Seismotectonic of a Locked Subduction Patch: The Southern Hikurangi Margin

<p>Subduction zones produce the largest earthquakes on the planet, where rupture along the plate interface can result in the release of stress over large areas, with up to tens of meters of slip extending from below the surface to the trench. The regional stress field is a primary control on the faulting process, ergo understanding the regional stress field leads to a better understanding of the current and future faulting in the area. Abundant new seismic and continuous Global Positioning System (cGPS) data in the southern North and northern South Island, New Zealand, make it possible to characterize stress and strain parameters throughout the southern Hikurangi subduction zone. Stress orientations calculated within the subducting plate, the overriding Australian plate, and due to gravitational forces reveal that stress throughout the subducting system varies across the southern North Island. Margin parallel motion is being accommodated by shear deformation west of theWairarapa fault, whereas margin perpendicular motion is being accommodated east of theWairarapa fault. Stress parameters within the double Benioff zone (DBZ) were characterized in term of two bands of seismicity. In the deep region of the DBZ, inversion the upper band of seismicity shows down-dip tension, while the lower band shows compression. Tension in the upper band and compression in the lower band is consistent with bending stresses. In the shallow region of the DBZ, the inversion of both the upper and lower bands seismicity showed tension; this is indicative of slab pull. Shear-wave splitting of stacked waveforms of local earthquakes recorded on 291 three-component stations showed an average fast azimuth of N-S to NNE-SSW, west of theWairarapa fault. A fast azimuth orientation of N-S to NNE-SSW is sub-parallel to the local major faults. This indicates that the observed anisotropy west of theWairarapa fault is structurally derived. East of the Wairarapa fault, within the Wairarapa Basin, the average fast azimuth orientation isNNW-SSE. Because the fast azimuth orientation showed no dependence on station-earthquake distance, depth, or back azimuth and is perpendicular to major local faults; it has been interpreted as being reflective of the SHmax orientation. cGPS daily solutions for long-term and inter-slow slip events (inter-SSE) time periods show distinctly differing regions of shear strain rate in the southern North Island and northern South Island. Compression and positive (clockwise) rotation in the southern North and northern South Island was observed using both datasets. Inter-SSE time periods resulted in lower magnitude strain parameters than those calculated during time periods including SSEs. These datasets shows that strain parameters change on time scales of SSEs (< 10 years).</p>

Seismotectonic of a Locked Subduction Patch: The Southern Hikurangi Margin

<p>Subduction zones produce the largest earthquakes on the planet, where rupture along the plate interface can result in the release of stress over large areas, with up to tens of meters of slip extending from below the surface to the trench. The regional stress field is a primary control on the faulting process, ergo understanding the regional stress field leads to a better understanding of the current and future faulting in the area. Abundant new seismic and continuous Global Positioning System (cGPS) data in the southern North and northern South Island, New Zealand, make it possible to characterize stress and strain parameters throughout the southern Hikurangi subduction zone. Stress orientations calculated within the subducting plate, the overriding Australian plate, and due to gravitational forces reveal that stress throughout the subducting system varies across the southern North Island. Margin parallel motion is being accommodated by shear deformation west of theWairarapa fault, whereas margin perpendicular motion is being accommodated east of theWairarapa fault. Stress parameters within the double Benioff zone (DBZ) were characterized in term of two bands of seismicity. In the deep region of the DBZ, inversion the upper band of seismicity shows down-dip tension, while the lower band shows compression. Tension in the upper band and compression in the lower band is consistent with bending stresses. In the shallow region of the DBZ, the inversion of both the upper and lower bands seismicity showed tension; this is indicative of slab pull. Shear-wave splitting of stacked waveforms of local earthquakes recorded on 291 three-component stations showed an average fast azimuth of N-S to NNE-SSW, west of theWairarapa fault. A fast azimuth orientation of N-S to NNE-SSW is sub-parallel to the local major faults. This indicates that the observed anisotropy west of theWairarapa fault is structurally derived. East of the Wairarapa fault, within the Wairarapa Basin, the average fast azimuth orientation isNNW-SSE. Because the fast azimuth orientation showed no dependence on station-earthquake distance, depth, or back azimuth and is perpendicular to major local faults; it has been interpreted as being reflective of the SHmax orientation. cGPS daily solutions for long-term and inter-slow slip events (inter-SSE) time periods show distinctly differing regions of shear strain rate in the southern North Island and northern South Island. Compression and positive (clockwise) rotation in the southern North and northern South Island was observed using both datasets. Inter-SSE time periods resulted in lower magnitude strain parameters than those calculated during time periods including SSEs. These datasets shows that strain parameters change on time scales of SSEs (< 10 years).</p>

