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Eberhard Karls Universität Tübingen
Department of Geosciences
Bachelor Thesis
Subduction Zone Seiesmicity and Plate Geometry at
Plate Corners
written by Ilze Muceniece
(matriculation no 3550858)
Tübingen, August 29th 2015
First examiner: Prof. Dr. Todd A. Ehlers
Second examiner: Dr. Byron Adams
I hereby declare that I have written the Bachelor's Thesis on my own and have used no other than
the stated sources and aids.
_____________________________________________________________(Place, Date, Name)
Abstract
It is known, that some plate corners show high rates of erosion and strong deformation. To
get such a phenomenon, tectonics and climate have to be combined, therefore it is important to
know how the 3D-geometry of the subducted plate works. In this study, four plate corner regions in
Alaska, Cascadia, South America and Himalaya are considered. The position of earthquake
hypocenters, the model of subducted slab, Slab 1.0, as well as the crustal thickness is used to
make observations of seismicity and plate geometry at these regions. The crustal thickness map,
seismicity map, and slab depth map were created with ArcGis to gain a better overview. From this
study, one can see that the seismicity rates and earthquake depths vary in different regions, from
high seismicity rates and deep earthquakes in South America to shallow and few earthquakes in
Cascadia. Also the slab depth, slab form and crustal thickness vary with the regions. The results
for South America and Himalaya indicate a deep crust in these regions. The maximum crustal
thickness in South America and Himalaya is more than 70 kilometers, whereas it lies between 55
and 60 kilometers in Cascadia and Alaska. Curved slabs cause upper plate deformation and the
crust is thinner on the top of convex slab. These results can be used as an overview of the
subducted plate geometry and the seismicity of subduction zones.
3
Table of Contents
Abstract............................................................................................................................................ 3
1 Introduction................................................................................................................................... 5
2 Methods....................................................................................................................................... 6
2.1 Earthquake Data................................................................................................................... 6
2.2 Crustal Thickness.................................................................................................................7
2.3 Slab Data.............................................................................................................................. 7
2.4 Topographic Data.................................................................................................................. 7
2.5 Creation of Cross Sections...................................................................................................7
3 Results......................................................................................................................................... 8
3.1 Alaska................................................................................................................................... 8
3.1.1 Seismicity...................................................................................................................... 8
3.1.2 Slab Depth.................................................................................................................. 10
3.1.3 Crustal Thickness........................................................................................................10
3.1.4 Cross Sections............................................................................................................ 10
3.2 Cascadia............................................................................................................................. 13
3.2.1 Seismicity.................................................................................................................... 13
3.2.2 Slab Depth.................................................................................................................. 14
3.2.3 Crustal Thickness........................................................................................................14
3.2.4 Cross Sections............................................................................................................ 14
3.3 Himalaya............................................................................................................................. 19
3.3.1 Seismicity.................................................................................................................... 19
3.3.2 Crustal Thickness........................................................................................................23
3.3.3 Cross Sections............................................................................................................ 23
3.4 South America.................................................................................................................... 25
3.4.1 Seismicity.................................................................................................................... 25
3.4.2 Slab Data.................................................................................................................... 28
3.4.3 Crustal Thickness........................................................................................................28
3.4.4 Cross Sections............................................................................................................ 32
4 Discussion.................................................................................................................................. 33
5 Conclusions................................................................................................................................ 37
6 Acknowledgments...................................................................................................................... 37
Appendices:................................................................................................................................... 39
References:.................................................................................................................................... 40
4
1 Introduction
One assumes that erosion and tectonics are strongly coupled at plate corners (Willett et al,
2001), as some plate corners indicate rapid exhumation of metamorphic material (Finlayson et al,
2002, Enkelmann et al., 2010), and deformation(Zeitler et al.,2001). In this study, we focus on the
following four syntax regions: Alaska, Cascadia, Himalaya and South America. These regions are
considered to undergo different types of
collision and also possess different properties
of erosion rates and deformation. In the
regions of Alaska, Cascadia and South
America,
oceanic
crust
collides
with
continental crust, whereas Himalaya is the
result of a continent-continent collision. It is
Figure 1: Simplified schematic, illustrating the
geometry and structure within the syntaxis ( after
Bendick and Ehlers, 2014)
believed, that Himalaya and Alaska as well as
Cascadia have a rapid erosion rate and
deformation (Brandon et al., 1998, Zeitler et al.,2001, Enkelmann et al., 2010), while South
America does not show these properties.
