science 

USGS

for a changing world

Probabilistic Seismic Hazard 

Maps of Alaska

by Robert L Wesson1 , Arthur D. Frankel 1 , Charles S. Mueller1 , and 

Stephen C. Harmsen1

Open-File Report 99-36

1999

This report is preliminary and has not been reviewed for conformity with the U.S. 

Geological Survey editorial standards or with the North American Stratigraphic Code. 

Any use of trade, firm or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

1 Denver, Colorado
Abstract

Probabilistic seismic hazard maps have been prepared for Alaska portraying 

ground motion values (peak ground acceleration and spectral amplitude at 

periods of 0.2,0.3 and 1.0 seconds) at probabilities of exceedance of 2% and 10% 

in 50 years. Preparation of these maps followed the same general strategy as 

that followed for the U.S.G.S. seismic hazard maps of the contiguous United 

States, combining hazard derived from spatially-smoothed historic seismicity 

with hazard from fault-specific sources. Preparation of the Alaska maps 

presented particular challenges in characterizing the hazard from the AlaskaAleutian megathrust. In the maps of the contiguous United States the rate of 

seismicity for recognized active faults was determined from slip rates estimated 

from geologic data. This approach is not appropriate for the megathrust because 

it has been demonstrated that a significant fraction of the subduction occurs 

aseismically. The characteristic earthquake hypothesis, based on recurrence rates 

determined from geologic data, is appealing for the portion of the megathrust 

that ruptured in the 1964 Alaskan earthquake, but is shown to be inappropriate 

for the western portion of the megathrust by the recent large earthquakes in the 

region which did not follow the characteristic model. Consequently the hazard 

from the western portion was estimated based on a truncated Gutenberg and 

Richter model derived from historic seismicity, and the hazard for the 1964 zone 

was estimated from a combination of a Gutenberg and Richter model derived 

from historic seismicity and the characteristic earthquake hypothesis with 

recurrence rates estimated from geologic data. Owing to geologic complexity 

and limited data, hazard models of the easternmost portion of the megathrust in 

the vicinity of Yakataga are the least satisfactorily constrained. Hazard is 

estimated for the recognized crustal faults of the Denali, Fairweather-Queen 

Charlotte and Castle Mountain fault systems based on available geologic slip 

rates. Hazard from other sources is estimated from spatially smoothed historic 

seismicity. Disaggregations of the hazard for Anchorage, Fairbanks and Juneau 

reveal the dominant sources of the hazard at each location.
Introduction

Alaska is the most seismically active state in the United States, and in 1964 

the site of one of the largest earthquakes since the beginning of instrumental 

recording. Although the current population of the region is small by comparison 

with, say California, the consequences of a large earthquake in the region could 

be much greater now than at the time of the 1964 Alaskan earthquake. The 

probabilistic seismic maps we have prepared are intended to extend those 

prepared by Frankel et al. (1996) for the 48 contiguous states, and with soon-tobe-published maps for Hawaii. Our methodology follows the basic approach of 

Cornell (1968). These maps are intended to summarize the available quantitative 

information about seismic hazard from geologic and geophysical sources. Full 

color maps at a scale of 1:7,500,000 are available as U.S. Geological Survey 

Miscellaneous Investigations Series 1-2679 (Wesson et al., 1999).

The process of preparing these maps included a workshop held in 

Anchorage in the fall of 1996 attended by many scientists and engineers involved 

in aspects of earthquake research and structural engineering practice in Alaska. 

Preliminary calculations of hazard were presented for discussion and a number 

of recommendations were made that affected the subsequent preparation of the 

map. Draft maps were posted on the World Wide Web and circulated for 

comment in the fall of 1997. The maps presented here have benefited greatly 

from both the original workshop in 1996 and from the review comments 

received.

The strategy for preparing these maps is similar to that for recently prepared 

seismic hazard maps of the contiguous United States (Frankel et al., 1996). The 

maps presented here include maps for peak ground acceleration and 1.0, 3.3 and 

5.0 Hz spectral acceleration at probabilities of exceedance of 10% in 50 years 

(annual probability of 0.0021) and 2% in 50 years (annual probability of 0.000404).

The historical instrumental seismicity of Alaska and the Aleutians for 

earthquakes greater than or equal to magnitude 6 is shown in Figure 1. The 

preparation of the earthquake catalogs used for the analyses in this report (and 

shown in Figure 1) is discussed in a companion report, Mueller et al., 1998). 

Clearly the majority of the seismicity in the region is associated with the AlaskaAleutian 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 

and islands. Additional significant seismicity occurs along the northwestwardstriking system of right-lateral strike-slip faults extending southeastward 

through and offshore of the panhandle of southeast Alaska. This system of 

faults forms the northeast boundary of the Pacific plate. Additional seismicity 

occurs in central Alaska.

The estimated rupture zones of the largest earthquakes in this century are 

shown in Figure 2 (Plafker et al., 1993). 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 Komandorsky Islands, near the Shumagin 

Islands, and near Cape Yakataga (Sykes, 1971; Davies et al., 1981). Near the 

Komandorsky 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 et al., 1981; 

Taber et al., 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 else along the arc. Indeed, Cormier 

(1975) has argued that the region may be incapable of supporting a great 

earthquake. 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 in this century. These two regions have 

been identified as "seismic gaps," that is, the potential sites of future large 

earthquakes (Davies et al., 1981; Sykes, 1971, Lahr et al., 1980).

Characterization of Seismic Sources

The seismic potential of Alaska is captured through consideration of 

earthquake sources that can be explicitly identified, including the Alaska-Aleutian 

megathrust and active crustal faults with known slip rates; and earthquake 

sources that are characterized by spatially smoothed historical seismicity, 

including shallow earthquakes from sources not included above, and deeper 

earthquakes (focal depths of 50 to 120 km). Hazard calculated from all these 

sources was combined as shown in Figure 3. Spatially-smoothed seismicity was 

used to estimate the hazard from shallow crustal earthquakes in the magnitude 

range 5.0 to 7.3 based on a Gutenberg-Richter model. For the deeper 

earthquakes, hazard was estimated for the magnitude range 5.0 to 7.0 based on a 

Gutenberg-Richter model with the parameters estimated from the spatiallysmoothed seismicity in the two depth intervals, 50 to 80 km, and 80 to 120 km.

