Week 1: Strength of the continental
2000- 2001 Istanbul
Week 2a: Imaging the Deep Seismic Structure Beneath a Mid-Ocean
Week 2b: Lithospheric strength in
related to seismogenic layer thickness
Lithospheric strength and its relationship to Te and Ts!
With the possibility of estimating Te and Ts at oceanic
and continental lithosphere, the paper shed some light on the relationship
between the elastic thickness and the seismogenic thickness claiming that
Te >>Ts in continental lithosphere, due to its different rheology,
but not in the oceanic lithosphere, due it relatively simple structure. It
shows that Te and Ts are different in the ways they contribute to strength
of earth’s lithosphere
activity in the oceanic lithosphere is limited to a depth range of around
At such depth range a
semi-brittle/semi ductile strain rate dependent plastic flow takes over.
Frictional component doesn’t present an important factor at such depths.
In short, at depth where ductile behavior is dominant earthquakes are
rare, whether it is related to oceanic or continental lithosphere.
Is there a difference in the mechanism
that originates shallow and deep earthquakes (Al-Omar)?
In general shallow earthquakes are related to the
absolute rock strength and deep seismic activity is not related to
frictional sliding that follows Bayerlee’s law. Hence, deep earthquakes
are weakly related to absolute rock strength.
Could Te and Ts follow each other and Te is always less than
conducted recently, the above is possible with reexamining the data and
trying to determine the, accurately, the depth associated with earthquakes
epicenter and Moho depths. They found that Te and Ts follow each other in
different regions. That entails the strength lies on the uppermost layers
of the continental crust, but it doesn’t say much about the oceanic crust
since it is bound by different factors.
What are the factors on which elastic thickness depends
Elastic thickness depends on
mineralogy, temperature and state of stress of the lithosphere.
How can you estimate elastic thickness
from the gravity data and topography (Akram)?
There are two approaches, 1. bouger coherence 2. Free air
Bouger coherence measures the correlation of topography and
bouger gravity as a function of wavelength where loads are supported
predominantly by stress. Free-air admittance is the transfer function
between free air gravity and topography.
How'll you interpret (Akram)?
i) Te nearly equals Ts
ii) Te >> Ts
iii) Te < Ts
and iii) Effective strength of the lithosphere lies in the
ii) Strength of the lithosphere is not limited to
the seismogenic layer. Strength of the lithosphere can also reside in any
layer that is aseismic. This also gives support to a strong mantle concept
if we consider it aseismic.
MEM method, the “no-load” approach and the topography method for the
estimation of Te, and state what author implied out of comparing the Te
values predicted from these methods (El-Hussein)?
(1) MEM (Maximum Entropy Method), uses spectral
estimates, which calculated in boxes that are moved step step-wise a cross
the study area.
(2) The “no-load” approach uses only the flexure
and gravity anomaly to one side of a load, in order to derive Te.
(3) Topography method uses topography to define load, the flexure
and gravity anomaly to one side of, and beneath a load is used to estimate
The author implied that the Te value can not be exactly
estimated. However, an idea of its extent can be figured out using
Why the author
considered that the sub-crustal mantle is and important contributor to the
support of long term beds in both the oceans and continents (El-Hussein)?
In oceans, oceanic Te studies suggest
that thermal cooling, which strengthens the lithosphere dominates over
that of load induced stress relaxation, which weakens it such that the
mantle becomes increasingly more involved in the support of loads with
In continents, stresses generated by flexure are
large enough to cause earthquakes in the uppermost brittle part of the
continental crust. They may not be sufficient to overcome the brittle
strength of the continental sub-crustal mantle when stresses encounter it.
Hence, again the mantle contributes as a support.
How in contrary to Ts, Te reflects the integrated strength
of the entire lithosphere (El-Hussein)?
oceanic lithosphere, the potential brittle zone extends to the
brittle-ductile zone (BDT), which may be as deep as 50 km. This is because
there is no intermediate ductile layer that prevents stresses from being
propagated into surrounding competent layers. As a result, the stresses
generated by flexure accumulate locally and if they exceed the confining
pressure, cause earthquakes.
