1. Introduction 2. Geologic mapping

6. Remaining questions and future work

3. GPS data

4. Other geophysical data 5. Interpretations and revised tectonic model140° W142° W144° W

60° N

59° N

l 0 50

km

Elevation (m)

-5500

0

Yakataga-Chaix Hills Fault

Malaspina

FaultEsker Creek Fault

Fairweather Fault

Yakutat FaultForeland Fault Zone

??

Pamplona

ZoneU1420

U1421

200 km

130° W140° W150° W

60° N

55° N

l0 100 20050

km

1964

Aleutian Trench

Yakutat Terrane

Fig. 1bIcy Bay, Yakutat Bay

Paci�c Plate

North American Plate

~4.5 cm/year

Queen Charlotte-

Fairweather FaultTransition Fault

l

Post-1899 dated trees

Logan Beach

Plafker and Thatcher (2008)

Plafker and Thatcher (2008)

Plafker and Thatcher (2008)

EEE

E

EEEEEE

EEEEEEE

EEE

EEEEEE

EEEEEE

EEEE EEE

EEEEEEE

EEE

EEEEEEE

EEE

EEE

EE

EEE

E

EE

E

EE

EE

E

E

E

EE

EEEEEE

E

EEE

E

EEEE

E

E

E

E

EEEE

E

E

EEE E

EE

E

EE

EEE

E

EE

E

EEE

E

EE

E

EEE

E

EE

E

E

E

E

E

E

E

E

E

3.61

2.84

2.29

0.32.84

1.88

0.32.08

0.99

0.32.16

2.31

0

0

0

0

-0.61

-0.3

-1.52

-0.3

14.4

14.4

0.52.7

43.8

4.44.6

3.42.7

3.2

2.2

0.9

139° W140° W

60° N

59.5

° N

l0 10 205

kmFig. 2b

EE

Fig. 2a

Yakutat oblique thrust? Uplift measured following M8.2 event of 10 Sept 1899Range front topography

Uplift measured following M8.2 event of 10 Sept 1899Mapped during cruise EW0408 Topographic expression

Plafker and Thatcher (2008)Boundary transform?

Fairweather transform M7.9 event of 1958Topographic expression, o�shore seismic crossings

Doser (2010)

Tarr and Martin (1912); Plafker andThatcher (1982); Plafker and Thatcher(2008)

Esker and Bancasthrusts

M8.1 event on 10 Sept 1899 caused a linear north-south shoreline, as well as ~10m (Esker) and ~9m (Bancas) uplift across Disenchantment Bay coseismicwith the 10 Sept 1899 event

Plafker and Thatcher (1982); Plafker andThatcher (2008)

Cotton et al. (2014) Dextral o�set subglacier drainage valley

Malaspina Geodetic modelingAftershocks of M7.4 Saint Elias earthquake of 1972 O�shore seismic crossings

Elliott et al. (2013)Savage et al. (1986); Estabrooket al. (1992)

Foreland Fault Zone Geodetic modeling Uplift of a beach berm ca. 1899 (tentative correlation)

Elliott et al. (2013)Bruhn and Shennan (personal commun.)

Otmeloi thrust Deformation following M8.1 event of 10 Sept 1899 Tarr and Martin (1912); Plafker andThatcher (1982); Plafker and Thatcher(2008)

SEBay

NWBay

Modi�ed from Bruhn et al., 2012; italics indicate evidence compiled for this study; strikethrough indicates poor/no constraints

Fault Constrained by/rationale for displacement References

Fairweather Fault

(main fault plane;

transform)

Yakutat Fault(thrust?)

Boundary Fault(transpressive?)

Increasingshortening/transpression

NNorth American Plate

Yakutat Terrane

Relative plate motion

~4.5 cm/yr

?