Alaska-Aleutian megathrust

The Alaska-Aleutian megathrust has been responsible for several of the 

largest earthquakes known in instrumental seismology, including the 1964 Prince 

William Sound (Mw 9.2) and 1957 Aleutian (Mw 9.1) earthquakes (Figure 2). For 

purposes of this analysis two different segmentation models of the megathrust 

with corresponding recurrence assumptions have been considered.

Segmentation models

In the first segmentation model (Model I), the megathrust has been divided 

into three parts as shown in Figure 4. The Western Zone includes the reach of 

the megathrust along the western and central Aleutians from about 170.2° E to 

161.7° W. The Eastern Zone extends eastward and includes the reach along the 

eastern Aleutians, the Alaska Peninsula and Prince William Sound, from about 

161.7° W to 144.2° W. Finally, the Yakataga Zone, extends further to the east, 

across the Copper River delta to Yakutat, that is, to about 139.5° W. In view of 

the uncertainties in the earthquake potential in the region of the Komandorsky
Islands, no segment of the megathrust was explicitly modeled west of 170.2° E. 

These boundaries are interpreted to be the limits of the possible rupture surfaces 

of significant earthquakes associated with the megathrust.

In the second segmentation model of the megathrust (Model II), shown in 

Figure 5, four segments are considered. This model is similar to the first, in that 

the Western Zone in Model I generally corresponds to Zone A in Model II, and 

the Yakataga Zones in the two models are identical. The primary differences 

between Models I and II arise in the region between the eastern Aleutians and 

Prince William Sound. In Model II, the Eastern Zone of the Model I is divided 

into Zones B and C. The portion of the Eastern Zone that ruptured in the 1964 

earthquake is identified as Zone C, and the remainder, including the portions of 

the Eastern Zone that ruptured in the 1938 earthquake and that portion between 

the 1938 and 1957 earthquake ruptures are included in Zone B.

It seems reasonable to conclude that an earthquake of magnitude 9.2 is the 

maximum to be expected from the megathrust, but it is not clear whether the 

potential for an earthquake of this magnitude extends throughout the Eastern 

Zone of Model I (that is as far west as the limit of the continental crust near 

161.7° W), or whether this potential is limited the rupture area of the 1964 

earthquake. The occurrence of the 1938 earthquake (Mw 8.2) off the lower 

Alaskan Peninsula argues that the behavior of this portion of the region may not 

be characteristic with a magnitude of 9.2. Model II is intended to that this 

possibility into account, confining the characteristic magnitude 9.2 to the 1964 

aftershock zone.

In Model I the boundary between the Western and Eastern zones, and in 

Model II the boundary between Zones A and B, is taken as the approximate limit 

of continental crust north of the Aleutian Arc. This segment boundary was 

suggested by several participants at our workshop. Specifically, we set the 

boundary at the eastern edge of the 1946 earthquake. The selection of this 

boundary is based on the assumption that it is extremely unlikely that a large 

earthquake could rupture through this region. A consequence of prohibiting 

rupture through a boundary is that the calculated hazard shows a saddle in the 

region of the boundary. The saddle occurs because the hazard is calculated from 

a sequence of floating rupture zones that are offset incrementally along the 

megathrust. Because no rupture zones are allowed to cross the segment 

boundary, a site at the end of the segment is immediately adjacent to only one 

floating rupture, and is at increasing distances from all other floating ruptures as 

they are offset along the megathrust. In contrast, higher hazard is calculated at 

the center of a segment where a site is immediately adjacent to many floating 

rupture zones. Reviewers objected to the presence of the saddle, but were 

unwilling to abandon the concept of a segment boundary in this region. To 

resolve this incompatibility, in Model I the Western and Eastern Zones were 

allowed to overlap by 200 km. The resulting hazard is generally constant along 

the strike of the megathrust through this region. The eastern limit of the Eastern 

Zone is taken to be the approximate eastern limit of the aftershocks of the 1964 

earthquake (and of the well-defined Alaska-Aleutian Benioff zone). The western
boundary of the Western Zone is taken as the western limit of the 1965 Rat 

Islands earthquake rupture zone (approximately 170° E).

Although a variety of more detailed segmentation models have been 

proposed for the megathrust zone, large earthquakes, particularly in the western 

and central Aleutians, notably the 1986 earthquake (Mw 8.0) near Adak Island, 

have tended to occur without particular regard for the proposed boundaries.

The southern or updip boundaries of the Western and Eastern Zones in 

Model I and Zones A, B and C in Model II are defined by the so called "seismic 

front" that is, the presumed updip limit of that part of the megathrust capable of 

producing a significant earthquake. The seismic front is generally defined by the 

southern limit of well-recorded seismicity (Engdahl, written communication, 

1997) and by the break in slope at the lower edge of the shelf on the northern 

side of the Aleutian Trench. (This break approximately follows the 400 m depth 

contour, c.f. Plafker et al., 1993). The updip boundary is taken to be at a depth of 

20km.

The northern or downdip boundaries of the Western and Eastern Zones, 

except in the area of the Alaskan Peninsula and Cook Inlet, is taken as the 50 km 

depth contour of earthquakes in the Benioff zone (Plafker et al., 1993). Boyd et 

al. (1995) observed that standard earthquake locations in the central and western 

Aleutians are biased as much as several 10's of km to the north, owing to the 

influence of early arrivals at seismograph stations in Europe to which wave 

propagation is along anomalously high-velocity paths down the subducting slab. 

