U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1043

Revision of Time-Independent Probabilistic Seismic Hazard 

Maps for Alaska
Revision of Time-Independent Probabilistic 

Seismic Hazard Maps for Alaska

By Robert L. Wesson, Oliver S. Boyd, Charles S. Mueller, Charles G. Bufe, 

Arthur D. Frankel, and Mark D. Petersen

 Open-File Report 2007–1043

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Wesson, Robert L., Boyd, Oliver S., Mueller, Charles S., Bufe, Charles G., Frankel, Arthur D., Petersen, Mark D., 2007, 

Revision of time-Independent probabilistic seismic hazard maps for Alaska: U.S. Geological Survey Open-File Report 

2007-1043.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic and Seismologic Setting..............................................................................................................3

Methodology...................................................................................................................................................3

Hazard Model ........................................................................................................................................7

Smoothed Seismicity...................................................................................................................7

Changes for 2007.................................................................................................................7

Fault Sources..............................................................................................................................12

Megathrust Sources .................................................................................................................12

Improvements in Methodology Adopted from the 2002 Revision ...............................................14

Innovations in Methodology .............................................................................................................14

Regional Discussion....................................................................................................................................14

Alaska-Aleutian Megathrust.............................................................................................................14

Eastern Aleutians to Prince William Sound ..........................................................................14

Semidi Region.............................................................................................................................16

Shumagin Gap ............................................................................................................................16

Western Aleutians .....................................................................................................................16

Komandorski Islands Region ...................................................................................................16

Yakataga Region..................................................................................................................................16

Detailed Model of Megathrust .........................................................................................................16

Transition Fault....................................................................................................................................17

Fairweather and Queen Charlotte Faults........................................................................................17

Central Denali, Totschunda, and Eastern Denali Faults ...............................................................18

Castle Mountain Fault ........................................................................................................................20

Kodiak Crustal Faults..........................................................................................................................20

Other Sources Modeled with Smoothed Seismicity.....................................................................20

Strike-Slip Faults Subparallel to Arc ......................................................................................20

Cook Inlet.....................................................................................................................................20

Tintina/Kaltag Fault....................................................................................................................20

Region Between the Denali and Tintina/Kaltag Faults ........................................................21

Attenuation Relationships ..........................................................................................................................21

Calculation Procedure ................................................................................................................................21

Resulting Maps.............................................................................................................................................21

Discussion.....................................................................................................................................................28

Conclusions...................................................................................................................................................28

References Cited..........................................................................................................................................30

Figures

1. Schematic map of Alaska and Aleutians..................................................................................2

2. Instrumental seismicity of Alaska and the Aleutian Islands from the declustered

consolidated catalog....................................................................................................................4
iv

3. Rupture areas of large earthquakes in Alaska and the Aleutian Islands

from 1900 to 2004...........................................................................................................................5

4. Centroid moment tensors from Harvard catalog for events with

M0•5x1025 dyne-cm for the period 1977–2004 ...........................................................................6

5. Hazard Model for Alaska and Aleutians...................................................................................8

6. Gridded incremental values of 10a-b*6 ....................................................................................9

7. Segmentation of megathrust. ...................................................................................................15

8. Active crustal faults in south-east Alaska..............................................................................18

9. Active crustal faults in south-central Alaska.........................................................................19

10. Probabilistic ground motion with a 2-percent probability of exceedance

in 50 years for peak ground acceleration...............................................................................24

11. Probabilistic ground motion with a 10-percent probability of exceedance

in 50 years for peak ground acceleration...............................................................................25

12. Probabilistic ground motion with a 2-percent probability of exceedance in

50 years for peak ground acceleration at a larger scale in the Aleutians........................26

13. Percentage changes in the 2007 map relative to the 1999 map for

south-central Alaska..................................................................................................................27

Tables

1. Characteristics of active faults assumed for hazard analysis............................................13

2. Attenuation relationships assumed for various seismic sources ......................................22

3. Input files, key parameters, and hazard codes used in preparing the maps ...................23

4. Selected ground-motion values and percentage changes from the 1999

map at representative cities and towns in Alaska and the Aleutians. ..............................29

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 
Abstract

We present here time-independent probabilistic seismic 

hazard maps of Alaska and the Aleutians for peak ground 

acceleration (PGA) and 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0 second 

spectral acceleration at probability levels of 2 percent in 

50 years (annual probability of 0.000404), 5 percent in 50 

years (annual probability of 0.001026) and 10percent in 50 

years (annual probability of 0.0021). These maps represent a 

revision of existing maps based on newly obtained data and 

assumptions reflecting best current judgments about methodology and approach. These maps have been prepared following the procedures and assumptions made in the preparation 

of the 2002 National Seismic Hazard Maps for the lower 

48 States. A significant improvement relative to the 2002 

methodology is the ability to include variable slip rate along 

a fault where appropriate. These maps incorporate new data, 

the responses to comments received at workshops held in 

Fairbanks and Anchorage, Alaska, in May, 2005, and comments received after draft maps were posted on the National 

Seismic Hazard Mapping Web Site. These maps will be 

proposed for adoption in future revisions to the International 

Building Code. In this documentation we describe the maps 

and in particular explain and justify changes that have been 

made relative to the 1999 maps.

We are also preparing a series of experimental maps of 

time-dependent hazard that will be described in future documents.

Introduction

Probabilistic seismic hazard maps for Alaska and the 

Aleutians were prepared in 1985 (Thenhaus and others, 

1985) and 1999 (Wesson and others, 1999a 1999b). The 

aim of the present effort is to revise and extend the previous maps taking into account new and improved information 

about the earthquake hazard in the region and improvements 

in methodology. The most significant development since 

the preparation of the 1999 maps was the occurrence of the 

November 3, 2002, Denali earthquake (Mw 7.9), which was 

accompanied by thrust and right-lateral strike-slip surface 

fault ruptures extending 340 km along the Susitna Glacier, 

central Denali, and Totschunda faults, reaching a maximum 

of about 8 m (Eberhard-Phillips and others, 2003; Crone and 

others, 2004; Haeussler and others, 2004). The earthquake led 

to a significant geologic effort (Schwartz and others, 2005a 

and b; Matmon, 2006) that will result in a vastly improved 

understanding of both the long-term slip rates on these faults 

and the earthquake history extending back about 2,000 years. 

These efforts have led to improved estimates of slip rates on 

the Denali and Totschunda faults. (Selected faults and geographic locations are shown in figure 1). New paleoseismic 

data are also available for the Castle Mountain fault, which, 

after the Alaska-Aleutian megathrust zone, appears to be the 

most significant threat to the most heavily populated part 

of Alaska near Anchorage (Haeussler and others, 2002). 

New geologic information has also been obtained on Quaternary faults exposed on Kodiak Island (Carver and others, 

2003), and slightly revised slip rates are available for the 

Fairweather­–Queen Charlotte fault system and the Transition 

fault (DeMets and Dixon, 1999; Freymueller and Fletcher, 

1999; Fletcher and Freymueller, 2003). In addition, earthquake catalogs have been extended, and improved earthquake 

locations and new geophysical studies have contributed to 

improved understanding of the geometry of the megathrust 

zone and hazard in the Cook Inlet region (Doser and others, 

2004; Haeussler and others, 2000; Ratchkovski and Hansen, 

2002b). Methodological improvements underlying these maps, 

relative to 1999, principally following the 2002 National Map, 

include the explicit inclusion of epistemic uncertainty, new 

attenuation relationships, and log-log interpolation (rather than 

log-linear interpolation) of probabilistic ground-motion values 

between the directly calculated values. Improvements specific 

to the Alaska maps include improved characterization of the 

subduction zone and minor improvements in the approach to 

the calculation of parameters representing smoothed seismicity. We also consider the partial seismic decoupling along the 

Transition fault (fig. 1). The ability to calculate probabilistic 

ground motion assuming a spatially variable slip rate on a 

fault enables us to capture more realistically the hazard along 

the southeastern Denali fault and in a few other areas. These 

changes will be discussed in detail herein.

Revision of Time-Independent Probabilistic Seismic 

Hazard Maps for Alaska

By Robert L. Wesson, Oliver S. Boyd, Charles S. Mueller, Charles G. Bufe, Arthur D. Frankel, and

Mark D. Petersen
160oE

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Komandorski Islands

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Transition Fault

Tintina Fault

Denali Fault

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Fairweather-Queen Charlotte 

Prince William Sound Cook Inlet

K.I. F.N.C. F.

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KodiakIsland

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S.G.F.

A

l

a

s k a

- A l e u t i a n M e g a

t

h

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t

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Figure 1. Schematic map of Alaska and Aleutians showing selected crustal faults used in calculation of seismic hazards (shown in red) and geogratext. Neither the Tintina fault (shown in yellow) nor the Susitna Glacier fault (denoted by “S.G.F.”) are explicitly included in the hazard map. The Kodiafaults are denoted by “K.I. F.” and “N.C. F.”, respectively. The offshore extent of these faults is poorly understood.
To gather new data and to provide an opportunity for 

discussion among experts on these issues, two workshops were 

held in Alaska in May 2005, and one meeting with Canadian 

colleagues was held in June 2005. Additional comments 

were obtained in response to the posting of draft maps on the 

National Seismic Hazard Mapping Web Site during the fall of 

2005.