Preliminary Model of P-Wave Tomography Beneath Central Java using FMTOMO

The tectonic setting of Java Island is mainly controlled by the collision of Indo-Australian plate subducting the Eurasian plate. The high collision activity of Eurasian and Indo-Australian plates often causes megathrust earthquakes and the rise of arc magmatism that includes volcanic eruption. This study aims to determine the tectonic pattern beneath Central Java based on P-wave tomography inversion. We used the fast-marching method as ray tracing and subspace inversion to image subsurface velocity model to a depth of 150 km. The data used in this study are catalogue events data derived from a temporary seismometer network MERAMEX installed around central Java and DOMERAPI installed surround Mt. Merapi and Mt. Merbabu. We also include events collected from the International Seismological Centre. In total, we processed 563 earthquake events to illustrate velocity structures under central Java. The checker-board model shows that good resolutions can be identified at shallow depth, including offshore south Java contributed from Ocean Bottom Seismometer data. In vertical axis, good resolution models can be expected down to a depth 150 km following rich events from the Benioff zone. Current P wave model show a distinct low velocity zone under Mt Merapi that can be seen down to a depth of 40 km, suggesting a possible separated deep magma reservoir. To the south of Mt Merapi area also shows a low-velocity band that may be related with the southern mountain arc. Additionally, the northern part of Mt. Merapi displays a band of strong low-velocity anomaly to the East and West with the anomaly in the Eastern Part seems to have a deeper extension to a depth of ~50 km. We related this anomaly with Merapi Lawu Anomaly and Kendeng basin. Our results show a similar result with the previous tomography models in this region.

Preliminary Results of Double Difference Tomography at Sunda-Banda Arc

The Sunda-Arc transition to the Banda Arc is located on the south of the Flores Island, Indonesia, where the Australian lithosphere is moving to the north direction. On-going subduction process dictates the tectonic setting though some studies also suggest a collision and obduction may occur in the past due to of plate buoyancy. This area has active seismicity with frequent large magnitude events. To better understand the tectonic system in this region, we performed double-difference tomography inversion using regional events. We obtained the data catalog from the Indonesian Agency of Meteorology, Climatology, and Geophysics ranging from 116° to 125° east longitude and -6.5° to 12.5° latitude. We collected 4312 events data, detected from 15 stations from January 2015 to December 2019. Final relocated hypocenters showed a reduced fixed-depth problem and a more clustered event, although some deep events disappear. Most events are related to the subducting Benioff zone with some clustered events in the northern area may be related to back-arc thrust. We also observed clustered events near active volcano region and reduced shallow seismicity region to the west of the Timor Island. Resolution test using the checkerboard and Derivative weigh Sum (DWS) shows that fair P wave resolution can be achieved until 300 km, although a smearing start to show at a deeper depth. However, due to lack of arrival S wave data, the resolution test suggest good resolution can only be seen until a depth of 100 km. Tomogram P and S wave models show a clear dipping subducting slab from south to North down to a 250 km. We also spot a fast velocity band near the Timor Island area that similar to the previous tomography study, interpreted as sliver forearm. We spotted a band of lower Vp, lower Vs and higher Vp/Vs at shallow depth close to the volcanic line and we interpreted this as a zone of higher temperature, that may relate to magmatic activity in this region. We also noticed a zone of low velocity and higher Vp/Vs that may relate with dehydration and partial melting. However, we feel this still uncertain due to low Vs resolution.

Initial Result of P Wave Tomography Model in Sunda-Banda Arc Transition using FMTOMO

Our study area is located near island Sumbawa, Sumba, Flores, West Timor, Indonesia and East Timor, popularly known as Sunda-Banda arc transition zone. The tectonic setting is mainly controlled by the movement of the oceanic lithosphere Indo-Australian plate subducting the Eurasian plate and Northward migration of Australian continental lithosphere into western Banda-arc in the region of Flores, Sumba and Timor island. We tried to image velocity structure beneath these regions using regional events and tomography inversion model. We collected 5 years of regional events from the Indonesian Agency of Meteorology, Climatology and Geophysics. In total, we reserved 3186 events recorded on 29 stations. For data processing, we used fast marching method as ray tracing between sources and receiver. We then employed subspace inversion as the tomography procedure to estimate the best velocity model representing the tectonic model in the region. Hypocenter data distribution is concentrated on shallow parts of the region and along the Benioff zone down to a maximum depth of 400 km. One of challenge of this study is that although events are abundance, the stations used are mostly located onshore and does not extend in the south-north direction that leads us to under determined problem in the inversion process. However, checker-board models show most our target area can be retrieved to its initial model with sign of smearing effects shown start from a depth of 50 km. After six iteration and optimized selection of damping and smoothing parameters, we observed low velocity anomaly under Bali, Lombok, Sumba, East Nusa Tenggara at shallow depth that may be related with volcanic activity. Deeper low anomaly can also be seen that may be related with partial melting process. A band of fast velocity is clearly seen that goes deepen to the north depicting subducting slabs own to a depth of 300 km. We also observed a possible of fast velocity in the northern part of our stations at shallow depth that we believe may represent the back arc thrust.

Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95MoNIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (–0.18‰ to +0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (–0.65‰ to –0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis.

Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision

The migration of arc magmatism that is a fundamental aspect of plate tectonics may reflect the complex interaction between subduction zone processes and regional tectonics. Here we report new observations on volcanic migration from northwestern Sumatra, in the westernmost Sunda arc, characterized by an oblique convergent boundary between the Indo-Australian and Eurasian plates. Our study indicates that in northwestern Sumatra, volcanism ceased at 15–10 Ma on the southern coast and reignited to form a suite of active volcanoes that erupt exclusively to the north of the trench-parallel Sumatran fault. Younger volcanic rocks from the north are markedly more enriched in K2O and other highly incompatible elements, delineating a geochemical variation over space and time similar to that in Java and reflecting an increase in the Benioff zone depth. We relate this mid-Miocene volcanic migration in northwestern Sumatra to the far-field effect of propagating extrusion tectonics driven by the India-Eurasia collision. The extrusion caused regional deformation southward through Myanmar to northwestern Sumatra and thus transformed the oblique subduction into a dextral motion–governed plate boundary. This tectonic transformation, associated with opening of the Andaman Sea, is suggested to be responsible for the volcanic migration in northwestern Sumatra.

Slow deformation event between large intraslab earthquakes at the Tonga Trench

Slow deformations associated with a subducting slab can affect quasi-static displacements and seismicity over a wide range of depths. Here, we analyse the seismotectonic activities in the Tonga subduction zone, which is the world’s most active area with regard to deep earthquakes. In our study, we combine data from global navigation satellite systems with an earthquake catalogue. We focus on the deep earthquakes that are below 400 km at the lower part of the Wadati–Benioff zone. We find that trenchward transient displacements and quiescence of deep earthquakes, in terms of background seismicity, were bounded in time by large intraslab earthquakes in 2009 and 2013. This “slow deformation event” between 2009 and 2013 may have been triggered by a distant and shallow M8.1 earthquake, which implies a slow slip event at the plate interface or a temporal acceleration of the subduction of the Pacific Plate. These findings provide new insights into the relationship between shallow and deep earthquakes in the subduction zone.

The role of the remarkable Seram-Kumawa strike-slip fault in the tectonic process of the northern Banda Arc system

&lt;p&gt;&lt;span&gt;The Banda Arc system is sited in a junction of &lt;/span&gt;convergence between the Eurasian, Indo-Australian, Philippine and Pacific plates&lt;span&gt;. It &lt;/span&gt;has a remarkable 180&amp;#176; curve in the Benioff zone&lt;span&gt;. Two fundamental ideas have been invoked to explain this significant subduction-arc orientation change: (1) bent subduction zone around the Banda Sea (Hamilton, 1979; Spakman and Hall, 2010; Hall, 2012)&lt;/span&gt;, or (2) oppositely dipping subduction zones (Cardwell and Isacks, 1978; McCaffrey, 1989), b&lt;span&gt;ut no general agreement exists as to the cause of this curvature. However, a WNW-trending strike-slip fault, i.e. Seram-Kumawa fault, is observed at the north-eastern end of the Arc, cutting through the Seram accretionary wedge, prism and trench and seems to continue on the subducting plate (Hall et al., 2017). This fault is either inactive or locked temporarily at the present day, because there are very few strike-slip events along its trend while there are many thrust earthquakes on its north and northwest side. A few essential questions remain unanswered about this fault in relation to the evolution of the Banda Arc. For instance, what is the origin of this fault, what role does it play in the tectonic processes and large earthquakes along the Banda Arc. Could this fault eventually break-up the Banda Arc? What is its tectonic implication on the evolution of other highly curved subduction-arc systems? To address these questions, we will carry out a comprehensive investigation into active tectonics and seismicity occurrence along the northeast Banda Arc using high-resolution bathymetry, 2D marine seismic profiles and earthquake data. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Reference:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;&lt;span&gt;Cardwell, R.K. and Isacks, B.L., 1978. Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions. Journal of Geophysical Research: Solid Earth, 83(B6): 2825-2838.&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Hall, R., 2012. Late Jurassic&amp;#8211;Cenozoic reconstructions of the Indonesian region and the Indian Ocean. Tectonophysics, 570: 1-41.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Hall, R., Patria, A., Adhitama, R., Pownall, J.M. and White, L.T., 2017. Seram, the Seram Trough, the Aru Trough, the Tanimbar Trough and the Weber Deep: A new look at major structures in the eastern Banda Arc.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Hamilton, W.B., 1979. Tectonics of the Indonesian region. US Govt. Print. Off.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;McCaffrey, R., 1989. Seismological constraints and speculations on Banda Arc tectonics. Netherlands Journal of Sea Research, 24(2-3): 141-152.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Spakman, W. and Hall, R., 2010. Surface deformation and slab&amp;#8211;mantle interaction during Banda arc subduction rollback. Nature Geoscience, 3(8): 562.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;