Bretwood Higman, PhD
Andrew Mattox
This is the first version of this report.
It is still under revision.
There are no more recent drafts.
A subtle line crossing the tundra near the Pebble Prospect may be evidence of an active fault. During nine days of fieldwork in 2009 we dug trenches and ground-truthed aerial photos along this line. Our results suggest that earthquakes may have occurred in this area in the geologically recent past. However, this preliminary study was unable to eliminate other possible interpretations. For further information visit our possible Lake Clark Fault page.
During an additional five days of fieldwork, we examined ancient beaches along Kamishak Bay that were uplifted more than 10 meters above the current beach. The uplift of this beach terrace, and associated deformation of the terrace, may have resulted from earthquakes near Kamishak Bay. Visit our possible faults on Kamishak Bay page for details.
This field research is ongoing. We plan to return to both of these field sites in the summer of 2010. Details are available on our summer 2010 field plan page. Following further work we will generate an updated report.
All of the main pages of this report are arranged in the outline at left. This overview page along with our possible Lake Clark Fault page and the possible faults on Kamishak Bay page provide the overall results of our work, and are intended to be understandable to the general public. Links to technical sections explain our work in scientific detail. Highlighted links are part of this report, whereas normal links reference other articles on Ground Truth Trekking's website that are not part of this report. Links to outside pages are denoted with a small arrow ().
This website structure is an experiment, seeking to create a report both accessible and understandable to readers with a non-geological background, but also amenable to rigorous examination and scientific review by geologists.
The tectonic deformation of southern Alaska is driven by collision of the Pacific Plate with the North American Plate. This map shows some of the prominent known faults that bound different fragments of crust in southern Alaska. Arrows depict how the pieces are moving relative to North America (longer arrows mean faster motion). Black dots mark the two field areas discussed in this report.
Faults in the area around Lake Iliamna, along with most of Western Alaska, have not been studied in detail. Lake Iliamna straddles the west edge of the band of earthquakes and volcanoes that comprises the Pacific 'Ring of Fire'. It's the sort of place where active faults are likely, but few earthquake geologists have worked in the field here, and few instruments have been deployed to measure plate motion or earthquakes.
Geologists do know something about tectonic plates in the larger region surrounding Lake Iliamna and the base of the Alaska Peninsula. There is good evidence that most of southcentral Alaska, called the 'Southern Alaska Block' (Haeussler, 2008) is a section of the earth's crust that is moving westward relative to the rest of North America. The Denali Fault, in the Alaska range, is the main fault that this block moves along, but there are other faults, including the Castle Mountain Fault just north of Anchorage, that allow it to deform and move westward. Additionally, there is evidence that the crust beneath the Bering Sea (called the 'Bering Block', Mackey et al., 1997), is rotating clockwise relative to North America and eastern Russia. And the Pacific Plate, which extends all the way from the Gulf of Alaska down into the South Pacific, is sliding northwards, beneath continental crust that forms Alaska.
These three pieces of the earth's crust surround our field sites. To the east is the Southern Alaska Block, to the northwest is the Bering Block, and to the south is the Pacific Plate. It is unclear whether the Lake Iliamna region is part of the Bering Block or Southern Alaska Block. It may even be that the Bering Block and Southern Alaska Block have no distinct boundary between them (Redfield et al., 2007).
Also, there are known faults in the area that were once active, and which may or may not currently be active. The Lake Clark Fault (Haeussler et al. 2004), an extension of the Castle Mountain Fault, extends southwest from Lake Clark Pass down through Lake Clark. The Bruin Bay Fault branches from the Castle Mountain and Lake Clark faults near Tyonek, and runs south along the Cook Inlet coast into Katmai National Park.
Given the lack of instruments and geological fieldwork in the area it is very possible that subtle evidence of activity on these faults and others has simply been missed.
In our fieldwork, we explored possible evidence for geologically recent activity on both of these faults. We undertook this fieldwork to address the large uncertainties about tectonics in this region, which are particularly relevant to the prospects for industrial development in the area, such as the proposed Pebble Mine.
Black dots mark field sites, black lines mark our trackline (including flights), and blue outlines mark areas of detailed aerial imagery analysis
Faults are deeply penetrating fractures in the Earth’s crust. Faulting results when stress builds up and causes the rock to shear. A fault is 'active' if that process of building stress is ongoing, and earthquakes are thus likely in the future. This is differentiated from inactive faults, where stress is no longer concentrated and earthquakes no longer occur.
The main way geologists assess whether a fault is active is by attempting to determine whether it has produced earthquakes in the geologically recent past (the past few hundred thousand years). If an earthquake has occurred on a fault recently, then the stress field that leads to earthquakes is likely still there, and will lead to earthquakes in the future. Given that most major faults only have an earthquake every few hundred to a few thousand years, and we have only been measuring earthquakes for the last century, it is common that no direct measurements or observations exist of earthquakes on a particular fault. Instead we must rely on evidence that has survived since the most recent earthquake. This geologic evidence becomes part of a detective story - clues that can show an earthquake has occurred.
Many types of evidence can reveal ancient earthquakes. In our fieldwork, we focused on three in particular:
When an earthquake occurs on a fault, the rock on either side of the fault shears, moving in different directions. For some earthquakes, especially the largest earthquakes, the shearing action of the fault extends up to the ground surface, leaving visible scars and disturbance. Shearing action along the fault breaks the soil and rock at the ground surface, opening cracks and shifting layers.
In some places a gully may form across a fault, and then shearing along that fault can cut the gully and move the two halves in opposite directions. The water flowing through them may keep the two halves connected, but give them a 'zig-zag.'
If an earthquake moves the land upwards at a beach, the water will form a new beach, leaving the old one as evidence of the uplift.
Continue to our specific results on the possible Lake Clark Fault and faults near Kamishak Bay.