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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.

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?

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,
Goetz

webpages: laboratoire http://www.dstu.univ-montp2.fr/
personell http://www.dstu.univ-montp2.fr/PERSO/bokelmann/index.html
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Week 4: Earthquake Conversation

A brief abstract is given by Al-Omar as:

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:

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.

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.

Best,

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:
http://quake.usgs.gov/~ross

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

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.

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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 1: Strength of the continental lithosphere

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

Week 3: Which forces drive North America?

Week 4: Earthquake Conversation

A brief abstract is given by Al-Omar as:

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

Week 6: Evolution of the Lithosphere Beneath the Rocky Mountains

Week 7: The crustal structure of the interior Arabian Platform

Don’t you think that the results obtained for Arabia is less accurate, and this less reliable than other regions in this study (Busfar)?

Week 8a: Continental Lithosphere of New Zealand

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

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

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Week 10 a: Gravity anomalies in the larger earthquake zones

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