Structural Geology and the Seismotectonics of the 2004 Great Sumatran Earthquake

Abstract

The paper sets out a method for structural analysis of seismotectonic data using centroid moment tensors and associated hypocenters from the Global Centroid Moment Tensor project, here illustrated for aftershocks from the 2004 great Sumatran earthquake. We show that the Sumatran segments of the megathrust were subject to compression in a direction near to orthogonal with the margin trend, consistent with the effect of relative movement of the adjacent tectonic plates. In contrast, the crust above the Andaman Sea segments was subject to margin‐orthogonal extension, consistent with motion toward the gravitational potential well accumulated due to prior lateral (westward) rollback of the subducting edge of the northward moving Indian plate. Since this potential well is largely defined by topography, this episode of margin‐orthogonal extension is at least in part “gravity driven.” It did not last long. Within 15 months, an earthquake cluster across an Andaman Sea spreading segment showed a return to kinematics driven by relative plate motion. The transition can be explained if fluid activity temporarily reduced basal friction (or effective stress) but then led to healing so that the megathrust once again began to develop friction‐locked segments. The influence of slab rollback is in developing a gravitational potential well facing the megathrust, hence drawing the overriding crust toward it in the immediate postrupture phase while the megathrust is in a weakened state. Plate tectonics dominates during interseismic gaps, once the megathrust heals, and regains frictional resistance.

Lineaments and earthquake ruptures on the East Japan megathrust

“…Typically, a high degree of kinematic coordination is observed in movements associated with particular spatial clusters of earthquakes (Lister et al, 2014;Lister and Forster, 2018). This patterning is responsible for the orientation clusters on lower hemisphere stereographic projections when slip lines and/or fault plane poles are plotted for individual spatial clusters (Fig.…”

“…Jackson, 2012;Scholz, 2014;Tsuji et al, 2013). Moreover, the implied switch in geodynamic mode represents abrupt transitions from mode-I to mode-II behavior on the megathrust, where: (i) mode-I involves the classic "push from behind" action (Scholz, 2014), and leads to crustal shortening; and (ii) mode-II requires movement on a weakened basal thrust, with motion associated with concomitant crustal extension in the overriding crust (Lister and Forster, 2018). In the latter circumstance, motion above the megathrust must be driven by gravity, at least during the aftershock sequence, and this requires the rupture to have offered negligible resistance.…”

“…Zhao et al (2011) consider structural heterogeneity of the megathrust and the 2011 earthquake mechanism. The approach adopted here is outlined in Lister et al (2014), and in Lister and Forster (2018), who applied similar methods to seismotectonic analysis of the significance of fault movements during the 2004 Great Earthquake in the Andaman Sea.…”

“…The eQuakes program allows ready analysis of the geometry of earthquakes (Lister et al, 2014;Lister and Forster, 2018) and thus allows recognition of patterns in the type of faulting taking place during an earthquake sequence. Figure 1A shows epicenters for reverse faults and thrusts (red), normal faults (light blue), and strike-slip faults (purple).…”

“…In almost all cases, the inherent geometrical ambiguity present in focal plane solutions could be resolved by bringing geology into the equation (Lister et al, 2008(Lister et al, , 2014Lister and Forster, 2018). Earthquakes that result from normal faults (Jackson, 1987) typically have a slip line that plunges at ~60°, whereas classification of the fault plane normal and associated slip line for thrust faults can assume that the fault plane dips gently.…”