Stress change and effective friction coefficient along the Sumatra‐Andaman‐Sagaing fault system after the 26 December 2004 (M_w= 9.2) and the 28 March 2005 (M_w= 8.7) earthquakes

Abstract

The 2004 Aceh and 2005 Nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused large stress perturbations which significantly altered seismic activity in the Sumatra‐Andaman region. Using both detailed mapping of failure planes and various slip distributions, we calculate this stress change along the Sumatra‐Andaman‐Sagaing fault system from central Sumatra to southern Myanmar. The static Coulomb stress change ΔCFF and the observed seismic activity are in very good agreement with a Coulomb index ∼ 20% greater than the one obtained for random events. Compared to previous studies, this high Coulomb Index confirms two important issues on the use of static stress change criterion: unsuited to study near‐field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months. The calculated ΔCFF distribution suggests that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and on the Sagaing fault, while failure is encouraged along the transform Andaman zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system, and the offshore thrust faults west of Sumatra Island. The maximum value of ∼15–20 bar (1.5–2 MPa) for ΔCFF is reached in the northern part of the Sumatra fault system. This high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region. Our results are also consistent with temporal evolution of both earthquakes’ location and focal mechanism prior to and after the events. In particular, we explain the occurrence and the mechanism of seismic swarms observed in the central Andaman rift and along the west Andaman fault. Finally, our calculations reveal that the seismicity in the Andaman rift zone can only be explained if μ′ > 0.5. This result leads to two end‐member models: one with a constant and high fault friction and one with spatial variations, for which friction may depend on either the nature of the lithosphere (oceanic versus continental) or the fault type.

Stress imparted by the great 2004 Sumatra earthquake shut down transforms and activated rifts up to 400 km away in the Andaman Sea

“…Cattin et al (34) also calculated the static stress transferred to the backarc by the 2004 mainshock, finding a Coulomb stress increase on the transforms and a decrease on the rifts at 11°-15°N lat., opposite to our results. Although we use the same source (9), our interpretations of the backarc faults differ.…”

“…As a result of their fault interpretation, they infer fault friction >0.5, A B while we infer 0.2-0.4, consistent with studies of the other seafloor spreading centers. Finally, Cattin et al (34) consider 2 y of post-mainshock events (36), while we use 5-7 y of relocated events (16); the longer period permits identification of the seismicity rate drop along the transforms. The remote changes in seismicity rate have implications for seismic hazards.…”

Earthquake frequency-magnitude distribution and fractal dimension in mainland Southeast Asia

“…According to Brown and Leicester (1933), an M w -8.0 earthquake (number 2 in Figure 1) in zone C was created by the Sagaing fault zone on May 23, 1912, as were a number of other major earthquakes (Kundu and Gahalaut 2012). Zone D, the Andaman Basin off the coast of Andaman Sea, is occupied by transform fault systems consisting of the Andaman transform zone and Central Andaman rift in addition to the West Andaman and Seuliman faults (Cattin et al 2009). Tectonically, zones C, D, and E include the plate boundary between the Burma and Sunda plates (Kundu and Gahalaut 2012).…”

Co-seismic offsets due to two earthquakes (M w 6.1) along the Sumatran fault system derived from GNSS measurements

“…The GSF system has a right-lateral component that accommodates a significant amount of the strike-slip component of the oblique convergence between the Australian/Indian and Sunda plates (Fitch 1972;Sieh and Natawidjaja 2000;Genrich et al 2000). Following the 2004 Sumatra-Andaman earthquake, previous studies had expected earthquakes to be triggered on the GSF McCloskey et al 2005;Cattin et al 2009). The reasoning is based on the Coulomb failure function CFF in the northern part of the GSF system due to the 2004 Sumatra-Andaman and 2005 Nias earthquakes Cattin et al 2009).…”

“…Following the 2004 Sumatra-Andaman earthquake, previous studies had expected earthquakes to be triggered on the GSF McCloskey et al 2005;Cattin et al 2009). The reasoning is based on the Coulomb failure function CFF in the northern part of the GSF system due to the 2004 Sumatra-Andaman and 2005 Nias earthquakes Cattin et al 2009). Cattin et al (2009) suggested that CFF increases by approximately 2 MPa on the northwestern part of the Sumatran fault, resulting in a greater seismic hazard potential for this region that may trigger a fault slip in the northwestern part on the Sumatran fault in the near future.…”