In continents, however, there are
more ductile layers which may decouple the competent parts of lithosphere
and cause smaller stresses for the same amount of flexure. Furthermore,
small flexures and long loading times suggest that most continental
lithosphere will deform at rates that are significantly smaller than
oceanic lithosphere, which further reduces stress levels.
What is the elastic thickness of the
lithosphere (Tå) for oceans and continents (Bulaihed)?
The elastic thickness of the lithosphere (Tå) is in
the range 2-50 km for oceans and up to 80 km and higher for continents.
What is the Byerlee’s law of frictional
brittle failure, which characterizes deformation in the uppermost part of
that strength linearly increases with pressure and depth.
How we can determine the flexural
rigidity of the lithosphere (Bulaihed)?
By the brittle and ductile properties of the
constitutive rocks that comprise it.
Week 3: Which forces drive North
the forces you think can be responsible for driving plates
Buoyancy and viscosity
2. Trench suction
4. slab pull
5. thermal convection
If the north American plate is
driven from below then what should happen to the motion of North America
should slow down and finally come to rest.
Presenter's respond: I agree with slowing of motion
and disagree with its stopping.
lithospheric thickness effect degree of coupling with mantle
thicker the lithosphere, the stronger the coupling.
we test the assumption that hold simple shear responsible for lithospheric
If this assumption is true, then fast axes must have a dip
angle shallower than 45 degree from the horizontal.
Presenter's respond: I agree, as explained in Figure
plate is driven by side forces or by below then where do you think
orogenesis to occur (Akram)?
Orogenesis will occur on side toward which
root is moving (see Fig.1).
Presenter's respond: I agree,
while orogenesis will occur on the contracting side.
Stations in western United
States have northeast trending fast directions. What does this fact
I would believe that the observed variation in fast
direction would be more dependent on the decoupling factor. So I would
suggest that western United States has thinner lithosphere and thus is
less influence by the flowing mantle beneath it, or in other words, it is
more weakly coupled with the mantle than other parts of the United States,
and more influenced by gravity pull and drag.
Presenter's respond: I agree, and the coupling
relation with lithosphere thickness is illustrated in Figure
came first gravity drag-pull, or mantle flow (Busfar)?
I think this question
supports the idea that mantle flow does indeed play a role in moving the
lithosphere. Because I would assume that during the early stages of the
birth of earth, the whole earth had fairly the same surface. So how did we
get the drag-pull effect if nothing was being subducted (dragged by
gravity) beneath the other? This might bring up the idea that probably the
mantle flow caused the plates to break up and then the effect of drag-pull
Presenter's respond: I think it’s a logical prediction. However, it is very hard
to understand the whole phenomenon initiation.
Will north America really stop
don’t think that it will completely stop, because simply you will still
have the mantle flow, and buoyancy effects acting on everything on the
face of the earth.
Presenter's respond: I agree, while plates all over the world will be
moving and the driving forces like mantle convection will still be
author used a technique based on angular variations of P-wave delays
To determines both the azimuth and the dip angles for a set
of stations in North America.
What the anisotropic layers that the deep Canadian
Shield consists of (Bulaihed)?
The shallower one has subvertical foliation
plane, and the deeper one has subhorizontal foliation
Presenter's respond: I agree, as mentioned by the author referring to his paper with
Silver in 2000.
Why North America has slowed dramatically throughout the past 100
Because the south western motion of North America currently
places the western part of the stable continent over the downwelling, and
if the motion continues and the stable continent centers itself over the
downwelling, the lateral force acting on it will be zero, and the motion
Presenter's respond: I agree. However I disagree with its stopping, while
plates all over the world will be moving and the driving forces like
mantle convection will still be there.
summary given by Al-Omar on the above paper as:
Two views attempting to
explain the driving forces behind the motion of earth’s plate, plate
tectonics. The one theory relate the motion of the plates to convection
and the other view relate the motion of the plates to a combination of
“ridge push” and “slab pull” by the subducting plates. Through studying
the deformation, and the orientation of the deformation, of the deep
continental roots, the paper endeavors to proof that mantle plays a major
role in plate motion. The technique used was based on angular variations
of P-wave delays to determine the orientation, azimuth and dip angle, of
Does the orientation of minerals supports the mantle role in plate
Yes, Studies shoes that in North America the mantle
movement is faster than the overlain plate. It also shows that arrivals of
from southwestern direction are faster than arrivals from other direction.
