Tedi Yudistira

Limits of the seismogenic zone in the epicentral region of the 26 December 2004 great Sumatra-Andaman earthquake: Results from seismic refraction and wide-angle reflection surveys and thermal modeling

The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated around 30 km depth and ruptured 1300 km of the Indo‐Australian–Sunda plate boundary. During the Sumatra‐OBS (ocean bottom seismometer) survey, a wide‐angle seismic profile was acquired across the epicentral region. A seismic velocity model was obtained from combined travel time tomography and forward modeling. Together with reflection seismic data from the SeaCause II cruise, the deep structure of the source region of the great earthquake is revealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, and the subducting slab can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km from the trench axis, below the fore‐arc basin. A high‐velocity zone at the lower landward limit of the ray‐covered domain, at 22 km depth, marks a shallow continental Moho, 170 km from the trench. The deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault. Assuming 100°–150°C as its updip limit, the seismogenic zone is predicted to begin 5–30 km from the trench. The downdip limit of the 2004 rupture as inferred from aftershocks is within the 350°–450°C temperature range, but this limit is 210–250 km from the trench axis and is much deeper than the fore‐arc Moho. The deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate.

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...

The preliminary results: Seismic ambient noise Rayleigh wave tomography around Merapi volcano, central Java, Indonesia

Ambient noise tomography is relatively a new method for imaging the shallow structure of the Earth subsurface. We presents the application of this method to produce a Rayleigh wave group velocity maps around the Merapi Volcano, Central Java. Rayleigh waves group velocity maps were reconstructed from the cross-correlation of ambient noise recorded by the DOMERAPI array which consists 43 broadband seismometers. In the processing stage, we first filtered the observation data to separatethe noise from the signal that dominated by the strong volcanic activities. Next, we cross-correlate the filtered data and stack to obtain the Green’s function for all possible station pairs. Then we carefully picked the peak of each Green’s function to estimate the dispersion trend and appliedMultiple Filter Technique to obtain the dispersion curve. Inter-station group velocity curvesare inverted to produceRayleigh wave group velocity maps for periods 1 to 10 s. The resulted Rayleigh group velocity maps show the interesting features around the Merapi Volcano which generally agree with the previous studies. Merapi-Lawu Anomaly (MLA) is emerged as a relatively low anomaly in our group velocity maps.

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...

The application of ambient noise tomography method at Opak River Fault region, Yogyakarta

In seismology, earth’s subsurface imaging is usually use earthquake or explosive as its main sources. Ambient noise tomography is a relatively new technique to image velocity variations from natural vibrations of Earth. In this study, ambient noise tomography method is applied to Opak River Fault region, Yogyakarta, Indonesia. The fault existence was previously assumed as the source of Yogyakarta earthquake on 27 May 2006 which make it interesting to be further investigated. This research uses temporary seismometer array managed by Geo Forschungs Zentrum (GFZ) and consists of 12 stations for three months recordings (June, 1st to August, 31st 2006). Interstation Green’s function is calculated using crosscorrelation process of two records data. Then, group velocity curve is determined by Multiple Filtering Technique (MFT). The set of interstation group velocity curve are then used to create group velocity maps as a function of periods. The obtained maps show velocity contrast variations in the research area ranging from 0.4 – 2.3 km/s. The Opak River Fault is clearly indicated by velocity contrast in the western part of the research area. There is also a velocity contrast that indicated the existence of another fault located ±11 km distance eastward of Opak River Fault. This fault is possibly associated with the source of Yogyakarta earthquake on 27 May 2006.In seismology, earth’s subsurface imaging is usually use earthquake or explosive as its main sources. Ambient noise tomography is a relatively new technique to image velocity variations from natural vibrations of Earth. In this study, ambient noise tomography method is applied to Opak River Fault region, Yogyakarta, Indonesia. The fault existence was previously assumed as the source of Yogyakarta earthquake on 27 May 2006 which make it interesting to be further investigated. This research uses temporary seismometer array managed by Geo Forschungs Zentrum (GFZ) and consists of 12 stations for three months recordings (June, 1st to August, 31st 2006). Interstation Green’s function is calculated using crosscorrelation process of two records data. Then, group velocity curve is determined by Multiple Filtering Technique (MFT). The set of interstation group velocity curve are then used to create group velocity maps as a function of periods. The obtained maps show velocity contrast variations in the research area...