Previous studies reveal, that tectonic processes, specifically the geometry of subducting
plates, are important to get such a pattern of deformation and exhumation. (Bendick and Ehlers,
2014). It is also known, that the subducted plate has a 3D-geomety. (Mahadevan et al, 2010).
Figure 1 demonstrates how an upwards convex subducted plate can affect the deformation of the
upper plate. Observations indicate, that the geometry of indenters at syntaxes approaches the
shape of a plunging ellipsoid (Bendick and Ehlers, 2014). One of the 3D slab models, slab 1.0, is
based on seismic data, namely earthquake hypocenters and seismic velocity anomalies (Hayes et
al., 2012).
The subduction zone is strongly connected to seismicity, as about 95% of all earthquakes
occur at these zones (Frisch and Meschede, 2005). Subduction zones are the only zones in which
not only shallow earthquakes (up to about 70 kilometers deep) occur, but also intermediate (70-300
5
kilometers deep) and deep earthquakes (300-700 kilometers deep) (Frisch and Meschede 2005).
Shallow earthquakes are strongly related to horizontal compression and coupling-decoupling
mechanisms of the plates, while intermediate ones are the result of decompression. Only a small
number of earthquakes occur in depths of more than 20 kilometers, because of the ductile
deformation of the continental crust. (Frisch and Meschede, 2005)
This study deals with subduction zone seismicity and plate geometry at the four plate
corners: Alaska, Cascadia, Himalaya and South America. Different data sets such as crustal
thickness, earthquakes hypocenters and slab model are combined to make statements about
seismicity, geometry of the subducted and upper plate deformations.
2 Methods
For this study the geographic information system software package ArcGis, in particular
ArcMap, was used. For the creation of cross sections the program Matlab was used. Matlab was
used since it provides an easy graphical display utility as well as easy loading of large data sets
and their processing. ArcGis was used to delineate maps of seismicity, slab depth and crustal
thickness of the plate corner regions considered in this study. For the basic map the National
Geographic basemap of ArcGis was used. This map was selected because of good spatial data
2.1 Earthquake Data
The earthquake data were taken from the U.S. Geological Survey Science Data Catalog
(USGS). The USGS Earthquake Data Catalog is freely accessible and contains a large amount of
earthquake data. In this study only earthquakes with a magnitude of at least 5 have been
considered, because earthquakes with a magnitude of 5 are possible to register worldwide, and
therefore the location and depth can be determined with sufficient precision. Due to the small
number of earthquakes in the Cascades, those with a magnitude greater than 3.5 were taken into
account in this region. The earthquake data were collected for every region separately. For this
study, the earthquake depth data were used, which is given in kilometers.
6
2.2 Crustal Thickness
The crustal model Crust 1.0 (Laske et al., online data) shows the thickness of the earth's
crust. It is available as image file with a resolution of 1×1 degrees. The crustal thickness is given in
kilometers. Data were gathered from seismic experiments and averaged globally for similar
geological and tectonic settings. These averages were used to assign structure to regions without
seismic information. (Laske et al., online data)
2.3 Slab Data
The data set Slab 1.0 (Hayes et al., online data) was also used for the model. Slab 1.0 is a
three-dimensional compilation of global subduction geometries, separated into regional models for
each major subduction zone. Each model is based on a probabilistic non-linear fit to data from a
combined catalog consisting of several independent data sets: historic earthquake catalogs, CMT
solutions, active seismic profiles, global plate boundaries, bathymetry and sediment thickness
information (Hayes et al., online data). There are no slab data existing for the Himalaya. The depth
of slab is given in kilometer units.