This supports the idea of driving force from below
How is that related to the
thickness of the lithosphere (Al-Omar)?
Variation in the thickness corresponds to the
speed of the mantle movement.
Presenter's respond: I agree, and this is
illustrated in Figure (2).
How is the dip direction supports the claim
The dip direction depends on the driving force of the plate
motion. If the plate motion was caused by mantle convection than the dip
is in the direction of the of the movement. However, if the plate movement
was caused by slab pull and ridge push thean the dip orientation is the
Instruction's comment: The GPS studies are evident on the present-activity
of North America. How a discussion on the past deformational process (100
m.y.) "slowing" and future form of deformation "stopping" by current
deformation rates may be acceptable?
looks like an interesting seminar that you are having there, and
a selection of interesting papers. Now what concerns the motion of North
America, I don't think that the deformation would stop; on the other hand
the absolute motion of the North American plate with respect to the deep
mantle probably will. The case of the North American plate is a rather
particular one, since that plate finds itself above a well-documented
downwelling. In that case, forces driving the plate from below converge
toward the downwelling from both sides. That is why the North American
continent cannot escape slowing down if the plate is indeed driven from
below. Relative plate motion (plate tectonics..) would continue, with
surface deformation and earthquakes.
Week 4: Earthquake Conversation
A brief abstract is given by Al-Omar as:
The idea in this paper is easy to follow due its logical
progression. An earthquake causes tremors that add stress to an earthquake
prone area close by. Faulty planes respond to any thing that would
increase the stress causing them to grind and slid shaking the earth above
them. The cause of the added stress might as well be another earthquake
“near by”. If the theory is sound then the world could be alerted of the
approximate time and location of the next destroyer.
Is there enough evidence of such theory other than the ones related
to the San Andrea’s fault (Omar)?
The paper seems only to talk about that part of the
Presenter's respond: Well, the paper does seem to talk about only the San Andreas Fault
area, But the people are working on stress changes for different areas,
not only on strike slip regimes, but also on thrusting and normal faulting
Would it matter if the earthquake is a long a strike-slip fault
formation or underneath a convergent zone (Omar)?
The paper doesn’t specifically take about the different kinds of
earthquakes. However, it seems that the author believes of the simple
theory that any where in the world if there two earthquake-prone fault
then they’ll eavesdrop on each other.
Presenter's respond: The stress conditions will vary from regimes to regimes, and also
the changes in coulomb failure will vary, depending on the strike slip,
thrust or normal regimes. Like for the same order magnitude stress
changes, that is, the coulomb failure stress change produced by the source
earthquake, being the order of magnitude 10-2 MPa, if the ambient stress
level is high ( as for strike slip earthquakes), there is almost no
evident effect produced by such small perturbation. On the other hand, if
the ambient stress level is low (as for thrust earthquakes), the small
stress perturbations play a mole important role.
Could this production be the next forecast after the daily news,
next to weather forecast (Omar)?
Possibly, nowadays earthquakes are happening all over the
world. Although, the cause of these earthquakes might be follow other
reasons, such as the mantle loosing it viscosity.
happen when the shear stress exceeds the frictional resistance on the
fault or when the stress pressing the two sides of the fault
rocks on either side will slip past each other suddenly, releasing
tremendous energy in the form of an earthquake.
Presenter's respond: I agree, as whenever the shear
stress exceed the friction, slip will occur and cause an
Explain how come in Turkey and in southern California that even
tiny stress changes can have momentous effects, both calming and
Seismicity never shuts off completely in the shadow zones, nor does
in turn on completely in the trigger zones .Instead the rate of seismicity
merely drops in the shadows or climbs in the trigger zones relative to the
preceding rate in the area.
Presenter's respond: Up to some
extent I agree with your answer , But what i think is that earthquake from
the main shock can both reduce and enhance the regional stress
conditions/. In case if it raises, the earthquake will happen and when it
decreases, there are very less chances that we'll have an earthquake
there. That's the calming and catastrophic effects of the main earthquake,
that either it can cause some earthquakes on other faults or it can
inhibit the seismicity on other faults.
What the author and his colleagues observe after
mapping the locations of Landers, Big Bear and hundreds of other
California earthquakes (Bulaihed)?
