Earthquake-Induced 

JL Liquefaction ' ' JL "

Features in the Coastal Setting of South 

Carolina and in the Fluvial Setting of the 

Hew Madrid Seismic Zone

V-.S. GE.OLO6ICAL St|RV;EY PR<5 EESS J Q N AL tAPER 1.5-04
Earthquake-Induced Liquefaction 

Features in the Coastal Setting of South 

Carolina and in the Fluvial Setting of the 

New Madrid Seismic Zone

By S.F. OBERMEIER, R.B. JACOBSON, J.P. SMOOT, R.E. WEEMS, G.S. GOHN, 

J.E. MONROE, and D.S. POWARS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1504

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1990
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid

seismic zone / by Stephen F. Obermeier ... [et al.]. 

p. cm. (U.S. Geological Survey professional paper ; 1504) 

Bibliography: p.

Supt. of Docs. no. : I 19.16 : 1504 

1. Geology South Carolina Atlantic Coast. 2. Geology Missouri-New Madrid Region. 3. Soil liquefaction. 4. Geology.

Structural. I. Obermeier, Stephen F. II. Series: Geological Survey professional paper ; 1504. 

QE162.A85E37 1990

557.57-dc20 89-600177

CIP

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
CONTENTS

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

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

Coastal South Carolina ........................................................ 2

Regional Setting Geology and Liquefaction Susceptibility.. 3

Characteristics of and Criteria for Sand-Blow Formation..... 4

Filled Sand-Blow Craters......................................... 5

Vented-Sand Volcanoes ........................................... 8

Features Showing Evidence of Lateral Spreads or Ground

Oscillations........................................................... 11

Regional Distribution of Sand Blows............................... 12

Other Possible Origins.................................................. 13

Features of Weathering Origin ...................................... 14

Overview of the South Carolina Liquefaction Study........... 18

Earthquake Ages................................................... 19

Shaking Severity Estimation ................................... 19

Holocene Earthquake Shaking ................................. 19

New Madrid Seismic Zone................................................... 20

Regional Geologic and Seismotectonic Setting................... 22

Characteristics of Quaternary Alluvium..................... 23

Seismotectonic Setting............................................ 24

Characteristics of and Criteria for Earthquake-Induced

Liquefaction Features ......................................... 24

Vented-Sand Volcanoes .......................................... 24

New Madrid Seismic Zone Continued

Characteristics of and Criteria for Earthquake-Induced

Liquefaction Features Continued

Fissures............................................................ 26

Intruded Features.............................................. 27

Features of Unknown or Nonearthquake Origin................ 34

Sand Boils............................................................ 34

Mima Mounds ....................................................... 34

Load Structures.................................................... 35

Other Features ..................................................... 38

Local Geologic Controls on Production of Vented Sand ...... 38

Topstratum Thickness............................................ 38

Topstratum Lithology............................................. 38

Substratum Grain Size............................................ 39

Resistance of Source Sand to Liquefaction................. 39

Overview of Studies........................................................... 39

Geologic Criteria ......................................................... 39

Types of Earthquake Features....................................... 40

Geologic Controls......................................................... 40

Suggestions for Future Research ........................................ 41

Relevance of Liquefaction Features...................................... 42

References Cited............................................................... 42

ILLUSTRATIONS

[Plate in pocket]

Page

PLATE 1. Typical load structures produced by rapid sedimentation, observed in trench near Marked Tree, Arkansas.................. In pocket

FIGURE 1. Map showing 1886 and older sand-blow sites ................................................................................................................ 2

2. Schematic cross section of representative barrier showing sediment types, ground-water table locations, filled sand-blow

	craters, and Bh soil horizons................................................................................................................................ 4

3. Sketch and photograph of craters produced by the 1886 earthquake................................................................................. 6

4. Schematic cross section of normal type of filled sand-blow crater..................................................................................... 7

5. Ternary diagram showing grain sizes of paired samples at identical depths inside and outside liquefaction craters ................... 7

6. Schematic cross sections showing sand-filled fissures interpreted as resulting from liquefaction and flowage during the

	1886 earthquake..............................................................................................................................................^ 9

7. Schematic vertical section of representative vented-sand volcano..................................................................................... 10

8. Photograph of vented-sand volcanoes that have coalesced to form a continuous sheet of sand on the ground surface................. 11

9. Schematic cross section of filled sand-blow crater, illustrating aspects associable with downslope movement........................... 12

10. Schematic cross section showing pedogenic tonguing of BE- and E-horizon sand into underlying B horizon and a graph

	showing particle-size data for a fractured pedogenic tongue ...................................................................................... 15

11. Sketch of cross section through a white, pedogenic sand tongue at site HW, near Hollywood, S.C........................................ 16

12. Sketch of vertical section through BE' horizon at site HW, near Hollywood, S.C............................................................... 18

13. Map showing late Quaternary alluvial deposits of St. Francis and Western Lowlands Basins................................................ 21

14. Map showing area covered by vented sand, estimated epicenters of strongest 1811-12 earthquakes, and faults and fault zones.. 22

15. Schematic east-west cross section showing geologic and ground-water setting of St. Francis Basin ....................................... 23

16. Block diagram showing the configuration of the buried New Madrid Rift Complex .............................................................24

ill
IV CONTENTS

17. Map of northern Mississippi Embayment showing earthquake epicenters, plutons, rift boundaries, and faults ......................... 25

18. Photograph showing stratigraphy of vented-sand volcano with organic-rich silt between two fining-upward sequences of sand... 26

19. Photographs and line drawings of an "eruptive vent" that cut stratified deposits of a vented-sand volcano.............................. 27

20. Aerial photographs showing vented sand, interpreted as the product of liquefaction and flowage during the 1811-12

earthquakes ...................................................................................................................................................... 28

21. Photographs showing sand dikes and sills, interpreted as having originated by liquefaction and flowage during the

1811-12 earthquakes........................................................................................................................................... 31

22. Sketch and photographs of section showing earthquake-induced intrusions in Holocene sediments and underlying

Wisconsinan braided-stream sands observed in ditch about 15 km northwest of Marked Tree, Ark.................................. 32

23. Aerial photograph of mima mounds in the northern part of St. Francis Basin..................................................................... 36

24. Sketches showing pseudonodules formed in a shaking experiment by Keunen..................................................................... 37

25. Schematic drawing of the development of load-casted ripples, caused by ripple crests sinking into soft mud............................ 37

TABLE

Table 1. E stimated relative susceptibility of saturated cohesionless sands to liquefaction during strong seismic shaking........................... 4
EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN THE

COASTAL SETTING OF SOUTH CAROLINA AND IN THE FLUVIAL

SETTING OF THE NEW MADRID SEISMIC ZONE

By S.F. OBERMEIER/ R.B. JACOBSON/J.P. SMOOT/ R.E. WEEMS,* G.S. GoHN, 1 J.E. MONROE,* and

D.S. POWARS 1

ABSTRACT

Many types of liquefaction-related features (sand blows, fissures, 

lateral spreads, dikes, and sills) have been induced by earthquakes in 

coastal South Carolina and in the New Madrid seismic zone in the 

Central United States. In addition, abundant features of unknown and 

nonseismic origin are present. Geologic criteria for interpreting an 

earthquake origin in these areas are illustrated in practical applications; these criteria can be used to determine the origin of liquefaction 

features in many other geographic and geologic settings.

In both coastal South Carolina and the New Madrid seismic zone, the 

earthquake-induced liquefaction features generally originated in clean 

sand deposits that contain no or few intercalated silt- or clay-rich 

strata. The local geologic setting is a major influence on both development and surface expression of sand blows. Major factors controlling 

sand-blow formation include the thickness and physical properties of 

the deposits above the source sands, and these relationships are 

illustrated by comparing sand blows found in coastal South Carolina (in 

marine deposits) with sand blows found in the New Madrid seismic zone 

(in fluvial deposits). In coastal South Carolina, the surface stratum is 

typically a thin (about 1 m) soil that is weakly cemented with humate, 

and the sand blows are expressed as craters surrounded by a thin sheet 

of sand; in the New Madrid seismic zone the surface stratum generally 

is a clay-rich deposit ranging in thickness from 2 to 10 m, in which case 

sand blows characteristically are expressed as sand mounded above the 

original ground surface.

Recognition of the various features described in this paper, and 

identification of the most probable origin for each, provides a set of 

important tools for understanding paleoseismicity in areas such as the 

Central and Eastern United States where faults are not exposed for 

study and strong seismic activity is infrequent.

INTRODUCTION

The near-surface and surface expressions of 

earthquake-induced liquefaction are highly dependent on 

the local geologic setting. This paper discusses the types 

of features, primarily sand blows, that formed on level or 

nearly level ground in two very different geologic setManuscript approved for publication April 20, 1989.

1U.S. Geological Survey, Reston, Va.

2U.S. Army Corps of Engineers, Memphis, Term.

tings, one in which the parent sediments have a marine 

or near-marine origin (coastal South Carolina) and one in 

which the parent sediments have a fluvial origin (New 

Madrid seismic zone). The diverse types of sand blows 

and the equally diverse controls on their formation 

became apparent during field studies recently conducted 

to locate regions subjected to strong earthquake shaking, 

as indicated by the presence of liquefaction-induced 

features. In coastal South Carolina, field studies focused 

on sand blows of very late Pleistocene to Holocene age, 

as well as features caused by the 1886 Charleston earthquake. In the New Madrid seismic zone, the search was 

largely restricted to sand blows caused by the 1811-12 

earthquakes. In both areas, features of nonearthquake 

origin occur that might be confused with earthquakeinduced features. Geologic criteria have been developed 

by which the earthquake-induced liquefaction features 

can usually be distinguished. ("Liquefaction" in this 

paper is defined as transformation of water-saturated 

sediments from the solid to liquid state as a consequence 

of increased pore pressure; this transformation indicates 

that grains are suspended by pore water. "Fluidization" 

is defined as the process of both suspension and transport 

of grains by pore water.)

The scope of the paper includes both intruded features 

(for example, dikes and sills) and vented features (for 

example, sand blows). The term "sand blows" is used to 

indicate features formed where earthquake shaking 

causes liquefaction at depth followed by the venting of 

the liquefied sand and water to the surface. Features 

described as sand boils in this paper form in the absence 

of earthquake shaking and involve transport of sediment 

to the surface by artesian flow (springs). This distinction 

in terms is made because the processes within the ground 

that lead to development of earthquake-induced features 

differ from processes not related to earthquakes. Sand 

blows are thought to often form in response to liquefaction that is in turn followed by segregation of sand and
2 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

water at depth, upward development of a vent, and 

finally the violent discharge of soil and water that soon 

diminishes to an ebbing flow (Scott and Zuckerman, 

1973). (A slightly different model for some sand deposits 

is proposed in this paper.) In contrast, sand boils form by 

the downward development of a vent that typically does 

not enlarge violently and that continues to flow as long as 

high artesian pressure remains, commonly for days or 

years. This difference in development processes between 

sand boils and sand blows can often be distinguished in 

tiie field by use of techniques discussed in this paper.

COASTAL SOUTH CAROLINA

The strongest historic earthquake in the Southeastern 

United States took place in 1886 near Charleston, S.C. 

Throughout much of the epicentral region, an area about 

35 km wide and 50 km long, the Modified Mercalli 

intensity ranged from IX to X (Bollinger, 1977). The 

estimated body-wave magnitude (mb) was between 6.6 

and 7.1 (Nuttli, 1983a). The potential for a future earthquake having the strength of the 1886 earthquake is a 

major concern in engineering design in the Southeast. 

The concern is reinforced by a 300-year historical record 

of continuing weak seismic activity near Charleston. The 

source of the earthquakes in the Charleston area remains 

unknown, and seismotectonic hypotheses are widely 

disparate, despite many geologic, geophysical, and seismic studies during the past decade. No faults or fault 

systems have been identified that adequately explain the 

large 1886 Charleston earthquake or the other smaller, 

historic earthquakes that have occurred throughout 

much of South Carolina (Hays and Gori, 1983; Dewey, 

1985; Science News, 1986). Because direct evidence of 

seismotectonic conditions is lacking and because the 

historic earthquake record is too limited to provide a 

dependable basis for estimating the frequency of moderate to strong earthquakes, we undertook a search for 

pre-1886 sand blows.

Results of the search are shown on figure 1. Figure 1 

shows the approximate boundary of the 1886 Charleston 

earthquake meizoseismal zone; the sites conspicuous in 

1886 for development of many sand blows, described as 

"craterlets" by Button (1889); and the sites of pre-1886 

sand blows that we discovered. The unshaded portion of 

figure 1 encompasses the area that was searched for sand 

blows. Radiocarbon ages of sand-blow materials at site 

HW show that at least three pre-1886 Holocene earthquakes have produced sand blows in that area (Weems 

and others, 1986). It is not yet known how many earthquakes are represented by the sand blows at the widely 

scattered sites on figure 1, nor are the seismic source 

zones known for these prehistoric sand blows. What is 

known is that the prehistoric sand blows extend far 

beyond the limits of 1886 sand blows (see fig. 1) in

78° 34°

*

V

- - - Approximate boundary 

of 1886 meizoseismal 

zone (Bollinger. 1977)

= Areas conspicuous in 1886 

for craters (Dutton, 1889)

 Pre-1886 sand-blow site 

designation (this study)

Outer limit of historically 

documented 1886 sand 

blows (Georgetown)

ff

$

£

|FLT~ ~~^ , |________;_____________I

INDEX MAP SHOWING 

LOCATION OF STUDY AREA

30 KILOMETERS

FIGURE 1. 1886 and older sand-blow sites. Unshaded onshore region 

is predominantly of marine sediments younger than about 240,000 

years. Shading pattern denotes region of older marine sediments 

that was not reconnoitered. Younger fluvial sediments occur locally. 

All sand-blow sites in the region with no shading are in marinerelated deposits, and all sites in region with shading are in fluvial 

deposits. Numerous sites discovered in and near the 1886 meizoseismal zone are not shown because of lack of space. Abbreviations 

adjacent to sand-blow sites are specific site designations.

sediments having approximately the same liquefaction 

susceptibility (Obermeier and others, 1986; unpublished 

data) and that the strongest Holocene shaking has probably been near Charleston (Obermeier and others, 1989). 

All the pre-1886 sand blows that we found have no 

expression on the ground surface that is discernible by 

onsite examination or on aerial photographs. The sandblows are seen only where exposed in walls of excavations at least 1.5 m deep, typically in drainage ditches
COASTAL SOUTH CAROLINA

and borrow pits. At most sites shown on figure 1, at least 

three or four sand blows are exposed within a few 

hundred meters of one another. The following section 

focuses on the geologic setting in which these sand blows 

originated.

REGIONAL SETTING-GEOLOGY AND LIQUEFACTION 

SUSCEPTIBILITY

In South Carolina, the coastal region is known locally 

as the "low country" because it has low local relief (1-3 

m) and low elevation (0-30 m) and because vast expanses 

of swamp and marshland are under water much of the 

year. Most of the Carolina low country is covered by a 5- 

to 10-m-thick blanket of unconsolidated Quaternary 

marine and fluvial deposits, which lies on semilithified 

Tertiary sediments (McCartan and others, 1984). The 

Quaternary sediments primarily occur as a series of six 

well-defined, temporally discrete, interglacial beaches 

and associated back barrier and shelf deposits that form 

belts subparallel to the present shoreline. The oldest 

beach deposits are farthest inland and are at the highest 

altitudes; younger beach deposits are progressively 

closer to the ocean and are at successively lower altitudes. Most beach deposits are 8 to 15 m thick.

Cutting across these marine and marginal-marine 

deposits at nearly right angles are four major rivers, the 

Savannah, Edisto, Santee, and Pee Dee (fig. 1). Bordering these rivers are fluvial terrace sediments of rather 

limited extent that consist almost exclusively of clean 

sand (that is, sand without clay, silt, or gravel).

Figure 1 shows the approximate areal extent of the 

marine-related deposits (beach, shelf, and open-sound 

back barrier) designated as Ql, Q2, and Q3 by McCartan 

and others (1984). The part of figure 1 containing units 

Ql, Q2, and Q3 is shown without shading. Q3 deposits 

are about 200,000 to 240,000 years old (Szabo, 1985) and 

are present as far as 20 to 40 km inland from the modern 

coast. The intervening Q2 deposits are about 80,000 to 

130,000 years old (Szabo, 1985). Unit Ql is closest to the 

ocean and is made of younger deposits. The search for 

sand blows was generally restricted to units Q2 and Q3. 

Older units have such a low susceptibility to liquefaction 

(due to effects of chemical weathering) that the likelihood 

of forming sand blows has been extremely low during the 

late Pleistocene and Holocene. Because unit Ql generally 

has such a high ground-water table, the possibility of 

finding exposed sand blows was quite limited.

Formation of sand blows in any geologic setting 

depends primarily on the depth to the water table, the 

properties and thickness of materials in the depth range 

susceptible to liquefaction during shaking, and the thickness and characteristics of sediments above the zone that 

was liquefied during shaking. Specific relations between 

liquefaction susceptibility and subsequent formation of

sand blows in the South Carolina low country are as 

follows:

(1) A water table very close to the ground surface 

usually greatly increases the susceptibility to liquefaction, even in comparison with a water table at depths of 

only 3 to 4 m. The modern water-table depth is generally 

0 to 1.5 m throughout the low country and is typically a 

subdued mimic of the surface topography. The water 

table is deepest under hilltops and may come to the 

surface in swales. As surface elevations decrease toward 

the ocean, the water table is generally nearer the surface; within 15 km of the ocean, the water table is rarely 

deeper than 1.5 to 2 m.