2.4 Topographic Data
For the topography a combination of GTOPO30 and ETOPO1 was used. The topographic
data GTOPO 30 (USGS) have a 30 arc seconds resolution. GTOPO30 only covers continents,
therefore another data set was needed to show the ocean floor. For this purpose the data set
ETOPO1 (NOAA), compiled from a large number of global and regional data sets, was used. It
includes land topography and ocean bathymetry. For illustration of land topography GTOPO30 was
used and ETOPO1 was used for oceanic topography. The original topography layer was a WGS84
projection and the coordinates were given in decimal degrees.
7
2.5 Creation of Cross Sections
To create the cross sections, a combination of ArcMap and Matlab was used. In the first
step, small regions, shown in Table 1 (in Appendix) as regions B and C, through which the profiles
were constructed were transformed into UTM coordinates. This step was necessary so that all
numbers, coordinates, elevation and earthquake depth had the same unit (meters). To get an
elevation profile, the layer GTOPO30 and ETOPO1 and the tool Profile Graph in 3D Analyst was
used. The obtained values were exported to an Excel worksheet. To get an earthquake depth
profile, the tool Point Profile in 3D Analyst was used. The horizontal extent of the point profile
depends on the region. In Alaska it is 60 kilometers, in Cascadia 80 kilometers, in South America
100 kilometers, in Himalaya for first both cross sections 120 kilometers.
3 Results
In this chapter, a closer look is taken at the four regions of the study. In particular, the
relationships between earthquakes hypocenters and slab depth as well as crustal thickness were
considered, as well as cross sections and topography.
3.1 Alaska
3.1.1 Seismicity
Figure 2 illustrates the seismicity of the region of Alaska. It shows, that the earthquake depth
in this region reaches more than 150 kilometers. Moreover, it indicates that most earthquakes are
located along the trench and to be more precise, at the north of the trench. The earthquakes that
occur along the trench show a distinct trend of the depth of earthquakes increasing with the
distance from the trench. It also can be seen, that the earthquakes at the east of the trench are
located in greater distance from the trench compared to the earthquakes at the west of the trench.
There are also earthquakes located further away from the trench, which are mainly up to 15
kilometers deep, but with some exceptions, where the depth reaches 15 to 40 kilometers. They
mostly occur at the north side of the trench, but also eastwards from 145° W.
8
A
A
B
B
Figure 2: (A) Seismicity map of Alaska. ; (B) Subregion of Alaska. Earthquakes are colored by depth
(in kilometers)
9
3.1.2 Slab Depth
Figure 3 demonstrates the slab depth and seismicity of Alaska. It also shows that the slab is
located along the trench, north of the trench. The slab depth varies from 20 to 260 kilometers.
Moreover, it indicates that the maximal slab depth on the west of the slab is up to 260 kilometers,
whereas to the east of the slab, it is from 65 to 150 km deep. The slab at the east is wider than at
the west of the trench. Figure 3 also indicates that most earthquakes are located on the slab,
except for some with depths up to 40 kilometers, that are located further away from the trench.
3.1.3 Crustal Thickness
In addition to seismicity and slab depth, also the crustal thickness of Alaska was observed.
Figure 4 shows that the crustal thickness of Alaska lies between 20 and 50 kilometers, whereas the
greater thickness values occur between 60°-65°N and about 155°-145°W. Figure 4 also indicates
that the depth of the earthquakes further away from the trench is equal to or less than the crustal
thickness.
3.1.4 Cross Sections
Figure 5 indicates the topography and the depth of earthquakes across the cross section
illustrated in Figure 2B. The maximum of the topography is at about 300 kilometers from the
trench. Figure 5 shows the trend, that the depth of earthquakes also increases with an increasing
distance from the trench. However, there are five exceptions: one earthquake at about 350
kilometers, one at about 500 kilometers and three earthquakes at about 600 kilometers away from
the trench.
