Figure 3.9. Short period vertical records at six \,'WSSN stations, for the September 12, 1965 earthquake. If the second arrival is sP, the focal depth is about 16 kilometers. (A minute mark occurs during the POD record.)
seconds of the long period component could be matched by the simple three rays ~odel, this phase is unlikely to be due to near source effects. It should then be one of the free surface reflections, and its delay time should yield the focal depth. pP, the obvious candidate, implies a focal depth of about 24 km. The synthetic long period seismograms for this depth did not match well. On the other hand, if the later phase is sP, then the focal depth would be approximately 16 km, in general accord with that obtained from the synthetics. If so, then pP is presumably buried in the signal associated ,,'ith the first arrival.
This is not purely a radiation pattern effect, as pP should be sizable at several of these stations. This later phase appears with the same time delay on short period records of the two smaller earthquakes;
again suggesting similar focal depths for all events.
TECTONIC U1PLICATIO:lS
TIle Chagos Bank swarm represents an extreQely unusual phenomenon.
Its location on the steep scarp face of the Chagos Bank is not easily explained by conventional tectonic models.
One possibility is that the Chagos Bank SHarm is related to the Vema Fracture Zone, as the swarm's location is directly along the
trend of the fracture zone. This would be difficult to reconcile ,,·ith the earthquake mechanisms, as strike-slip faulting ,,"ould be the
logical result. Noreover, the fault planes , rather than paralleling the fracture zone trend, cut across it.
An alternate hypothesis would suggest that the earthquakes are related to the steep scarp on the Chagos Bank. Fisher et al.'s [1971]
model, in uhich the Cbagos Bank and Hascarene Plateau \Jere torn apart, implies that the ocean floor at the base of the scarp is much younger than the elevated area. Such a situation could lead to differential subsidence and dip slip faulting along a plane roughly parallel to the Chagos Bank. All three mechanisms shOl' faulting almost
perpendicular to the Chagos Bank.
Thus the swarm cannot be easily related to any of the bathymetric features in the area. On the other hand, i t should probably not be considered an lIintraplatell event, generated by the forces applied to the plate as a "hole. In such a case. so close to the spreading ridge, the intraplate stress axes would presumably be controlled by the
spreading direction [Forsyth and Uyeda, 1975; Richardson et a1., 1976; Fujita and Sleep, 1978] . This does not seem the case, which is
hardly surprising for earthquakes on an anomalous structure like the Chagos-Laccadive Ridge.
As these earthquakes seem inexplicable in terms of standard tectonic models, an unusual model may be tenable. Perhaps, at depths
\-lithin the lithosphere, an east-\Jest tending fracture remaining from the separation of the Chagos Bank and Hascarene Plateau is still active. Although this is not the current spreading direction, such a fracture could have formed in the early stages of the hreakup. The existence of nor~dl faulting, and swarm-type seismicity ~ay even suggest ~aterial
being intruded into such a fracture. In the absence of any surficial expression, of course, such a feature must remain hypothetical.
The seismicity and mechanisms on the Chagos-Laccadive Ridge seem quite dif ferent from those on tbe morphologically similar Ninetyeast
Ridge [Stein and Okal, 1978]. This may be an artifact of the sampling period, since historical (from 1918 on) earthquakes are knm<n on the Ninetyeast Ridge, while this study considered only recent (1963-1976) events to avoid mislocated Central Indian Ridge earthquakes. Barring this effect, i t appears that the Chagos-Laccadive Ridge is generally aseismic, except for the knot of earthquakes on the Chagos Bank; the Ninetyeast Ridge has a continuous seismic zone along a large fraction
of its length. The Ninetyeast Ridge is probably still an active tectonic feature; the Chagos-Laccadive Ridge seems dormant except for this one region on the Chagos Bank.
CO)lCLUSION
The earthquake s"arm that occurred on the Chagos-Laccadi ve Ridge between 1965 and 1970 represents the only seismicity from 1962 to 1976 on an otherwise aseismic ridge. These earthquakes represent almost pure normal
faulting on an east-west trending plane, which cannot be easily
reconciled with the present day tectonics of the area. A still-active fracture at depth, remaining from the breakup of the Chagos Bank and Nascarene Plateau, offers a possible explanation of this unusual swarm.
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