(2) Clean sands are generally the only materials 

observed to have liquefied in the Charleston region. At 

one site (site SAN, fig. 1) the source sand bed contains as 

much as 3 to 5 percent clay; typically there is less than 1 

percent silt and clay at all other sites. The liquefied sands 

are generally fine-grained, well-sorted (that is, uniformly graded) beach sands.

Principal properties of sand that control liquefaction 

susceptibility during shaking are degree of compaction or 

state of compactness (known as "relative density" by 

geotechnical engineers), sand-grain size and sorting, and 

cementation of the sand at grain-to-grain contacts.

The state of compactness is commonly a reflection of 

the energy (or mode) of deposition. To illustrate, beach 

deposits laid down in a high-energy environment such as 

a pounding surf zone are generally more dense than sand 

deposited in a quiet zone away from the influence of the 

surf. Higher density makes the surf-zone deposits more 

difficult to liquefy. Locally within the surf zone, though, 

there are many regions of low-energy deposition where 

the sand is not so compacted by wave pounding. In 

addition, for a given relative density, the fine-grained, 

well-sorted sands of ancient and modern beaches 

throughout the low country are much more susceptible to 

liquefaction than standard sands used for engineering 

analysis (Cullen, 1985), and this increased susceptibility 

due to grain-size effects and lower local compactness 

must account in part for the widespread liquefaction 

during the 1886 earthquake.

Cementation in near-surface subaerial environments is 

chiefly a reflection of the age of the deposit. Even slight 

cementation by agents such as silica, calcite, or clay 

dramatically reduces liquefaction susceptibility; thus liquefaction potential is related to age of deposits in many 

geologic settings (Youd and Perkins, 1978), and this 

relation is particularly apparent in the low country. 

Table 1 lists deposits in the low country shown in figure 

1 in terms of depositional environment (type of deposit) 

and age and provides an estimate of their relative 

susceptibilities to liquefaction during strong seismic 

shaking.
4 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

.' ;Barrier-bar deposit, 

' uniformly sized sand

Near-shore deposit 

of shelly, silty, 

and clayey sand

FIGURE 2. Schematic cross section of representative barrier showing sediment types, ground-water table locations, filled sand-blow craters, and 

Bh (humate-rich) soil horizons. Modern shoreline is located southeastward. Lagoonal clay deposit at left is younger and lower in elevation than 

the barrier-bar deposit.

Features large enough to be interpreted as possibly 

having an earthquake origin in the low country were 

found only in sand deposits having a total thickness 

exceeding 2 to 3 m; within this 2- to 3-m-thick deposit, 

the thinnest individual source stratum was 0.3 m.

(3) The local geologic setting has a major role in the 

formation of earthquake-induced sand blows. The geologic setting most frequently associated with sand blows 

is the crest or flank of Pleistocene beach ridges, where a 

thin surficial cover of a clay-bearing sand or humate-rich 

sand overlies clean sand. According to first-hand observations of effects of the 1886 earthquake by Earl Sloan, 

"these craterlets are found in greatest abundance in belts 

parallel with (beach) ridges and along their anticlines" 

(Peters and Herrmann, 1986, p. 68). A schematic cross 

section through a typical low-country beach ridge, such 

as the ridges described by Sloan, is presented in figure 2. 

To a much lesser extent, sand blows have been found in 

back-barrier environments.

A thin clay-bearing stratum (or other stratum having 

very low permeability) above the liquefied zone is generally an important control on development of sand blows 

(Scott and Zuckerman, 1973; Obermeier, 1988; Ishihara, 

1985). A veneer of nonliquefiable sediment, 1 to 2 m 

thick, aids greatly in the formation of and recognition of 

sand blows in the low country. On the other hand, a 

clay-bearing or other low-permeability stratum thicker 

than 4 to 5 m prevents sand-blow formation at the great 

majority of sites in the low country; a thickness greater 

than 2 to 3 m seriously impedes such formation.

CHARACTERISTICS OF AND CRITERIA FOR SAND-BLOW 

FORMATION

Two kinds of pre-1886 sand blows have been recognized: (1) filled sand-blow craters (craterlets) and associated sedimentary structures and (2) sand volcanoes that 

have vented to the surface, leaving relict sand mounds. 

The crater-type sand blows generally occur only where a 

surficial soil having less than several percent clay has 

formed on a parent material of clean sand. Where the 

surficial soil is richer in clay, vented-sand volcanoes are 

much more likely to form.

Exact locations were not known for 1886 craterlet sand 

blows when our study was initiated in 1983. Only one 

1886 vented type of sand volcano had been discovered 

and examined (Cox, 1984). Thus it has been necessary to 

locate sedimentary features that display structures consistent with an earthquake origin and to develop criteria 

for interpreting whether or not these features have an 

earthquake origin.

TABLE 1. Estimated relative susceptibility of saturated cohesionless 

sands to liquefaction during strong seismic shaking

[Source: Youd and Perkins (1978). Strong seismic shaking is determined by two parameters, peak horizontal acceleration and duration 

of largest acceleration. For a mb = 5 earthquake, strong seismic 

shaking is defined as an acceleration of about 0.2 g for at least several 

seconds; for a stronger earthquake, the threshold acceleration is 

about 0.15 g for a duration of 10 seconds; for a much stronger and 

longer duration earthquake, the threshold acceleration can be less 

than 0.1 g (T.L. Youd, Brigham Young University, oral commun., 

1985)]

Liquefaction susceptibility for deposits of various ages

Types of deposit

Beach (low wave

Beach (high wave

pripyoTr^

River channel ....

Holocene

<500yr

High

High 

High

Moderate 

Very high 

High

<500yr

Moderate

Moderate 

Moderate

Low 

High 

Moderate

Pleistocene

Low

Low 

Low

Very low 

Low 

Low

Pre-Pleistocene

Very low

Very low 

Very low

Very low 

Very low 

Very low
COASTAL SOUTH CAROLINA

Geologic criteria that we have developed for interpreting whether near-surface features are earthquakeinduced sand blows generally have four elements:

1. The features have sedimentary characteristics that 

are consistent with an earthquake-induced liquefaction 

origin; that is, there is evidence of an upward-directed, 

strong hydraulic force that was suddenly applied and was 

of short duration.

2. Characteristics such as shape, width, and depth are 

consistent with historical observations of liquefaction 

during the 1886 earthquake.

3. The features are in ground-water settings where a 

suddenly applied, strong hydraulic force of short duration could not be reasonably expected except from 

earthquake-induced liquefaction. In particular, these 

settings are extremely unlikely sites for artesian springs.

4. Similar features occur at multiple locations, preferably within a few kilometers of one another, having 

similar geologic and ground-water settings. Where evidence of age is present, it should support the interpretation that the features formed in one or more discrete, 

short episodes that individually affected a large area and 

the episodes were separated by long time periods during 

which no such features formed.

As fewer of these criteria are satisfied, the confidence 

in an earthquake origin generally diminishes. Subsequent sections describe the application of the four criteria.

Whether earthquakes or other mechanisms induce 

small liquefaction-flowage features is often impossible to 

determine. Small synsedimentary liquefaction-flowage 

features such as dikes and sills as much as 0.25 to 0.5 m 

long and a few centimeters thick are not unusual in 

point-bar deposits and in beach surf zones. Very small 

features similar to sand blows are also common in beach 

surf zones. These small features result from a variety of 

forces, including dynamic wave loadings and static 

slumping. An earthquake origin was considered a possible mechanism in this study only if the flowage features 

were much larger than these small features and if, in 

addition, the flowage features cut the surface soil profile.

FILLED SAND-BLOW CRATERS

All filled sand-blow craters belong to a single morphologic group having many common features in the fill 

sediments, although systematically occurring variants 

also occur. The normal type of filled sand-blow crater is 

discussed first. In a later section there is discussion of 

two variants that may indicate association with 

earthquake-induced landslides.

Almost all pre-1886 sand-blow sites shown on figure 1 

have sand blows whose original morphology and size are 

comparable to the 1886 craters described by Button

(1889), except that the craters are now filled with 

sediment. Figure 3 shows craters that represent 

moderate- to large-sized craters produced by the 1886 

earthquake. A crater is a hole at the ground surface that 

forms as liquefied sand vents to the surface. In the 

process, the forceful upward surging of sand and water 

also scours and enlarges a hole and deposits sediment 

beyond the crater. When flowage stops, a surficial sheet 

of ejected sand and soil surrounds the rim of the crater, 

and clasts of dark soil are commonly scattered along the 

base of the steep wall (fig. 3). Examination of photographs shows that, in 1886, the surficial sheets commonly 

appear to have had thicknesses near crater rims of about 

15 to 20 cm and maximum diameters rarely exceeding 

about 3 to 4 m. The maximum reported thickness of 

vented sand was 1 m, and the maximum crater diameter 

was about 6 m (Button, 1889).

About 75 percent of the sites on figure 1 are on barrier 

and nearshore marine sands. The sands are almost 

exclusively well-sorted, fine- to medium-grained quartz, 

with less than 5 percent heavy minerals and mica 

(McCartan and others, 1984; Gelinas, 1986). Surface 

weathering has imposed a soil profile on these marinerelated sands. The soils are classified as spodosols, 

alfisols, and ultisols, but most sand blows occur in poorly 

drained areas where humic spodosols (humods) have 

formed. Humods are characterized by a thin (<10- to 

15-cm) surficial A horizon (organic matter and several 

percent sand) overlying a thin (<10- to 15-cm), very light 

gray E horizon; the E horizon overlies a thick (0.5- to 

1.5-m) black Bh horizon (humate-enriched sand containing a few percent clay) and a variably thick (0.1- to 1-m), 

gray to light-yellow B-C horizon (transition zone 

between B and C horizons). The B-C grades down into 

C-horizon sands (parent material), which are very light 

gray in the upper 1 to 2 m and grade down to a greenish 

hue. Beneath the upper 0.5 to 1 m of the C horizon, the 

original bedding consists of thin, horizontal, black heavymineral laminae about 0.25 to 0.5 cm apart. The sandblow features cut the solum and the C horizon. The 

source sand beds that liquefied during earthquake shaking generally occur within the depth range of 3 to 10 m.

Figure 4 is a vertical section of a filled sand-blow 

crater that is representative of the type observed at 

almost all of our study sites (fig. 1). The figure illustrates 

characteristics that we consider to be compatible with an 

earthquake-induced liquefaction origin. In figure 4, the 

soil horizon is cut by an irregular crater, which is filled 

with stratified to structureless (that is, nonstratified) 

and graded sediments. The fill materials are fine- to 

medium-grained sand and clasts from the Bh, B-C, and C 

soil horizons and sand from depths much below the 

exposed C horizon. Walls of the crater are generally
6 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

FIGURE 3. Craters produced by the 1886 earthquake. A, Sketch from 

a photograph of an 1886 crater (sand blow at Ten Mile Hill, near the 

present Charleston airport). Note that the crater contains sand 

sloughing toward the lowest parts and that there is a constructional 

sand volcano located in the right part of the crater (at arrow). The

smooth and sharp when viewed up close, especially in the 

lower part of the crater. Walls of some craters are very 

jagged, however.

The Bh horizon generally is much thinner over the 

central part of the filled crater than on the sides, and the 

Bh horizon of the laterally adjoining undisturbed soil is 

abruptly thicker. Clay content in the Bh horizon in the 

crater is much less than in the undisturbed soil. Figure 5 

is a sand-silt-clay ternary diagram of particle sizes of 

pairs of samples taken at identical depths inside and 

outside the liquefaction craters. The lines connecting 

pairs of samples show that crater samples have consistently less clay and silt than the adjacent undisturbed 

soil. The Bh horizon on the filled crater typically is 

thicker, is more clay rich, and has better developed soil 

structure (that is, peds) with increasing age; craters 

older than about 5,000 to 10,000 years have Bh horizons 

that approach the thickness and development of those in 

laterally adjacent undisturbed soils.

A sequence of five layers, each having specific characteristics, occurs beneath the Bh horizon in the filled 

crater. The sedimentary characteristics of these layers 

are less distinct in older craters because of pedogenesis 

in the crater deposits. Layer 5 (fig. 4) is a structureless 

(that is, massive), gray, humate-stained sand, which 

overlies a thinly (2- to 3-mm) laminated sequence of 

alternating light- and dark-colored sands Gayer 4). The 

lamina typically are discontinuous and irregular in thickness, as illustrated in figure 4. The dark color is generally

crater is surrounded by a thin blanket of sand partly veneered with 

cracked mud. B, Photograph of typical crater produced by the 1886 

earthquake. Note the thin blanket of ejected sand around the crater 

and sand and clasts of dark soil within the crater. (Photograph from the 

archives of the Charleston museum.)

imparted by humate staining; the dark lamina may 

contain appreciable amounts of silt and clay. The basal 

bed of this sequence (layer 4) is clay-rich in perhaps 10 

percent of the filled craters and is rarely thicker than 1 

cm. The basal bed sharply overlies a medium-gray structureless sand (layer 3). Layer 3 contains many small 

clasts (1- to 5-mm diameter) of Bh material, charcoal, and 

wood. This clast-rich layer grades down into a structureless sand zone (layer 2) containing many intermediatesized clasts (5- to 20-mm diameter) of friable Bh material 

and occasional extremely friable clasts of light-colored 

sand. The clasts of Bh material and sand have, respectively, the same color, mineralogy, consistency (that is, 

resistance to being crushed between fingers), and friability as the adjoining sands of the B-C and C horizons. 

Layer 2 grades down into layer 1, containing densely 

packed intermediate-sized (1-5 cm) and large-sized (>5 

cm) clasts of Bh material in a structureless sand matrix; 

the large clasts have diameters exceeding 25 cm in many 

filled craters. Many of the clasts in layer 1 have their long 

axes vertically oriented. At and below the thinly stratified sequence (layer 4), the sides of the bowl are sharply 

defined by a color boundary and by the presence of clasts 

within the bowl.

Beneath the bowl are vents containing structureless 

clean sand. Sides of the vents sharply cut bedding in the 

C horizon.

The matrix sand in layers 1 through 3 contains an 

extremely small percent of clay-sized material and clay
COASTAL SOUTH CAROLINA

Nonbedded 

organic-matter-rich san

/ Layer 4: 

Bedded sequence ' - . Layer 5: Structureless sand .'.-.' :

/ . ..- '. ' . .". . .. '.'' . '. // of alternating clean 

 '-.' '.'." ' -. '-.; ' ' .' '.'.' ' .' ' ./ ' ' . - Y/ancl organic-matterrich sand

Layer 3: Sand and 

many small clasts

1: Sand and large clasts"^ ~ ~ 

__ __ - - vents containing ^~of Bh-horizon material'-^

 - - structureless, clean sand <^'

 o

15 INCHES 

II I __

 I I } }

 0 30 CENTIMETERS 

__ NO VERTICAL EXAGGERATION

 / ___ . ___ ___ . _Q Bedded clean sand -

FIGURE 4. Schematic cross section of normal type of filled sand-blow crater. Letters correspond to soil horizon designations. The filled crater 

in this figure much predates the 1886 earthquake, based on thickness of Bh horizon in the filled crater.

10% silt

100% sand 0% silt

o undisturbed soils 

 liquefaction features

10% clay

FIGURE 5. Ternary diagram showing grain sizes of paired samples at 

identical depths inside and outside liquefaction craters.

minerals. Both the percent of clay minerals and percent 

of clay- and silt-sized material in layers 1 through 3 are 

much less than in the laterally adjoining undisturbed 

B-C and C horizons, particularly for craters formed less 

than a few thousand years ago (Gelinas, 1986). The 

maximum grain size of the matrix sand in layers 1 

through 3 is greater than in the laterally adjoining and 

overlying undisturbed materials in some filled craters.

The following phases have been interpreted in the 

formation of the filled sand-blow craters: (1) after 

earthquake-induced liquefaction at depth, a large hole is 

excavated at the surface by the violent upward discharge 

of the liquefied mixture of sand and water; (2) a sand rim 

accumulates around the hole by continued expulsion of 

liquefied sand and water after the violent discharge; (3) 

sand, soil clasts, and water are churned in the lower part 

of the bowl, followed by settling of the larger clasts and 

formation of the graded-fill sequence of sediments; and 

(4) the crater is intermittently filled by adjacent surface 

materials to form the thin stratified-fill sequence, during 

the weeks to years after the eruption. In the craters
8 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

predating the 1886 earthquake, the sand blanket ejected 

from the crater is indistinguishable in the field from the 

surface and near-surface (A, E, and Bh) soil horizons, 

because the blanket has been incorporated within these 

soil horizons.

This interpretation of origin of the filled craters is 

based in part on comparison with photographs and 

descriptions of 1886 craters. The filled craters are similar 

to the 1886 craters shown in figure 3 in that the dimensions (diameters) at the surface are about the same and 

the depths to the contact of the stratified fill to the 

graded fill (see fig. 4) are about the same. The lowermost 

stratified layers of filled craters (layer 4) are composed of 

humate-stained, slightly clayey or clean sand that generally contains a few clasts of Bh horizon material and, in 

a few filled craters, clasts of C horizon sand. (See fig. 4.) 

Presence of the intact very weak, friable clasts of C 

horizon sand suggests very rapid initial infilling of the 

crater, as illustrated in figure 3J5. In the filled craters, 

clasts of C-horizon material first must have been ejected 

onto the adjacent ground surface or walls of the crater 

before falling back or washing back into the bottom of the 

crater and being deposited within the beds of layer 4. We 

know of no other reasonable explanation of the means by 

which these clasts could have been transported to the 

stratified zone.

