The belove paper will be presented by BUSFAR who is the graduate student on February 28, 2004. The paper could be download from the site as: Lithospheric strength and its relaionship to the elastic and seismogenic layer thickness
Also, presentation form prepared by Busfar is present as online from the site:
The professional background of author for the paper of week is given in below.
The main focus of my research has been to use geological and geophysical techniques to study the Earth's crust and upper mantle beneath the world's ocean basins and their margins. Seismic reflection and refraction profile, gravity and magnetic, and swath bathymetry data acquired on research vessels have been used to determine information on the structure, tectonic evolution and sea-floor processes at oceanic islands and seamounts, aseismic ridges, deep-sea trench island-arc systems, and passive continental margins. By comparing observations of the structure of oceanic islands and passive margins to predictions of simple thermal and mechanical models, constraints have been placed on the response of the oceanic crust and upper mantle to long-term (i.e. > 1 Ma) geological loads. Results show that the oceanic crust and upper mantle is capable of supporting volcanic and sedment loads for long periods of geological time by flexing over broad regions of the ocean floor. A major part of my research has been to quantitatively understand the phenomena of flexure, how it depends on load and plate age, and how it contributes to the stratigraphic "architecture" of rift-type basins in continental margins, the growth and decay of oceanic islands, the structural styles that develop in mountain belts, and the evolution of planetary landscapes. Current research is focused on the elastic and seismogenic layer thickness and the information that these parameters provide on the mechanical properties, rheology, and strength of the Earth's lithosphere.
This message has been edited by DEPREM on May 8, 2005 12:07 AM This message has been edited by DEPREM on Feb 24, 2005 5:56 AM This message has been edited by DEPREM on Feb 21, 2005 9:54 AM This message has been edited by DEPREM on Feb 21, 2005 9:51 AM
Lithosphereic strength and its relationship to Te and Ts!
With the possibility of estimating Te and Ts at oceanic and continental lithosphere, ths 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
Questions:
1)Why Seismic activity in the oceanic lithosphere is limited to a depth range of around 15km?
Ans: 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.
2)Is there a difference in the mechanism that originates shallow and deep earthquakes?
Ans: 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.
3)Could Te and Ts follow each other and Te is always less than Ts
Ans: 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.
Q.1 what are the factors on which elastic thickness depends?
Ans. Elastic thickness depends on mineralogy, temperature and state of stress of the lithosphere.
Q.2 How can you estimate elastic thickness from the gravity data and topography?
Ans. 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.
Q.3 How'll you interpret
i) Te nearly equals Ts ii) Te >> Ts iii) Te < Ts
Ans. i) & iii) Effective strength of the lithosphere lies in the seismopgenic 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.
This message has been edited by DEPREM on Feb 26, 2005 8:28 AM
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.
Answer:
(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.
Question #2:
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?
Answer:
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.
Question #3:
How in contrary to Ts, Te reflects the integrated strength of the entire lithosphere?
Answer:
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.
Muhammed Q#1:What is the elastic thickness of the lithosphere (Tå) for oceans and continents? A#1: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. HAB: I agree. Q#2:What is the Byerlee's law of frictional brittle failure, which characterises deformation in the uppermost part of lithosphere? A#2:It suggests that strength linearly increases with pressure and depth. HAB: I agree. Q#3:How we can determine the flexural rigidity of the lithosphere ? A#3:By the brittle and ductile properties of the constitutive rocks that comprise it. HAB: I agree. Osman Question #1:
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.
Answer:
(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. HAB: I agree.
Question #2:
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?
Answer:
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. HAB: I agree.
Question #3:
How in contrary to Ts, Te reflects the integrated strength of the entire lithosphere?
Answer:
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. HAB: I agree.
Q.1 what are the factors on which elastic thickness depends?
Ans. Elastic thickness depends on mineralogy, temperature and state of stress of the lithosphere. HAB: I agree, and would add age.
Q.2 How can you estimate elastic thickness from the gravity data and topography?
Ans. 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. HAB: I agree.
Q.3 How'll you interpret
i) Te nearly equals Ts ii) Te >> Ts iii) Te < Ts
Ans. i) & iii) Effective strength of the lithosphere lies in the seismopgenic 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. HAB: I agree. Waleed With the possibility of estimating Te and Ts at oceanic and continental lithosphere, ths 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 Questions:
1) Why Seismic activity in the oceanic lithosphere is limited to a depth range of around 15km? Ans: 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. HAB: I agree.
2) Is there a difference in the mechanism that originates shallow and deep earthquakes? Ans: 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. HAB: I agree, however I don't agree with the author's conclusion that the mechanism should be different because of presence of aftershocks. In other words, just because we don't get aftershocks from deep earthquakes doesn't mean that the mechanism that generates deep earthquakes is different than that that generates shallow earthquake.
3) Could Te and Ts follow each other and Te is always less than Ts Ans: 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. I agree.
