Isotropic and anisotropic seismic tomography using active source and earthquake records

115 Pages·2018·5.345 MB·English

by  Lin, Yu-Pin

Springer Theses
Recognizing Outstanding Ph.D. Research
Yu-Pin Lin

Isotropic and
Anisotropic Seismic
Tomography Using
Active Source and
Earthquake Records
Springer Theses
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Yu-Pin Lin
Isotropic and Anisotropic
Seismic Tomography Using
Active Source
and Earthquake Records
Doctoral Thesis accepted by
National Taiwan University, Taipei, Taiwan
123
Author Supervisors
Dr. Yu-Pin Lin Prof. LiZhao
Department ofEarth Sciences, Southern Institute of Earth Sciences
California Earthquake Center Academia Sinica
University of SouthernCalifornia Taipei
LosAngeles, CA Taiwan
USA
Prof. Shu-Huei Hung
Department ofGeosciences
National Taiwan University
Taipei
Taiwan
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-10-5067-1 ISBN978-981-10-5068-8 (eBook)
DOI 10.1007/978-981-10-5068-8
LibraryofCongressControlNumber:2017943094
©SpringerNatureSingaporePteLtd.2018
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’
Supervisor s Foreword
One of the major goals of seismology is to gain knowledge on the internal struc-
tures of the Earth from seismic signals recorded by instruments deployed on the
surfaceoftheglobe. Eversincetheintroductionofdigitalseismometersinthelate
1970s, a huge amount of data has been collected which, with the availability of
increasinglypowerfulandaffordablecomputers,hasledtothecreationofnumerous
tomographymodels,allowingustopeekintotheEarth’sglobal,regionalandlocal
structures. In keeping with the improvement in seismic tomography models, our
perception of the Earth’s internal constitution and dynamics has been constantly
shaped and reshaped.
However, at the same time we have also seen attempts that are little more than
straightforwardapplicationsofanestablishedmethodtoamoreorlessthesameold
dataset(sometimeswithadditionsofanumberofearthquakesand/orstations).The
outcome of this practice is a simple accumulation of models for the same region
with little discernible advance in science. Take Taiwan as an example: an incom-
plete search of literature in the past two decades yields at least eight different
tomography models covering roughly the same area of Taiwan and its immediate
surroundings. They share some common large-scale features, but have drastic
differences in details. This situation is by no means unique to Taiwan. As “new”
models are created, there has clearly been a lack of (1) rigorous, objective and
quantitative assessments on the validity of the models and (2) exploration of new
techniques to qualitatively improve the robustness of tomography models. The
research presented in this thesis represents two efforts in those two aspects.
The first part of this thesis (Chaps. 2 and 3) describes the research in which a
dataset of the first (P-wave) arrival times from active sources were used to make
quantitativeassessmentsofexistingtomographymodelsforTaiwan.TheseP-wave
arrival times are regarded as “ground truth” because they are from active sources
(explosions) whoselocations and origin times areexactlyknown.Through tedious
manualpickingfromwaveformsrecordedatnearly1400sitesalloverTaiwan,the
author acquired more than 6000 P-wave arrival times, which were then compared
withpredictionsbyexistingtomographymodelstoevaluatetheperformanceofthe
models. Indeed, different levels of performance can be seen not only in different
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vi Supervisor’sForeword
tomography models but also in different parts of the same model. This “ground
truth”dataset was alsousedtoconducta tomography study for theshallow crustal
structure in Taiwan. The resulting model (presented in Chap. 3) has much lower
residual and good correlation with surface geology. It was nevertheless turned
down for publication at an international journal due to the objection by an
anonymousbut“well-knownexpertinactive-sourceseismology”.ShunryuSuzuki,
founderoftheSanFranciscoZenCenter,oncewrote,“Inthebeginner’smindthere
are many possibilities, in the expert’s mind there are few”.
The second part of this thesis (Chaps. 4 and 5) deals with the development and
application of a new, full-wave approach to imaging the three-dimensional aniso-
tropic structures using the widely available shear-wave splitting measurements.
Traditional modelling and interpretation of shear-wave splitting in terms of ani-
sotropy have been largely based on ray theory which presumes a uniform distri-
bution of anisotropy. The full-wave approach based on normal-mode theory
developed here leads to spatially varying sensitivities of shear-wave splitting on
anisotropicstructures.Theapplicationofthisapproachtodatacollectedataseismic
network in Southern California, coupled with a multi-scale inversion practice,
demonstrates that this new approach is very effective in imaging the three-
dimensional variations of anisotropy, which in turn provides strong seismic con-
straints on the state of stress and strain in the lithosphere and the dynamics of the
