred off the coast of Northern California about 140 kilometers WSW of Crescent City. What a fortuitous way to verify the strainmeter was functioning properly!

The figures at right show two 10-minute segments of data recorded by the Hoko Falls strainmeter, with each subfigure showing a different component of strain. The data were sampled 20 times per second and have been scaled to natural strain units. The peak strain amplitudes are about 30 nanostrain, or 30 parts per billion. The first figure shows the first arrival on a 10 minute time scale, while the second figure shows the next 10 minutes. These results have been minimally processed.

The last data figure at right shows the beginning of the grout curing process, which causes compression on all cells depending on the strength of the rock in each direction.