GEOLOGICAL SURVEY CIRCULAR 898

Summary of Workshops Concerning 

Regional Seismic Source Zones of Parts of 

the Conterminous United States, Convened 

by the U.S. Geological Survey 1979-1980, 

Golden, Colorado
Summary of Workshops Concerning 

Regional Seismic Source Zones of Parts of 

the Conterminous United States Convened 

by the U.S. Geological Survey 1979-1980, 

Golden, Colorado

Edited by Paul C. Thenhaus

GEOLOGICAL SURVEY CIRCULAR 898

J983
United States Department of the Interior

WILLIAM P. CLARK, Secretary

Geological Survey

Dallas L. Peck, Director

Free on application to the Branch of Distribution, Eastern Region, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22034
CONTENTS

Page

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

Introduction, by P. C. Thenhaus and F. A. McKeown .................. 1

Acknowledgments ....................................... 4

Great Basin seismic source zones Summary of workshop convened October 10-11, 1979,

by R. C. Bucknam and P. C. Thenhaus ........................ 4

Northern Rocky Mountains seismic source zones Summary of workshop convened

December 6-7, 1979, by F. A. McKeown, D. C. Ross, and P. C. Thenhaus ... 9 

Southern Rocky Mountains seismic source zones Summary of workshop convened January

23-24, 1980, by R. E. Anderson, W. P. Irwin, and P. C. Thenhaus ....... 14

Central Interior seismic source zones Summary of workshop convened June 10-11, 1980,

by F. A. McKeown, D. P. Russ, and P. C. Thenhaus ............... 19

Northeastern United States seismic source zones Summary of workshop convened September 10-11, 1980, by W. H. Diment, F. A. McKeown, and P. C. Thenhaus . 24 

Summary, by P. C. Thenhaus ................................ 31

References cited ........................................ 32

ILLUSTRATIONS

FIGURES 1-11. Maps:

1. Five regions of the conterminous United States discussed at seismic source zone workshops . . 2

2. Index of the Great Basin region ................................... 5

3. Seismic source zones of the Great Basin .............................. 7

4. Index of Northern Rocky Mountain region ............................. 10

5. Seismic source zones of the Northern Rocky Mountains ..................... 13

6. Index of the Southern Rocky Mountains region .......................... 16

7. Seismic source zones of the Southern Rocky Mountains ...................... 17

8. Index of the Central Interior .................................... 21

9. Seismic source zones of the Central Interior ............................ 22

10. Index of the Northeastern United States .............................. 26

11. Seismic source zones of the Northeastern United States ..................... 27

TABLES

Page

TABLE 1. Estimated maximum magnitudes and recurrence rates from geologic data for earthquakes M8>7 in the Great

Basin seismic source zones .......................................... 8

2. Estimated maximum magnitudes (M8) and estimated relative recurrence of maximum magnitude events for

the source zones of the Northern Rocky Mountains region ........................ 14

3. Estimated maximum magnitudes (Ms) and estimated relative recurrence of maximum magnitude events for

the source zones of the Southern Rocky Mountains region ........................ 18

4. Estimated maximum magnitudes (Mb) and estimated recurrence rates for maximum magnitude events for

the seismic source zones of the Central Interior region .......................... 23

5. Estimated maximum magnitudes for seismic source zones of the northeastern United States and adjacent

Canada ...................................................... 28

III
Summary of workshops concerning regional seismic source

zones of parts of the conterminous United States,

convened by the U.S. Geological Survey,

1979-1980, Golden, Colorado

Edited by Paul C. Thenhaus

ABSTRACT

Workshops were convened by the U.S. Geological Survey 

to obtain the latest information and concepts relative to defining seismic source zones for five regions of the United States. 

The zones, with some modifications, have been used in preparation of new national probabilistic ground motion hazard maps 

by the U.S. Geological Survey.

The five regions addressed are the Great Basin, the Northern Rocky Mountains, the Southern Rocky Mountains, the 

Central Interior, and the northeastern United States. Discussions at the workshops focussed on possible temporal and spatial variations of seismicity within the regions, latest ages of 

surface-fault displacements, most recent uplift or subsidence, 

geologic structural provinces as they relate to seismicity, and 

speculation on earthquake causes.

Within the Great Basin region, the zones conform to areas 

characterized by a predominance of faults that have certain 

ages of latest surface displacements. In the Northern and 

Southern Rocky Mountain regions, zones primarily conform to 

distinctive structural terrane. In the Central Interior, primary 

emphasis was placed on an interpretation of the areal distribution of historic seismicity, although geophysical studies in the 

Reelfoot rift area provided data for defining zones in the New 

Madrid earthquake area. An interpretation of the historic seismicity also provided the basis for drawing the zones of the 

New England region.

Estimates of earthquake maximum magnitudes and of recurrence times for these earthquakes are given for most of 

the zones and are based on either geologic data or opinion.

INTRODUCTION

By P. C. Thenhaus and F. A. McKeown

Under the Earthquake Hazards Reduction act 

of 1977 (Public Law 95-124, Executive Office of 

the President, 1978) the U.S. Geological Survey 

has been given the responsibility for producing 

earthquake hazard and seismic risk maps on both 

a regional and national scale. The maps are intended to aid in the mitigation of the short-term 

earthquake hazard to buildings of standard construction. The preparation of these maps requires 

completion of three major tasks: (1) delineation of 

seismic source zones, (2) analysis of the recurrence

interval of earthquakes in each of the zones, and 

(3) calculation of cumulative probability distributions of expected acceleration exceedences for 

points in the region. A major part of the first task 

in the preparation of the new maps is to incorporate the most recent information and ideas related 

to the seismicity, geology, and geophysics of various parts of the country. This report is primarily 

a description of the first task the delineation of 

seismic source zones.

In an effort to utilize the most recent, pertinent 

information from a variety of disciplines and for 

a variety of areas across the country, informal 

workshops were convened by the U.S. Geological 

Survey in Golden, Colo., in late 1979 and early 

1980. The number of workshops convened was limited by budget considerations. A difficult decision, 

therefore, was to determine which regions of the 

United States would benefit greatest from these 

workshops. Seismic source zoning of the east and 

west coasts had been addressed in studies completed just prior to the planning of the 1979-1980 

seismic source zone workshops (Perkins and 

others, 1979; Perkins and others, 1980; Thenhaus 

and others, 1980). A predecessor to the 1979-1980 

workshops was convened in September 1978 at the 

U.S. Geological Survey's office in Woods Hole, 

Mass., to gather recent information that might 

have a bearing on seismic source zones for the 

eastern United States. Source zones defined as a 

result of information acquired at that meeting are 

discussed by Perkins and others (1979). Because 

source zones for the east and west coasts incorporated the most recent information available as of 

1978-1979, they were excluded from consideration 

in planning the 1979^1980 workshops. Throughout 

the remaining part of the United States, the general governing principle was to choose those regions where geological, geophysical, and seismological research of the past 5 years contributed
 -4.^

I/ NORTHERN ^

NTAINS | /

*U

FIGURE 1. Five regions of the conterminous United States discussed at seismic source zone workshops convene

Survey, 1979-1980. Numbered zones are shown on figure indicated in parentheses.
significantly to the understanding of the seismotectonics of the region. The regions chosen 

were the (1) Great Basin, (2) Northern Rocky 

Mountains, (3) Southern Rocky Mountains, (4) 

Central Interior, and (5) northeastern United 

States (fig. 1).

The workshops provided a forum for (1) presenting and discussing current research, some of which 

is in a formative stage, (2) speculating on the nature of the earthquake-generating process operating on a regional scale, (3) voicing concerns and 

recommendations for various seismic source zones, 

and (4) suggesting various treatments of these 

zones in application to probabilistic hazard maps.

Prior to each workshop, participants were asked 

to ponder a number of considerations in relating 

geological, geophysical, and seismological information to seismic source zones. These considerations 

are enumerated here as they were the focus of 

the discussions at the workshops.

1. A clear distinction should be drawn (or at least 

attempted) between estimating ground motion in 

the short term (say 50 years) and the long term 

(say 10,000 years). To what extent can our present 

understanding of regional seismotectonics be applied to this problem?

2. Will the spatial distribution of earthquake activity in the region remain stationary or change in 

the next 50 years, or in the next 10,000 years? 

If there are changes, what will be the directions 

and rates? Is the Neogene tectonic history of use 

to predict changes in the distribution of earthquake activity?

3. With regard to the previous question, will large 

shocks recur within that zone in the next 10-500 

years or will other areas become active? What 

other areas might become active?

4. To what extent is it now possible to estimate 

frequency of occurrence of earthquakes in a region 

by means other than the magnitude distribution 

of earthquakes (for example, by using data on average rates of slip on faults)?

5. Can a relationship be drawn between the age 

of mapped faulting in various parts of a region 

and (1) current seismicity, (2) seismicity in the 

next 50-500 years, and (3) seismicity in the next 

10,000 years? Is there a difference in the preceding relationships for (1) earthquakes of magnitude 

M<6.5, and (2) earthquakes M>6.5? 

6. Can faults in different parts of the region be 

characterized by differences in style, such as, in

length, continuity, mode of displacement, or in location with respect to range fronts or within basins, and so forth? If differences can be recognized, is there any relationship to seismicity patterns or size of earthquakes? 

7. Is there strong evidence that various structural, 

tectonic, or geophysical features, including average elevation, heatflow, gravity or magnetic gradients, or volcanic centers, are correlative with 

seismicity and, therefore, indicate constraints on 

the distribution of seismicity?

Zones resulting from these considerations are 

shown in figures 3, 5, 7, 9, and 11. The zones 

are numbered consecutively (1 through 75) in accord with the chronological order of the workshops 

for easy reference to the source zone descriptions. 

Some source zones are duplicated among workshop summaries and tables 1, 2 and 3 because of 

the overlap of the three western United States 

regions (see fig. 1). Information requested from 

the workshops, other than outlines of source 

zones, were estimates of maximum magnitude and 

recurrence of large events for the zones. This information is summarized in tables 1 through 5. 

Geologic data that support these estimates are 

available for some zones; for other zones for which 

little or no pertinent data are available, estimates 

of the magnitude and recurrence are made relative 

among the zones. Outside the Basin and Range 

and the New Madrid regions, these estimates, although intuitive, still are informative when made 

by a group of knowledgeable researchers. Interestingly, an opinion as to the reasonableness of 

areas belonging to the same (or different) zones 

commonly is based on an intuitive distinction of 

both maximum magnitude and earthquake recurrence among different areas.

We must point out that the zones derived from 

the workshops are not necessarily the zones used 

in the national seismic hazard maps (Algermissen 

and others, 1982). A brief description of those 

zones accompanies the maps.

The configurations of the source zones in this 

report represent current thinking in seismic regionalization and can be expected to change in the 

future as research pertinent to the subject 

evolves. Therefore, just as important as the exact 

zone configurations outlined here, is the need to 

indicate the current trends in thinking. Discussions at the workshops covered a wide scope, 

ranging from detailed accounts of geologic data 

about earthquake occurrences on a particular fault
to relatively unconstrained speculation on regional 

tectonics and underlying earthquake causes. Indeed, participants were encouraged to range as 

far as they could to construct a reasonable 

hypothesis. The detailed, better founded hypotheses are represented in the source zone maps and 

are documented in the accompanying source zone 

descriptions. They are, as would be expected, the 

information that the groups were comfortable 

using as a basis for zoning. Many other ideas that 

related to uncertain geologic associations with 

seismicity, or ideas that may have involved 

speculative causal mechanisms are not represented in the source zone maps. However, each 

idea has some degree of merit, and, limited as it 

might be at present in scope or application, with 

further study may result in a useful zoning principle. Accordingly, the "Workshop Summary" sections in this report reflect the nature of these 

more speculative ideas and discussions.

The source zones described herein represent 

solely the views of the various committees. They 

do not represent the authors' viewpoints nor are 

they to be interpreted as an official position of 

the U.S. Geological Survey.