mantle flow in the asthenosphere.
At the present time, full-wave seismic tomography is one of the most active
researchfrontiersinseismology.AnothernewgenerationofEarthmodelsarebeing
created offering new insights into the inner workings of our planet. I am confident
that the author of this thesis has much to contribute in her future endeavours.
Taipei, Taiwan Li Zhao
February 2017
Abstract
Thisthesisinvolvestworegionalseismictomographystudies:Onefortheisotropic
P-wave velocity in the upper crust beneath Taiwan using first-arrival times from
active sources; the other for the shear-wave anisotropy structure under Southern
CaliforniausingSKSsplittingintensitymeasurements.Intheisotropictomography
forTaiwan,weusewaveformrecordsfromthe10explosionsin2008conductedby
the TAiwan Integrated GEodynamics Research (TAIGER) project. A large dataset
of high-quality ground-truth first-arrival times are hand-picked from the
active-source records at *1400 sites throughout Taiwan, which greatly enhance
ourcapabilitytodeterminethecrustalvelocitybeneathTaiwanwithunprecedented
accuracy and resolution, especially along the two north-south and two east-west
island-wide linear transects with densely deployed receivers. At first, these
first-arrivaltimes areusedtoevaluate theexistingtomographymodelsforTaiwan.
Results show that tomography models obtained from traditional travel time inver-
sions provide consistent and qualitatively correct locations of larger-scale velocity
perturbations. However, small-scale features are inconsistent among different
models, and their velocity perturbations are mostly underestimated. Then we use
our ground-truth first-arrival times to refine the P-wave velocity model. With a
trial-and-error procedure, we acquire the best 2D models along a number of
shot-to-stationprofilesbyfittingthefirst-arrivaltimes.Finallyapartitionmodelling
approach is employed to invert for a 3D model in northern Taiwan based on a
collection of the crisscrossing 2D models that densely transect across the region.
The resulting structural variations in our 3D model correlate remarkably well with
the surface geological features that are distinctly shaped by the orogenic and tec-
tonic history in Taiwan.
IntheanisotropictomographyforSouthernCalifornia,ourpurposeistoresolve
thespatialvariationofanisotropyintheuppermantlewhichplaysanimportantrole
in our understanding of the Earth’s internal dynamics. Shear-wave splitting has
always been a key observable in the investigation of upper-mantle anisotropy.
However,theinterpretationofshear-wavesplittingintermsofanisotropyhasbeen
largely based on the ray-theoretical modelling of a single vertically incident plane
SKSorSKKSwave.Inourstudy,weusesensitivitykernelsofshear-wavesplitting
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viii Abstract
to anisotropic parameters calculated by the normal-mode theory, which automati-
callyaccountsforthefull-waveeffectsincludingtheinterferenceofSKSwithother
phases of similar arrival times, the near-field effect, and multiple reflections in the
crust.Thesefull-waveeffectscanleadtosignificantvariationsofSKSsplittingwith
epicentral distance and are neglected in ray theory. We image the upper-mantle
anisotropyinSouthernCaliforniausingnearly6000SKSsplittingdataandtheir3D
full-wave sensitivity kernels in a multiscale inversion enabled by a wavelet-based
model parameterization. We also appraise our inversion by estimating the spatial
resolution lengths using a statistical resolution matrix approach, which shows the
finest resolution length of *25 km in regions with better path coverage. The
anisotropic model we obtain displays the structural fabrics in relation to surface
geologic features such as the Salton Trough, the Transverse Ranges and the San
Andreas Fault. The depth variation of anisotropy does not suggest a strong
decoupling between the lithosphere and asthenosphere. At long wavelengths, the
orientations of the fast axis of anisotropy are consistent with the absolute plate
motion in the interiors of the Pacific and North American plates.
(cid:1) (cid:1) (cid:1)
Keywords TAIGER project Active sources First-arrival times Seismic
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
tomography Taiwan Shear-wavesplitting Anisotropy Multiscaletomography
(cid:1)
Full-wave sensitivity kernels Southern California
Parts of this thesis have been published in the following journal articles:
1. Lin, Y.-P., Zhao, L., Hung, S.-H. (2014b) Full-wave multiscale anisotropy
tomography in Southern California, Geophys. Res. Lett., 41, doi:10.1002/
2014GL061855. (Reproduced with Permission)
2. Lin, Y.-P., Zhao, L., Hung, S.-H. (2014a) Full-wave effects on shear wave
splitting, Geophys. Res. Lett., 41, 799-804, doi:10.1002/2013GL058742.
(Reproduced with Permission)
3. Lin, Y.-P., Zhao, L., Hung, S.-H. (2011) Assessmentof tomography models of
Taiwan using first-arrival times from the TAIGER active-source experiment,
Bull.Seism.Soc.Am.,101(2),866–880,doi:10.1785/0120100244.(Reproduced
with Permission)
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