The presence of friable, angular clasts of C- and 

Bh-horizon material in the graded-fill portion is consistent with a short-lived, churning type of upwelling from 

the vent. Water commonly flows for a day or so from 

earthquake-induced sand blows. The violent, boiling 

phase is much shorter in duration. Hence, the presence 

of friable clasts argues against a long-term artesian 

spring origin for these features; a spring-induced longterm churning phase, lasting days, that slowly diminishes in flow would abrade, round, and (or) disintegrate 

the clasts of Bh- and C-horizon material. Short-term, 

nonearthquake-related springs have been eliminated as a 

possible mechanism by which craters are formed along 

the crest of the beach ridges, because such springs 

cannot form in this topographic-geologic setting. (This 

point is discussed in detail later.)

Our interpretation of the origin of the craters is also 

supported by the presence of sand-filled tabular fractures, whose overall shape and dimensions strongly 

suggest that they are "incipient craters." These fractures 

are rather common at some places in the meizoseismal 

zone of the 1886 earthquake, where craters are plentiful. 

Figure 6 shows V- and U-shaped fractures (fig. 6A also 

shows a connecting vent) that are filled with sand we 

bslieve was transported upward from depth, on the basis 

of the freshness of minerals in the fractures. The fractures, which are tabular, generally widen with depth

until they connect to a single, near-vertical large sandfilled fissure (that is, a vent). The sand-filled fractures 

probably represent the early phase of development of 

craters; for the features in figure 6, however, the upward 

forces were too weak to excavate the overlying material.

It is possible that liquefaction led to the production of 

craterlets because of a fortuitous combination of sediment properties in and above the zone that liquefied 

during earthquake shaking. The source beds that liquefied were exceptionally susceptible of liquefaction, in 

that generally they were very loose (engineering sense), 

fine-grained, uniformly sized, and free of clay (Dickenson 

and others, 1988); these properties would cause the 

source beds to liquefy abruptly and, once liquefied, to 

flow readily (Seed and others, 1983; Youd, 1973). We 

suspect that the liquefied sand strata suddenly applied a 

large point force to the overlying sediment (through a 

hole or weak zone such as that left by a decayed root), 

causing a V- or U-shaped crack to form, through which 

the liquefied sand violently vented because of its exceptional ability to flow. The V- and U-shaped cracks 

occurred because overlying sediment is humate cemented, has no pronounced planes of weakness, and is 

very brittle; the process is similar to formation conchoidal fracture in an isotropic, brittle medium, caused by 

the application of a point load.

In summary, the filled craters that we found have a 

morphology that is consistent with descriptions and 

photographs of craters caused by the 1886 earthquake. 

The general geologic setting (Pleistocene beach crests 

and flanks) and the locations of swarms of sand blows 

reported by geologists immediately after the 1886 earthquake (Peters and Hermann, 1986) are very near or 

coincident with two sand-blow sites found during this 

study (HW and ARP on fig. 1). The morphology of the 

walls of the craters, the stratigraphy of the fill of the 

craters, and the evidence that they formed in relatively 

sudden, discrete episodes serve to demonstrate that the 

filled craters are the result of earthquake-induced liquefaction. Alternate origins, such as short- or long-term 

artesian springs, ocean wave pounding, thrown trees, or 

other mechanisms that have the potential to create 

similar features are discussed in detail in the section 

entitled "Other Possible Origins."

VENTED-SAND VOLCANOES

Figure 7 is a schematic cross section of a pre-1886 

vented-sand volcano. Where the zone that liquefied at 

depth is overlain by a thick clay-rich stratum, a sand 

blow typically is expressed on the ground surface as a 

mound of sand. The thickest part of the mound ranges 

from a few centimeters to as much as 0.25 m. At a few
COASTAL SOUTH CAROLINA

C-horizon sand~~ - -

BANK 

OVERHANG

// Brownish-black, 

massive Bh-horizon sand

Sand-filled fissure Sand-filled fissure

___ 

 -- clasts of - .-

--^- Bh sand

 C-honzon sand - - ;,

^~_^ sand rr^ir^rr_^

 -~ C horizon n^m_^.

FLOOR OF DITCH 0 30 CENTIMETERS

NO VERTICAL EXAGGERATION

FIGURE 6. Schematic cross sections showing sand-filled fissures interpreted as resulting from liquefaction and flowage during the 1886 

earthquake. Light-yellow, intruded sand in fissures is determined to have been vented from a depth of 6 m, on the basis of grain-size 

and mineralogical data. A, V-shaped sand-filled fissure and vent. Fissure cuts soil horizon developed on pre-1886 filled sand-blow 

crater. B, U-shaped sand-filled fissure.
10 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

Soil rich in organic matter <L///x//^/&/

zJaint bedding plane 

Clasts of clay-bearing stratum maximum depth^ 

: to unweathered 

clean sand in vent:

Jnweathered clean 

 -to highly weathered 

zr: clay-bearing

Unweathered 

clean sand, 

ZLZT^^T structu reless

FIGURE 7. Schematic vertical section of representative vented-sand volcano.

localities, mounds have discernible bedding. The mounds 

are generally thickest above their junctions with the 

widest steeply dipping sand-filled dikes (that is, feeder 

vents) that extend downward through the clay-bearing 

stratum. Often the sand in the mound and in the dikes 

contains clasts of the clay-bearing stratum that have 

been torn from the walls of the vent. The sand in the 

dikes is structureless and has no discernible bedding; the 

sand in the central parts of the dikes may be better 

sorted and coarser than that near the edges. Dike widths 

range from about 0.25 to 1 m.

An 1886 sand blow of this type was described by Cox 

(1984). Similar sand blows have been observed at many 

places around the world, including the New Madrid 

seismic zone (to be discussed later). Figure 8 shows a 

variant of this type of sand blow, in which the vented 

sand has coalesced into a continuous surface cover; some 

of the white surface sand is pedogenic E horizon, which 

is virtually indistinguishable from the older, vented 

sand. The volume of fluidized sand expelled to the 

surface has been so large at some places that the 

clay-bearing stratum has downdropped noticeably. At 

the site shown in figure 8, for example, comparison of soil 

structure and clay content of the undisturbed soil with 

the clay-bearing stratum above the hand shovel showed 

that the stratum above the hand shovel had been downdropped about 1 m.

Near-surface sediments in the depth range of 1.5 to 2 

m are commonly so intensely weathered and discolored 

that textural analysis is required to determine a possible 

origin by venting. A useful (but not sufficient) test for 

venting is comparison of the coarsest sand fraction in the 

suspected vent with the grain size of sand in the laterally 

adjoining clay-bearing stratum or soil horizon. An earthquake origin for the sand is not considered likely unless 

the coarsest sand in the dike is significantly different in 

size from that in the crosscut clay-bearing stratum. Such 

a textural difference cannot be the result of soil-forming 

processes. In addition, there must be no possibility that 

the coarsest sand fraction has been introduced from 

above the vent. At three vented-sand volcano sites 

shown on figure 1 (sites CH, BR, and SAN) that have 

been interpreted as earthquake induced, other evidence 

of venting is also present. Examples include clay-bearing 

clasts in the sand mound combined with sand-filled sills 

and dikes that widen downward and, at depth, extend 

into slightly weathered and unweathered sediments. At 

some places, the sand-filled dikes and sills cut through 

ground that appears to have been shattered (that is, 

irregularly, intensively fractured and intruded), which is 

very suggestive of forceful intrusion.

Features similar to the vented-sand volcanoes, but for 

which springs or other nonearthquake sources cannot be 

easily eliminated as possible origins, are common in
COASTAL SOUTH CAROLINA 11

FIGURE 8. Vented-sand volcanoes that have coalesced to form a continuous sheet of sand on the ground surface. At least two 

episodes of venting separated by long periods of time are represented. Note irregular pattern of fracturing. (Shovel is 

60 cm long.)

many lowland areas. These sites have not been included 

on figure 1.

FEATURES SHOWING EVIDENCE OF LATERAL SPREADS OR 

GROUND OSCILLATIONS

Lateral spreads8 were commonplace features of the 

1886 earthquake (Peters and Herrmann, 1986). Most of 

these spreads developed in fluvial terrace deposits, bordering streams in low areas. Obvious surface evidence of 

lateral spreads has not persisted to the present, and 

ditches and pits are so rare in the wet locales where 

lateral spreads formed that none has been found. However, along the flanks of some Pleistocene beaches, 

reverse shears are present that may indicate an incipient 

formation of lateral spreads or ground oscillations. Shear 

displacements commonly range from 1 to 4 cm. Slope of 

the ground surface is typically less than 1 percent for 

hundreds of meters downslope or upslope. These slopes 

are so gentle and the possibility of high artesian pressures is so remote that gravity-induced slumping is 

virtually impossible. Some of these reverse shears 

almost certainly have an origin in earthquake-induced 

liquefaction. At one site, for example, reverse shears

3A lateral spread is a landslide formed by a laterally moving slab, commonly 

of large areal extent; earthquake-induced lateral spreads commonly form where 

surface slopes range between 0.5 and 5 percent (Youd, 1978). The presence of 

lateral spreads generally indicates a large areal extent of liquefied material.

dipping in opposite directions (toward one another) 

formed about 10 m apart in the stratum that liquefied 

during shaking, and sand blows traceable to this liquefied 

stratum formed between the shears; the most likely 

mechanism that could have formed the opposite facing 

shears was alternating directions of ground movement, 

caused by earthquake oscillations, in the liquefied stratum. (Such a mechanism is illustrated as fig. 2-11 in 

"Liquefaction of Soils during Earthquakes," published 

by the National Academy Press, Washington, D.C., in 

1985.)

Reverse shears can occur as isolated features but are 

generally found in association with sand blows. Figure 9 

shows the typical relationship for filled craters. A 

reverse shear is present on the downslope side of the 

crater, and the upthrown block cuts the C, B-C, and Bh 

horizons. The shear formed prior to venting of sand from 

the source stratum at depth, because the vent is not cut 

or distorted.

At some sites, the shears along crater edges could 

have formed only in response to earthquake-induced 

lateral spread movement because the shears are traceable into and along the bedding of the stratum that 

liquefied. Only rarely is an exposure deep enough to 

allow determination of whether the shear goes into a 

stratum that liquefied, and thus an earthquake origin 

cannot be confidently assigned to all sites. However, we 

are of the opinion that these reverse shears along crater 

edges suggest an earthquake origin, even where the
12 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

Downslope (0.5-2 percent) 

A

..Layer 5: Structureless sand -'.;";

ayer 4: Bedded sequence 

of alternating clean and/

Layer 3: Sand and 

many small clasts

: Layer 2: Sand and . 

. . scattered clasts of .

"-. " ' .:." "-.' ' clean sand and -'; 

Bh-horizon material!

' ".' ( I ' ' ' ~ " 

..' ."j^r.r-^r: Layer 1: Sand and large clast

-of Bh-horizon materialrzirr^ir

_ Vents containing ~^ 

structureless, clean sand

FIGURE 9. Schematic cross section of filled sand-blow crater, illustrating aspects possibly associable with downslope movement. A reverse shear 

soil is along the lower left side of the filled crater; an under-turned edge forms the lower right part of the filled crater beneath the B-C and 

C soil horizons; large clasts along the right side are much higher than in other soil parts of the filled crater.

shears have formed on gently sloping ground (less than 2 

percent) as much as 5 m below the beach crest.

Another aspect of filled craters that possibly indicates 

an incipient lateral spread is also illustrated by the 

position of the clasts along the right side of the crater of 

figure 9. Large clasts occupy the region from the vent to 

the vicinity of the bedded sequence (layer 4). The lower 

part of this clast zone turns beneath the B-C and C 

horizons. These clast relations are found in perhaps 25 to 

50 percent of craters having reverse shears. The consistent positional relationship of underturned edge with 

clasts along the crater edge suggests control by minor 

downslope movement, which took place while the sediments in the crater were still fluid enough to segregate 

slightly.

REGIONAL DISTRIBUTION OF SAND BLOWS

The regional distribution of sand blows is systematic 

and predictable in coastal South Carolina, in terms of size 

and number of sand blows. This distribution conforms

with the pattern that would be anticipated from an 

earthquake origin.

Numerous worldwide observations show that the number, size, and type of earthquake-induced liquefaction 

features are a reflection of three principal factors. Of 

primary importance are (1) severity of shaking (controlled by earthquake accelerations and duration (Seed 

and Idriss, 1982)), (2) liquefaction susceptibility (controlled largely by state of compactness and cementation 

(Youd and Perkins (1978)), and (3) local geologic controls 

(affected primarily by depth to water table, thickness of 

impermeable sediments above liquefied stratum, deformation properties, permeability of sediments above liquefied stratum, and permeability of liquefied stratum 

(Ishihara, 1985; Obermeier, 1989; Obermeier and others, 

1986)). The local geologic controls on the Pleistocene 

beaches in coastal South Carolina are constrained within 

a very narrow range because (1) bedrock accelerations 

should have been amplified about the same amount at 

many places by previous earthquakes, (2) the liquefaction susceptibility of the source sand beds has been about 

the same, and (3) local geologic controls on morphology
COASTAL SOUTH CAROLINA 13

and size of sand blows on beach ridges has been about the 

same, for a time period extending at least throughout the 

Holocene (Obermeier and others, 1989; also discussed 

later). Thus, in the vicinity of the epicentral region, the 

sand blows should generally be largest (have largest 

diameters) and be most abundant. This hypothesis is 

confirmed in the Charleston region by observations of 

effects of the 1886 earthquake (Peters and Herrmann, 

1986). Sand blows produced by the 1886 earthquake were 

abundant within the 1886 meizoseismal zone, which was 

a region about 35 km wide and 50 km long. Beyond this 

zone, the sand blows were smaller and scattered. Only 

rare, small sand blows formed more than 10 km beyond 

the meizoseismal boundary. Because all the factors 

involving sand-blow production are about the same in 

many places throughout coastal South Carolina, this 

same relation of sand blows to epicenter location should 

hold true for pre-1886 Holocene earthquakes.

In particular, it has been found that for craters having 

one apparent age (based on soil profile) and whose 

diameters (measured near the ground surface) are less 

than about 1 m, there are at most two or three craters 

exposed in a nearby 1-km-long ditch cutting across beach 

ridge deposits. Wherever maximum diameters of craters 

having one apparent age are about 2 m, there are more 

(as many as 5 to 10) craters exposed in a nearby 

1-km-long ditch. Wherever maximum diameters of craters having one apparent age are 3 m or larger, there are 

a greater number (as many as 20) exposed in a nearby 

1-km-long ditch. This well-defined relation between 

number of sand blows and sand-blow diameters is consistent with an earthquake origin in our opinion.

We emphasize that a single, isolated feature that 

appears to be earthquake-induced would not be interpreted by us to be compelling evidence for prior earthquake shaking. Compelling evidence requires at least 

several features, scattered over a region of at least 

several square kilometers.

OTHER POSSIBLE ORIGINS

Origins other than earthquakes for filled sand-blow 

craters and vented-sand volcanoes that have been considered include compaction-induced dewatering and softsediment deformation; artesian springs; landslides; fillings in decayed stump and root holes; ground disruption 

by fallen, root-wadded trees; and liquefaction caused by 

storm-induced, ocean-wave pounding. Criteria for 

assessing each of these potential sources are discussed 

below.

Compaction-induced dewatering and deformation. 

Syndepositional and postdepositional dewatering by 

compaction occurs in sediments during or shortly after 

their deposition. This dewatering generally takes place

in response to rapid deposition of sediments (especially 

coarser, denser sediments) above very soft, clay-rich 

sediments, which causes buildup of pore-water pressure 

and gravitational instability (Dzulynski and Walton, 

1965; Alien, 1984; Lowe, 1975,1976); deposition of silts or 

fine sands rapid enough to cause buildup of pore-water 

pressure can also lead to instability (Dzulynski and 

Smith, 1963; Sanders, 1960). The kinds of sedimentary 

structures formed by these processes include load structures, dish structures, convolute lamination, sand dikes, 

and faults (Pettijohn and Potter, 1964). In our study, 

earthquake-interpreted features generally have soil horizons that are much thinner and less well developed (and 

thus younger) than soil horizons on laterally adjoining, 

undisturbed parent sediments; in the relatively few 

instances where both the parent sediments and crosscutting features have essentially the same degree of soil 

development, the earthquake-interpreted features contain clasts of Bh material at depths generally about 0.5 m 

below the laterally adjoining Bh material in the parent 

sediment. The apparent difference in age between the 

parent sediment and filled-crater or vented-sand volcano 

determined on the basis of soil development, combined 

with lack of reason to suspect sudden, nonseismic surface 

stressing or long-term pore-water pressure buildup, is 

sufficient reason to eliminate postdepositional dewatering as a source mechanism.

Artesian springs. The regional and local topographic 

and ground-water setting of many sites rules out any 

significant likelihood for the occurrence of a sudden, 

strong increase in the hydraulic force of a spring. The 

beach crests are generally flat lying for many kilometers 

along their crests, and the crests are well above the 

lagoonal deposits. Where filled craters are found on 

ancient beaches, a short- or long-term spring origin is not 

believed possible if the following conditions are met: (1) 

the filled craters cut humate-rich sandy soils that are also 

cut with numerous, highly permeable sand-filled root 

holes and burrows that extend well into the C horizon, (2) 

the filled craters are much above the lagoonal deposits 

adjoining the beach (as illustrated in fig. 2), and (3) the 

filled craters are within 1 to 2 m of the beach crest. 

