Nanang T. Puspito

Mantle structure and seismotectonics of the Sunda and Banda arcs

Abstract We have examined the mantle structure and seismotectonic features of the Sunda and Banda arcs, Indonesia, based on the P-wave tomographic images, focal mechanism solutions, gravity anomaly and heat-flow data. On the basis of slab morphology and seismicity, we can divide the arcs into three parts, the Western Sunda, Eastern Sunda, and Banda arc. The slab-like tomographic image penetrates to a depth of about 500 km below the Western Sunda arc where seismicity does not exceed a depth of 250 km. In the Eastern Sunda arc, where a seismic gap exists between 300 and 500 km depths, the slab appears to be continuous and to penetrate into the lower mantle. Beneath the Banda arc, with seismicity down to a depth of about 650 km, the slab dips gently and does not appear to penetrate into the lower mantle. The positive gravity anomaly shows a systematic pattern, namely, the anomaly along the Eastern Sunda arc is larger than that in the Western Sunda and the Banda arcs. Along the back-arc side of the Sunda and Banda arcs, the heat flow decreases from the west to the east. Seismic strain release from the shallow earthquakes calculated from the CMT solutions show the strain axes to be oblique to the structural trends. The CMT solutions show that the Eastern Sunda arc is characterized by normal earthquakes along the trench and back-arc thrusting earthquakes north of the volcanic line. In the Western Sunda and the Eastern Sunda arcs, earthquakes of the down-dip extension type dominate the slab down to a depth of 200 km while down-dip compression earthquakes occur below 500 km depth. In the Banda arc, deep earthquakes show down-dip extension to a depth of 500 km; below this depth the state of stress is not clearly defined.

Three-dimensional P-wave velocity structure beneath the Indonesian region

Abstract We present the P-wave seismic tomography image of the mantle to a depth of 1200 km beneath the Indonesian region. The artb inversion method is applied to a dataset of 118,203 P-wave travel times of local and teleseismic events taken from ISC bulletins. Although the resolution is sufficient for detailed discussion in only a limited part of the study region, the results clarify the general tectonic framework in this region and indicate a possible remnant seismic slab in the lower mantle. Structures beneath the Philippine Islands and the Molucca Sea region are well resolved and high-velocity zones corresponding to the slabs of the Molucca Sea and Philippine Sea plates are well delineated. Seismic zones beneath the Manila, Negros and Cotabato trenches are characterized by high-velocity anomalies, although shallow structures were not resolved. The Molucca Sea collision zone and volcanic zones of the Sangihe and Philippine arcs are dominated by low-velocity anomalies. The Philippine Sea slab subducts beneath the Philippine Islands at least to a depth of 200 km and may reach depths of 450 km. The southern end of the slab extends at least to about 6°N near southern Mindanao. In the south, the two opposing subducting slabs of the Molucca Sea plate are clearly defined by the two opposing high-velocity zones. The eastward dipping slab can be traced about 400 km beneath the Halmahera arc and may extend as far north as about 5°N. Unfortunately, resolution is not sufficient to reveal detailed structures at the boundary region between the Halmahera and Philippine Sea slabs. The westward dipping slab may subduct to the lower mantle although its extent at depth is not well resolved. This slab trends N-S from about 10°N in the Philippine Islands to northern Sulawesi. A NE-SW-trending high-velocity zone is found in the lower mantle beneath the Molucca Sea region. This high-velocity zone may represent a remnant of the former subduction zone which formed the Sulawesi arc during the Miocene. The blocks along the Sunda and Banda arcs are less well resolved than those in the Philippine Islands and the Molucca Sea region. Nevertheless, overall structures can be inferred. The bowl-shaped distribution of the seismicity of the Banda arc is clearly defined by a horseshoe-shaped high-velocity zone. The tomographic image shows that the Indian oceanic slab subducts to a depth deeper than 300 km i.e., deeper than its seismicity, beneath Andaman Islands and Sumatra and may be discontinuous in northern Sumatra. Along southern Sumatra, Java and the islands to the east, the slab appears to be continuous and can be traced down to at least a depth of the deepest seismicity, where it appears to penetrate into the lower mantle.

Geoelectric potential difference monitoring in southern Sumatra, Indonesia — Co-seismic change—

Five geoelectric potential difference (electric field, here after) monitoring stations have been in operation since September 1997 in anarea near Liwa town, southern Sumatra, Indonesia, to examine the relationship between electric field changes and earthquakes. Short-term electric field variations were found to correspond mainly to geomagnetic activity, while long-term variation was mostly gradual shift and was clearly correlated neither precipitation nor ground water level variations. Co-seismic electric field changes ranging between 1 and 8 mV were observed for five mb > 5 earthquakes at multiple stations during September ∼ December 1997. The epicenters of the earthquakes were in the Indian Ocean within about 170 km from the monitoring sites.