In considering the appropriate northern boundaries of the zones, hypocenters of 

earthquakes relocated by Engdahl (written communication, 1997) were 

compared with the map and depth contours of Plafker et al. (1993) and found to 

be in good agreement. In the eastern Alaskan Peninsula and Cook Inlet regions, 

the aftershocks of the 1964 earthquake did not extend as far downdip (or north) 

as the 50 km contour, and the boundary was taken as the approximate northern 

limit of the aftershocks of the 1964 earthquake (Plafker et al., 1993). In this 

region the boundary was assumed to lie at a depth 40 km. Between about 155.0° 

W and 159.2° W the depth to the boundary increases smoothly westward from 

40 to 50 km. The assumption about the location of the downdip or northern 

limit of the megathrust in the Cook Inlet area is important because it 

significantly affects the estimation of hazard in the area of Anchorage.

Although the Yakataga segment is clearly the location of significant northsouth convergence and the site of very large earthquakes (e.g. 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. 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.
Recurrence Assumptions

Although the rate of convergence across the Alaska-Aleutian megathrust is 

relatively well known, the fraction of the convergence that is accommodated by 

large earthquakes is significantly less than one. This fraction varies with position 

along megathrust zone, but is poorly known. It appears to range from 10% or 

less to near 100% (c.f. Pacheco et al., 1993). Thus, estimates of the rate of large 

earthquakes based on the known convergence rate alone are unrealistically 

large, well above the observed rate of large earthquakes over the last century.

Other than the plate convergence rate, the only data available to estimate 

recurrence along the entire megathrust is the instrumental seismicity catalog. At 

present geologic data for recurrence exists only for the 1964 zone. Plafker and 

Rubin (1994) estimate that seven or eight events with displacements similar to 

1964 are reflected in the stratigraphic sequence in the Copper River delta in the 

~5600 years preceding 1964. These data suggest a recurrence time for 

earthquakes of magnitude 9.2 of 700 to 800 years. The occurrence of the 1938 

earthquake (Mw 8.2) off the lower Alaskan Peninsula indicates that very large 

earthquakes with magnitudes less than 9.2 occur as well. Therefore, one must 

ask the question, "What is the largest earthquake that might have occurred 

within the 1964 zone, but would not be reflected in the stratigraphy of the 

Copper River delta?" Our estimate is that an earthquake with a magnitude as 

large as 8 could occur without causing sufficient vertical displacement in the 

region of the Copper River delta to be reflected in the stratigraphy.

In view of the limited geologic data, recurrence assumptions for most of the 

megathrust are based on instrumental seismic data. Hazard was estimated 

assuming a Gutenberg-Richter recurrence model for magnitude 7.0 to 9.2 in the 

Western Zone of Model I and Zone A of Model II. Similarly, a GutenbergRichter recurrence model was assumed for magnitudes 7.0 to 8.5 in Zone B of 

Model II, for magnitudes of 7.0 to 8 in the Eastern Zone of Model I and Zone C 

of Model II, and for magnitudes 7.0 to 8.1 in the Yakataga Zone. The parameters 

a and b in the Gutenberg-Richter relations was estimated in each of these regions 

from the historical seismicity data. In addition a characteristic earthquake of 

magnitude 9.2 with a recurrence time of 750 years was assumed for the Eastern 

Zone of Model I and for Zone C of Model II.

Smaller Earthquakes

Hazard for earthquakes in the magnitude range 5.0 to 7.0 was calculated 

from a Gutenberg-Richter recurrence relation determined for each of the 

megathrust source zones, a and b- values were determined for each of the 

source zones of the megathrust. These seismicity parameters were determined 

from maximum-likelihood fits of log N versus magnitude for shallow events 

greater than magnitude 4.5, the minimum magnitude of completeness in the 

region since 1964. Hazard was calculated using the fl-value and fr-value for each 

zone. No smoothing was applied to the edges of these source zones. Details 

may be found in the companion report by Mueller et al. (1998).
Active Crustal Faults

Although considerable information is available about a few active crustal 

faults in Alaska, there are certainly many more faults with unknown slip rates. 

Faults included explicitly in the map are shown in Figure 6. (Note that the 

seismic hazard associated with faults not explicitly included in the map is 

captured to a large degree by the smoothed seismicity model described below.) 

To be included in this map a fault must have an estimated slip rate. As in our 

treatment of western U.S. faults in the national maps (Frankel et al., 1996), we 

divide the faults into two types: A (characteristic) and B (hybrid). The A-type 

faults are faults with "known" segmentation. We use a characteristic rupture 

model for the A-type faults in which rupture occurs only as the largest 

earthquake estimated for each fault segment. B-type faults have "unknown" 

segmentation, so we use two equally-weighted recurrence models. For these 

hybrid faults, we calculated hazard using 50% weight for the characteristic 

earthquake model and 50% weight for a truncated Gutenberg-Richter frequencymagnitude relation. We used a minimum magnitude of M6.5 and a maximum 

magnitude of Mchar for the Gutenberg-Richter hazard calculation. In general, the 

Gutenberg-Richter recurrence model yields higher hazard for a given fault than 

the characteristic model, because of the more frequent occurrence of moderatesized earthquakes in the Gutenberg-Richter model. Use of this model is intended 

here to account for the possibility that the crustal faults will rupture in segments 

smaller than their entire length.

Recurrence times (characteristic) and a- values (G-R) for the each fault were 

determined from their slip rates, not from the seismicity surrounding the fault. 

Characteristic magnitudes were determined from fault lengths using the 

relations of Wells and Coppersmith (1994). See Frankel et al. (1996) for the 

formulas deriving the a- values and characteristic earthquake rates from the 

geologic slip rate and area of fault. The fault widths of crustal faults were 

estimated by assuming a maximum faulting depth of 15 km. These crustal faults 

were taken to be vertical, except for the Castle Mountain fault (dips 75° to North) 

and the Transition fault (dips 10° to North).

Table 1 identifies the faults (and fault segments) included in the map, and the 

assumptions made about them. The slip rates are from Nishenko and Jacob 

(1990) and Plafker et al. (1993) except as noted below. The recurrence times in the 

table are for the characteristic earthquakes. Again, the hybrid faults also use a 

Gutenberg-Richter recurrence model which will produce more frequent 

recurrence for earthquakes between M6.5 and Mchar.