Short-term springs induced by a hurricane deluge (or any 

other mechanism) have not been observed where these 

criteria are met.

Short-term springs are suspected to be rather common 

in lowland areas of lagoonal deposits, however. Features 

interpreted as earthquake-induced sand volcanoes are 

restricted to sites where artesian springs are thought to 

have been unlikely, and in addition, there is other 

evidence for an earthquake mechanism, such as fractured ground (as illustrated in fig. 8). Typical sites are 

elevated locations near deeply entrenched rivers, where 

artesian pressures would have been relieved by lateral
14 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

flow to the rivers rather than by vertical flow to higher 

elevations, which would have been required to form the 

volcanoes.

Landslides. All sites on figure 1 are on level or 

nearly level ground, hundreds of meters from any steep 

slopes. Downslope movement on these nearly level sites 

could be initiated only by seismic shaking, considering 

relations between surface topography, ground-water 

setting, and strength of materials.

Fillings in root holes. Holes caused by decayed 

stumps and roots, and later filled with clean sand, are 

common in the study area. Although the filled holes are 

generally circular in plan view, many display poorly 

defined layers of clay mineral segregation (due to weathering) and gradually taper downward; they do not have 

the well-defined laminar bedding of the filled craters and 

underlying graded zone of sediments. Typically, maximum depths of root penetration are about 2 to 3 m.

Ground disruption by trees. Tree throw in the South 

Carolina low country is generally restricted to hardwood 

species having wide, shallow root systems. These trees 

may blow over in wind storms and rip up a wad of 

sediment caught within their root mats, thus creating 

pits and mounds. Taprooted pines and cypress trees very 

rarely tend to throw; instead, these species break off 

near the ground surface. Pits excavated by hardwood 

tree throw tend to be shallow (usually less than 50-100 

cm deep); when filled, they do not contain sediment 

introduced from depth, and they almost always lack the 

orderly internal stratigraphy of liquefaction craters. At 

some few places, though, pits excavated by thrown trees 

can be distinguished from craters excavated by 

earthquake-induced liquefaction only by the presence of 

feeder vents or by the presence in the crater of sediment 

(generally sand or silt) that has been introduced from 

strata beneath the base of the crater. Verification that 

sediment has been introduced from depth may require 

mineralogical, weathering, and grain-size analysis (Gelinas, 1986).

Ocean-wave pounding. The disruption of subaerially 

formed soils shows that liquefaction occurred long after 

deposition of the parent sediments. Storm-induced, 

ocean-wave poundings are not a credible mechanism of 

liquefaction at most of the widely scattered sites because 

the elevations are well above modern sea level (up to 15 

m) and are too far inland. In addition, no sedimentary 

records indicative of inland surges of the ocean, such as 

soil horizons buried by storm-deposited sediments, have 

been found.

FEATURES OF WEATHERING ORIGIN

A wide variety of features produced by chemical 

weathering mimic earthquake-induced liquefaction features. Such weathering features include the white, Ehorizon sand that commonly blankets the surface; pedogenic tongues; and BE or fragipan horizons. 

Distinguishing liquefaction features from weathering 

features is much more difficult where older liquefaction 

features have been extensively weathered.

A loose, clean, white sand blanket covers large parts of 

the South Carolina low country that is underlain by 

sandy sediments of barrier beach and nearshore marine 

origin. In undisturbed soil profiles, the white sand underlies a thin, 0- to 15-cm-thick, dark-gray A horizon. 

Although some of this white sand has been periodically 

remobilized by surficial processes, it is a pedogenic E 

horizon, formed by weathering and leaching of the underlying sandy sediments (Gamble, 1965). The pedogenic 

origin of the E horizon is demonstrable by its eluvialilluvial relationship with the underlying B horizon. Clays 

and labile minerals have been removed from the E 

horizon, and weathering products have been deposited in 

the B horizon; however, particle-size distribution and 

resistant heavy-mineral percentages are nearly identical 

in both horizons. In addition, the boundary between the 

E and B horizons is commonly abrupt and irregular and 

is characterized by narrow, near-vertical tongues of 

white E horizon penetrating downward into the B horizon. Laterally continuous, interpenetrating boundaries 

of this type are more likely to be indicative of geochemical leaching rather than clastic deposition. At some sites 

resistant sedimentary features, such as thin pebble beds, 

pass through the horizon boundary, showing that it is not 

a sedimentary contact.

In summary, although a blanket of ejected white sand 

often exists at earthquake liquefaction sites, all white 

sand blankets are not necessarily formed by earthquake 

liquefaction. In younger liquefaction features, bedding 

within ejected sand blankets helps to demonstrate liquefaction origin if other fluvial and eolian origins can be 

rejected. With age, the usefulness of this criterion 

decreases as soil mixing by flora and fauna destroys 

bedding, making ejected sand blankets appear superficially like massive pedogenic E horizons.

The pedogenic boundary between E and B horizons 

can be gradational or quite sharp and can also be highly 

convoluted. Gamble (1965) describes boundaries between 

loose, white E-horizon sand that grades over 1 to 2 mm 

of depth to brown, clayey sand. The boundary also 

commonly has 2- to 3-cni-wide, 5- to 10-cm-long tongues 

of E horizon descending vertically into the underlying B 

horizon. Locally, we have observed tongues of E-horizon 

sand that extend more than a meter into thick, red to 

brown (7.57/2 5/6-57/2 5/8), clayey B horizon (fig. 10). 

Tongues of this size and shape can give the impression of 

fractured and brecciated ground and might be mistaken 

for liquefaction features unless examined carefully for 

sedimentary characteristics.
COASTAL SOUTH CAROLINA 15

Ground surface

Massive white sand

Massive, light-gray to

light-yellowish'brown

sarid to loamy sarid

Sharply bounded bodies of

strong brown foamy sand

Massive, loose, - - - - - 

light-gray sand -- - --

/\ Samples from B horizon 

C_y Samples from tongues

20 INCHES

50 CENTIMETERS

NO VERTICAL EXAGGERATION

O

100

90

80

70

60

50

40

30

20

10

A samples from B horizon 

O samples from tongues

1 2 3 

Particle size, phi units

FIGURE 10. Schematic sketch and grain-sized data for pedogenic 

tonguing. A, Pedogenic tonguing of BE- and E-horizon sand into 

underlying B horizon of an exposure in a drainage ditch near 

Rincon, Ga. (near Savannah). B, Cumulative particle-size distributions for sand fraction samples from Rincon, Ga., shown in A. 

B-horizon samples (triangles) are virtually indistinguishable from 

tongue samples (circles), indicating that the tongues are probably 

not vented sand.

The single field criterion that is most useful for distinguishing pedogenic tongues from liquefaction features is 

the downward closing of the narrow, nearly vertical sand 

bodies. Pedogenic tongues almost always close downward. Alternatively, if they connect at depth with a 

source bed, an earthquake-induced liquefaction origin is 

possible. Unfortunately, many exposures are too shallow 

to allow use of this criterion.

Pedogenic tongues can range in morphology from 

tubular (Gamble, 1965) to planar (defining B horizon 

polygons; Nettleton and others, 1968a). Hence, tongue 

morphology is tenuous evidence for pedogenic or earthquake origin.

An example of pedogenic tonguing is shown in figure 

10A, which is a schematic drawing from an exposure in a 

drainage ditch near Rincon, Ga. The tongues extend
16 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

White, massive, well-sorted sand, 

very loose and friable . . .

-Grayish-black, massive Bh-honzon sand,

< _-/ *~~ ~^~ -< ^x very firm

/^||Si®liS%>v.-

"Lightto" da*gray 

welMsortedsand^rm

FIGURE 11. Cross section through a white, pedogenic sand tongue from an exposure in a drainage ditch at site HW, near Hollywood, S.C. Prime 

designations on E and Bh horizons indicate the lower horizons of a bisequal weathering profile. Bhir is iron-enriched Bh material.

downward from a massive, pale-brown to gray E horizon and narrow with depth. Tongues may form preferentially along soil structure polygon boundaries (ped 

boundaries) or where tap roots have penetrated the B 

horizon. Once formed, tongues become the loci of concentrated weathering because the higher permeability of 

the leached E-horizon sand guides infiltrating solutions 

through the clayey B horizon. Increased weathering in 

the tongues may alter the particle-size distribution of the 

original sediment either by producing or destroying 

minerals in the finer particle-size fractions. Therefore, a 

comparison of the particle-size gradation exclusive of the 

clay and silt fractions (less than 63 micron) may show 

whether the tongue sand has been injected from a 

separate source. Sand size and cumulative particle-size 

distributions of six samples taken from areas marked in 

figure 10A are shown in figure 105. The extreme similarity between the tongue and the nontongue portions 

strongly suggests that the tongues were weathered in 

place, although there is a remote possibility that injected 

sand could have the same sand-particle-size distribution. 

Alternatively, a liquefaction origin would be more 

strongly suggested if the tongue sand-particle-size distribution was distinctly different from that of the adjacent soil.

Other evidence for pedogenic versus liquefaction origin of tonguelike features is the mineralogy of the sand 

fraction. Because weathering is concentrated in the 

E-horizon tongues, the presence of abundant fresh, labile 

minerals, relative to adjacent soil mineralogy, would be

good evidence for injection. Conversely, if the tongue 

and adjacent soil have similar mineralogies, or if the 

tongue has more highly weathered minerals than the 

adjacent soil, a pedogenic origin is more probable. By 

using mineralogic criteria, Gelinas (1986) assigned a 

liquefaction origin to sand vents at the McL, SAN, and 

CH sites (fig. 1) by demonstrating that suspected vent 

sands have greater abundances of easily weathered 

feldspars and hornblendes than the adjacent undisturbed 

soil.

Another type of pedogenic tonguing is sketched in 

figure 11 from an exposure in a drainage ditch near site 

HW (fig. 1), where the ditch crosses the crest of a 

relatively well drained beach ridge. At least 10 features 

of this size and orientation were exposed in a 60-m 

section of the ditch. In vertical exposures, 10- to 30- 

cm-thick tongues of white sand dip gently downslope. 

The tongues are surrounded by black to dark-orange, 

humate and iron-oxide-stained sand. Some of the nearhorizontal parts of the tongues can be traced for distances up to 7 m. At the upslope end of each tongue, the 

feature abruptly turns up, breaks through the overlying 

Bh soil horizon, and flares toward the surface. At the 

surface, the tongue is continuous with white sand of the 

E horizon. The downslope end of the tongue is commonly 

terminated by black-and-orange-stained rind that is continuous with the Bh horizon; in other cases the tonguerind contact becomes diffuse downslope.

The white sand tongue features were originally interpreted by us as intrusions of liquified sand (Gohn and
COASTAL SOUTH CAROLINA 17

others, 1984, p. 11); however, subsequent observations 

have led to the conclusion that the tongues are weathering features unrelated to liquefaction events. Essentially, they are extreme convolutions in the pedogenic E 

horizon caused by concentrated infiltration of soil solutions at places where taproots of trees have broken 

through brittle, relatively impermeable Bh horizons. 

This interpretation relies on several field observations:

 The tongues are continuous with white E-horizon 

sand overlying the Bh horizon. Pedogenic origin of 

the E horizon sand is established by satisfaction of 

the sedimentologic and mineralogic criteria discussed 

previously.

 The rind surrounding the white tongues is zoned into 

black Bh and dark-orange, iron-enriched Bh material 

(Bhir) stripes identical to the sequence found at the 

top of undisturbed B horizons. The vertical sequence 

of horizons in an undisturbed soil profile shows that 

dark organic compounds are deposited closer to the 

surface than iron oxyhydroxides after solutions infiltrate through the overlying E horizon. A similar 

zonation in the rind surrounding the tongues suggests 

that soil solutions are migrating laterally along the 

tongues and outward toward their margins.

 The sharp to diffuse contacts between the tongues 

and surrounding sands show no appreciable sandgram-size variation across them. The only difference 

between the tongues and the rinds is that the tongues 

contain black and orange colloidal material that 

bridges and coats sand grains.

 No bedding is present in the tongues.

 The flared portion of each tongue (the portion closest 

to the surface) is characterized by evidence of a 

taproot, which is either a rotted root in-place or a 

narrow V-shaped zone of loose, organic-stained sand 

subjacent to the white sand tongue. E-horizon 

tongues were found in all stages of development, 

from incipient leaching and bleaching surrounding 

areas adjacent to relatively fresh taproots to wider 

and more deeply bleached zones that formed as the 

taproots became more decayed.

 The tongues all dip downhill in the direction of the 

low-gradient, shallow ground-water flow system. 

Mottles around rootlets in the C horizon are also 

elongated in downhill directions. Coincidence of the 

tongue orientation with ground-water flow direction 

suggests that after the infiltrating solutions break 

through the Bh horizon where taproots have 

decayed, the infiltrating solutions are entrained in 

the shallow ground-water flow system. Dissolved 

organics and iron oxyhydroxides in the ground water 

are then deposited downgradient as a rind adjacent to 

the E-horizon tongues.

Another category of pedogenic feature that might be 

confused with earthquake-induced liquefaction is the BE 

or BE' horizon. (The prime designation is used to denote 

the lower sequence of eluvial and illuvial horizons in a 

bisequal profile, in the general order A, E, Bh, E', BE', 

B (t,x), C.) Bisequal profiles with BE' horizons form at 

the transition between poorly drained and moderately 

well drained landscape positions (Nettleton and others, 

1968a,b). BE or BE' horizons are transitional between 

eluviated E horizons and illuvial B, Bt (argillic), or Bx 

(fragipan) horizons and are characterized by irregular to 

prismatic bodies of clay-rich sand surrounded by leached, 

clean sand. Studies of this type of weathering profile in 

the North Carolina coastal plain indicate that the clean 

sand forms from the progressive destruction of a clayrich Bt horizon (Daniels and others, 1966; Nettleton and 

others, 1968a, b; Steele and others, 1969). In this process, a Bt horizon is leached at the depth of a fluctuating 

water table, which removes clay and labile minerals and 

leaves patches of white to gray quartz sand that often 

coalesce to define a polygonal pattern in plan view. This 

horizon, composed of patches of leached and unleached 

material, is termed an E' or BE' horizon, depending on 

the extent of leaching. With continued leaching of clay 

and labile minerals, the soil volume decreases, resulting 

in collapse and the formation of a dense Bx horizon.

In the ditch at the HW site, bisequal soils occur 

between red, oxidized spodosols on beach ridge crests 

and black, organic-rich spodosols in interridge depressions. For the example shown in figure 12, the BE' 

horizon is a layer of pale-brown, clayey sand and white 

sand. The top of the layer is 1 to 2 m beneath the ground 

surface, and the layer is 0.3 to 0.5 m thick. Both above 

and below the layer is light-colored, well-sorted, fine- to 

medium-grained quartz sand. This sand cuts vertically 

across the clay layer at many places and creates irregular 

masses of the brown clayey sand. In three dimensions it 

can be seen that the sand forms irregular vertical walls. 

In vertical section, the sand walls are typically about 5 to 

10 cm wide but can be as much as 30 cm wide. They are 

spaced at both irregular and at regular intervals. The 

contacts of the sand and the brown clayey sand are 

typically sharp with regard to both color and texture. 

The sand-grain sizes in both the leached sand and clayey 

sand masses are essentially the same.

Sharp contacts between the leached quartz sand areas 

and the pale-gray to pale-brown clayey material originally suggested that this feature was formed by some 

type of ground disruption, possibly liquefaction. However, the similarity with features attributed to pedogenesis elsewhere and the consistent occurrence in horizons 

underlying undisturbed pedogenic horizons led us to 

interpret these features as pedogenic.
18 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

Bank overhang

NO VERTICAL EXAGGERATION

Massive Bh horizon sand;

grayish-black grading down

to dark-reddish-brown

White, massive, well-sorted sand

Palebrown, 

massive, 

clayey 

sand Palebrown, 

massive, 

clayey

sand

White, well-sorted sand, 

locally stained pale brown

FIGURE 12. Vertical section through BE' horizon at site HW, near Hollywood, S.C.

OVERVIEW OF THE SOUTH CAROLINA LIQUEFACTION 

STUDY

Pre-1886 sand blows were first discovered in 1983 

(Gohn and others, 1984; Obermeier and others, 1985) 

near the town of Hollywood (site HW, fig 1). There, the 

sand blows are exposed in an unusually deep ditch (up to 

3 m) for a distance of 5 km. Over a hundred sand blows 

have been discovered in this ditch, and the depth of the 

ditch has permitted detailed study of entire features to 

depths that include source vents. For these reasons, site 

HW is generally used for the descriptions of filled craters 

in this paper. Since the discovery at Hollywood, features 

we interpret as sand blows have been found at sites 

throughout much of coastal South Carolina.

Features at all sites shown on figure 1 are interpreted 

to be of earthquake origin, although the confidence level 

differs for various sites. Sites where we have greatest 

confidence of an earthquake origin are those where the 

following features occur: (1) craters have formed on 

topographically high beach crests, (2) numerous craters

are present near where craters were reported to have 

been abundant during the 1886 earthquake, (3) ground 

oscillation shears have formed in opposite directions, (4) 

lateral spreads that could not be gravity-induced have 

formed and have shears traceable into a liquefied stratum, or (5) shattered ground is cut by numerous sandfilled dikes in settings where high artesian pressures 

could not have been involved. Sites for which we have 

the highest confidence of an earthquake origin are BR, 

AR, HW, ARP, RRR, CH, FM, WV, SAN, OL, and 

SOPO.