Geologic and Seismologic Setting

Alaska is the most seismically active State in the United 

States. The 1964 Prince William Sound earthquake (Mw 9.2) 

is one of the three largest earthquakes to occur since the beginning of instrumental recording in the late 1800s. (The other 

two are the 1960 Chilean earthquake and the 2004 Sumatra–

Andaman Islands earthquake.) The 2002 Denali fault earthquake (Mw 7.9) is the largest onshore earthquake to strike the 

United States since the San Francisco earthquake of 1906. 

Although the population of Alaska remains small, the potential 

for very significant impacts on important natural-resource production and transportation facilities, on critical military facilities, and on the more populated regions of the State from a 

large earthquake must be taken very seriously. Fortunately, the 

2002 earthquake occurred in a sparsely populated region of the 

State, and the impact was relatively modest. The Denali fault 

earthquake was accompanied by fault rupture over a distance 

of 340 km, with offsets up to 8 m. Although the Trans-Alaska 

Pipeline was offset about 5 m, the pipeline survived the event 

owing to careful engineering and preparation for fault rupture 

and ground-shaking hazard (Cluff and others, 2003; Hall and 

others, 2003) preventing untold economic losses and environmental damage.

The instrumental seismicity of Alaska and the Aleutians 

for earthquakes since 1900 greater than or equal to magnitude 

5.5 is shown in figure 2. Clearly, most of the seismicity in the 

region is associated with the Alaska-Aleutian megathrust fault 

extending eastward along the Aleutian arc into south-central 

Alaska. The northwestward-moving Pacific plate is subducted 

along this megathrust beneath the North American plate, giving rise to the Aleutian trench, islands, and related volcanic 

activity. Additional significant seismicity occurs along the 

Denali fault in south-central Alaska and along a northwestward-striking system of right-lateral strike-slip faults extending southeastward through and offshore from the panhandle 

of southeast Alaska (Page and others, 1991). The southeastern 

portion of this system of faults forms the northeast boundary 

of the Pacific plate. Additional seismicity also occurs elsewhere in central Alaska.

The estimated rupture zones of the largest earthquakes 

in this century are shown in figure 3 (Plafker and others, 

1993; Ratchkovski and others, 2004). During this century, 

virtually the entire plate boundary from the westernmost 

Aleutian Islands to the Queen Charlotte Islands off British 

Columbia has ruptured in large to great earthquakes. The only 

exceptions are areas near the Komandorski Islands, near the 

Shumagin Islands, and near Cape Yakataga (Sykes, 1971; 

Davies and others, 1981). Near the Komandorski Islands, 

historical records of large earthquakes in 1849 and 1858 

at the extreme western end of the arc have been judged as 

insufficient to conclude that plate-margin-rupturing earthquakes have occurred there (Sykes and others, 1981; Taber 

and others, 1991). At this location, subduction is occurring 

at a highly oblique angle, and it has been argued that the 

recurrence properties of large earthquakes here may differ 

significantly from those elsewhere along the arc. Indeed, 

Cormier (1975) has argued that the region may be incapable 

of supporting a great earthquake, although in light of the 2004 

Sumatra–Andaman Islands earthquake, this interpretation 

must be revisited. In the vicinity of the Shumagin Islands, 

that is, in the region between the 1957 and 1938 earthquakes, 

it has been argued that no great earthquake has occurred in 

this century. Similarly, the vicinity of Cape Yakataga has 

experienced no great earthquakes since 1899 or before. These 

two regions have been identified as “seismic gaps”— that is, 

the potential sites of future large earthquakes (Sykes, 1971; 

Lahr and others, 1980; Davies and others, 1981). Although 

concern remains about the possibility of a large earthquake in 

the Yakataga region, recent analysis of GPS geodetic measurements in the Shumagin Islands suggests that very little 

strain is currently accumulating there and that slip along the 

megathrust may be accommodated aseismically (Freymueller 

and Beavan, 1999).

The broad scale stress system causing earthquakes in 

Alaska and the Aleutians can be seen in figure 4 in which are 

plotted the centroid moment tensors from the Harvard catalog 

(http://www.globalcmt.org/CMTsearch.html) for significant 

earthquakes since 1977 together with motion vectors for the 

Pacific plate relative to the North American plate from NUVEL 

1-A (DeMets and others, 1994). Evident in figure 4 is the 

gradual transition from perpendicular subduction in the Prince 

William Sound and eastern Aleutian regions to the oblique 

subduction in the far western Aleutian and Komandorski Island 

region.

Methodology

The overall methodology used in the preparation of these 

maps is the same used in the previous Alaska map (Wesson and 

others, 1999a, 1999b) and in the 1996 and 2002 probabilistic 

seismic hazard maps for the contiguous 48 States (Frankel and 

others, 1996, 2002) based on the basic approach of Cornell 

(1968). The methodology combines estimates of the frequencies and magnitudes of earthquakes from potential sources with 

empirical relationships for the attenuation of strong ground 

motion with distance from the source to make a quantitative 

estimate of the ground motion at each point on the map for 

various probability levels, for example, the ground motion with 

a 2-percent probability of exceedance in 50 years.

Methodology 
 160° E

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 170° W 160° W 150

° W

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° W

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° W

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 0-33 

M=9 8 7 6

Figure 2

Figure 2. Instrumental seismicity of Alaska and the Aleutian Islands from the declustered consolidated catalog. Earthquakes shown have magnitudranging from 1990 to 2004.
160 ̊E

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8

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Figu

Figure 3. Rupture areas of large earthquakes in Alaska and the Aleutian Islands from 1900 to 2004.
4

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74

75

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Figure 4. Centroid moment tensors from Harvard catalog for events with M0•5x1025 dyne-cm for the period 1977–2004. Also shown are pvectors for the motion of the Pacific plate relative to North America from the NUVEL 1a model. Numbers indicate rates of plate motion in year. Other plate motion models such as REVEL (Sella and others, 2002) show very similar results.
Hazard Model

For purposes of preparing the hazard map, the potential 

sources of earthquakes in Alaska and the Aleutians are treated 

in three categories: (1) uncharacterized and unrecognized 

faults, which are represented by smoothed seismicity; (2) 

active faults with known parameters (that is, slip rate, recurrence interval, and so forth) recognized in the interior of 

Alaska and onshore and offshore regions of southeast Alaska; 

and (3) the Alaska-Aleutian megathrust associated with the 

subduction of the Pacific plate. Because the map makes a 

quantitative estimate of ground motion, each source included 

in the map must have some degree of quantitative information 

allowing estimation of the frequencies and sizes of earthquakes that it might generate.

The overall strategy for combining the hazards from these 

sources is summarized in figure 5, to which we will make reference in the following sections. We now briefly review how 

each of the three source types is characterized and then discuss 

each of the source regions in more detail.

Smoothed Seismicity

An updated and revised catalog, including earthquakes 

in the time interval intervening since the 1999 maps, was 

prepared and used to calculate updated estimates of the 

smoothed seismicity. We combined catalogs from Engdahl 

and Villaseñor’s International Association for Seismology 

and Physics of the Earth’s Interior (IASPEI) project, Stover and Coffman’s Seismicity of the United States, the U.S. 

Geological Survey Preliminary Determination of Epicenters 

(PDE), the International Seismological Centre (ISC), and the 

Alaska Earthquake Information Center (AEIC). A subjective 

ranking of source catalogs was made to select the best focal 

coordinates and moment magnitude for each event from 1898 

through 2004. Listed moment magnitudes were preferred and 

used directly. Other magnitudes were converted to moment 

magnitude by using published relations (Utsu, 2002; Sipkin, 

2003). Moment magnitude is referred to simply as magnitude in the following discussion. The catalog was declustered 

using the Gardner and Knopoff (1974) algorithm following 

the procedure used in the 1999 Alaska map and the 2002 map 

for the lower 48 States. For 12 of the largest mainshocks1 for 

which published aftershock studies were available, aftershocks were removed from the catalog within zones based 

on the published studies rather than the default circular zones 

specified by Gardner and Knopoff. (In the interest of transparency and reproducibility we chose the simpler Gardner and 

1

Aftershocks were removed based on zones defined in published studies for 

the following earthquakes and intervals: 1949 0822 Queen Charlotte (M8.1), 

1,000 days; 1957 0309 Great Aleutian (M8.6), 1,500 days; 1958 0710 Fairweather (M7.7), 500 days; 1964 0328 Prince William Sound (M9.2), 1,500 

days; 1965 0204 Rat Islands (M8.7), 1,300 days; 1972 0730 Sitka (M7.6), 

500 days; 1979 0228 St. Elias (M7.5), 600 days; 1986 0507 Andreanof Island 

(M8.0), 1,000 days; 1987 1130 Gulf of Alaska (M7.9), 750 days; 1988 0306 

Gulf of Alaska (M7.8), 750 days; 2002 1103 Denali fault (M7.9), 500 days.

Knopoff approach over methods that require the setting of tuning parameters to determine the clustering.) The 1898–2004 

catalog with magnitude ≥ 4.0 contained about 21,000 records. 

After declustering, about 7,500 mainshocks remained: about 

450 from Engdahl and Villaseñor, about 220 from Stover & 

Coffman, about 4,270 from PDE, about 2,070 from ISC, and 

about 500 from AEIC.