Earthquake hypocenter relocation using double difference method in East Java and surrounding areas

Determination of precise hypocenter location is very important in order to provide information about subsurface fault plane and for seismic hazard analysis. In this study, we have relocated hypocenter earthquakes in Eastern part of Java and surrounding areas from local earthquake data catalog compiled by Meteorological, Climatological, and Geophysical Agency of Indonesia (MCGA) in time period 2009-2012 by using the double-difference method. The results show that after relocation processes, there are significantly changes in position and orientation of earthquake hypocenter which is correlated with the geological setting in this region. We observed indication of double seismic zone at depths of 70-120 km within the subducting slab in south of eastern part of Java region. Our results will provide useful information for advance seismological studies and seismic hazard analysis in this study.

Seismicity studies at Moluccas area based on the result of hypocenter relocation using HypoDD

The precise hypocenter was determined location using double difference method around subduction zone in Moluccas area eastern part of Indonesia. The initial hypocenter location from MCGA data catalogue of 1,945 earthquake events. Basically the principle of double-difference algorithm assumes if the distance between two earthquake hypocenter distribution is very small compared to the distance between the station to the earthquake source, the ray path can be considered close to both earthquakes. The results show the initial earthquakes with a certain depth (fix depth 10 km) relocated and can be interpreted more reliable in term of seismicity and geological setting. The relocation of the intra slab earthquakes beneath Banda Arc are also clearly observed down to depth of about 400 km. The precise relocated hypocenter will give invaluable seismicity information for other seismological and tectonic studies especially for seismic hazard analysis in this region.

Hypocenter Relocation of Earthquake Swarm in West Halmahera, North Molucca Region, Indonesia by using Double-Difference Method and 3D Seismic Velocity Structure

The earthquake swarm events sequence occurred in west Halmahera, north Molucca, Indonesia for the period of October 2015 to February 2016 as reported by Meteorological, Climatological, and Geophysical Agency (BMKG) of Indonesia. There were tenths swarm events with Magnitude larger than four in the region during the period. In this study, we used the earthquake catalog data compiled by BMKG to improve the location of swarms event in west Halmahera, north Molucca, Indonesia. We relocated 86 swarm events by applying teleseismic double-difference method and 3D seismic velocity model. The focus depth of swarm events mainly concentrated at depth of 5 to 12 km at south-east of Jailolo volcano. Our preliminary interpretation the earthquake swarms may be related to the stress change around the deep magma region of the volcano.

Preliminary result of 3-D attenuation tomography beneath Sunda Strait and western part of Java

Sunda strait lies in the transition zone of two different subduction system that is almost perpendicular to the subduction in southern Java and oblique subduction in western Sumatra. The series of major disasters such as earthquakes and volcanic eruptions is a manifestation of its tectonic setting complexity. We used selected waveform data from724 earthquakes that occurred around the Sunda strait and western part of Java from 2009 – 2015 recorded by 21 MCGA network stations to construct three dimensional image of seismic attenuation. Frequency independent attenuation operators (t*) was determined using spectral fitting method for P and S wave arrivals, respectively. The inversion was performed using simul2000 algorithm to image the lateral and vertical variations of Qp and Qs value in the Sunda Strait and western part of Java. Some interesting features such as subducting slab, mantle wedge, magma chamber under volcanos and fault zones can be imaged well.

Preliminary results of local earthquake tomography around Bali, Lombok, and Sumbawa regions

Bali, Sumbawa, and Lombok regions are located in active tectonic influence by Indo-Australia plate subducts beneath Sunda plate in southern part and local back-arc thrust in northern part the region. Some active volcanoes also lie from eastern part of Java, Bali, Lombok and Sumbawa regions. Previous studies have conducted subsurface seismic velocity imaging using regional and global earthquake data around the region. In this study, we used P-arrival time from local earthquake networks compiled by MCGA, Indonesia within time periods of 2009 up to 2013 to determine seismic velocity structure and simultaneously hypocenter adjustment by applying seismic tomography inversion method. For the tomographic inversion procedure, we started from 1-D initial velocity structure. We evaluated the resolution of tomography inversion results through checkerboard test and calculating derivative weigh sum. The preliminary results of tomography inversion show fairly clearly high seismic velocity subducting Indo-Australian and...

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...