All other sites on figure 1 are filled craters that are 

more than several meters below the beach crest, causing 

the confidence level to be lower. Some craters at site 

MYRB have reverse shears, however, an occurrence that 

makes an earthquake origin seem likely.

Elimination of all sites from figure 1 except those in 

which we have the highest confidence does not affect our 

interpretation of Holocene seismic activity (discussed 

subsequently). Our interpretation about earthquake
COASTAL SOUTH CAROLINA 19

ages and relative severity of shaking is based only on 

data from the sites for which we are most confident of an 

earthquake origin.

EARTHQUAKE AGES

Craters are typically the only features for which 

radiocarbon ages related to earthquake ages can be 

generated, because other liquefaction-related features 

are not found in association with preserved organic 

matter. Three methods have been used to bracket the 

times of crater formation (Weems and others, 1986): (1) 

radiocarbon ages of woody material (tree limbs or pine 

bark) that fell into the open crater soon after crater 

formation, (2) dating of roots sheared off at the edge of 

the crater (predating crater formation) and dating of 

roots that grew into the stratified fill portion of the 

crater (postdating crater formation), and (3) dating of 

clasts of Bh material that fell into the graded fill zone of 

the crater. The first method yields a highly accurate age 

for the time of earthquake occurrence, whereas the other 

two yield a broad range of possible ages. Sufficient data 

have been collected at site HW (near Charleston; fig. 1) 

to show that at least three pre-1886 earthquakes produced sand blows within the past 7,200 years. Radiocarbon dating of a clast of pine bark in a crater at site ARP 

(also near Charleston) independently verifies the middle 

of these three events. The only definitive statement 

about earthquake recurrence that can presently be made 

is that near Charleston there have been at least four 

sand-blow-producing (mb probably >5.5, discussed later) 

earthquakes within the past 7,200 years (including the 

1886 event). Accurate ages of crater formation have been 

obtained from some sites more than 100 km from 

Charleston. These dates differ from ages near Charleston, thereby suggesting that the craters far from 

Charleston originated from epicentral regions also far 

from Charleston. At many sites far from Charleston, 

there are at least two generations of craters that are long 

separated in time of formation.

SHAKING SEVERITY ESTIMATION

Insufficient radiocarbon ages have been collected from 

liquefaction features throughout the Carolina coastal 

region to define epicentral regions of separate earthquakes. Adequate data have been collected, however, to 

estimate the relative shaking severity throughout the 

coastal region during the Holocene. This estimate is 

provided by measurement of the number and size of 

craters at the sites shown on figure 1.

The methodology for estimating shaking intensity is 

based on the premise that the number and size of 

liquefaction features are greatest where earthquake shaking has been strongest for a fixed geologic setting and

liquefaction susceptibility. The condition of a fixed geologic setting is met almost ideally on many of the 

Pleistocene beaches, as discussed in an earlier section.

The condition of a fixed liquefaction susceptibility is 

also almost certainly satisfied at the widely scattered 

sites on figure 1. Source sands typically are loose (based 

on limited Standard Penetration Test data and numerous 

observations of ease of augering) and have about the 

same thickness. Moreover, the thickness and properties 

of nonliquefiable sediments overlying the source stratum 

lie within a narrow range. It is also a certainty that 

recurrences of liquefaction do not greatly diminish the 

potential for formation of more large craters in loose 

sands. This statement is verified by the observation that 

at site HW there are many large craters that formed in 

each of at least three generations of Holocene earthquakes, with each generation widely spaced in time. 

Thus, at sites in beach deposits on figure 1, liquefaction 

susceptibility is generally high and has not been greatly 

reduced by previous occurrences of liquefaction.

The other major variable, depth to the water table, is 

about the same from site to site (very shallow depth) and 

probably has been shallow throughout the Holocene 

(Obermeier and others, 1987). Evidence for location of 

the water table throughout the Holocene is provided by 

location of the base of the Bh horizon. (See fig. 4 for 

location of this horizon at a typical filled crater.) The 

maximum depth of the seasonal water table during the 

Holocene is marked very nearly by the base of the Bh 

horizon. (The Bh is defined as the subsoil zone of 

accumulation of organic matter and is formed in these 

soils at the lower limit of vertical infiltration of water.) 

Throughout the coastal region, the base of the Bh 

(generally 0.6 to 1 m below land surface) is nearly 

coincident with the present-day water table. Radiocarbon ages from the basal Bh horizon are 5,000 to 10,000 

years at site HW (Weems and others, 1986, p. 7). 

Because these ages are mean residence times of organic 

matter in a dynamic system characterized by continuing 

vertical infiltration of younger organic matter, some of 

the organic matter has been there even longer. Thus, it 

can be concluded that the water table has been very 

shallow throughout the Holocene over wide areas of the 

South Carolina Coastal Plain.

HOLOCENE EARTHQUAKE SHAKING

Both the abundance and diameters of pre-1886 

Holocene craters are greatest within the 1886 meizoseismal zone for a given age of craters. On the basis of these 

criteria, we know that shaking has been much weaker 

north of the Santee River (Obermeier and others, 1989). 

Intermediate shaking has taken place between Charles-
20 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

ton and the Santee River and also between Beaufort and 

the Savannah River.

Confidence in this interpretation is high for the area 

between Charleston and Wilmington, because of the 

hundreds of kilometers of ditches we searched. Our 

confidence is also high for the 1886 meizoseismal zone and 

for the area between Beaufort and the Savannah River. 

Our confidence is not nearly as high for the area between 

the Beaufort and the Edisto River nor for the area south 

and southeast of the 1886 meizoseismal zone; this lower 

confidence is caused by the limited number of ditches and 

pits available for inspection.

Whether or not the pre-1886 Holocene shaking in the 

1886 meizoseismal zone is associable with earthquakes 

stronger than the 1886 event can be determined only by 

additional radiocarbon ages for craters at sites far 

beyond the 1886 meizoseismal zone.

Based on worldwide observations in the field, the 

minimum earthquake strength required for liquefactioninduced features is mb equal to approximately 5; such 

features are rare for mb less than 5.25 to 5.5 (Carter and 

Seed, 1988). It is likely that an earthquake slightly 

stronger than about 5.5 is adequate to produce numerous 

small liquefaction features in coastal South Carolina 

because of the exceptional liquefaction susceptibility of 

many marine sand deposits there (discussed previously 

and discussed in Dickenson and others, 1988).

Many of the pre-1886 craters near Charleston that we 

have observed probably were not produced by earthquakes as small as mb 5.5, however. Many pre-1886 

craters are large (diameters of as much as 3-4 m are not 

uncommon) in comparison to historical descriptions of 

1886 craters (Button, 1889) and our observations of the 

sizes of 1886 craters. As a result, we suspect that some of 

the pre-1886 craters were formed by earthquake shaking 

that was stronger at those sites than in 1886. Direct 

comparison of 1886 and pre-1886 crater sizes cannot be 

used for estimating earthquake magnitudes, however, 

because of the lack of knowledge about distances 

between craters and their associated epicentral regions 

and lack of knowledge about changes in liquefaction 

susceptibility caused by a previous occurrence of liquefaction. About the only comment that can be made with 

much assurance is that the prehistoric craters were very 

likely the result of earthquakes stronger than mb 5.5; 

some of the earthquakes probably were much stronger 

because of the widespread distribution of large craters.

NEW MADRID SEISMIC ZONE

The succession of great earthquakes collectively designated as the New Madrid earthquakes of 1811-12 

caused severe and widespread ground failure throughout

a large area near the Mississippi River in southeastern 

Missouri, northeastern Arkansas, western Kentucky, 

and western Tennessee. The earthquakes caused multitudes of fissures and sand blows (Fuller, 1912), set off 

numerous landslides (Fuller, 1912; Jibson, 1985), and 

caused localized doming and submergence of the ground 

surface (Russ, 1982). Earthquake effects were particularly severe in the St. Francis Basin (fig. 13). According 

to the earliest extensive documentary account (Fuller, 

1912), three major shocks occurred: December 16, 1811; 

January 23, 1812; and February 7, 1812. The surfacewave magnitudes (Ms) of these earthquakes are estimated to have been between 8.3 and 8.8 (Nuttli, 1983a). 

Estimated Modified Mercalli (MM) intensities are XI to 

XII throughout large areas near the epicenters (Nuttli, 

1973); these intensities are regional values based largely 

on historical accounts of liquefaction-induced ground 

failure. No faults associated with the 1811-12 earthquakes have been found that cut through alluvium, loess, 

or older strata to the surface. The epicenters are estimated to lie within the large area of sand blows near the 

Mississippi River, shown on figure 14. This interpretation of epicenters is based on the study by Fuller; a study 

of MM intensities by Nuttli (1973); recent geologic, 

geophysical and modern seismicity studies (McKeown 

and Pakiser, 1982; Crone and others, 1985); and 

engineering-geologic studies of factors controlling the 

distribution of sand blows (Obermeier, 1989). (The epicentral regions during the 1811-12 earthquake and the 

region of epicenters on figure 14 define the "New Madrid 

seismic zone.")

Figure 14 is a map by Obermeier (1988) that shows the 

percent of the ground surface covered by sand vented to 

the surface in the St. Francis Basin. The map is the latest 

of a series of maps (Fuller, 1912; Saucier, 1977; Heyl and 

McKeown, 1978) showing sand blows throughout the 

alluvial lowlands of the basin. Significant differences 

appear between each of the maps, particularly north of 

the town of New Madrid. These differences occur for a 

number of reasons, the most likely being that (1) some 

surface soils are so sandy that sand blows can be 

observed only on the older aerial photographs used by 

Obermeier, (2) the modern farming practice of landleveling has destroyed many sand blows since the 1950's, 

(3) some nonearthquake features cannot be distinguished 

from sand blows except by field excavations, and (4) the 

map by Obermeier extends more to the limits of ground 

covered by vented sand, whereas the earlier maps 

emphasize areas of abundant sand blows.

Sand-blow deposits and other manifestations of 

liquefaction-induced flowage occurred far beyond the 

limits of sand-blow deposits shown on figure 14, but 

beyond these limits the deposits were generally
NEW MADRID SEISMIC ZONE 21

EXPLANATION

Alluvium on small streams

Mississippi River meander belts

Late Wisconsinan glacial outwash

Early Wisconsman glacial outwash

Undifferentiated pre-Wisconsinan

depositions! terraces

Uplands

FIGURE 13. Late Quaternary alluvial deposits of St. Francis and Western Lowlands Basins (from Saucier, 1974).

restricted to modern flood-plain sediments adjoining 

rivers. These very young (.late Holocene) sediments 

generally have higher liquefaction susceptibility than the 

late Wisconsinan and early Holocene fluvial deposits, 

which underlie the area of sand-blow deposits shown on 

figure 14. Liquefaction features were especially commonplace in alluvium along some of the small streams of the 

Western Lowlands Basin (fig. 13), particularly those 

nearest Crowleys Ridge. Sand blows were reported 

(McDermott, 1949) in alluvial deposits as far north as 

Cahokia, 111. (which is very near to St. Louis, Mo.), and 

as far northeast as the Wabash River valley (Street and

Nuttli, 1984). Some possible sand blows were formed in 

some of the river valleys beyond the limits of sand blows 

on figure 14. For example, Dickey (1985) reports features on aerial photographs in the Western Lowlands 

that appear to be sand-blow deposits. At many places, 

though, sand-blow deposits on modern flood plains have 

been covered by a veneer of alluvium deposited since the 

1811-12 earthquakes.

In the New Madrid area, visible sand-blow deposits 

are still plentiful on the ground surface, and the epicentral regions are relatively well established. These characteristics permit a more detailed evaluation of geologic
22 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE 

91° 90°

37C

36C

EXPLANATION

Alluvium with more than

25 percent of ground surface

covered by sand-blow deposits

Epicenters for three largest

1811-1812 earthquakes 

according to Nuttli (1979)

Fault Fault zone

20 MILES

20 KILOMETERS

FIGURE 14. Map showing area covered by vented sand (Obermeier, 1988), estimated epicenters of strongest 1811-12 earthquakes (from Nuttli,

1979), and faults and fault zones (from Hamilton and Zoback, 1982).

controls on the formation of liquefaction features than is 

possible in coastal South Carolina.

REGIONAL GEOLOGIC AND SEISMOTECTONIC SETTING

The St. Francis Basin is a topographic lowland containing 30 to 60 m of late Quaternary alluvium, which 

originated from deposition associated with the Mississippi and Ohio Rivers and their tributaries. The lowlands 

are typically very flat, having about 0.3 to 2 m of local 

relief. Numerous small scarps, 2 to 6 m high, result from 

depositional or erosional processes. The water table is

very high and is within 1 to 2 m of the ground surface at 

many places. Elevations in the lowlands exceed 100 m 

near Cairo, 111., and decrease southward to less than 60 

m near Memphis, Tenn.

Widespread floods were common prior to the construction in this century of manmade levees along the Mississippi River. Standing water occupied the lower parts of 

the lowlands throughout much of the year before the 

levees were built and large drainage ditches were excavated. Until about 50 years ago, dense forests and 

cypress swamps covered most of the basin; today the 

forests have been replaced by large fields of soybeans, 

rice, and other row crops.
NEW MADRID SEISMIC ZONE 23

Braided stream deposits Meander belt deposits

T

Dominantly clay-rich 

sediments

T

' .Medium-grained sand .' : ". \ : .... .'. ' Pleistocene-;.'". .: 

substratum:'.' '"-.-.' :

artesian seepage area Generally 0-3 km clay-rich 

Tertiary deposits

FIGURE 15. Schematic east-west cross section showing geologic and ground-water setting of St. Francis Basin.

CHARACTERISTICS OF QUATERNARY ALLUVIUM

Cyclic Pleistocene glaciations directly and indirectly 

controlled the origin, character, and distribution of virtually all the Pleistocene deposits in the basin (Saucier, 

1974). Although continental glaciers did not extend into 

the St. Francis Basin, they supplied large volumes of 

glacial meltwater and outwash to southward-flowing 

river systems. Up to 60 m of valley fill composed of very 

coarse grained, well-graded (engineering sense) glacial 

outwash (gravel and sand) were first deposited by 

aggrading braided streams. After maximum aggradation, numerous braided-stream terraces were deposited. 

The ancestral Mississippi River changed from a braided 

to a meandering regimen in early Holocene time, and 

since that time, slow aggradation has taken place in most 

parts of the valley. Most meander-belt deposits are 

medium-grained, well-graded sand; locally, silts and 

clays have been laid down.

Sikeston Ridge (fig. 13), the highest terrace of St. 

Francis Basin, is early Wisconsinan in age, while the 

other braided-stream terraces are late Wisconsinan. The 

earlier braided-stream deposits are topographically 

higher, have greater relief, and have sandier surfaces 

than younger deposits. The older braided-stream deposits occur in several terrace sublevels separated by 2 to 6 

m. These deposits generally are capped with 3 to 7 m of 

silty or clayey sediments (the topstratum) containing 

some thin sand strata. The topstratum abruptly overlies 

the clean, outwash sands and gravels (the substratum) 

over large areas except at the highest sublevels and 

interfluves. The topstratum was deposited either by 

relatively slack streams as individual braided-stream 

levels were successively abandoned or by overbank 

deposition during widespread flooding of the Mississippi 

River and local streams that now occupy the topographic 

lows. Substratum sands and gravels generally are found

in intercalated strata and lenses ranging from a few 

centimeters to a meter thick and tend to become coarser 

with depth.

The Mississippi River meander belts represent successive courses formed by lateral migration of the river. 

Most of the meander belt consists of point-bar "accretion" topography of parallel arcuate ridges and swales, 

abandoned channels in various stages of filling, and 

natural levees. Point-bar deposits are generally clean, 

well-graded sands4 that are capped by the topstratum 

except locally at the highest elevations. Point-bar deposits of the Mississippi River typically have 2 to 3 m of local 

relief. Many abandoned channels are filled with 30 m or 

more of soft clays and silts and are swampy and densely 

forested.

Large backswamp areas are found along the margins 

of some Mississippi River meander belts (Saucier, 1974). 

Backswamp deposits were formed by seasonal floodwaters depositing silts and clays in low areas of the flood 

plains during the Holocene. The deposits average 12 m in 

thickness but are as much as 18 m thick.

The ground-water setting is generally simple, as 

shown in figure 15. Major rivers such as the Mississippi 

and St. Francis both supply and drain large volumes of 

water in the substratum. Locally, especially within 0.5 to 

1 km adjacent to levees, artesian conditions occur many 

years during flooding. Beyond this area of flood-related 

springs near levees, the streams have cut through the 

topstratum, thus preventing artesian conditions. In addition, throughout the St. Francis Basin many topographically elevated areas make artesian conditions impossible. Present ground-water conditions are shown in a 

fairly detailed manner in a ground-water report by

4In this paper, the clean sands beneath the fine-grained cap are referred to 

as the "substratum," and the fine-grained cap as the "topstratum," irrespective of 

origin as meander-belt or braided-stream deposits.
24 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

Surface projection 

of buried rift complex

FIGURE 16. Configuration of the buried New Madrid Rift Complex 

(from Graile and others, 1984). The structurally controlled rivers, 

Paleozoic rocks in the cratonic sedimentary basins, and the Mississippi Embayment, all associated with the buried rift complex, are 

also shown. Dark areas indicate intrusions near the edge of the 

buried rift.

Krinitzsky and Wire (1964); detailed geologic maps by 

Saucier (1964) and Smith and Saucier (1971) can be used 

to help determine present and Holocene ground-water 

conditions. No evidence indicates that ground-water 

conditions (especially the piezometric surface) were significantly different during the past few thousand years in 

the St. Francis Basin, except locally near manmade 

levees during times of flooding.