Overall completeness levels were determined from the 

catalog: magnitude ≥ 4.5 since 1964, magnitude ≥ 6.0 since 

1932, and magnitude ≥ 6.9 since 1898. To model the smoothed 

seismicity, we assumed an exponential magnitude-frequency 

distribution,

log10 n = a – bM,

where n is the number of earthquakes per year in a magnitude 

interval, M is magnitude, and a and b are parameters, and 

used Weichert’s (1980) method for estimating a and b values 

in catalogs with variable completeness levels. Earthquakes 

with depth 0–50 km and associated with the megathrust were 

treated separately, as described herein. The remaining earthquakes were considered in three groups: those with depth 0–50 

km not associated with the megathrust, those with depth 50–80 

km, and those with depth 80–120 km. Earthquake activity 

rates were calculated in each cell of a 0.1o-long × 0.1o-lat grid 

and smoothed using a two-dimensional Gaussian function with 

correlation distance g (Frankel, 1995): b = 0.816 and g = 75 

km for earthquakes with depth ≥ 50 km not associated with 

the megathrust, b = 0.858 and g = 50 km for earthquakes with 

depth > 50 km and ≥ 80 km, and b = 1.007 and g = 50 km for 

earthquakes with depth > 80 km and ≥ 120 km. The megathrust was divided into several subzones, and b values and 

average a values were estimated for earthquakes with depth 

≥ 50 km within each subzone. The subzone average seismicity 

rate was prorated into each grid cell, giving a uniform (rather 

than gridded and smoothed) spatial pattern. This approach 

is described in more detail in the section “Detailed Model of 

Megathrust.”

These grids, shown in figure 6, are used to estimate the 

hazard for earthquakes not associated with known faults. As 

shown in figure 4, these grids are used (1) for earthquakes in 

the megathrust zones with depths less than 50 km and magnitudes up to 7.0, (2) for earthquakes not located in the megathrust zones with depths from 0 to 50 km and magnitudes up to 

7.3, and (3) for earthquakes deeper than 50 km and magnitudes 

up to 7.5.

Changes for 2007

The new catalog has been extended from the 1996 

through the end of 2004. Improvements since 1999 have been 

made both in the choice of source catalogs and in catalog 

processing, notably magnitude conversion and aftershock 

analysis for the largest mainshocks. Incorporation of the 

Engdahl and Villaseñor catalog, with many carefully relocated 

earthquakes, is a major improvement. The DNAG (Decade of 

North American Geology) source catalog was used in 1999 but 

is not included in the updated catalog. This change accounts 

Methodology 
5.0≤M≤7.0* M≥7.0

Spatially Smoothed Seismicity

Shallow earthquakes (0-50 km)

5.0≤M≤7.0

Deeper earthquakes (50-80 km)

5.0≤M≤7.0

Deepest earthquakes (80-120 km)

5.0≤M≤7.0

Megathrust sources

5.0≤M≤7.0

*An upper limit of magnitude 7.3 was adopted toaccommodate the possibility of a background earthquake ofthat magnitude.

Megathrust Zone

7.0≤M≤8.0

Equal likelihood throughout megath

rate determined from average of 

determined from instrumental 

M≥8.0

Rates and maximum magnitudes 

segment. Equally weighted with ch

Prince William Sound and Kodia

Faults with “known” segm

(“Type-A”)

Characteristic model using slip 

recurrence data to obtain recurrenc

earthquake.

Faults with “unknown” seg

(“Type-B”)

Equal weights on characteristic m

truncated Gutenberg-Richter m

maximum magnitude using slip 

recurrence data to obtain recurrenc

earthquake.

Figure 5. Hazard model for Alaska and Aleutians.

Figure 5. Hazard Model for Alaska and Aleutians.
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10.000 x 10-4

10a-b*6 /100km2/yr (i,0.1), d≤50 kilometers

A

Figure 6. Gridded incremental values of 10a-b*6 (the number of M6.0 earthquakes per 100 square kilometers per year) fassociated with the megathrust with depths 0–50 kilometers, (B) earthquakes with depths 50–80 kilometers and (C) earth80–120 kilometers.
GMT Feb 28 08:10

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10a-b*6 /100km 2/yr (i,0.1), 80<d2120 C
for some differences that are visible in low-hazard areas of the 

maps, where a single earthquake can make a clear difference. 

For example, two prominent hazard bull’s-eyes (albeit at low 

levels of probabilistic ground motion) in the 1999 maps near 

160°W., 50°N. in the Gulf of Alaska correspond to magnitude 

4.5 and 4.8 earthquakes in 1982 from the DNAG catalog. 

These earthquakes are not listed in any other authoritative 

catalog, and the corresponding hazard disappears in the new 

maps.

Fault Sources

Information to characterize fault sources comes from 

a variety of types of studies. Ideally, one would like a long 

history of the origin times, magnitudes, rupture lengths, and 

displacements along each fault. Realistically, in the best cases, 

such as the preliminary results from the recent studies of the 

central Denali fault, we know the average slip rate over several 

thousand years and the approximate origin times of events at 

several discrete points along the fault. There is typically some 

uncertainty about the correlation of event times at adjacent 

and nearby sites. Event frequencies and size distributions are 

extracted from such data by variations of a strategy called 

“moment balancing.” The moment of an earthquake is equal 

to the product of the area of slip times the average slip on that 

area times the elastic rigidity. The total moment released over 

an extended time, T, is equal to the product of the average slip 

rate on the fault, the time T, the area of the fault surface, and 

the elastic rigidity. Thus, from the slip rate and knowledge of 

the geometry of the fault, we know the sum of the moments 

of all the earthquakes, assuming that all the slip takes place 

during earthquakes. Incorporating the geologically measured 

long-term slip rate and whatever information is known about 

the timing and size of past earthquakes, together with generic 

models of the magnitude-frequency distribution of earthquakes 

on faults, we estimate the frequency of various magnitudes for 

each fault.

The generic models of the magnitude frequency distribution commonly used in USGS seismic hazard studies are the 

“characteristic” model, in which we assume that virtually all 

the seismic moment is released in repeated occurrences of 

similar earthquakes which are approximately the largest that 

can be expected for the fault, and the “truncated GutenbergRichter” model. It is well known that, over time, the earthquakes in a region follow a statistical relationship called the 

Gutenberg-Richter relation which states that the logarithm of 

the number of events in each magnitude interval is equal to 

a constant (the a-value) minus another constant (the b-value) 

times the magnitude. Typically, the b-value is near 1, leading to the rule of thumb that one observes 10 times as many 

magnitude-six earthquakes as magnitude-seven earthquakes, 

and so forth. The application of the Gutenberg-Richter relation 

to a single fault, as opposed to a region, tends to estimate too 

many smaller earthquakes. A partial fix to this problem has 

been to use a truncated distribution considering only earthquakes between, say, magnitude 6.5 and some maximum 

magnitude. Commonly, this truncated distribution still predicts 

a higher ratio of lower magnitudes to higher magnitudes than 

is observed on a single fault. In contrast, the characteristic 

model, in its most simple application, predicts fewer earthquakes on a fault than are generally observed (see for example, 

Field and others, 1999; Frankel and others, 2002).

The approach taken in the preparation of past USGS 

hazard maps was to define two types of faults. The first, “Atype” faults, were assumed to be well characterized and to 

follow characteristic behavior. The second, “B-type” faults, 

were modeled with 50-percent characteristic behavior and 50-

percent truncated Gutenberg-Richter behavior. The properties 

of the faults characterized in the new map are summarized in 

table 1. A slight variation of this approach was taken for the 

Fairweather–Queen Charlotte fault system in 2007. In 1999 

the fault segments were assumed to be characteristic and the 

segment boundaries were taken as fixed. It appears that the 

strongest evidence for these segment boundaries is the extent 

of the most recent earthquakes. Thus, in 2007, characteristic 

behavior on the segments was equally weighted with earthquakes in a truncated Gutenberg-Richter model in which the 

earthquakes are allowed to float without respect to segment 

boundaries. Hence, the Fairweather–Queen Charlotte fault system is no longer an “A-type” fault in the sense used previously, 

reducing the dependence of the resulting hazard calculations 

on the assumption of imperfectly known segment boundaries.

Megathrust Sources

Application of the moment-balancing strategy of estimating the frequency of earthquakes on the Alaska-Aleutian 

megathrust is not appropriate. Although the overall rate and 

direction of slip along the megathrust is well known—for 

example, about 5 cm/year approximately perpendicular to the 

fault in the region of Prince William Sound— a significant but 

poorly known fraction of the slip occurs without earthquakes. 

This is known as aseismic slip. The fraction of the total slip 

that is accommodated by aseismic slip also seems to vary with 

position along the megathrust. Over 3–5-year time periods, 

most if not virtually all the slip is accommodated by aseismic 

slip in the Shumagin Island region (Freymueller and Beavan, 

1999) and almost no aseismic slip occurring in the Prince 

William Sound region (Cohen and Freymueller, 2004). There 

are also some indications that the amount of aseismic slip may 

vary with time (Buergmann and others, 2005). In short, means 

of estimating the frequency of large earthquakes along the 

megathrust must be found elsewhere.

For the eastern part of the megathrust, geologic studies 

within the delta of the Copper River by Plafker and Rubin 

(1994) and on the arms of Cook Inlet by Combellick (1994) 

and Bartsch-Winkler and Schmoll (1992) provide estimates 

of the origin times and frequencies of great earthquakes as 

reflected in subsidence in the sediments and peats. These 

estimates yield an average recurrence time of about 750 years 

for 1964-type events. Recent results with high-accuracy dating 

from the Girdwood area along Turnagain Arm of Cook Inlet 

12  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
 Table 1. Characteristics of active faults assumed for hazard analysis. 