Estimates of the slip rate for the Totschunda fault ranged from 8 to 15 

mm/yr (Plafker et al., 1993). We adopted a "mean" value of 11.5 mm/yr.

We note that we treated the Transition fault as an A-type fault, even though 

its segmentation is unknown. In initial hazard calculations 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 (J. Lahr, 

pers. comm.), and we were concerned that one of the more poorly known faults
had a higher hazard than the megathrust zone. Consequently, 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 et al. (1997) 

subduction zone interface attenuation relation when estimating ground motions 

for the Transition fault (see attenuation section below). This produces somewhat 

lower ground motions than do attenuation relations for crustal earthquakes.

Of particular interest is the Castle Mountain fault, passing about 40 km from 

Anchorage. The fault has been considered in two segments the western, or 

Susitna segment, and the eastern or Talkeetna, segment (Detterman et al., 1994). 

Along the Talkeetna segment there is no evidence for surficial displacement 

younger than Pleistocene (Detterman et al., 1976), but Lahr et al. (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 et al., 1974, 1976; Bruhn, 1979). These studies, 

however, have served only to put wide limits on the slip rate. We chose a value 

of 0.5 mm/yr and a maximum magnitude of 7.5 leading to a recurrence time of 

1300 years (R. Updike, oral communication, 1997).

Determination of Seismicity Parameters from 

Spatially-smoothed Seismicity

For the smoothed seismicity calculation, the shallow events (focal depth <50 

km) in the areal source zones of the megathrust were first removed from the 

catalog. Next the catalog was divided into shallow, deep (focal depth 50-80 km) 

and deeper (focal depth 80 to 120 km) events, fr-values were determined 

separately for the shallow, deep and deeper events using the maximum 

likelihood method (Weichert, 1980) for events with magnitudes greater than 4.5. 

We found b-values of 0.87 for the shallow seismicity, 1.2 for the deep seismicity 

(50-80 km depth), and 1.15 for the deeper seismicity (80-120 km depth).

Using the approach of Frankel (1995), a-value grids were calculated using the 

maximum-likelihood formula from Weichert (1980). These 0-value grids were 

then smoothed with Gaussian smoothing functions (correlation distance of 75 

km) and the hazard was calculated by summing the frequencies of exceedance 

for all of the grid cells. This was done separately for the shallow and two deep 

cases. These a-value grids were then used as the basis to calculate the hazard 

arising from earthquakes in the magnitude range 5.0 to 7.0 (5.0 to 7.3 for the 

shallow case).

Source Finiteness in Hazard Calculation

In all the calculations of hazard, the treatment of source finiteness varied 

with magnitude range. For events between M5.0 and M6.5, we assumed point 

sources. For events from M6.5 to M7.0 (M6.5 to M7.3 in the case of shallow 

earthquakes) we used finite faults of arbitrary strike. For events greater than 

magnitude 7.0 in the megathrust zone, we used floating rupture zones offset 

incrementally along the megathrust.
For the crustal faults, when using the Gutenberg-Richter recurrence model, 

we floated the rupture zones along the fault. The floating rupture zones along 

the crustal faults and the megathrust cause a tapering of the hazard at the ends 

of the faults.

Attenuation Relationships

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 30m. This 

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

represents a typical western U.S. "firm-rock" 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 U.S. 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.

Discussion of Maps

Probabilistic seismic hazard maps for Alaska calculated as described above 

are shown in Figures 7-10, for peak ground acceleration, 5.0, 3.3, and 1 Hz 

spectral acceleration, and for 10% and 2% probability of exceedance in 50 years.

Hazard is highest in the coastal regions adjacent to the megathrust and the 

Transition Fault and in regions adjacent to the Denali and Fairweather-Queen 

Charlotte fault systems at all periods and probability levels. In the interior of 

Alaska, away from the Denali fault, hazard is dominated by the spatiallysmoothed seismicity. The region of lowest hazard in Alaska is along the 

northern coast adjacent to the Arctic Ocean. The hazard associated with the 

Castle Mountain fault is overwhelmed by the megathrust at most periods and 

probability levels, but can be seen on the map of peak ground acceleration with 

2% probability of exceedance in 50 years (Figure 7b.) In general hazard in the 

higher hazard regions of Alaska is comparable to areas of higher hazard in 

California (Frankel et al., 1996).

Disaggregations by magnitude and distance to the source of the hazard are 

shown in Figures 11, 12 and 13 for Anchorage, Fairbanks and Juneau for peak 

ground acceleration and 5 Hz and 1 Hz spectral acceleration at the 2%-in-50 year 

probability level. The disaggregation plots include the summary joint 

distribution statistics, the mean magnitude and distance ("mbar" and "dbar"), and 

the modal magnitude and distance ("mmode" and "dmode").

The disaggregations for Anchorage indicate the role of great earthquakes 

(magnitude 9 at a distance of about 50 km) relative to the other sources (Figure 

11). The relative contribution of the great earthquakes increases with increasing 

period (decreasing frequency) until the great earthquakes dominate at a

10
frequency of 1 Hz. On these plots hazard from the Castle Mountain fault at a 

distance of about 40 km is combined with the hazard from earthquakes in the 

magnitude range up to 7.5 on the megathrust. The other significant 

contributions to the hazard arise from the shallow smoothed seismicity (shown 

at a distance of about 20 km) and the two deeper zones of smoothed seismicity ( 

show at distances of 60 and 90 km).

The disaggregations for Fairbanks (Figure 12) show that the hazard is 

dominated by local earthquakes. The influences of the Denali fault at distance of 

about 135 km and the megathrust at a distance of about 350 km are only 

apparent on the disaggregation for 1 Hz.

The disaggregations for Juneau (Figure 13) show the relative contributions 

of the local shallow seismicity and large earthquakes on the South Denali 

(Chatham Strait) fault at a distance of about 80 km. The relative contribution of 

the large earthquakes on the South Denali increase with increasing period 

(decreasing frequency).

Although somewhat more detailed, the current maps for peak ground 

acceleration are generally similar to the maps prepared by Thenhaus et al. (1985) 

for the corresponding probability levels, although significant differences do exist. 

