The 2003 Mid-Indian Ocean Earthquake: An Unusual Earthquake in Oceanic Lithosphere

The 2003 Mid-Indian Ocean Earthquake: An Unusual Earthquake in Oceanic Lithosphere (Top) Isotherms (at 100° C intervals) for a simple half-space cooling model of the Indian Plate lithosphere. The horizontal axis is plotted in terms of age of the younger side of the fracture zone and distance from the Carlsberg Ridge (red). The age and depth range of the strongest asperity of the 2003 earthquake is indicated by the gray rectangle. (Bottom) Slip distribution of the 2003 Mid-Indian Ocean earthquake obtained from inversion of teleseismic body waves, plotted as distance from the Carlsberg Ridge and depth below the seafloor. The hypocenter position is shown by the star.
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We analyze the source process of the large earthquake (Mw 7.6) that occurred in the central Indian Ocean on July 15, 2003. This earthquake ruptured a fossil fracture zone within the Indian Plate. Its epicenter lies within 20 km of the Carlsberg Ridge separating the Somalian and Indian plates, but the rupture directivity was unilateral away from the spreading ridge. Although the earthquake occurred in a remote area far from land, the density and quality of stations within the current IRIS Global Seismographic Network is sufficient to resolve many details of the rupture process.
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Analysis of broadband body waves (P and SH) at 1-150 s period reveal an unusual rupture process. The source duration of longer than a minute is more than twice as long as expected from earthquake scaling relations, yet ~80% of the moment release occurred in two energetic asperities near the end of the rupture (see figure). Very little slip occurred near the hypocenter, or close to the Carlsberg Ridge. The two energetic asperities are located in lithosphere with an age of 7 Ma or greater as estimated from the spreading rate of 36 mm/yr.
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The rupture process of the 2003 earthquake is strikingly similar to the March 20, 1994, earthquake that occurred along the Romanche transform in the central Atlantic Ocean. While some studies have concluded that oceanic strike-slip earthquakes are characterized by high apparent stress (e.g., Choy and McGarr, 2002), Perez-Campos et al. (2003) analyzed several oceanic earthquakes with unusually long source durations (including the Romanche earthquake) and suggested that these long-duration events are the result of a slow slip process and low apparent stress. They concluded that the average apparent stress of oceanic strike-slip earthquakes is not significantly higher than those occurring within continents. We searched for a slow rupture component to the 2003 Mid-Indian earthquake using long-period spectra and the method of Abercrombie and Ekström (2001) and could find no evidence for slow slip.
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We suggest that the long source duration of the 2003 earthquake is due only to nucleation close to the active Carlsberg Ridge in very young lithosphere. Young oceanic lithosphere may be unable to sustain slip in a large event due to steady release of strain in aseismic creep events, and large strike-slip earthquakes may occur only in the central portions of long transforms or intraplate regions (in lithosphere with an age larger than 7 Ma). These earthquakes typically rupture energetic asperities like those which failed in the 2003 earthquake, and lead to the observation that oceanic strike-slip earthquakes have the largest apparent stresses among the global population of shallow earthquakes (Choy and McGarr, 2002).
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Abercrombie, R.E., and G. Ekström, Earthquake slip on oceanic transform faults, Nature, 410, 74-77, 2001.
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Choy, G.L., and A. McGarr, Strike-slip earthquakes in the oceanic lithosphere: Observations of exceptionally high apparent stress, Geophys. J. Int., 150, 506-523, 2002.
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Perez-Campos, X., J.J. McGuire, and G.C. Beroza, Resolution of the slow earthquake/high apparent stress paradox for oceanic transform fault earthquakes, J. Geophys. Res., 108(B9), 2444, doi: 10.0129/2002JB002312, 2003.

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