Fault

(segment)

Mchar

(Mmax)

Slip rate 

(mm/yr)

Recurrence time1 for 

characteristic

earthquake

(years)

References

Queen Charlotte 8.1b 49 155 1

Fairweather,

offshore 7.7b 49.5 100 1

Fairweather,

onshore 8.0b 48 150 1

Denali, southeast 7.9 8.4–2 1,065–4,465 2

Denali,

central 7.9 1–9.4–14.4 15,305–1,630–1,065c21,430–2,280–1,490d 2

Totschunda 7.9 6 1,490 2

Castle Mountain 7.1 0.5–2.9–0.5 4,255–730–4,255 3

Transition 8.2b 6 325 (see text) 

Kodiak Island 7.5 1 4,435 4 

Narrow Cape 7.5 2 2,220 4 

References

1DeMets and Dixon (1999); Fletcher and Freymueller (2003) 

2Schwartz and others (2005a, 2005b) 

3Willis and others (in press) 

4Carver and others (2003) 

Notes

aRecurrence times are estimated from the rate of seismic moment release for earthquakes of the 

characteristic magnitude required to balance the observed geologic slip rate. They represent recurrence 

within any section of fault length equal to Wells and Coppersmith (1994) surface rupture length. Ranges 

in recurrence time correspond to the ranges in slip rate along the fault. 

bCharacteristic magnitudes estimated from fault length using the Wells and Coppersmith (1994) 

relations.

cRecurrence times for ruptures on the Central Denali-Eastern Denali system. 

dRecurrence times for ruptures on the Central Denali-Totschunda system. 

Methodology 13
(Hamilton and Shennan, 2005; Hamilton and others, 2005; 

Shennan and Hamilton, 2006) indicate six great earthquakes in 

a period of 3,300 years for an average recurrence time of about 

650 years. This value is adopted here.

The Kodiak Island region of the megathrust ruptured during 

the 1964 earthquake and is inferred to have also ruptured in previous 1964-type earthquakes. Based both on historical records (see 

for example, Nishenko and Jacob, 1990) and geologic evidence 

(Carver and others, 2003; Sauber and others, in press), this region 

has also been the locus of more frequent ruptures.

The western Aleutians display recurrence behavior that 

is decidedly not “characteristic,” as will be discussed in more 

detail herein.

To model the behavior of the megathrust, we generally 

followed the approach used in 1999 of combining models for 

the characteristic behavior on the Prince William Sound portion of the megathrust with truncated Gutenberg-Richter models for all portions of the megathrust, the parameters for which 

were estimated from the statistics of instrumentally observed 

earthquakes. The procedure and the modest changes adopted 

for 2007 are discussed in more detail herein.

Improvements in Methodology Adopted from the 

2002 Revision

In the process of preparing the 2002 update of the seismic 

hazard map of the lower 48 States, Frankel and others (2002) 

made certain improvements in details of the methodology used 

relative to the 1996 map of the lower 48 States and the 1999 

map of Alaska and the Aleutians. These improvements have 

been adopted in the preparation of the new maps of Alaska and 

the Aleutians. Principal among these changes were the incorporation of epistemic uncertainty about maximum magnitudes on 

fault segments and new attenuation relationships. The attenuation relations will be discussed in more detail. For discussion of 

the treatment of epistemic uncertainty and other minor improvements in the methodology, please see Frankel and others (2002).

The hazard codes of Frankel and others (1996, 2002) 

calculate the frequencies (and probabilities) of exceedance of 

a predetermined set of ground-motion ordinates. To produce 

the map it is necessary to interpolate within this relationship 

to obtain the probabilistic ground-motion values at each point 

on the map for the desired probability levels. For the 1996 

map for the lower 48 States and the 1999 map for Alaska, this 

interpolation was done in linear space. For the 2002 lower 48 

map and the current Alaska map, this interpolation is done in 

log space. Interpolation in log space is judged to represent the 

variation in ground motions between the calculated ordinates 

within the range of the values of interest.

Innovations in Methodology

We have also modified the hazard programs to allow 

for position-dependent probabilities owing to variable slip 

rates along faults. This has allowed us to vary the probability 

for large earthquakes, for example, along the eastern Denali 

fault, from a value that reflects 7 mm/yr of right-lateral slip 

where it meets the central Denali fault to one that reflects 2 

mm/yr at its southern end (Matmon and others, 2006; Plafker 

and others, 1993). This modification has also allowed us to 

cascade earthquakes between the central and eastern Denali 

and the central Denali and Totschunda faults. Another reason 

for this modification in methodology is to facilitate calculations of earthquake probability involving stress transfer from 

regional earthquakes, which produce position-dependent 

stress changes.

Regional Discussion

Alaska-Aleutian Megathrust

In the following discussion of the Alaska-Aleutian 

megathrust, reference is made to subregions that are indicated 

in figure 7.

Eastern Aleutians to Prince William Sound

The evidence from geologic studies in the Kodiak area 

suggests that the underlying portion of the subduction zone 

ruptures not only in great earthquakes such as the 1964 

earthquake (Mw 9.2), but also more frequently in somewhat 

smaller events (Carver and others, 2003; Sauber and others, in 

press). Consequently, the 1964 segment used in the 1999 map 

has been subdivided to allow for the rupture of the Kodiak 

zone alone about as frequently as it ruptures together with the 

Prince William Sound portion of the 1964 zone. The maximum magnitude for the Kodiak segment rupturing alone is 

estimated as Mw 8.8, based on the idea that such an earthquake could have up to one-half the area and one-half the slip 

of the 1964 earthquake.

Recent detailed studies of earthquakes associated with 

the megathrust in the Anchorage region (Doser and others, 

2004; Ratchkovski and Hansen, 2002b) suggest that the depth 

of the interface is somewhat less and extends farther north 

than that assumed in the 1999 maps; thus, we have changed 

the geometry of the megathrust in the model to reflect better 

located seismicity.

14  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
160 170 180 -170 -160 -150 -140 -13

506070

Western Aleutians

Semidi

Prince

William

Sound

Kodiak

Shumagin Komandorski

Yakataga

Megathrust Sub-zones

Figure 7. Segmentation of megathrust. 
Semidi Region

The Semidi region lies between the Kodiak region and the 

Shumagin gap. It was the location of an Mw 8.2 earthquake in 

1938. GPS geodetic observations (Fletcher and others, 2001) 

indicate the plate interface is fully coupled in this region, in 

contrast to the largely decoupled region in the Shumagin gap 

to the west.

Shumagin Gap

New geodetic evidence that the subduction zone in the 

region of the Shumagin gap is largely decoupled (Freymueller 

and Beavan, 1999) tends to support the assumption made in 

the 1999 maps that great earthquakes are extremely unlikely to 

rupture through a region near the Shumagin Islands and near 

the western end of the Alaska Peninsula. Thus, while earthquakes as large as Mw 8 have been observed and are consequently assumed in the map in this region, larger earthquakes 

are assumed not to rupture through this region.

Western Aleutians

In the 1999 map the western Aleutians were characterized with a truncated Gutenberg-Richter behavior because it 

was noted that the 1986 Mw 8.0 and 1997 Mw 7.9 appeared to 

rerupture a portion of the rupture area of the 1957 earthquake 

(Boyd and others, 1995), in apparent violation of classic “characteristic” behavior (as did the 2003 Mw 7.8 as well). The 

work of Johnson and others (1994) supports this assessment. 

Although Johnson and others revised downward the magnitude of the 1957 earthquake to Mw 8.6 (the value adopted in 

the catalog for the current map), they specifically concluded 

that the 1986 earthquake represented a repeated rupture of the 

main asperity ruptured in 1957.

Komandorski Islands Region

As discussed previously, in the 1999 map, following the 

arguments in Cormier (1975), Sykes and others (1981), and 

Taber and others (1991), the assumption was made that, owing 

to the highly oblique angle of subduction, the Komandorski 

Island region was incapable of supporting a great earthquake. 

This region is, in significant ways, analogous to the tectonic 

setting along the Andaman trench north of Sumatra. In both 

cases the current plate motions are highly oblique to the trench 

with the development of significant strike-slip faulting behind 

the arc. Despite this highly oblique orientation, rupture in the 

2004 Sumatra–Andaman Islands earthquake extended several 

hundred kilometers into this region of oblique slip, albeit with 

a considerably lower slip than observed in the regions of more 

near-trench perpendicular convergence farther to the south 

(Lay and others, 2005). Thus it seems imprudent to reject the 

notion that seismic slip along the Komandorski Island section 

of the arc is possible. Focal mechanisms in the region (fig. 4) 

show both strike-slip and thrust character with the strike-slip 

mechanisms occurring particularly to the north of the arc 

where Geist and Scholl (1994) report geologic evidence of 

strike-slip faulting.

In the current map, we consider the possibility of thrust 

earthquakes along the westward extension of the megathrust 

west of Attu both independent of and in combination with the 

megathrust farther to the east. Possible strike-slip earthquakes 

in the region are considered to be modeled by the smoothed 

seismicity, as discussed in more detail herein.