YakutatBay

Disenchantment

Bay

EC BP

YF

BFFF

??

lN

140° W142° W

59.5° N60° N

l4010

km

Imagery sources: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS,USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community

0 20

FFZMF

CH

BPEC

FFBF

YFOF

4 Sept 1899 M8.1

10 Sept 1899 M8.2

YakutatBay

IcyBay

SW NE

200

300

400

500

100

TWTT

(ms

belo

w 0

)

VE ~7.5:1

~100

m

1 km600

N S

200

300

400

500

100

TWTT

(ms

belo

w 0

)

VE ~6:1

~100

m

1 km600

N S

400

600

800

200

TWTT

(ms

belo

w 0

)

VE ~7.5:1

~200

m

2 km

l0 5 10 15 20

km

140° W 139° W

Fig. 4

b

Fig. 4c

Fig. 4a

60° N59.5° N

BP

ECFF

BF

YF

OF?

4d

FF

BF

YF

Yakutat Terrane

4 Sept 1899 (?)

Elliott et al. (2013)

Elliott et al. (2010)

Elliott et al. (2010)

Fig. 4a

Revisiting the 1899 earthquakes of Yakutat Bay, Alaska using new and existing geophysical data

Maureen LeVoir Walton (maureenlwalton@utexas.edu)1, Sean S. Gulick1, Peter J. Haeussler2

21NH23B-1883

ReferencesBruhn, R. L., J. Sauber, M. M. Cotton, T. L. Pavlis, E. Burgess, N. Ruppert, and R. R. Forster (2012), Plate margin deformation and active tectonics along the northern edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska, and Yukon, Canada, Geosphere, 8(6), 1384-1407.Cotton, M. M., R. L. Bruhn, J. Sauber, E. Burgess, and R. R. Forster (2014), Ice surface morphology and �ow on Malaspina Glacier, Alaska: Implications for regional tectonics in the Saint Elias orogen, Tectonics, 33(4), 581-595.Davies, J., L. Sykes, L. House, and K. Jacob (1981), Shumagin seismic gap, Alaska Peninsula: History of great earthquakes, tectonic setting, and evidence for high seismic potential, J. Geophys. Res., 86(B5), 3821-3855.Doser, D. I. (2006), Relocations of earthquakes (1899–1917) in south-central Alaska, Pure and Applied Geophysics, 163(8), 1461-1476.Doser, D. I. (2010), A Reevaluation of the 1958 Fairweather, Alaska, Earthquake Sequence, Bull. Seismol. Soc. Am., 100(4), 1792-1799.Elliott, J., J. T. Freymueller, and C. F. Larsen (2013), Active tectonics of the St. Elias orogen, Alaska, observed with GPS measurements, J. Geophys. Res., 118(10), 5625-5642.Elliott, J. L., C. F. Larsen, J. T. Freymueller, and R. J. Motyka (2010), Tectonic block motion and glacial isostatic adjustment in southeast Alaska and adjacent Canada constrained by GPS measurements, J. Geophys. Res., 115(B9), B09407.Estabrook, C. H., J. L. Nábělek, and A. L. Lerner‐Lam (1992), Tectonic model of the Paci�c‐North American Plate Boundary in the Gulf of Alaska from broadband analysis of the 1979 St. Elias, Alaska, earthquake and its aftershocks, J. Geophys. Res., 97(B5), 6587-6612.Plafker, G., and W. Thatcher (1982), Geological and geophysical evaluation of the great 1899-1900 Yakutat Bay, paper presented at Alaska earthquakes (abstract), paper presented at AGU Chapman Conference on Fault Behavior and the Earthquake Generation Process, AGU, Snowbird, Utah.Plafker, G., and W. Thatcher (2008), Geological and geophysical evaluation of the mechanisms of the great 1899 Yakutat Bay earthquakes, Geophysical monograph, 179, 215-236.Savage, J., M. Lisowski, and W. Prescott (1986), Strain accumulation in the Shumagin and Yakataga seismic gaps, Alaska, Science, 231(4738), 585-587.Tarr, R. S., and L. Martin (1912), The Earthquakes at Yakutat Bay, Alaska, in September, 1899, US Gov't. Print. O�.

A series of large earthquakes ruptured in September of 1899 in the tectonically- complex region of southeastern Alaska. Though poorly understood, the two largest events (magnitude 8+) may have ruptured some of the Yakutat-North America plate boundary. Over 14 m of uplift in Yakutat Bay apparently coseismic with the largest event (Mw 8.2 on 10 Sept 1899; see Fig. 2b) leads to questions about how local fault structure relates to the 1899 earthquake series and regional hazard.

