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Possible evidence for an active Lake Clark Fault near the Pebble Prospect

Along a line between Lake Clark and Lake Iliamna, there are two locations where a faint line is visible crossing the tundra. These linear features may be evidence of a section of the Lake Clark Fault that has generated large earthquakes in the time since glaciers last scoured this landscape. During our summer 2009 fieldwork, we visited both of these features to look for evidence that would help distinguish whether the linear features were actually the result of earthquakes, or whether they were instead traces of another geologic process.

We chose to study these two features based on a careful study of aerial and satellite photos of the area. They are roughly in line with each other, and in line with the length of Lake Clark. Given that the Lake Clark Fault is thought to run along the length of the lake, this made them likely candidates. Also one of the features (Braid-scarp) crosses braided river channels that are young, probably less than 20,000 years old. And the other (Slope-disruption), is exceptionally long and linear and associated with unusual slumps that might be generated by strong shaking near a fault.

Braid-scarp

The first linear feature, the 'Braid-scarp' is a step or 'scarp' stretching across about 2.5 kilometers of flat tundra. It's divided in half by a series of depressions and ponds a little more than a kilometer wide. The flat plain shows the faint traces of braided river channels.

The evidence of glaciers and rivers reworking the land surface helps estimate the age of the braid scarp. The scraping of the glacier left furrows in the hills, and rivers flowing around blocks of ice as the glacier retreated left distinctive pits and lakes called 'kettles'. This shows that this area was buried in ice in the recent geologic past. Curving braided river channels visible in aerial photos showed that after that glacier retreated, this area was flooded by a large river that left a nearly flat plain of sand, gravel, and cobbles. Since the Braid-scarp crosses this plain, it must have formed during or after the time when the river was here, and after the time when the glaciers retreated. This means it probably formed within the last 20,000 years, after the maximum extent of glaciers during the last ice-age. Based on our field survey, the only explanations that seem plausible are:

• The scarp formed when the river eroded the lower part of the plain.

• The plain was broken and shifted vertically during an earthquake.

We dug two trenches across the feature, each about 4m long, 1m wide, and 1-2m deep. Layering was apparent in the deposit but difficult to follow consistently, which made interpretation more difficult. In both trenches, a vertical band in the middle of the trench lacked any visible layering. This disturbance might have either arisen from shearing of the soil during an earthquake, or by disruption of the soil caused by the erosional edge of a river. Though we could not conclusively eliminate the latter possibility, we could much more easily explain what we saw with disruption during an earthquake on a fault underlying the Braid-scarp.

A detailed explanation of this feature and its interpretation can be seen on our Braid-scarp page.

Slope-disruption

Northeast of the Braid-scarp, roughly 9 km away along roughly the same line is a 3 km-plus long linear disturbance along a broad hill-slope. We called this feature the 'Slope-disruption.' This feature is clearly visible on aerial photos, outlined by abrupt changes in vegetation. We followed the feature on the ground for its entire length, and it is variably an abrupt break in slope, a trough, or a gully.

Unlike the Braid-scarp, this feature could not have been generated by erosion from a stream or river, since it rises and falls over undulations in the slope, and water draining along it switches directions multiple times.

• It is a moraine left by the edge of a vanished glacier.

• It is the trace of a fault.

It seems unlikely that this feature is a glacial moraine because it continues straight down into the lowlands, departing the hill-slope it initially parallels, where the hill curves away from it. A glacier would be expected to curve around the slope, leaving a similarly curved moraine.

Additionally, four gullies that cross the Slope-disruption appear to be horizontally offset by approximately 120 meters. This offset would be extremely large if the gullies were offset just by earthquakes in the last 20,000 years since the most recent glacial maximum. Investigating this further, we dug pits in the area, revealing the ground surface was draped with roughly 1 meter of fine wind-blown silt or 'loess', which is typically carried by wind from bare ground along active glaciers. The thickness of this accumulation, suggests that some of the immediate area around the Slope-disruption may not have been covered by glaciers 20,000 years ago. Instead it may have been much longer since this location was last glaciated, likely 100,000 years or more, giving plenty of time for 120 m of offset. Note that even 100,000 years ago is recent geologically speaking, and the offset of 120 m would not have occurred all at once so the most recent earthquake must have been much more recent.

Finally, we identified a series of small landslide slumps in the area, just uphill of the linear feature. Earth movements such as these are frequently correlated with earthquakes, particularly in close proximity to the active fault, but are indirect and uncertain evidence of earthquakes. However, these slumps occurred on a lower angle slope (less than 10%) than would usually collapse in this way, and they are absent from similar slopes in the same general area. The slumps indicate this slope is particularly prone to failure, perhaps because it is cut by a fault that creates earth shaking strong enough to trigger slumping.

Are the Braid-scarp and Slope-disruption proof that the Lake Clark Fault is active?

The evidence we present here is circumstantial. The simplest explanation for the linear features we found is that they are the surface traces of active motion of the Lake Clark Fault. However, we could not credibly rule out all other explanations. In summary:

• Sediment layers beneath the Braid-scarp are disrupted. This disruption does not match the usual evidence caused by river erosion, but we couldn't rule out such an interpretation. If it isn't the trace of a river bank, then the only currently plausible explanation is that it is a fault.

• The Slope-disruption may be either a fault or a glacial moraine. Slumping and offset gullies are consistent with the fault hypothesis.

• Both features are in rough alignment, and are in-line with the Lake Clark Fault where it is mapped further to the northeast.

Together, these pieces of evidence suggest active faulting, but they are far from proof. Next summer we will return to dig bigger trenches across the Braid-scarp, apply ground-penetrating radar to the feature if possible, and possibly look further at the Slope-disruption.

A technical summary of this interpretation including other hypotheses is presented on our possible interpretations table.

What are the implications for the Pebble Prospect and local communities if this fault is active?

This fault lies near a large copper and gold mining prospect called the Pebble Prospect. The Pebble Partnership's initial assumption of the Lake Clark Fault's position put it 18 miles from the proposed mine, but the possible fault we mapped during our 2009 fieldwork lies just 5 miles from their planned pit and other facilities. Shaking from earthquakes increases near the source fault, so this suggests that the Lake Clark Fault is a potentially much greater concern for the mine than they originally assumed. At this time, Pebble Partnership has released no supporting evidence for their estimate of the fault's location.

This is particularly critical for the design of the tailings impoundment facility. The preliminary design includes multiple 700 foot earthen dams intended to contain the metal sulfide tailings generated by the mine. To prevent the release of billions of tons of acid and metal generating material into downstream waters, these dams will need to remain intact in perpetuity, both during mine operation and following mine closure. To address the greater hazard posed by a fault nearer than previously thought, dams and other structures at the facility would need to be built to withstand a higher intensity of shaking than initially estimated.

For communities near the fault line, particularly Nondalton and Igiugig (both within a few miles of this trace), the fault could present a major hazard. Though it may be generations before the next earthquake occurs here, it could also happen any time. Some suggestions for preparing for earthquakes in your area are provided by the Alaska Earthquake Information Center.