Yakataga Region

Although the Yakataga segment is clearly the location 

of significant north-south convergence and the site of very 

large earthquakes (for example, 1899, 1979), the details of the 

faulting are poorly understood. Several east-trending, northdipping thrust faults are inferred to exist beneath the heavily 

glacier-covered region. Although some studies have been 

carried out in this region since 1999 (Pavlis and others, 2004), 

it is not possible at this time to construct a better model of the 

faulting, and the model assumed is that used in the 1999 map. 

As a proxy for a more detailed understanding, a flat fault surface (that is, with a 0° dip) at a depth of 15 km was assumed, 

extending from 59.1° to 61.0° N. and from 139.5° to 145.4° 

W. This is nearly identical to the dislocation model proposed 

by Sauber and others (1997, 1998; see also Sauber and Molnia, 2004) to explain GPS observations in this region. Sauber 

and others (1997) estimate that sufficient strain has accumulated in this region since 1899 to generate an earthquake of 

magnitude Mw 8.1. They also note that there is evidence for 

about 15 mm/yr of right-lateral slip in the regions a few tens of 

kilometers north of Cape Yakataga.

Detailed Model of Megathrust

In view of the incremental improvements in understanding of the earthquake-generating characteristics of the megathrust zone, the recurrence model of the zone contains significant modifications from the model used in 1999. Although 

the results of these modifications to the calculated hazard are 

relatively subtle, the changes result in a somewhat clearer, 

more supportable model. As previously discussed, we consider 

truncated Gutenberg-Richter behavior alone in the western 

Aleutians, but a combination of Gutenberg-Richter and characteristic behavior in the eastern Aleutians–Kodiak–Prince 

William Sound region.

Parameters to describe the Gutenberg-Richter behavior 

for each of the regions along the megathrust are derived from 

the historical instrumental seismic record. The megathrust was 

divided into six zones as shown in figure 7. The a- and b-values 

were calculated for the shallow earthquakes (0–50 km) in each 

zone and for the aggregate, excluding the Yakataga zone. The 

a- and b-values for the individual zones were used to calculate 

the rates of earthquakes with magnitudes greater than 5. The 

16  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
a- and b-values for the aggregate of the five western zones were 

used to estimate the rate of earthquakes in the magnitude range 

7–8 throughout the western zones. Earthquakes in this magnitude range have been observed throughout the megathrust (with 

the possible exception of the Komandorski Islands and Prince 

William Sound regions). The steps taken to characterize the 

hazard in each of these regions may be summarized as follows:

Calculate the b-values for

earthquakes with depth 50 km except the megathrust;

earthquakes with depth 50 km for whole megathrust except Yakataga;

earthquakes with depth 50 km for Komandorski, Western Aleutian, Shumagin, Semidi, 

Kodiak+Kenai/Prince William Sound, and 

Yakataga subzones individually, all earthquakes 

with depth >50 and 80 km; and

all earthquakes with depth >80 and 120 km.

Calculate and smooth gridded 10a values.

Smooth (75 km) for earthquakes with depth 50 

km, skip megathrust cells when smoothing cells 

outside megathrust.

Smooth (50 km) for earthquakes with depth >50 

and 80 km.

Smooth (50 km) for earthquakes with depth >80 

and 120 km.

Calculate average (rather than gridded) 10a value for 

whole megathrust except Yakataga (depth 50 km), 

then prorate into each cell.

Calculate average (rather than gridded) 10a value 

for each megathrust subzone (depth <50 km), then 

prorate into each cell.

Calculate hazard for depth 50 km and magnitude 

range 5–7 from Steps 2a and 4 with corresponding 

b-values.

Calculate hazard for depth >50 and magnitude range 

5–7.5 from Steps 2b and 2c with corresponding bvalues.

Calculate hazard for depth 50 km and magnitude 

range 7–8 from Step 3 with corresponding b-value.

Hazard for magnitudes greater than 8:

Komandorski subzone: none

Western Aleutian subzone: magnitude range 

8–9.2 with the a-value calculated in Step 4 and 

the b-value calculated in Step 1

1.

a.

b.

c.

d.

2.

a.

b.

c.

3.

4.

5.

6.

7.

8.

a.

b.

Shumagin subzone: none

Semidi subzone: magnitude range 8–8.5 with 

the a-value calculated in Step 4 and the b-value 

calculated in Step 1

Kodiak subzone alone: characteristic magnitude 

8.8 with 650-year recurrence

Kodiak+Kenai/Prince William Sound subzone: 

characteristic 9.2 (1964-type) with 650-year 

recurrence

Transition Fault

The Transition fault is a thrust/oblique slip fault and as 

such is an important component of the tectonics in the shift 

from strike-slip displacement along the western margin of the 

Pacific plate off southeast Alaska to the thrust displacement 

along the Alaska-Aleutian megathrust to the west. Owing to 

the fault’s location completely beneath the Gulf of Alaska, the 

fault is insufficiently understood.

In 1999, we treated the Transition fault as an A-type fault, 

even though its segmentation is unknown. In initial hazard 

calculations for the 1999 map using a hybrid approach for this 

fault, we found that the Transition fault produced the highest hazard in the map. The slip rate of the Transition fault is 

highly uncertain, and we were concerned that one of the more 

poorly known faults had a higher hazard than the megathrust 

zone. Thus, we used only the characteristic rupture model for 

the Transition fault, which produces a lower hazard than the 

hybrid approach. Also, we used the Youngs and others (1997) 

subduction zone interface attenuation relation when estimating ground motions for the Transition fault (see “Attenuation 

Relationships” section). This produces somewhat lower ground 

motions than do attenuation relations for crustal earthquakes.

In 1999 a slip rate of 10 mm/yr was adopted. Review 

of the plate tectonic and geodetic constraints on the slip rate 

(DeMets and Dixon, 1999; Fletcher and Freymueller, 2003) 

suggests that a slip rate of 12 mm/yr is more appropriate. The 

low number of large earthquakes on the fault, however, seem 

to be at odds with this slip rate, although it is possible that 

some the large earthquakes in 1899 could be associated with 

this fault (Doser, 2006). On the subduction zone to the west 

it is clear that a variable, but perhaps substantial, fraction 

of the slip is accommodated as aseismic slip (Pacheco and 

others, 1993). By analogy, but somewhat arbitrarily, we have 

therefore allowed 50 percent of the slip across the transition 

fault to be accommodated aseismically and assume that 6 

mm/yr is released in characteristic events of magnitude 8.2 

every 325 years.

Fairweather and Queen Charlotte Faults

Improved understanding of the segmentation along the 

Fairweather–Queen Charlotte fault system (fig. 8), plate 

c.

d.

e.

f.

Regional Discussion 17
motion studies specific to the North American and Pacific 

plates (DeMets and Dixon, 1999), and GPS studies (Freymueller and Fletcher and 1999; Fletcher and Freymueller, 2003; 

Mazzotti and others, 2003; Smith and others, 2003), have led 

to several modifications of the seismic hazard attributable to 

the Fairweather and Queen Charlotte faults.

In 1999, the boundary between the onshore and offshore 

Fairweather fault segments were drawn on the basis of the notations shown on the map of Plafker and others (1993). These 

extents did not coincide with the extent of aftershocks for these 

earthquakes drawn on the same map. Following the arguments 

of Bufe (2005) our segment boundaries have changed to reflect 

the aftershock distributions, moving the segment boundary 

about 100 km to the southwest. This leads to longer onshore 

and shorter offshore Fairweather fault segments.

Plate-motion studies using geologic and GPS constraints 

(DeMets and Dixon, 1999) clearly show a reduction in relative 

plate velocity between the North American and Pacific plates 

along the Queen Charlotte fault relative to velocities used in 

the 1999 hazard maps. In 1999 we adopted a value of 58 mm/

yr. According to the aforementioned studies, this value has 

been reduced to 49 mm/yr and increases by 1 mm/yr toward 

Eastern Denali/Chatham

 Strait

Fairweather Onshore

Fairweather Offshore

Queen Charlotte

Figure 8

Figure 8. Active crustal faults in south-east Alaska.

the northwestern end of the Fairweather fault. Because of the 

partitioning of strain between the eastern Denali (2 mm/yr) 

and Fairweather faults (Fletcher and Freymueller, 2003), we 

have adopted a value of 48 mm/yr for the onshore portion of 

the Fairweather fault.

Mazzotti and others (2003) suggested that a significant 

component of convergence perpendicular to the fault exists at 

the southern end of the Queen Charlotte system. This convergence is not explicitly modeled in the preparation of the map, 

although to some degree the hazard can be taken as modeled 

by the smoothed seismicity.

Central Denali, Totschunda, and Eastern Denali 

Faults

Results of geologic studies carried out since the 2002 

Denali earthquake have required changes to the models of 

the central and eastern Denali and Totschunda faults (fig. 9) 

relative to what was used in 1999 (Schwartz and others, 2005a 

and b; Matmon and others, 2006). The best current information suggests that the slip rate on the central Denali increases 

in an eastern direction along the fault from about 9 mm/yr near 

18  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
the Parks Highway to about 14 mm/yr at the junction with 

the Totschunda fault. Current understanding suggests that this 

slip rate is portioned into about 8 mm/yr on the eastern Denali 

fault and about 6 mm/yr on the Totschunda fault. The slip rate 

on the eastern Denali fault continues to decrease with distance 

to the east to values of 2 mm/yr or less in Canada near Kluane 

Lake, although Fletcher and Freymueller (2003) suggest that 

it may be higher. In the 1999 map, constant slip rates of 10 

mm/yr were assumed for the central Denali fault, 2 mm/yr for 

the eastern Denali, and 11.5 mm/yr for the Totschunda. One 

further implication of the new information is that ruptures 

extending eastward from the central Denali fault are about 

equally likely to continue onto the eastern Denali and Totschunda faults.

