Paleoseismic Trenching in Eastern Washington

Mar 5, 2023
8 min read

Updated: Jan 28

Locations of fault trenches in Eastern Washington. Trenched faults in the Yakima Fold Belt. Locations of fault trenching investigations by USGS, Washington Geological Survey, DOGAMI, USBOR, and others. Most trenches were excavated in Columbia River Basalt. The Spencer Canyon trench, epicenter of the 1872 North Cascades quake, lies outside the YFB in older crystalline rocks of the Cascade Range. The Gales Creek trench, near Portland, is not shown on the map. Trench studies have established no connection between Pleistocene clastic dikes in the Channeled Scablands and the movement of Quaternary faults east of the Cascades. Trenches are typically no more than a few meters deep and a few tens of meters long. Cost of a large trench can easily exceed a hundred thousand dollars, requires heavy equipment, and several weeks of staff time. Trenches are refilled once the it is logged (gridded, photographed, sketched). Basemap by Czajkowski and Bowman (2014).

Earthquakes sell newspapers. Fault studies in Eastern Washington are driven by the need to protect hydroelectric dams, highways, and nuclear waste facilities at the Hanford Site. Newspaper articles about Washington's earthquake hazards entertain the public and insure agencies tasked with seismic hazard messaging retain their funding. No liquefaction produced by strong shaking (magnitudes >7.0) has been found in trenches excavated across fault scarps in the Yakima Fold Belt. Its possible that all the ridges of the region rose to their current heights one magnitude 5.9 quake at a time. Figure credit: Seattle Times/Terry Tolan.

Unfaulted Missoula flood gravel truncates the Arlington-Shutler Butte Fault. The Arlington-Shutler Butte Fault is well exposed along I-84 west of Arlington, OR. The oblique-slip fault is part of the Yakima Fold Thrust system, mapped as a down-to-the-northeast normal fault and as a right-lateral strike-slip fault. In outcrop, it cuts Miocene basalt and a sedimentary interbed, but is truncated by Missoula flood boulder gravel and Holocene loess. In the photo above, the unfaulted flood gravel fills a swale incised into faulted bedrock. The fault last moved during early to middle Pleistocene time (<780,000 years) and is capable of producing 3.2-3.6 magnitude quakes. It strikes northwest crosses the Columbia River, expresses as a topographic lineament, and links Jones Canyon with Old Lady Canyon. The fault is No. 847 according to USGS and No. 81 according to Geomatrix Consultants. Location: Lat 45.7062, Lon -120.2904, 166m elevation.

Trench through Lind Coulee Fault West at O'Sullivan Dam. Sand-filled fractures exposed in the Lind Coulee trenches post-date faulting (GEI/West & Shaffer, 1988, Plate 13). No indication is given whether they are sheeted (e.g., Touchet-type dikes). They are sourced in an old flood bed, descend into fractured basalt, and parallel nearby shear planes. The "dikes" intrude the shear zone which predates undeformed sediments above (lacustrine silt, a younger flood deposit, loess). Unlikely the features are products of liquefaction. Sheeted dikes intrude an older shear zone in basalt at Gable Mountain (see trench log above). Loess-filled fractures (passive, gravity infill) appear to be quite young, formed sometime after deposition of youngest flood bed and the development of its soil. Paleosols are shown with a vertically striped pattern. More on Lind Coulee Fault and local stratigraphy here: https://www.skyecooley.com/single-post/lind-coulee-fault-at-o-sullivan-reservoir

Spencer Canyon Trench near Entiat, WA (1872 Chelan/North Cascades earthquake). No liquefaction features were observed in the Spencer Canyon Trench No. 2 according to USGS logs (Sherrod and others, 2015; Brocher and others, 2017; Brocher and others, 2018; Sherrod and others, 2021). Vertically-elongate structures shown in the figure above are sediment-filled root casts.

According to the Olympia-based Washington Standard, shaking caused by the 1872 quake was "insignificant" in Puget Sound, but near-apocalyptic east of the Cascades,

It appears that our earthquake experience…although it awakened considerable interest in the future state [of Washington], was insignificant compared to that of our neighbors east of the mountains, who were forced to believe at the time that the end of all things sublunary had indeed come.

