Download Defining the Moho boundary using earthquake PmP reflections in

Defining the Moho boundary using earthquake PmP reflections in
order to investigate arc-continent collisional deformation within Taiwan.
Trevor Thomas
University of Southern California
Submitted as a Senior Thesis Requirement for the B.S. in Geological
Sciences Degree
Abstract
Along the east Asian continental margin, the young Philippine Sea plate routinely
subducts under continental Eurasia. The island of Taiwan marks the location where a
subduction reversal is taking place and the Luzon island arc impinges with the
continental Eurasian plate. The ongoing orogenic evolution of Taiwan involves a
complex interaction often considered the type example of arc-continent collision.
Tectonic models to explain how this collision occurs and produces associated mountain
building, of which Taiwan's is one of the fastest rising in the world, range from thinskinned to lithospheric-scale end-members. In the thin skinned model, subduction of
continental Eurasian mantle and lower crust is separated from a deforming crustal wedge
by a plate boundary decollement. In the latter, deformation of crust and mantle occurs
within a vertically contiguous system, with progressive thickening of continental mantle
beneath the thickened core of the mountain belt. The 3D geophysical signatures are
fundamentally different between these models, and the crust-mantle Moho boundary
serves as an important geometrical constraint.
A joint USA-Taiwan multidisciplinary project known as TAIGER (Taiwan
Geodynamics Research; sponsored by NSF Continental Dynamics and the Taiwan
National Science Council) carried out seismological experiments in 2008-2009 with
observational targets to image large scale seismic structure and reflectivity strain markers.
The experiments were designed to collect both passive and controlled seismic sources.
Because of the high rates of seismicity in Taiwan, the active source experiments utilized
continuous recording as much as was possible, and passive seismic deployments were
overlapped with the active source experiments. Local earthquakes can supplement
controlled sources, multichannel processing and imaging methods can be applied to dense
array passive source data, and controlled source data with known source origin times and
locations are easily incorporated into regional seismic 3D tomography and earthquake
location studies. The important consequence of this combined-source ("crossover") data
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is the enhancement of spatial and temporal resolution in seismological imaging of the
earth's crust and mantle lithosphere.
We use PmP phases obtained from crustal earthquakes to identify the depths and
configuration of the Eurasian Moho that is being deformed via arc-continent collision.
These earthquakes were recorded by dense portable arrays (arrays of 2 km and 200 m
spacing) used during the controlled source experiments. Within these "earthquake shot
gathers", crustal Pg and PmP phases are clearly visible. We attempt arrival time raytracing and tomographic inversion methods. Thus, by treating earthquake profile data
like controlled source data, we can constrain the irregular geometry of the Taiwan Moho.
Introduction
Seismic tomography studies have traditionally been classified into two separate
categories, according to energy source. Active-source seismology utilizes explosions or
other human-caused ground disturbances in the creation of seismic energy, while passivesource seismology employs earthquakes as the seismic source. We jointly analyze active
and passive source Pg-phase data recorded into the same seismometer array in an attempt
to constrain a subsurface velocity model of northern Taiwan that better represents true
subsurface characteristics than models that use only one of the above sources. Then,
because many of our recordings show clear PmP arrival phases, we attempt to use PmP in
defining the geometry of the Mohorovicic discontinuity (Moho) in the context of our Pgconstrained velocity model.
Background
Taiwan sits at the convergent boundary of the Eurasian and Phillipine tectonic
plates [Tsai et al., 1977]. The island owes its existence to the collision of these plates.
Fast uplift rates experienced by the Central Range in Taiwan serve as an easily
observable surface manifestation of this collision [Chai, 1972]. The deformation
kinematics at the Taiwan locus are poorly understood, but understanding this plate-plate
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collision is crucial to the safety of millions of Taiwanese who live in densely populated
centers atop this seismically-explosive region.
