Abstract
Landward increase of surface velocity has been found for segments adjacent along-strike to megathrust faults after the 2003 Tokachi-oki and the 2011 Tohoku-oki earthquakes, NE Japan. A similar increase of landward velocities was reported for the segments to the north of the rupture of the 2010 Maule earthquake, Chile. We utilize available GNSS data to find such changes for six megathrust earthquakes in four subduction zones, including NE Japan, central and northern Chile, Sumatra, and Mexico to investigate their common features. Our study showed that such increase, ranging from a few mm/yr to ~1 cm/yr, appeared in adjacent segments following the 2014 Iquique (Chile), the 2007 Bengkulu (Sumatra), and the 2012 Oaxaca (Mexico) earthquakes in addition to the three cases. The region of the increased landward movements extends with spatial decay and reach the distance comparable to the along-strike fault length. On the other hand, the temporal decay of the increased velocity is not clear at present. The degree of increase seems to depend on the earthquake magnitude, and possibly scales with the average fault slip in the earthquake. This is consistent with the simple two-dimensional model proposed earlier to attribute the phenomenon to the enhanced coupling caused by accelerated subduction. However, these data are not strong enough to rule out other possibilities.
Highlights
Classical concept of the movement of forearc at convergent plate boundaries during a seismic cycle is the alternation of slow interseismic landward movement and sudden coseismic trenchward movement
A diagram indicating the postseismic enhanced coupling In Section 4, we show a set of figures as shown in Fig. 4, for each megathrust earthquake
Lubis et al (2013) calculated postseismic crustal movements of the 2007 earthquake by the viscoelastic relaxation, and the results showed insignificant movement around the MLKN station, and this support our interpretation that the velocity change of MLKN
Summary
Classical concept of the movement of forearc at convergent plate boundaries during a seismic cycle is the alternation of slow interseismic landward movement and sudden coseismic trenchward movement. Heki and Mitsui (2013) reported unexpected increase of the landward movements of forearc GNSS stations in segments adjacent alongstrike to the megathrust rupture after the 2003 Tokachi-oki earthquake (Mw 8.3), and possibly after the 2011 Tohoku-oki earthquake (Fig. 1). To explain the postseismic increase of landward velocity, Heki and Mitsui (2013) hypothesized that the coseismic stress drop modified the force balance acting on the slab around the ruptured segment and induced accelerated subduction of the oceanic plate (Anderson, 1975). We try to find common features and discuss if the compiled data support a certain model, e.g. the slab acceleration model by Heki and Mitsui (2013)
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