A letter from Klikitat county says that the earthquake...was very violent in that vicinity, but did no damage. The writer…formerly of this county, gives a very amusing account of the conduct of [our reporter] Mr. Shazer at the time. Greatly excited he sprung from bed, and ran out to his chicken-coop, and soon returned with the gratifying information that the chickens were all safe! Earthquakes will never injure such men. - Washington Standard, 11 January 1873

Finley Quarry at Horse Heaven Hills (Wallula Fault Zone). The exposure was initially investigated in the late 1970s by Kienle/Foundation Sciences, Inc. Consultants (Foundation Sciences, 1980). Farooquoi and Thoms (1980) observed "thin clastic dikes of sand" and "clastic dikes of very light terracotta-colored silt" intruding zones of older fault breccia in the basalt. Identical dikes intrude identical fault breccia at Umapine Quarry.

Finley Quarry reinvestigated by USGS. The orange feature labeled 15 is an old clastic dike, not a liquefaction feature. It cuts two silt-pebble diamicts (13 and 16), which are most likely silty flood beds misinterpreted as loess. Figure by Sherrod et al. (2016).

Smyrna Bench paleoseismic log for Trench No. 1, north flank Saddle Mountains. No clastic dikes or liquefaction evidence found in loess, colluvium, or paleosols that overlie sheared basalt. Sketch from Bingham et al. (1970, Plate 4) redrawn by me in 2022.

Buroker Fault roadcut interpreted by Foundation Sciences (1980, Fig. 7B). A small reverse fault exposed in a roadcut six miles east of Walla Walla was investigated in the late 1970s by Rockwell (1979), Foundation Sciences (1980), Farooqui and Thoms/Shannon & Wilson Consultants (1980, Figs. 10, 11, 12) for Washington Public Power Supply System. This sketch by Foundation Sciences shows the fault offsetting Miocene basalt of the "Dodge" flow, post-basalt stream gravel, oxidized Palouse loess with a caliche stringers. Younger gray and dark brown loess units are not cut by the fault. Clastic dikes are indicated in the Palouse loess, but they do not appear in the sketch. The Dodge basalt flow is part of the lower Wanapum. The Buroker is Quaternary Fault 578a in Lidke's compilation. Russell Creek Rd southeast of Walla Walla, WA (NW1/4 SW1/4 Sect 31 T7N R37E).

Buroker Fault interpretation by Farooquoi and Thoms (1980, Fig. 11). A different interpretation of the same roadcut shows the fault offsetting weathered Miocene basalt and reddened Pleistocene Palouse loess. Throw on the fault is 56 cm. The young, tan Holocene loess (<11,000 years) is not cut by the fault. According to Swanson in Rockwell (1979), the fault cuts "fluvial gravels and an older loess, but does not deform overlying young loess." Farooquoi and Thoms identified no clastic dikes, but note some "fractures are lined with caliche". I visited the site a few years ago. The exposure is small, isolated, and mostly covered.

Smyrna Bench trench log for Trench No. 2, north flank Saddle Mountains. Conspicuous vertical features are loess-filled tension cracks (Bingham and others, 1970 Plate 6). They are not vertically-laminated clastic dikes nor products of liquefaction, but features formed by passive (gravity) infill of vertical openings formed by sub-horizontal block sliding (mass wasting) in the Ringold Fm. Block sliding at Smyrna Bench is a local phenomenon, a product of its peculiar geology. Shear displacement is not associated with the loess-filled cracks, only tensile opening (Mode I). According to the trenching project geologist John Bingham, "In both trenches [3N and 3S], the [Ringold] fanglomerate is broken by the separation cracks similar to those in trench 2. Some of these are filled with loess; others contain fragments derived from the walls of the crack. Several of the cracks show some stratigraphic offset, but no gouge zones or slickensides were found." I have labeled units in red text for clarity.