Two observations from the 2002 Denali earthquake and 

the resulting studies motivated modifications to the standard 

approach to calculation for the National Seismic Hazard Map. 

First, as previously noted, the 2002 earthquake branched 

from the central Denali fault to the Totschunda fault, but new 

geologic evidence implies that the continuation of rupture onto 

the eastern Denali fault is about equally likely. Second, the 

slip rate on the eastern Denali fault, and to a lesser extent, on 

the central Denali fault, clearly varies with position along the 

fault. The first of these issues was addressed by considering 

Central Denali

Castle Mountain

Totschunda

Eastern Denali

Susitna Glacier

Figure 9

two fault models, one including the eastern Denali fault at its 

full slip rate plus the central Denali fault with 8/14 of its slip 

rate, and another model with the Totschunda fault together 

with the central Denali fault with 6/14 of its slip rate. The 

proportions for the central Denali were derived from the relative partitioning of slip onto the eastern Denali and Totschunda 

faults. Then, in the hazard codes, earthquakes are allowed to 

float the full length of the combined faults in each model.

The second modification incorporated into these calculations was the ability to change the frequency of earthquakes 

of any particular magnitude in proportion to the observed slip 

rate. For example, the characteristic magnitude assumed for 

the central and eastern Denali fault model was Mw 7.9. In 

the hazard calculations, the frequency of such earthquakes is 

assumed to be five times greater along the reach of the fault 

characterized by a slip rate of 10 mm/yr as contrasted with the 

reach of the fault characterized by a slip rate of 2 mm/yr. Similarly, when considering earthquakes of lower magnitudes, the 

same ratio is applied. Although some might argue that portions 

of the faults characterized by higher slip rates might also have 

a different magnitude distribution (likely with higher magnitudes), we judge the current modification to be a significant 

improvement over the existing methodology and leave further 

improvements for subsequent revisions of the hazard maps.

Figure 9. Active crustal faults in south-central Alaska. The yellow line shows the surface ruptures associated 

with the 2002 Denali fault earthquake.

Regional Discussion 19
We also take advantage of these modifications to address 

the hazard at the western end of the active Denali fault. In 

the 1999 map, the end of the active fault was taken to be at 

about longitude 150.2° W. Published sources reviewed in 

the new Quaternary Fault and Fold Database of the United 

States. (http://earthquake.usgs.gov/regional/qfaults/) provide 

evidence for extending the active fault to the west with a slip 

rate gradually tapering to zero. Thus we have tapered the slip 

rate on the Denali fault linearly from 9.4 mm/year at about 

150.2°W. to 0 mm/year at about 154.7°W.

These modifications allowing the use of positiondependent probabilities along faults also permit application of time-dependent hypotheses such as the variation of 

slip-deficit and Coulomb stress changes along faults. We are 

currently working on experimental time-dependent hazard 

maps exploiting these capabilities.

Castle Mountain Fault

The Castle Mountain fault (fig. 9) is particularly important to estimation of the seismic hazard in the vicinity of 

Anchorage because it is located at a distance of only about 40 

km from the city and even closer to the developing areas to the 

north of the city. In the 1999 map, the fault was considered as 

two segments, the western or Susitna segment and the eastern 

or Talkeetna segment (Detterman and others, 1974). Along the 

Talkeetna segment there is no evidence for surficial displacement younger than Pleistocene (Detterman and others, 1976), 

but Lahr and others (1986) describe an earthquake of Ms 5.2, 

which indicated slip at a depth of 13 to 20 km along the segment. In contrast, along the Susitna segment, no significant 

earthquakes have been instrumentally located, but geologic 

studies indicate Holocene surface displacement (Detterman 

and others, 1974, 1976; Bruhn, 1979). At the time of preparation of the 1999 maps, these studies, however, had served only 

to put wide limits on the slip rate. At the time, we chose a 

value of 0.5 mm/yr and a maximum magnitude of 7.5, leading 

to a recurrence time of 1,300 years.

The new geologic data for the Castle Mountain fault near 

Houston, Alaska, on the Susitna segment (Haeussler and others, 2002; Willis and others, in press) suggest slip rates as high 

as 3.2 mm/yr, compared to the value of 0.5 mm/yr assumed 

in 1999, and an average recurrence time of about 700 years 

based on the interpretation of four paleoearthquakes. We have 

assumed a rate of 2.9 mm/yr, and the estimate of characteristic or maximum magnitude has been reduced from 7.5 to 7.1. 

These changes led to a faster slip rate and somewhat smaller, 

but more frequent events, and significantly increased the seismic hazard from this fault.

Kodiak Crustal Faults

Work by Carver and others (2003) on the southeastern 

edge of Kodiak Island indicates a series of left-lateral strike-slip 

faults trending northeast, subparallel to the subduction-zone 

trench. Enough information has been compiled on these faults 

to warrant their inclusion into the current set of maps. We have 

assigned a slip rate of 1 mm/yr to the Narrow Cape fault and 

2 mm/yr to the Kodiak fault and allowed both characteristic 

and Gutenberg-Richter behavior with magnitudes of 7.5 and 

6.5–7.5, respectively. It is possible that some of this observed 

movement could be coincident with large subduction-zone 

events.

Other Sources Modeled with Smoothed 

Seismicity

Strike-Slip Faults Subparallel to Arc

At the western end of the Aleutian arc in the vicinity of 

Bering Island, where the relative direction of plate movement 

between the Pacific and North American plates is nearly parallel to the arc, it is clear that a very significant portion of this 

slip is partitioned along right-lateral strike-slip faults both to 

the north and south of the Aleutian arc (c.f. Geist and Scholl, 

1994). Near Bering Island (166.5° E.) recent GPS studies 

suggest that as much as 50 mm/yr is accommodated on strikeslip zones to the north of the island (Buergmann and others, 

2005). Earthquake focal mechanisms and geologic studies, 

especially from bottom and sub-bottom profiling, suggest that 

these faults extend eastward along the arc at least as far as 

160° W. From geometric considerations it seems likely that 

the amount of slip that is partitioned into the strike-slip faults 

decreases eastward as the portion of the slip tangential to arc 

decreases; that is, as the angle between the plate motion and 

the arc increases from nearly zero at the western end of the 

arc to nearly perpendicular at the eastern end.

Slip rates are unknown for individual faults within this 

group, and thus, hazard must be captured in the smoothed 

seismicity.

Cook Inlet

Recent studies by Doser and others (2004) and Haeussler and others (2000) have investigated earthquakes that 

are associated with crustal faults and folds in the Cook Inlet 

region. Other than for the Castle Mountain fault, insufficient 

information exists to explicitly characterize the seismic hazard from individual faults in this region. Significant hazard 

is captured in the maps, however, through the smoothed 

seismicity.

Tintina/Kaltag Fault

The Tintina fault (fig. 1) is a major right-lateral fault 

about 1,500 km long, extending from north-central Alaska 

southeastward to British Columbia. The fault displaces midCretaceous rocks and lower Paleozoic facies boundaries by 

450 km (Roddick, 1967; Gabrielse, 1985). To the southeast 

20 Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
the fault is associated with the Rocky Mountain Trench. Little 

earthquake activity is associated with this fault in the Yukon 

(Hyndman and others, 2005). Although there is evidence for 

late Pleistocene movement along the Kaltag fault (the westward extension of the Tintina) and possibly Holocene activity 

along the Medicine Lake lineament (Plafker and others, 1993), 

there is insufficient information to include the fault explicitly 

as a source. Some earthquake potential from this fault is captured in the smoothed seismicity.

Region Between the Denali and Tintina/Kaltag 

Faults

The region of interior Alaska between the Denali and 

Tintina/Kaltag faults experienced several earthquakes in the 

magnitude 7 range during the 20th century and, in addition, 

has a number of young faults. The earthquakes include the 

events of July 22, 1937, October 16, 1947, and April 7, 1958 

(Fletcher and Christensen, 1996). Many of the smaller earthquakes in the region are concentrated in three diffuse bands 

striking north-northeast. These bands have been termed the 

Minto Flats, Fairbanks, and Salcha seismic zones (Page and 

others, 1995; Ratchkovski and Hansen, 2002a). To date, none 

of these bands have been clearly associated with a geologic 

fault. There are, however, a number of northeast- to northnortheast-striking faults along the north side of the Denali 

fault with evidence of youthful activity. Primary examples of 

these include the Donnelly Dome and Canteen faults. Several 

of these faults have received recent attention (for example 

Bemis, 2004; L.S. Cluff, G. Plafker, G. Carver, oral commun., 

2005), but as of this writing, insufficient information is available to include any individual fault explicitly; thus, the hazard 

they present is captured in the smoothed seismicity.

Attenuation Relationships

The set of attenuation relations used in this revision of the 

map is shown in table 2.

Relative to 1999, the set of attenuation relationships 

used for the 2002 National Seismic Hazard Map for the lower 

48 States and also used here on average tend to give slightly 

lower ground motions than the set used in 1999 (Frankel and 

others, 2002). One important exception to this is that the addition of the Sadigh relation for the shallow subduction earthquakes tends to increase ground motions.