ACKNOWLEDGMENTS

Thanks are due to all the participants of the 

workshops for their contribution in this effort, 

particularly for their willingness to supply or to 

discuss unpublished data and ideas. Their unselfishness in this respect has led to the use of the 

most up-to-date information for delineation of seismic source zones. The following workshop summaries and zone descriptions have been excerpted, 

either in part or in whole, from workshop summaries prepared by the Recorder of each workshop. The Recorders' extra efforts are greatly appreciated as are the efforts of Frank McKeown 

for organizing the workshops.

A special thanks is extended to Frank 

McKeown, Porter Irwin, Otto Nuttli, John Bell, 

Michael Stickney, Yngvar Isachsen, Gabriel Leblank, William Diment, Anthony Qamar, Allan 

Sanford, S. T. Algermissen, and David Perkins 

for helpful comments during the preparation of 

this text.

GREAT BASIN SEISMIC SOURCE ZONESSUMMARY OF WORKSHOP 

CONVENED OCTOBER 10-11, 1979

By R. C. Bucknam and P. C. Thenhaus

PARTICIPANTS

R. C. Bucknam ....... U.S. Geological Survey (USGS)

(Moderator and Recorder) 

S. T. Algermissen ........................ USGS

R. E. Anderson ......................... USGS

W. J. Arabasz .................. University of Utah

J. W. Bell ...... Nevada Bureau of Mines and Geology

L. S. Cluff ............. Woodward-Clyde Consultants

D. M. Perkins ........................... USGS

A. S. Ryall .................. University of Nevada

P. C. Thenhaus .......................... USGS

R. E. Wallace ........................... USGS

C. M. Wentworth ........................ USGS

M. L. Zoback ........................... USGS

WORKSHOP SUMMARY

A substantial part of the workshop was spent 

discussing the significance of spatial and temporal 

variation of seismicity along both the Nevada Seismic Zone (NSZ) and the Wasatch fault (see fig. 

2 for location). Ryall (1977) argued two points that 

were reiterated at the workshop: (1) that, based 

on the distribution of late Quaternary faulting in 

Nevada, the seismic activity in the western Great 

Basin migrates with time, and (2) that the sites 

of large historic events are probably the least 

likely areas to have large events in thejuture 

based on the distribution and recurrence characteristics of recent seismicity in the vicinity of the 

past large events. A clear division of opinion 

existed as to whether the present state of knowledge of the seismotectonics in the western Great 

Basin allows delineation of areas where seismic activity could be expected to increase in the future 

as opposed to areas where seismicity could be expected to decrease. Seismologic data indicate a decreasing rate of low-magnitude seismicity along 

faults that have had historic rupture, indicating 

that perhaps the microseismicity is part of a waning aftershock pattern. Also, clusters of seismicity 

are prominent at the ends of certain faults that 

have late Quaternary but not historic displacement. Other late Quaternary faults, however, have 

moderate levels of seismicity along their length, 

perhaps indicating stress accumulation on these
41°

CALIFORNIA NEVDA

SEISMIC 

ZONE

STILLWATER R ANGE

_____I 

FIGURE 2. Index map of the geographic region addressed at the Great Basin seismic source zon

geologic, and geographic features that are referenced in the text. Two-degree topographic qua

nary faults have been mapped. Dashed line indicates geographical extent of the region considere
particular faults and possibly impending fault rupture. Although the concept generally was accepted 

as having merit, the unclear nature of the seismotectonic cycle in the western Great Basin precluded its use in a form of detailed zoning of individual faults. The high incidence of historic faulting in the NSZ is apparently without precedent 

in Holocene time, indicating perhaps that rather 

broad zones in the western Great Basin are "uniformly" activated but only for geologically short 

periods of time. The concept of zones of reactivation that contain a number of faults involves still 

other considerations for determining temporal and 

spatial variation of seismicity. A suggestion was 

made that perhaps the NSZ has run the course 

of a seismic cycle in the western Great Basin and, 

therefore, that large earthquakes should not be 

expected there in the future. An objection was 

that large events could not be precluded on rangebounding faults next to those faults that have ruptured in historic time. Also, how likely are areas 

outside the NSZ but still within the area of Holocene faulting to be reactivated in the near future? 

Discussions at the workshop did not resolve any 

of the myriad questions raised through considerations of spatial and temporal variations of seismicity in the western Great Basin. The source zones 

(fig. 3) do not represent any special consideration 

along the Nevada Seismic Zone.

Along the Sierra Nevada-Great Basin boundary 

zone (SNGBZ), west of the Nevada Seismic Zone, 

Ryall and Van Wormer (1980) noted that existing 

seismic hazard maps do not show as high a hazard 

as fault-scarp studies, rate of uplift, and instrumental seismicity indicate (see Slemmons and 

others, 1979; Bell and Slemmons, 1979; Sanders 

and Slemmons, 1979). This area is of considerable 

significance as it includes the Reno-Carson City 

area which is the most heavily populated area in 

western Nevada. Ryall and Van Wormer (1980) 

have demonstrated reasonably the low estimation 

of ground acceleration hazard along the SNGBZ 

in existing hazard maps (Algermissen and Perkins, 

1976; Applied Technology Conference, 1978) and 

therefore have called for a reconsideration of these 

hazard maps for that region.

The Wasatch fault at the east margin of the 

Basin and Range province in Utah poses two problems that were discussed at the workshop. First, 

the entire fault zone has evidence of late Quaternary displacement, and at two sites shows evidence of repeated Holocene movement (Swan and 

others, 1980). These Holocene displacements

reasonably can be inferred to represent M>7 

events, yet in historic times no earthquake 

larger than M=5.5 has occurred on the fault 

(Arabasz and others, 1980). Second, two areas 

along the fault show anomalously low instrumentally detected seismic activity (Arabasz and 

others, 1980; Smith and Sbar, 1974; Smith, 1972) 

but these same areas have geologic evidence of 

repeated Holocene movement (Swan and others, 

1980). The implication is that seismicity is periodic 

along the fault. Pavlis and Smith (1980) have 

suggested that the Wasatch fault is segmented 

and bounds independent crustal blocks along its 

length. Possibly, block adjustments could control 

the spatial character of the seismicity along the 

Wasatch fault.

Arabasz and others (1980) provided an excellent 

discussion of considerations involving seismic 

"gaps" along the Wasatch fault. They distilled the 

arguments down to the fact that if seismicity is 

periodic along the Wasatch fault, the seismic cycle 

has yet to be defined with any confidence. Available geologic data are not adequate to determine 

whether seismic potential is increasing or decreasing along individual segments of the fault.

The primary seismic zonation of the Basin and 

Range region has been on the basis of broad regions that are relatively homogeneous with respect to the age of most recent surface faulting 

within certain broad age categories (Holocene, no 

Holocene/late Pleistocene, no late Quaternary). 

The zones can be inferred to represent distinctive 

regional variations in the rate of occurrence of 

large (M>7) earthquakes as averaged over 

thousands of years (Bucknam and others, 1980). 

The approach used does not preclude the presence 

of a younger fault within a region of predominantly older faults. Individual faults can be defined 

as source zones where the details of their late 

Quaternary history are known well enough to assign a specific magnitude and recurrence interval 

to them. The recurrence estimates in table 1 are 

minimum estimates. The recurrence estimates of 

events assumed to be M>7 are made on what is 

thought to be a nearly complete record of these 

events to the extent that the events are preserved 

in the geologic record as fault scarps.

DESCRIPTION OF INDIVIDUAL SOURCE ZONES

For documentation of the ages of faulting in 

western Utah and northeastern Nevada, the 

reader is referred to figure 2 on which is outlined
areas of I°x2° topographic quadrangle maps in commun., 1980; T. P. Barnhard and R. C.

which morphometric scarp studies have been made Bucknam, written commun., 1980; Bell, 1981). The

(Bucknam and Anderson, 1979b; Anderson and seismic source zones as defined at the workshop

Bucknam, 1979; R. L. Dodge and others, written are shown in figure 3. Table 1 lists estimated

120° 115° 110°

100 200KILOMETERS

FIGURE 3. Seismic source zones of the Great Basin. Numbers refer to descriptions of seismic source zones given in text. 

Unconnected lines indicate that boundary continues beyond the area considered at the Great Basin workshop. Dashed 

line indicates geographical extent of the region considered in the workshop.
TABLE 1. Estimated maximum magnitudes and recurrence 

rates from geologic data for earthquakes Ms>7 in the 

Great Basin seismic source zones

[Ms, surface wave magnitude; leaders ( ) indicate that no value was given at 

the workshop]

Zone No.

(from

fig. 3)

1 

2

3

4

5

6

7

8

9

10

11

12

13

14

Estimated

maximum

magnitude

(Mg )

6.5

7.0

 

7.5

8.0

7.5

__

7.5

 

7.5

 

7.75

 

Estimated geologic

rate for earthquakes

M >7.0 given as

number of events per

10 years per 10

square kilometer

 

 

 

 

"l~T, 2 3.4

1 0.7

 

*2.4

 

 

 

For western Utah zones from Bucknam and others (1980).

o

For Wallace (1977) study area shown on figure 1.

maximum magnitudes and geologic recurrence 

rates for events M>7.0 in the zones.

Zone 1. A zone of late Quaternary faulting located along the western flank of the Sierra 

Nevada.

Zone 2. A zone of no late Quaternary faulting.

Zone 3. A zone of late Quaternary faulting 

north of the Sierra Nevada.

Zone 4. Central Sierra Nevada zone. Late Cenozoic deformation in this zone is restricted to 

broad uplift and westward tilting in the northern 

part with more intense warping and tilting in the 

southern part (Christiensen, 1966).

Zone 5. This zone trends northerly along the 

Sierran front along a series of en-echelon warps 

and faults to the vicinity of Reno, Nev. A basis 

for the western boundary was poorly defined; in 

a general way, the zone encloses the western extent of the en-echelon segments. The northwestern and southeastern boundaries of the zone were 

supplied by J. W. Bell (written commun., 1981).

Zone 6. This zone includes Owens Valley, 

Panamint Valley, and Death Valley. The boundary 

within the Sierran block has been drawn along the 

"frontal fault" to the vicinity of Bishop, Calif. 

From Bishop north, the Sierran front diverges 

from a linear north-northwest trend and steps 

westerly on a series of warps and en-echelon 

faults. (See Bateman, 1965, for a good account of

the structural style in this area.) The northern 

part of the boundary extends to the west side of 

the White Mountains and then joins the northern 

end of Fish Lake Valley.

Zone 7. This zone includes the Nevada Seismic 

Zone (NSZ) which, except for the historic activity, 

does not appear to have been more active in Holocene or late Quaternary time than the rest of the 

region. The zone boundary represents a region 

within which Holocene faulting is widely distributed. Note, though, that determination of faulting 

as Holocene outside the region studied by Wallace 

(1979; see fig. 1) is largely subjective and is not 

well documented. The rate of occurrence of magnitude 7-7.5 events within Wallace's study area 

is 3.4/104 yr/104 km2. Wallace believed that the 

density of Holocene age scarps in his study area 

is at least as great, or perhaps greater, than that 

of the entire region. The rate of faulting, therefore, may represent an upper bound. The small 

branch of zone 7 extending from western Nevada 

into northeastern California includes the Walker 

Lane in zone 7. Holocene age faulting is recognized along the Walker Lane (J. W. Bell, written 

communs., 1981, 1982).

The significance of the Nevada Seismic Zone received considerable discussion but was not resolved definitively. Ryall argued that the occurrence of major events in the NSZ has lowered the 

likelihood of large earthquakes in the rupture 

zones of these events (Ryall, 1977; Ryall and Van 

Wormer, 1980). In general, the group believed 

that the concept was of merit but some would prefer to show some indication of the high rate of 

historic seismicity. One participant believed that 

large events cannot be precluded on other major 

faults adjacent to the zone of large historic events, 

such as the west side of the Stillwater Range. The 

point clearly was not resolved, although there was 

not strong pressure to show the Nevada Seismic 

Zone as a separate zone. The map was left with 

much of western Nevada as a single zone.

