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From its first eruption at the end of August 2010, Mount Sinabung has been being seismically active. Thousands of micro earthquakes have occurred in the magma itself, in hydrothermal systems, and along nearby tectonic faults. The installation of a three-component seismometer network around the Sinabung volcano from October 2010 to December 2011 has encouraged more detailed study of the source characteristics of micro earthquakes. In this study, we conduct a focal mechanism analysis of volcanic tectonic (VT) activity during this period. This study delineates three kinds of faults around the volcano. First, a possible oblique-strike-slip fault that was “cut” by the volcano is confirmed. The fault parameters are a strike of 44.76°, a dip of 67.83° and a rake of 19.7°. Second, a normal-faulting zone associated with Kawar Lake is parametrized by a strike, dip and rake of 5.8°, 77.4°, and − 113.2°, respectively. Third, a clear hidden oblique strike-slip was found at the north-west part of the summit. The fault lineation is shown clearly by its hypocentre distribution and is parameterized by its strike of 213.15°, dip of 77.72°, and rake of − 20.04°.

Generally, analysis of geological structure in the geothermal field is carried out using remote sensing methods, geological mapping, and gravity analysis. These methods can provide the distribution of geological structure on the surface but cannot provide information on whether the structure is active or not. This study aims to utilize improved hypocenter location which can be used to delineate the distribution of active faults. This is useful for obtain a better interpretation of geological structures in geothermal areas. Hypocenter locations are determined by picking the arrival time of the P-wave and S-wave. This method sometimes leads to uncertainty of hypocenter location due to subjectivity of picking arrival time. The double-difference method is applied with additional input pairing events from waveform cross-correlation (WCC) to overcome the uncertainty of hypocenter location. The result shows 3 microearthquake clusters formed in the western, central, and eastern parts of the “X” Geothermal Field. The cluster formed in the western area shows that there are two orientations of conjugated hypocenter distribution with the orientation of NW-SE and NE-SW. These hypocenters reveal the presence of geological structures which provide permeability pathways for fluids rising in the western part of the geothermal field. Multiplet-clustering analysis in the middle cluster which coincides with the reservoir zone shows two different sub-clusters indicating the possibility of two different sources mechanism working in the area. By integrating all the analysis and hypocenter distribution, these methods provide a better interpretation of structure’s distribution in the “X” Geothermal Field.

DOI: 10.17014/ijog.8.1.109-117 The cross-correlations of ambient seismic noise at Sinabung Volcano were analyzed from February 2013 to February 2014. Many eruptions occurred during these periods, started on 15 September 2013. Looking at the variations in the coda of the correlations, two types of measurements can be distinguished associated to two types of changes: relative velocity variation and waveform decoherence. The drop of relative velocity variations and waveform decoherence were observed for each station pair of Sinabung one to two months before the first eruption. These changes in accordance to the deformation of the Sinabung edifice were estimated from geodetic measurements, since an analysis of baseline change between GPS stations indicated an inflation of the volcanic edifice prior to September 2013 eruption. The monitoring of relative velocity variations and decoherence provides insights into the ongoing processes in the volcanic edifice to assist in determining the level of volcanic activity.

An Mw 7.8 tsunami earthquake ruptured the shallow part of the Mentawai megathrust on 25 October 2010 and generated a large tsunami that hit the Mentawai Islands. The earthquake continued a partial rupture sequence of the 2007 Mw 8.5 and 7.9 Bengkulu earthquake and the 2008 Mw 7.2 North Pagai earthquake. The aftershock activity gives the opportunity to investigate the geometry of the subduction interface, which is critical to understanding the propagation and termination of a large earthquake. However, it is necessary to have a precise location and mechanism of the aftershock distribution to clearly image the fault geometry. In this preliminary study, we have applied the Cut-and-Paste (CAP) focal mechanism inversion method to 20 aftershocks (M ≥ 4.9) of the 2010 Mentawai earthquake. The seismic waveforms recorded by the BMKG broadband seismic network on the regional distance were used in the inversion. They successfully obtained ten focal mechanisms more than that provided in the GCMT catalog. The result exhibits a distinctly different thrust and normal faulting aftershocks activity. The thrust faulting aftershocks are mainly distributed in the downdip of the Mentawai 2010 mainshock rupture, indicating the stresses that have not been released during the mainshock. In the shallow region near the trench, the aftershocks are dominated by a normal faulting mechanism which shows the tensile stress caused by the bending of the subducting plate.

