[kuo animation]Solid Earth Geophysics         Instructor: Dr. Ali O. Oncel                         Location: Building 26, Room:319 , Office hours:  Sunday and Monday.  2:00 pm-4:00pm

Title of weekly reading and presentation

Solid Earth Geophysics


Online Presentation

Feb 21 Strength of the continental lithosphere  Discussion 1


Feb 28 Imaging the Deep Seismic Structure Beneath a Mid-Ocean Ridge Discussion2


Mar 7 Which forces drive North America? Discussion3 El-Hussein
Mar 21

Earthquake Conversation    

Discussion4 Akram
Mar 28

Mantle, topography and rift-flank uplift of Arabia

Discussion5 Al-Bulaihed
 Apr 16

 Evolution of the Lithosphere Beneath the Rocky Mountains:

Discussion6 Akram
Apr 25 The crustal structure of the interior Arabian Platform Discussion7 Al-Bulaihed
May 2

 Continental Lithosphere of New Zealand   

Discussion8 Busfar
Low-frequency earthquakes beneath Mt. Fuji, Japan El-Hussein
May 9 Arabia-Eurasia convergence in the Zagros-Makran zone Discussion9 Akram
GPS constrains on Africa (Nubia) and Arabia Plate motions Al-Bulaihed
May 16 Gravity anomalies in the larger earthquake zones Discussion10 El-Hussein 
Uppermantle discontinuity structure in a Subduction Zone Busfar

Forum Discussion: Solid Earth Geophysics

syllabus | homework | links | homeu

Week 1:  Strength of the continental lithosphere 

What are the differences between the conventional view of the continental lithosphere, and the new view claimed by the author (El-Hussein)?

The conventional concept is based on assuming that the continental lithosphere is formed of a weak lower crust lying between a strong upper crust and strong uppermost mantle and the mantle is the strongest part of the lithosphere. This view was resulted from studying the depth distribution of earthquakes, combined with an extrapolation of laboratory rock mechanics experiments to geological conditions. Hence, based on this view, it was know for along time that, in most places, earthquakes on the continents are confined to the upper half of the crust. When rare earthquakes occur in the uppermost mantle in few areas, the conventional view attributes this to a strength contrast between the upper mantle and the generally aseismic lower crust.

The author proposed view for the continental lithosphere, is that the behavior of the continental lithosphere is dominated by the strength of the upper seismogenic layer. Hence, the seismogenic layer may be the only significant source of strength in the continental lithosphere, and that the upper mantle beneath the continents is relatively weak. In this view the author claimed that patterns of surface faulting on the scale of a few hundred kilometers are likely to be controlled by the anisotropic strength of crustal block and their intervening faults. In addition, transient lower-crustal flow, of the type associated with metamorphic core complexes, is likely to be controlled by the input of igneous melts and fluid into the lower crust.

Explain how the author tried to prove that the behavior of the continental lithosphere is dominated by the strength of its upper seismogenic layer (El-Hussein)?.

The author started his argument with highlighting that our current views of continental tectonics are confused by not knowing what really controls the pattern of deformation we see at surface. Author argued also with and example of earthquakes in the foreland of the Himalaya, within the underthrusting Indian shield. The depths of these earthquakes were estimated by waveform modeling or by direct identification of the surface reflection phased pP and sP. These depths were found to lie at or above the estimates of the Moho depth. Since most of these earthquakes are relatively small, with source dimensions of order 5 km, this results in uncertainties in Moho and centroid depths, and the author claimed that these depths may not be all above the Moho depth while we know that the lower crust is seismically active. The other argument that if the deeper earthquakes were at the top of a separate strong upper mantle layer, they should show extensions, not shortening, and the single thick seismogenic layer is partly responsible for the large fault areas and moments of the biggest earthquakes in the Indian shield, such as the 1897 earthquake beneath the Shillong plateau, whose fault plane ruptured between 9 and 45 km depth, and the 2001 Bhuj earthquake in Gujarat. In addition, author claimed that an alternative interpretation of the gravity could be that all or most of the elastic strength lies in the mantle, rather than the crust, and that earthquakes are an indicator of the frictional stability rather than strength and that the continental mantle could still be strong despite being aseismic.

What are the possible implications if the author view is correct (El-Hussein)?

In addition to the stated reevaluation of the conventional view, the implications will be that flexure of the Indian shield is likely to be major support of the topography in the Himalaya and southern Tibet. In addition, it will be normal that the regional patterns of active faulting at the surface were dominated by the strength of the crustal blocks and the interactions between them. If the new views proposed by the author are correct, the detailed patterns of faulting on the scales of interest to most tectonic and structural geologists (say, 100-400 km) are likely to be controlled predominantly by the strength of the crustal blocks and the faults that bound them. Nevertheless, it will be more likely that the high elevations in the region are supported by the flexure of the Indian shield, with the entire overlying region 300-400 km north of Himalayan front falling towards India, caused arc-normal slip vectors on the
thrusts and arc-parallel extension behind.