Zone 8. A zone of no late Quaternary faulting.

Zone 9. This zone is defined on the basis of 

widespread faults of late Quaternary age. Holocene faulting is not known but may be present. 

Faults here tend to be short; Wallace believed that 

the maximum magnitude expected for this region 

shoald be less than that of zone 6 to the west.

"Zone 10. A zone of no late Quaternary faulting. (Zones 10 through 14 are from Bucknam and 

others, 1980.)

8
Zone 11. A zone of Holocene faulting. 

Zone 12. A zone of no Holocene, but late Pleistocene faulting.

Zone 13. Wasatch fault, multiple Holocene 

movement. 

Zone 14. A zone of Holocene faulting.

NORTHERN ROCKY MOUNTAINS

SEISMIC SOURCE ZONES SUMMARY

OF WORKSHOP CONVENED

DECEMBER 6-7, 1979

BY F. A. McKeown, D. C. Ross, and P. C. Thenhaus

PARTICIPANTS

F. A. McKeown .............. U.S. Geological Survey (USGS)

(Moderator) 

D. C. Ross .......................................................... USGS

(Recorder) 

S. T. Algermissen ................................................ USGS

R. C. Bucknam .................................................... USGS

J. M. Dewey ........................................................ USGS

M. D. Kleinkopff .................................................. USGS

L. C. Pakiser ....................................................... USGS

D. M. Perkins ...................................................... USGS

T. A. Qamar ................................ University of Montana

M. W. Reynolds ................................................... USGS

W. P. Rogers ....................... Colorado Geological Survey

R. G. Schmidt ...................................................... USGS

P. C. Thenhaus .................................................... USGS

C. M. Wentworth ................................................. USGS

(Observer) 

I. J. Witkind ....................................................... USGS

WORKSHOP SUMMARY

Systematic regional mapping of Quaternary 

faults in the Northern Rocky Mountains region 

has not been done. However, Witkind (1975a, b, 

1976) has summarized information from many 

sources on late Cenozoic faulting. Pardee (1950) 

mapped displacements of range-bounding faults in 

western Montana of late Tertiary to recent age, 

but many of these scarps need to be reevaluated 

in light of the new understanding of fault-scarp 

morphology and methods of study (Wallace, 1977; 

Bucknam and Anderson, 1979a, b). If a systematic 

search were made for young faults, probably many 

more would be found, and ages of faulting could 

be revised on a number of known faults.

Because of this lack of regional faulting information, workshop participants adopted a zoning 

rationale based on information on the age of 

youngest faulting within regions of similar tectonic 

setting and structural style. This approach is not

zoning on the spatial extent of different ages of 

latest displacements as is true for the Basin and 

Range province. Instead, the spatial extent of a 

particular age of faulting (known from specific 

faults within a distinctive structural region) is inferred by assuming that the age of faulting is 

characteristic of a region of similar structure. Emphasis is placed on the spatial distribution of historic activity in drawing only one zone zone 21. 

The remaining zones generally conform to distinctive structural terranes under the assumption that 

distinctive structural terranes also have different 

seismotectonic characteristics that govern present-day seismicity.

Considerable discussion was devoted to the significance of the Lewis and Clark line (fig. 4). Features defining the lineament are the St. Marys 

fault trend and parallel faults to the south. These 

faults extend across western Montana and into 

Idaho (Witkind, 1977). There is evidence of right 

lateral movement of Pleistocene age on the St. 

Marys fault; sand boils and faulted talus cones are 

evident along its trace. Some strike-slip movement 

of similar age on faults paralleling the St. Marys 

trend is suspected. The zone bends to the south 

near Helena where there has been historic seismicity (Eppley, 1965; Stickney, 1978). The 1935 

earthquake had a strike-slip focal mechanism 

(Smith and Sbar, 1974) and is consistent with 

geologically inferred type of fault movement. Pardee (1950) stated that the 1935 series of earthquakes (M=6.25, M=6.7) at Helena, Mont., probably were associated with a fault at the south end 

of Prickly Pear basin as indicated by surface effects and locations from instrumental records. He 

noted, though, that no surface displacement was 

found from the earthquakes. Pardee (1926) located 

the 1925 event (M=6.75) 80 km southeast of 

Helena. This location coincides with a projection 

of the St. Marys trend to the southeast. Freidline 

and others (1976) defined a northwest-trending 

zone of seismicity between Helena and Marysville 

that had both strike-slip and normal faulting indicated by focal mechanisms. They suggested an association on a regional scale between seismicity 

and northwest-trending zones of weakness in the 

basement. One such zone, the Lake Basin lineament, coincides with a possible extension of the 

Lewis and Clark line to the southeast and reasonably might extend the structure into the northern 

parts of Wyoming. Stickney (1978) also noted a 

northwest trend in earthquakes of magnitude

9
116" 114' 112' 110' 108* 108° 104° 102°

FIGURE 4. Index map of the geographic region addressed at the Northern Rocky Mountains seismic source zone 

workshop showing selected topographic, geologic, and geographic features that are referenced in the text. 

Dashed line indicates geographical extent of the region considered in the workshop.

greater than 2.5 recorded between July, 1974, and 

March, 1977. The trend is about 80 km wide and 

extends from Helena, Mont., to Flathead Valley. 

Fault-plane solutions and hypocenter distributions 

in that part of the trend covered by the Helena 

seismograph array indicate earthquake swarms on 

northwest-trending normal faults and northeasttrending oblique-slip faults. He noted that the 

lengths of active, continuous faults are short (less 

than 10 km), which suggests faulting along

preexisting zones of weakness that bound crustal 

blocks.

The committee considered the possibility of the 

Lewis and Clark line being a separate source zone. 

The lack of late Quaternary faults north of the 

line suggests that faults there are less recently 

active than are those near Helena; although extensional tectonism has affected both areas north and 

south of the line, it is much more impressive to 

the south. Also, gravity and magnetic signatures

10
differ on either side of the line. An interesting 

fact is that the Marysville geothermal area lies 

along the Lewis and Clark line near Helena. Such 

geothermal anomalies may be indicative of a deep 

crustal flaw that provides an avenue for the ascent 

of hydrothermal fluids. However, the lineament 

itself is not clearly a primary source for historic 

earthquakes. Areal rates of seismic activity to the 

north in western Montana appear to be higher.

Other topics of significant discussion were the 

Snake River Plain, the Uinta Mountains, and the 

seismic source zones of Wyoming.

In southeast Idaho, formation of the Snake 

River Plain has progressed from southwest to 

northeast since Miocene time, and perhaps is related to the gross movement of the North American plate over a mantle thermal anomaly (Suppe 

and others, 1975; Smith, 1978; Christiansen and 

McKee, 1978; Armstrong, 1978). The Yellowstone 

area now forms the advancing front of tectonism 

to the northeast. Strong northwest-oriented gravity gradients in the older, thicker western part 

of the plain indicate that perhaps the plain has 

been superimposed over older northwest-trending 

Basin and Range structures. These structures are 

evident both north and south of the basalt cover 

of the plain. Holocene "pull-apart" structures form 

an arcuate pattern across the plain so as to follow 

the grain of the Basin and Range structures to 

the north and south (Prinz, 1970). These "pullaparts" show no apparent lateral or vertical displacement across them (M. A. Kuntz and H. R. 

Covington, oral commun., 1979). The relatively 

warmer, younger, eastern part of the Snake River 

Plain has been aseismic in historic time. The aseismieity may be due to the young crust being unable 

to store enough elastic energy for the generation 

of earthquakes. The older western part of the 

plain, however, has a thicker crust (see Hill and 

others, 1961; LaFehr and Pakiser, 1962; Hill, 1963; 

Hill and Pakiser, 1967). At Shoshone, Idaho, on 

the western part of the plain, (fig. 4) an intensity 

VII was felt from an event occurring on 

November 11, 1905. Greensfelder (1976) indicated 

that the epicenter probably was located within 24 

km of Shoshone. Not known is what structure may 

have been the earthquake source. In the workshop 

there was a general consensus that the western 

part of the plain has a higher potential for earthquakes than the eastern part because of the different crustal rheologies.

Discussion of the Uinta trend centered around 

its eastern margin. The structure has an anomalous east-west strike as compared with the northwest trends of Laramide-age structures. Geophysical evidence indicates that there is a distinct difference in crustal thickness north and south of the 

trend; therefore, the Uinta trend may be a fundamental crustal structure. Geophysically, the Uinta 

trend can be traced eastward into Colorado to the 

south end of the Park Range, near Steamboat 

Springs. There, the Neogene-age structures of the 

northern extension of the Rio Grande rift swing 

west along the Uinta trend (Tweto, 1979), indicating perhaps a fundamental influence on younger 

tectonism. Quaternary faulting has been noted on 

a north-south oriented uplift extending south from 

the Uinta extension in Colorado (M. West, oral 

commun., 1980). Although the Uinta trend apparently has influenced the structure of northeastern 

Utah and northwestern Colorado for a long span 

of time, its influence on present-day seismicity is 

unclear. Lack of seismological and other supporting evidence precludes the identification of any obvious relationship.

Other discussions centered around the source 

zones of Wyoming. Witkind (1975a) has noted that 

ages of latest displacements of east-trending faults 

in central Wyoming are Pliocene. Similar easttrending faults occur in the southern Bighorn 

Mountains and in the Owl Creek Mountains. Zones 

26 and 27 were drawn on this basis, although there 

was considerable argument as to whether a Pliocene age of latest displacements is significant to 

present-day seismicity. One point of view holds 

that, regardless of the ages of tectonism, the area 

of most recent tectonism in a region should be considered the most hazardous. Another point of view 

holds that any tectonism older than Quaternary 

is, by itself, irrelevant to the relative hazard 

among areas. The argument was not resolved 

clearly, although the zones defined by Pliocene 

faulting were retained.

An alternative zoning procedure suggested by 

some committee members was that the large 

Laramide-age basins be separated from a large 

zone encompassing the central Rocky Mountain region.

R. B. Smith (written commun., 1979) recommended an east-trending zone from Norris Geyser 

Basin in Yellowstone National Park west to 

Hebgen. This zone would include the Centennial 

Valley frontal faults and other east-trending tectonic features along the southern Montana State 

line.

11
DESCRIPTIONS OF INDIVIDUAL SOURCE ZONES

The following seismic source zones are shown 

in figure 5. Table 2 lists estimated maximum magnitudes and estimated relative recurrence times 

of the maximum magnitudes of earthquakes for 

the individual zones.

Zone 15. This zone is ambiguous as it is defined primarily by its contrast with the surrounding zones. On the southwest, the zone boundary 

is drawn along the north edge of the Columbia 

Plateaus province. On the west, the boundary separates this zone from the Cascade Mountains of 

north-central Washington.

Zone 16. This zone encompasses a northwesttrending group of faults that had pre-middle Pleistocene movement. It coincides with an area of 

Basin and Range type normal faulting that may 

be a continuation of the Rocky Mountain trench 

into Montana (Mudge, 1970).

Zone 17. The disturbed belt of western Montana. The zone is characterized by large imbricate 

thrust sheets of the Northern Rocky Mountains 

(Mudge, 1970).

Zone 18. Idaho batholith exclusive of the Challis geothermal area (zone 21).

Zone 19. This zone has a structural style similar to that of zone 20 and similarly, has Quaternary faults. However, the rate of seismic activity 

is noticeably lower than in zone 20 to the east, 

or in zone 16 to the north.

Zone 20. This zone is characterized by mixed 

east-, north-, and northwest-trending structure 

and both normal and strike-slip focal-plane solutions (Smith and Sbar, 1974; Qamar and Hawley, 

1979).