Movement on this convergent boundary is largely accommodated by two major
subduction zones. The southern portion of Taiwan is composed of the Phillipine Sea plate
overriding an eastward-subducting Eurasian plate slab. In contrast, the Phillipine Sea
plate subducts northward below the Eurasian plate just east of northern Taiwan, defining
the Ryukyu island arc [Chai, 1972]. The combination of these subduction geometries
poses the question: what happens along the plate boundary between these subduction
zones, i.e. in the middle of Taiwan? It is clear that compression is driving the ascension
of the central range in this region, but how is slip accommodated? These are some
important questions that need to be answered in understanding Taiwan geodynamics.
It is likely that the Moho is highly deformed at this plate boundary due to the
above collisional processes, and that it dips down several kilometers beneath the eastern
half of the island to accommodate the Central Range mountain roots above it. Previous
studies find this mountain root but place it at various offsets to the east of Taiwan [Lin,
2005; Shih et al., 1998; Yeh et al., 1998; Yen et al., 1998]. Because PmP phases captured
by our seismograms can be more easily characterized than other lower-crustal phases,
Moho geometries extrapolated from these phases can serve as a reference horizon for
deformational trends in the lower crust. These trends should help determine stress
accommodation along the plate boundary and should help elucidate the subduction
geometry of northward-trending Phillipine Sea plate subduction beneath the Eurasian
plate.
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TAIGER
The TAIGER (Taiwan Integrated Geodynamics Research) project is a joint U.S.Taiwan program that uses several geophysical techniques to understand the geodynamics
of Eurasion plate subduction, Philippine Sea plate subduction, mountain-building of the
Central Range and crustal deformation at depth in the Taiwan orogen [Okaya et al., 2009].
TAIGER carried out two separate instrument deployments in 2008 and 2009, placing
dense land-based seismometer arrays across the whole width and length of terrestrial
Taiwan. In addition, ocean bottom seismometer (OBS) arrays were placed in-line with
land arrays, recording marine reflection profiles up to 100 km from the island. The 2008
land arrays, which are the interest of this study, consisted of 5 transects spanning the
entire width and length of the island (Figure 1). Two arrays extended North-South along
the island and had a seismometer spacing of 2 km between each instrument. The other
arrays consisted of three East-West transects: one placed in the north part of the island,
another in the south, and one along mid-latitude Taiwan. Instrument spacing within these
arrays was very dense, with only 200 m between each seismometer. All five arrays were
placed to overlap in such a way that their data provides 3D control on subsurface
properties
The arrays were set out with the dual purpose of gathering active-source and
passive earthquake-source data arriving during the timeframe of the experiment. Instead
of only recording during active-source events as would normally be the case for activesource studies, seismometers were allowed to run on “continuous” mode, thus recording
over a much larger span of time. This resulted in the recording of many earthquake events
that originated all over Taiwan but are most densely concentrated along plate boundaries.
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The geographic location and depth of these events indicates they originated as a result of
subduction-slip events in the zone of north-trending Phillipine plate subduction along the
coast of northeast Taiwan, and within east-trending subduction of the Eurasian plate
beneath Phillipine plate in the southern portion of Taiwan.
Crossover Studies
We have treated recorded earthquake data as additional “shot” gathers to use
along with active-source data in constraining Taiwan subsurface seismic velocities.
Traditionally, active-source and passive-source studies are carried out separately, but we
find that a more accurate picture of the subsurface is acquired by reinforcing one method
with the other. We refer to the use of such dual-sourced data as “crossover studies”
[Okaya et al., 2009]. In areas of high seismicity, such as Taiwan, many earthquakes can
be recorded if seismometers are left on “continuous” mode. The main benefit of seismic
crossover studies is the rich dataset they provide. Explosion seismic gathers elucidate
upper-crustal features well, while deeper earthquake sources define features at greater
crustal depths.
This study tests the above assertion of crossover studies and helps to uncover
subsurface properties of Taiwan within the context of TAIGER, adding to the TAIGER
group’s ongoing geodynamical interpretation of the Taiwan Orogen. We specifically
focus on data gathered in the northernmost east-west seismometer land array, referred to
as the North/T6 transect. Its location is particularly useful for crossover analysis: the east
end of the array sits right beside a very “hot” zone of earthquake activity that originates
via Phillipine plate north-trending subduction beneath the Eurasion plate. We attempt to
use these earthquakes in addition to 2008 active sources in defining northern Taiwan’s
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subsurface, jointly testing the validity of crossover studies as well as adding to
TAIGER’s story of the Taiwan Orogen.
