seismic properties and processes along the subduction plate interface

253 Pages·2015·10.72 MB·English

by  Stephen Hicks

S P P
EISMIC ROPERTIES AND ROCESSES
A
LONG THE
S P I :
UBDUCTION LATE NTERFACE
T F 2010 M 8.8 M ,
HE EBRUARY AULE
W
C
HILE EARTHQUAKE

Thesis submitted in accordance with the requirements of the
University of Liverpool
for the degree of Doctor in Philosophy (Earth Sciences) by
Stephen Paul Hicks
August 2015
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A
BSTRACT
The seismogenic zone of subduction margins has the potential to generate some of the
world’s largest earthquakes. A detailed study of the 2010 M 8.8 Maule, Chile rupture
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has enabled interpretation of the controls that govern subduction zone seismic
behaviour across the earthquake cycle. In this thesis, we focus on two aspects of the
central Chile margin: (1) imaging physical properties in the forearc and along the plate
interface; (2) assessing source complexity of megathrust ruptures.
We exploit a dataset of seismic body wave onset times from local aftershocks
recorded on a temporary network to derive a 3-D seismic velocity model of the Maule
rupture area. We image the main domains of the subduction zone and find a high
velocity anomaly located along the plate interface, which we initially interpret as a
subducted topographic high. We then develop a second, more accurate velocity model
that uses an improved arrival time dataset together with observations from ocean-
bottom seismometers. This refined model gives a sharper view of both the plate
interface close to the trench, and the marine forearc. We show that ancient blocks of
dense mantle in the lower forearc may have decelerated slip during the Maule
earthquake and contributed to its nucleation. Furthermore, we infer that fluid saturated
sediments inhibited significant slip close to the trench.
We study source processes of a large aftershock of the Maule sequence, the
2011 M 7.1 Araucania earthquake, by inverting local seismic waveforms for a
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multiple point-source faulting solution. We find this earthquake constituted rupture on
the plate interface followed by almost instantaneous slip along a normal fault in the
overriding plate: the first observation of its kind. The second rupture of this closely-
spaced doublet was hidden from teleseismic faulting solutions, and may have been
dynamically triggered by S-waves from the first event.
Overall, our work highlights the role played by the upper plate in subduction
zone seismogenesis. We suggest that seismic velocities can help to characterise the
behaviour of future large megathrust earthquakes. We show that the potential hazard
posed by closely-spaced doublets involving the upper plate should be accounted for in
real-time tsunami warning systems by using local waveform anaylsis.
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D
ECLARATION
I declare that the work presented in this thesis is the result of my own work. The
material contained in the thesis has not been presented, nor is currently being
presented, either wholly or in part, for any other degree or qualification.

Stephen Paul Hicks
21 August 2015

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“Science has fallen into many errors – errors which have
been unfortunate and useful rather than otherwise, for
they have been the steppingstones to truth”

– Jules Verne
Journey to the Center of the Earth
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T C
ABLE OF ONTENTS

Abstract .................................................................................................................... 3
Declaration ............................................................................................................... 5
Table of Contents ..................................................................................................... 9
List of Figures ........................................................................................................ 13
List of Tables.......................................................................................................... 25
List of Enclosures ................................................................................................... 27
List of Abbreviations.............................................................................................. 29
Acknowledgements ................................................................................................ 31
Preface .................................................................................................................... 33
1. Introduction .................................................................................................. 35
1.1 Megathrust earthquakes ........................................................................ 35
1.2 The 2010 M 8.8 Maule, Chile earthquake ........................................... 40
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1.3 Aims and objectives of this thesis ......................................................... 44
1.4 Thesis organisation and publication status of chapters ......................... 47
2. Upper Plate Versus Lower Plate: Physical Controls on Megathrust
Earthquake Rupture Processes ................................................................... 51
2.1 Structural domains above and below the megathrust ............................ 52
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Table of Contents
2.2 Structural and compositional influences on megathrust seismogenic
behaviour .............................................................................................. 62
2.3 Limitations and remaining questions ................................................... 69
2.4 Summary .............................................................................................. 73
3. Seismic Imaging of Subduction Zone Structure using Local Earthquake
Tomography ................................................................................................. 75
3.1 Fundamentals of seismic tomography .................................................. 75
3.2 Seismic ray tracing ............................................................................... 77
3.3 The coupled hypocentre-velocity problem ........................................... 78
3.4 Inversion for 1-D velocity structure ..................................................... 81
3.5 3-D tomographic inversion ................................................................... 83
3.6 Illuminating the subduction megathrust using local earthquake
tomography: past applications .............................................................. 88
3.7 Summary .............................................................................................. 90
4. The 2010 M 8.8 Maule, Chile Earthquake: Nucleation and Rupture
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Propagation Controlled by a Subducted Topographic High ................... 93
Published paper abstract ............................................................................... 94
4.1 Introduction .......................................................................................... 94
4.2 Tomographic Inversion Scheme ........................................................... 96
4.3 Results of 3-D Tomographic Inversion ................................................ 97
4.4 Discussion .......................................................................................... 100
4.5 Conclusions ........................................................................................ 103
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