As in the 1999 map, the reference site condition is the 

NEHRP B/C boundary, which corresponds to an average 

shear-wave velocity of 760 m/sec in the top 30 m. This is the 

same site condition used in the 1996 national maps. This site 

condition represents a typical Western United States “firmrock” site. Table 2 shows the ground-motion relationships 

used in the calculations. These are the same relations used 

in producing the 1996 hazard maps for the Western United 

States. For crustal faults we used different ground-motion 

values for thrust faults and for strike-slip faults, using the 

values specified in each attenuation study. For the deep earthquakes, we assumed a focal depth of 60 km for earthquakes 

in the depth interval 50 to 80 km and a focal depth of 90 km 

for earthquakes in the depth interval 80 to 120 km. In the 50- 

to 80-km range, the distribution of earthquakes is strongly 

biased toward the shallower depths, justifying the choice of 

an assumed depth less than the midpoint of the interval. In 

the 80- to 120-km range, the distribution of earthquakes in 

the Kodiak and Prince William Sound regions is also biased 

toward shallower depths, and a similar argument can be 

made. Further to the west along the arc, the distribution is 

more uniform with depth in this interval, although the number of events is fewer than in the Kodiak and Prince William 

Sound regions. For shallow earthquakes, focal depths of 5 

km were assumed.

Calculation Procedure

The overall calculation procedure is summarized in 

table 3, which describes the input files, key input parameters, 

and the hazard codes used. In general, hazard curves relating 

ground motions to frequencies of exceedance are generated for 

the series of sources previously outlined. The hazard curves 

at each latitude and longitude are combined, and a groundmotion value for a desired frequency of exceedance corresponding to a particular probability level is interpolated from 

the curves. The resulting ground-motion values constitute the 

probabilistic seismic hazard maps.

Resulting Maps

Figures 10–13 show the resulting maps for the entire 

region of Alaska and the Aleutians. Figure 10 shows the 

maps for the probability level of 2 percent in 50 years for 

peak ground acceleration (PGA) (fig. 10A), 0.2 sec spectral 

amplitude (SA) (fig. 10B), and 1.0 sec SA (fig. 10C). Figure 

11 shows the same PGA and SA for the probability levels of 

10 percent in 50 years. Figure 12 shows PGA at 2 percent in 

50 years at an expanded scale for the Aleutians, south-central 

Alaska and the 1964 earthquake zone, and southeast Alaska.

Resulting Maps  21
 Table 2. Attenuation relationships assumed for various seismic sources. 

 [%, percent] 

Source Attenuation Relationship Weight 

Megathrust and 

Transition fault 

Youngs et al. (1997) (interface) 

Sadigh et al (1997) 

50% each1

Crustal faults Abrahamson and Silva (1997) 

(rock)

Boore et al (1997) (NEHRP B/C)

Sadigh et al. (1997) (rock) 

Campbell and Bozorgnia (2003) 

Site conditions as in Frankel and 

others (2003) 

25% each 

Shallow smoothed 

seismicity 

(including in zone 

of megathrust) 

(depth = 0–50 km) 

Same as for crustal faults, but 

with corrections for random 

mechanisms 

(see Frankel and others, 2003) 

25% each 

Deep earthquakes 

(depth= 50–80 km) 

Youngs et al. (1997) (intraslab;

depth fixed at 60 km) 

Atkinson and Boore (2003) 

(global)

50% each 

Deeper earthquakes 

(depth= 80-120 km) 

Youngs et al. (1997) (intraslab;

depth fixed at 90 km) 

Atkinson and Boore (2003) 

(global)

50% each 

1

 At distances greater than 70 km, full weight is given to Youngs et al. (1997) 

 Table 2. Attenuation relationships assumed for various seismic sources. 

 [%, percent] 

Source Attenuation Relationship Weight 

Megathrust and 

Transition fault 

Youngs et al. (1997) (interface) 

Sadigh et al (1997) 

50% each1

Crustal faults Abrahamson and Silva (1997) 

(rock)

Boore et al (1997) (NEHRP B/C)

Sadigh et al. (1997) (rock) 

Campbell and Bozorgnia (2003) 

Site conditions as in Frankel and 

others (2003) 

25% each 

Shallow smoothed 

seismicity 

(including in zone 

of megathrust) 

(depth = 0–50 km) 

Same as for crustal faults, but 

with corrections for random 

mechanisms 

(see Frankel and others, 2003) 

25% each 

Deep earthquakes 

(depth= 50–80 km) 

Youngs et al. (1997) (intraslab;

depth fixed at 60 km) 

Atkinson and Boore (2003) 

(global)

50% each 

Deeper earthquakes 

(depth= 80-120 km) 

Youngs et al. (1997) (intraslab;

depth fixed at 90 km) 

Atkinson and Boore (2003) 

(global)

50% each 

1

 At distances greater than 70 km, full weight is given to Youngs et al. (1997) 

22  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
Table 3. Input files, key parameters, and hazard codes used in preparing the maps.

[km, kilometer; °, degrees; mm/yr, millimeters per year]

1

Input file Source Magnitude

range

a b Length (km) Depth 

(km)

Dip

(°)

dmag Hazard

program

Assumed

depth

Attenuation relationships Weight

megaAll.out_revF.in Entire Aleutian arc G-R

7 to 8 3.540 0.689 20-50 0.1

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

megaeast.out_revF.in Eastern Aleutians

and 1964 zone

Characteristic

9.2 6.387 1.000 20-42 0.0

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

megaA.out_revF.in Western Aleutians G-R

8 to 9.2 3.160 0.660 20-50 0.1

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

megaF.out_revF.in Far Western Aleutians G-R

8 to 9.2 2.690 0.773 20-50 0.1

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

megaB.out_revF.in Semidi region G-R

8 to 8.5 2.400 0.710 20-50 0.1

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

megaD.out_revF.in Kodiak segment Characteristic

8.8 5.987 1.000 20-42 0.0

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

yak.out_revF.in Yakataga G-R

7 to 8.1 2.180 0.666 15 0.1

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

trans.out_revF.in Transition zone Characteristic

8.2 5.687 1.000 0-15 0.0

hazSUBXv3 Calculated 2

Youngs and others (1997) (interface)

Sadigh and others (1997)

1.0

AKML75.out_revF.in Shallow earthquakes

Depth 0 to 50 km

Smoothed at 75 km

G-R

5 to 7.3

GridsR4/

a_000d050_xmt.out 0.816 0-50 0.1

hazgridXv3 5 km Same as crustal faults but with 

corrections for random mechanisms

1.0

AKMT.out_revF.in Source zones corresponding

to the megathrust

G-R

5 to 7

GridsR3/

a_000d050_mt.out_sw 0-50 0.1

hazgridXv3 5 km Same as crustal faults but with 

corrections for random mechanisms and 

half thrust for Boore and Sadigh

1.0

AKDEEP.out_revF.in Deep earthquakes 

Depth 50 to 80 km

G-R

5 to 7.5

GridsR3/

a_051d080.out_sw 0.858 50-80 0.1

hazgridXv3 60 km Youngs and others (1997) (intraslab)

Atkinson and Boore (2002) (global)

1.0

AKDEEPER.out_revF.in Deeper earthquakes

Depth 80 to 120 km

G-R

5 to 7.5

GridsR3/

a_081d120.out_sw 1.007 80-120 0.1

hazgridXv3 90 km Youngs and others (1997) (intraslab)

Atkinson and Boore (2003) (global)

1.0

AKF5.out_revF.in

Kodiak Island fault,1 mm/yr

Narrow Cape faults, 2 mm/yr

Characteristic

7.5

7.5

3.198

3.465

0.870

0.870

208.82

193.20

15

15

90

90

0.0

0.0

hazFXv6 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

AKF4.out_revF.in

Kodiak Island fault, 1 mm/yr

Narrow Cape faults, 2 mm/yr

G-R

6.5 to 7.5

6.5 to 7.5

2.476

2.743

0.870

0.870

208.82

193.20

15

15

90

90

0.1

0.1

hazFXv6 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

AKF1.out_revF.in

Queen Charlotte, 49.0 mm/yr

Fairweather offshore, 49.5 mm/yr

Fairweather onshore, 48.0 mm/yr

Characteristic

8.1

7.7

8.0

6.4923e-3

9.8952e-3

6.5652e-3

466.65

176.85

341.03

15

15

15

90

90

90

0.0

0.0

0.0

hazFXv6 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

AKF0.out_revF.in

Queen Charlotte and

Fairweather at 49 mm/yr

G-R

7.6-8.1 4.635 0.870 987.81 15 90 0.1

hazFXv6 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

AKF2.out_revF.in

Western + Eastern Denali

1-9.4-14.4,8.4-2 mm/yr

Western Denali + Totschunda

1-9.4-14.4 mm/yr, 6 mm/yr

Castle Mtn, 0.5-2.9-0.5 mm/yr

Characteristic

7.9

7.9

7.1

4,53.82225(3.58817) - 4.79538(4.56130) - 

4.98061(4.74653), 4.74653 - 4.12328

3.54382(3.16361) - 4.51695(4.13674) - 

4.70219(4.32197), 4.32197

3.12184 - 3.88527 - 3.12184

0.870

0.870

0.870

1570.5

827.2

189.5

15

15

15.529

90

90

-75

0.0

0.0

0.0

hazFXv7 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

AKF3.out_revF.in

Western + Eastern Denali

1-9.4-14.4,8.4-2 mm/yr

Western Denali + Totschunda

1-9.4-14.4 mm/yr, 6 mm/yr

Castle Mtn, 0.5-2.9-0.5 mm/yr

G-R

6.5 to 7.9

6.5 to 7.9

6.5 to 7.1

4,53.04528(2.81119) - 4.01841(3.78432) - 

4.20364(3.96956), 3.96956 - 3.34631

2.76685(2.38664) - 3.73998(3.35977) - 

3.92521(3.54500), 3.54500

2.51945 - 3.28288 - 2.51945

0.870

0.870

0.870

1570.5

827.2

189.5

15

15

15.529

90

90

-75

0.1

0.1

0.1

hazFXv7 Calculated Abrahamson and Silva (1997) rock

Boore and others (1997) NEHRP B/C

Sadigh and others (1997) rock

Campbell and Bozorgnia (2003)

0.5

1

To Determine the output file names, replace 'in' with 'pga', '010', '020', '030', '050', '100', and '200'.