Specifically, we attempt to define the Moho geometry beneath the Taiwan Orogen.
We believe trends in Moho depth are closely linked to the large deformational trends of
the orogen, making defining Moho geometry a research priority. With this geometry
constrained, we can further add validity to certain representative geodynamical models of
Taiwan while throwing out others that do not fit this new analysis.
This study also aims to allow the TAIGER project to increase the safety of
millions of Taiwanese. Data from the North/T6 transect is a priority for analysis because
this array sits closer to the most populous regions of Taiwan (e.g. Taipei metropolitan
area) than any of the other land-based arrays, thus having the largest potential in aiding
hazard mitigation that impacts the largest number of people.
Main Hypothesis
Using PmP arrival phases from both active and passive sources supplemented by
Pg-generated shallow-depth constraints, we should be able to constrain the depth of the
Moho beneath Taiwan. If PmP phases are found to be recorded by both ends of the T6
array, we can use them to define the east and west-dipping flanks of a presumably
deepening Moho beneath Taiwan’s Central Range.
Dataset
Our dataset consists of the recordings of 4 explosions and 17 earthquakes into the
T6 array that spans northern Taiwan. The array consists of 456 seismometers at a spacing
of 200 m (Fig.2). It begins on the west side of the island at approximately 121° East, 25°
North and trends southeast for approximately 100 km to the eastern shore. Seismometer
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offsets from a straight-line that contains both array endpoints varies up to a maximum of
approximately 9 km perpendicular to the North/T6 transect. These offsets arise due to
terrain and infrastructure constraints on seismometer deployment locations. Data-gaps
occurred as a result of seismometers failing to record or recording at a quality too
substandard for proper analysis. The largest recording gap is approximately 50 successive
seismometer traces in length and exists within the western end of all gathers (Fig. 2). This
gap is large enough to disrupt a correct interpretation of arrival trends along its length,
but all other gaps are much shorter and do not pose this problem.
Most seismic traces from across the array clearly show Pg and Sg wave first
arrivals (Fig.2). These phases are less visible within traces farthest from their respective
sources. When plotted in a reduced travel-time scale of t – x/6 seconds, Pg phases from
explosions mostly trend horizontally with time, indicating Pg phase velocities of ~6 km/s.
Earthquake Pg phases show a decreasing reduced travel-time with distance when plotted
in this format, indicating >6 km/s average earthquake Pg phase velocities (Fig. 3). This is
expected due to the greater depth of earthquake sources. Raypaths from earthquakes
should generally cover faster-velocity Earth along the deepest parts of path since they
originate at greater depth than those of explosions.
In addition to Pg and Sg phases, PmP phases are also clearly visible in recordings,
albeit fewer PmP than Pg and Sg phases can be pinpointed. Out of the 4 explosion and 17
earthquake recordings, PmP have thus far been clearly identified within 1 explosion
gather (Fig. 2) and 2 earthquake gathers (Fig. 3). Two recordings have PmP arrivals in
seismometers located west of array center, while the other has recorded PmP east of
array-center. PmP phases have thus far
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only been discovered in recordings of MW >2.0 earthquake events. Because other
recorded earthquakes are similarly geometrically situated to PmP-producing events, it is
assumed that MW < 2.0 earthquakes lack the energy required to generate PmP arrivals.
Methods
Determining Relevant Sources (Earthquakes)
In total, 3200 earthquakes occurred during the 2-month 2008 TAIGER
seismometer deployment period. Seismometers were set to record during specific “time
windows” which represented planned and auxiliary times for the detonation of explosions.
We searched through the February-March Taiwan earthquake catalogue from the Taiwan
National Weather Bureau for earthquake event times that fell within recording windows.
The North/T6 transect recorded at specific times during a week-long period, significantly
reducing the number of possible earthquake candidates from the 2-month catalogue list.
