Nugroho D. Hananto

Deformation and Slip Along the Sunda Megathrust in the Great 2005 Nias-Simeulue Earthquake

Seismic rupture produced spectacular tectonic deformation above a 400-kilometer strip of the Sunda megathrust, offshore northern Sumatra, in March 2005. Measurements from coral microatolls and Global Positioning System stations reveal trench-parallel belts of uplift up to 3 meters high on the outer-arc islands above the rupture and a 1-meter-deep subsidence trough farther from the trench. Surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward. A saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 Sumatra-Andaman rupture. The southeastern edge abuts a predominantly aseismic section of the megathrust near the equator.

Enhanced reflectivity of backthrusts in the recent great Sumatran earthquake rupture zones

In the last five years, there have been three great megathrust earthquakes in the Sumatra subduction zone, and a part of the zone is still locked. We have carried out deep seismic reflection surveys along the SW Sumatran margin and have imaged backthrusts along four profiles down to 15–20 km depth. We find that the seismic images of the backthrusts in the 2004 and 2007 great earthquake ruptured zones are brighter than those in the regions where the subduction zone is still locked. We suggest that this enhanced reflectivity is due to the increase of fluid contents along the reactivated backthrusts during or soon after the great earthquakes. If this interpretation is valid, then the backthrusts observed in the forearc basins could be used to monitor fluid movement from mantle wedge and earthquake precursors before and after an imminent great earthquake in the Sumatra locked zone.

The discovery of a conjugate system of faults in the Wharton Basin intraplate deformation zone

A conjugate system of faults (shear and reactivated fracture zones) accommodates intraplate deformation in the Wharton Basin. The deformation at well-defined, narrow plate boundaries depends on the relative plate motion, but how the deformation takes place within a distributed plate boundary zone remains a conundrum. This was confirmed by the seismological analyses of the 2012 great Wharton Basin earthquakes [moment magnitude (Mw) 8.6], which suggested the rupture of several faults at high angles to one another. Using high-resolution bathymetry and seismic reflection data, we report the discovery of new N294°E-striking shear zones, oblique to the plate fabric. These shear zones are expressed by sets of normal faults striking at N335°E, defining the direction of the principal compressional stress in the region. Also, we have imaged left-lateral strike-slip faults along reactivated N7°E-oriented oceanic fracture zones. The shear zones and the reactivated fracture zones form a conjugate system of faults, which accommodate present-day intraplate deformation in the Wharton Basin.

Crustal Structure Along Sunda-Banda Arc Transition Zone from Teleseismic Receiver Functions

We analyzed receiver function of teleseismic events recorded at twelve Indonesian-GEOFON (IA-GE) broadband stations using nonlinear Neighbourhood Algorithm (NA) inversion and H-k stacking methods to estimate crustal thickness, Vp/Vs ratios and S-wave velocity structure along Sunda-Banda arc transition zone. We observed crustal thickness of 34–37 km in Timor Island, which is consistent with the previous works. The thick crust (> 30 km) is also found beneath Sumba and Flores Islands, which might be related to the arc-continent collision causing the thickened crust. In Timor and Sumba Islands, we observed high Vp/Vs ratio (> 1.84) with low velocity zone that might be associated with the presence of mafic and ultramafic materials and fluid filled fracture zone. The high Vp/Vs ratio observed at Sumbawa and Flores volcanic Islands might be an indication of partial melt related to the upwelling of hot asthenosphere material through the subducted slab.

Crustal structure beneath two seismic stations in the Sunda-Banda arc transition zone derived from receiver function analysis

We analyzed receiver functions to estimate the crustal thickness and velocity structure beneath two stations of Geofon (GE) network in the Sunda-Banda arc transition zone. The stations are located in two different tectonic regimes: Sumbawa Island (station PLAI) and Timor Island (station SOEI) representing the oceanic and continental characters, respectively. We analyzed teleseismic events of 80 earthquakes to calculate the receiver functions using the time-domain iterative deconvolution technique. We employed 2D grid search (H-κ) algorithm based on the Moho interaction phases to estimate crustal thickness and Vp/Vs ratio. We also derived the S-wave velocity variation with depth beneath both stations by inverting the receiver functions. We obtained that beneath station PLAI the crustal thickness is about 27.8 km with Vp/Vs ratio 2.01. As station SOEI is covered by very thick low-velocity sediment causing unstable solution for the inversion, we modified the initial velocity model by adding the sediment thic...

Study on 2-D Crustal Shear Wave Splitting Tomography along The Sunda-Banda Arc Transition Zone

The Sunda-Banda Arc transition zone is an active region characterized by a change in tectonic regime from subduction of Indo-Australia oceanic lithosphere along the eastern part of Sunda Arc to collision of the Australian continental crust with islands arc in the western part of the Banda Arc. This complicated tectonic setting causes this area is an ideal place to study the crustal deformation along the plate boundary. The density contrast between the Australian continental crust and Indo-Australia oceanic crust in the transition zone may cause large stresses around the boundary between them. These plate boundary forces may control the distribution pattern of the deformation in the subduction to collision transition zone. The geometry of this deformation can be investigated using shear wave splitting (seismic anisotropy) study. We conduct shear wave splitting measurements from local earthquakes recorded at 17 broadband seismic stations around the Sunda-Banda arc transition zone. The 2D delay time tomography is then applied to determine the first order approximation of lateral varying anisotropic layers due to the local effect of geological structures. We observe strong anisotropy regions which coincide with the geological features as possible causes of anisotropy in the Sunda-Banda Arc transition zone. For instance, the high anisotropy zone found in Timor Island can be related to the alignment of metamorphic and igneous rocks, whereas the high anisotropy area around Sumba Island might correspond to the interaction of Sumba basement with the Australian margin increasing the frictional strength at the plate boundary.

A study on crustal shear wave splitting in the western part of the Banda arc-continent collision

We analyzed shear wave splitting parameters from local shallow (< 30 km) earthquakes recorded at six seismic stations in the western part of the Banda arc-continent collision. We determined fast polarization and delay time for 195 event-stations pairs calculated from good signal-to-noise ratio waveforms. We observed that there is evidence for shear wave splitting at all stations with dominant fast polarization directions oriented about NE-SW, which are parallel to the collision direction of the Australian plate. However, minor fast polarization directions are oriented around NW-SE being perpendicular to the strike of Timor through. Furthermore, the changes in fast azimuths with the earthquake-station back azimuth suggest that the crustal anisotropy in the study area is not uniform. Splitting delay times are within the range of 0.05 s to 0.8 s, with a mean value of 0.29±0.18 s. Major seismic stations exhibit a weak tendency increasing of delay times with increasing hypocentral distance suggesting the main ...