The largest earthquake in Nevada in 65 years drew plenty of attention at the California Geological Survey (CGS) – not because it caused significant damage – but because it helps paints a clearer picture of what’s going on under the Earth’s surface in the east/central part of our state.
That’s important for an institution that creates maps and provides data to protect California residents and property in future large quakes.
“The earthquake likely occurred east of a previously mapped fault and is likely the extension of that fault,” said Tim Dawson, a senior engineering geologists with the CGS Seismic Hazards Program. “It seems clear that this zone of faults helps connect the active faults in eastern California to some of the major active faults in Nevada, so it is geologically interesting to have this zone produce earthquakes.”
On May 15, the magnitude 6.5 earthquake hit in a remote part of west-central Nevada. The nearest community is Tonopah, 35 miles southeast. The quake didn’t kill or seriously hurt anyone, but the Nevada governor declared a state of emergency mainly because of the damage done to the major route from Reno to Las Vegas. More than 6,500 aftershocks – including one magnitude 5 — have since followed.
The day after the earthquake, Dawson and two CGS colleagues, Judy Zachariasen and Gordon Seitz, were in the area of the big quake’s epicenter to lend assistance to the Nevada Bureau of Mines and Geology (NBMG) and the USGS in cataloging the temblor’s impact.
“This surface rupture appears to be the eastern extension of the Candelaria Fault that trends towards Mono Lake,” said Seitz who spent several days in the area. “There has been increased seismic activity in that area. The relationship (of that activity) to this earthquake is part of the question we are trying to answer.”
The faults in that region are complicated, like pieces of a shattered mirror. Earth scientists aren’t sure how it all fits together, so any anomaly gets their attention. But they do know this much: what happens in Nevada doesn’t always stay in Nevada.
“Unfortunately, faults don’t always respect state borders,” Dawson said.
On April 11, a magnitude 5.3 quake occurred near Mono Lake. Just over a month later, the big Nevada quake struck only 35 miles to the east. Probably not a coincidence; as Seitz alluded to, different sections of the same fault may be the culprit.
Dawson called the space and time that elapsed between the quakes a “seismic gap.”
“We think it is part of a sequence of earthquakes that are related to each other,” he said. “After that earthquake, we considered sending someone out to the field but, because of the size of the earthquake, we didn’t think it would have ruptured to the surface. Also, we were trying to minimize travel due to the shelter-in-place order.”
Surface fault rupture occurs when an underground fault causes either vertical or horizontal disruption on the ground. In some cases, the rupture is large enough to render structures or infrastructure unusable. That’s one of the seismic hazards mapped by CGS to aid local governments in land-use planning.
“One of our goals was to document whether this (Nevada) quake caused surface rupture on the California side and to assess the seismic gap for the potential to rupture into California,” Dawson said.
Last Fourth of July, an M6.4 earthquake struck near the community of Ridgecrest. The next day, an even larger quake struck nearby. CGS was concerned something similar might happen in Nevada — that the magnitude 6.5 event was just a foreshock for a larger quake that could cause damage in California.
Due to the number of aftershocks and the fact they are not subsiding as quickly as typical for a quake of this size, the USGS estimates a higher-than-normal possibility of another large quake until June 4: a 45 percent likelihood of an M5 or higher, a 7% chance of an M6 or higher, and a 1-in-100 chance of an M7.
So far, the eastern end of the fault in Nevada has had more of the larger aftershocks, so the seismologists and geologists in Nevada are keeping an closer watch on that area.
Because of the lack of fault rupture in California, it’s unlikely CGS will produce new Seismic Hazard Zone maps as a result of this earthquake. But it will share all the data it collected with its USGS and Nevada partners and maybe got another piece of the puzzle that is the geology of the area.
“It was an opportunity to work with our new data-collection tools and to learn more about the geology of that region,” Dawson said. “Fortunately, no one was hurt, and that’s the best kind of earthquake.”