Another constraint we imposed on earthquake candidates is perpendicular distance from
the North/T6 transect. Earthquakes occurring at too great a distance from the array would
likely fail to propagate sufficient energy for our studies, or would fail to be recorded by
the array at all. More importantly, earthquakes occurring at too great a perpendicular
distance from the array might propagate waves through too much Earth outside of the
cross-section we attempt to image. This would add 3D data to an analysis that can only
represent data in 2D, corrupting our results. To this end we chose to focus on earthquakes
with epicenters within a 15 km perpendicular distance from the array (Fig. 4). A simple
Matlab algorithm was devised and implemented to match earthquakes from the catalogue
to time-window and perpendicular distance criteria. In total, 21 earthquake events fit our
constraints. Recordings of these events were subsequently extracted from TAIGER’s
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database of 2008 land-array recordings. Extracted records were then viewed and the
quality of recorded phases was assessed. Four of the recorded events were found to be
unusable in our study due to recording errors and data corruption. Thus, our total number
of earthquake sources is 17 earthquakes (Fig. 4).
Time-Picking of Arrival Phases
We used the VISTA seismic package to pick arrival times for seismic phases
recorded by the array. We put raw recordings through a sequence of processing and
scaling in order to accentuate seismic phases for easier recognition. We first applied DC
bias removal, which corrected for a constant amplitude shift that is imparted by the
recording instrumentation. We next applied automatic gain control (AGC) which boosts
signal strength via a sliding-window local-normalization process. AGC attempts to make
important low-energy, long-distance signals strong enough to see in trace-plot view. Next,
an Ormsby filter with low to high trapezoid filter values of approximately 10-15-35-45
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Hz was applied. This combination of frequencies gave the best results for viewing Pg
first arrival phases (Fig. 5). To enhance PmP phases, lower Ormsby-filter defining
frequencies of 5hz, 8hz, 10hz and 13hz were used. Next, a manual picking procedure for
Pg first-arrival phases was performed on all 17 earthquake records with the aid of the
VISTA package. The previous processing steps accentuated first-arrivals (Pg) clearly
enough that picking within each trace was mostly unambiguous, giving only a small
estimated average pick-time error of approximately ±10ms. PmP phases became easily
identifiable once traces were plotted in reduced travel time.
Because all of the earthquakes, with the exception of earthquake 9, occurred near
the eastern end of the T6 array, recording quality was highest in traces recorded at this
end and, due to limits on energy propagation, traces belonging to the western half of the
array recorded much fainter signals. In fact, rarely were signals strong enough within the
westernmost quarter of the array to be identified when plotted in VISTA. Thus, this limit
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on energy propagation causes our analysis to best constrain subsurface properties of the
eastern half of the array. This also limits any constraints for defining deeper depths: the
farther away a seismometer is from its source, the deeper the raypath must be for it to
travel to this instrument. Thus, we do not achieve recordings of deepest-path arrivals.
Modeling
We used a Fortran-based raytracing and velocity inversion algorithm developed
by a TAIGER colleague to analyze arrival times (H. Van Avendonk, 1998). Several
procedural steps and definitions relevant to the code follow.
First we built an initial velocity model to be used for raytracing. This involves
setting up several text files that define velocity model parameters. To do this we define
prominent subsurface reflectors, or horizons, within a “horizon” file. We fix particular x
and z coordinate points (in km) for each horizon, and points in between fixed points are
interpolated according to the two nearest-defined points within the same horizon. A
necessary companion file to the horizon file is the velocity file, which characterizes the
starting velocities of the model. It holds columns of velocity values defined for each
depth. The code performs interpolations of non-specified velocities according to the
horizon file, thereby constructing a velocity grid. Velocity interpolation within each
model layer depends only upon those velocities defined within the layer, and all other
velocities are masked until the code reads in velocities for the next layer.
We define our initial velocity model to be 150 km wide by 50 km deep (Fig. 6).