Fig. 1a: Regional tectonic context. 1964 rupture outline from Davies et al., 1981. Shows location of Fig. 1b.

CONCLUSIONS:

1) No major active fault systems located o�shore crossing Bay (Fig. 4a, 4b, 4c)

2) Uplift related to GIA may have in�uenced some measurements by Plafker and Thatcher (Fig. 2c, 3c)

3) 10 Sept 1899 coseismic (or postseismic) slip on the southeast side of the Bay seems to be minimal; primary slip likely occurred on the NW side of the Bay (e.g. Fig. 2c)

4) Dextral transpression likely dominates on the SE side of Yakutat Bay with a strain-partitioning, horsetail-type termination of the Fairweather strike-slip fault (Fig. 5a); convergence likely dominates in the NW

1) Can we link o�shore deformation structures related to subduction (i.e., Pamplona Zone) to onshore-o�shore structures in and around Yakutat Bay and Icy Bay? Is the Foreland Fault Zone mappable? --> Has been proposed through the USGS Earthquake Hazards Program (EHP); see proposed Icy Bay trackline (yellow) in Fig. 6a.

2) Is the fault system that ruptured during the 10 Sept 1899 event completely onshore or blind?

3) Why was there a tsunami if 1899 rupture was onshore? Was the tsunami due to slumping?

4) Was slip on the southeast side of the bay coseismic or postseismic with the 10 Sept 1899 event, and what was the mechanism of stress transfer across the Bay?

5) Can we model (e.g. Coulomb) loading onto local faults caused by the 4 Sept 1899 event?

Fig. 2c: Uplift data collected by Tarr and Martin (measured in 1905, published 1912; red +) and by Plafker and Thatcher (measured in 1980, published in 2008; black +) in Yakutat Bay with selected values labeled (meters). Discrepancies may be due to e�ects of glacial isostatic adjustment (GIA; see Fig. 3c). Subsidence measured by Tarr and Martin likely indicates non-tectonic sur�cial slumping (Plafker and Thatcher, 2008). Fig. 2d shows conceptual tectonic model based on uplift contours.

Fig. 3a, 3b: Tectonic block models by Elliott et al. (2013) and Elliott et al. (2010) showing best-�t fault geometry and relative motion between modeled blocks on the northwest (Fig. 3a) and southeast (Fig. 3b) sides of Yakutat Bay. Model is constrained by GPS data and known fault geometries. Relative motion values are in mm/year. Values of interest circled in blue and pink, respectively.

Fig. 3c: Glacial isostatic adjustment (GIA) model for southeastern Alaska based on best �t to GPS data, published by Elliott et al. (2010). Contours indicate uplift in mm/year and vectors indicate horizontal motion due to GIA.

Fig. 4a shows a seismic re�ection line from survey EW0408. Fig. 4b, 4c show post-stack time migrations from a 2012 seismic re�ection survey shot aboard the USGS R/V Alaskan Gyre. Data were shot using a mini-GI gun (peak frequency ~250 Hz) and a 24-channel streamer. Line locations are shown in Fig. 4d (yellow) along with remaining 2012 tracklines (white) and original Plafker and Thatcher (2008) fault traces (black dashes). Pink and blue traces show topographic expressions of local faults and represent our interpretation of actual fault geometry.

Fig. 1b: Local fault structure (geometry from Bruhn et al., 2012) with the epicenters for the two largest 1899 events (locations from Doser, 2006). MF - Malaspina Fault. CH - Chaix Hills Fault. FFZ - Foreland Fault Zone. EC - Esker Creek Fault. BP - Bancas Point Fault. FF - Fairweather Fault. BF - Boundary Fault. YF - Yakutat Fault. OF - Otmeloi Fault.

1a

1b

2a 2c

2d

3b

4a

4c

4b

5a

2b

3c

3a

terminal moraine

glacial advance surface

glacial retreat sequence

lateral moraine

glacial retreatsequence

glacial retreatsequence

glacially-carved surface

lateral moraines

glacially-carved surface

smaller glacially-carved channels

6a

Fig. 4a

Fig. 4b Fig. 4c�at-lying sediments

What ruptured on 10 Sept 1899?