1
Paleoseismic investigations
Arnfried Becker, Institut für
r Geophysik, ETH-Hön
nggerberg, CH-8093 Zürich
CONTENTS
•
•
•
•
•
•
Introduction
Basic concepts
Geological archives
o Active fault-scarps
o Liquefaction in flood-plain deposits
o Lacustrine deposits, sub-aquatic slumps
o Caves
o Rockfall deposits
Dating methods
Magnitude/Intensity estimates
Epicentre estimate
Keywords: paleoseismology, fault-scarp, colluvial wedge, trenching,
liquefaction, seismite, speleothems, rockfall deposits, age dating,
magnitude, epicentre
Introduction
Paleoseismology is the study of prehistoric earthquakes based on
the interpretation of the geological record that these earthquakes
have left behind (Krinitzsky & Slemmons 1990, McCalpin 1996,
Wallace 1981). Paleoseismological investigations are particulary
needed in regions with a short instrumental and historical record
such as
America and Australia and large parts of Africa, were
written communications exist for not longer than 200 to 500 years.
In these regions seismologist soon reach the limits of the written
seismic record, if no other sources of information were developed.
Thus, it comes as no surprise that, despite some initial research
efforts
in
other
regions
of
the
world,
systematic
paleoseismological investigations started in the western part of
the United States. Most of the early stimulating work in this
field of research was related to safety assessments of nuclear
power plants and other lifeline buildings. In Central Europe the
historical record goes back for 1000 years, in the Mediterranean
for 2000 years and more and in China for up to 4000 years
(although reasonably complete only for 1000 years). Seismologists
and local authorities in some of these regions developed a feeling
of safety; the thinking was that strong earthquakes will either
not or rarely happen so that damage of facilities and many
2
casualties are unrealistic scenarios. However, paleoseismological
research in so-called stabile plate interiors such as Australia
and the Eastern US could demonstrate that there is potential for
strong earthquakes even in these regions, although average
recurrence times for mainshocks are much longer (and mostly longer
than the written historical record) than along active plate
boundaries. A M = 7 or M = 8 earthquake in a region were nobody
expects such a shock would probably result in numerous fatalities
and damage to facilities because safety guidelines for buildings
would not have faced the possibility of such strong earthquakes.
After all, not earthquakes kill people, building do. The possible
expectation of strong earthquakes also in regions which are
currently not suffering strong earthquake shocks stimulated also
paleoseismological investigations in different parts of Central
Europe such as Switzerland. The following description will be
restricted to the need of paleoseismological investigation in
Switzerland leaving out methods and geological archives which
cannot be used here.
Basic concepts
Primary targets for paleoseismological research are active faultscarps that show evidence for coseismic movements (Fig.~1). This
evidence generally can only be discovered by trenching, i.e.
digging a trench perpendicular (or oblique) to the active faultscarp and subsequent geological mapping of the trench faces.
Active fault-scarps have the advantage that the epicentre of the
paleo-earthquake can be directly determined with some degree of
reliability. In addition a magnitude estimate based on the length
of the reactivated fault section and the offset, can be derived.
It is also possible to date the event by dating organic matter
embedded in the colluvial wedge at the foot of the fault scarp.
However, these data only supply information about the seismicity
along a single fault. If there is no clear evidence that this
fault is the only seismic active fault in the region, co-seismic
movements along faults in the vicinity would not express
themselves in this paleoseismic record. Therefore, secondary
targets have to be taken into account, which are susceptible to
the effects of seismic shaking and which record all strong
paleoseismic events in a region independently of the individual
fault responsible for a single earthquake. Most promising in this
respect are lake deposits. Because of there depositional
environment a continuous sedimentary record can be investigated
which covers in general several thousand to ten thousand years, in
Switzerland mostly 10’000-15’000 years. Other secondary targets
could be flood-plain deposits, caves and rock fall deposits. All
these secondary geological archives are sensitive to earthquake
shaking; however they are also susceptible to non-seismic
disturbances. One of the difficulties of these secondary archives
is to distinguish between features related to earthquakes and
those independent of seismic events.