Zone 21. Challis geothermal area. This region 

is characterized by swarm events that typify areas 

of high geothermal anomalies. Faults are present 

in Eocene volcanics. The petrology of the volcanic 

and plutonic rocks is similar to that of the rocks 

in the Yellowstone area and may indicate that the 

Challis area is an old analog to the Yellowstone 

area.

Zone 22. Trenching studies in this zone 

(Malde, 1971) indicated faulting from Pleistocene 

to at least 10,000 years before the present.

Zone 23. This zone is the volcano-tectonic area 

of Yellowstone National Park. The zone has a high 

heat flow and may have a low maximum magnitude.

Zone 24. Western Snake River Plain. The 

western plain has an older, thicker surface than 

the eastern part of the plain.

Zone 25. Eastern Snake River Plain. The surface is generally 20,000-50,000 years old (based 

on studies of undisturbed loess) and has young volcanic features superimposed upon it. Limited 

breakage consisting of pull-apart structures with 

no apparent displacement (Prinz, 1970; M. A. 

Kuntz, and H. R. Covington, oral commun., 1979) 

is evident on this surface. The breaks form an arcuate pattern across the plain (Printz, 1970). The 

aseismicity may be attributable to a warm, thin 

sialic crust incapable of sustaining earthquake 

stresses.

Zone 26. This zone encompasses the Bighorn 

Mountains, Absaroka Range, Wind River Range, 

and the Owl Creek uplift of northwestern Wyoming. The zone excludes the east-trending faults 

that terminate each of these ranges to the south.

Zone 27. This zone includes the east-trending 

faults of central Wyoming that cut late Pliocene 

but not Pleistocene sediments in the Picket Lake 

area. Faults in the southern Bighorns and the Owl 

Creek area have much the same pattern. The area 

of east-trending faults may be potentially more 

hazardous than the rest of Wyoming (see previous 

discussion), even though youngest documented 

movement is Pliocene.

Zone 28. The boundary of this zone is poorly 

constrained, but structural style here is different 

from that of the Snake River Plain to the north. 

The north-northwest-trending faults in the zone 

apparently had no late Quaternary'movement.

Zone 29. Faults in this area have the same 

Basin and Range character as those north and 

south of the Snake River Plain. Southeast Idaho 

has well-developed extensional tectonic features. 

This north-south-trending, normal extensional 

faulting extends south past the west end of the 

Uinta Mountains in Utah.

Zone SO. No distinction was made in this zone 

between age of faulting or tectonic style.

Zone 31. Bear Lake area (Utah). Age of faulting is Holocene. On the basis of scarp heights of 

more than 1 m and on lengths of fault segments, 

magnitude 7 + earthquakes can be expected in this 

zone.

Zone 32. Wasatch fault zone. Multiple Holocene offsets are evident along the fault.

Zone 33. Uinta Mountains. Geophysical data

12
115°

0 100 200 MILES 

11 i

x 0 100 200 KILOMETERS

40° -

FIGURE 5. Seismic source zones of the Northern Rocky Mountains region. Unconnected lines indicate a continuation of the 

boundary beyond the area considered at the workshop. Dashed line indicates geographical extent of the region considered 

in the workshop.

13
TABLE 2. Estimated maximum magnitudes (Mg) and estimated relative recurrence of maximum 

magnitude events for the source zones of the Northern Rocky Mountains region

[Mg, surface wave magnitude; leaders ( ) indicate that no value was given at the workshop]

Zone No.

(from

fig. 5)

15

16

17

18

19

20

21

22

23

24

25

26

27

Estimated

maximum

magnitude

(Mg )

6.0

6.5-7.0

6.0

5.0

7.0

7.5

6.0-6.5

7.5

6.5

6.5

6.0

6.0

6.0

Estimated

relative

recurrence

C

B

C

D

B

A

B

B

A

C

C-D

C

C

Zone No.

(from

fig. 5)

28

29

30

31

32

33

34

35

36

37

38

39

Estimated

maximum

magnitude

(Mg )

 

6.5-7.0

6.0

7.0+

7.75

7.0

6.5-7.0

6.5

6.5

6.5

7.0-7.5

6.5

Estimated

relative

recurrence

 

B

D

A

A+

C

AC-D

C

C

A-B

 

A relative scale agreed upon by committee. A+ is assigned to the 

Wasatch fault zone as the zone of highest likelihood of seismic events of 

maximum estimated magnitude during a 50-year exposure time. Decreasing in 

likelihood are categories A, B, C, and D. The following numerical values were 

assigned by committee consensus: A=40, B=10, C=3, and D=l. In other words, a 

zone rated A would have 40 times higher likelihood of a maximum estimated 

magnitude earthquake than a zone rated D within the return period.

indicate different crustal thickness north and 

south of an eastward extension of the Uinta Mountains. Also, Neogene structures bend westward 

along an extension of the Uinta Mountains to the 

south end of the Park Range in Colorado. The Rio 

Grande rift structures to the south terminate at 

this eastward extension. Historic seismicity provides no compelling evidence that these features 

control present-day seismicity.

Zone 34. This zone encompasses a north-southtrending group of normal, extensional faults that 

terminate the Colorado Plateaus to the northwest. 

The faults extend beyond the west end of the 

Uinta trend.

Zone 35. This zone is defined by the Mullen 

Creek-NashiFork shear zone (Houston and McCallum, 1961) that juxtaposes strongly contrasting 

Precambrian rocks. This feature is a fundamental 

basement trend that may represent an old plate 

boundary. The zone includes a possible extension 

of the feature to the northeast, the Hartville fault 

(Droullard, 1963).

Zone 36. This zone is a subdivision of the Park 

Range at the north end of the Sangre de Cristo

Range. Quaternary-age fault displacements are 

evident in the zone.

Zone 37. This zone is the Front Range of the 

Northern Rocky Mountains. Quaternary-age fault 

displacements are evident in the zone.

SOUTHERN ROCKY MOUNTAINS

SEISMIC SOURCE ZONES SUMMARY

OF WORKSHOP CONVENED

JANUARY 23-24, 1980

By R. E. Anderson, W. P. Irwin, and P. C. Thenhaus

PARTICIPANTS

R. E. Anderson .............. U.S. Geological Survey (USGS)

(Moderator) 

W. P. Irwin ......................................................... USGS

(Recorder) 

S. T. Algermissen ................................................ USGS

E. H. Baltz ......................................................... USGS

R. C. Bucknam .................................................... USGS

J. W. Dewey ....................................................... USGS

S. M. Dubois ....... Arizona Bureau of Geology and Mining

Technology 

L. H. Jaksha ....................................................... USGS

14
M. N. Machette ................................................... USGS

F. A. McKeown ................................................... USGS

W. R. Muehlberger ........... University of Texas at Austin

D. M. Perkins ...................................................... USGS

W. P. Rogers ....................... Colorado Geological Survey

A. R. Sanford ......... New Mexico Institute of Mining and

Technology 

G. R. Scott .......................................................... USGS

P. C. Thenhaus .................................................... USGS

0. G. Tweto ........................................................ USGS

M. West ................. Water and Power Resources Service

R. L. Wheeler ..................................................... USGS

WORKSHOP SUMMARY

As in the Northern Rocky Mountains, systematic morphometric studies of fault scarps have not 

been made throughout the Southern Rocky Mountains. The age-of-faulting information that is available has been assumed to hold throughout distinctive tectonic areas, the boundaries of which included consideration of basin and mountain blocks, 

alinements of volcanic centers, possible buried 

magma chambers, basement configuration, and 

physiography.

Expression of late Paleogene and Neogene tectonic movements in reactiviated older structures 

is widespread in the Southern Rockies (Tweto, 

1979; Kirkham and Rodgers, 1978, 1981). The 

most impressive tectonic movements, however, 

are associated with the formation of the Rio 

Grande rift (see fig. 6 for location). The structure 

of the Rio Grande rift and its associated seismicity 

therefore dominated the discussions of seismic 

source zones in this region.

Although significant faulting has occurred within the rift in late Quaternary time, the low level 

of recent seismicity within the rift does not appear 

to represent sustained long-term regional tectonic 

activity. This observation has prompted Cordell 

(1978) and Sanford and others (1979) to suggest 

that the current low level of seismicity is not representative of the long-term trends. From microseismic data collected in recent years, Sanford and 

others (1979) noted that the current seismicity 

level in the rift is comparable to those levels of 

the neighboring Colorado Plateaus and High 

Plains provinces. Sanford and others (1979) have 

related much of the current microseismicity to 

movement of magma in the crust. However, they 

noted that the historic record of earthquakes from 

1849 through 1961 indicates that the Rio Grande 

rift was by far the most active area in New 

Mexico. Their listing of six intensity VII events 

shows that all these events occurred within the

rift. Earthquakes of this large size probably are 

reported completely from 1869 and therefore 

should be free of biased reporting due to nonuniform population density. The period 1962-1977 

is one of low seismicity compared with the seismicity of previous years. Sanford and others (1979) 

concluded that the seismicity in the rift is episodic, 

related perhaps to episodic spreading of the rift.

Sanford and others (1981) noted that on the Colorado Plateau in New Mexico, seismic activity is 

relatively high along the eastern and northwestern 

margins of the San Juan basin. Also, an epicentral 

trend crosses the southern part of the Colorado 

Plateau and extends through the Rio Grande rift 

into the High Plains of northeastern New Mexico. 

The trend is coincident with a trend of Pliocene 

to Pleistocene volcanic centers that define the 

Jemez lineament.

DuBois and Smith (1980) recently have published the results of their investigation of the 1887 

(estimated M=7.2) Sonoran earthquake. The epicenter was about 60 km south of Douglas, Ariz., 

in the State of Sonora, Mexico. Surface rupturing 

extended about 50 km north of the epicentral area. 

A critical task for seismic zoning is to define the 

area over which a similar event might reasonably 

be expected in the future. Sumner (1976) has 

noted a seismicity trend that extends from the 

area of the Sonoran event northwestward across 

the San Francisco volcanic field and into the western Grand Canyon region of Arizona (fig. 6). However, it remains to be established that seismic potential is similar along the entire seismicity trend 

that includes such a diversity of geologic structure. The workshop participants chose a geologic 

approach to zoning this area as described in the 

following descriptions of source zones.

DESCRIPTION OF INDIVIDUAL SOURCE ZONES

The seismic source zones as they were defined 

at the workshop are shown in figure 7. Table 3 

lists maximum estimated magnitudes and relative 

recurrence estimates of the maximum magnitudes 

for the zones of the Southern Rocky Mountain region.

Zone 38. Sangre de Cristo Range. The zone 

contains the Sangre de Cristo fault that bounds 

the San Luis Valley on the northeast. The fault 

may be potentially active (Kirkham and Rogers, 

1978, 1981).

Zone 39. Uncompahgre Plateau and San Juan 

volcanic field. The northeastern and southwestern

15
102

SAN FRANCISCO 

VOLCANIC FIELD

FIGURE 6. Index map of the geographic region addressed at the Southern Rocky Mountains seismic source zone workshop 

showing selected topographic, geologic, and geographic features that are referenced in the text. Dashed line indicates 

geographical extent of the region considered in the workshop.

flanks of the Uncompahgre Plateau are structurally complex. On the northeast flank, only one 

fault can be proved to have moved in Quaternary 

time (Kirkham and Rogers, 1978) although other 

faults are suspect. Quaternary faults also exist on

the southwestern flank (Kirkham and Rogers, 

1978).

Zvne 40. Golden fault. Kirkham (1977) 

documented evidence of recurrent Quaternary displacement on the fault. If the small size of the

16
110° 105°

40

35'

30'

100 200MILES

0 100 200 KILOMETERS

FIGURE 7. Seismic sources zones of the Southern Rocky Mountains region. Unconnected Unes indicate that boundaries extend 

beyond the area considered at the workshops. Queried lines indicate uncertainty that the zone is a significant seismic 

source. Dashed line indicates geographical extent of the region considered in the workshop.

zone limits its usefulness to seismic zoning, it Zone 41. Rampart Range fault. Delineation of

could be deleted in favor of a larger zone of this zone is based on evidence of Quaternary disQuaternary faulting that covers all the Colorado placement along the fault (Kirkham and Rogers,

Front Range. 1978, 1981).

