The 2004 Aceh‐Andaman Earthquake: Early clay dehydration controls shallow seismic rupture

Abstract

[1] The physical state of the shallow plate-boundary fault governs the updip extent of seismic rupture during powerful subduction zone earthquakes and thus on a first order impacts on the tsunamigenic hazard of such events. During the 2004 Mw 9.2 Aceh-Andaman Earthquake seismic rupture extended unusually far seaward below the accretionary prism causing the disastrous Indian Ocean Tsunami. Here we show that the formation of a strong bulk sediment section and a high fluid-pressured pred ecollement, that likely enabled the 2004 rupture to reach the shallow plate-boundary, result from thermally controlled diagenetic processes in the upper oceanic basement and overlying sediments. Thickening of the sediment section to >2 km 160 km seaward of the subduction zone increases temperatures at the sediment basement interface and triggers mineral transformation and dehydration (e.g., smectite-illite) prior to subduction. The liberated fluids migrate into a layer that likely host high porosity and permeability and that is unique to the 2004 rupture area where they generate a distinct overpressured pred ecollement. Clay mineral transformation further supports processes of semilithification, induration of sediments, and coupled with compaction dewatering all amplified by the thick sediment section together strengthens the bulk sediments. Farther south, where the 2005 Sumatra Earthquake did not include similar shallow rupture, sediment thickness on the oceanic plate is significantly smaller. Therefore, similar diagenetic processes occur later and deeper in the subduction zone. Hence, we propose that shallow seismic rupture during the 2004 earthquake is primarily controlled by the thickness and composition of oceanic plate sediments.

Subduction zone megathrust earthquakes

“…Various diagenetic and metamorphic processes that affect the friction and effective stress in the shallow megathrust materials play important roles in defining the updip edge of seismicity (Moore and Saffer, 2001). Thick and strong sediments in the 2004 Sumatra earthquake rupture zone may have allowed very shallow rupture for that event, in contrast to the deeper slip of the 2005 Nias earthquake (Geersen et al, 2013). Large variations of subducted sediment along-strike in northern Hikurangi may affect the locations of shallow slow-slip events there (Bell et al, 2010;Eberhart-Phillips and Bannister, 2015).…”

“…Many studies have focused on the frictional stability of various clay-rich and carbonate sedi ments observed entering subduction zones to identify the conditions under which velocity weakening behavior is possible (e.g., den Hartog et al, 2012;den Hartog and Spiers, 2013;Ikari et al, 2013;Kurzawski et al, 2016). Geersen et al (2013) inferred that the clay-rich sediment composition in the 2004 Sumatra earthquake rupture zone was as important as the thickness in controlling the shallow rupture. Ikari et al (2015aIkari et al ( , 2015b demonstrated that samples of the smectite-rich fault material obtained from the shallow drilling of the 2011 Tohoku earthquake zone can deform in the laboratory at a variety of rates, from fast seismic slip to slow SSE speeds, to produce the range of slip behaviors observed in that zone.…”

Chapter 13 Anatomy of the Andaman–Nicobar subduction system from seismic reflection data

“…Such a high velocity requires a high compaction due to burial depth. Geersen et al (2013) suggested that the large thickness of the sediments also increases the temperature at the sediment -basement interface and triggers mineral transformation and dehydration (smectite to illite) prior to the subduction, increasing the updip limit of seismogenic zone, therefore allowing the 2004 Sumatra -Andaman rupture to propagate to the subduction front and hence enhancing the tsunami produced by the earthquake.…”

Controls on spatial and temporal evolution of prism faulting and relationships to plate boundary slip offshore north‐central Sumatra

“…The thickness of sediments on the incoming plate is regionally controlled by proximity to the Bengal‐Nicobar fan source and locally by oceanic basement topography. Sediment thickness at the trench varies from ~1–3 km near the Batu Segment Boundary Zone to >4 km northwest of Sumatra [ Dean et al , ; Geersen et al , ; Gulick et al , ; McNeill and Henstock , ].…”