They notice a remarkable pattern in the distribution not only of
true aftershocks but also of other, smaller earthquakes that follow a main
shock by days, weeks or even years.
Are earthquakes the only mechanism in which stresses are generated
and released (Busfar)?
believe that there are other factors that contribute to the generation
and/or release of stress. One might be the behavior of the mantle beneath.
This includes the convection regimes and the distribution of mass within
Could we estimate the stresses in the earth accurately? In other
words, how accurate is the “Coulomd stress change” map that the author
presented in his paper? Are they accurate enough for the purpose of
earthquake forecasting (Busfar)?
I am not sure how accurate these maps are because I
don’t know the method used to generate them, however, the author did
mention that he believes that very small change in stress regime (as
little as 1/8 of the pressure required to inflate a car’s tire!!) could
trigger as earthquake. If that is the case, then could these maps be
reliable for the purpose of earthquake forecasting? I tend to believe
the capital of Turkey (Busfar)?
Ankara! Not Istanbul!
Explain how aftershocks generated in North America as per the
author interpretation (El-Hussein).
Along the San Andreas Fault, for instance, the plate carrying North
America is moving south relative to the one that underlies the Pacific
Ocean. As the two sides move in opposite directions, shear stress is
exerted parallel to the plane of the fault, as the rocks on opposite sides
of the fault press against each other; they form second stress,
perpendicular to the fault plane. When the shear exceeds the frictional
resistance on the fault or when the stress pressing the two sides of the
fault together is eased releasing tremendous energy. But because stress
cannot simply disappear, it must distribute some where along the same
fault or to nearby faults, which causes aftershocks.
What is the renewal probability
is a more refined forecast that predicts that the chances of a damaging
shock climb as more time passes since the last one struck. This based on
the assumption that stress along a fault increases gradually in the wake
of a major earthquake.
is main aspect of the author’s forecast method
builds the probabilities associated with earthquakes interactions on top
of the renewal method by including the effects of stress changes imparted
by nearby earthquakes.
Ali, This is great. Your students did a
wonderful job with the paper. The SciAm editor added that Istanbul was the
capital of Turkey, and she is very embarrassed by this mistake.
any of your students want to go deeper, they can check my web site below,
which includes the most recent work on this subject and a new paper (Toda
et al, JGR, 2005) with 3 animations included.
R o s
Ross S. Stein
U.S. Geological Survey, MS 977
Road, Menlo Park, CA 94025
Tel: 1 650 329 4840 Fax: 1
650 329 5143
Papers, animations, software, teaching
Week 5: Mantle, topography and rift-flank uplift
think that the failure of this model in predicting the large peak in the
African residual topography is an indication that the theory is incorrect
shortcoming of this method might be due to different conditions that exist
in the African plate and thus causes the large peak in the African
residual anomaly. Another possibility is that it might be due to
shortcomings in the model itself caused by using simplified models in
computing the final results.
the author neglect the effect caused by heterogeneity of the mantle below
670 km depth (Busfar)?
author claims that taking into consideration the effect of the upper 670km
of the crust and upper mantle and deleting the effect of heterogeneity
caused by depths larger than 670km improves the result of the calculated
topography compared to the observed topography, however, I am curious to
know the results of neglecting different depths. In other words, what
would the results be if we only included the effect of the upper 500km,
400km, 300km and so on? Maybe this will result in a better approximation,
or give us a better understanding to come up with a better, or rather,
Describe the mean features of
models that used to explain rift-flank uplift (El-Hussein).
Thermal Models: Uplift can
result from depth dependent stretching or from heating of flanks by
small scale convection.
Mechanical Models: Indicate
that upward flexure may occur if the lithosphere maintains finite
strength during rifting.
Geometric Models: Explain the
symmetry of uplift in terms of a single low angle detachment penetrating
the entire lithosphere.
Melt Process Models: Extensive
flank uplift may also result from magmatic underplating due to
asthenospheric partial melting.
Illustrate the main steps used
by the author in formulation of his model (El-Hussein).
What the author claimed about
the tilting of the Arabian platform (El-Hussein)?
Prescribing density field
within the model domain by using results from seismic tomography.
- The governing equations are solved for instantaneous flow fields
throughout the domain.
