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Take a small detour off the Parks Highway near Houston, Alaska, and walk along a section of the Castle Mountain fault. 

The map¹ and GoogleEarth photo below indicate the location of the fault as it passes through Houston.  This fault is the only one known in the greater Anchorage, Alaska, area, with historical seismicity and a Holocene (created within the past 11,500 years) fault scarp ("cliff" produced by faulting rather than erosional forces).  The fault is considered capable of producing a magnitude 6.9 to 7.3 earthquake² sometime during the near future.

The Castle Mountain fault is a right lateral strike-slip fault that runs east-northeast along its 200 km length. The term strike-slip means the plates (land masses) on opposite sides of the fault move horizontally.  "Right lateral" means the plate on the side of the fault opposite the observer moves to the right when the fault slips.  An animation of a strike-slip fault and other types of faults may be found by clicking here.

The fault is observed easily from the air because of a prominent change in vegetation on the upthrown, north side of the fault.  Seismologists dug trenches across the fault at several locations to learn how often and how much the fault slips when it triggers an earthquake.  The scientists determined that the Castle Mountain fault has slipped three times in the past 2145 years:  2145–1870, 1375–1070, and 730–610 years before present (B.P.), indicating an average recurrence interval of ~700 years.  Given that it has been  610 to 730 years since the last significant earthquake caused by this fault, a significant earthquake may occur in the near future.  The dates are given in ranges due to the uncertainty in dating sedimentary and dead vegetative evidence found at various levels in the trenches.  One interesting aspect of Castle Mountain fault earthquakes is that they all appear to have occurred during April to October due to evidence of ground liquifaction (movement of water saturated soils when shaken) which indicates the ground was not frozen during the events.³

Seismologists identified a postglacial (created after the pre-Holocene glaciers retreated) outwash channel that has been offset by approximately 36 meters during the past 12,400 years due to ongoing fault slippage³.  In recent times, the fault produced light to moderate magnitude 5.7 and 4.6 earthquakes in 1983 and 1996, respectively.⁴  Did these quakes relieve some of the strain along the fault or did they serve to "wind the clockspring" at another location?  Only time will tell.

For more information about Alaskan earthquakes and earthquake preparedness, visit the Alaska Earthquake Information Center website.

To receive credit for this cache, e-mail the answers to the following questions.  Logs not followed up by an e-mail will be deleted.  

1. The past is often a predictor for future events along faults. Using the average fault movement per year after the glaciers retreated, how much would you expect the fault to move during the next major siesmic event?

2. Walk between N61° 37.324'  W 149° 47.329' and N61° 37.326'  W 149° 47.309', and determine and report the height of the tallest scarp (vertical surface expression of the fault) that you observe.  Why do you think the scarp is taller at that location? 

3. Why isn't the fault readily apparent along its entire 200 km length?  (What ongoing natural phenomenon is at work?)

¹GIS Coverages of the Castle Mountain Fault, South Central Alaska, Keith Labay and Peter J. Haeussler, U.S. Geological Survey Open-File Report 01-504, http://pubs.usgs.gov/of/2001/of01-504.

²Holocene Slip Rate for the Western Segment of the Castle Mountain Fault, Alaska, Julie B. Willis, Peter J. Haeussler, Ronald L. Bruhn and Grant C. Willis, Bulletin of the Seismological Society of America; June 2007; v. 97; no. 3; p. 1019-1024; DOI: 10.1785/0120060109 © 2007 Seismological Society of America, www.bssaonline.org/cgi/content/abstract/97/3/1019.

³Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain fault near Houston, Alaska, Geological Society of America Bulletin, Article: pp. 1296–1310, Volume 114, Issue 10 (October 2002), Peter J. Haeussler, Timothy C. Best, and Christopher F. Waythomas.

⁴USGS Study Shows that Anchorage Area Fault is Capable of a Large Quake, U.S. Geological Survey News Release, Oct. 23, 2002,www.scienceblog.com/community/ older/archives/E/usgs308.html.

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