Geology Reference
In-Depth Information
Figure 3.6. Earthquakes along the Mid-Atlantic Ridge. Open symbols show loca-
tions (epicentres) of earthquakes, and solid symbols with arrows show the sense
of slip inferred from fault plane solutions. The fracture zones (oriented east-west)
have earthquakes mainly between segments of the ridge crest, and only rarely on
the extensions beyond ridge crests. These locations and the sense of slip on the
active segments are consistent with Wilson's transform fault hypothesis. From
Sykes [33]. Copyright by the American Geophysical Union.
a distinctive four-lobed pattern. In two opposite lobes, the waves that arrive first
are compressional. In the intervening two lobes, the 'first arrivals' are dilatational.
These waves spread through the Earth's interior in all directions. With a global
distribution of seismographs, it is possible to sample these waves with sufficient
density to reconstruct the orientation of the lobed 'radiation pattern' and the orien-
tation of its two 'nodal planes'. One of these planes corresponds to the fault plane,
and the other is perpendicular, though you can't tell which is which just from the
seismic waves. It is also possible to infer directly the orientation of stresses at
the earthquake source. The result of this determination was called a 'fault plane
solution'.
Sykes knew that some previous fault plane solutions on ridges were suggestive,
but that he could get much more reliable results from the new global seismographic
network. This he did. He found that, for earthquakes located on segments of ridge
crest, the solutions indicated normal faulting, consistent with the ridge crest being
extensional. Earthquakes located on active segments of fracture zones had one