- Dynamic topography is computed by applying surface normal
stresses output from the convection code to a model of elastic beam
Author stated that the
topography dynamically supported by large scale viscous flow in the mantle
is responsible for the dramatic tilting of the Arabian platform. The
tilting also is enhanced by seismically mantle beneath northeast Arabia,
which acts to dynamically depress the overlying plate in this
do you mean by seismically fast mantle (Akram)?
Cold Dense mantle, as it
is dense therefore velocities will be fast in that.
In the mega-plume area, would
you expect high velocities or low velocities (Akram)?
Low velocities, as due
to temperature, the material will become less denser to raise upward and
the velocities will be decreased in that zone.
Why don't the thermal and
mechanical models offer a good explanation for tilting of Arabia
these models are largely concerned with flank uplift within a few hundred
kilometers of the rift basin, therefore, these models don't provide a good
explanation for Arabian tilt.
What does thermal model say about the rift-flank
According to thermal model, uplift can result from
depth-dependent stretching or from the heating of flanks by small-scale
Week 6: Evolution of the Lithosphere Beneath
the Rocky Mountains
Figure 2 shows a large lateral velocity variation in the upper
mantle. The author suggests that these differences could be a reflection
of the temperature variation within the asthenosphere. Could there be
other factors that contribute to the velocity variation within the
I tend to believe that lateral variation in density, which
could be caused by physical compression or simply chemical change of the
mantle’s content, would contribute to the variation of P-wave
Presenter's respond: What I think, that these chemical changes
also come from temperature variation like when oceanic (basaltic)
crust tend to subduct beneath some oceanic / continental crust down to
mantle, due to temperature and pressure, it goes under metamorphism and
changes to high density eclogite. So these density variations in these
depths are also because of temperature variations.
The article suggests
that western North America (e.g., from the Canadian shield to the Pacific
plate margin) contains the largest mantle-velocity gradient on Earth. If
this is true, what do you think causes this phenomenon
The article suggests that western North America
(e.g., from the Canadian shield to the Pacific plate margin) contains the
largest mantle-velocity gradient on Earth. If this is true, what do you
think causes this phenomenon (Busfar)?
This is most likely, or at least largely, due
to the nature of the geological setting in the Rocky Mountains area where
there is a dramatic change in the lithosphere density going from fast,
cratonic, cold, and dense lithosphere mainly on the east to slow,
orogenic, hotter, and less dense lithosphere mainly on the west. This
abrupt change in the nature of the lithosphere over a relatively (relative
to other parts of the world) short distance causes this unique situation
where the lithosphere’s velocity varies
What do you think is the origin of the high velocity
lower crustal layer?
It could be due to the intrusion of other high density
materials during the Proterozoic Eon. Other possibilities include that it
had a more complex origin than currently assumed, or concentration of
refractory residues of partial melting.
Why authors selected North America for such study
Because it contains one of the thickest mantle on the
planet, and western North America (e.g., from the Canadian shield to the
Pacific plate margin) contains the largest mantle velocity gradient on
Earth. In addition, Gradation from fast to slow upper mantle velocity
structure occurs over a remarkably short distance in the Rocky
How the Proterozoic lithosphere of Colorado and New Mexico differs
from lithosphere beneath the Archean core of the continent
1. In thickness.
2. The strongly segmented nature.
Long term fertility for magnetism.
4. Its relative
How Moho layer formed at Rocky Mountains as per the authors' point
of view (El-Hussein)?
It has formed diachronously and by combination of processes
including original arc development and subsequent magmatic underplating
and to be the product of progressive evolution of the
What does the combined geophysical and geologic data from the
CD-ROM experiment provide (Al-Bulaihed)?
They are providing a high-resolution,
multiscale image of the lithosphere of the Rocky Mountain
What does the integrated data set for the Cheyenne belt, the
Farwell-Lester Mountain zone, and the Jemez lineament, and their
corresponding velocity anomalies in the mantle (to >200 km) indicate
They are controlled by Paleoproterozoic subduction
zones that were active during collisions of juvenile
What are the two provocative and testable hypotheses concerning
lithospheric evolution (Al-Bulaihed)?
