Geoscience Reference
In-Depth Information
Figure 9.41.
A schematic transform fault showing the types of earthquake which
would occur at each location. Earthquakes A and B on the Romanche and Chain
transform fault (Fig. 9.33) have the anomalous geometries illustrated here. (From
Engeln
et al
.(1986).)
quite variable; often 10-50 microearthquakes per day are recorded over regions
40-50 km long, though this has been observed to vary by up to two orders of mag-
nitude during a few months on adjacent transform faults. Earthquake swarms are
common. Figure 9.41 shows the type of earthquake which could occur along a
transform fault and at its intersections with the mid-ocean ridge. Notice in partic-
ular that anomalous fault-plane solutions could occur at the inner corner between
the ridge and the transform fault. Figure 9.33 shows how fault-plane solutions
along the equatorial Atlantic transform faults conform to this model.
Depths determined for microearthquakes and for teleseismically recorded
events are generally shallower than 7-9 km. This confirms that the faulting is
occurring above the 400-600
◦
C isotherms (half-space and cooling-plate models,
Section 7.5.2, are indistinguishable at these young ages). In 1925, a magnitude-
7.5 earthquake occurred on the eastern half of the Vema Transform; that was
the largest Atlantic transform earthquake recorded to date. The largest recorded
strike-slip earthquake, a magnitude 8.0, occurred in 1942 on the Andrew Bain
transform fault.
9.6 Subduction zones
9.6.1 Introduction
A
subduction zone
,aconvergent plate boundary, is the zone where old, cold
lithospheric plate descends (is subducted) into the Earth's mantle. The surface
expression of a subduction zone is the deep
oceanic trench
on the oceanic plate