10
A
A
B
Figure 3: (A) Slab depth map of Alaska; (B) Subregion of Alaska, Earthquakes and slab depth are
colored by depth. Depth is given in kilometers
11
A
A
B
B
Figure 4: (A) Crustal thickness map of Alaska ,(B) Subregion of Alaska
12
Figure 5: Earthquake depth data and topography in kilometers across the cross section D-D'
3.2 Cascadia
3.2.1 Seismicity
As Figure 6 demonstrates, the earthquake depth in Cascadia is mainly up to 40 kilometers.
Earthquakes with depths of 40 to 65 kilometers, and one earthquake being more than 65
kilometers deep, are also present, but more unfrequent. Earthquakes with depths up to 40
kilometers occur in the entire region, however, they are mainly concentrated in two regions. One
region is located between 125°-130°W and 47°-50°N and the other about 123°-130°W and 40°45°N. Earthquakes with depths of up to 65 kilometers can be observed at the Olympic mountains
and Vancouver island (see Figure 6B).
13
3.2.2 Slab Depth
Figure 7 illustrates the slab depth and the seismicity of Cascadia. Furthermore it shows, that
the slab is located along the plate boundary. Figure 7 shows the trend, that the slab depth
increases with the distance from the plate boundary reaching the maximum depth of 100
kilometers. Furthermore, one can see, that the lines, indicating the depth of the slab, are not
always at the same distance from each other. For example, the lines indicating a depth up to 40
kilometers are closer together at the north of the slab model than the ones at the Olympic mountain
region. In contrary, the lines showing a depth of up to more than 65 kilometers are closer together
in the Olympic mountain region than at the ends of the slab model. One can see, that the lines
indicating a depth of more than 40 kilometers, bend at about 47°N. Figure 7 also demonstrates,
that earthquakes with a depth of more than 40 kilometers, possess a depth equal to the depth of
the slab.
3.2.3 Crustal Thickness
Figure 8 demonstrates the crustal thickness of Cascadia. Beside that, it shows, that the
crust is mainly 20 to 35 kilometers thick. However, at Vancouver island and between about 120°125°W and 114°-117°W, the crustal thickness reaches up to 45 kilometers. Moreover, Figure 8
indicates, that most seismic depths are less or near to crustal thickness, except the earthquakes
with depths of more than 40 kilometers. )
14
A
A
B
B
Figure 6: (A) Seismicity map of Cascadia. ; (B) Subregion of Cascadia. Earthquakes are colored by
depth (in kilometers)
15
A
A
B
B
Figure 7: (A) Slab depth map of Cascadia; (B) Subregion of Cascadia, Earthquakes and slab depth
are colored by depth. Depth is given in kilometers
16
A
A
B
B
Figure 8: (A) Crustal thickness map of Cascadia ,(B) Subregion of Cascadia
17
3.2.4 Cross Sections
Figure 9 illustrates the topography and the depth of earthquakes across the cross section,
as shown in Figure 6B. It can also be seen, that the first earthquake occurs at a depth greater than
40 kilometers. They generally show the trend, that the depth of earthquakes also increases with an
increasing distance. Earthquakes with depths of less than 30 kilometers can also be observed.
There are two earthquakes at about 300 kilometers from plate boundary, 340 kilometers (it is
approximately 10 kilometers deep) and three at about 350 to 370 kilometers.(they are less than 10
kilometers deep
Figure 9: Earthquake depth data and topography in kilometers across the cross section
E-E'
18
3.3 Himalaya
3.3.1 Seismicity
Figure 10 shows the seismicity of the Himalaya. Moreover it indicates, that the depth of
earthquakes in the Himalaya is up to more than 150 kilometers. The earthquakes are mainly
located along the Main Boundary Thrust and northwards from the Main Boundary Thrust. Most
earthquakes occur at depths of between 0 and 40 kilometers. Some earthquakes with depths of
more than 40 kilometers are located at the Main Boundary Thrust and northwards from the Main
Boundary Thrust. However, most of the earthquakes having a depth of more than 40 kilometers are
located at the west and east of the Himalaya. Figure 10B demonstrates, that the majority of
earthquakes in this region are 15 to 40 kilometers deep. The ones with depths of more than 40
kilometers are mainly located between about Southwards 26°N and about 94°-97°E.