The western end of the North/T6 transect lies at 0 km in the far left end of the model,
while the eastern end of the array ends at 100 km. This leaves 50 km for the input of
offshore-earthquake locations. Also, 50 km depth is very likely deeper than we can
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reasonably image, but this leaves room for the deepest earthquake we use which lies at 45
km depth. The defining feature of the model is our placement of the Moho, which we
represent as a jump in seismic velocity from 6.8 km/s to 7.8 km/s. We define its geometry
based on the assumption that the Moho will “dip” slightly underneath the Central Range
of Taiwan. This dip would isostatically accommodate the mountain roots of the central
range. We represent this by defining the Moho with these three points: 30 km depth at the
eastern and western ends of the model, and 33 km depth in the center of the model, below
the Central Range (Fig. 6).
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Raytracing and Inversion
Using explosion and earthquake pick files picked and extracted from the VISTA
seismic package, we are able to perform raytracing of recorded events through our initial
velocity model with H. Van Avendonk’s code (Figs. 7-10). Certain parameters need to be
specified in the code depending on desired input and output. Different phases had to be
identified as either reflected or direct phases, and reflection layers had to be specified as
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well. We found the raytracing module of the program to run fairly smoothly.
The next step in creating a velocity model with real-Earth characteristics is
running inversions on our raytrace model. These inversions should better fit our velocity
model to observed arrival times. They approximate discrete velocity-cells by the use of a
least-squares regression method that calculates velocity based on all through-going rays.
We created a UNIX master-shell that interacts with the code in order to run a suite of
inversions. The first iteration performs a joint inversion on both explosion and earthquake
Pg phase arrivals simultaneously. The resulting velocity model from this inversion is then
used as the starting velocity model for a PmP raytrace and successive PmP arrival phase
inversion.
As constraining Moho geometry is the overarching goal of this research, it is
important that we do so as accurately as possible. Thus, we employ Pg from explosions
and earthquakes to constrain shallow velocity layers, allowing for a more realistic-fit of
PmP phase velocities at depth. Once an inversion of PmP arrivals has been performed,
the resulting velocity model with newly-constrained Moho is used as the input for
another Pg phase inversion. Our shell dictates that this output is used as input for another
PmP inversion, and so on, for a total of 7 complete Pg + PmP iterations of inversion.
Results
This study validates the use of crossover-studies in seismic imaging work. It
shows that supplementing active-sources with passive-sources gives seismological
studies much better constraints than does using one method alone. In fact, it leads one to
question why only active sources would be utilized in a seismically-active region for
seismic imaging. If earthquakes occur in the region of study, seismic-array experiments
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should record them so that they become an additional constraint on velocity tomography.
Without using them, researchers are choosing to ignore some of the physical realities of
their area of inquiry.
Our discovery of clear PmP reflections within explosion and earthquake trace
gathers shows that the Moho can be imaged directly. This result motivates Moho imaging
via PmP reflections in 2009 TAIGER deployment seismic data.
In this study we tested several combinations of arrival time inversions. The
resulting velocity models suffered from a flaw that has yet to be resolved. After one
iteration of our inversion, the velocity model changes so that the next iteration of
raytracing refracts multiple rays within one layer. This concentrates rays from subsequent
raytraces into discrete pathways. Because this phenomenon localizes all traced-rays along
just a few paths, it has a severe coarsening-effect on all successive inversions.
Discussion
Problems / Sources of Error
Earthquakes in this study have not been precisely located. They were located by
permanent seismic-monitoring stations in Taiwan, which may give an error in location of
± several kilometers. This would cause arrival time picks to be significantly off, leading
to raytracing that either takes a faster or slower path to make up for this data error. This
in turn compromises the quality of our tomographic inversions. An earthquake location
inversion could be performed by the use of the widely-used raytracing and inversion code
built by Zelt [Zelt, C. A. and Smith, R. B., 1992]. This would help eliminate any errors
due to earthquake location uncertainty.
