Strain deformation in the northern Indian Ocean

Abstract

Seismicity, focal mechanisms and moment tensors are used to estimate the strain deformation for earthquakes, which occurred during the last 80 yrs in the northern Indian Ocean (up to 38°S). The shear strain deformation values are 0.47 × 10 −8 near the Owen's fracture zone, 0.26 × 10 −8 in the north Carlsberg Ridge, 0.51 × 10 −8 in the south Carlsberg Ridge, 4.92 × 10 −8 in the Chagos Laccadive Ridge, 0.21 × 10 −8 in the Rodriguez fracture zone, 0.9 × 10 −8 in the southwest Mid Indian Ocean Ridge, 0.16 × 10 −8 in the southeast Mid Indian Ocean Ridge, 0.15 × 10 −8 in the Ninety-East Ridge and 2.11 × 10 −8 in the Sunda Arc. The high shear strain obtained in the area 6°S–10°S and 65°E–72°E suggests that the earthquakes here are characterized by high creep strength (20 Kbars) of material in the vicinity of the earthquake epicentral regions. The slip rates obtained from the seismic moment are found to vary from 0.30 to 9.85 mm/yr in different parts in the northern Indian Ocean. The maximum slip rate is obtained for the Chagos Laccadive Ridge. Moment tensor values obtained in different parts of the northern Indian Ocean suggest that the E-W extension and the N-S compression are predominant. Vertical compression (crustal thinning) is observed in the western areas, whereas vertical extension (crustal thickening) occurs in eastern parts of the northern Indian Ocean. The tension and compression strain deformation values estimated in different parts of the northern Indian Ocean support the hypothesis of ocean-floor spreading and continental collision of the Indian and Eurasian plates. The principal stress directions obtained from the moment tensor of earthquakes show that the compressive stress field is acting approximately in the alignment of the ridge and that the extensional stress is perpendicular to the ridge axis. Vertical compressions are more prominent near the Owen's fracture zone, in the Chagos Laccadive Ridge and in the southwest Mid Indian Ocean Ridge, whereas vertical extension is prominent in the Sunda Arc and the Ninety-East Ridge.