3.3.2 Crustal Thickness
Figure 11 shows the crustal thickness of the Himalaya. The crustal thickness in the
Himalaya varies form 30 to more than 70 kilometers. Figure 11 also indicates, that most
earthquakes have a depth less or equal to the crustal thickness, except the earthquakes with
depths greater than 40 kilometers at the West and East regions of the Himalaya. Figure 11B
shows, that the crust at the south of the region is 25-30 kilometers thick, and it gets thicker towards
the North, whereas the thickness of the crust increases more slowly between 93°-95°E.
19
A
A
B
B
Figure 10: (A) Seismicity map of Himalaya. ; (B) Subregion of Himalaya. Earthquakes are colored by
depth (in kilometers)
20
A
A
B
B
Figure 11: (A) Crustal thickness map of Himalaya ,(B) Subregion of Himalaya
21
3.3.3 Cross Sections
Figure 12 and Figure 13: illustrates the topography and locations of the earthquake foci
across the cross sections of Figure 10. Figure 12 shows the cross section from F to F', while
Figure Figure 13: shows the cross section from F to F'' Figure 12 demonstrates, that the maximum
of the topography is reached at 700 km away from the beginning of cross section. Moreover, it
shows that most earthquakes are located between approximately 10 to 50 kilometers deep. Some
earthquakes occuring at depths to about 100 km from the beginning of the cross section, indicate
the trend that, the depth of the earthquake increases with the distance across the cross section.
They possess a depth of 30 to 70 kilometers. There are also many earthquakes with a depth of 25
kilometers. They are located all over the cross section.
Figure 12: Earthquake depth data and topography in kilometers across the cross section F-F'
22
Figure 13: indicates that the maximum of the topography is reached at 800 km away from the
beginning of the cross section. Nearly all earthquakes, that occur to 400 km across the cross
section, indicate the trend of the depth increasing with the distance. They are 40 to 140 kilometers
deep. There are also many earthquakes located between 300 and 600 kilometers and with a
depth of 10 to 60 kilometers. Many of these earthquakes have depths of 25 or 30 kilometers. Two
earthquakes with depths of approximately 10 kilometers are located at 800 km along the cross
section.
Figure 13: Earthquake depth data and topography in kilometers across the cross section F-F''
23
3.4 South America
3.4.1 Seismicity
Figure 14 shows the seismicity of South America. Moreover it demonstrates that the
earthquake depth in this region is from 0 to more than 300 kilometers. Most of earthquakes are
located along the trench and show a trend, that with increasing distance from trench, the depth of
earthquakes also increases. There are some earthquakes with depths up to 65 kilometers
concentrated between 40°-45°S. Figure 14 also indicates that some regions have deep
earthquakes, that occur far away from the trench. One of these regions is at about 10°S, the other
between 15° and 30°S. (see Figure 14B)
3.4.2 Slab Data
The Figure 15 demonstrates the slab depth model and seismicity of South America.
Moreover it shows, that the slab is located along the trench. Figure 15 demonstrates the trend, that
the slab depth increases with the distance from the trench. The slab has depth from 20 to 740
kilometers. It can also be seen, that the lines, showing the depth of slab is not always at the same
distance from each other. For example, the lines showing the depth 66 to 150 kilometer at
approximately 30°S are further away from each other (and curved) than the same lines at 20°S.
(Figure 17) One can see, that the most of earthquakes are located along the slab.
3.4.3 Crustal Thickness
Figure 16 shows the crustal thickness of South America. Moreover it shows that the crustal
thickness of South America increases from east to west. The mainly crustal thickness in South
America is from 35 to 55 kilometers. On the east side from South America the crustal Thickness
can reach more than 70 kilometers (see Figure 16). The region, with the biggest distance from
trench with thick crust are shown in Figure 16. Northwards and Southwards the region with the
thick crust becomes smaller. Only a few earthquakes has depth smaller or equal with the crustal
thickness.