(Thenhaus et al. did not estimate the hazard in the area of the Aleutian Islands.) 

In general the values on the current map for the 10%-in-50-year probability level 

are somewhat lower than those on the Thenhaus et al. map in south central and 

southeast Alaska, but somewhat higher in the offshore regions above and 

adjacent to the megathrust. The values on the current 2%-in-50-year map are 

similar to those on the Thenhaus et al. map in south central and southeast 

Alaska, but somewhat higher in the offshore regions above and adjacent to the 

megathrust. For example, for the Anchorage area at the 10%-in-50-year 

probability level, the current map indicates a peak ground acceleration of 37% g, 

in contrast to about 45% for the Thenhaus et al. map. At the 2% in 50 year 

probability level, the current map indicates about 65% g for the Anchorage area, 

as contrasted with ~67% g on the Thenhaus et al. map. Table 3 compares 

estimates of the estimated peak ground accelerations from the current maps and 

the Thenhaus et al. maps at nine locations in Alaska.

Conclusions and Issues Requiring Future Work

Alaska has some of the areas of highest seismic hazard in the United States. 

In contrast to California where most of the regions of highest hazard occur in 

relatively narrow zones and are often associated with nearly vertical faults, most 

of the hazardous regions in Alaska occur in association with relatively shallow 

dipping faults leading to much larger affected areas. The principal sources of 

seismic hazard in Alaska are the Alaska-Aleutian megathrust and the Transition 

fault (both relatively shallow dipping and affecting very large areas), and the 

Fairweather, Queen Charlotte and Denali faults (near vertical faults leading to 

relatively narrow zones of high hazard.)

11
Obviously, there are many aspects of the methodology and input 

information that can be debated. In constructing these maps we have attempted 

to include those elements of geologic and geophysical information and 

interpretation for which a community consensus exists. We have also attempted 

to indicate explicitly those areas where we have been forced to rely on judgment 

or assumption.

Future seismic hazard maps would benefit from additional information in 

several areas. Additional understanding of the characteristics of faulting in 

subduction zones, especially the 1964 zone would be very helpful. Tectonic 

understanding of the region between the 1964 zone and the Fairweather fault, 

including the Yakataga gap and the Transition fault would be extremely 

valuable. More measurements of slip rates on the crustal faults are needed. 

Finally, new insights into the segmentation of the megathrust and the expected 

magnitude distribution of earthquakes would have a large impact.

Specific methodological issues that warrant further consideration include: 1) 

Segmentation boundaries on faults characterized by large earthquakes cause 

saddles in hazard with lower hazard near boundary, because of the tapering of 

hazard caused by floating rupture zones. We dealt with this saddle in the hazard 

in an ad hoc manner by overlapping the zones. This issue requires additional 

consideration over the long run. 2) We used historical seismicity rather than 

convergence rates to establish rates of large (M>7) earthquakes in the 

megathrust source zones. It would be desirable to rationalize the seismicity rates 

with the convergence rate through some quantitative mechanism. 3) Future 

segmentation models for the megathrust could benefit greatly from additional 

tectonic insight. 4) Use of time-dependent probabilities.

Acknowledgments

We thank the participants of the September 1996 workshop on seismichazard mapping in Alaska for their valuable suggestions and guidance. Many of 

their comments were applied to make the draft maps. We particularly thank Max 

Wyss and Stefan Wiemer of the Geophysical Institute and John Lahr of the USGS 

for their help with the treatment of earthquake catalogs (see accompanying 

documentation). George Plafker, Randy Updike, and Don Wells provided useful 

discussions. C.B. Grouse, K.H. Jacob and E.V. Leyendecker provided helpful 

reviews. Ken Rukstales prepared the full color 1:7,500,000 scale maps for 

publication.

References

Boore, D., W. Joyner, and T. Fumal (1997). Equations for estimating horizontal 

response spectra and peak acceleration from Western North American 

earthquakes: a summary of recent work, Seism. Res. Letts., v. 68, no. 1, pp. 

128-153.

12
Boyd, T.M., E.R. Engdahl, and W. Spence, (1995). Seismic cycles along the 

Aleutian arc: Analysis of seismicity from 1957 through 1991, J. Geophys. 

Res., v. 100, pp. 621-644.

Bruhn, R.L. (1979). Holocene displacements measured by trenching the Castle 

Mountain fault near Houston, Alaska, in Short Notes on Alaska Geology, 

Alaska Division of Geological and Geophysical Surveys, Geologic Report 61, 

Anchorage, Alaska, 4 p.

Campbell, K. and Y. Bozorgnia (1994). Near-source attenuation of peak 

horizontal acceleration form worldwide accelerograms recorded from 1957 

to 1993, in Proceedings of Fifth U.S. National Conference on Earthquake 

Engineering, Earthquake Engineering Research Institute, Oakland, 

California, III, pp. 283-292.

Cormier, V. (1975). Tectonics near the junction of the Aleutian and KurilKamchatka arcs and a mechanism for the middle Tertiary magmatism in the 

Kamchatka basin, Geol. Soc. Amer. Bull., v. 86., pp. 443-453.

Cornell, A. (1968). Engineering seismic risk analysis, Bull. Seism. Soc. Am., v. 58, 

pp. 1583-1606.

Davies, J.N., L. R. Sykes, L. House, and K. Jacob (1981). Shumagin seismic gap, 

Alaskan peninsula: History of great earthquakes, tectonic setting and 

evidence for high seismic potential, J. Geophys. Res., v. 86, pp. 3821-3855.

Detterman, R.L., G. Plafker, T. Hudson, R.G. Tysdal, and N. Pavoni (1974). 

Surface geology and Holocene breaks along the Susitna segment of the 

Castle Mountain fault, Alaska, U.S. Geol. Surv. MF Map 618,1 sheet.

Detterman, R.L., G. Plafker, G.T. Russell, and T. Hudson (1976). Features along 

part of the Talkeema segment of the Castle Mountain-Caribou fault system, 

Alaska, U.S. Geol. Surv. MF Map 638,1 sheet.