SEISMOTECTONIC SETTING

The St. Francis Basin lies in the northern Mississippi 

Embayment, which is a broad, southward-plunging syncline (fig. 16). The northern embayment is underlain by a 

late Precambrian intraplate rift that has been active 

periodically since its formation (Braile and others, 1984). 

Within this rift lies the New Madrid seismic zone. This 

zone is the most seismically active area in the United 

States east of the Rocky Mountains. Continuing fault 

movement, in response to regional compressive and 

perhaps thermal stresses, is the most likely cause of 

modern seismicity (Braile and others, 1984).

Since 1812, at least 20 earthquakes having estimated 

body-wave magnitudes between 3.8 and 6.2 have 

occurred in the region (Nuttli, 1982). Most estimates for 

the return period of great earthquakes are between 500 

and 700 years (Hopper and others, 1983).

Figure 17 shows locations of instrumentally recorded 

earthquakes and principal structural elements in the 

New Madrid seismic zone. The trend of modern epicenters extending from Marked Tree, Ark., to Ridgley, 

Tenn., coincides at least approximately with the epicenters interpreted by Nuttli (1979) of the three principal

shocks of the 1811-12 earthquakes. Subsurface faults are 

inferred by seismic-reflection data to lie beneath the 

most seismically active areas (O'Connell and others, 

1982; Crone and others, 1985).

CHARACTERISTICS OF AND CRITERIA FOR EARTHQUAKEINDUCED LIQUEFACTION FEATURES

Reports of level-ground and near-level ground failure 

features made shortly after the 1811-12 earthquakes 

noted great multitudes of vented-sand volcanoes, linear 

fissures up to 6 m deep and hundreds of meters long, 

craters many meters in diameter, and lateral spreads 

hundreds of meters long (for example, see Penick (1976)). 

Fuller (1912) described vicinities where these features 

were especially abundant. Individual vented-sand volcanoes and some long linear fissures through which sand 

vented are the only features that are still readily visible 

on the ground surface. Intruded dikes and sills are 

common in walls of deep (>3 to 4 m) drainage ditches in 

the clay-rich topstratum. We have not found other types 

of level-ground failure features such as deep craters or 

lateral spreads having downdropped blocks at the head 

such as those described by Fuller (1912, p. 48), primarily 

because we have not searched extensively in areas where 

these features would have formed (that is, locations in 

sandy topstratum for craters and locations near streams 

or scarps for lateral spreads). Limited observations in 

the New Madrid seismic zone by Wesnousky and others 

(1987) and by us of the characteristics of sand blows 

formed in a sandy topstratum indicate that open, deep 

craters are atypical forms. Instead, intrusions that 

extensively shattered the ground near the surface locally 

erupted to form shallow craters (up to 2 m deep).

The geologic criteria (previously discussed) for interpreting an earthquake origin with regard to specific 

features are identical in both the South Carolina and 

New Madrid earthquake areas.

VENTED-SAND VOLCANOES

The vented-sand volcanoes in the New Madrid area are 

similar to, though much larger than, those discovered in 

South Carolina. (See fig. 7.) Individual vented-sand 

volcanoes induced by the 1811-12 earthquakes are domelike accumulations of clean sand on the ground surface. 

Fuller (1912, p. 79) noted that "the normal blow is a patch 

of sand nearly circular in shape, from 8 to 15 feet across, 

and 3 to 6 inches high." Such small sand blows as Fuller 

described can rarely be found at present. The sand blows 

that are now obvious range from about 0.3 to 1.3 m in 

height at the center and thin to a feather-edge at a 

diameter of 20 to 60 m.

Most vented-sand volcanoes have a well-defined internal stratigraphy. Along the base of the vented-sand
NEW MADRID SEISMIC ZONE 25

92°00'

380OCT -

91°00' 90°00' 89°00' 88°00'

37°00' -

36°00'

35°00' TENNESSEE 

MISSISSIPPI

EXPLANATION

Plutons

Earthquake epicenters

Faults

Rift boundaries

FIGURE 17. Northern Mississippi Embayment (shaded area) showing earthquake epicenters, plutons, rift boundaries, and faults (from Zoback

and others, 1980).

volcano deposits, where the vented sand spread over the 

ground surface, many irregular 1- to 3-cm-long clasts of 

topstratum clay (generally a blue-gray, highly plastic 

clay) are scattered throughout the sand. The clasts are 

largest and most plentiful closest to the vent, which is 

beneath the central part of the dome. The basal part of 

the sand-blow deposit contains numerous pieces (1-3 cm 

in diameter) of rounded charcoal fragments and other 

very low density materials vented to the surface from the 

substratum. This basal part contains and is overlain by a

very clean, generally medium- to coarse-grained sand. 

This sand grades upward to a much finer sand, which is 

capped by a clayey, organic silt stratum, 0.5 to 4 cm thick 

(fig. 18); this cap may also contain multiple very thin 

(1-mm-thick) clay-rich layers (Saucier 1989). The organic 

matter in the silt is made up of small pieces of coal, 

lignite, and wood. The fining-upward sequence, from the 

basal clay clast-bearing sand to the organic silt stratum, 

represents the transition from the initially rapid, turbulent eruption to the final ebbing flow out of the vent.
26 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

FIGURE 18. Stratigraphy of vented-sand volcano showing organicrich silt between two fining-upward sequences of sand. Presence of 

two fining-upward sequences is inferred to represent two separate 

liquefaction events or reactivation during a single earthquake. 

Height of shovel is 40 cm.

Locally, near the vent, the beds may dip steeply toward 

the vent if flowage has been sufficiently slow. (Dipping 

beds in an excavation through vented-sand volcano 

deposits are shown in fig. 5 in an article by NewmannMahlkau (1976)). If a violent eruption has taken place, 

the lower and central portion of the vented-sand volcano 

may be sheared and disrupted. Figure 19 shows 

earthquake-induced "eruptive vent" in sand-blow deposits of the 1811-12 earthquakes.

A basal bed, several centimeters thick, of pea-sized 

clean gravel is the coarsest basal material that we have 

found in the sand blows. Within a sand matrix, isolated 

gravels up to 3 cm in diameter have been vented; these 

coarsest materials were vented only near the earthquake 

epicenters. Sand blows far from the epicenters, near the 

boundary of sand-blow occurrences (fig. 14), are rather 

small and are made up of finer grained material. Here the 

basal sands are often fine-grained and fine upward to a 

silty sand.

At least two fining-upward sequences characteristically occur in vented-sand volcanoes near epicentral 

regions; each sequence represents a separate occurrence 

of venting induced by the 1811-12 earthquake. 5 However, generally only one sequence occurs near the 

regional boundary of sand blows, which is shown in 

figure 13.

Not only are the largest sand blows near the epicentral 

regions, but in some places the vented sand coalesces 

into continuous sheets. The sheets are up to 2 m thick

6Interpretation of 1811-12 earthquakes is based on the absence of a soil 

profile in any of the fining-upward sequences.

and at several places blanket tens of square kilometers 

(for example, just south of the Missouri-Arkansas boundary and near the December 16, 1811, epicenter shown on 

fig. 14).

Vented-sand-blow deposits are obvious on aerial photographs at many places. Figure 20 shows some examples. The light-colored spots show vented sand blows, 

and the light-colored linear features show fissures 

through which sand has vented. (Some of the linear 

features near streams probably mark edges of lateral 

spreads, but we have not confirmed this suspicion in our 

study, and therefore in this paper such features are 

simply called fissures). Figures 20A and 205 illustrate 

how sand blows typically occur at irregular, almost 

erratically spaced intervals, and the crazing pattern of 

vented sand suggests that the ground is fractured. 

Figure 20C illustrates that where less sand is vented to 

the surface, the venting is more erratic and is restricted 

to localized areas and that relatively small differences in 

site characteristics become important controls on vent 

locations. Comparison of figures 20A and 205 illustrates 

that sand-blow deposits may be more apparent in the 

older photographs than the more recent photographs.

Some surficial features unrelated to earthquakes have 

a similar appearance to earthquake-related features 

shown in figure 20C. An earthquake origin was attributed to questionable features only if the features had a 

concentration of irregular, large (2-3 cm) clay clasts at 

the base (implying a strong hydraulic force); had a 

fining-upward clean sand (implying diminishing flow 

within a short time period); were irregularly spaced 

(suggesting ground fracturing); and (4) were located 

where spring flow from uplands or from beneath levees 

could not have occurred. (Sand boils formed by flow 

beneath levees are discussed later.)

Only limited data have been collected to characterize 

vent shapes and sorting and flow structures in the vent 

filling. The vents are frequently steeply to vertically 

oriented and are fissure shaped. Vents generally appear 

to be filled with a structureless mixture of silt, sand, and 

clay clasts ranging from sand sized to having a length of 

up to 20 cm. The clasts have been transported up the 

vent and are derived from sidewalls and beds at depth.

FISSURES

Earthquake-induced fissures presently appear on the 

ground surface as lines of continuously or discontinuously 

vented sand. The fissures are common on level ground 

far from any (erosional) scarps but are especially commonplace on those parts of terraces immediately above 

scarps and on river banks. Vented sand locally forms 

small ridges, but the ridges are generally not as high as 

nearby vented-sand-volcano deposits and sometimes are
NEW MADRID SEISMIC ZONE 27

barely discernible. Widths of fissure openings tend to 

range between 0.3 m to a feather edge, most being small;

Plow zone

50 CENTIMETERS

the wider openings almost certainly indicate association 

with lateral spreads.

Fissure patterns can be highly sensitive to the local 

geologic setting. Fissures have a strong tendency to 

follow the crests of point-bar deposits; on braided-stream 

deposits having essentially a uniformly thick topstratum, 

the fissures are commonly oriented randomly (Obermeier, 1989). Topstratum thickness appears to be an 

important control on development of fissures; more fissures develop at locations having a thin topstratum.

Long irregular fissures through which sand has 

vented, such as those shown in figures 20A and 205, 

have not been observed to have originated by springs in 

the alluvial lowlands of the St. Francis Basin. Slumps 

and lateral spreads along rivers (generally caused by 

rapid hydraulic drawdown after floods) can generate 

fissures, but we have not observed sand flowing to the 

surface through the fissures except for small amounts 

associated with water oozing through the slide mass. In 

addition, aseismic lateral spreads and slumps that we 

have observed do not extend back from banks along large 

rivers more than 50 m. Thus we believe that fissures 

associated with vented sand that is located more than 50 

m from present or former banks of rivers can generally 

be attributed to seismically induced slope instability. 

Confidence in this interpretation increases with increasing width of the sand-filled fissure and with increasing 

distance from the banks. In order to confidently attribute 

an earthquake mechanism to a specific vented-sand fissure, however, the location of the river at the time of 

fissure formation must be established, an undertaking 

that may be quite difficult.

Fissures formed on terraces far from streams can be 

attributed to earthquakes more confidently. In interpreting an origin for any feature, it is still necessary to 

assess ground-water conditions at the site and ascertain 

that the fissures could not have originated by some 

process related to syndepositional deformations.

INTRUDED FEATURES

Intruded features are sand-filled cracks that do not 

reach the surface, and therefore the features are visible

FIGURE 19. "Eruptive vent" that cut stratified vented deposits of 

a vented-sand volcano (from Haller and Crone, 1986). A, Number 

102 is above a severely disrupted zone. Distances between vertical 

edges of photograph is approximately 1 m. B, Number 61 is at 

contact between vented sediments and the original ground surface. 

Distance between vertical edges of photograph is approximately 1 

m. C, Line drawing of "eruptive vent" in B. Heavy lines are 

stratigraphic contacts; fine lines show laminations within the 

"eruptive vent," dashed where discontinuous. Open circles are 

aligned clay fragments.
28 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

MILE

0

FIGURE 20. Aerial photographs showing vented sand, interpreted as 

the product of liquefaction and flowage during the 1811-12 earthquakes. Note pattern of crazing in figures 20A and B, which is 

especially indicative of ground breakage caused by earthquakeinduced liquefaction. A, Taken in 1941, showing more than 25 

percent of ground surface covered by vented sand (small white

1 KILOMETER

spots and thin linear zones) near Marked Tree, Ark. B, Taken in 1959, 

showing more than 25 percent of ground surface covered by vented 

sand (small white spots and thin linear zones) near Portageville, Mo. C, 

Taken in 1940, showing localized venting of sand near Reelfoot Lake, 

Tenn. Regions with vented sand are outlined.
NEW MADRID SEISMIC ZONE 29

1MILE

1 KILOMETER 

FIGURE 20. Continued.

only in exposures cut into the subsurface. A simple, very 

common type of intrusion having an earthquake origin is 

a near-vertical, wedge-shaped, sand-filled dike. At many 

places in the region of vented sand, the clay-rich topstratum is cut by vertical dikes that are spaced tens to 

hundreds of meters apart. The dikes extend upward from

the substratum. Dike widths are several centimeters 

near the substratum; widths commonly narrow upward 

to an apex near the ground surface. The sand in the dike 

is generally structureless and can contain clasts of topstratum carried upward; the larger clasts are often 

oriented vertically.
30 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

1 MILE

1 KILOMETER 

FIGURE 20. Continued.

Combinations of dikes and sills are also common in the 

topstratum. The sills generally follow bedding planes or 

other horizontal planes of weakness in the topstratum. 

Sills tended to form in thin beds of silt or sand beneath

only slightly deformed clay beds, but locally thin clay 

beds can be strongly warped. Figure 2LA shows a typical 

relationship of near-vertical dikes connected to a laterally extensive sill. (The sill extends beyond the photo-
NEW MADRID SEISMIC ZONE 31

graph and is at least 25 m long.) The internal layering of 

this sill is also typical of many of those we have studied. 

Individual laminae are composed of small pieces of charcoal or of much finer grained sand and silt. (See figs. 21B 

and 21C.) A single laminae of silt or very fine sand can 

have a length as long as 10 cm. Lamina made up of pieces 

of charcoal can be much longer, and lengths of 300 cm are 

not unusual. In some places, the sills are structureless, 

having no traces of bedding, and contain many clay clasts 

in a sand matrix; in other places, sills have graded 

bedding with clay clasts concentrated along the base. 

The absence of bedding or the presence of graded 

bedding seems quite understandable; however, the presence of planar strata of fine sand and silt within a much 

coarser sand cannot be explained by us with any degree 

of confidence. (Possibly the planar stratification develops 

by repeated intrusions of fluidized sand into an open 

crack.) Most certainly, however, sills having stratified 

bedding are common earthquake-related features that 

resulted from intrusion of liquefied substratum sands. 

(Sills that have stratified and graded bedding originating 

from earthquake-induced liquefaction of much deeper 

source beds have also been observed in the South Carolina study at sites HW and BLUF on fig. 1.)

The clay clasts in the sills are generally angular, 

suggesting a brittle or shattering mode of breakage of 

the clay stratum from which the clasts were derived. 

This brittle mode of breakage also suggests that when 

the stratum was "shattered" the clay stratum was dewatered significantly from the initial depositional condition. Such shattering indicates forceful intrusion and 

thus is not equivalent to the process causing formation of 

syndepositional soft-sediment deformation features (discussed later). We have observed that the shattered clay 

strata can have a consistency that is so soft that a person 

can force his thumb a few inches into a stratum.

Sills in the topstratum at depths of about a meter or 

less below the ground surface may be very wavy in 

vertical section and can thin and thicken dramatically 

within a meter, producing bulges at the surface. Sills can 

be as much as 0.5 m thick near the surface. Such large 

thicknesses seem less common at greater depths.

Earthquake-induced sand dikes that cut the upper 

substratum sands and the lower topstratum are shown in 

figure 22A, which is a schematic depiction of a vertical 

ditch bank exposed about 15 km northwest of Marked 

Tree. The exposure also illustrates many of the other 

common forms of intrusions in the topstratum. In the

FIGURE 21. Sand dikes and sills, interpreted as having originated 

by liquefaction and flowage during the 1811-12 earthquakes. A, 

Overview and line drawing of typical dikes and sills. Rectangle 

shows area of figure 21B. B, View showing detailed layering in sill. 

C, Very close view showing detailed layering in sill.

-.. .r«*.T.jy»»<-r

50 INCHES

100 CENTIMETERS

- - '--r^'

.v <4

2 INCHES

5 CENTIMETERS
32 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

GROUND SURFACE BED

Clay, medium gray,

blocky; altered by

soil processes

Sand, fine- to mediumgrained, clean, orangish 

brown, massive

Medium sand Clay, pale-blue, blocky

micro-fractured, well

laminated at base

Sand, medium- to coarse-grained 

clean, prominent foreset beds

Sand, fine-grained, medium brown 

silty, contains planar crossbeds

Clay, highly plastic, grading up

to sand, fine-grained, silty,

bluish-gray, massive

Clay, sandy, coarsening upward to

sand, fine-grained, bluish-gray,

Medium sand weakly laminated

Sand, very fine grained, silty,

bluish-gray, well laminated at base

with carbonaceous partings

composed of charcoal grains

Sand, medium-grained, clean

buff-brown with yellow stains

crossbedded, contains wood

(dated at 12,000 years

before present)

FIGURE 22. Section showing Holocene sediments (topstratum) and 

underlying Wisconsinan braided-stream sands (substratum) in ditch 

about 15 km northwest of Marked Tree, Ark. Earthquake-induced 

intrusions cut section at many places. A, Schematic diagram of 

stratigraphic relationships and earthquake-induced liquefaction features (numbered 1 through 5). Feature 1 Dike of medium sand that cuts 

substratum and topstratum. Features 2 and 3 Intruded dikes and sills of 

massive, clean, medium sand. Feature 4 Dike of medium sand and large 

clasts from bed G. Feature 5 Dikes of medium sand, truncated by 

unconformity. Feature 6 Pseudonodules collapsed into bed D.
NEW MADRID SEISMIC ZONE 33

intntisioo 

structure

Bed C

.small dike 

(feature 5)

FIGURE 22. Continued. .8, Photo graph of beds A-D, showing sand 

intrusion (feature 3). Knife is 12 cm long. C, Photograph of vents 

(feature 1) cutting substratum sand (bed A) and bed B. Knife is 12 cm 

long. D, Photograph of plan view of bed B, showing intrusion 

structures caused by feature 1; the plan view is in the area of the knife

oriented vertically in figure 22B. Knife is 12 cm long. E, Photograph of 

part of feature 4, showing clay clasts in sand matrix intruded into bed H. 