1. the lithospheric mantle in the
southern Rocky Mountains preserves old subduction structures, is thick
(>200 km) and has been persistently weak, and
2. the lowermost
crust is a record of progressive evolution of the lithosphere and has
grown through several underplating and/or intrusive
Week 7: The crustal structure of the interior
What do you suggest that
implications of more stations to record earthquake data in Saudi Arabia
will help in imaging the crustal structures? And What do you think that
the crustal model suggested by the author and described in Table2 can be
improved as it already shows a lot of velocity variations in the crustal
Well, definitely by
increasing the number of recording stations we mean that more information
we get about earthquakes. By interpreting this valuable earthquake
information, we can better image the earth’s structure. If we integrate
different information obtained through recent techniques like Receiver
function studies, stress distribution, seismic tomography etc. we can
improve our results as well as make them accurate.
When do the Nyquist frequency
cause aliasing while digitization? What do you think (Akram)?
Well I think that when
the Nyquist frequency fN will be greater than the highest frequency in the
function, then it’ll cause the aliasing effect.
If we consider the assumptions
regarding the vertical velocity gradient and lateral velocity variations,
the solutions to elastodynamic
equation becomes easy or very difficult
difficult to solve, if not, then impossible.
Explain briefly Thomson and
Haskell matrix method (El-Hussein)?
The method provides calculations of responses
of any number of horizontal layers to incident plane waves ate any angle
of incidence, by using products of 4x4 matrices, whose elements are
functions of the parameters of each layer and boundary
How was the Arabian Platform divided structurally
Interior homocline, which is 400 km wide belt of
sedimentary rocks dipping gently.
Interior platform, 400 km wide where
sedimentary rocks dip away from the shield at low angles.
depressions, found around interior homocline and interior
What are the main results
generated by the author's model (El-Hussein)?
The model claimed that the
Arabian crust consists of five distinct layers with thicknesses 3 km, 10
km, 8 km, 15 km and 10 km, from top to bottom, with P-wave velocities of
5.6 km/s, 6.3 km/s, 6.6 km/s, 6.9 km/s and 7.6 km/s. For Moho, the
velocity is 8.3 km/s for upper mantle and 46 km depth.)
Why do plates move with
different velocities (Busfar)?
I believe that the two forces, namely
slab pull and slab suction that causes plates to move at subduction zones
are responsible for plates moving at different velocities. At subduction
zones plates are being subducted faster and thus the plates move with
relatively larger velocities. Whereas at divergent boundaries, the slab
pull effect is minimal because it is remote and thus we observe smaller
values for the plate velocities.
Don’t you think that the results
obtained for Arabia is less accurate, and this less reliable than other
regions in this study (Busfar)?
Due to the fact that
Arabia is an area on minimal seismic activity relative to the region, it
is not as widely covered by seismic stations and other areas such as
Turkey. Therefore I would tend to be suspicious about the results
concluded for Arabia.
Why is the DSF rate higher in comparison with
regional GPS studies (Busfar)?
This could be due to an active opening
of the gulf of Suez rift but it could also be due to miscalculation of the
DSF using the available data.
Week 8b: Low-frequency earthquakes
beneath Mt. Fuji, Japan
What do you think that why
Mt.Fuji has been chosen for this study (Akram)?
Well, what I think is
it's the eruption rate that is the most important factor in selecting
Mt.Fuji. Now it didn't erupt anything for 300 years. That's why it is
studied to understand the mechanism of recent earthquakes around it. When
it'll erupt in the future.
Can Island arc type volcanoes erupt basaltic products
Yes, at ocean-ocean convergent boundaries, magmas
originating from the partial melting of the mantle give rise to volcanic
island arc erupting mostly basaltic lavas. Magmas formed from the
ocean-continental convergence are mixtures of basalts from the mantle,
remelted felsic continental crust, and material melted from the top of
subducted plate. So in both cases, basaltic products are the
you find anything in this paper which can be helpful in the future
prediction about Mt. Fuji (Akram)?
Well according to me, yes we do, as
tomographic studies suggest a low velocity zone about 25 km below the Mt.
Fuji. And they also suggested that this magmatic upwelling is the cause of
these MLF's. In the future what i think that this magmatic material will
come to the surface as eruption.