Frankel, A. (1995). Mapping seismic hazard in the Central and Eastern United 

States, Seism. Res. Letts, v. 66, no. 4, pp. 8-21.

Frankel, A., C. Mueller, T. Barnhard, D. Perkins, E. Leyendecker, N. Dickman, S. 

Hanson, and M. Hopper (1996). National seismic-hazard maps: 

Documentation June 1996, U.S. Geological Survey Open-File Report 96-532, 

110 pages.

Lahr, J.C., C.D. Stephens, H.S. Hagewara, and J. Boatwright (1980). Alaska 

seismic gap only partially filled by 28 February 1979 earthquake, Science, 

v207, pp. 1351-1353.

Lahr, J. , R.A. Page, C.D. Stephens, and K.A. Fogelman (1986). Sutton, Alaska, 

earthquake of 1984: Evidence for activity on the Talkeema segment of the 

Castle Mountain fault system, Bull. Seism. Soc. Am., v. 76, pp. 967-984.

Mueller, C., A. Frankel, J. Lahr, and M. Wyss 1998). Preparation of earthquake 

catalogs for the national seismic hazard maps: Alaska, U.S. Geological 

Survey Open-File Report, in preparation.

Nishenko, S.P., and K.H. Jacob (1990). Seismic potential of the Queen CharlotteAlaska-Aleutian-seismic zone, J. Geophys. Res., v. 95, pp. 2511-2532.

Pacheco, J., L. Sykes, and C. Scholz (1993). Nature of seismic coupling along 

simple plate boundaries of the subduction type, J. Geophys. Res., v. 98, pp. 

14,133-14,159.

Page, R.A., N.N. Biswas, J.C. Lahr, and H. Pulpan (1991). Seismicity of continental 

Alaska, in Slemmons, D.B., Engdahl, E.R., Zoback, M.R., and Blackwell, D.D.,

13
eds., Neotectonics of North America: Boulder, Colorado, Geol. Soc. Am.,

Decade Map Volume 1, pp. 47-68. 

Plafker, G., L. Gilpin, and J. Lahr (1993). Neotectonic map of Alaska, in Geology

of Alaska, vol G-l of Geology of North America, Geological Society of

America, Boulder, CO. 

Plafker, G. and M. Rubin, (1994). Paleoseismic evidence for "yo-yo" tectonics

above the eastern Aleutian subduction zone: coseismic uplift alternating

with even larger interseismic submergence, in Proceedings of the Workshop

on Paleoseismology, C. S. Prentice, D.P. Schwartz and R.S. Yeats,

Convenors, USGS Open-file report 94-568, p.155-157. 

Sadigh, K., C. Chang, J. Egan, F. Makdisi, and R. Youngs (1997). Attenuation

relationships for shallow crustal earthquakes based on California strong

motion data, Seism. Res. Letts., v. 68, no. 1, pp. 180-189. 

Sykes, L.R. (1971). Aftershock zones of great earthquakes, seismicity gaps, and

earthquake prediction for Alaska and the Aleutians, J. Geophys. Res., v. 76,

pp. 8021-8041. 

Sykes, L.R., J.B. Kisslinger, L. House, J.N. Davies, and K.H. Jacob (1981). Rupture

zones and repeat times of great earthquakes along the Alaska-Aleutian arc,

in Simpson D.W., and R.G. Richards, eds., Earthquake prediction: American

Geophysical Union, pp. 73-80. 

Taber, J.J., S. Billington, and E.R. Engdahl, (1991). Seismicity of the Aleutian arc,

in Slemmons, D.B., Engdahl, E.R., Zoback, M.R., and Blackwell, D.D., eds.,

Neotectonics of North America: Boulder, Colorado, Geol. Soc. Am., Decade

Map Volume 1, pp. 29-46. 

Thenhaus, P.C., J.I. Ziony, W.H. Diment, M.G. Hopper, D.M. Perkins, S.L.

Hanson, and S.T. Algermissen (1985). Probabilistic estimates of maximum

seismic horizontal ground acceleration on rock in Alaska and the adjacent

continental shelf, Earthquake Spectra, v. 1, pp. 285-306. 

Youngs, R., S. Chiou, W. Silva, and J. Humphrey (1997). Strong ground motion

attenuation relationships for subduction zone earthquakes, Seism. Res.

Letts., v. 68, no. 1, pp. 58-73. 

Weichert, D. (1980). Estimation of earthquake recurrence parameters for unequal

observation periods for different magnitudes, Bull. Seism. Soc. Am., v. 70,

pp. 1337-1356. 

Wesson, R.L., A.D. Frankel, C.S. Mueller, S.C. Harmsen (1999). Seismic-Hazard

Maps for Alaska and the Aleutian Islands. U.S. Geological Survey

Miscellaneous Investigations Series 1-2679, 2 sheets.

14
Figure Captions

Figure 1. Instrumental seismicity of Alaska and the Aleutian Islands from the 

consolidated catalogs. Earthquakes shown have magnitudes, Mw>5.5, and 

dates ranging from 1880 to 1996. (Mueller et al., 1998).

Figure 2. Rupture areas of large earthquakes in the Alaska Aleutian region 

during this century (Plafker et al., 1993). Note that virtually the entire 

boundary between the Pacific and North American Plates has ruptured 

during this period with the exceptions of 1) the western most Aleutians 

(~168°E), 2) the Shumagin gap (~160°W), and 3) the Yakataga gap (~142°W). 

See text for discussion.

Figure 3. Hazard Model for Alaska.

Figure 4. Segmentation Model I for the Alaska-Aleutian Megathrust 

Figure 5. Segmentation Model II for the Alaska-Aleutian Megathrust 

Figure 6. Active crustal faults identified in Alaska (Plafker et al., 1993).

Figure 7a. Peak ground acceleration (%g) with 10% probability of exceedance in 

50 years.

Figure 7b. Peak ground acceleration (%g) with 2% probability of exceedance in 

50 years.

Figure 8a. 5 Hz acceleration (%g) with 10% probability of exceedance in 50 years. 

