ELKO – Earthquakes have been big news after deadly ones hit Japan and Ecuador, but in a state where mines are prevalent even small quakes can be a hazard.
According to U.S. Geological Survey data, Nevada is the fourth-most seismically active state. It falls behind Alaska, California and Hawaii.
In a week's time, Nevada had 54 quakes of magnitude 1.5 or greater. There were 232 in one month and 3,039 seismic events in the last year, including one north of Carlin, near several of Newmont Mining Corp.’s and Barrick Gold Corp.’s surface and underground mines.
According to USGS there have been 12 seismic events near Carlin since November and they ranged between 1.7 to 3.4 magnitude.
These numbers may seem small but a 3.2 magnitude quake caused Barrick employees to stop work. The quake hit at 10:24 a.m. on Feb. 28 a little more than 25 miles northwest of Carlin and about a half-mile deep.
“Goldstrike reported that the earthquake was felt throughout the underground and the mine was evacuated,” said Barrick Manager of Communications and Corporate Affairs Leslie Maple.
Newmont’s mining operations in the area were not impacted.
Maple said many times when a quake happens it will affect Barrick or Newmont mines but usually not both because of where the faults are in the ground.
The largest quake to hit Nevada in the past 20 years was the 6.2 in 2008 near Wells, according to USGS data. Since it was far from most of the mines it didn’t affect them, but that doesn’t mean large quakes don’t hit the Silver State.
The largest known earthquake in Nevada was a magnitude 7.1 on Oct. 3, 1915, in Pleasant Valley, almost 40 miles southeast of Winnemucca. At Kennedy, two adobe houses were destroyed, mine tunnels collapsed and concrete mine foundations were cracked. In Winnemucca, buildings were damaged and in Battle Mountain water tanks were thrown down. The quake was felt from Oregon to California and beyond Salt Lake City.
“One of the most striking effects of this earthquake was the large increase (and decrease) in the flow of springs and streams throughout northern Nevada,” stated USGS.
A similar size quake hitting Carlin could have devastating effects on the town and the surrounding mines.
Ground Monitoring Helps With Safety
“Newmont has a team of geotechnical experts who oversee ongoing monitoring programs as part of our normal mining operations – for both surface and underground mines,” said Newmont Director of Communications and External Relations for North American Region Rhonda Zuraff. “Regarding earthquake response, Newmont has a region-wide post-earthquake geotechnical procedure which includes, among other things, employment of ground checks.”
Barrick and Newmont work together to monitor ground conditions, but they've only been partnered since 2014.
Goldstrike Underground Division Manager Roger Hoops said the equipment was installed after the mine experienced a couple seismic events close together. Employees asked how the company knew it was safe to go back in and Hoops said Barrick decided to talk to Newmont and the Nevada Seismological Laboratory at the University of Nevada, Reno and USGS.
Goldstrike also uses sensors from the Institute of Mining Seismology to monitor in the underground, and employs an engineer dedicated to tracking and monitoring seismic activity in the area. Four units were installed on Barrick property and three units were installed on Newmont property, said Zaheer Jamkhana, senior mining engineer for Goldstrike underground division.
The equipment picks up all ground movement, including blasts, but it can tell the difference between a controlled explosion and a seismic event, Jamkhana said.
The system can pick up events five miles from the site, and because they have the monitoring equipment seismologists can tell the mine how close the earthquake was to the underground. The seismic event on Feb. 28 was 275 feet from one of Goldstrike's haulages and 963 feet from the shaft," said Amanda Norfleet, Goldstrike chief geologist for the underground.
When to Evacuate
The size of an event and its magnitude determine if the mine needs to be closed for a time, but generally if it is felt throughout the underground it is evacuated, said Miguel Lamadrid, Goldstrike superintendent of technical services for the underground.
After the event is over, the mine is inspected. Since the miners know where the quake occurred, the inspectors can pay particular attention to the areas closest to the seismic event, Lamadrid said.
The inspection usually takes about four hours, he said. After the Feb. 28 event, crews found a small crack and a monitoring system was put in, but there hasn't been any movement.
The ground support systems in Barrick and Newmont underground mines are engineered to provide some protection. Jamkhana said the underground support is designed to move with an earthquake.
“The ground support systems in use are considered ideal for small to moderate ground motion, however, larger events can require up to entire mine evacuations,” Zuraff said.
All underground mines are designed with escapeways either by a portal exit or shaft. If an underground mine cannot be evacuated quickly, both companies maintain refuge chambers at various locations.
“These chambers provide emergency shelter including, air, water, food and communication equipment and would be utilized if miners are not able to safely exit the mine,” Maple said.
While earthquakes are more likely to affect the underground, both Newmont and Barrick constantly monitor their pit highwalls. The surface mines have similar safety and evacuation protocols if movement is detected.
Hoops said the bigger concern on site is usually the concrete structures on the surface because they aren't designed to move.
“Goldstrike is the most often impacted by seismic events based on the geography in the area,” Maple said. “So far, they have not experienced damage as a result of an earthquake.”