What do you think the NIED stands for
National institute for Earth Science and Disaster
9a: Arabia-Eurasia convergence in the Zagros-Makran
What are the tectonic study and fault slip vector analyses
indicated that two tectonic regimes have occurred successively since the
Miocene within a consistent regional NE trending compression:
upper Miocene to Pliocene tectonic regime characterized by partitioned
deformation, between reverse faulting and en echelon folding.
(2) A NE
trending σ1 axis transpressional regime homogeneously affecting the region
since upper Pliocene.
What you think the
paper's main result (El-Hussien)?
study provides evidence for a wide zone of fault deformation accommodating
the oblique convergence of the Arabian-Eurasian plates and the relative
motion of Zagros and Makran wedges. This domain is made up of five major
fault zones: the Minab, Zendan, Palami, Sabzevaran and Jiroft fault zones
and the minor Kahnuj fault.
What are the hypotheses can explain the apparent faulting
regime change from EW to NE-SW σ1 (El-Hussein)?
Either the shortening direction has really changed
(i.e., as a result of regional/local rotation of stress axes), or the
rocks in which they are measuring their shortening directions have
Week 9b: GPS constrains on Africa (Nubia) and
Arabia Plate motions
you suggest that implications of more stations to record earthquake data
in Saudi Arabia will help in imaging the crustal structures? And what do
you think that the crustal model suggested by the author and described in
Table2 can be improved as it already shows a lot of velocity variations in
the crustal layers (Akram)?
Well, definitely by increasing the number of recording
stations we mean that more information we get about earthquakes. By
interpreting this valuable earthquake information, we can better image the
earth's structure. If we integrate different information obtained through
recent techniques like Receiver function studies, stress distribution,
seismic tomography etc. we can improve our results as well as make them
When do the Nyquist frequency cause aliasing while digitization?
What do you think (Akram)?
Well I think that when the Nyquist frequency fN will be
greater than the highest frequency in the function, then it'll cause the
If we consider the assumptions regarding the vertical
velocity gradient and lateral velocity variations, the solutions to
elastodynamic equation becomes easy or very difficult (Akram)?
Very difficult to solve,
if not, then impossible.
Are GPS readings alone
appropriate to determine plate motions (Busfar)?
I think other measures
should be taken into consideration when constructing a model for plate
motion. GPS readings are very accurate compared to other methods but are
have been taken since the past few decades and no GPS data is available
before that period. However, other readings such as magnetic anomalies
span a wider range of time but the down point in this particular method is
that older readings were taken using instruments that are less accurate
than the ones available nowadays and thus generally tend to be less
accurate than GPS readings.
How would we be able to construct a reliable model
for plate movement when geodetic coverage is not equally distributed over
the earth (Busfar)?
I believe that the studies conducted using the currently
available geodetic data would be much improved and even perhaps arrive at
different results and hence conclusions if geodetic data is obtained
equally over the entire earth. In other words, it doesn’t sound appealing
to draw conclusions and come up with theories when regions are covered by
millions of measurements and other parts of the world are only slightly
covered by very few limited measurements.
Week 10 a: Gravity anomalies in the larger
seismic velocities increase abruptly in the upper mantle
think Phase transition is one of the reasons for the above increase, but
there might be other reasons like the presence of cold mantle than the
surroundings can also cause an increase in velocities.
What makes the Olivine to change
into other minerals in the deep subsurface (Akram)?
Well, what I think that
temperature increase is the main reason for that but we cannot reject the
pressure variations with depth. These also cause the change of olivine
into other minerals.
What do you think the objective of this paper
me, in this paper, an attempt is made to study the lateral variations in
depths of discontinuities in the region of Tonga subduction
this paper, author used Pds. What is it (Akram)?
What I can understand is
it is used for the P to S converted phases with depth, as d in this
notation is representing the conversion depth.
What the authors used to study
the discontinuities near the Tonga subduction zone (Bulaihed)?
They used data recorded
by a joint PASSCAL land and sea deployment of temporary seismometers in
What is the objective of this study (Bulaihed)?
The objective of this
study is to investigate lateral variations in depth of the discontinuities
in the region of the subducted slab.
I agree and would
add that they wanted to study the geometry of the Tonga slab at
the authors observations of the seismic discontinuities within the region
of the Tonga slab consistent with (Bulaihed)?
They are consistent with
discontinuity deflections in the manner expected for olivine transitions
as perturbed by cool material.