Gulf of Corinth

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Feb 22, 2014 (3 years and 5 months ago)

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CorSeis Final Scientific Report


Brunel University



Overview


The Brunel group were principally involved in earthquake geology studies along the Eliki Fault with
the aim of shedding light on its recent fault history. The fault itself is known to comprise
two discrete
seismogenic segments. The eastern segment ruptured in 1861 and fault trenching studies (some of
them undertaken within the Corseis programme) have identified at least two previous surface
-
faulting
ruptures along it. In contrast, no historical
rupture is known for the western segment. Moreover,
artificial terracing of the slopes for agriculture, and construction of the main Athens
-
Patras highway
have meant that palaeoseismic studies have not been carried out along the western segment. Thus,
whil
e geological studies on the eastern segment focus on tying together and refining a detailed history
of past surface
-
faulting earthquakes, studies on the western segment seek to resolve whether or not
this fault has experienced recent (Holocene) tectonic ac
tivation.


The western fault segment has been the principal focus of the Brunel research team. Here, because
intense human modification had meant that standard palaeoseismic studies could not be used, recourse
was made to two indirect methods of investiga
tion: (1) ground
-
penetrating radar studies across the
suspected fault trace; and (2) shallow borehole studies in the hangingwall of the fault to detect
sedimentary evidence for past abrupt environmental change. In addition, the group undertook




Detection o
f buried fault scarp on the west Helike fault with Ground Penetrating Radar
(GPR)



Evidence for abrupt environmental change from coring in the Eliki hanging wall and
coastal plain



First application of 210Pb geochronology to paleoseismology on the colluvial

edge of
the Heliki fault scarp


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Fig. 1: Map of study sites .



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1. Ground Penetrating Radar Studies


In order to assess whether the western Eliki Fault has experienced recent tectonic reactivation,
ground
-
penetrating radar surveys were made across a ste
pped sequence of morphological scarps along
the main escarpment base. GPR investigations were focussed on the Nikolaiika Fan (NF), part of the
Kartoula Fan (KF) complex, above the Eliki Plain. In the field, morphological scarps often coincide
with agricult
ural terrace edges and so, in some cases may be of anthropogenic rather than tectonic
origin. It is to ascertain whether these surficial scarps relate to concealed near
-
surface faults that
recourse was made to ground penetrating radar studies.


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a
)
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Fig. 2. (a) Location map of the three GPR profiles undertaken across possible tectonic scarps on the Nikolaiika
Fan. (b) GPR survey on GPR1 showing the position of two scarps in the adjacent fields.


GPR surveys using PulseEKKO 50, 100 and 200 MHz antenna
e were collected on three roads cutting
across the fan surface which run perpendicular to 0.5
-
1.0 m high surface scarps identified in the field.
On one road (L1) profiles were collected at 50, 100 and 200 MHz. These profiles were collected
across two scarp
s which were visible in the field to the side of the road. On the other roads profiles
were only collected at 50 MHz. Data was post processed using Gradix 1.10 software. Around 250 m
of profiles were taken to image the faults beneath the Nikolaiika Fan.


The results (Woodward & Stewart in prep) reveal near
-
surface tectonic disturbances on all profiles. In
most cases, the subsurface disturbances can be correlated with breaks in slope in the surrounding
fields. The faults are manifest in three ways: 1) visib
le offsets in near
-
surface strata; 2) concave
features in the stratigraphy, representing zones of fracture and warping; 3) At depths of 2
-

5 m, where
some fault planes are steeply dipping, faults are imaged on GPR profiles as incoherent scattering from
ru
pture planes.


The GPR profiles confirm recent (Late Holocene) tectonic activity under the Nikolaiika Fan. Such
recent activity in front of the footwall of the Western Eliki Fault may suggest that recent earthquake
activity has been concentrated on fault
splays from the main footwall fault, rather than along the
footwall fault plane itself.


Scarp

Scarp

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3

L2
L1
L3
0
30
60
15
45
Distance along profile (m)
0
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8
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(
m
)
OS
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DW
S
S
OS
IS
TP

Fig 3. GPR profiles showing inferred faults that in many cases can be correlated with surface
breaks in slope.



