Dr.Ali Osman Oncel

Positions Held


Awards, Fellowship, Grants 

Courses Thought

Business Address

Peer Reviewer



Week 1:  Strength of the continental lithosphere 

Assistant Professor

2004-         KFUPM

Associate Professor

2000- 2001  Istanbul University


1990-2001 Istanbul University


Week 2a: Imaging the Deep Seismic Structure Beneath a Mid-Ocean Ridge

Week 2b: Lithospheric strength in related to seismogenic layer thickness

Lithospheric strength and its relationship to Te and Ts!

Abstract by Al-Omar:
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

Why Seismic activity in the oceanic lithosphere is limited to a depth range of around 15km (Al-Omar)?

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 Ts (Al-Omar)?

 From studies 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 (Akram)?

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 admittance
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


i and iii)  Effective strength of the lithosphere lies in the seismogenic layer.
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.

Compare the 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 Te

The author implied that the Te value can not be exactly estimated. However, an idea of its extent can be figured out using different methods.

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 thermal age.

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)?

In 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 lithosphere (Bulaihed)?

It suggests 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 America?

Which are the forces you think can be responsible for driving plates (Akram)?
 Buoyancy and viscosity
2. Trench suction
3. ridge push
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 (Akram)?
It should slow down and finally come to rest.
Presenter's respond: I agree with slowing of motion and disagree with its stopping.

How would lithospheric thickness effect degree of coupling with mantle (Akram).
The thicker the lithosphere, the stronger the coupling.

How can we test the assumption that hold simple shear responsible for lithospheric deformation (Akram).

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 (3)

If the 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 suggest (Busfar)?

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 (2).

What 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 came in.

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 (Busfar)?

I 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 there.

Why the author used a technique based on angular variations of P-wave delays (Bulaihed)?

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 plane.

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 m.y. (Bulaihed)?

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 will stop.

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.

A brief 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 the deformation.

Does the orientation of minerals supports the mantle role in plate motion (Al-Omar)?

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 (Al-Omar)?

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 opposite.

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?

Authors's respond:

[Picture of GB]Dear Dr. Öncel,
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.

Best regards,

webpages: laboratoire http://www.dstu.univ-montp2.fr/
personell http://www.dstu.univ-montp2.fr/PERSO/bokelmann/index.html


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 world.

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 regimes.

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.   

What happen when the shear stress exceeds the frictional resistance on the fault or when the stress pressing the two sides of the fault (Bulaihed)?

The 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 earthquake.

Explain how come in Turkey and in southern California that even tiny stress changes can have momentous effects, both calming and catastrophic (Bulaihed)?

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)?

I 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 the earth.

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 otherwise.

What is 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 forecast (El-Hussein)?

It 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.

What is main aspect of the author’s forecast method  (El-Hussein)?

He builds the probabilities associated with earthquakes interactions on top of the renewal method by including the effects of stress changes imparted by nearby earthquakes.

Author's respond:
Ross's pictureAli, 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.

If 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 s
Ross S. Stein
U.S. Geological Survey, MS 977
345 Middlefield Road, Menlo Park, CA 94025
Tel: 1 650 329 4840    Fax: 1 650 329 5143

Papers, animations, software, teaching tools:


Week 5: Mantle, topography and rift-flank uplift of Arabia 

Do you 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 (Busfar)?

The 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.

Why did the author neglect the effect caused by heterogeneity of the mantle below 670 km depth (Busfar)?

The 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, alternative explanation.


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).

  • 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 deformation.
What the author claimed about the tilting of the Arabian platform (El-Hussein)?
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 area.

What 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 (Akram)?

As 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 uplift (Akram)?

According to thermal model, uplift can result from depth-dependent stretching or from the heating of flanks by small-scale convection.


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 Asthenosphere (Busfar)?
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 velocity.

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 (Akram)?

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 dramatically.

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 (El-Hussein)?

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 Mountains.

How the Proterozoic lithosphere of Colorado and New Mexico differs from lithosphere beneath the Archean core of the continent (El-Hussein)?

1. In thickness.
2. The strongly segmented nature.
3. Long term fertility for magnetism.
4. Its relative weakness.

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 lithosphere.

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 region.
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 (Al-Bulaihed) ?
They are controlled by Paleoproterozoic subduction zones that were active during collisions of juvenile terranes.
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 events.


Week 7: The crustal structure of the interior Arabian Platform

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 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 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 (Akram)?

Very 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 conditions.

How was the Arabian Platform divided structurally (El-Hussein)?

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.
Intrashelf depressions, found around interior homocline and interior platform.

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 8a: Continental Lithosphere of New Zealand

What are large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault shown (Bulaihed)?

Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion.

What happen for oceanic lithosphere when we have much greater creep strength of olivine than of crustal minerals (Bulaihed)?

It makes oceanic lithosphere strong in the depth range where continental lithosphere appears to be weakest.

What are the two processes that can align anisotropic minerals (Bulaihed)?

 (1) rotation of crystals in a finite strain field and (2) dynamic recrystallization.

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 (Akram)?

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 outcome.

Do 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 (Akram)?

National institute for Earth Science and Disaster prevention.


Week 9a: Arabia-Eurasia convergence in the Zagros-Makran zone

What are the tectonic study and fault slip vector analyses indicated (El-Hussein)?

Analyses indicated that two tectonic regimes have occurred successively since the Miocene within a consistent regional NE trending compression:
(1) An 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)?

The 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
rotated clockwise.


Week 9b: GPS constrains on Africa (Nubia) and Arabia Plate motions 

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 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 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 (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 earthquake zones

Week 10b: Upper mantle discontinuity structure in a Subduction Zone

Why the seismic velocities increase abruptly in the upper mantle (Akram)?

I 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 (Akram)?

To me, in this paper, an attempt is made to study the lateral variations in depths of discontinuities in the region of Tonga subduction .

In 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 1993-1995.

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.

Presenter's respond: I agree and would add that they wanted to study the geometry of the Tonga slab at depths.
What are 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.



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Dr. Ali O. Oncel