Geodesy in the Eastern Aleutians, Alaska Peninsula

and Shumagin Region:current state of knowledge

Ken Hudnut, USGS

Workshop on SoCal Tsunami Scenario Source:Tsunami Source Working Group andMulti-Hazards Demonstration Project

9 February 2010U. S. Geological SurveyMenlo Park, California

Chernabura, PBO photo

Defined … “Shumagin Gap” … Re-defined

Shumagin Gap - McCann et al., Pageoph 1979

Geodesy weighs in… 1) tiltmeters and sea level gauges for long-baseline tilt

Science 21 October 1983: Vol. 222. no. 4621, pp. 322 – 325 DOI: 10.1126/science.222.4621.322

Tilt and Seismicity Changes in the Shumagin Seismic Gap J. BEAVAN, E. HAUKSSON, S. R. MCNUTT, R. BILHAM, and K. H.

JACOB Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964

Changes in the ground surface tilt and in the rate of seismicity indicate that an aseismic deformation event may have occurred between 1978 and 1980 along the plate boundary in the eastern Aleutians, Alaska, within the Shumagin seismic gap. Pavlof Volcano was unusually quiescent during this period. The proposed event would cause an increase of stress on the shallow locked portion

of the plate boundary, bringing it closer to rupture in a great earthquake.

NEHRP-funded0.5 microradian

per decade5 x 10-8/yrtilt rates

Geodesy weighs in… 2) geodolite(and early survey-mode GPS)

“where’s the beef?”Science 7 February 1986: Vol. 231. no. 4738, pp. 585 – 587 DOI: 10.1126/science.231.4738.585

Strain Accumulation in the Shumagin and Yakataga Seismic Gaps, Alaska J. C. SAVAGE , M. LISOWSKI , and W. H. PRESCOTT

Strain accumulation during the 1980-85 interval has been measured by means of trilateration surveys in the Shumagin and Yakataga seismic gaps, which are the two regions identified as the most likely sites for the next great thrust earthquakes along the Alaska-Aleutian arc. No significant strain accumulation was detected in the Shumagin gap, but experience at similar subduction zones and simple models of the subduction process suggest that a measurable amount of strain should have accumulated. The most likely explanation of the observation is that subduction there is either aseismic or episodic. The strain accumulation measured in the Yakataga gap is consistent with that expected for the plate convergence rate, although the direction of maximum compression may suggest a somewhat more oblique convergence than expected.

Lisowski et al. (JGR 1988) and thenLarson & Lisowski (GRL 1994) confirmed using EDM & GPS

TI-4100

Blast from the past - the Dark Ages… before GPS!

1st paper, Hudnut & Taber, GRL 1987Cut the ‘Shumagin gap’ into two small parts …

West (170 km) - only MS ~8.0 max; not tsunamigenic?

… yes, I would liketo do field work inAlaska again …

circa 1985

inferreddifferentstressesE vs. W

Geodesy – GPS & PBO

PBO

survey-mode GPS or

continuous GPS station:all face

challenges

Brian CoylePBO AK

Freymueller & Beavan (GRL 1999) – no strain in Sanak Islands

Peninsula

Chirikof Is.

Shumagin Islands

Sanak Is.Unimak Is.

Alaska

Much greater range in trench normal distances.

Courtesy ofJeff Freymueller

Geodesy weighs in, part 3 … GPS … “it’s complicated”

Arc velocity (mm/yr)

West = 2.1

South = 3.4

Trench parallel = 3.6

Trench normal = 1.9

Total Velocity = 4.0

#1#2

#3

#4

Fault planes

Peninsula

Alaska

Alaska Peninsula Dislocation Model Results

Fault geometry based on Fournier and Freymueller [2007]

SW arc translation velocity of ~4 mm/yr

Decrease in coupling from east to west

Cross and Freymueller [2008]

Model Results

26 km

26 km

20 km

~0 km

Fournier and Freymueller [2007]

Locked and Creeping Segments:so, is Eastern Aleutians source reasonable?

Freymueller et al. [2008]

Latest GPS indicates from middle of Unalaska (~166°) to Shumagins (~159°)is less than about 30% seismically coupled (W~100 km), but …

Aseismic area can potentially generate a very large slow source

[Scotch Cap Light, 1946 event]

USCG 1966 photo, from Okal et al. (BSSA 2003)

Looking NW at Scotch Cap station. The white arrows indicate the ruins of the concrete lighthouse destroyed by the tsunami (right arrow) and of a wooden predecessor (built in 1903, decommissioned in 1940; left arrow). The circled white building was the radio communication facility, flooded by the tsunami [Sanford, 1946], whose ruins constitute Location 4. The three buildings in the fore- ground on the bluff postdate the tsunami. The dashed line and the black arrows show the inundation line identified by debris deposited by the tsunami. Maximum runup is found at Location 3.