How the depths of earthquakes in India and Southern Tibet have been determined (Bulaihed)?

By waveform modeling or by direct identification of the surface reflection phases Pp and Sp.


Presenter's Respond:

I agree, yet, the paper doesn’t take about the percent error in the data. It just takes it into a fact that the data are better for modeling.  


Why the effective elastic thickness, Te, is smaller than the seismogenic thickness, Ts (Bulaihed)?

For two reasons

1) The top few kilometers, especially in sediment thick foreland basins are unlikely to contribute much to the elastic strength.

2) Te reflects the ability of the lithosphere to support loads over several million years, whereas the loading and unloading associated with the earthquakes cycle happen on a much shorter time scale, over which the lithosphere might appear to be stronger.


Where earthquakes on the continent are confined, in most places (Bulaihed)?

Earthquakes on the continents are confined to the upper half of the crust


Presenter's Respond:

I agree: the paper, however, trying to proof that the lower crust is part of the seismogenic layer and not the upper mantle.


How will you define the strength of the lithosphere (Akram)?

It can be defined as the vertical integration of the differential stress required to trigger either brittle failure or the flow failure of rocks. Failure can occur by power law creep or Dorn law creep at high temperature, low strain rate, or by frictional sliding at low temperature and high strain rate, in which case the differential stress depends of the tectonic regime.


Presenter's Respond:

In this paper differential stress was associated with wetness which was an indication of the weakness of the lower mantle. Hence, the lower the stress the weaker the layer, the unlikely it’s contribution to the strength of the continental lithosphere. Nevertheless, the paper claims that due to the weakness of the upper mantle, it isn’t part of the seismogenic layer.


What is the effect of temperature for moho on lithospheric strength (Akram)?

As any material, the strength of rocks decrease as temperature increases. The graph* shows the evolution of the integrated strength of the continental lithosphere as the temperature at the moho (TMoho) increases. This graph shows that when the temperature increases from 500 to 700ºC, the strength decreases by a factor of ~20. This suggests that for similar composition and thickness, the continental lithosphere in the Archaean was much weaker that its modern counterpart. It shows also that abobe ~700ºC the strength does not change much as the TMoho increases.The strength of the continental lithosphere depends so much on the temperature at the Moho (TMoho) that the Moho temperature can be used as a proxy for the lithospheric strength


 Presenter's Respond:

The mention of temperature was analogues to the Homologous temperature, the ratio of actual temp to melting temp. it seemed that the author didn’t put emphasis on temperature when it comes continental lithosphere. The author associated temperature effects on the oceanic lithosphere.


Why continents are more complicated than oceans. In general, continental convergence zones should have intermediate or deep focus earthquakes or not (Akram)?

Continental crust is much thicker and less dense than oceans with earthquakes and faults distributed over wide areas and not confined to the narrow plate boundaries that typify the oceans. That's why continents are more complicated than oceans.
Continental lithosphere is much less denser than the upper mantle, it is not subducted and a wadati benioff zone is not formed. As a result, continental convergence zone does not , in general, have the intermediate or dep focus earthquakes.


What is the effect of topography on crustal thickness and depth of moho (Akram)?

According to classical studies of isostacy, the higher the topography, the thicker the crust and deeper the Moho ( roots of the mountains).


Presenter's Respond:

In this paper the technique used was modeling of the flexural free-air gravity ignoring topography. Hence, an assumption was made conveying the idea that the plate is bent only by loads and couples on its end. Two theories associated with the thickness of the crust. The first one is that isostacy theory. And the second is the flat base model. I guess the author adapted the second one. The author then mad the measurements for the model to fall on a flat datum.


Which other factor than the temperature is related to the lithospheric strength (Akram)?

Water contents as the presence of its minimum amount reduces the creep strength.


Presenter's Respond:

I agree, However, remember that it’s a theoretical experiment and it should not be taken as a proof of the claim of weak upper mantle.


In the area where the Indian plate collides with the Eurasian plate, how would you explain the idea that the deeper earthquakes would have shown extension (normal faulting) and not shortening (thrust faulting) have these deep earthquakes had been at the top of a separate strong upper mantle layer (Busfar)?

 I didn’t quite understand this concept, however, the best explanation I came up with is that if these earthquakes where at the top of a separate strong upper mantle layer then due to the fact that the continental lithosphere is thicker towards the north than it is in the south, and thus, a “heavier” load is applied on the solid upper most mantle as we move towards the north, and since the fault dips towards the north, then we would expect the “heavier” part (north) to move down and the lighter part (south) to move up so that we would end up with a normal fault (extension).

How do we get normal faulting at the shallow events (about 20 km deep) (Busfar)?

I am guessing that maybe as we move away (upward) from the subduction zone we start getting normal faults as a result of the underlain reverse faults which are triggered by the deeper earthquakes.

In figure 3B, why is the gravity profile decreasing as we move to the north (Busfar)?

The subsiding Indian plate is dipping towards the north which decreases its effect on our gravity measurements as we move to the
north because it gets deeper.



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.