M7.6 earthquake Carlsberg Ridge

Felt strongly in Maldives.

The July 15, 2003, M 7.6 earthquake on the Carlsberg Ridge occurred as the result of shallow transform faulting within a mid-ocean ridge system, located in the Arabia Sea between India and Northern Africa. The ridge marks the boundary between the India and Nubia (Africa) plates. Focal mechanism solutions indicate that rupture occurred on either a left-lateral northwest-striking fault or a right-lateral northeast-striking fault; the latter is consistent with the orientation of the local plate boundary. Of the two possible fault orientations, finite-fault modeling of globally recorded seismic data is also more consistent with slip on the northeast-striking (right-lateral) fault. At the location of the earthquake, the India plate is moving to the northeast away from the Nubia plate at a rate of 33 mm/yr.

The Carlsberg Ridge is a slow-spreading ridge with rough topography and a depth that ranges from 1,700 to 4,400 m. Mid-ocean ridges are divergent plate boundaries, where two tectonic plates move apart from each other. New oceanic crust is formed as magma rises up between the two diverging plates. Active spreading ridges are offset by transform faults, where plates slide horizontally past each other neither destroying nor forming crust. This gives the plate boundary a zigzag pattern. Ocean ridges represent the longest linear uplifted features of the Earth’s surface and are marked by a belt of shallow earthquakes. Earthquakes can be caused by the release of tensional stress in the uplifted ridge or by the horizontal movement of plates along the transform faults.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip events of the size of the July 15, 2003, earthquake are typically about 140x20 km (length x width); modeling of this earthquake implies dimensions of about 100x20 km, well to the northeast of the hypocenter.

Few large earthquakes occur along mid-ocean ridges. Only three events greater than M 6 have occurred within 400 km of the July 15th event over the preceding 40 years; all were shallow M 6.0 earthquakes. Due to the remote location of these far from population centers that might be vulnerable to earthquake shaking, none have recorded damage or casualties.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)