24
Figure 14:
25
A
Figure 14: (A) Seismicity map of South America. ; (B) Subregion of South America. Earthquakes are colored by depth (in
kilometers)
C
B
B
Figure 15
26
C
B
B
Figure 15: (A) Slab depth map of South America; (B) Subregion of South America, Earthquakes and slab depth are colored by
depth. Depth is given in kilometers
A
Figure 16:
27
C
B
B
Figure 16 : (A) Crustal thickness map of South America ,(B) Subregion of Sotuth America
A
Figure 17: Seismicity, slab depth and crustal thickness maps of subregion of South-America (depth
in kilometers)
28
3.4.4 Cross Sections
Figure 18 demonstrates the topography and the earthquake depth across cross section
shown in Figure 14B. It also demonstrates, that the maximum elevation compared with the
earthquake depth is near to flat. Though can be still seen, that the maximum of elevation is at
about 300 and 600 kilometers. Moreover Figure 18 shows the trend, that with increasing distance
from the trench the depth of earthquake also increases, there are a many earthquakes that are up
to 200 km deep and they are located up to 400 km from the trench. And there are also some deep
earthquakes that are 500 to 600 kilometer deep and are located 800 to 900 kilometer from the
trench.
Figure 18: Earthquake depth data and topography in kilometers across the cross section G-G'
29
4 Discussion
In Alaska the earthquakes occur mainly along the subduction zone, which is located along
the Aleutian trench (Figure 2). The region in the east of 140°W is aseismic. Most earthquakes are
associated with subduction zones. Only few ones are located away from this zone with depths of
up to 15, some with up to 40 kilometers, all being crustal earthquakes and not directly associated
with subduction zones. Based on the slab model, it appears that the slab is upwards convex at the
plate corner. At this region it is also shallower than the rest of the slab with the result, that this
earthquakes are not that deep and the seismic zone is wider. The seismic activity at the east is
somewhat smaller than the activity at the west of the slab, and they are more dispersed along a
longer distance. The convergence velocity of the North American plate (this plate is considered to
be fixed) and the Pacific plate is about 58 mm/yr at the east of the slab and increases towards the
west to 74 mm/yr (based on MORVEL plate velocity model (DeMets et al.,2010) North American
plate is fixed), giving the impression, that a greater convergence velocity causes more and deeper
earthquakes. The convergence direction is NNW, which corresponds to the deformation directions
of the subducted plate. The highest elevation of Alaska is at Mt. McKinley in Alaska Range with a
value of more than 6000 meters. The crust (Figure 4) below is only 35 to 45 kilometers thick. It also
corresponds to the fact, that the subducted plate is curved and has the flattest slab at this region.
The crust under the Alaska Range is between 25 and 45 kilometers thick. The Alaska mountain
range is curved, representing the form of a lab curvature. It shows, that high elevation can be
connected to 3D geometry of a subducting plate.
In Cascadia the earthquakes occur along nearly all regions.(Figure 6). The earthquakes are
mainly shallow with depths of up to 15, some up to 40 kilometers. The majority of them is located
along the divergent plate boundaries of Juan de Fuca, and transform faults. Figure 6
demonstrates, that there is a large aseismic zone across the subduction zone. The first
earthquakes, relatead with subduction zone, occur only after 200 km from the plate boundary due
30
to a flat slab. Only few earthquakes, mainly with depths of 15 kilometers, located mainly at Olympic
mountains and Vancouver Island regions, are directly related to the subduction zone. One can see,
that the slab is upwards convex and is stretched in NE direction under the Olympic mountains and
eastern part of Vancouver island. The majority of earthquakes associated with subduction zones
occur where the curved slab is getting steeper. The convergence velocity of the North American
plate and the Juan de Fuca plate is about 44 mm/yr at the north of the slab and decreases towards
the south to 32 mm/yr (based on MORVEL plate velocity model (DeMets et al.,2010), North
American plate is fixed). Most earthquakes occur at the Olympic Mountains, which also
corresponds to the biggest convergence velocity in this region. The convergence direction is NE,
and it corresponds to the direction in which the slab is stretched. In the region, where the
subducted slab is the flattest (under the Olympic mountains) the crust is only 25-35 kilometers
thick. The highest elevation there is 2428 meters (Mount Olympus). However, this is not the
highest point of elevation in Cascadia. The highest point of elevation can be found at the Cascade
Range (Mount Rainier, 4392 meters). The crust below the Cascade Range is 45-50 kilometers
thick. Its position also corresponds to a curved plate. Nevertheless, the highest elevation occurs,
where the slab gets steeper again.