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Another source of error exists in our choice of earthquake sources. We used
earthquakes located up to a distance of 15 km perpendicular to the array (Fig. 4). This
distance, equivalent to ~15% of the length of the North/T6 transect, represents a
compromise between achieving accuracy by the use of sources in-line with the array and
achieving accuracy by using a more robust number of sources. Waves from earthquakes
located at too-great a perpendicular distance from the array travel through an Earth
volume far enough outside of our 2D cross-section to invalidate inversion results. Thus
inversions should be run with only array-in-line earthquakes in order to see the effects
that array-offset earthquakes have on the model. Then a decision can be made regarding
the best choice of sources.
Our initial velocity model could be improved by performing some small forwardmodeling steps. The initial model fits explosion arrivals well, but is not as accurate in
fitting earthquake arrivals. This most likely indicates that model velocities at depth are
ill-defined. By observing the error in raytracing earthquake arrivals, adjustments to the
initial velocity model could be made, giving us a more realistic starting-point for
inversions of arrival time data. In addition, forward-modeling steps could also be taken to
better define a starting Moho geometry. A few successful inversions of PmP arrivals will
be able to tell us if our initial Moho configuration is real enough to provide a good
starting point for inversion.
There could be more accurate methods of picking arrival times available. We
were only able to perform manual picks, potentially leading to human error in picking. It
is likely that if we use an automatic picking procedure our picks will be more statistically
precise, and it is possible that they would be more accurate as well. Thus the VISTA
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program we are using may not be the best choice for picking. A popular alternative would
be the Antelope seismic software.
As introduced in the section on methods, we have not had much success running
raytrace-inversions on our initial velocity model. We may have inputted bad values for
starting parameters, or there could be problems with the data itself. One issue we have
still not resolved is that of setting correct χ² values. These values are a measure of
inversion roughness that determines the fineness of adjustments that can be made to the
model via inversions. Each inversion requires the user to input a value of χ², which the
van Avendonk code then uses to “target” the actual value used in inversion computations.
We have worked with the code’s author on this issue with little success thus far. With a
better understanding of the meaning and use of χ² we should be able to fix this issue and
complete a full run of inversions commanded by our UNIX master-shell.
Future Work
The first step in furthering this study is to complete the inversion runs that we
have set up. Debugging of either the inversion code, or the input master shell, or perhaps
both, need to be performed to ensure the correct input and setup parameters. Once we
have results, we can further question their validity and try to minimize sources of error
such as those described in the previous section. We will also be able to compare our
results to other studies to see if our work fits with results of previous work, such as the
depth and geometry of moho characterized by Wu and Kuo-Chen [Wu et al., 2009; KuoChen et al., 2010].
Crossover-studies and PmP reflection analysis need to be performed on data
gathered during the 2009 TAIGER deployment. Land arrays in the 2009 deployment are
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placed along the same transects as land arrays from the 2008 deployment, and thus a full
analysis should incorporate data from both deployments, resulting in one model with
robust constraints. 2009 land arrays recorded continuously for 3 months, potentially
capturing arrivals from 3783 earthquakes (Fig. 11), which is significantly more than 2008
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arrays recorded. According to the 15 km perpendicular distance criteria for earthquake
source candidates used in this study, 758 earthquakes could be used for analysis of the
2009 North/T6 transect alone, as opposed to just 17 in the 2008 North/T6 transect; Fig.
12 shows the usable 2009 earthquakes. In addition to the North/T6 transect, we have also
identified 405 earthquake source candidates for analysis of the 2009 South/T4b transect
(Fig. 13). Similar analysis to that performed on the North/T6 transect should be
performed on the South/T4b transect as well.
Conclusions
We find that mixing active and passive sources in tomographic studies of the crust
improves resolution and Earth coverage, and should become the standard for wide-angle
reflection studies in seismically-active regions. We also conclude that PmP reflection
phases from our data show up clearly enough to perform direct imaging of the Moho, a
procedure that can be performed in future studies of other tectonic regions as well.
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Acknowledgements
-Professor David Okaya (USC Earth Sciences Dep.), who agreed to be my advisor for
this research and taught me a whole lot about computing and seismology.
-USC Department of Earth Sciences for summer funding support through the Earth
Science Research Assistant Program (ESRAP).
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