Seismic structure along transitions from flat to normal subduction

188 Pages·2014·16.36 MB·English

Seismic structure along transitions from flat to normal
subduction: central Mexico, southern Peru, and southwest
Japan
Thesis by
Sara L. Dougherty
In Partial Fulfillment of the Requirements
for the Degree of
Doctor of Philosophy
California Institute of Technology
Pasadena, California
2014
(Defended May 23, 2014)
ii
°c 2014
Sara L. Dougherty
All Rights Reserved
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Acknowledgments
First and foremost, I would like to sincerely thank my advisors, Rob Clayton and Don Helmberger,
withoutwhomthisthesiswouldnothavebeenpossible. Dontaughtmetoappreciateeverywiggleof
aseismogramandintroducedmetocomplexwaveformsandthefine-scalestructurestheycanresolve.
Rob taught me to appreciate the bigger picture and to explore the possible tectonic implications
of observed fine-scale structures. Their support and guidance throughout my time at Caltech has
been immensely valuable. I greatly appreciate Rob’s patience, encouragement, and accessibility,
especially as this thesis was coming together.
I would like to thank my academic advisor, Jennifer Jackson, for her assistance with selecting
courses and applying for candidacy and for the thought-provoking questions that she poses in my
thesis committee meetings. I am also grateful to Joann Stock for serving on my committee and
always providing insightful comments.
The former grad students and postdocs who helped me in the initial stages of my research and
along the way deserve special thanks: Alex Song, Risheng Chu, Min Chen, YoungHee Kim, and
Ting Chen. Shengji Wei has also been a great help. I would like to thank Xyoli P´erez-Campos and
Steve Skinner for insightful comments and valuable discussions.
I am grateful for the support staff in the Seismo Lab and Tectonics Observatory, especially
Rosemary Miller, Donna Mireles, Viola Carter, Evelina Cui, and Lisa Christiansen. Thanks also to
Mike Black for all the IT support.
SpecialthankstoJasonSaleebyandJean-PhilippeAvouacforleadingthePahoehoefieldtripto
HawaiiandthedivisionenrichmenttriptoGreece,respectively,bothofwhichweregreatexperiences.
Finally, my deep thanks to my husband, Walt Stelz, for his continuous support and understand-
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ing, especially during the busy times. His ability to always make me laugh has been invaluable, and
the occasional coding advice didn’t hurt either.
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Abstract
The fine-scale seismic structure of the central Mexico, southern Peru, and southwest Japan subduc-
tion zones is studied using intraslab earthquakes recorded by temporary and permanent regional
seismic arrays. The morphology of the transition from flat to normal subduction is explored in cen-
tralMexicoandsouthernPeru,whileinsouthwestJapanthespatialcoincidenceofathinultra-slow
velocity layer (USL) atop the flat slab with locations of slow slip events (SSEs) is explored. This
USL is also observed in central Mexico and southern Peru, where its lateral extent is used as one
constraint on the nature of the flat-to-normal transitions.
In western central Mexico, I find an edge to this USL which is coincident with the western
boundaryoftheprojectedOrozcoFractureZone(OFZ)region. Forwardmodelingofthe2Dstructure
of the subducted Cocos plate using a finite-difference algorithm provides constraints on the velocity
and geometry of the slab’s seismic structure in this region and confirms the location of the USL
edge. I propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South
Cocos plate along the projection of the OFZ, by a process analogous to that which occurred when
the Rivera plate separated from the proto-Cocos plate 10 Ma.
In eastern central Mexico, observations of a sharp transition in slab dip near the abrupt end of
theTransMexicanVolcanicBelt(TMVB)suggestapossibleslabtearlocatedwithinthesubducted
South Cocos plate. The eastern lateral extent of the USL is found to be coincident with these
features and with the western boundary of a zone of decreased seismicity, indicating a change in
structure which I interpret as evidence of a possible tear. Analysis of intraslab seismicity patterns
and focal mechanism orientations and faulting types provides further support for a possible tear
in the South Cocos slab. This potential tear, together with the tear along the projection of the
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OFZ to the northwest, indicates a slab rollback mechanism in which separate slab segments move
independently, allowing for mantle flow between the segments.