In Himalaya the earthquakes are mainly located along and northwards from the Main
Boundary Thrust. Most earthquakes indicate depths of between 0 and 40 kilometers, and only few
earthquakes reach depths of more than 150 kilometers, but never 200 kilometers, which is also
typical for continent-continent collisions (Artemieva et al., 2015). Some earthquakes with depths of
more than 40 kilometers are located at the Main Boundary Thrust and northwards from Main
Boundary Thrust. However, most earthquakes having depths of more than 40 kilometers are
located at the west and east of the Himalaya. Since the slab model slab 1.0 does not include
models for this subduction zone, it is difficult to state much about the slab geometry. If we take a
closer look at Figure 12,some earthquakes, that occur at about 100 km from the beginning of the
cross section, show the trend of the depth of the earthquake increasing with the distance across
the cross section. Furthermore, their depth is greater than that of the crust, which could imply, that
31
these earthquakes are associated with subduction zones. However, the crustal thickness data
resolution is too uncertain for such a small scale to make reliable statements. The other
earthquakes in this cross section appear to be crustal earthquakes. Figure 13: indicates, that
nearly all earthquakes, that occur as far as 400 km across the cross section, show the trend of the
depth increasing with the distance. Furthermore, the crustal thickness is less than the earthquake
depth, which could signify, that also these earthquakes are associated with the subduction zone.
The other ones appear to be crustal earthquakes. The convergence velocity of Eurasian plate and
Indian plate is about 35 mm/yr at the west of the Himalaya and increases towards the east to 50
mm/yr (based on MORVEL plate velocity model (DeMets et al.,2010), Eurasian plate is fixed). ,
The convergence direction is NNE at the plate corner.
The South American earthquakes are concentrated mainly along the Peru-Chile trench and
to the west from the trench. The remaining regions indicate low seismicity to aseiesmicity. In this
region across the subduction zone earthquakes with depths of up to 600 km occur. Beside those
earthquakes, there are some with depths up to 65 kilometers concentrated between 40°-45°S that
can be associated with the border of Nazca Plate and its transform fault. The majority of the other
earthquakes are directly associated with the subduction zone. Most of the other earthquakes are
directly associated with the subduction. Figure 15B shows, that the subducted slab is curved,
however it is steep and not stretched in some directions. As shown in Figure 14 some regions are
locations of deep earthquakes, that occur far away from the trench. One of these regions is at
about 10°S, the other between 20° and 30°S. Figure 18 Demonstrates, that there is no seismicity
between depths of 250 and 500 kilometers, which corresponds to the fact, that the Oceanic slabs
subducting beneath continents are not seismogenic at depths of 250–500 kilometers (Artemieva et
al., 2015). The convergence velocity of the South American plate and the Nazca plate is about 68
mm/y at the north of the plate corner region and increases to 74 mm/yr to the south (based on
MORVEL plate velocity model (DeMets et al.,2010), South American plate is fixed). According to
Figure 14 it does not seem, that there is more seismicity in regions with higher convergence
velocity. However, in this study only earthquakes with magnitude over 5 are considered, so it may
32
not reflect the real seismic activity. The elevation above the South American plate corner is more
than 6000 meters and there, the crust is 50 to more than 70 kilometers thick.
