Field studies of active faults
in the Lake Iliamna region
Bretwood Higman, PhD
Andrew Mattox
- Overview
- Possible Lake Clark Fault
- Possible faults in Kamishak Bay
- Summer 2010 field plans
- Technical notes:
- Maps:
- Figures:
Report Status
This is the first version of this report.
It is still under revision.
There are no more recent drafts.
Possible offset on the Bruin Bay Fault
The uplifted beach terrace along Kamishak Bay provides evidence of deformation both because its existence is evidence of uplift, and because it is no longer flat. A relatively abrupt change in elevation near the position of the Bruin Bay Fault may be evidence that that fault is active, and is at least evidence for some sort of tectonic deformation. The terrace was once flat, since it was eroded by the ocean. So any change in elevation along its length must be the result of localized uplift of subsidence.
Our measurements of the beach terrace near Amakdedori Beach and just south of there show uplift of 20 m or more. Further south the uplift is less, around 15 m. This discontinuity in elevation occurs near the Bruin Bay fault, but not right on it. The fault runs just a few hundred meters north of our southernmost high measurement of uplift (24.75 m). Another 2 km south of here we measured uplift at 16.83 m. It may be that this apparent offset in the terrace results from motion on a splay of the fault. Faults are commonly not a simple plane, but instead a network of fractures, and a given earthquake might occur on any given branch or 'splay'. So if the earthquake that offset the Kamishak beach terrace here occurred on a more southerly splay, then it could nicely explain this observation.
We surveyed an ancient beach above the modern beach of Kamishak bay (using a horizon-line survey method). Its elevation is quite variable, and may be related in part to motion on the Bruin Bay Fault. Aerial observations reveal that the terrace continues at a similar elevation further south and east, but it does not continue north into Bruin Bay.
The main possible alternative explanation for this apparent offset is some sort of error in our measurement methods or geomorphic identification of uplifted beach features. Our Coastal Survey Methods page describes some of the potential problems with our survey methods. To check these results, we will be returning to the field to survey using a mapping grade GPS in 2010. There are more details on this survey on our 2010 Field Plans page.