Author’s Note: This part of a weekly series on geology for a class I am taking this semester at BYU-Idaho. This week’s prompt required students to look at earthquake patterns in the area they live in. Other posts from this assignment can be found under the “Geology 111” category.
Earlier this semester, I wrote an article for this assignment entitled Earthquakes in the Cascadia Subduction Zone. That article discussed the difference between earthquakes such as the 2001 Nisqually earthquake and the megathrust earthquake expected to strike the region eventually.
This article will focus on earthquake patterns in and around the Mid-Columbia Basin found east of the Cascades. I could have done it based on the Reno-area, but as my wife and I are planning to move to Boardman next month, I thought this would be more fun.
First, we’ll take a look at earthquakes in the area in the past 30 days (these are M1.0+ for the last week and M2.5+ for quakes older than a week old):
There are a grand total of two earthquakes shown on this map. They are:
- M1.3 south of Kennewick (archive) last Friday
- M1.7 north of Benton City (archive) early this morning
Taking a closer look, we can eliminate the M1.3 from Friday. That guy was an explosion, probably associated with one of the quarries near Locust Grove Road, so really we only have one true earthquake on the map.
Our M1.7 north of Benton City is located on the slopes of Rattlesnake Mountain, which was built by an active fault line. This earthquake was located about 17 km below the surface. The movement on this fault is slow, less than 0.2 cm/yr, but movement is movement and this movement causes strain on the surrounding rocks.
This group of faults, known as the Olympic-Wallowa Lineament, run from near Port Angeles to the Wallowa Mountains in northeast Oregon. It is responsible for a large number of topographic features along its length, including Tiger Mountain, Badger Mountain, and the Horse Heaven Hills. The portion in the Mid-Columbia is also known as the Yakima Fold Belt.
The abundance of fault lines in the region have the potential to create earthquakes that can be felt by humans, some of which could even reach moderate strength. It is not unusual, for example, for there to be small earthquake swarms on the Hanford Site about five miles north of the WSU Tri-Cities campus. One such swarm occurred in 2009 (archive), producing over 280 earthquakes – the largest of which was a M2.9. Other swarms in this area have produced earthquakes in excess of M4.0.
Earthquake swarms on the Hanford Site are generally pretty shallow, leading geologists to believe that they are not associated with the larger, deeper faults that I mentioned earlier. The basalt in the region is slowly being compressed in a north-south fashion, causing small failures in the underlying rock. Water table fluctuations and irrigation may also be a factor in these swarms.
For as interesting as these Hanford swarms are, there is a bigger threat to the Mid-Columbia. Located northwest of Walla Walla is the Hite Fault, which is believed to be capable of producing earthquakes in excess of M7.0. It is believed that the Hite Fault is the boundary between the more stable portion of the North American plate to the east and portions of crust that have been “welded” onto the North American plate as a result of the Juan de Fuca Plate subducting underneath the Pacific Northwest. It runs nearly parallel to the Blue Mountains, but is located in the rolling hills of Walla Walla County.
A moderate-size earthquake on the Hite Fault has the potential to cause significant damage in Walla Walla, Pendleton, and La Grande with lesser amounts of damage in the Tri-Cities, Hermiston, and Pullman as shown on the map above. One could say a Hite Fault earthquake is our version of “the big one.”
Accurate prediction of when earthquakes will happen doesn’t exist. There’s no reason to freak out about the possibility of this earthquake happening, just as there’s no reason to live in constant fear of the Cascade volcanoes. In fact, we don’t have any evidence of any major events on this fault in the last 2.5 million years.
So if we can’t come up with any evidence of an earthquake on this fault, why worry about it? Two reasons come to mind for me. First, we just don’t know a whole lot about this fault in general. We don’t know how much it moves, if any, for example. Second, with the potential of a good-size earthquake, it never hurts to be aware of it and to be prepared.
The Washington State Department of Natural Resources (archive) modeled some of the impacts of a M6.8 on the 19 mile long fault line in 2012. The more interesting impacts from such an event include:
- Possible economic damage exceeding $800 million.
- Walla Walla schools may only be 42% functional the day after.
- Possible landslides, especially if the ground is saturated.
- Possible underwater landslides in rivers, concentrated at river deltas.
- Possible ground liquefaction, particularly along the Touchet and Walla Walla Rivers.
These are estimations to help emergency managers prepare for an earthquake on the Hite Fault, but it is not my intention to scare anyone by discussing this earthquake.
Larger earthquakes are not confined to the Hite Fault. In 1936 an earthquake centered near Milton-Freewater was felt as far away as Spokane and caused damage in Milton-Freewater, Athena, and Walla Walla. This earthquake was later estimated at M6.1. Another earthquake, estimated as M6.0, occurred west of Walla Walla in 1882. (Source – dead link)
Many people living in the Mid-Columbia have likely never felt an earthquake in the area. I have lived in the Tri-Cities off and on for years and never experienced one, but it is important to be aware of what’s going on around us (and in the ground below us).
