Geophysical Studies Reveal Potential Quake Hazard in Spokane Area

Posted: 03 Jan 2014 09:00 AM PST

SPOKANE, Wash. —Preliminary interpretations of a recent geophysical survey confirm the presence of earthquake faults and reveal the possibility of previously unknown faults beneath the greater Spokane area.

During the spring of 2013, the U.S. Geological Survey undertook an airborne magnetic survey over the Spokane area to help understand the geologic reasons for a series of earthquakes and ground uplift that occurred several years earlier.

Low-Flying airplane for mapping fault zones

The Spokane aeromagnetic survey was flown with a Cessna 180, owned and operated by a private company under contract to the USGS, and specially modified for low-altitude geophysical surveys. The magnetic sensor is located in the "stinger" protruding from the rear of the aircraft. Credit: Michael Hobbs , EDCON-PRJ, Inc.

A sequence of 105 small (less than magnitude 4) earthquakes occurred beneath Spokane in 2001, accompanied by a small increase (1/2 inch) in ground elevation. The shape of the deforming ground surface was consistent with movement on a northeast-trending fault beneath Spokane, yet no known mapped faults in Spokane have been active in the last 1.6 million years. To find the Spokane fault that caused 105 earthquakes in 2001, and evaluate its potential for large earthquakes in the future, scientists turned to geophysical tools to "see" below the surface. Knowing a fault's length, depth, and history of earthquakes helps us prepare for future earthquake hazards.

"As the director of Spokane Emergency Management, we work diligently on preparing our community for disasters which includes identifying hazards in our area. The USGS has performed studies and their preliminary findings will help us develop effective mitigation strategies," said Spokane County Sheriff Ozzie Knezovich.

In an "aeromagnetic survey," a magnetic sensor, towed by an airplane, is flown back and forth close to the ground and along closely spaced, parallel lines, recording the minute changes in the magnetic field. Geologic processes often bring together rocks with slightly different magnetic properties, and these variations cause very small magnetic fields above the Earth's surface. The differences in the magnetic field are called "anomalies." Credit: USGS

"Geologic processes often bring together rocks with slightly different magnetic properties, and these variations cause very small differences in magnetic fields above the Earth's surface," explained the study's lead author, USGS geophysicist Richard Blakely. "In an aeromagnetic survey, a magnetic sensor is flown back and forth close to the ground and along closely spaced, parallel lines. After data processing, we arrive at a map of 'magnetic anomalies' reflecting the variations in the magnetic signal of the rocks and geology of the upper part of the Earth's crust."

Movement along a fault is an example of a geologic process that brings together different kinds of rocks. On a magnetic anomaly map, faults often appear as linear features, even though the faults themselves may be concealed below sedimentary deposits, vegetation, or urban development.

Illustration showing Spokane earthquakes

A sequence of 105 small (less than magnitude 4) earthquakes occurred beneath Spokane, Washington in 2001. The yellow circles on this map are location of epicenters in the Spokane area, with symbol size proportional to earthquake magnitude. The Latah Creek fault passes beneath Spokane but is not known to be active.

Several linear patterns are evident in the new Spokane data. Two prominent anomalies in the southeast corner of the survey extend across the entire area. One of these is caused by the Cheney fault zone; the other is probably caused by a similar, previously unrecognized zone of faults and fractures. The Latah Creek fault and other known faults appear in the new data as complex alignments of magnetic anomalies.

One alignment of magnetic anomalies is of particular interest: It crosses Spokane in a northeast direction, passes through the zone of earthquakes, and coincides with the Spokane fault postulated from ground deformation. There are many geologic reasons for linear magnetic anomalies, and the presence of unrecognized faults is just one possibility. Future USGS studies, including airborne LiDAR surveys and ground-based geologic investigations are planned to confirm the Spokane fault.

The bright colors on this map represent the magnetic field of the upper part of the Earth's crust in the greater Spokane , Washington area. The complex pattern of anomalies throughout the southwest part of the survey is caused by flood basalts of the Columbia River Basalt Group. These lava flows do not extend to the northeast part of the survey, where subdued magnetic anomalies reflect older crustal rocks. Several linear magnetic anomalies are evident on the map. Two prominent anomalies in the southeast corner extend across the entire survey. One of these is caused by the Cheney fracture zone; the other is probably caused by a similar, previously unrecognized zone of faults and fractures. The Latah Creek fault and other known faults appear in the new data as complex alignments of magnetic anomalies. Credit: Richard Blakely, USGS.