3
Fig. 1 Co-seismic effects due to strong earthquakes: surface rupturing
along an active fault, liquefaction and soft-sediment deformations in
flood-plain and lake deposits, damage of speleothems in caves, land
slides and rockfalls along steep slopes (after Levret & Combes 1997).
Geological Archives
Fault-scarps
The most prominent features in paleoseismological research are
active fault-scarps. The reason is obvious: active fault-scarps
are the clearest expression of fracturing in the earth’s crust,
most likely related to earthquakes. However, co-seismic rupturing
4
is not the only possibility to create such fault-scarps.
Therefore, one of the major tasks for paleoseismologists is to
find evidence for co-seismic rupturing and give estimates for the
percentage of aseismic creep on the total offset along the faultscarp. Most experience has been built-up by the investigation of
active fault-scarps in the extensional regime of the Basin-andRange Province in the western US. Co-seismic fault-scarps related
to normal faulting events are therefore well understood, although
the features seen in the semi-arid regions of the western US are
sometimes difficult to transfer into regions with a more humid
climate and more intensive disturbance by human activities. Less
well known are coseismic deformations along thrust faults, because
these features are less frequent at the earth’s surface and faultscarps are frequently not very prominent due to splays and the low
dip angle of the fault plane. Strike-slip faults can frequently be
seen as clear tectonic lines in the topography, mostly due to
weathering and erosion. However, it is very difficult to give
proofs for individual co-seismic offsets because strike-slip
faults will not necessarily generate prominent vertical offsets
along the trace of the fault-plane.
Fig. 2 Generalized topographic profile of a fault-scarp. A — upper
original surface, B1 and B2 are two bevels, C - free face, D - colluvial
wedge, E - lower original surface, H - total vertical offset of the fault
scarp, h - height of the free face (after Qidong & Yuhua 1996).
Repeated co-seismic slip along a normal fault can already be seen
in the development of bevels above the fault-scarp (Fig.~2). These
are erosional features related to the degradation of the slopes of
preceding offsets along the fault-scarp. The steepest part of the
fault-scarp is the free-face, were the in situ rock surface can be
seen. The height of the free-face largely depends on the material
properties of the in situ rock. Young, unconsolidated rocks with
low cohesion are not able to keep a steep newly formed fault scarp
stabile for a long time. In that case the free face will collapse
soon, creating a colluvial wedge of coarse-grained material
without layering at the foot of the scarp. After stabile
conditions at the fault scarp have re-established, erosional
5
degrading of the fault-scarp will add sediment to the colluvial
wedge, which can be clearly distinguished from the underlying
debris (debris facies) by its reduced grain-size and its layering
(wash facies). If no renewed offset along the fault-scarp has
happened, soil formation can commence. A new seismic event that
reactivates the fault-scarp sufficiently, will create a new coarse
grained layer, followed by sediments of the wash facies, and
finally a soil again. Therefore, the investigation of the
colluvial deposits at the base of fault-scarps are most important
for the deciphering of the history of co-seismic slip events along
an active fault. A method commonly used for the investigation of
the colluvial wedges is trenching, the digging of a trench across
the fault-scarp and the colluvial wedge. This investigation allows
a separation of the individual colluvial wedges, most clearly if
both the sediments of the debris and the wash facies are developed
(Fig.~3). That is only possible if the recurrence times for
surface ruptures along the fault-scarp are sufficiently long
allowing the generation of the sediments of the wash facies as
well. If the recurrence time intervals are to short it is
difficult to distinguish between individual colluvial wedges. In
principal the number of individual colluvial wedges allows the
counting of earthquake surface ruptures along the fault with a
sufficient, i.e. significant surface offset.
Fig. 3 Schematical sketch of sequential stages in the evolution of a
normal fault scarp. A - initial faulting, B - collapse of free face and
generation of the debris facies colluvial wedge (triangles) , C — wash
facies colluvium (stippled) buries debris, scarp stabilizes, soil forms,
D - second faulting event creates new free face, E - newly formed debris
covers earlier wedge and soil, F - wash facies colluvium and soil develop
on second wedge. G - Example for a graben where colluvial wedge
interfingers with sag pond muds (horizontal dashes) (after McCalpin
1989).