17
TABLE 3. Estimated maximum magnitudes (MJ 

and estimated relative recurrence of maximum 

magnitude events for the source zones of the 

Southern Rocky Mountains region

[Ms, surface wave magnitudes; leaders ( ) indicate that no values 

were given at the workshop]

Zone No. 

(from 

fig. 7)

38 

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

Estimated 

maximum 

magnitude 

(Mg )

7.0-7.5 

6.5

 

 

6

7.0-7.5

7.0-7.5

 

6.0

6.0

 

7.0-7.5

7.0-7.5

7.0-7.5

6.5-7.0

5.5-6.0

6.0

Relative 

recurrence 

rate 1

A 

D

 

 

C

B

A

 

C

C

 

A

A

D

B

D

D

Relative recurrence is based on a 

dimensionless scale and indicates a 

subjective likelihood of a zone having 

the maximum magnitude earthquake take 

place in a 50-year period of interest.

Zone 42. Jemez lineament, east of the Rio 

Grande rift. The zone includes the Cimarron and 

Raton-Clayton-Capulin volcanic fields. In terms of 

magnitude and frequency of earthquakes, this 

zone is inferred to be similar to zone 48.

Zone 43. This is an elongate zone that follows 

the trend of the Rio Grande rift and includes the 

Trans-Pecos area of west Texas. Evidence for late 

Quaternary faulting does not exist everywhere in 

this zone, but in some parts the evidence is widespread. In the Albuquerque-Belen basin, Bachman 

and Machette (1977) have identified a widespread 

geomorphic surface that is 5-6 xlO6 years old. This 

and younger surfaces are displaced by perhaps a 

hundred faults including scarps along about 50 km 

of fault trace at the eastern margin of the Albuquerque basin. Detailed studies by Machette 

(1978) indicated at least four displacement events 

during Quaternary time on a single fault in the

northern Albuquerque basin. Although systematic 

morphometric studies have not been made, widespread, easily recognized scarps in the TransPecos area (Muehlberger and others, 1978) 

suggest late Quaternary faulting there also. Justification for the placement of the zone boundaries 

comes, in part, from recognition of the Rio Grande 

rift as a series of structurally integrated basins 

that have an overall integrity and continuity imposed across a wide variety of older structures. 

The boundaries of the Trans-Pecos part of the 

zone are determined by the distribution of a coherent system of north- and northwest-trending 

scarps. Linear dry lakes in that region parallel the 

scarp trends and suggest perhaps a Holocene 

structural control possibly by faulting.

Zone 44. Evidence for Holocene faulting in 

this zone is mainly from studies by Michael 

Machette who has found evidence for offset of Holocene deposits of as much as 6 m on the La Jencia 

fault a major basin-margin structure. The fault 

shows evidence of young rupture for 35 km but 

not all the rupture may be related to a single seismic event (Machette, 1980). A. R. Sanford (oral 

commun., 1980) noted that seismic activity has not 

been detected along the La Jencia fault and that 

a geodetic network installed across the fault by 

J. Savage showed no indication of extension.

Zone 45. Delineation of this zone is based on 

a lack of evidence for late Cenozoic faulting in the 

central part of the Colorado Plateaus province.

Zone 46. This zone is the Jemez lineament 

west of the Rio Grande rift. The zone is drawn 

on an alinement of Pliocene-Holocene volcanic centers that extends diagonally across Arizona and 

New Mexico.

Zone 47. Delineation of this zone is based on 

evidence of minimal faulting in Pleistocene lavas 

in the San Francisco volcanic field (E. Wolfe and 

G. Ulrich, unpub. data). A few faults in the northern part of the volcanic field may have moved during late Quaternary time, but they average only 

about 2 km in surface trace and have small displacement. In general, there is a conspicuous lack 

of large-magnitude faulting associated with volcanism in the San Francisco field. Infrequent large 

seismic events probably have an upper-bound 

magnitude of 5.5-6.0.

Zone 48. Delineation of this zone is based on 

a lack of evidence for late Quaternary faulting 

within a tectonic province that was deformed by 

late Cenozoic faulting.

18
Zone 49. A recently published study by 

DuBois and Smith (1980) examined about 90 percent of the historic record of the 1887 Sonoran 

earthquake (estimated M=7.1). The earthquake 

produced surface breakage over a distance of 

about 50 km. Studies of young faulting in that area 

are in progress. Geomorphic evidence suggests the 

possibility of four events on the 1887 earthquake 

fault, but whether a prehistoric Holocene event 

occurred is not known. The zone is extended into 

Arizona on the basis of the possibility of Holocene 

surface faulting in southeasternmost Arizona according to R. C. Bucknam and S. M. DuBois (oral 

commun., 1980).

Zone 50. Evidence for Holocene faulting in 

this zone is based mainly on unpublished studies 

by L. Gile, who first demonstrated the presence 

of mid-Holocene faulting, and by Machette (1980) 

who has found evidence for 3-4 m of displacement 

where the Cox Ranch fault trace crosses Holocene 

fans. The mid-Holocene scarp is locally 9-10 m 

high and the fault shows evidence of breakage for 

at least 25 km (W. Seager, written commun., 

1980). Faults other than the La Jencia and Cox 

Ranch in zones 44 and 51 have "reported" but undocumented Holocene displacement.

Zone 51. This zone includes an area of the 

Trans-Pecos, Texas, for which no fault scarps cutting Quaternary units have been identified but 

which has major late Cenozoic faults cutting rocks 

of early Oligocene age and older. It is similar to 

zone 49.

Zone 52. This,zone includes the terrane commonly referred to as the western Grand Canyon 

region in the transition zone between the Basin 

and Range and Colorado Plateaus provinces. The 

location of the boundary is not narrowly constrained. Evidence for Quaternary faulting relates 

to (1) Quaternary lava flows that are offset by 

widely spaced normal faults in the western Grand 

Canyon region (Hamblin, 1970; Koons, 1945; Anderson, 1978), (2) offset of Quaternary alluvium 

in northwestern Arizona (I. Lucchitta, written 

commun., 1980), and (3) displacement of Quaternary alluvium in Chino Valley (I. Lucchitta, oral 

commun., 1976).

Zone 53. Delineation of this zone is based on 

lack of evidence for late Cenozoic faults.

Zone 54. Delineation of this zone is based on 

lack of evidence for Quaternary faulting, although 

the area was deformed in late Cenozoic time.

CENTRAL INTERIOR SEISMIC SOURCE

ZONES SUMMARY OF WORKSHOP

CONVENED JUNE 10-11, 1980

By F. A. McKeown, D. P. Russ, and P. C. Thenhaus

PARTICIPANTS

F. A. McKeown .............. U.S. Geological Survey (USGS)

(Moderator) 

D. P. Russ .......................................................... USGS

(Recorder) 

S. T. Algermissen ................................................ USGS

R. E. Anderson ................................................... USGS

R. C. Bucknam .................................................... USGS

T. C. Buschbach .................... Nuclear Regulatory Agency

A. J. Crone ......................................................... USGS

D. W. Gordon ...................................................... USGS

R. M. Hamilton .................................................... USGS

T. G. Hildenbrand ................................................ USGS

A. C. Johnston . Tennessee Earthquake Information Center 

M. F. Kane ......................................................... USGS

G. R. Keller .................. University of Texas at El Paso

F. J. Mauk ................................. University of Michigan

0. W. Nuttli ................................... St. Louis University

D. M. Perkins ...................................................... USGS

K. M. Shedlock .................................................... USGS

R. G. Stearns ............................... Vanderbilt University

P. C. Thenhaus .................................................... USGS

R. L. Wheeler ..................................................... USGS

WORKSHOP SUMMARY

A consensus was that the central United States 

zones should be defined on patterns of historical 

seismicity except those areas for which geological 

or geophysical knowledge allow delineation of 

zones on the basis of deep structure. Recent analysis of reflection profiles by USGS investigators has 

resulted in the identification of several deeply 

buried faults believed to be active. For the most 

part, however, little is known about buried faults 

in the central United States. Therefore, diverse 

factors such as gravity, aeromagnetics, and 

geologic province boundaries also were used to delineate seismic source zones.

A substantial part of the committee's discussions was on the characteristics of source areas 

and possible causes of earthquakes of the central 

United States. Subjects considered include the following:

1. The relation of seismicity to ancient rifts and 

aulacogens.

2. The relation of seismicity to plutons. Several 

investigators have suggested that stress is concentrated near the contacts of plutons with country

19
rock because of differences in their elastic properties.

3. The relationship of the Mississippi Embayment gravity field to thick masses of post-Paleozoic sediments indicates perhaps that during late 

Mesozoic time there were widespread intrusions 

of magma because of mantle upwarping. As these 

masses cooled, they contracted and subsided causing downwarping of the embayment. Loading by 

post-Paleozoic sediments caused further tectonic 

perturbations. Questionable, however, is if contraction on cooling would be sufficient to produce 

the net subsidence and if the isostatic phenomenon 

could operate in such a short time to produce the 

observed geology (W. J. .Hinze, oral commun., 

1980).

4. The enigma of the lack of moderate-size 

earthquakes on large fault zones such as the Ste. 

Genevieve (see fig. 8 for location). Some of these 

fault zones seem to be suitably oriented to the 

east-west stress field to have movement occur in 

them. Perhaps the question should not be why the 

northwest-trending structures are inactive, but 

rather, why only the northeast-trending structures are active. Although both directions are conjugate to an east-west stress field (and therefore 

theoretically would have an equal opportunity to 

be active), the northeast-trending structures may 

have a preferred structural fabric or other physical characteristic that make them preferentially 

active.

5. The suggestion of block uplift and tilting of 

the Ozarks as revealed by terrain relief studies. 

These studies suggest that the Ozarks may behave 

as a block bounded by northwest-trending faults. 

Uplift and tilting about a northeast-striking axis 

would depress the Mississippi Embayment, generating and concentrating stress along preexisting 

structural elements. Thus, modern stresses in the 

embayment may be related more closely to the 

tectonics of the Ozarks than to ambient stresses 

associated with a drifting continental plate. Interestingly, a projection of the lineament bounding 

the southwest edge of the inferred block is approximately coincident with the southwestern terminus 

of seismicity within the Reelfoot rift zone.

6. A possible relationship between seismicity 

and the intersection of the rift and the Pascola 

arch.

DESCRIPTION OF INDIVIDUAL SOURCE ZONES

The seismic source zones are shown in figure 

9. Maximum estimated magnitudes and recurrence 

estimates of the maximum magnitudes are listed 

in table 4.

Zone 55. This zone is the area of the largest 

earthquakes to affect the central United States 

(Nuttli, 1973, 1979; Nuttli and Herrmann, 1978). 

It is also the area of most frequent moderate seismicity (Stauder, 1982). Although there is little evidence for surface faulting within the zone (Russ, 

1979), subsurface faults, believed to be seismogenic, have been found recently (Zoback and 

others, 1980). The zone has been assigned a 

maximum estimated magnitude (mb) of 7.5 (see 

Nuttli, 1973) and a recurrence time of 600-700 

years (see Nuttli, 1974; Russ, 1979; Algermissen, 

1969, 1972). Zone 55 is defined as being generally 

coincident with the Reelfoot rift as identified by 

Hildenbrand and others (1977) and Hildenbrand 

and others (1980) on the basis of aeromagnetic 

anomalies. The northwest and southeast boundaries of the zone are drawn on differences in the 

pattern and intensity of the magnetic field and are 

believed to be the location of the border faults 

of the rift. Concealed plutons lie along these 

boundaries and may control the occurrence and 

distribution of nearby seismicity (Kane, 1977; 

McKeown, 1978). The southwest and northeast 

boundaries of the zone are not identified easily. 