2

Each relationship has equal weight less then 70 km distant from the source. Youngs is given full weight beyond 70 km.

3

Source zones corresponding to Western Aleutians (megaA and megaF), Eastern Aleutians (megaeast), Shumagin and Semidi (megaB and region between megaB and megaA), and Yakataga (yak).

4

The western Denali and eastern Denali are combined and added to the combination of western Denali and Totschunda.

5

Ranges of a value delimit the western to eastern ends of the fault segment. The value in parentheses represents 8.4/14.4 of 1-9.4-14.4 mm/yr for the central/eastern Denali and 6/14.4 of 1-9.4-14.4 mm/yr for the central Denali/Totschunda system.

GridsR3/

b_000d050_mt.out_sw

Resulting Maps  23
Figure 10

A. 

B. 

C. 

Figure 10. Probabilistic ground motion with a 2-percent probability of exceedance in 50 years for peak 

ground acceleration (A), 0.2 second spectral acceleration (B), 1.0 second spectral acceleration (C).

24  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
Figure 11. 

A. 

B. 

C. 

PGA with 10% probability of exceedance in 50 years

0.2 second SA with 10% probability of exceedance in 50 years

1.0 second SA with 10% probability of exceedance in 50 years

Figure 11. Probabilistic ground motion with a 10-percent probability of exceedance in 50 years for peak 

ground acceleration (A), 0.2 second spectral acceleration (B), and 1.0 second spectral acceleration (C).

Resulting Maps  25
A. 

B. C. 

Figure 12. Probabilistic ground motion with a 2-percent probability of exceedance in 50 years for peak ground acceleration at a larger scale in the AAlaska (B), and south-east Alaska (C).
Figure 13. 

TI % change of 2007 to 1999 PGA 2% in 50

Figure 13. Percentage changes in the 2007 map relative to the 1999 map for south-central Alaska.

Resulting Maps  27
Discussion

Overall, changes in the characterization of the subduction zone were modest, and although judged to have a more 

satisfactory justification in terms of observation and understanding, the hazard in the subduction zone was little affected. 

The largest changes in 2005 map, as contrasted with 1999, 

are in the vicinity of the Castle Mountain, eastern Denali, and 

Totschunda faults and the southeastern shore of Kodiak Island. 

Detailed comparison of the 2005 and 1999 maps for 2 percent 

in 50 years PGA for south-central Alaska is shown in figure 

13. It should be borne in mind that the net difference between 

the two maps is the result of many changes, mostly small. The 

changes aggregating to make the net differences in figure 13 

include:

Increased slip rate and decreased recurrence interval on 

Castle Mountain fault (increases hazard).

Increased slip rate and decreased recurrence interval on 

eastern Denali fault (increases hazard).

Extending the active Denali fault to the west with slip 

rate tapering to zero (increases hazard).

Decreased slip rate on Totschunda fault (decreases 

hazard).

•

•

•

•

New attenuation relations (mostly decreases hazard—

most visible in Yakataga region, but increases hazard for 

very shallow faults, for example the Transition fault).

Addition of Kodiak Island and Narrow Cape faults on 

and adjacent to Kodiak Island (increases hazard).

Slightly shallower geometry for megathrust interface 

under Cook Inlet (increases hazard).

Slightly lower slip rate for the southern Fairweather 

and Queen Charlotte faults (reduces the hazard).

Changes in the rates of earthquakes along the subduction zone (reduces hazard).

Comparison of selected ground-motion values at representative cities and towns in Alaska and the percentage change 

from the previous version of the map are shown in table 4.

Conclusions

We have prepared revised versions of the probabilistic 

seismic hazard maps for Alaska and the Aleutians. These 

maps will be proposed for adoption in future revisions to the 

International Building Code. Digital versions of these maps 

are available at http://earthquake.usgs.gov/research/hazmaps/.

•

•

•

•

•

28  Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska
Table 4. Selected ground-motion values and percentage changes from the 1999 map at representative cities and towns in Alaska and the Aleutians.

[%, percent; Hz, hertz; pga, peak ground acceleration]Comparison of 1999 to 2007 1999 2007 difference between 2007 an

pga 1Hz 5Hz pga 1Hz 5Hz pga 

2% in 50 10% 2% 10% 2% 10% 2% 10% 2% 10% 2% 10% 2% 10% 

City Lat Lon %g % change

Anchorage 61.17 -150.02 66.4 38.5 54.5 30.0 148.0 100.2 69.0 43.3 61.6 34.5 155.0 94.8 4% 12% 

Wasilla 61.62 -149.37 66.9 38.1 54.2 29.0 145.8 97.1 77.8 48.6 73.5 40.2 181.0 108.9 16% 28% 

Fairbanks 64.82 -147.87 43.0 21.4 30.6 14.2 110.9 49.4 40.7 20.0 30.3 15.1 96.5 46.6 -5% -7% 

Juneau 58.37 -134.58 28.1 13.9 29.7 15.3 63.1 30.0 24.6 12.3 29.0 16.1 57.8 28.1 -12% -12% 

Ketchikan 55.35 -131.70 13.3 7.1 16.8 9.2 26.3 13.0 11.4 6.7 19.2 10.7 25.6 14.3 -14% -6% 

Kodiak 57.75 -152.50 75.5 46.7 66.4 36.5 179.1 115.9 70.6 44.5 75.8 43.9 165.3 103.4 -6% -5% 

Bethel 60.78 -161.80 13.0 5.7 9.3 4.2 29.1 12.6 12.5 5.5 9.9 4.6 29.0 12.2 -4% -4% 

Nome 64.50 -165.43 21.9 9.4 14.0 5.9 50.9 21.6 22.8 9.9 15.9 6.8 53.8 22.5 4% 5% 

Kotzebue 66.87 -162.63 19.1 8.4 12.8 5.5 42.5 18.6 17.4 7.6 12.7 5.7 40.8 17.1 -9% -10% 

Barrow 71.30 -156.87 0.7 0.5 0.4 0.3 1.7 0.5 0.5 0.5 0.3 0.3 0.9 0.5 -29% 0% 

Prudhoe Bay 70.25 -148.33 11.0 4.8 7.3 2.8 25.7 10.7 12.9 5.4 9.2 3.8 30.0 11.9 17% 13% 

McGrath 62.97 -155.62 15.4 7.3 12.8 5.9 36.8 17.1 17.0 8.2 15.7 7.7 39.7 18.5 10% 12% 

Dillingham 59.05 -158.52 10.7 5.9 12.6 6.2 25.6 13.1 12.1 6.5 13.9 7.1 27.4 14.5 13% 10% 

Valdez 61.13 -146.35 64.8 36.6 53.9 29.8 144.7 95.6 64.2 38.0 65.7 32.9 153.3 87.7 -1% 4% 

Homer 59.63 -151.50 66.3 38.4 56.4 30.7 148.0 99.8 64.1 40.4 58.3 34.7 137.2 85.0 -3% 5% 

Kenai 60.57 -151.25 58.6 35.7 47.8 27.4 129.1 87.3 61.8 38.5 50.9 29.2 130.0 79.3 5% 8% 

Paxson 63.03 -145.50 35.0 19.4 38.7 19.0 83.8 42.2 33.4 19.9 40.5 21.8 78.5 45.6 -5% 3% 

Yakutat 59.52 -139.67 75.4 44.5 67.5 40.2 179.6 113.2 65.1 39.3 69.9 39.4 155.8 92.2 -14% -12% 

Haines Harbor 59.23 -139.43 75.7 43.0 68.5 39.3 181.5 110.8 70.5 41.2 81.5 41.2 168.8 96.9 -7% -4% 

Sitka 57.07 -135.35 37.2 24.2 50.5 29.9 98.1 50.6 31.8 20.8 46.1 28.1 76.4 48.2 -15% -14% 

Dutch Harbor 53.88 -166.53 66.5 38.9 57.5 31.3 150.4 103.4 63.5 39.6 53.7 32.5 143.0 86.7 -5% 2% 

Delta Junction 64.00 -145.50 26.2 13.6 25.3 13.0 58.8 29.3 26.6 13.9 27.0 14.8 62.1 31.8 2% 2% 

Tok 63.30 -143.00 20.5 12.1 27.5 13.6 43.9 26.4 19.5 11.6 26.3 14.4 46.1 26.8 -5% -4% 

Glennallen 62.00 -146.40 48.0 25.5 38.9 19.9 115.7 57.5 46.9 26.3 43.9 21.8 106.2 58.0 -2% 3% 
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