6
Trenching is also a useful tool for the investigation of faultscarps related to thrust faulting and has been used intensively in
China during the last decade. In principal this method can be used
with respect to thrusting events in a similar way like in the case
of normal faulting events. In the case of strike-slip faults, the
situation is different, because of no or only minor vertical
offsets along the fault a colluvial wedge will not develop.
However, a detailed investigation of the abutting relationships of
fractures and layers will also in case of strike-slip faults give
reliable results for the occurrence of individual fracture events.
Fig. 4 Composite block
Davenport et al. 1994).
diagram
of
liquefaction
structures
(after
Liquefaction in flood-plain deposits
Seismically induced liquefaction is usually caused by strong or
moderate earthquakes (magnitude threshold about 5 to 5.5). It is
accompanied by the ejection of sand and water at the land surface,
generating sandblows and small sand craters. Liquefaction is
caused by an increase of the pore water pressure during the
passage of seismically generated shear waves. If the pore water
pressure increases to a value which is equal to the confining
pressure, the effective stress drops to zero and the sediment will
7
become liquefied. Most susceptible for liquefaction are well
sorted, cohesionless, water-saturated sands. Generally, gravels
are not experiencing liquefaction, because water can escape during
the passage of seismic waves and no pore water over-pressure is
built up. On the other hand, for silt and clay the effects of
cohesion will avoid liquefaction. A high ground water table and
suitable sediments in a depth no deeper than 4 m covered by a 1-2
m thick slightly cohesive caprock are ideal conditions for
liquefaction and the formation of sand blows at the surface and
sand dykes in the caprock (Fig.~4). Flood-plain deposits along
major drainage systems or coastal planes frequently fulfil these
conditions and have been used for paleoseismic investigations, for
instance in the Mississippi valley near New Madrid, central USA,
and at Charleston on the east coast of North America. Liquefaction
phenomena are suitable for the detection of strong earthquakes in
a region. The liquefaction potential of a sediment remains even
after several liquefying earthquake events have happened
(Obermeier 1996), which gives this method potential for the
reconstruction of the history of strong earthquakes in a region.
Lacustrine deposits, sub-aquatic slumps
Lakes are an environment of continuous sedimentation over time
spans covering several 1000 or even several 10000 years.
Therefore, lakes are interesting as a geological archive, which
has the potential to register strong pre-historic earthquakes over
a long time span. The effects of an earthquake can directly create
in situ deformations related to fracturing, liquefaction, or all
kind of soft sediment deformations mostly related to density
contrasts. Most famous are fault-graded beds, ball-and-pillow
structures and dish structures. Despite of these in situ
deformation features, earthquakes may also trigger secondary
features related to subaquatic sliding and slumping or seiches
which can be seen in the sedimentary record as turbidites and
homogenites. However, most of the features ascribed here to
earthquakes may also be caused by non-seismic processes. Therefore
a careful investigation of the event horizons in a lake has to be
carried out along with correlating events across the boundaries of
individual sedimentary basins based on age datings to constrain
the findings as seismites.
Caves
Earthquakes may also be documented in caves due to the damage they
cause (Fig.~5). Particulary susceptible for earthquake damage in
caves are spelaeothems (stalagmites, stalagtites, soda straws);
however, earthquakes may also cause a collapse of parts of the
caves, for instance of the ceiling. Broken stalagmites and
stalagtites embedded in cave deposits, fallen blocks from the
ceiling
overgrown
by
spelaeothems, regrowths of broken
speleothems, growth-anomalies of soda straws, changes in the
8
growth pattern of speleothems in a cave can have different causes,
such as local cave collapse, floods in the cave, rock mass
instabilities and vandalism. Nevertheless, earthquakes may also
have caused these deformations. Again, it is nessesary to perform
a detailed investigation to distinguish between non seismical and
seismical destruction features.
Such an investigation primarily
relies on age dating which may show a regional synchronous damage
event and a comparison with other paleoseismic indicators
independently of the findings in caves.