Although most of the seismicity associated with 

zone 55 does not extend south of about Marked 

Tree, Ark. Gat 35°30' N.) (Stauder, 1982), the rift 

continues its geophysical expression to at least lat 

34°30' N. At this location, the rift boundaries become poorly defined and gravity data show a 

northwest-trending zone of many areally small but 

intense highs that are tentatively interpreted to 

be plutons. The northeast boundary of the gravity 

highs is nearly coincident with what traditionally 

has been mapped as the buried Ouachita front. 

The change in the character of the gravity is used 

as the southwestern boundary of seismic source 

zone 55 because recent studies throw doubt upon 

the existence of the so-called buried Ouachita front 

(a boundary previously suggested as the southwestern limit of modern seismicity). The implication is that large earthquakes can occur along the 

entire length of the rift. The rift initially continued 

farther south than lat 34°30' N. but burial or tectonism has masked or destroyed the structure

20
 f- ^7"

FIGURE 8. Index map of the geographic area addressed at the Central Interior seismic source zone workshop showing 

topographic, geologic, and geographic features that are referenced in the text. Dashed line indicates geographical 

extent of the region considered in the workshop.

south of this location. The extension of the zone 

to the southwest invoked dissenting opinions because the southwestern part has been much less 

active in historic times relative to the northeastern part. An alternative might have been to divide 

the zone into two parts, both having the same estimated maximum magnitude, but with the southwestern part having a recurrence time about double the 600- to 700-year estimate for the northeastern part. The cause of the cessation of seismicity 

just south of Marked Tree, Ark., is unknown. 

Whether it marks a change in subsurface structure

or is the result of spatial variation in seismic activity within the zone (that is, the zone presently 

is quiescent, but periodically may be active) remains to be determined. Because of these uncertainties and because the rift structure with which 

many of the earthquakes seem to be associated 

extends at least to the vicinity of Stuttgart, Ark., 

the committee decided that zone 55 should be retained as drawn. The northeastern boundary of 

zone 55 is defined as the southwesternmost of a 

series of prominent northwest-trending 

aeromagnetic anomalies that truncate the rift

21
KENTUCKY ^ VIRGINIA y ^- 

NORTH 

TENNESSEE CAROL INA

- V

ALA BAMA \ GEORG I A

100 200 MILES

0 100 200 KILOMETERS

(\

FIGURE 9. Seismic source zones of the Central Interior. Unconnected lines extend beyond the region considered at the Central 

Interior workshop. Dashed line indicates geographical extent of the region considered in the workshop.

about 15 km southwest of Paducah, Ky. The fault zone. Although Hildenbrand and others

southwesternmost anomaly is believed to be along (1980) projected the rift a short distance northeast

the subsurface extension of the Ste. Genevieve of the anomalies, the anomalies have been selected

22
TABLE 4. Estimated maximum magnitudes

and estimated recurrence rates far maximum 

magnitude events for the seismic source zones 

of the Central Interior region

[mb, body wave magnitude; leaders ( ) indicate that no values were 

given at the workshop]

Zone No.

(from

fig. 9)

55

56

57

58

59

60

61

62

63

Estimated

maximum

magnitude

(mb )

7.5

6.5

6.5

6.5

5.5-6.0

 

 

 

 

Estimated

recurrence

(in years)

^00-700

2 1000

2 1000

22000

 

 

 

___

 

^Frorn Russ (1979).

2From Nuttli and Hermann (1978).

as the source zone boundary because they represent a prominent shift in the orientation and character of deep geologic structure and because the 

areas of intense modern seismicity are southwest 

of the anomalies.

Zone 56. This zone is identified on the basis 

of aeromagnetic, gravity, crustal-seismic, and 

basement-rock studies. As with zone 55, zone 56 

is a relatively narrow northeast-trending feature 

characterized by subdued magnetic relief within 

the zone and greater magnetic relief outside the 

zone. Magnetic highs interpreted to be plutons 

bound the northwest and southeast margins of the 

feature. Braile and others (1980) suggested that 

the feature may be an offset continuation of the 

structure inferred to be a rift (Reelfoot rift) in 

the Mississippi Embayment. The southwest 

boundary of the zone is expressed geologically by 

the east-southeast-striking Cottage Grove-Rough 

Creek fault zones and geophysically by a prominent east-southeast-trending magnetic lineament. 

The northeast boundary of the zone is set arbitrarily at about lat 39° N. where geophysical expression of the feature is lost. The Wabash Valley fault 

zone is situated in the southern half of source zone 

56, which geologically is part of the Illinois basin. 

The Wabash Valley faults strike obliquely to the 

trend of zone 56. No Holocene surface offsets have 

been reported in the fault zone. Earthquakes in 

this zone frequently are deeper (>15 km) than 

those to the south in zone 55. Some of these earthquakes are along the margin of the Fairfield basin 

(a flexural zone) and are not on Wabash Valley 

faults. The estimated maximum magnitude (mb) 

for earthquakes in zone 56 is 6.5 and the recurrence time is 1000 yr (see Nuttli and Herrmann, 

1978).

Zone 57. This zone encompasses the St. Francois Mountains and surrounding regions. Its 

boundary, however, is not based upon physiography or structure but rather on the spatial pattern of seismicity which takes the form of a ring 

surrounding the mountains (Nuttli, 1979). Earthquakes in the center of the ring are not as common 

as those along the perimeter. The southern part 

of the zone has been extended to the south in 

order to include a number of events in northeast 

Arkansas with 4.0<mb<4.9. Zone 57 has been assigned a maximum estimated magnitude (mb) of 

6.5 and a recurrence time of 1000 years (Nuttli 

and Herrmann, 1978).

Zone 58. This zone includes the St. Louis, Mo., 

area and much of south-central Illinois. It is 

situated on the deepest part of the Illinois basin. 

The zone is delineated, however, solely on the 

basis of seismicity. Several events with 

5.0<mb<5.9 have occurred here. The north-central boundary of the zone has been shifted slightly 

to the north in order to include two events with 

5.0<mb<5.9 (Nuttli, 1979). According to Nuttli, 

the largest earthquakes generally occur in the 

eastern half of the zone. Zone 58 has been assigned a maximum estimated magnitude of 6.5 

(Nuttli and Herrmann, 1978) and a recurrence 

time of 2000 yr.

Zone 59. This zone embraces the area of the 

Anna, Ohio, earthquakes. Several faults have been 

mapped in zone 59, and those striking northwest 

appear to be the active ones. A number of the 

earthquakes have occurred near the Anna-Champaign fault (Mauk and others, 1979). Presently unclear is whether the faults are related to the 

Findlay arch or to the buried glacial Teays River 

valley. Two earthquakes (mb=3-3.4) have been recorded in the zone in the past 6 years. Most of 

the larger earthquakes (MM VII-VIII) occurred 

in the 1930's.

Zone 60. This zone is a north-northeast trending area of seismicity in central and eastern Ohio 

and northeastern Kentucky that has had earthquakes with magnitudes between 3<mb<5.3. The 

northern part of the zone is parallel with the regional strike of Paleozoic rocks; the southern part

23
includes elements of the Kentucky River fault 

zone, the Bryant Station-Hickman Creek fault and 

the Rome trough. The zone has been extended to 

the south to include the area of the Sharpsburg, 

Ky., earthquake sequence of July-August, 1980.

Zone 61. Delineation of this zone is based entirely on seismicity and includes most of northern 

Illinois and a small part of southern Wisconsin. 

Earthquakes of 3.0<mb<5.9 have occurred in the 

area in historic times. The eastern boundary was 

drawn arbitrarily through the center of Lake 

Michigan. There is no apparent reason, however, 

for separating the events of zone 61 from the 

events of zone 63.

Zone 62. This zone is distinguished on the 

basis of its lack of seismicity. According to Nuttli 

(1979), no earthquakes with mb>3.0 have occurred 

in this region in historic times. Zone 62 includes 

east-central Illinois, northern Indiana, and part of 

western Ohio.

Zone 63. This zone is a background zone that 

encompasses the area included in the Central Interior United States study area not designated a 

formal seismic source zone. Earthquakes occur in 

the area, although most have mb<4.0 (Nuttli, 

1979). Nuttli (1979) stated that the maximum estimated magnitude (mb) for earthquakes in this area 

is 5.5. Nuttli adds, however, that the larger earthquakes probably are associated with minor active 

structures and that in regions not associated with 

these minor structures the maximum-magnitude 

earthquake may be reduced to 4.5. Probably the 

most questionable part of this zone is the wedgeshaped area in southern Illinois and western Kentucky. Much of this particular area lies within the 

highly faulted Kentucky fluorspar district (New 

Madrid system of Heyl and Brock, 1961; and Heyl 

and McKeown, 1978) and along strands and splays 

of the Cottage Grove-Shawneetown-Rough Creek 

fault zones. These structures generally have been 

included as part of the 38th Parallel lineament. 

Although these faults are some of the longest and 

most prominent in the Central Interior United 

States, they have not been grouped into a designated source zone because there have been no historic large earthquakes on or near them (Nuttli, 

1979). Nevertheless, the potential for earthquakes 

may be significant. The wedge-shaped area separates by only a small amount the two most hazardous zones in the region. Faults within the Kentucky fluorspar district have the same optimal 

orientation to the modern stress field (to permit

movement) as faults in zone 55. However, faults 

in zone 55 may not be related genetically to those 

in the wedge-shaped area. Zone 55 faults are associated with the Reelfoot rift (Zoback and others, 

1980) whereas faults in the Kentucky fluorspar 

area may have formed during the uplift and subsidence of a regional dome in Pennsylvanian time 

(Heyl and Brock, 1961; Krausse and Treworgy, 

1979). The prominent northwest-trending structural elements in the wedge-shaped area probably 

separate or uncouple faults in zone 55 from those 

in zone 56, and from those in the wedge-shaped 

area itself. A difference in opinion did exist at the 

workshop as to whether the wedge area should 

be deleted in favor of connecting zones 55 and 56.

NORTHEASTERN UNITED STATES

SEISMIC SOURCE ZONES SUMMARY

OF WORKSHOP CONVENED

SEPTEMBER 10-11, 1980

By W. H. Diment, F. A. McKeown and P. C. Thenhaus

PARTICIPANTS

F. A. McKeown .............. U.S. Geological Survey (USGS)

(Moderator) 

W. H. Diment ...................................................... USGS

(Recorder) 

S. T. Algermissen ................................................ USGS

P. W. Basham .............. Energy, Mines and Resources of

Canada, Ottawa 

R. D. Brown ....................................................... USGS

E. F. Chiburis ................................. Ensco, Incorporated

J. W. Dewey ....................................................... USGS

R. H. Fakundiny ............... New York State Museum and

Science Service 

J. Harbour .............. U.S. Nuclear Regulatory Commission

Y. W. Isachsen ................. New York State Museum and

Science Service 

M. F. Kane ......................................................... USGS

G. Leblanc ..................... Weston Geophysical Corporation

P. H. Osberg .................................. University of Maine

D. M. Perkins ...................................................... USGS

P. W. Pomeroy ............................... Roundout Associates

N. M. Ratcliffe .................................................... USGS

R. W. Simpson .................................................... USGS

P. C. Thenhaus .................................................... USGS

C. M. Wentworth ................................................. USGS

WORKSHOP SUMMARY

The committee consensus was to zone the region 

primarily on the distribution of historic seismicity. 

We noted that the spatial distribution of seismicity 

in the Northeast during last two decades was essentially the same as that revealed by the compila24
tions of W.E.T. Smith (1962, 1966), except that 

some of the recent epicenters seem to be more 

closely bunched in areas of high seismicity. The 

closer bunching can be explained easily by imperfections in the earlier record. For small magnitude 

events, Dewey and Gordon (1980) noted that about 

half of the instrumentally recorded earthquakes 

that they have studied are in persistent source 

zones whereas the remainder are more than 20 

km from other earthquakes.

