Geophysicists from Potsdam have established a mode of action that can explain the irregular distribution of strong earthquakes at the San Andreas Fault in California. As the science magazine "Nature" reports in its latest issue, the scientists examined the electrical conductivity of the rocks at great depths, which is closely related to the water content within the rocks. From the pattern of electrical conductivity and seismic¹ activity（地震活动） they were able to deduce that rock water acts as a lubricant（润滑剂） . Los Angeles moves toward San Francisco at a pace of about six centimeters per year, because the Pacific plate with Los Angeles is moving northward², parallel to the North American plate which hosts San Francisco. But this is only the average value. In some areas, movement along the fault is almost continuous, while other segments are locked until they shift abruptly³ several meters against each other releasing energy in strong earthquakes. After the San Francisco earthquake of 1906, the plates had moved by six meters.

The San Andreas Fault acts like a seam（接缝） of the Earth, ranging through the entire crust and reaching into the mantle⁴（地幔） . Geophysicists from the GFZ German Research Centre for Geosciences have succeeded in imaging this interface⁵ to great depths and to establish a connection between processes at depth and events at surface. "When examining the image of the electrical conductivity, it becomes clear that rock water from depths of the upper mantle, i.e. between 20 to 40 km, can penetrate⁶ the shallow areas of the creeping section of the fault, while these fluids are detained in other areas beneath an impermeable⁷ layer," says Dr. Oliver Ritter of the GFZ. "A sliding of the plates is supported, where fluids can rise."

These results suggest that significant differences exist in the mechanical and material properties along the fault at depth. The so-called tremor⁸（颤动） signals, for instance, appear to be linked to areas underneath⁹ the San Andreas Fault, where fluids are trapped. Tremors¹⁰ are low-frequency vibrations¹¹ that are not associated with rupture¹² processes as they are typical of normal earthquakes. These observations support the idea that fluids play an important role in the onset¹³ of earthquakes.