Figure 8b. 5 Hz acceleration (%g) with 2% probability of exceedance in 50 years.

Figure 9a. 3.3 Hz acceleration (%g) with 10% probability of exceedance in 50 

years.

Figure 9b. 3.3 Hz acceleration (%g) with 2% probability of exceedance in 50 

years.

Figure lOa. 1 Hz acceleration (%g) with 10% probability of exceedance in 50 

years.

Figure lOb. 1 Hz acceleration (%g) with 2% probability of exceedance in 50 

years.

Figure lla. Disaggregation of hazard at Anchorage for peak ground 

acceleration at 2%-in-50 year probability level.

Figure lib. Disaggregation of hazard at Anchorage for 5 Hz ground 

acceleration at 2%-in-50 year probability level.

15
Figure lie. Disaggregation of hazard at Anchorage for 1 Hz spectral 

acceleration at 2%-in-50 year probability level.

Figure 12a. Disaggregation of hazard at Fairbanks for peak ground acceleration 

at 2%-in-50 year probability level.

Figure 12b. Disaggregation of hazard at Fairbanks for 5 Hz ground acceleration 

at 2%-in-50 year probability level.

Figure 12c. Disaggregation of hazard at Fairbanks for 1 Hz spectral acceleration 

at 2%-in-50 year probability level.

Figure 13a. Disaggregation of hazard at Juneau for peak ground acceleration at 

2%-in-50 year probability level.

Figure 13b. Disaggregation of hazard at Juneau for 5 Hz ground acceleration at 

2%-in-50 year probability level.

Figure 13c. Disaggregation of hazard at Juneau for 1 Hz spectral acceleration at 

2%-in-50 year probability level.

16
Table Captions

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

Table 2. Attenuation models assumed for various seismic sources assumed for 

hazard analysis.

Table 3. Comparison of peak ground acceleration for selected locations in Alaska 

between the current maps and those prepared by Thenhaus et al. (1985).

17
%\

ii
180

170°W 160°W
Hazard Model for Alaska

5.0 < M < 7.0*

Spatially Smoothed Seismicity

Shallow Earthquakes (0-50 km) 

5.0 < M < 7.3

Deeper Earthquakes (50-80 km) 

5.0< M < 7.0

Deepest Earthquakes (80-120 km) 

5.0 < M < 7.0

Megathrust source zones 

5.0 < M < 7.0

M>7.0

Megathrust Zone

Equal weights on two segmentation models with recurrence rates for truncated GutenbergRichter determined from historical seismicity and recurrence rate for Characteristic Model in

1964 zone determined from geologic studies

Faults with "known" segmentation (A)

Characteristic Model using slip rates to obtain 

___recurrence of maximum earthquake___

*An upper limit of magnitude 7.3 was adopted for the shallow earthquakes to accomodate the 

possible occurrence of a background earthquake of that magnitude.

Faults with "unknown" segmentation (B)

Equal weights on Characteristic Model and truncated Gutenberg-Richter for 7.0 to maximum magnitude, and using slip rates for recurrence
1°

170°W160

Yakataga 

Zone 

v G-R 7-8.1

^Tastern Zone 

G-R 7-8 

CharST.2

Western Zone 

CrR 7-9.2

780

170°W 160°W
a

N

Yakataga 

Zone 

G-R 7-8.1

i X Zone C 

G-R 7-8 

vChar 9.2

180

170°W 160°W
760

Fairwr ^er. o 

mm/'y

150°W 140°W
%g

190° 200° 

Peak Ground Acceleration (%g) with 10% Probability of Exceedance in 50 Years

30

25

20

1510

987

6

5

4

3

2

1

0
%g

190° 200° 

Peak Ground Acceleration (%g) with 2% Probability of Exceedance in 50 Years

350

200

160120

80

6050

4030

20

18

161412

10

86

420

1
ifa

%g

190° 200° 

5 Hz Spectral Acceleration (%g) with 10% Probability of Exceedance in 50 Years

350

200

160

120

80

6050

403020

18

1614

1210

8

6

420

«
%g

o

eP

180

190° 200°

5 Hz Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 Years

600

400320

240

160120100

8060

4036

32

282420

1612

84

0

H I O
00

%g

I

750

190° 200°

3 Hz Spectral Acceleration (%g) with 10% Probability of Exceedance in 50 Years

350

200

160120

80

6050

40

30

20

1816

141210

86

4

20
%g I

180

190° 200°

3 Hz Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 Years

600

400320240

160120100

8060

403632

282420

1612

840

H la
%g

190° 200° 

1 Hz Spectral Acceleration (%g) with 10% Probability of Exceedance in 50 Years

180100

80

60

40

3025

20

1510

987

65432

1
%g

180

190° 200°

1 Hz Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 Years

60

50403020

18

16

141210

8

6420
Anchorage AK Seismic Hazard

Pk Gr Accel 0.612 g PE = 2% per 50 yr

Mbar6.77dbar41.4kmMmode 6.76 dmode 16.5 km (13.9% contribution)

RTOiiMar11 10:151 Preiim run 2% PE/50 yr. PGA 0.6115 g. Red: log(PGA) gt mu+2slgma; Orange 1 - 2; Yellow 0-1;
Anchorage AK Seismic Hazard

5 Hz SA 1.37 g PE = 2% in 50 years

Mbar 7.00 dbar 38.6 kmMmode 9.20 dmode 57.9 km (17.0% contribution)

iC^jjg Mar 1208:47] SelsHaz 2% PE/50 yr. 5 Hz 1.367 g. Red: log(SA) gt mu+2slgma; Orange 1 - 2; Yellow 0 -1;
Anchorage AK Seismic Hazard

1 Hz SA 0.511 g PE = 2% per 50 yr

Mbar 7.76 dbar 57.9 kmMmode 9.20 dmode 57.9 km (34.1% contribution)

Revised March 1998

Mfl|jMar11 09:581 SeisHaz 2% PE/50 yr. 1 Hz 0.511 g. Red: log(SA) gt mu+2slgma; Orange 1-2; Yellow 0 -1;
Fairbanks AK Seismic Hazard