Temporal changes in activity have been noted 

in some areas. For example, the seismicity in eastern Massachusetts in recent years was low with 

respect to that of the historic record. In examining 

the seismicity of southern New England, Shakal 

and Toksoz (1977) found that the seismicity was 

higher in the period 1725-1824 than in the following 100 years. Also, there was little activity prior 

to the Attica, N.Y. earthquake (I0=VIII) of 1929. 

In this sense, the border (PQ/ME) earthquake of 

1973 (Wetmiller, 1975) also was something of a 

surprise.

Problems with focal depths and focal-plane solutions were reviewed. The problems pose a severe 

handicap in relating seismicity to geologic structure. Measurements of focal depths, and, to a 

somewhat lesser extent, focal-plane solutions are 

of questionable reliability except in those areas 

covered by dense networks, such as the Ramapo 

fault area (Aggarwal and Sykes, 1978) (see fig. 

10 for location), Blue Mountain Lake (Sbar and 

others, 1972; Sbar and Sykes, 1977), Attica, sometimes (for example, Fletcher and Sykes, 1977; 

Herrmann, 1978), and La Malbaie (Leblank and 

Buchbinder, 1977). For a seismotectonic interpretation of seismicity in New England, the depth 

of the foci must be known whether it is 1 or 10 

km or even deeper. Depths of hypocenters make 

a difference as to which structures might be 

causal. Ratcliffe (1971) outlined the intricate history of deformation in the area of the Ramapo 

fault system, and, at the workshop, stressed the 

wide range of structures that could be seismogenic 

in the 1-10 km depth interval in which earthquakes have been observed (Aggarwall and Sykes, 

1978).

A good illustration of the importance of reliable 

depth determinations comes from the studies of 

Bollinger and his collegues in Giles County, Va. 

Their detailed network reveals focal depths between 5 and 25 km (Bollinger and Wheeler, 1980). 

Moreover, the trend of epicenters is north-northeast which is in accord with the structural grain 

to the north of the region but which is discordant 

with the northeast structural trends in the vicinity 

of the earthquakes. The suggestion is that the 

seismicity is controlled by older and deeper structures that have little manifestation at the surface 

(Wheeler and Bollinger, 1980).

Another aspect of the association of geologic 

conditions with seismicity is that earthquakes appear to occur in crystalline rock; that is, in highly 

metamorphosed or igneous rocks. This association 

seems to be so in the northeast, except where solution mining of salt in the overlying sediments is 

involved (Fletcher and Sykes, 1977) or where 

there are other manmade perturbations. A 

rationale (for example, Diment, 1980) is that the 

sediments are too thin, too soft, or too decoupled 

from the basement for sufficient strain to accumulate within them to produce significant earthquakes. Basement is exposed in much of the northeast including the Adirondack Mountains and most 

of New England and appears to be peppered with 

shallow earthquakes, judging from instrumental 

determinations and the prevalence of earthquake 

sounds which some would attribute to shallowness 

of foci (Sbar and others, 1972; Anderson and 

Fletcher, 1976). The fact remains, however, that 

some earthquakes seem to be of mid-crustal depth 

or deeper. Some have suggested (for example, 

Sbar and Sykes, 1977; Acharya, 1980) that these 

are the regions where large earthquakes are likely 

to occur. This suggestion is plausible but one that 

remains to be evaluated more fully. A hypothesis 

was suggested relating seismicity to residual pore 

pressure and higher porosity in alkalic intrusives 

compared with rocks of normal alkali content.

DESCRIPTION OF INDIVIDUAL SOURCE ZONES

The seismic source zones for the northeastern 

United States are shown in figure 11. Table 5 lists 

the estimated maximum magnitudes assigned to 

the zones. No recurrence estimates of these 

maximum magnitudes were made due to a lack 

of geologic data to support such estimates.

Zone 64. Offshore zone. The northwestern 

boundary of this zone corresponds roughly to the 

western edge of zone 3 in a report of seismic hazard for the east coast of the United States by Perkins and others (1980). The boundary roughly coincides with the western edges of deep Jurassic ba25
74°

46'

44?

CHARLEVOIX 

SEISMIC ZONE"

TREAL g*f

 

----A _.. n^T i

\ < * r*

1DACK I » \ J

BLUE MOUNTAIN \ /

OLAKE ^RE . \JtLWT 

/1\ I ^cm8tj|FAULF1 t

LOWWLLE ^ 

/" BOONVILLE

M. r\IS\ _

^<

\ GETTYSBUR 

x.Q&<;iNx^,

FIGURE 10. Index map of the geographic area addressed at the Northeastern seismic source zone workshop showing 

selected topographic, geologic, and geographic features referenced in the text. Dashed line indicates geographical 

extent of the region considered in the workshop.

sins (Klitgord and Behrendt, 1979) that underlie 

the continental shelf and slope.

Recent earthquakes near the Bermuda Rise 

(Nishenko and Kafka, 1980) are a reminder that 

the oceanic plate is not inactive. Indeed, the

Grand Banks earthquake of 1929, which occurred 

about 700 km to the east of the area covered by 

the map, was east of the western edge of the Jurassic basins. This earthquake was assigned a 

magnitude of 7.2. Intensity IV effects were felt

26
75C 70° 65C

CANADA

200 MILES

100 200 KILOMETERS 

\

FIGURE 11. Seismic source zones of the northeastern United States and adjacent Canada. Unconnected lines indicate that 

boundaries continue beyond the area considered at the workshop. Dashed line indicates geographical extent of the region 

considered in the workshop.

in the United States about 1,000 km from the epi- ter magnitude 7 have occurred in this zone. The 

center. zone's historical and instrumental seismicity is as 

Zone 65. Charlevoix zone. Earthquakes esti- high as, or maybe exceeds, that of the New Mamated to have equalled or slightly exceeded Rich- drid region of the Mississippi Embayment. The

27
TABLE 5. Estimated maximum 

magnitudes for seismic source 

zones of the northeastern United 

States and adjacent Canada

[Leaders ( ) indicate that no value was given at 

the workshop]

Zone No.

(from

fig. ID

64 

65

66

67

68

69

70

71

72

73

74

75

Estimated

maximum

magnitude

7.5

6.0

7.0

6.0

5.5

6.5

6.5

6.5

6.0

6.0

5.0

Magnitude scale not 

specified at the workshop

zone is drawn essentially as presented by Basham 

and others (1979). They noted that if the zone was 

drawn on the basis of distribution of microearthquakes (Leblank and others, 1973; Leblank and 

Buchbinder, 1977) the zone would be smaller. The 

microearthquakes extend to depths of about 20 km 

and appear to be confined to the Precambrian 

crystalline rocks that dip southeasterly under Paleozoic sediments and metasediments. Microearthquake data obtained during the past 4 years confirm the earlier observations. Stevens (1980) reassessed the locations of the larger instrumentally 

recorded earthquakes (1924-1978) and found them 

to fall largely in the zone of microseismicity (70 

km long) with concentrations at both ends. On the 

basis of Dewey and Gordon's (1980) instrumental 

relocations, the major shocks tend to occur near 

the northeastern end of the zone.

Basham and others (1979, table 3) and Weichert 

and Milne (1979, table 2) both used a maximum 

magnitude of 8.0 for their probabilistic studies. At 

the workshop, a maximum magnitude of 7.5 was 

suggested, apparently to represent more recent 

thinking about the maximum size of intraplate 

earthquakes and to bring the value more in line 

with those assigned in the central United States.

Zone 66. St. Lawrence River zone. There are 

regions along the St. Lawrence River valley, both

to the northeast and southwest of the Charlevoix 

zone, that are considerably less seismic than the 

Charlevoix zone. However, these regions probably 

are considerably more seismic than the 

background zones to the northwest and southeast. 

Thus, a zone has been introduced that extends on 

the northeast from the lower St. Lawrence zone 

(Basham and others, 1979), (off fig. 12 to the 

northeast) to the southwest through the Charlevoix zone to the western Quebec zone. 

Zone 67. Western Quebec-northern New York 

zone. This zone is drawn essentially as outlined 

by Basham and others (1979) who were careful 

to state that the zone is "simply intended to encircle concentrations of seismicity."

Basham and others (1979, fig. 11) defined a zone 

of higher seismicity within the western Quebec 

zone (as they did within the Charlevoix zone) and 

call it the "Gatieau Triangle" (from Forsyth, 1977). 

Near the United States border it is about half the 

width of the western Quebec zone.

The northwestern edge of the western Quebec 

zone could be extended far to the northwest on 

the basis of the known seismicity and in accordance with the highly speculative notions of Diment and others (1980) and Muller and others 

(1980). These authors suggested that zones of high 

seismicity may occur along, or may be bounded 

by, certain northwesterly trending lineaments on 

gravity and magnetic maps.

The western Quebec zone is terminated in 

northern New York near the northern edge of the 

high Adirondack Mountains.

Some of the earthquakes in the western Quebec 

zone are rather deep. The ManisaM earthquake 

of 1975 was assigned a depth of 17 km (Horner 

and others, 1978) and the St. Donat earthquake 

of 1978 a depth of 7 km (Horner and others, 1979). 

Moreover, a systematic change of focal depth has 

been suggested across the western Quebec zone 

from upper-crustal depth north of Montreal to 

mid-crustal depth north of Ottawa (Horner and 

others, 1979). The focal depths of some northern 

New York earthquakes are thought to be of midcrustal depth (Aggarwal and Yang, 1977).

Zone 68. The Adirondack Mountains and environs. Until recently the seismicity of the Adirondacks has been portrayed as low, perhaps because 

of the low density of population and because most 

of the seismicity is distributed around the margins 

of the Adirondacks. Recent studies indicate an 

abundance of small earthquakes in the Adirondack

28
Mountains (Aggarwal and Sykes, 1978) although 

the principal anorthosite bodies tend to be aseismic (Aggarwal and Yang, 1977).

No large earthquake has been attributed to this 

region, nor have any been attributed to its 

periphery, except to the north, which falls in the 

western Quebec zone. A maximum magnitude of 

6 seems appropriate.

Serious attempts have been made to relate seismicity to geologic structure in this region (for example, Isachsen and McKendree, 1977a-d; 

Pomeroy and Fakundiny, 1976). So far the results 

are most interesting, but rather equivocal. The 

maximum compressive stress, however, has been 

shown to have an east-northeast direction (Sbar 

and Sykes, 1973, 1977) as it seems to have in those 

regions west of the Ordovician suture (for example, Sykes, 1978), however it might be defined.

The southwestern limit of seismicity falls close 

to a northwesterly trending lineament constructed 

by Diment and others (1980) and Muller and 

others (1980) on the basis of terminations of gravity and magnetic features in the region. This lineament has not been examined in detail as yet. The 

Lowville earthquake of 1853 (Coffman and von 

Hake, 1973) and the Booneville earthquake of 1980 

(Kafka and others, 1980) appear to occur along it. 

Moreover, the focal-plane solution for the Boonville earthquake suggests a northwesterly trending plane with northeasterly directed compressive 

stresses, as do most earthquakes in the Adirondacks.

Some earthquakes in the Adirondacks are 

known to be shallow (<3.5 km) as at Blue Mountain Lake (for example, Sbar and others, 1972; Anderson and Fletcher, 1976), but they cannot be 

related unequivocally with geologic structure 

(Isachsen and Geraghty, 1979).

Zone 69. Niagara-Attica Zone. Basham and 

others (1979) defined a zone of diffuse seismicity 

extending from the Niagara peninsula to Attica 

and a little beyond. Because the zone is delineated 

mainly on the basis of historical seismicity, its possible extent into Lake Ontario and Lake Erie is 

not well known, although results from the LDGO 

net do show epicenters in the western part of 

Lake Ontario (Sykes, 1978). Because many of the 

small earthquakes near Hamilton and Toronto and 

on the Niagara peninsula were the result of shallow pop-ups, the zone might be restricted to western New York.