2. Shallow Coring on the Eliki Plain


Shallow (<10 m
) coring campaigns in two locations attempted to establish a depositional record of
fault activity along the western Eliki Fault based on detecting abrupt environmental changes in the
adjacent coastal plain. One location was a former lake located in the up
per part of the plain in
immediate hangingwall of the fault. Here a series of boreholes revealed surficial silts and clays typical
of a low
-
energy depositional setting. However, subsequent laboratory analysis of samples found that
oxidation of the sediment
s had ensured there was no preservation of sensitive palaeoecological
indicators (pollen, ostracods etc) of the depositional environment. Moreover, no material suitable for
radiometric dating was obtained. This environment, probably a seasonal playa, was c
onsidered
unlikely to preserve a useful sedimentological record of tectonic movements.


A second and more favourable depositional environment was found in the lower part of the Eliki
deltaic plain. According to historical accounts of the 373 BC and 1861 ea
rthquakes, surface faulting
on the Eliki Fault is accompanied by the metre
-
scale tectonic subsidence and marine inundation of the
coastal plain. An integrated programme of GPR surveys and shallow boreholes, accurately levelled in
with respect to sea level,

were carried out. Stratigraphic descriptions were made in the field and basic
geotechnical tests were made on the intact core material (e.g. penetrometer measurements of bearing
strength; shear vane measurements of shear strength). Subject to the limitati
ons for core sample
collection imposed by the archaeological permit under which the group was working, core material
was obtained for laboratory analyses (particle size, trace element) and, where appropriate, radiometric
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dating (
137
Cs,
210
Pb,
14
C). GPR sur
veys in the coring sites proved to be unsuccessful in imaging the
near
-
surface stratigraphy; in the heavily farmed fields of the plain the GPR profiles were found to be
excessively degraded by offline reflectors (roots of olive trees, metal poles, irrigati
on channels,
telegraph poles).


Trochoidea pyramidata
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Pisidium
sp
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sp
Planorbis
sp.
Planorbis
sp.
P
EAT

CLASTS

IN

CLA
Y
Rumina decollata
Bithynia tentaculata
M
UD

PELLET
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Lymnaea
sp.
C
ORE

LOS
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P19
P20
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Fig. 4. (a) Mechanical coring at P13 in the Eliki coastal plain. (b) Summary stratigraphy of the cores P19 and
P20 showing a transition from basal lagoonal muds into a wetland peaty soil (black unit) followed by an abrup
t
and erosional return to a lagoonal depositional environment.


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Trochoidea pyramidata
1600 ± 40 C BP
[5th-6th Centuries cal. AD]
C -8.2‰
14
13

Zonitidae
Zonitidae
Zonitidae
Zonitidae
Pisidium
sp.
Zonitidae
Zonitidae
P15
P13
P12
P18
P16
P17
L
L

Fig. 5 Correlative summary of core stratigraphies in the transect P15
-
P17. Two abrupt environmental changes
are recognised, one at ~0 m and one at ~
-
2 m. Sedimentological, geochemical and
microfossil analysis of P18
indicates that at ~2 m below sea level a basal high
-
energy shoreline deposit abruptly changes into a low
-
energy
lagoonal(?) deposit; a
14
C age for a terrestrial gastropod shell indicates that the changeover occurred in the
last
2000 years.


The results from the sediment cores show a complex stratigraphy typical of late Holocene deltaic
plains but do exhibit evidence of abrupt environmental changes. In places, there is stratigraphic
evidence of
a transition from basal lagoonal mud
s into a wetland peaty soil followed by an abrupt and
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erosional return to a lagoonal depositional environment. Although consistent with a sudden deepening
of the coastal plain, the date of the transition is awaiting radiometric dating.
Along a separate cor
e
transect,
two abrupt environmental changes are recognised, one at ~0 m and one at ~
-
2 m.
Sedimentological, geochemical and microfossil analysis indicates that at ~2 m below sea level a basal
high
-
energy shoreline deposit abruptly changes into a low
-
energ
y lagoonal(?) deposit. This change
postdates a

terrestrial shell age of 5
th
-
6
th

centuries AD and may correspond to sudden subsidence event
in Roman or Byzantine times, consistent with published coastal studies (Stewart 1996), archaeological
evidence (Mouya
ris
et al
. 1992; Katsonopoulou 1998, Soter & Katsonopoulou 1999) and
palaeoseismic investigations (Koukouvelas
et al
. 2001).