Pk. Gr. Accel. 0.414 g PE = 2% in 50 years

Mbar 6.20 dbar 19.2km

Mmode 6.24 dmode 16.9 km (23.3% contribution)

M Mil Mar 12 08:581 SeisHaz 2% PE/50 yr. Peak Ground Accel 0.414 g. Red: log(SA) gt mu+2slgma; Orange 1 - 2; Yellow 0-1;
Fairbanks AK Seismic Hazard

5 Hz SA 0.933 g PE = 2% in 50 years

Mbar 6.25 dbar 19.4 kmMmode 6.25 dmode 17.2 km (24.0% contribution)

11:581 NnlSelsHaz 2% PE/50 yr. 5 Hz 0.933 g. Red: log(SA) gt mu+2slgma; Orange 1 - 2; Yellow 0 1;
Fairbanks AK Seismic Hazard

1 Hz SA 0.292 g PE = 2% in 50 years

Mbar 6.68 dbar 29.4 kmMmode 6.75 dmode 18.1 km (28.9% contribution)

UJ

MfljJMarH 11:51 | Prelim run 2% PE/50 yr. 1 Hz 0.292 g. Red: log(SA) gt mu+2sigma; Orange 1 - 2; Yellow 0 -1;
Juneau AK Seismic Hazard

Pk. Gr. Accel. 0.234 g PE = 2% in 50 years

Mbar 7.19 dbar 55.4 kmMmode 8.0 dmode 34.9 km (13.4% contribution)

uo

MfliJMar1208:44| NnlSelsHaz 2% PE/50 yr. Pk Gr Accel 0.234 g. Red: log(PGA) gt mu+2slgma; Orange 1 - 2; Yellow 0 -1;
Juneau AK Seismic Hazard

5 Hz SA 0.528 g PE = 2% in 50 years

Mbar7.21 dbar60.4kmMmode 8.2 dmode 90.3 km (14.5% contribution)

11:21 | NnlSelsHaz 2% PO50 yr. 5 Hz 0.528 g. Red: log(SA) gt mu+2slgma; Orange 1 - 2; Yellow 0 -1;
Juneau AK Seismic Hazard

1 Hz SA 0.229 g PE = 2% in 50 years

Mbar 7.65 dbar 70.4 kmMmode 8.0 dmode 35.0 km (17.6% contribution)

O

11:03| NatlonalSelsHaz 2% PE/50 yr. 1 sec SA= 0.229 g. Red:log(SA) gt mu+2slgma; Orange 1 - 2; Yellow 0 -1; Green -1 to 0
Table 1. Fault Characteristics

Fault

(segment)

Queen Charlotte

Fairweather,

offshore

Fairweather,

onshore

Denali, southeast

Denali,

central

Totschunda

Castle Mountain

Transition

Mchar

(Mmax)

(from

length)

8.1

7.9

7.8

8.12

8.0

7.7

7.53

8.2

Slip rate

(mm/yr)

58

52

52

2

10

11.5

0.5

10

Recurrence

time1 for

characteristic

earthquake

(years)

130

120

110

1900

700

400

5000

200

Characteristic (A)

or

Hybrid (B)

A

A

A

B

B

B

B

A

References

Nishenko and

Jacob (1990)

Nishenko and

Jacob (1990);

Plafker et al.

(1993)

Nishenko and

Jacob (1990)

Plafker et al.

(1993)

Plafker et al.

(1993)

Plafker et al. 

(1993)

(See text)

(See text)

(See text)

1 Recurrence times are estimated from the rate of seismic moment release for earthquakes of the characteristic 

magnitude required to balance the observed geologic slip rate, and are rounded to two significant figures.

2 On the basis of length alone the southeast Denali fault would give a magnitude exceeding 8.1. However, 

there seems to be no historical precedent for a continental, strike-slip fault generating an earthquake with a 

moment magnitude, Mw, exceeding 8.1.

3 On the basis of length alone, the Castle Mountain fault would yield a magnitude of 7.8. In view of the 

uncertainties of the length, slip rate and other seismic characteristics of the fault, a lower value of 7.5 was 

adopted. Because this lower estimate leads to more frequent earthquakes, and thus higher estimates of 

hazard, this is considered an appropriately conservative assumption.
Table 2. Attenuation Relationships

Source

Megathrust and 

Transition Fault

Crustal faults, 

shallow

earthquakes

Deep earthquakes 

(depth= 50-80 km)

Deeper 

earthquakes 

(depth= 80-120 

km)

Attenuation 

Relationship

Youngs et al. 

(1997) (Interface)

Boore et al (1997) 

NEHRP B/C

Sadighetal. (1997) 

rock

Campbell and 

Bozorgnia (1994) 

soft-rock

Boore et al. (1997)

Sadighetal. (1997)

Youngs et al. 

(1997) (Intraslab; 

depth fixed at 60 

km)

Youngs et al. 

(1997) (Intraslab; 

depth fixed at 90 

km)

Frequencies

pga, 1, 3.33, 5 Hz

PSa

1 , 3.33, 5 Hz

pga, 1,3.33, 5 Hz

pga, 1,3.33, 5 Hz

Weight

100%

33 1/3% each

50 % each

100%

100%
Location

Table 3. Comparison of Peak Ground Accelerations of 

Current Maps with Thenhaus et al. (1985)

Lat. Long.

10% in 50 years 2% in 50 years

Current Thenhaus Current Thenhaus

Map# etal.* Map# et al.*

Anchorage 

Barrow

Bethel

Cold Bay 

Fairbanks

Juneau 

Kodiak

Nome

Sitka

61.2 

71.3

60.8

55.2 

64.8

58.3 

57.8

64.5

57.1

149.9 

156.8

161.8

162.7 

147.7

134.4 

152.4

165.4

135.3

38

6

27 

21

13 

46

9

22

-45 

<4

-6

-15 

-22

-10 

-60

-8

-40

66 

1

13

48 

43

25

75

22

35

-67 

<4

-13

-25 

-37

-15 

-65

-15

-40

# Calculated values

* Values interpolated visually from published map