The intensity VIII Attica earthquake of August

12, 1929 is the largest earthquake of the region. 

Street and Turcotte (1977) assigned an mb=5.2. 

Herrmann (1978) found focal depths between 2 and 

3 km for two more recent events (1966 and 1967) 

near Attica and suggested that the anomalously 

high intensity of the 1929 shock also might be the 

result of such shallow focal depth.

Zone 70. The Clarendon-Linden fault. The 

north-northeast-trending Clarendon-Linden fault 

is close to Attica and the 1929 earthquake could 

have occurred along one of its strands, but this 

is not entirely clear. Certainly, the small earthquakes (1972-1975) induced by solution mining of 

salt did occur along a strand of the Clarendon-Linden system but they ranged in depth from 0.5 to 

1.1 km (Fletcher and Sykes, 1977, fig. 8). A focal 

mechanism with thrust motion on a nodal plane 

nearly parallel to the fault was obtained from 

these earthquakes (Fletcher and Sykes, 1977). The 

mechanisms obtained from the 1966 and 1967 

events also yield a north-northeast-trending nodal 

plane, but require approximately equal components of right-lateral and reverse faulting along 

this nodal plane (Herrmann, 1978).

The Clarendon-Linden fault falls on the west 

flank of a pronounced gravity and magnetic feature that extends north-northeast to northeast 

across Lake Ontario (Diment and others, 1974; 

Bothner and others 1980). This geophysical anomaly must owe its presence to contrasts primarily 

within the Precambrian basement. However, the 

Precambrian structures may have influenced the 

development of the Paleozoic Clarendon-Linden 

fault.

Many earthquakes in the zone clearly are not 

related to the Clarendon-Linden fault. An eastwest trend has been noted by many, a northwest 

trend by some (Diment and others, 1980), an association of earthquakes with mafic plutons as revealed by gravity and magnetic anomalies (Kane, 

1977), and an association of earthquakes with 

edges of blocks as defined by geophysical and 

stratigraphic studies (Fakundiny, this workshop). 

The Precambrian basement of northwestern New 

York appears somewhat anomalous with respect 

to the surrounding region in that local but intense 

gravity and magnetic highs are more common (Revetta and Diment, 1971). The character of the signatures appears to extend northward into Lake 

Ontario.

Zone 71. Southeastern New York and northern New Jersey. This zone was drawn on the basis

29
of historical and recent instrumental seismicity 

(Aggarwal and Sykes, 1978; Sykes, 1978; Chiburis 

and Ahner, 1980). The seismicity in a part of the 

zone appears to be closely related to the 

northeasterly trending Ramapo fault (TriassicnJurassic) or to earlier faults that may date back to 

late Precambrian time. However, there are many 

historic as well as instrumentally recorded earthquakes in the general area that are well outside 

the Ramapo zone (W.E.T. Smith, 1962, 1966; 

Sykes, 1978; Chiburis and Ahner, 1980). Indeed, 

some epicenters extend a considerable distance 

offshore. Thus, the zone has been extended 

offshore to the arbitrary offshore boundary of zone 

64 (see previous discussion). Perhaps it extends 

farther into the ocean where it might be associated 

with transform faults, although which of the many 

faults may be seismogenic is not clear from the 

maps of Klitgord and Behrendt (1979). The southern termination of the zone corresponds to the 

change in structural trends from southwest to 

west-southwest, and to a reduction in seismicity. 

The northern termination corresponds to the intersection of the northeasterly structural trends 

with the more northerly trends of the Berkshire 

Hills, and to a reduction in seismicity. There is 

evidence for significant minor seismicity along the 

Hudson River valley and the zone might be extended to the north, perhaps to join with the areas 

of moderate seismicity in, and peripheral to, the 

southern and central Adirondack Mountains 

(Pomeroy and Fakundiny, 1976).

Zone 72. The Boston-Ottawa trend. Over the 

years, several suggestions have been made that 

link a diffuse northwesterly trending zone of seismicity (W.E.T. Smith, 1962, 1966) that extends 

from offshore through eastern Massachusetts, 

southeastern New Hampshire and into Canada in 

the region of Montreal and Ottawa, with an ill-defined zone of Mesozoic alkaline magmatism (Diment and others, 1972; Sbar and Sykes, 1973; 

McHone and others, 1976; McHone,, 1977; Sykes, 

1978). Although the notion may have some merit, 

it has some significant imperfections: (1) there is 

a gap in seismicity in Vermont, although results 

from recently established stations indicate that 

this area may not be aseismic (Chiburis and 

Ahner, 1980); (2) if the orientation of the principal 

compressive stress in the coastal zone (west-northwest) is correct, and, if the orientation of this compressive stress is east-northeast in the Canadian 

Shield, the Adirondack Mountains, and the Appalachian platform, there is a zone in which the 

orientation of the stress directions must change. 

The zone of change must intersect the Boston-Ottawa trend, probably in Vermont in the region of 

relative low seismicity. There was little discussion 

of these ideas and uncertainties among the participants. However, some participants insisted that 

the seismicity in southern New Hampshire and 

northeastern Massachusetts is higher than in adjacent areas along the coast to the north and to the 

south.

Zone 73. Northeastern New England. This 

large zone could be subdivided if we had more information and greater insight, but at this moment 

it would be difficult to subdivide with a systematic 

rationale. A number of comments were made 

about the seismicity of the region and its possible 

relation to geologic structure, but time was short 

and the comments were brief and sometimes conflicting.

1. Although the historic seismicity near the 

coast is somewhat high relative to that of the interior (W.E.T. Smith, 1962, 1966; Coffman and 

Von Hake, 1973; Stover and others, 1977), the 

contrast may be due to the fact that the interior 

was settled late (U.S. Geological Survey, 1970) 

and remains sparsely populated.

2. The border (PQ, ME) earthquake of 1973 of 

magnitude mb**5 (Wetmiller, 1975) was something 

of a surprise and is a principal reason for extending the zone so far to the northwest.

3. Recent focal mechanisms for this zone 

(Graham, 1978; Pulli and Toksoz, 1980) are sufficiently different that they do not necessarily support the notion of uniform west-northwest-trending compressive regime but rather suggest a complex stress pattern along coastal New England.

Zone 74. Southeastern New England. Seismic 

activity seems to be clustered loosely in the vicinity of the Triassic-Jurassic grabens, but area! distribution may be higher a little to the east in 

southern Connecticut, which might raise the question of the relation of seismicity to strands of the 

Lake Char and Honey Hill fault zones or possibly 

to the northeasterly trending lineaments in this 

region.

Zone 75. Zone of background seismicity. 

Large areas have been assigned to this category 

(mb=5) of low historic and instrumental seismicity. 

An enumeration of ideas expressed at the workshop and in the literature may be useful:

1. Many believed that earthquakes in the zone

30
are attributed to shallow phenomena and have 

been expunged from the record. However, many 

quarry blasts remain unculled from the record (an 

important problem inasmuch as they outnumber 

natural events by more than a hundred to one in 

some regions). Moreover, some earthquakes 

caused by collapse of subsurface workings, popups due to -natural unloading or quarrying 

(Pomeroy and Fakundiny, 1976), and solution mining or reinjection of wastes remain in the record.

Although most of these earthquakes are consequences of man's activities, some actually are 

indicators of high stress levels near the surface. 

The latter may be regional in extent and should 

not be ignored entirely in seismic zoning.

2. The aseismicity of certain regions may be due 

to the presence of shallow sediments that are so 

soft or so decoupled that they cannot accumulate 

sufficient strain to produce shallow earthquakes 

(for example, Diment, 1980). However, this does 

not mean that deeper earthquakes might not occur 

in such regions.

3. Within the zones of background seismicity are 

areas that are more seismic than others, but that 

are not recognized as such because consideration 

of larger variations obscures them. Sometimes 

these subtle variations of seismicity within 

"background regions" represent extensions of 

more obvious trends into regions where seismogenic structures are present at greater depths 

(for example, the possible extension of the Attica 

zone to the southeast).

SUMMARY

By P. C. Thenhaus

The procedures used in delineating seismic 

source zones are ill defined. No single standard 

exists by which source zones across the nation can 

be drawn, primarily because of the nonuniform 

level of pertinent seismological, geological, and 

geophysical information available for areas of vastly differing tectonic and geologic settings. The 

equivocal association of seismicity with geologic 

structure throughout most of the United States 

compounds the problem. The zones described 

herein, resulting from a general consensus of each 

of the committees, illustrate three useful approaches in defining regional seismic source zones. 

Each of the approaches represents the different 

level of understanding of seismotectonics in any 

one of the regions. They are: (1) zoning on individual faults, or area! extent of faulting where the 

faults have geologically young displacements, or 

have distinct association with seismicity; (2) zoning 

primarily on regional structural style, particularly 

where regional seismicity is associated strongly 

with distinctive structural terrane; and (3) zoning 

on areal distribution of historic seismicity. Typically, some combination of the three approaches 

is used to best define the zones of a region; however, one approach usually predominates.

Comparing the regions discussed in this report, 

much more recent faulting information is available 

on the Great Basin region than on either the 

Northern or Southern Rocky Mountain regions. 

This detailed information allows geologic estimates 

of earthquake recurrence for high-magnitude 

events. These estimates have high uncertainty, 

but are informative and useful for comparison with 

statistically derived estimates of recurrence.

Where recency of faulting information was available for the Rocky Mountain regions (primarily in 

the Rio Grande rift and the Intermountain Seismic 

Belt), it has been taken into account in defining 

zones. Large areas exist, though, that have not 

been studied in this respect. The zoning philosophy, therefore, has been modified for the Rocky 

Mountain regions to include zones defined by similar structural or tectonic setting. The available 

data on ages of latest faulting has been assumed 

to hold throughout a distinctive structural region. 

Note that this second approach is different from 

that used in the Basin and Range where the zone 

boundaries conform to areas characterized by certain ages of latest fault displacements.

In contrast to the western United States, 

throughout most of the eastern United States specific seismotectonic structures are unknown. Also, 

the diversity of geologic and tectonic terrane is 

not nearly as marked as in the west. These facts 

pose major handicaps in attempting to define 

zones primarily on geologic information. Accordingly, in the Central Interior and the northeastern 

United States, a third approach for developing 

source zones is used that has primary emphasis 

on the spatial distribution of historic seismicity.

Considerations for defining seismic source zone 

boundaries within regions parallel those for devising a zoning technique for the region. Within a 

region, a nonuniform level of pertinent information 

exists among source zones being considered under 

a single zoning technique. This fact bears on the

31
certainty of the zone boundaries within each region. Illustrating this intraregion variation, the 

given zone boundaries within the Basin and Range 

have a higher degree of certainty at the eastern 

and western margins than in the central area. The 

reasons for this are obvious: (1) the margins of 

the Basin and Range are the most seismically active, and (2) the hazard threatens the population 

which is more densely concentrated at these margins. Therefore, more research effort has been 

concentrated in these areas. This effort results in 

more and better quality data on ages of latest displacements.

In contrast, certainty of zone boundaries defined 

under the remaining two techniques of zoning (use 

of structural provinces and spatial distribution of 

seismicity) can be assessed only according to the 

extent that they appear to reasonably organize 

historic seismicity. Because these techniques are 

not based on geologic effects of earthquakes (that 

is, number and size of fault scarps), their certainty 

cannot be assessed in terms of completeness or 

quality of a particular kind of quantitative data 

set. Judgments on both the tectonics of an area 

and the historic record of events are involved. It 

is because of these judgments that the source 

zones represent a general but qualified consensus 

on the part of the workshop participants. There 

usually exists dissenting opinions on the part of 

a few as to the reasonableness of some zones.

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