1)According to the definition of the Wadati-Benioff Zone, earthquakes can have a focus no deeper than 670km below the surface of the earth because beyond a depth of 670km, the rocks are able to deform without breaking (ductile deformation), i.e. bend or flow easily without breaking! WHY? (I thought only the asthenosphere is behaving this way!) 670km deep is far below the asthenosphere layer...why are earthquakes able to occur in the asthenosphere (about 70-250km deep) but never below a depth of 670km which is part of the rigid upper mantle?

2) The following figure is showing the San Andread Fault, with Pacific plate on the left and North American plate on the right! There are 2 small arrows pointing correctly along the fault! But I am wondering if the 2 big arrows are pointing in the wrong direction...should both arrows be pointing the other way around, ie same as the small arrows? (I remember seeing a map showing that the Pacific plate moves northwest realtive to North American plate, or does my memory serves me wrong?)
http://hk.geocities.com/vkfchanhk/earthsci3.JPG

Can somebody take a look and explain it to me, please? :)

locked

Why no earthquakes can have a focus deeper than 670km below the surface of the earth? Can someone explain please?

Earthquakes occur because the rocks are brittle--if the rock is too warm to be brittle (which happens below 670 km) no earthquakes--the rock flows, albeit slowly, as the mantle is churned by convective currents driven by heat from the core. Get the September issue of Scientific American, if you can find it. There's a whole series of articles about the latest ideas in earth science.

Hi Xylene,

But I thought that only the asthenosphere is behaving that way (ductile, flowing solid), and below the asthenosphere (below ~250km-350km), the upper rigid mantle is back to rigid again! If the earthquakes are to stop occuring, would the sites be at the asthenosphere? What is special there when you reach a depth of below 670km that stops earthquakes to happen completely?

The aesthenosphere reacts to weight being applied to it (such as icesheets being laid down on Antarctica or Greenland). In both those cases, the aesthenosphere gets out of the way. When the ice-load is removed, the soft material of the aesthenosphere moves back into the space it occupied previously, and lifts the land up slowly. Northern England and Canada are still reacting to the removal of the Laurentian and Fenno-Scandian ice-sheets about 14,000 years ago.

The mantle material, on the other hand, goes through phase transitions as you go deeper. The material gets hotter and softer as you do further down and approach the centre of the Earth. At 670 km or thereabouts the heat gets so intense that despite the enormous pressure at that depth, the rock becomes fluid enough to flow rather than fracturing.

The thing is (to continue) that the mantle is divided in two in its behaviour. The upper mantle (above 670 km) is comparitively rigid, and is therefore prone to suffer earthquakes; the entire lower mantle is being heated continuously by radioactively-induced warmth from the outer core. An enormous convective current of slowly flowing rock is constantly circulating within the lower mantle. When tectonic plates are completely consumed into the crust, they can cause the overlying continent to sink below sea-level. This happened to the North American continent millions of years ago when the now-vanished Fallaron Plate dived under the west coast of what's now the US. What remains of the plate is now being finally consumed within the mantle, about 1500 km below the east coast of the US.

"The pressure in the mesosphere (=lower mantle) is so great that even though the rock is hot, it is solid and considerably more rigid than the rock on top of it" (I checked out many web sites, and they are saying the same thing -- lower mantle has solid, rigid rocks)

The thing I don't get is...when you go below 670 km in depth, the rocks get too ductile that no breaking can occur...why?
At least, the rocks below 670 km is way more rigid that those in the asthenosphere (~70-300 km deep), why can earthquakes with a focus of 250km deep (right at the asthenosphere layer) still possibly occur when the rocks are plastic there? And why can an earthquake with a focus 800 km deep never ever occur where the rocks there are more rigid than those above?

Wait a moment...am I going on the wrong track? I was thinking about the earth's interior layer instead of the the subducted slab...I remember saying a diagram showing the Benioff Zone, when the subducted plate goes below 670 km deep, it can't subduct further, but kind of flows like fluid and sinks into the deep mantle...why??? Is it because the relatively cool subducting plate gets too hot and therefore melts, and becomes ductile rather than brittle???

Your last sentence there hit it on the spot, kingwinner; the descending crust becomes too ductile to break, and instead bends and flows.

"Wadati-Benioff (W-B) zones, the inclined bands of seismic activity generally associated with oceanic trenches, volcanic arcs, and young mountain belts, mark the positions of descending oceanic plates. At depths generally greater than about 40-55 km, seismic failure in W-B zones represents rupture occurring within slabs"
http://www.agu.org/revgeophys/kirby01/node1.html

"A Benioff zone (also Benioff-Wadati zone or Wadati-Benioff zone) is a deep active seismic area in a subduction zone. Differential motion along the zone produces deep seated earthquakes. They develop beneath volcanic island arcs and continental margins above active subduction zones. The deep earthquakes along the zone allow seismologists to map the three dimensional surface of a subducting slab of oceanic crust and mantle. The term was named for the two seismologists, Hugo Benioff of the California Institute of Technology, and Kiyoo Wadati of the Central Meteorological Observatory of Japan who independently discovered the zones." source: Wikipedia

"Benioff Zone: an interaction of a downgoing oceanic crustal plate with a continental plate. These earthquakes can be produced by slip along the subduction thrust fault or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle. Also known as the Wadati-Benioff zone."
also has a diagram of it: http://earthquake.usgs.gov/image_glossary/benioff.html

"At first glance it may seem paradoxical that in a dynamic system dominated by plate convergence, this convergence does not control structural style. Early hypotheses on subduction zones predicted abundant and obvious compressional features, and the fact that there is little evidence for compression should be enough to discredit the hypothesis of plate subduction and hence plate tectonics. Even the convergent features of the lower continental or arc slope could be the lower portions of slumps, as suggested above by McNeil....The earthquakes that define the Wadati-Benioff zone are pointed to as proof that one plate is sliding below an upper plate. However, there are many complications with this simple interpretation. The Wadati-Benioff zone comes in many patterns that change along strike. It can even be regionally horizontal, as in the Peru-Chile subduction zone. Seismic gaps commonly occur in the Wadati-Benioff zone near the trench and at intermediate depths, which seems anomalous, especially at shallow depths where the frictional force should be substantial. Wadati-Benioff zones can be vertical, like the Mariana subduction zone. Most interesting is that at intermediate levels the underthrust shearing of one plate past another does not seem to occur; it is more the opposite with a tendency for tension. And most anomalous is that the direction of earthquake fault motion is usually not in the plane of the Wadati-Benioff zone but at an oblique angle to it. Furthermore, some deep quakes occur relatively distant from the Wadati-Benioff zone. Double seismic planes are a further complication. There is still a theoretical problem understanding how deep quakes can occur at all when the mantle should be ductile below about 100 km. All these complications point to another mechanism. Vertical tectonics in which the island arc represents upwelling of hot mantle, while the trench and Wadati-Benioff zone represent a response to this upwelling, seems to fit most of the observations, including heat flow data and gravity anomalies."
http://www.answersingenesis.org/tj/v16/i1/plate_tectonics2.asp