The central Indian Ocean exhibits a higher level of earthquake activity than is normally associated with a region distant from conventional plate boundaries. This anomalous seismicity has been the subject of previous studies which assigned large focal depths (as deep as 40km below the seafloor), despite crustal faulting and folding observed on seismic reflection profiles. I re-examine the distribution and source parameters of teleseismically recorded earthquakes and relate them to the tectonic setting of the Central Indian Ocean Basin. Previous studies have used long-period waveform modelling, which gives unreliable focal mechanisms for small earthquakes (<I>m<SUB>b</SUB></I><SUB> </SUB><5.5), and poor resolution of depth for shallow earthquakes (<100km). Zero-phase broad-band conversions from short period and broadband waveforms are used with a detailed velocity structure derived from seismic reflection data obtained by the <I>Marion Dufresne</I> in the Central Indian Basin, to assign the more accurate focal depths (±3km). Measurements of the relative amplitudes of <I>P</I> arrivals, and surface reflections from the seafloor and sea surface, are used to constrain fault plane solutions even when <I>P</I>-wave first motion polarities cannot be determined. Unlike in other oceanic intraplate settings, the seafloor reflection is small and unclear, due to high sediment deposition in the Bengal Fan. The earthquakes have either thrust or strike-slip mechanisms and their depths are in agreement with previous studies, with no depths shallower than 22km below the seafloor in the Central Indian Ocean Basin or Wharton Basin. Earthquakes in the Bay of Bengal are shallower, possibly due to loading by Fan sediments raising lithospheric stresses beyond failure. The directions of the compressional axes of the earthquakes are in agreement with those predicted by plate models. The seismicity shows a concentration between the Afanasy Nikitin Seamount and the Ninety East Ridge. Other earthquakes are evenly distributed between 10°N and 10°S, and 80°E and 95°E. The existence of an aseismic region west of 80°E suggests that there are low strain rates in the west Central Indian Ocean Basin, associated with the proximity of the proposed Euler pole of rotation between separate Indian and Australian plates.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:642187 |
| Date | January 1998 |
| Creators | Buchanan, Sarah |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13246 |
Page generated in 0.0019 seconds