In southern Peru, observations of a gradual increase in slab dip coupled with a lack of any gaps
orverticaloffsetsintheintraslabseismicitysuggestasmoothcontortionoftheslab. Concentrations
of focal mechanisms at orientations which are indicative of slab bending are also observed along the
change in slab geometry. The lateral extent of the USL atop the horizontal Nazca slab is found to
be coincident with the margin of the projected linear continuation of the subducting Nazca Ridge,
implying a causal relationship, but not a slab tear. Waveform modeling of the 2D structure in
southern Peru provides constraints on the velocity and geometry of the slab’s seismic structure and
confirms the absence of any tears in the slab.
InsouthwestJapan,IestimatethelocationofapossibleUSLalongthePhilippineSeaslabsurface
and find this region of low velocity to be coincident with locations of SSEs that have occurred in
this region. I interpret the source of the possible USL in this region as fluids dehydrated from the
subducting plate, forming a high pore-fluid pressure layer, which would be expected to decrease the
coupling on the plate interface and promote SSEs.
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Contents
Acknowledgments iii
Abstract v
List of Figures x
List of Tables xiii
1 Introduction 1
2 Seismic structure in central Mexico: Implications for fragmentation of the sub-
ducted Cocos plate 7
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2 1D Velocity Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.3 Ultra-slow Velocity Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.4 2D Velocity Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.5 Seismicity and Slab Dip Across USL Edge . . . . . . . . . . . . . . . . . . . . 27
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.6 Supplemental Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
viii
3 Seismicity and structure in central Mexico: Evidence for a possible slab tear in
the South Cocos plate 48
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3 Tectonic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.4 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.4.2 Slab Dip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.4.3 Ultra-slow Velocity Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.4.4 Seismicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.5 1D Velocity Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.4.6 2D Velocity Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.7 Supplemental Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
4 Seismic structure in southern Peru: Evidence for a smooth contortion between
flat and normal subduction of the Nazca plate 107
4.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.3 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.3.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.3.2 Seismicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.3.3 Slab Dip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4.3.4 Slab Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.3.5 Ultra-slow Velocity Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
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4.3.6 2D Velocity Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
4.6 Supplemental Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
5 Further evidence for P-wave complexity in a region with slow slip events 158
5.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
5.3 Data and Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
5.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
5.5 Discussion and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
5.6 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
6 Conclusions 173
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List of Figures
1.1 Overview map of study locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1 Map of western central Mexico with event and station locations . . . . . . . . . . . . 11
2.2 1D P- and S-wave velocity models tested for western central Mexico . . . . . . . . . . 14
2.3 Comparison of 1D modeling results for the five models tested in western central Mexico 15
2.4 1D modeling results for the ncM velocity model . . . . . . . . . . . . . . . . . . . . . 16
2.5 Profile across the MARS array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.6 Raypath schematic and examples of complex P waveforms in western central Mexico . 19
2.7 Maps of local S-to-P conversion points in western central Mexico . . . . . . . . . . . . 21
2.8 2D velocity model across the MARS array . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.9 2D modeling results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.10 Comparison of 2D modeling results for five different models . . . . . . . . . . . . . . . 25
2.11 Seismicity and slab dip across the western USL edge . . . . . . . . . . . . . . . . . . . 28
2.12 3D schematic of two-tear plate model . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.13 Supplemental: 1D modeling results for the SoCal velocity model . . . . . . . . . . . . 37
2.14 Supplemental: 1D modeling results for the SoCalx velocity model . . . . . . . . . . . 38
2.15 Supplemental: 1D modeling results for the Kim et al. (2010) velocity model . . . . . . 39
2.16 Supplemental: 1D modeling results for the Song et al. (2009) velocity model . . . . . 40
3.1 Map of eastern central Mexico with event and station locations . . . . . . . . . . . . . 52
3.2 Topographic-bathymetric map of eastern central Mexico . . . . . . . . . . . . . . . . . 54
3.3 Lateral variation in slab dip across eastern central Mexico . . . . . . . . . . . . . . . . 59