The science behind the science...

Conservative plate margins

In some situations, the positioning of constructive margins and the pattern of convection in the asthenosphere causes adjacent plates to move laterally (sideways) against each other. In these cases, crustal material is neither created nor destroyed. These margins are termed conservative margins, and are actually very large-scale transform (strike-slip) faults. The best example is along the San Andreas fault line on the western coast of the USA (pictured left). The San Andreas fault represents the boundary of the Pacific plate, which is moving northwards relative to the adjacent North American plate. At present, the relative movement between the plates is approximately 28mm per year.

The fault became active about 25-35 million years ago, and since then it has been estimated that there has been as much as 1500km of total lateral offset between the two plates. Only about 300km of this movement has been along the San Andreas fault itself - the rest of the movement has been accommodated along associated fault lines. The complexity of the fault zone is due to the brittle nature of the upper layers of the continental crust, and it is in this region that fault movement produces frequent shallow focus earthquakes. The major earthquakes in the upper layers of the crust are generated because movement along the fault is intermittent rather than continuous - friction locks the plates together until stress builds up to a critical level. At this point, there is a sudden release of energy as the plates unlock and move past each other, and an earthquake is generated. Plotting the location of these locked zones helps geologists to predict where the focus of the next major earthquake might be. Looking for “seismic gaps” along the fault - where movement has not occurred for a long time - can pinpoint areas where the plates are locked together but are under great strain.

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