3.
210
Pb geochronology of sediments suspected of reactivation in the 1861 earthquake


In its first application to palaeoseismic
studies, we show that that 210Pb dating, supported by 137Cs
dating, is potentially useful for estimating the ages of very young (less than 150 years) colluvial
deposits associated with recent earthquake activity. Colluvial deposits exposed in two fault tre
nch
sections were sampled for laboratory assay and analysed to confirm interpretations that they postdated
the 1861 surface rupture. Only one of the two sites, the Eastern Eliki Fault Trench 1A, was found to
provide an appropriate sediment sequence for dat
ing via 210Pb and 137Cs. Here, a soil horizon was
found to have been buried by alluvial sedimentation in the late AD 1800s, consistent with reactivation
during the 1861 earthquake (Cundy & Stewart in prep.).


The results show that the precision of the dat
es obtained was found to depend largely on the date of
occurrence of the earthquake event, and on the local sedimentary setting. The half
-
life of 210Pb (22.3
years) usually limits its dating range to 150 years (ca. 6


7 half lives), hence it is only usefu
l in dating
events which have occurred since ca. 1850. Extrapolation of the chronology to examine earlier events
is not possible due to potential variations in sediment accumulation rate over time. Since errors on
210Pb dates generally increase as the exce
ss 210Pb activity decays to low values (i.e. in older
sediments), dating precision is likely to be better for more recent earthquake events, particularly those
occurring after 1954 where the 210Pb ages can be corroborated by 137Cs dates. Where possible,
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0Pb and 137Cs dating should be used in combination to confirm dates, and fully assess variations in
sediment input and erosional processes. The results suggest that this novel geochronological technique
can be applied more widely to date colluvial deposits

typically encountered in fault
-
trench studies.

EEF1A
-120
-100
-80
-60
-40
-20
0
0.000
0.010
0.020
0.030
0.040
210Pbtotal (Bq/g)
Depth (cm)


Fig. 6(a) View of an infilled ground fissure along a bedrock fault scarp that has been dated by
210
Pb
geochronology to older than the last 150 years. (b)
Based on the simple mod
el of
210
Pb dating, a preliminary
sediment accumulation rate of 0.8 cm/y (95% confidence interval = 0.6


1.3cm/y) is inferred.
137
Cs dating,
when completed, will be used to corroborate this accumulation rate. Based on the
210
Pb
-
derived sediment
accumulat
ion rate, the top of the buried soil horizon dates to AD 1887 (95% confidence interval: AD 1850


AD
1930). Hence, it is probable that the soil horizon was buried by alluvial sediments in the late AD 1800s, possibly
as a result of movement on the adjacent
fault during the 1861 earthquake.

B

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References


Cundy, A.B. & Stewart, I.S. (in prep) Dating recent sediments on the Eliki fault, Gulf of Corinth: the first
palaeoseismological application of
210
Pb and
137
Cs dating.

Katsonopoulou, D. 1998. The first excav
ation at Helike: Klonis Field.
In
: Katsonopoulou, D., Soter, S.,
Schilardi, D. (Eds)
Ancient Helike and Aigialia
. Proceedings of the Second International Conference,
Aigion, 125
-
145.

Koukouvelas, I.K., Stamatopolous, L., Katsonopoulou, D., Pavlides, S., 20
01. A paleoseismological
investigation of the Eliki fault, Gulf of Corinth, Greece.
Journal of Structural Geology
, 23, 531
-
543.

Mouyaris, N., Papastamatiou, D. & Vita
-
Finzi, C. 1992. The Helice fault? Terra Nova, 4, 124
-
129.

Noller, J.S. 2000. Lead
-
210 Geo
chronology.
In
: Noller et al. (eds)
Quaternary Geochronology: methods and
applications
, American Geophysical Union.

Soter, S., 1998. Holocene uplift and subsidence of the Helike Delta, Gulf of Corinth, Greece.
In
: Stewart, I.S. &
Vita
-
Finzi, C. (Eds)
Coast
al Tectonics
, Geological Society, London, Special Publication, 146, 41
-
56.

Soter, S., Katsonopoulou, D., 1999. Occupation horizons found in the search for the Ancient Greek City of
Helike.
Geoarchaeology
, 14(6), 531
-
563.

Stewart, I., 1996. Holocene uplift

and palaeoseismicity on the Eliki Fault, western Gulf of Corinth, Greece.
Annali di Geofisica
, 39, 575
-
588.

Woodward, J. & Stewart, I.S. (in prep). Imaging near
-
surface tectonic structures using ground
-
penetrating radar:
western Eliki Fault, Gulf of Corin
th, Greece. (
To be submitted in next month to Tectonophysics.)