The occurrence of deep-focus earthquakes is unusual and has become one of mysterious seismic phenomena. The behavior of deep earthquake occurrence is still poorly understood. The geodynamical process of a deep earthquake must have a fundamental difference of physical processes from a shallow or intermediate earthquake. There are two main processes of deep earthquake occurrence, water infiltration into the subduction zone (dehydration embrittlement) and transformational faulting of metastable olivine. In Indonesia, the Java subduction zone is known as the deep earthquake source region. We will present preliminary results from the study of characteristic and geodynamical processes of deep earthquake events. We used catalog data from ISC-EHB and mechanism data from global CMT catalog within study area 3° – 13°S and 104,5° – 115°E. The activity of deep earthquakes in Java subduction showed 3 main characteristics i.e the seismicity decreases exponentially in 60 – 300 km depth, lack of earthquake activity due to hole structure in 330 – 500 km and increasing earthquake activity in 500-600 km depth due to transformational faulting of metastable olivine. Earthquakes’ focal mechanism most varies at shallow depth, subsequently, focal mechanism transition occurred from reverse at 60-300 km to normal at >300 km depth.

The implementation of the Seismic Quiescence Index (SQI) into seismic hazard study and analysis mapped around the off southern coast of Sumatra - West Java. The 15 years of Region Time Length (RTL) were used in this study. To construct the SQI, first of all, we divided the period of the declustered shallow earthquake catalog of observation into two parts, i.e., a period of 1978 to 1992 and 1993 to June 2006. Two rate models were then developed to estimate the probability of a given magnitude and a specified time window. The result was then used to determine the probability difference between the two periods of observation and SQI estimation. The SQI then used better to understand the pattern of seismicity changes for PSHA. To construct the model for SHA is done by developing the seismicity rate model of the period 1978 to June 2006 and then weighted by the SQI index. Furthermore, the median Peak Ground Acceleration (PGA) of 10% probability exceedance level for 50 years is estimated and mapped. By incorporating an amplification factor, the PGA value on the surface can be determined. The amplification factor is calculated using the Horizontal-Vertical Spectral Ratio (HVSR) method. This result might be useful for better understand the future PSHA and mitigation before a large earthquake.

The rupture processes of two sequentialearthquakes have been inverted from teleseismic data. The first event released a total seismic moment of 7.9×1018 Nm (Mw 6.5) and the slip distribution shows three asperities, 1.5 m at the shallowside, 0.7 m at the rightsouth-east deep side and 0.5 m atthe north-west deep side. The second event had one asperity with 1.7 m slip and released a seismic moment of 7.5×1018 Nm (Mw 6.5). In both cases, maximum slip occurred above the hypocenter which was responsible for the surface displacement pattern.

The Bandung basin is a large basin in Indonesia surrounded by mountains that are associated with faults. There is the possibility of earthquakes generated by these faults shaking populated areas in the basin. The consequences will be worse because the shaking is amplified by the sediment layer of the basin. We have estimated the amplification of SH-waves generated by the Lembang fault using Haskell’s method for multilayer models. The pattern of amplification is a decreasing value with increasing distance from the Lembang fault. This pattern is valid for low-frequency incident waves. For higher-frequency incident waves, the pattern looks more complicated. Fortunately, there are many areas with low amplification values. Hopefully, this result will help the local government in making decisions regarding construction planning in this region. Of course, the final objective is to reduce earthquake risks.

We have characterized 27 earthquake-generated tsunamis from 1991 to 2012 in Indonesia, based on source parameter analysis. This includes the focal mechanism derived by W phase inversion analysis, the ratio ( Θ ) between the seismic energy ( E ) and the seismic moment ( Mo ), the moment magnitude ( Mw ), the rupture duration ( To ) and the distance of the hypocenter to the trench. Most of the earthquakes (24 events) were tsunamigenic earthquakes with various fault types, a shallow focal depth (12 km ≤ D ≤ 77.8 km), a small to large magnitude (6.6 ≤ Mw ≤ 9.0), a low ratio of seismic energy to seismic moment (-5.8 < Θ < -4.9), a short to long rupture duration (27 s ≤ To ≤ 257 s), a small to large tsunami height (0.1 m ≤ H ≤ 50.9 m) and a short to long distance from the hypocenter to the trench (10 km < HT ≤ 230 km). Three tsunami earthquakes were characterized by a thrust fault mechanism, a very shallow depth ( D ≤ 20 km), a moderate magnitude (7.5 ≤ Mw ≤ 7.8), a very low ratio of seismic energy to seismic moment ( Θ ≤ -5.8), a long rupture duration (99 s ≤ To ≤ 135 s), a large tsunami height (7.4 m ≤ H ≤ 14 m) and a short distance from the hypocenter to the trench ( HT ≤ 20 km).