Knife is 12 cm long. F, Photograph of dike of unknown origin (feature 5), 

cutting through bed C and truncated by bed D. Coin diameter is about 

1 cm.
34 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

upper part of the substratum (bed A, fig. 22A), small 

dikes branch out from a large dike (feature 1), cut 

through the basal topstratum bed (bed B) at horizontal 

intervals of 0.5 to 1 m, and extend upward about 0.5 to 1 

m. However, a few dikes and sills have intruded at much 

higher levels (features 2, 3, and 4). At places nearby (not 

shown) dikes extend to the surface, and sand has been 

vented to produce sand blows.

The dikes and sills shown on figure 22A as features 1 

through 4 contain clean, structureless, medium-grained 

sand. The edges of the intrusions are very sharp in 

clay-rich beds (such as beds C, D, and G). Edges are 

generally less distinct in beds of clean permeable sand. 

Dikes commonly terminate in a permeable sand bed (bed 

H); near the terminus there are often numerous clasts of 

clay-rich material from the subjacent bed.

Features labeled 1 through 4 in figure 22 are probably 

earthquake-induced because (1) they are widely distributed (common for tens of kilometers), (2) they contain 

evidence of intrusion by large volumes of watersaturated sediment (dikes and sills are commonly up to 

15 cm wide), (3) there is evidence of forceful intrusion 

(clean, medium-sized sand containing large clay clasts), 

and (4) they are found in sites where artesian conditions 

are unlikely. Because of the lack of weathering of sand in 

the intrusions near the ground surface, we believe that 

features 1 through 4 probably formed during the 1811-12 

earthquake.

The dike shown as feature 5 has an uncertain origin. 

Three small dikes that are truncated at the contact of 

beds C and D were exposed in a 25-m section exposed 

along the ditch but were not found in other nearby 

exposures of beds C and D nor in the lowermost portion 

of the topstratum at exposures further away (where beds 

C and D cannot be traced). The dikes contain a large 

amount of silty fine sand and cannot be traced far down 

into substratum sands. From this information, we interpret that the dikes quite possibly originated due to 

springs formed near the base of a streambank or, less 

likely, as slump-related features.

Feature 6 (pseudonodules) is discussed in the next 

section of the paper.

FEATURES OF UNKNOWN OR NONEARTHQUAKE ORIGIN

The principal features in this category include sand 

boils, mima mounds, load structures, sand dunes, "deer 

licks," and sand ridges on point-bar deposits. Some of 

these features can be distinguished from sand blows on 

aerial photographs. Others may remain uncertain in their 

origin even after field studies.

SAND BOILS

During floods that occur every few years, sand boils 

originate due to artesian flowage beneath levees. Sand 

boils are common in lowlands near levees along the large 

rivers, particularly the Mississippi River (Kolb, 1976), 

and are generally restricted to the areas within 0.5 to 1 

km of the levees. (See fig. 15.)

The sedimentary structures and morphologies of sand 

boils are very similar to and may not be distinguishable 

from those of (earthquake-induced) sand blows. For 

example, both processes vent sand to the surface from 

source beds at depth. The stratigraphic position of the 

bed that provided the sand can be used to indicate origin 

at some places. In a sand boil, the sand on the surface 

commonly can be traced to a sand bed that immediately 

underlies the cohesive (and thus relatively impermeable) 

topstratum. In contrast, the source bed for a sand blow 

may be much deeper. By itself, this criterion is of limited 

value and generally is best used in a context that includes 

tests to determine the earthquake-induced liquefaction 

susceptibility of the various beds (discussed later).

The shape of the vent in plan view may help distinguish between sand boils and sand blows. Both sand boils 

and sand blows can have circular vents; however, many 

sand blows have linear vents that follow large fissures. 

Where large sand-filled fissures widen downward and 

extend for tens to hundreds of meters laterally, they are 

probably vents associable with earthquake-induced liquefaction (providing the possibility of nonseismic slumping can be eliminated as the source of the fissures).

Sand boils have vents whose diameters range from 3 

cm to greater than 1 m. Numerous small sand-filled 

fissures or tubes can be connected to the main vent of a 

sand boil; clay clasts up to 7 cm in diameter, derived from 

sidewalls, have been observed within the vent. The 

upper parts of the vents (as deep as 1 m or more below 

the original ground surface) of sand boils may contain 

stratified deposits of sand layered with other materials 

that fell into an open hole. A nonearthquake origin can be 

proven at some places because of the age of the materials 

in the vents. For example, the presence of agricultural 

produce such as rice or soybeans far down in the vent 

virtually eliminates an association with historic earthquake origin in the New Madrid region, especially if the 

vent is within 1 km of a levee. (Soybeans and rice have 

been cultivated only within the past 50 years or so, much 

postdating the last liquefaction-producing earthquake, 

the 1895 Charleston, Mo., earthquake.)

MIMA MOUNDS

Mima mounds (sometimes called prairie mounds) are 

domes of sand-rich material, usually less than 30 m in 

diameter and 1 m high, that were not formed by venting;
NEW MADRID SEISMIC ZONE 35

in exceptional cases the domes are as much as 1.7 m high 

but have diameters of only 10 to 12 m. Mima mounds are 

present in alluvial lowlands (especially in the northern 

parts of the St. Francis and Western Lowlands Basins, 

north of the town of New Madrid) and are also in upland 

areas west of the Western Lowlands (fig. 13). The origins 

of mima mounds are not understood. In many upland 

areas, the mima mounds are formed on nonliquefiable 

deposits and therefore are of nonearthquake origin. 

Mima mounds on alluvial lowlands can be identified as 

not resulting from earthquakes if excavation shows an 

absence of vents to connect the mounds to source beds 

below (Fuller, 1912, p. 80).

On aerial photographs, mima mounds can often be 

distinguished from sand blows by criteria illustrated in 

figure 23, which is an aerial photograph of mima mounds 

taken in the northern part of the St. Francis Basin. 

These criteria are regularity of spacing, alignment, and 

size. In contrast, sand blows are typically irregularly 

spaced, have widely differing sizes (heights and diameters) at locations nearby, and are not aligned along such 

precisely defined curves. Comparison of sand blows 

formed on point-bar deposits (fig. 20C) with figure 23 

illustrates the contrast.

Locally it can be difficult to distinguish between sand 

blows and mima mounds on aerial photographs, and 

excavation is required to examine for feeder vents or 

other evidence of origin. Mima mounds in the vicinity of 

the St. Francis Basin characteristically have welldeveloped soil horizons that obviously predate the 

1811-12 earthquakes and in addition are slightly 

cemented, making them difficult to auger using hand 

tools.

LOAD STRUCTURES

Many Holocene fluvial deposits in the Central United 

States have abundant syndepositional, soft-sediment 

deformation features known as "load-flow" or simply 

"load" structures. Load structures are bulbous downward intrusions of sandy or silty material into underlying 

weaker, finer grained muddy sediment. Two common 

types of load structures occur in the topstratum of the St. 

Francis Basin pseudonodules and load-casted ripples. 

Pseudonodules occur when overlying sandy or silty sediments become detached and sink to become isolated 

subspheroidal bodies in the underlying clayey bed. 

Pseudonodules are usually found laterally adjacent to 

other undetached load structures (Alien, 1984, p. 

359-360). Pseudonodules were experimentally produced 

in the laboratory by Kuenen (1958) by hammering a 

container in which sand overlies water-saturated, very 

soft clay. (See fig. 24.)

Sims (1975) correlated subspheroidal pseudonodules 

found in modern lake sediments, similar to those produced by Kuenen (and some possibly morphologically the 

same as those produced by Kuenen), with known earthquake events. Sims argued that structures such as those 

produced by Kuenen could be interpreted as earthquakeinduced if (1) they occur in a seismically active region, (2) 

they are restricted to specific stratigraphic horizons, (3) 

they are correlative over large areas within a sedimentary basin, and (4) there is no detectable influence of 

slope movement or failure. However, pseudonodules 

similar to Kuenen's can form without shaking by the 

gravity-induced instability of denser sediment overlying 

less dense sediment (Alien, 1984, p. 363). The degree of 

soft-sediment deformation is controlled by the difference 

in densities between the two adjacent layers and the 

strength of the underlying layer. Sand deposited rapidly 

over water-saturated, muddy or extremely soft clay is 

ideal for deformation.

In the case of load-casted ripples, sandy or silty 

intrusions form because of the unequal loading of migrating ripples of sand on a clayey mud substratum. Loadcasted ripples show progressively deformed radial internal lamination caused by the rotation of the ripple 

cross-laminations as the ripples sink (Dzulynski and 

Walton, 1965, p. 146-149). The asymmetry reflects current direction, as illustrated in figure 25. The development of load-casted ripples requires local deformation 

synchronous with deposition of overlying sand, and 

therefore load-casted ripples cannot likely be related to 

earthquake events.

The distinction between possible earthquake-induced 

load structures and those produced by rapid sedimentation is illustrated in a ditch exposure near Marked Tree, 

Ark. The section in plate 1A is composed of bluish clays 

interbedded with layers of sand, apparently related to 

intermittent crevasse delta sand deposition in a waterfilled swale as illustrated in plate IB. Small, deltalike 

sand bodies formed convex-upward sand lenses containing climbing ripple cross-laminations. The lenses are 

coarser upward, fine toward the edges, and are laterally 

adjacent to layers of pseudonodules (25-50 cm long and 

5-25 cm wide) made up of similar grain sizes. The base of 

each sand lens has load structures, including load-casted 

ripples (pi. IB ). The load structures make up a greater 

percentage of each sand lens as it thins toward the edge 

(see north-south view of pi. IB)- downstream, each sand 

lens apparently grades into the pseudonodules. (See 

east-west view of pi. I A.) The cross-lamination in the 

sand lenses has a gradational contact to the deformed 

ripple cross-laminae in the underlying load structures, 

whereas ripple cross-laminae above and laterally adjacent to the load structures are undeformed. (See northsouth view of pi. IB.) The pseudonodules are internally 

ripple cross-laminated, and some have load-casted 

ripples at the base. Isolated pseudonodules with the
36 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

1 KILOMETER

FIGURE 23. Aerial photograph of mima mounds in northern part of St. Francis Basin. Mima mounds often appear as trails of light dots and are

commonly formed on slightly elevated ridges of point-bar deposits.

internal fabric of load-casted ripples have an asymmetry 

reflecting a current direction about the same as that for

the sand lenses. The clay surrounding the pseudonodules 

has streaks of highly contorted silt, indicating flowage
NEW MADRID SEISMIC ZONE 37

5 INCHES

10 CENTIMETERS

FIGURE 24. Pseudonodules formed by shaking, from Keunen (1958). 

A layer of sand (unit 1) overlies very soft clay (unit 2) in A. With 

shaking, pseudonodules developed (B and O and became completely 

enclosed in clay (D and E). Note the destruction of layering from C 

to D. This deformation is due to sinking of the entire sand layer into 

the very soft clay and also to localized sinking caused by uneven 

loading.

around the pseudonodules. The lower portion of the 

sequence of sediments in the trench is brecciated and 

faulted; the breccia, including pseudonodules, is surFIGURE 25. Development of load-casted ripples, caused by ripple 

crests sinking into soft mud. Note the progressive tilt of the internal 

cross-lamination, which causes the downflow portion to be more 

steeply inclined than the upflow portion (modified from Dzulynski 

and Kotlarczyk, 1962). Load-casted ripples can also have an opposite 

sense of rotation (upflow more steeply inclined than downflow), as 

evidenced in field observations in the St. Francis Basin.

rounded by a coarse-grained sand that has been injected 

from beneath. These larger scale deformation features 

are truncated by the uppermost sand lens (sand unit X), 

which is also laterally equivalent to pseudonodules.

The load structures in plate IA are interpreted as 

synsedimentary in origin and not as earthquake-induced 

on the basis of the following criteria: (1) The pseudonodules are gradational into the sand lenses, including 

load-casted ripples, with the same sense of flow direction, (2) there are several layers of pseudonodules laterally equivalent to largely undeformed sand lenses, and 

(3) the recurring conditions of sedimentation (rapid progradational deposition of sand over soft clay) were con-
38 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

ducive to this type of soft-sediment deformation. 

Although load structures such as these may be formed by 

earthquakes, their repeated occurrence in this sequence 

(as also shown by Alien, 1984, figs. 9, 10; or in Coleman 

and Prior, 1980, figs. 15, 21, 22, and 32) and their 

apparent localized occurrence argue for a depositional 

origin. (The injection features, brecciation of clay, and 

faulting in this exposure that crosscut the syndepositional features are probably of earthquake origin.)

Pseudonodules formed by earthquake shaking would 

be similar in shape to those formed by depositional 

processes, because the mechanism of sinking is similar. 

The environmental conditions conducive to pseudonodule 

development are also ideal for the formation of convoluted bedding, fluidization dikes, and slumping. Any 

depositional environment in which coarse-grained sediment is introduced rapidly in otherwise quiet, standingwater conditions, such as fluvial overbanks, lacustrine or 

marine deltas, and submarine fans, may include these 

features as part of its regular sedimentary record. We 

suggest that the difference between syndepositional 

deformation and earthquake-induced deformation would 

be in the relationship of the pseudonodules to the surrounding sediments. Intense earthquake shaking might 

cause collapse of an entire sand layer into clay, rather 

than only the thin edge, and a small portion of the base of 

a sand lens; in addition, the structures should be widespread in deposits having the same age. A widespread 

distribution of load structures formed at the same time is 

difficult to prove in most fluvial depositional environments.

OTHER FEATURES

Field examination is often required to distinguish 

between sand blows and small aeolian sand dunes, particularly on high terraces having very sandy surface 

soils. Another feature requiring field examination is a 

"deer lick," which is a very light-colored area, up to 15 m 

in diameter, caused by a high sodium concentration in 

poorly drained silty soil on level ground (Brown and 

others, 1973). They are relatively common in western 

Tennessee.

Aerial photographs often show arcuate ridges of sand 

on modern and Holocene point-bar deposits, as illustrated in figure 20C. These sand ridges form where the 

fine-grained topstratum is especially thin, and thus more 

quickly eroded, or was never present over the point-bar 

deposits. Field excavations are often required to determine if the sands have been vented versus exposed by 

erosion.

LOCAL GEOLOGIC CONTROLS ON PRODUCTION OF 

VENTED SAND

Some of the geologic controls on production of vented 

sand during the 1811-12 earthquakes were first noted by

Saucier (1977) and were later described in much more 

detail by Obermeier (1988, 1989). Principal controls are 

topstratum thickness and lithology, substratum sand 

grain size (and, indirectly, permeability), susceptibility 

to liquefaction of source sand (as related to age of the 

sediment and measured by the Standard Penetration 

Test (SPT)), and earthquake accelerations. The influence 

of these parameters was determined by hundreds of test 

borings, which were used to formulate a general notion of 

the geologic setting where sand was vented and to 

back-calculate 1811-12 earthquake accelerations. Along 

the border where sand blows developed (fig. 14), the 

Simplified Procedure of Seed and Idriss (1982) was used 

to estimate peak earthquake accelerations. Accelerations 

along the border probably did not exceed 0.25 g (Obermeier, 1988, 1989).

Substratum sands in the St. Francis Basin are typically at least moderately dense (engineering sense). Nj 

values (SPT blow counts adjusted to an overburden 

stress of 1 ton/ft2) in the depth range most likely to 

liquefy (5-15 m) are rarely less than 10, and median 

values are generally about 25.

Other basic data that should be kept in mind during 

this discussion of geologic controls for liquefaction features of the 1811-12 earthquakes are that the substratum 

sands generally have a narrow range of thickness 

(between 25 to 40 m); the epicenters of the three major 

shocks in 1811-12 were almost certainly somewhere 

within the areas where large volumes of sand were 

vented; and the Ms values of the three major shocks were 

probably about 8.5 to 8.7.

TOPSTRATUM THICKNESS

In general, where nonliquefiable topstratum thickness 

exceeded a critical threshold, there was increasing 

resistance to development of sand blows and venting of 

large volumes of sand (Obermeier, 1989). A topstratum 

thicker than 10 m presented a major barrier to more than 

scattered venting, even in close proximity to the epicenter. No sand blows formed where the topstratum was 

thicker than about 15 m. Within 20 to 25 km of the 

epicentral regions, a 5- to 6-m-thick topstratum was 

insufficient to prevent severe, extensive venting. At 

more distant localities, near the border of sand blows, a 

3- to 4-m thickness of topstratum noticeably reduced 

sand-blow development. Where a thick topstratum 

restricted venting to the surface, intrusions within the 

lower part of the topstratum were still plentiful.