Wenas Creek trench logs. A scarp in the Wenas Creek Valley, identified from lidar imagery, was trenched by USGS in 2009. Two trenches, located 5 km apart, were opened in alluvial fan deposits revealing several small offsets in bedrock, young sediments, and soils. No evidence of liquefaction was found and no clastic dikes were logged. The red vertical features are small shear zones (gouge) mostly in the bedrock. PDF of the report is available online: https://www.usgs.gov/maps/paleoseismology-a-possible-fault-scarp-wenas-valley-central-washington

Gable Mountain Fault paleoseismic log for Trench No. 3 East Wall at Hanford Site, WA. No liquefaction evidence. A few clastic dikes found in other trenches nearby clearly originate in Missoula flood deposits and intrude downward into fractured basalt.

Gable Mountain Trench G-2. Sketch of trench wall containing a Touceht-type clastic dike (Bingham and others, 1970). In the diagram below, I've attempted to unravel the deformation history and clarify the timing of dike injection using crosscutting relationships and 5 time-slice snapshots. According to the geologists who logged the trench, both the older blue and younger pink dikes post-date faulting; it exploits the weakness created by the fault.

According to the Nuclear Regulatory Commission geologist on site (Justus, 21 Nov 1980), great care was taken in preparing trenches at Gable Mountain for viewing, Observations in the trenches were jointly made by staff from Golder Associates, Rockwell Hanford, USGS, and HRC. Features in the trenches were clearly marked by colored flags, walls had been gratifyingly cleaned, and walls were shored with obvious great care. General observations of the faults were as follows: apparent reverse slip sense, displacement of glacial floodwater deposits of up to several inches, apparent spatial continuity of faults and fault zones in basalt with faults in overlying sediments, at least one fault has splays and anastomosing segments, fault gouge in basalts are several feet thick in places, clastic dikes occur within and across fault zones, clastic dikes and basalts may be slickensided, at least one clastic dike was offset by a fault, apparent normal faults of less than about one inch displacement are associated with the reverse faults.

Philip S. Justus, the NRC geologist summarizes his observations following inspection of several open trenches at Gable Mountain (Justus, 1980). Field notes pertinent to clastic dikes are provided below. Key take aways are a.) the dikes at Gable Mountain are typical of those found throughout the region, b.) the dikes are sourced in flood deposits and descend into fractured basalt, c.) the dikes are not themselves fault-generated structures, but exploit bedrock faults (weaknesses), and d.) minor faulting offsets some dikes.

Flood deposits on Gable Mountain bear a close resemblance to typical Missoula flood deposits found elsewhere.
Two distinct cycles of [Ice Age flood] deposition are present on the north side of Gable Mtn; possibly three on south side.
Clastic dikes on Gable Mountain are similar in lithology and fabric to those found elsewhere in the Pasco Basin.
Clastic dikes associated with each overlying cycle are found in Trenches CD-8, G-2, and G-3.
The youngest clastic dikes originate from the base of the coarse upper unit of flood deposits which is bounded at the top by St. Helens S ash as found in Trenches CD-4 and G-1.
Clastic dikes in Trenches CD-5 and G-3 are displaced by shearing on the fault plane (CD-5) and in the hanging wall (G-3).
In Trench G-3 displacements in the flood deposits appear to post date the youngest clastic dike.
Shears, possibly associated with the thrust fault, appear to cross and slightly displace clastic dikes in the footwall in an area of Trench CD-6.
Clastic dikes along fault plane in Trench CD-6 have slickensides surfaces with stipe parallel to the dip of the fault.
Oriented slickensides in clastic dikes parallel to slickensides in gouge on fault breccia (Trench CD-5).
Wherever fine-grained material is present along fault plane, slickensides are present.

Undeforming the deformation. Structural reconstruction of a feature identified as a clastic dike (blue body) in the log of Gable Mountain Trench G2. The older, unsheeted "dike" does not have a clear source. The feature ostensibly intrudes a fracture in Floodbed A. Floodbed A is thickened in the fold hinge, consistent with oblique thrusting. The second episode of faulting segments the main body of the blue dike and folds the thinner, now detached portion (oblique faulting). Two episodes of movement are indicated prior to Time 5. Faulting does not offset the upper two floodbeds. Time 5 shows the deposition of two glacial outburst flood gravels. A sheeted clastic dike descends from the younger floodbed into brecciated material along the older fault. Both dikes appear to intrude weaknesses and neither seems to have formed because of faulting.