Active tectonics of the Hellenic subduction zone

178 Pages·2012·11.732 MB·English

by  Beth Shaw (auth.)

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Beth Shaw
Active Tectonics of the
Hellenic Subduction Zone
123
Author Supervisor
DrBeth Shaw Prof.James Jackson
StJohn’s College Bullard Laboratories
Universityof Cambridge Department of Earth Sciences
Cambridge Universityof Cambridge
UK Cambridge
e-mail: [email protected] UK
e-mail: [email protected]
ISSN 2190-5053 e-ISSN2190-5061
ISBN 978-3-642-20803-4 e-ISBN978-3-642-20804-1
DOI 10.1007/978-3-642-20804-1
SpringerHeidelbergDordrechtLondonNewYork
(cid:2)Springer-VerlagBerlinHeidelberg2012
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The object of my mission had sufficient
interest in itself to excite a feeling of enthu-
siasm, from an anticipation of future utility,
from the opportunity afforded me of studying
some of the grander features of nature, and
from the varied interests connected with the
examination and exploration of a new field.
Thomas A.B. Spratt
Travels and Researches in Crete, 1865
Publications resulting from this thesis
ThematerialdescribedinChap.2hasbeenpublishedas:Shawetal.2008,Eastern
MediterraneantectonicsandtsunamihazardinferredfromtheAD365earthquake.
Nature Geoscience 1, 268–276.
The material described in Chap. 3 has been published as: Shaw and Jackson
2010, Earthquake mechanisms and active tectonics of the Hellenic subduction
zone. Geophysical Journal International doi:10.1111/j.1365-246X.2010.04551.x
The material described in Chap. 4 has been published as: Shaw et al. 2010,
Radiometric dates of uplifted marine fauna in Greece: Implications for the inter-
pretationofrecentearthquakeandtectonichistoriesusinglithophagiddates.Earth
and Planetary Science Letters 297, 395–404.
Supervisor’s Foreword
Beth Shaw’s thesis illustrates very well the progress, and unexpected benefits, of
curiosity-driven science at its best, ranging widely over diverse fields of endeav-
our, from earthquake seismology, to coastal geomorphology, elastic dislocation
modelling and the implications of marine ecology and biogeochemistry for
radiocarbon dating. It is through making connections across such scientific dis-
ciplines thatourviewoftheplanetweliveoncan changeprofoundly.Itisnotan
easy course of research to pursue, and requires determination, and some confi-
dence,tofollowclueswithoutevenknowingwhattheyarecluesto.Thisapproach
isnotforthespecialistwhoiscomfortableonlywithinthewell-definedboundaries
ofaparticularprofessionaldiscipline,yetitismanifestlythewaytofindoutabout
a planet that is constantly carrying out experiments without the courtesy of
informing us first what those experiments are for.
The thesis began with a forensic investigation of a famous catastrophe of the
Byzantine world, in which a tsunami destroyed Alexandria, the Nile delta, and
other coastal settlements inSicily and the Adriatic in AD 365. The prime suspect
for causing the tsunami was an earthquake in the Hellenic subduction zone, but
it was also known that the 40 mm/year convergence between Crete and Libya
was mostly accommodated without earthquakes, which can only account for
about 10% of the necessary motion. There is nothing particularly unusual about
slip without earthquakes, even in other subduction zones, but it is generally
thought to be a characteristic of the slip surface itself, which is either dominated
by friction (and generates earthquakes) or not. If the Hellenic subduction zone
generated occasional earthquakes of magnitude 8 or greater it could evidently
behave in both ways, which was a puzzle. Another element in the mystery was
the remarkable observation, first recorded by Captain Spratt RN in 1865, that the
entire SW corner of Crete had been uplifted by up to 10 m since Roman times,
but by what mechanism? Following these clues and resolving these puzzles was
the first element of Beth’s thesis and, like any reviewer reluctant to give away
the plot of a detective story, I will not reveal the answer in this preface. It is
sufficient to point out that its discovery involved an unusually broad spectrum of