Compared to other four regions, South-America features the deepest seismicity and is the
only region, considered in this study, that has deep earthquakes, that are deeper than 300
kilometers. The maximal depth in other regions is obviously smaller. Himalaya and Alaska show
similar maximal earthquakes depths – more than 150 kilometers. However, Cascadia
demonstrates shallower earthquake depths, merely more than 65 kilometer deep. The plate corner
regions of Alaska and Cascadia feature convex, shallower slabs, whereas these properties cannot
be observed in South America. The slab of South-America is not only, deeper, but also steeper
than in other regions. If the earthquakes in Himalaya, discussed earlier, are really associated with
subduction zone, than the slab there is somewhat shallower than in South America, however
significantly steeper than the other ocean-continent collision regions considered in this study.
Along ocean-continent collisions, one can also see, that the convergence velocity correlates with
the seismic activity. Regions with lower convergence velocity, also indicate lower seismic activity
and shallower earthquake depths. Cascadia features a large aseismic zone away from the plate
boundary and Alaska to the east from 139°W. In Alaska, the highest elevation occurs at the place,
where the subducted plate is curved with the crust being the thinnest at the top of the convex plate.
In Cascadia the topography is the highest above the point, where the slab gets steeper again but
not at the middle of the curvature.
In this study one demonstrates with a simple method, that the subducted plate geometry
has influence on the subduction zone seismicity and the upper plate deformation. As the slab
model Slab 1.0 performs insufficiently while fitting overturned slabs and does not define slab
geometries beyond their seismic definition in the deep mantle, uncertainties may occur. The edges
of each slab model are governed by the extent of each slabs’ seismicity, and may not reflect the
true spatial dimensions of the slab in the mantle. Slab1.0 shows lack of resolution where slabs are
aseismic (Hayes et al, 2012)
33
Crust 1.0 possesses a 1×1 degree resolution, which is enough to determine the trend for long
scales, but working with small scales can produce noticeable uncertainties.
5 Conclusions
In summary one can observe, that the ocean-continent plate corners show more similarities
with each other than with the continent-continent plate corner. However, even ocean-continent
collisions do not always have the same properties. For example, Cascadia and Alaska possess
flat, curved slabs, whereas South-America shows steep slabs. The slab of the Himalayan region
probably indicates a slab gradient more similar to that of South America. South-America and
Alaska feature strong seismicity, located mainly along the subduction zone Nevertheless, Cascadia
shows low rates of seismicity. The seismic rate of the Himalaya lies between those of South
America and Cascadia. However only earthquakes considered with a magnitude of more than 5
can affect the statements about the rate of seismicity.
The results of this study provide a basic and very simple overview of the plate corner
geometry and seismicity of the plate corner regions. To investigate this topic further, more detailed
data is necessary. Furthermore, a higher resolution of the crustal thickness and slab models of the
Himalaya would be recommendable.
34
6 Acknowledgments
First of all, I would like to express my gratidtude to Todd Ehlers for giving me the opportunity
to investigate such an interesting topic and providing me with any information I needed for my
study. I also want to thank Matthias Schmiddunser for his friendly and patient assistance and for
answering all my questions. I am also very grateful to Byron Adams, Mareike Hoffmann, Manuel
Schmid for helping me with GIS. Next, I want to thank all members of the working group, especially
Jana Geller, Elena Grin, Jessica Starke and Sarah Falkowski for productive and helpful
discussions.
Special thanks to my parents and family being there for me and encouraging me. And to all
my friends for supporting me.
35
Appendices:
Table 1:Selected regions and decimal degree coordinates of each region.
Decimal degree coordinates
Alaska
Cascadia
Himalaya
South
America
West
East
North
South
A: Overview
-195.820
-130.430
67.576
48-633
B: Subregion
-154.284
-130.430
64.245
54.725
A: Overview
-133.462
-117.026
51.427
38.074
B: Subregion
-129.397
-119.048
50.695
43.181
A: Overview
71.455
101.777
40.313
24.678
B: Subregion
90.637
101.777
32.547
24.678
A: Overview
-90.352
-49.570
13.240
-58.631
B:Subregion
-79.805
-63.281
-12.726
-26.981
C:Subregion
-78.926
-63.281
-16.973
-56.487
36
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