Rupture characteristics of the 2003 M_w 7.6 mid‐Indian Ocean earthquake: Implications for seismic properties of young oceanic lithosphere

Abstract

[1] Analysis of broadband seismograms from the 15 July 2003 large earthquake (M 7.6) in the central Indian Ocean reveals an unusual source 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. These two asperities were located in lithosphere with an age of 7 Ma or greater. A previous study has suggested that strike-slip earthquakes in oceanic lithosphere having much longer than expected source durations also have a slow, dissipative rupture process characterized by low radiated seismic energy (and therefore low apparent stress). We find no evidence for a slow rupture process to the 2003 earthquake. Instead, the long duration appears to be due only to nucleation close to the actively spreading Carlsberg Ridge, in lithosphere younger than 7 Ma. Younger oceanic lithosphere may be able to generate small to moderate earthquakes but be unable to sustain slip in a large event due to steady release of strain in aseismic creep events. Large strike-slip earthquakes within oceanic lithosphere may occur only in the central portions of long transform faults or in intraplate regions, rupturing energetic asperities like those that failed in the mid-Indian Ocean earthquake and leading to the observation that oceanic strike-slip earthquakes have the largest apparent stresses among the global population of shallow earthquakes.

The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near‐orthogonal, young, oceanic mantle faults

“…As with the complexity of rupture, we should not be too surprised that the event ruptured into the mantle, below the ∼12 km deep Moho: Geophysical surveys in the Wharton Basin have shown evidence of faulting that extends beneath the oceanic Moho [e.g., Bull and Scrutton , ; Carton et al , ]. Other oceanic earthquakes have also been deep. For example, the 1989 M s 8.2 Macquerie Ridge earthquake (deepest slip 12–16 km) [ Anderson and Zhang , ], the 1998 M s 8.1 Antarctic Plate (centroid depths 16–26 km) [ Henry et al , ], the 2003 M s 7.6 Mid‐Indian Ocean (large slip ≥20 km depths) [ Antolik et al , ]. and various events on the Romanche transform (centroid depths of 7–20 km) [ Abercrombie and Ekström , ] have been estimated to rupture below the Moho to various extents. Unlike continental earthquakes, slip in oceanic earthquakes is expected to rupture into the mantle based on the composition of the oceanic mantle and its thermal profile.…”

“…In a global survey Choy and Boatwright [] concluded that oceanic strike‐slip events have the highest apparent stress of all the events they studied. Antolik et al [] discuss observations that large earthquakes occurring in oceanic lithosphere have often exhibited low aftershock to main shock energy ratios, which could suggest that complete stress drop is common for these types of events.…”

Seismic Rupture on an Oceanic-Continental Plate Boundary: Strike-Slip Earthquakes along the Queen Charlotte-Fairweather Fault

“…We perform a teleseismic finite-fault inversion following the approach of Antolik et al, (2000Antolik et al, ( , 2004Antolik et al, ( , 2006. Green's functions are computed with an oceanic source structure on top of the IASP91 model for the mantle (Antolik et al, 2006), and we account for attenuation using the QL6 model (Kennett and Engdahl, 1991;Durek and Ekström, 1996). We obtain waveforms and process them as outlined in the previous section, using the strike and rake of the fault plane of our preferred point-source models.…”

Using centroid time-delays to characterize source durations and identify earthquakes with unique characteristics

“…Rupture duration measurements provided in the literature for ! ≥ 6.5 earthquakes are compared with duration estimated from our time--shift measurements (Abercrombie et al, 2003;Antolik et al, 2006;Houston et al, 1998;Lay et al, 2010a;2010b;Lay et al, 2009;Sladen, 2009a;2009b;2008a;2008b;2007a;2007b;Kikuchi, written communication, 2001;Tanioka and Ruff, 1997;Tocheport et al, 2006;Yagi, 2004;2003;2002). R o m a n c h e 1 9 9 4 S a n t a -C ru z Is .1 9 9 9…”