Seismic microzonation of Bandung basin from microtremor horizontal-to-vertical spectral ratios (HVSR)

Bandung is located on a thick sedimentary basin, which is mainly composed of volcanic rocks and the depositional remnants of an ancient lake. The high population density and vital infrastructure, surrounding by potential sources of earthquakes make Bandung vulnerable to earthquake impact. To study the seismic response of Bandung, a microzonation study is needed so as to facilitate disaster risk assessment and mitigation. The parameters which are mapped for the purpose of microzonation are the distribution of the dominant frequency (F0), the amplification factor (Am) and seismic susceptibility (Kg). We use the HVSR analysis method with microtremor/ambient seismic noise data. The microtremor data were taken from 58 measurement points from the Bandung Seismic Experiment network run from March to October 2014 in the Bandung basin and surrounding areas. The results show that the dominant peak frequency in the studied area ranges from 0.195 Hz to 7.016 Hz, amplification (A) spans from 1.6 to 11.3, and the value of the seismic susceptibility index (Kg) ranges from 0.6 to 245.6. The spatial distribution of seismic susceptibility index indicates that almost all areas of the Bandung basin have high susceptibility to earthquake hazard. The highest susceptibility is found in the eastern part of the Bandung basin that includes Bojongsoang, Rancaekek, Ciparay, Rancasari, and Majalaya, while the lower susceptibility zones are scattered in the hills and mountains around the basin of Bandung.Bandung is located on a thick sedimentary basin, which is mainly composed of volcanic rocks and the depositional remnants of an ancient lake. The high population density and vital infrastructure, surrounding by potential sources of earthquakes make Bandung vulnerable to earthquake impact. To study the seismic response of Bandung, a microzonation study is needed so as to facilitate disaster risk assessment and mitigation. The parameters which are mapped for the purpose of microzonation are the distribution of the dominant frequency (F0), the amplification factor (Am) and seismic susceptibility (Kg). We use the HVSR analysis method with microtremor/ambient seismic noise data. The microtremor data were taken from 58 measurement points from the Bandung Seismic Experiment network run from March to October 2014 in the Bandung basin and surrounding areas. The results show that the dominant peak frequency in the studied area ranges from 0.195 Hz to 7.016 Hz, amplification (A) spans from 1.6 to 11.3, and the value...

The Preliminary Results of GMSTech: A Software Development for Microseismic Characterization

The processing of microseismic data requires reliable software for imaging the condition of subsurface related to occurring microseismicity. In general, the currently available software is only specific for certain processing module and developed by the different developer. However, the software with integrated processing modules will give a better value because the users can use it easier and faster. We developed GMSTech (Ganesha Microseismic Technology), a C# language-based standing-alone software consisting several modules for processing of microseismic data. Its function is to solve a non-linear inverse problem and imaging the subsurface. C# library is supported by ILNumerics to reduce time consumption and give good visualization. In this preliminary result, we will present four developed modules: (1) hypocenter determination, (2) moment magnitude calculation, and (3) 3D seismic tomography. In the first module, we provide four methods for locating the microseismic events that can be chosen by a user independently: simulated annealing method, guided grid-search method, Geigers method, and joint hypocenter determination (JHD). The second module can be used for calculating moment magnitude using Brune method and to estimate the released energy of the event. At last, we also provided the module of 3-D seismic tomography for imaging the velocity structures based on delay time tomography. We demonstrated the software using both a synthetic data and a real data from a certain geothermal field in Indonesia. The results for all modules are reliable and remarkable, reviewed statistically by RMS error. We will keep examining the software using another set of data and developing further modules of processing.

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.

Ambient Noise Tomography of Merapi Complex, Central Java, Indonesia: A Preliminary Result

Mt. Merapi is one of the most active and hazardous volcanoes not only in Indonesia but also in the world. Having a height of about 2968 meter above sea level it contains an active lava dome, which regularly produces pyroclastic flows and is categorized as a stratovolcano. It erupts on average every 2-5 years, in which thousands of people live on the flanks of the volcano. The last eruption occurred on 26 October 2010 and after the large eruption in 2010 the characteristic of Mt. Merapi was changed. Due to its uniqueness, Merapi is closely monitored by the many geoscientists, particularly through volcanological surveys.This study is concerned with the application of ambient noise tomography (ANT) to create Rayleigh wave group velocity maps around Mt. Merapi. The continuous data set is taken from the DOMERAPI project, which consists of temporary seismic array of 40 broadband seismometers for approximately ten months. The resulting group velocity maps of Rayleigh wave show an anomaly pattern that agrees with previous geological and geophysical study results. A pronounced, positive anomaly is clearly imaged with direction about 152°N beneath Mt. Merapi through to Mt. Merbabu. In addition, negative anomalies are observed in its east and west flanks.