observations, arguments and techniques, including earthquake seismology, GPS,
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viii Supervisor’sForeword
radiocarbon dating, dislocation and tsunami modelling and field observations of
geomorphology.
Solving this first mystery of course revealed others. It showed how the con-
vergencebetweenAfricaandCretewasaccommodatednearCrete,andhowitwas
abletogeneratetsunamis.Butwhatwasthetsunamipotentialoftherestofthearc
between the Ionian islands and Rhodes? How do the variations in earthquake
mechanisms relate to the motions revealed by GPS measurements? How does the
active faulting in the subduction zone connect with that in central Greece, and
what happens at the eastern and western ends of the arc? These questions were
answered in the second part of the thesis, which involved a re-evaluation of the
earthquakes in the entire arc, a major effort in observational seismology, and a
close examination of the GPS measurements over the last 10 years. Several
important new insights were revealed by this investigation, which represented a
stepchangeinclarifyingthepatternsofearthquakesandwhattheyweretellingus.
The final substantial element of the thesis involved an entirely different focus.
HavingestablishedthatSWCretewasindeedupliftedinasingleeventinAD365,
anoutstandingpuzzlewaswhysomeupliftedmarineshells,whichmusthavebeen
killedinthatevent,gavethe‘wrong’agewhencarbon-dated;usuallytoooldbyup
to1,000 years.Thiswasmorethanacuriositybecausetheshellsconcernedarethe
best and most commonly preserved organisms in uplifted coastlines around much
of the Mediterranean and Aegean, and are often used to estimate tectonic uplift
rates.TheAD365uplifteventallowedBethtoinvestigatethiseffectanddiscover
that it is related to the ecology and mode of life of the shells themselves, which
incorporate old (radiocarbon-dead) carbon from host limestone into their shells
duringgrowth.Quantificationofthiseffectrevealsthelimitationsoftheseshellsin
understanding uplift rates in much wider tectonic contexts than Crete.
Other questionsandpuzzleshave been revealedbythisworkandremaintobe
pursued.Buttheworkreportedherewassubstantiallycompletedin3 years,which
istheintendeddurationofaPh.D.study,andsoahalthadtobecalled.Butitisnot
a halt to the work’s influence: all of it has now been published in professional
journals, and I expect it will have an important effect on tectonic studies in the
Mediterranean andothersubductionzones for sometime. No onecould ask more
of a thesis.
Cambridge, 19 January 2011 James Jackson
Acknowledgements
Sincere thanks are due to my supervisor, James Jackson; without him this thesis
could not have been written. I am extremely fortunate in having been able to
discuss my work with Philip England, Dan McKenzie and Tom Higham, whose
comments and advice have dramatically improved this thesis.
ThetsunamimodellingdescribedherewascarriedoutbyMatthewPiggottand
colleagues at Imperial College, London. Michael Floyd and Jean-Matthieu Noc-
quetsuppliedtheGPSmeasurementsthatwereusedextensivelyinthisthesis.Nic
Ambraseys is an absolute mine of information concerning the historical record of
the Mediterranean, and it was a great pleasure to discuss ancient descriptions of
earthquakes with him. Microearthquake data was generously provided by Denis
Hatzfeld and Thomas Meier.
Special thanks are also due to Martin Brasier, Norman Charnley, Jill Darrell,
Owen Green, Liz Harper, Gideon Henderson, Mathew Lowe, Roberto Portela-
Miguez,RichardPreece,GarethRoberts,BrianRosen,AlastairSloan,PaulTaylor,
Alex Thomas, Kathie Way, Tim Wright, and the staff of the NERC Radiocarbon
centres in Oxford and East Kilbride.
I owe an enormous debt of gratitude to my friends in St John’s College and at
BullardLaboratories.Myfamilyhavebeenacontinualsourceofsupport,andhave
always encouraged me to be curious about the world around me. Finally, my
sincerethanksgotomyhusband,Richard,towhomIdedicatethisthesis.Without
his constant cheerfulness, encouragement and calm in the face of innumerable
minor Ph.D. disasters, this thesis would probably not exist.
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