Fig. 5 Different speleothems related to earthquakes: A and B — collapse
of stalagmites and growth of new concretions over the broken parts, C displacement along a fault, D - collapse of stalagmites caused by the
displacement of the adjacent wall, E - growth of stalagmites over
collapsed blocks from the ceiling, F - start of new concretioning over a
fallen block. Positions a and b indicate characteristic sampling points
referring to samples characterizing the time span immediately before and
after the earthquake (after Postpischl et al. 1991).
Rockfall deposits
All instabilities along steep valley incisions like slides, slumps
and falls can be triggered by strong earthquakes with magnitudes
of M ≥ 5 (Keefer 1984). However, in case of slope instabilities
also, mechanisms other than earthquakes are likely to cause slope
9
failures, like exceptional precipitation events, effects of frost,
over deepening of the valleys due to erosion. Just like earthquake
triggered events these may happen spontaneously. It is the task of
the paleoseismologist to find evidence for the triggering of such
slope instabilities due to earthquakes, a very ambitious task
considering that earthquake triggering has to be distinguished
from other causes that affect whole regions. Deposits related to
slope instabilities can support other paleoseismological findings,
for instance from active fault-scarps, flood plains, lakes, caves;
however, the results are difficult to interpret as long as they
remain for themselves.
Dating methods
The most prominent method for dating purposes in palaeoseismology
is the radiocarbon method. It may be used for samples of an age of
up to 60’000 years. Fault-scarps can be dated indirectly by
organic remains, for instance wood, charcoal, in the colluvial
wedge or organic remains in soils. Radiocarbon plays an important
role in dating of lake deposits, preferentially based on remains
of terrestric plants, i.e. wood, leaves, plant chuff, seeds.
Speleothems can be dated as well, although with much more care
because of the contamination of samples by ’old carbon’ from the
surrounding
carbonatic
rocks.
In
lake
deposits
also
palynostratigraphy (i.e. the vegetation history based on pollen)
for the Late Pleistocene and Holocene, sedimentation rates
(varves) or tephrostratigraphy (volcanic ash layers) can be used
for dating purposes. A standard method in speleology is the
Uranium/Thorium method, which allows dating back to 500’000 years.
Also amino-acid razemisation dating of bones has been frequently
used in speleology with variable success. The measurement of
exposure ages of newly formed rock surfaces, such as fault-scarps
or rockfall deposits, has been attempted with different methods.
Some of them are Beryllium-10, lichenometry or growth rates of
weathering crusts. In case of fault-scarps the degradation rate of
such scarps could be used successfully for (approximate) age
datings.
For paleoseismological purposes dating is fundamental, because one
of the strongest arguments for a seismic event in a region is the
synchronicity of presumed seismites in different geological
environments. A combination of different dating methods and
different kinds of deformation features supplies the strongest
argument for an exceptional palaeo-earthquake.
Magnitude/Intensity estimates
The estimate of palaeo-earthquake magnitudes or intensities is
based on regional calibration curves for recent earthquakes
causing surface-deformation features. Because deformation features
in sediments caused by earthquakes are strongly dependent on the
10
local and regional geology, it is important to build-up regional
calibration curves. Such calibration-diagrams, which convert the
degree of deformation and the regional extend of the deformation
features into magnitudes (intensities), have been established for
fault-scarps, liquefaction features and also rockfall deposits
(Davenport 1994, Keefer 1984, Wells & Coppersmith 1994). These
calibration diagrams can now be used to give an estimate for the
magnitude of a prehistoric earthquake, where only geometric
parameters are known, for instance the regional extend of certain
deformation features, the rupture length and height of a fault
reactivated during an earthquake. Examples for such conversions
for reactivated fault length, for the maximum extend of
liquefaction phenomena and various types of slides/falls are shown
in the Figs. 6,7 and 8.
Fig. 6 (a) Regression of surface rupture length on magnitude (M). Short
dashed line indicates 95% confidence interval. (b) Regression lines for
strike-slip, reverse, and normal-slip relationships (after Wells &
Coppersmith 1994).
