Seismic anisotropy in the Sumatra subduction zone

Abstract

[1] An important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting (SWS). In Sumatra, two temporary seismic networks were deployed between December 2007 and February 2009, covering the fore arc between the fore-arc islands to the back arc. We use SKS and local SWS measurements to determine the type, amount, and location of anisotropy. Local SWS measurements from the fore-arc islands exhibit trench-parallel fast directions which can be attributed to shape preferred orientation of cracks/fractures in the overriding sediments. In the Sumatran Fault region, the predominant fast direction is fault/ trench parallel, while in the back-arc region it is trench perpendicular. The trench-perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge, while the fault-parallel measurements are similar to the fault-parallel fast directions observed for two crustal events at the Sumatran Fault. This suggests that there are two layers of anisotropy: one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the Sumatran Fault. SKS splitting results show a NNW-SSE fast direction with delay times of 0.8-3.0 s. The fast directions are approximately parallel to the absolute plate motion of the subducting Indo-Australian Plate. The small delay times exhibited by the local SWS (0.05-0.45 s), in combination with the large SKS delay times, suggest that the anisotropy generating the teleseismic SWS is dominated by entrained flow in the asthenosphere below the slab.

Seismic Anisotropy Analysis Beneath Sumatra Revealed by Shear-Wave Splitting

“…However, this region shows more measurements that indicate an anisotropic layer. This result is similar to the finding of studies by Hammond et al (2010) and Collings et al (2013) which show that there is a thick anisotropic layer on overriding plate which is related to parallel fault layers. It is assumed that there is only one anisotropic layer beneath the Sumatra Backarc that is a subduction plate which is located beneath the Sumatra Backarc.…”

“…The shear-wave splitting measurement shows that the duration of the delay time is about 0.6 -1.4 s. This region shows the same result as those in the Sumatra Fault; that is a dominant polarization direction is parallel faults. This study also shows the same finding as that of by Collings et al (2013) which shows that in the region of the Sumatra Backarc, the dominant polarization direction is parallel with the Sumatra Fault. This region shows more fair duration compared to that in the Sumatra Fault showing a deformation from an extensive shift to the continental lithosphere.…”

“…This result does not suit the global observation of trench that had been conducted by Long and Silver (2008). However, this result is relevant to the result of studies conducted by Collings et al (2013) and Hammond et al (2010) which shows that the result of the share wave splitting observation conducted on the Mentawai Islands possesses a polarization direction which is parallel with the Absolute Plate Movements (APM). This finding indicates that there are anisotropic plates beneath the Mentawai Islands.…”

“…A study by Collings et al (2013) is one of the most detailed studies focusing on the subduction zone and fault in the north of Sumatra. In addition, a study by Hammond et al (2010) estimated anisotropic layers in Sumatra and Java by using a few stations.…”

Seismic Anisotropy in the Java‐Banda and Philippine Subduction Zones and its Implications for the Mantle Flow System Beneath the Sunda Plate

“…The resulting average δt is slightly smaller than the average value (1.25 s) from oceanic islands calculated by Király et al () from the Global Splitting Database (http://www.gm.univ-montp2.fr/splitting/) (Wustefeld et al, ) but which is in reasonable agreement with previous local studies, for example, 0.90 ± 0.14 s in the southernmost Indochina Peninsula (Yu et al, ), ∼1 s in the Sumatra wedge (specifically, measurements are made for the stations located above the 150‐km slab contour) (Hammond et al, ), and 0.34–1.08 s in the Banda wedge measured from local S splitting (Di Leo et al, ). The larger δt (of 1.6–2.4 s in Sumatra, Collings et al, ; ∼1.6 s in Java, Hammond et al, ; and 1.13–1.38 s in Banda, Di Leo et al, ) has been primarily attributed to subslab mantle flow, rather than the flow in the mantle wedge, and the larger δt all comes from SKS splitting measurements.…”