TOPSTRATUM LITHOLOGY

Even though the great majority of our field observations were at sites where the topstratum was dominantly 

clay, we observed enough areas where the surficial
OVERVIEW OF STUDIES 39

deposits were sand rich to make some general observations about the formation of craters and vented-sand 

volcanoes. The largest craters occurred where the topstratum was most sand rich at the ground surface. The 

stratigraphy in the craters is generally not well defined 

in comparison to craters in South Carolina. The largest 

clay clasts are at the base, and clast size decreases 

upward in a structureless sand matrix. The transition 

from graded zone to the overlying laminated bedding is 

difficult to find in some craters; the laminated bedding is 

generally defined only by thin, wispy layers of slightly 

different sizes of sand and silt. There is no evidence of 

organic material accumulating in an open hole, as in 

South Carolina.

We did not observe V- or U-shaped fractures in 

regions of craters, as in South Carolina. Partly for that 

reason, and partly because of the difficulty of making 

moderately dense, well-graded sand flow extensively 

after being liquefied, we suspect that the mechanism for 

formation of craters in the New Madrid region differs 

from the mechanism in South Carolina, where the eruptive craterlets were abundant. In the New Madrid 

region, the craters may have formed primarily because of 

abrasion and widening of the sidewalls as fluidized sand 

and water vented for a prolonged time. Similarly, large 

craters did not tend to form extensively in the clay-rich 

topstratum in the New Madrid region because of the 

difficulty of abrading the sidewalls.

SUBSTRATUM GRAIN SIZE

Medium-grained and coarse-grained, clean, wellgraded sands greatly predominate in the substratum, in 

the depth ranges that were liquefied in 1811-12. Other 

factors being equal, the most extensive development of 

sand blows corresponded to areas underlain by mediumgrained sand. Much smaller and more scattered sand 

blows developed over coarse-grained substratum sands 

than over medium-grained sands. Coarse-grained substratum sands have much higher permeabilities than the 

medium-grained sands; possibly the higher permeability 

permitted pore pressures of the liquefied sands to dissipate rapidly enough to curtail sand-blow development.

RESISTANCE OF SOURCE SAND TO LIQUEFACTION

Only small regional variations in the relative density of 

the late Quaternary substratum sands are found in the 

St. Francis Basin; these sands are typically at least 

moderately dense and thus moderately difficult to liquefy. However, historical accounts (Street and Nuttli, 

1984) and data by Fuller (1912) show that 1811-12 sand 

blows were common on modern flood plains far from the 

St. Francis Basin. The modern flood plains are underlain 

by younger and generally looser and more liquefiable

sand beds than those in the St. Francis Basin (Obermeier 

and Wingard, 1985), thus explaining the development of 

sand blows much further from the epicenters.

Liquefaction features in the New Madrid seismic zone 

have not been observed to be induced by numerous 

historic earthquakes having mb of 5.3 to 5.5, or by a 

single earthquake having mb of 6.0; in 1895, many sand 

blows (generally small) were produced by an mb 6.2 

earthquake whose epicenter was near Charleston, Mo. 

(Obermeier, 1988). These sand blows developed in alluvium that is slightly less susceptible to liquefaction than 

that at most other places in the New Madrid seismic 

zone, excluding the modern (<500 yr) point-bar and 

sand-bar deposits of the rivers in the region. Thus a 

reasonable earthquake threshold is approximately mb 6.0 

for small liquefaction features in braided stream and 

meander belt deposits, excluding very young alluvial 

deposits.

OVERVIEW OF STUDIES

Geologic criteria have been developed and geologic 

controls have been evaluated for earthquake-induced, 

level-ground liquefaction features in alluvium of coastal 

South Carolina and the New Madrid seismic zone. Many 

different types of earthquake features occur in these 

areas, in addition to features of unknown or nonseismic 

origin that might be interpreted as having an 

earthquake-induced origin.

GEOLOGIC CRITERIA

Assigning an earthquake origin to possible sand blows 

generally requires that four criteria be satisfied:

1. The features must have sedimentary characteristics 

that are consistent with earthquake-induced liquefaction 

origin; that is, there is evidence of an upward-directed, 

strong hydraulic force that was suddenly applied and was 

of short duration.

2. The features must have sedimentary characteristics 

that are consistent with historically documented observations of earthquake-induced liquefaction processes.

3. The features must occur in ground-water settings 

where suddenly applied, strong hydraulic forces of short 

duration could not be reasonably expected except from 

earthquake-induced liquefaction. In particular, such settings are extremely unlikely sites for artesian springs.

4. Similar features must occur at multiple locations, 

preferably at least within a few kilometers of one 

another, having similar geologic and ground-water settings. Where evidence of age is present, it should support the interpretation that the features formed in one or 

more discrete, short episodes that individually affected a
40 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

large area and that the episodes were separated by long 

time periods during which no such features formed.

As fewer of these elements are satisfied, confidence in 

interpretation of an earthquake origin generally diminishes. Considerable reliance has been placed on the 

second element for interpreting an earthquake origin in 

coastal South Carolina and the New Madrid seismic zone. 

If, however, all the other criteria are unequivocally 

satisfied outside these geographic areas, an earthquake 

origin can still be ascertained in areas that have not had 

historic earthquakes.

Criteria based on engineering analysis may be useful 

for diagnosis of the origin of features. To illustrate, if the 

source sand bed can be identified at a site of possible 

liquefaction, and the sand bed proves to be nonliquefiable 

on the basis of an engineering analysis, then an 

earthquake-induced origin can be eliminated. As another 

example, if mineralogical analysis associates vented sediment with a deep stratum of loose sand rather than with 

a shallower sand bed of much higher compactness, and an 

engineering analysis of nonearthquake-related groundwater flow forces shows that the shallower, more dense 

sand should have been transported up in preference to 

the deeper, loose sand, then an earthquake origin may be 

ascertained.

TYPES OF EARTHQUAKE FEATURES

In both coastal South Carolina and the New Madrid 

seismic zone, earthquake-induced liquefaction features 

originated in sand deposits that are relatively thick (3-50 

m) and contain no or few intercalated silt- or clay-rich 

strata. At the ground surface there is a cap that is much 

less permeable than either the subjacent or deeper 

source sand beds. Properties of the cap have a major 

effect on the surface expression of the sand blows. In 

coastal South Carolina, where the cap is generally a 

1-m-thick soil that is weakly cemented with humate, the 

sand blows are expressed as craters surrounded by thin 

sand sheets; in the New Madrid seismic zone, the cap is 

generally a clay-rich deposit about 2 to 10 m thick, and 

sand blows are expressed as sand volcanoes.

Both vented and intruded features can be related to 

earthquake-induced liquefaction. Features characterized 

by sand vented to the surface include more or less 

circular, individual sand blows and also long, irregular 

sand-filled fissures, hundreds of meters long. Where a 

clay-rich cap exceeds a critical thickness, sand is not 

vented to the surface; instead, earthquake-induced liquefaction is represented only by sand intrusions. 

Intruded dikes usually have a massive internal structure, 

whereas sills can contain both graded and laminar bedding in coastal South Carolina and the New Madrid 

seismic zone.

Lateral spreads produced by the 1811-12 earthquakes 

were common. (See Fuller, 1912, p. 48, for a schematic 

drawing.) We made no effort to locate lateral spreads in 

the New Madrid seismic zone, but evidence of lateral 

spreads is described in coastal South Carolina.

GEOLOGIC CONTROLS

The thickness of clay-bearing or impermeable surficial 

deposits can be an important control on development of 

surface venting. In the St. Francis Basin, our data show 

relations between the distance from the epicenter, thickness of topstratum, and development of surface venting 

produced by the 1811-12 New Madrid earthquakes; near 

the epicenters, a topstratum thickness more than about 

12 m prevented venting, whereas near the farthest limits 

of sand-blow development, a topstratum more than 4 to 

5 m thick greatly restricted development. In coastal 

South Carolina, a cap exceeding 3 to 5 m thick generally 

prevented development of sand blows for both 1886 and 

pre-1886 earthquakes. These differences in critical thicknesses between coastal South Carolina and the New 

Madrid seismic zone most likely reflect the influence of 

factors such as thickness of source sand bed, susceptibility of source sand bed to liquefaction, and earthquake 

magnitude. Similar relations between critical thickness 

of surficial cap and development of surface manifestations of liquefaction have been noted by Ishihara (1985).

The rate of expulsion of water to the surface while the 

sand blow was forming possibly controls whether sand 

blows formed as craters or as sand mounds, but this 

speculation has not been tested. Certainly, in coastal 

South Carolina, the sands are generally much looser than 

sands in the New Madrid seismic zone, and this looser 

state may have helped create a very thick, water-rich 

zone during shaking, which was probably the primary 

source zone during initial, very rapid explusion. Sands in 

the New Madrid seismic zone characteristically are much 

more permeable than the marine sands in coastal South 

Carolina, yet craters are not common features in the 

New Madrid seismic zone. Thus, the craters cannot be 

necessarily associated with higher permeability alone of 

source sands.

Vertical cracks formed by desiccation or other processes may predispose clay-rich sediments to form 

vented-sand volcanoes rather than craters. Vertical 

cracks are common in both coastal South Carolina and 

the New Madrid seismic zone.

Source beds for sand blows in the New Madrid seismic 

zone are predominantly medium- and coarse-grained 

sands; in the 1811-12 earthquakes, the coarse-grained 

and thus highly permeable source sands produced fewer 

and smaller sand blows than medium- to fine-grained 

source sands. Insufficient data are available to form
SUGGESTIONS FOR FUTURE RESEARCH 41

conclusions about influence of grain size on sand-blow 

size in coastal South Carolina, because almost all sands 

are fine grained.

Geologic age is an important control on liquefaction 

susceptibility in both the New Madrid seismic zone and 

coastal South Carolina. Youngest sediments are generally most susceptible to liquefaction, for a given mode of 

deposition.

SUGGESTIONS FOR FUTURE RESEARCH

In recent years it has become increasingly clear that 

detection of liquefaction features is valuable as a tool for 

understanding earthquake activity. Liquefaction features can be used to identify strong earthquake shaking 

and to date the strong shaking events. We have 

described our interpretations of earthquake- and 

nonearthquake-related features from field studies in the 

New Madrid seismic zone and in coastal South Carolina. 

Still, more work is needed to better identify potential 

earthquake hazards in these geographic areas.

In the study of New Madrid 1811-12 liquefaction 

features, much was learned that is relevant to any search 

for paleoseismic liquefaction in moderately thick fluvial 

sediments of the Central United States. Aerial photographs are useful in a search for features formed in 

1811-12, and earthquake deposits predating 1811-12 laid 

down on terraces having clay-rich surface soils may also 

be visible on such photographs. However, deposits laid 

down on terraces having sandy surficial soils have probably been so intensely reworked by wind that the deposits would be difficult, if not impossible, to identify. Aerial 

photographs should range from the earliest possible date 

(generally the late 1930's and early 1940's) to about the 

mid-1950's. The earlier photographs may show many 

features destroyed by modern farming, whereas more 

modern photographs generally have better overall clarity.

Field checking is generally required to determine the 

origin of features that on aerial photographs are suspected to have an earthquake origin. In particular, in 

point-bar deposits, field checking is often required to 

verify the origin of sand that appears to have been 

vented along slightly elevated, arcuate ridges. On terraces having sandy surface soils, sand blows are often 

indistinguishable from sand dunes, and distinctive signs 

of previous earthquakes may be restricted to long fissures that formed near and parallel to scarps along 

terrace sublevels.

Searches in drainage ditches and sand pits are probably the only means for identifying liquefaction features 

that much predate the 1811-12 earthquakes. The best 

areas to search can be predicted by evaluating the

influence of geologic controls such as sediment age, 

topstratum thickness, grain size of source beds, and 

proximity to suspected epicenters.

Any search for paleoseismic features in coastal South 

Carolina and in the Coastal Plain of the Eastern United 

States should also make extensive use of exposures in 

ditches; aerial photographs, for example, are useless for 

locating 1886 and pre-1886 sand blows in South Carolina. 

Determination of an earthquake or nonearthquake origin 

for disrupted ground features in the Coastal Plain generally requires a team of people having many skills. In 

addition to having an understanding of sedimentation 

processes, soil science, and engineering mechanics, there 

may also be a need for geochemistry studies because 

weathering in these humid environments produces 

bizarre features not previously described in the literature. We have described some weathered and disrupted 

ground features that could cause confusion, but our 

catalog is far from complete.

Research is also needed to provide criteria for distinguishing between features having an earthquakeinduced liquefaction origin and features having a shortterm spring origin. Locations where springs can be easily 

eliminated as a cause, such as along the crests of beach 

ridges of coastal South Carolina, are relatively sparse in 

the Eastern United States. Fluvial terraces in lowlands 

contain the only potentially liquefiable deposits in most 

places of interest to paleoseismicity. Possible indicators 

of origin include the ground failure mode; the nature of 

the filling in vents, dikes, and sills; geologic characteristics of the source beds; and engineering characteristics of 

the source beds. The relevance of each indicator is 

discussed below.

Ground failure mode. Ground failure mode includes 

features such as lateral spreads, single long fractures, 

and shattered ground at the ground surface. All of these 

features generally indicate an earthquake origin. 

Earthquake-induced liquefaction does not always produce these types of features, however, and in many cases 

they are not easy to recognize in ditch exposures even 

where they are known to have been relatively common, 

such as the meizoseismal region of the 1886 Charleston 

earthquake. Use of techniques that can rapidly develop a 

three-dimensional view, such as ground-penetrating 

radar, may prove to be of great value.

Nature of filling. Vents associated with an 

earthquake-induced liquefaction origin generally appear 

to be filled with a structureless mixture of sand and silt 

grains and clasts derived from sidewalls and beds at 

depth. This structureless mixture apparently represents 

transport through the vent as a slurry. In some places, 

particularly near the top of the vent, the finer grained 

material has been winnowed out, thus indicating transport on a grain-by-grain basis. For either the slurry or
42 EARTHQUAKE-INDUCED LIQUEFACTION FEATURES IN SOUTH CAROLINA AND THE NEW MADRID SEISMIC ZONE

the gram-by-grain mode of transport, it should be possible to place limits on the hydraulic forces that caused the 

transport and, by means of a flow-potential diagram, 

calculate whether nonearthquake flow forces could possibly have been the driving mechanism.

Numerous observations in the New Madrid seismic 

zone show a wide variation in the nature of sill fillings 

caused by earthquake-induced liquefaction. The fillings 

range from layered bedding to graded bedding to a 

structureless mixture of sand, silt, and clay clasts; for 

paleoseismicity interpretations, we suspect that the 

structureless mixture is associated only with the very 

forceful injection caused by earthquake-induced liquefaction, whereas layered and graded bedding may be associated with either a spring or earthquake-induced liquefaction origin.

In the New Madrid seismic zone, some of the larger 

sills within 2 m of the ground surface have domed the 

overlying beds as much as 0.3 m vertically over a 

horizontal distance of 5 m; such large doming may be 

unique to earthquake-induced liquefaction. The nature of 

the fillings in dikes may also be related to an earthquakeinduced liquefaction or spring origin. Relevance of the 

fillings in vents, dikes, and sills can be verified only by 

research in the field.

Geologic characteristics of source beds. Intuition 

suggests that earthquake-induced liquefaction should be 

much more effective than springs as a means to destroy 

original bedding over a large area. This suggestion has 

not been examined, however, and may be difficult to 

evaluate in practice. Earthquake-induced liquefaction 

would also probably produce many more widespread sills 

and dikes than springs.

Engineering characteristics of source beds. The 

source beds containing the material vented by springs 

should be located selectively with respect to the groundwater setting and flow forces. For springs, the source 

beds must be connected to the source of flowage in such 

a manner that flowage goes toward the vent, and the 

flowage forces must be large enough to carry material 

from the source bed. This source bed would be, in 

general, the uppermost stratum of wide lateral extent 

with respect to hydraulic connection and would also be 

the uppermost fine-grained, noncohesive sand stratum. 

The source bed with regard to earthquake-induced liquefaction commonly lies much below (several meters) the 

uppermost sands of wide lateral hydraulic connection, 

according to limited observations in deep ditches in the 

New Madrid seismic zone. Thus, permeability relations 

may be quite useful for determining the causative mechanism responsible for venting materials.

Standard Penetration Test data that show relative 

ability to liquefy during earthquake shaking should also 

prove useful for determining origin at some places.

Geologic investigation may also be required to determine 

origin in areas where Standard Penetration Test data 

indicate possible source beds that are thin (<1 m) and lie 

immediately beneath an impermeable cap, because 

(injected) sills beneath impermeable caps have been 

observed by us to approach a meter in thickness.

RELEVANCE OF LIQUEFACTION FEATURES

Recognition of the various features described in this 

paper, and identification of the most probable origin for 

each, provides a set of important tools for understanding 

the paleoseismicity in areas where faults are not obvious 

at the surface and where historic seismic activity is 

infrequent. Even where faults are available for study and 

offsets can be documented, there may be doubt that the 

offsets were associated with earthquakes; the presence 

of liquefaction-induced features is a means of verifying 

strong shaking.

The criteria we describe in this paper can be applied to 

many worldwide geologic settings. We caution, however, 

that verification of paleoseismic events becomes generally more difficult with increasing age of the event. The 

distinction between earthquake and nonearthquake liquefaction features is often impossible without knowledge 

of ground-water conditions. Application of the criteria to 

pre-Quaternary deposits may be extremely difficult, if 

not impossible, at many places.

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