Epicentre estimate
The clearest relationship between epicentre and palaeo-earthquake
is given in the case of a seismogenic fault creating a surface
rupture, since the active co-seismic fault-scarp is the epicentral
region. In case of liquefaction and soft-sediment deformations in
flood-plain and lake deposits the epicentral region has to be
mapped based on the decrease of deformation intensity with
increasing distance from the epicentre. The
most intensive
deformation one can expect in the epicentral region. With a
11
Fig. 7 Maximum distance to site of liquefaction versus earthquake
magnitude (m). Formula and solid line after Kuribayashi & Tatsuoka
(1975). Dashed-dot line fit after Tinsley et al. (1985). GR = Glen Roy
event, R = Roermond earthquake (after Davenport 1994).
sufficient number of localities showing deformation features in
the surroundings of an epicentre one should be able to map the
gradient in the deformation intensity and, thus, should be also
able to localize the epicentre. That procedure will only be
successful with an even distribution of localities in the
surroundings and good knowledge about the deformational behaviour
of the various sediments involved. In case of slope instabilities
it has also been attempted to map epicentral regions based on the
increase of the number of earthquake triggered slides, slumps and
falls. However, so far no consistent result can be given for such
a correlation with recent earthquake events, probably caused by
the fact that not only the type of rock defines the susceptibility
for slope instabilities but also structural and topographical
boundary conditions. However, just like outlined for the other
methods the estimate of the epicentral region will be more
reliable if the results of different independent methods are
combined.
12
Fig. 8 Maximum distance from fault-rupture zone to landslides of
different type in earthquakes of different magnitudes for A — disrupted
slide or fall, B - coherent slide, C -lateral spread or flow. D comparison of upper bounds of A, B and C (after Keefer 1984).
13
References
Davenport, C.A. (1994): Geotechnical consequences of ground
motion: hazard perspectives.- Geologie en Mijnbouw, 73: 339-356.
Keefer, D.K. (1984): Landslides
Geol. Soc. Am., 95, 406-421.
caused
by
earthquakes.- Bull.
Krinitzsky, E.L. & Slemmons, D.B. (1990): Neotectonics in
earthquake evaluation.- Geol. Soc. Am., Rev. Engng. Geol., vol.
VIII, 160 p.
Levret, A. & Combes, P. (1997): Recherche des s ismes historiques
et ant historiques: approaches m thodologiques.- In: Fiches, J.L., Helly, B. & Levret, A. (eds.): Arch ologie et sismicit .Sophia Antipolis (Edition APDCA), 15-39.
McCalpin, J. (1989): Current investigative techniques and
interpretive models for trenching active dip-slip faults.- In:
Watters,
J.
(ed.):
Engineering
Geology
and Geotechnical
Engineering.- Rotterdam (Balkema), 249-258.
McCalpin, J.P. (1996): Paleoseismology.- San Diego (Acad. Press),
588 p.
Obermeier, S.F., 1996: Use of liquefaction-induced features for
paleoseismic analysis - An overview of how seismic liquefaction
features can be distinguished from other features and how their
regional distribution and properties of source sediment can be
used to infer the location and strength of Holocene paleoearthquakes.- Eng. Geol., 44: 1-76.
Postpischl, D., Agostini, S., Forti, P. & Quinif, Y. (1991):
Palaeoseismicity from karst sediments: the Grotta del Cervo cave
case study (Central Italy).- Tectonophysics, 193: 33-44.
Qidong, D. & Yuhua, L. (1996): Paeoseismology along the rangefront fault of Helan Mountains, north central China.- J. geophys.
Res., 101 (B3): 5873-5893.
Wallace, R.E. (1981): Active faults, paleoseismology, and
earthquake hazards in the western United States.- In: Simpson,
D.W. & Richards, T.G. (eds.): Earthquake prediction: An
international review.- Am. Geophys. Union, Maurice Ewing Ser., 4:
209-216.
Wells, D.L. & Coppersmith, K.J. (1994): New empirical
relationships among magnitude, rupture length, rupture width,
rupture area, and surface displacement.- Bull. Seism. Soc. Am., 84
(4): 974-1002.