The main problem with the inversion of a low velocity medium is the application of an appropriate ray tracing method after choosing a suitable model parameterization. Block parameterization is not suitable, because it is not capable of representing the velocity model well. A large amount of blocks with a small grid size are needed to express the model well, but in that case, a ray coverage problem will be encountered. A knot-point parameterization model is better suited than a block model, because it can express the velocity model well, while the number of variables is much smaller. Ray calculation using the pseudo- bending method is not appropriate for the velocity model because of an instability problem at high velocity gradients. The crucial problem of this method involves the initial ray-path that is optimized in order to obtain the "true" ray, but does not satisfy the Fermat principle. These problems can be solved by applying the eikonal-solver method, because this can handle high-velocity gradients and does not need an initial ray path. Using a suitable model parameterization and appropriate ray tracing method, the inversion can obtain good results that fit the desired output. Applying a block model and the pseudo- bending method will not produce the desired output.

The Lembang fault considering vulnerable because of the site is near downtown Bandung. The west Lembang segment is the rural area of Cibodas. The two opinions is emerging, become normal fault or strike slip fault, based on recent earthquake (2010 onward). The geology data show the structure is observed by the outstanding topographic extended about 20 km, with volcanic rock very dominant. The seismology data is analysed for year 2011 in Mw less than 3, and classical fault plane solution is obtained. The gravity data is better acquired than 0.1 mGal, with elevation precision better than 60 cm. The upward Continuation, and second vertical derivative, is applied on data, as well as modelling 2.5D. The section is presenting in one section only, while data is getting in larger area. The seismology data show mostly for event is strike slip although in several parts having event normal. The gravity data is not ideal as graben like structure but rather locally low about 10 mGal amplitude. The combining the two methods suggested the slip zone or sag pond is more possible for the local anomaly. Besides, the Lembang fault is extended toward north rather to west or south.

Sumatera merupakan daerah dengan kondisi tektonik yang rumit dimana terjadi konvergensi lempeng India terhadap lempeng Eurasia dan terdapat beberapa sesar aktif. Untuk melihat keaktifan daerah Sumatera penulis menghitung stress drop di Sumatera dan membahas bagaimana kaitanya dengan tektonik secara regional. Stress drop diperoleh dengan menganalisis spektrum displacement observasi menggunakan metode inversi nonlinier Nelder Mead Simplex. Dari penelitian ini didapatkan bahwa stress drop pada daerah crustal segment Siulak menghasilkan nilai terbesar yaitu 39.26 Mpa dikarenakan daerah tersebut memiliki slip rate yang kecil jika dibandingkan dengan segment lainnya di Sumatera. Zona Ninety East Ridge menghasilkan nilai stress drop yang kecil 2.6 - 4.3 Mpa dikarenakan daerah ini merupakan daerah rekahan serta patahan. Daerah accretional wedge menghasilkan nilai stress drop tinggi 20.65 Mpa, merupakan lapisan sedimen tebal sehingga mampu menahan stress. Zona Forearc basin menghasilkan nilai stress drop kecil 0.49 Mpa karena merupakan lapisan sedimen yang mudah mengalami deformasi. Stress drop di Sumatera dipengaruhi oleh tipe tektonik, material batuan, serta slip rate masing-masing region di Sumatera.

2006年7月17日にインドネシアジャワ島南部で発生した津波地震は、パンガンダランでの現地調査の結果、地震の揺れは小さかったが、政府や住民の意識の低さが多大な被害をもたらしたことがわかった。津波警報システムの重要性を考える。, A tsunami earthquake (Mw 7.7) occurred south of Java on July 17, 2006. There was no ground motion damage from the earthquake but there was extensive damage and loss of life from the tsunami along the south coasts of Java, with extensive damage to wooden and unreinforced masonry buildings. There was no tsunami warning system in place, so escaping the large waves depended on the reaction of people to the weak earthquake shaking. This experience emphasizes the need for tsunami warning systems around the Indian Ocean.

A tsunami earthquake ( Mw = 7.7) occurred south of Java on 17 July 2006. The event produced relatively low levels of high-frequency radiation, and local felt reports indicated only weak shaking in Java. There was no ground motion damage from the earthquake, but there was extensive damage and loss of life from the tsunami along 250 km of the southern coasts of West Java and Central Java. An inspection of the area a few days after the earthquake showed extensive damage to wooden and unreinforced masonry buildings that were located within several hundred meters of the coast. Since there was no tsunami warning system in place, efforts to escape the large waves depended on how people reacted to the earthquake shaking, which was only weakly felt in the coastal areas. This experience emphasizes the need for adequate tsunami warning systems for the Indian Ocean region. Following the disastrous tsunamis of 26 December 2004 and 28 March 2005 in Indonesia, yet another earthquake caused a tsunami with a large number of deaths and vast property damage. The 17 July 2006 West Java earthquake ( Mw = 7.7) was located offshore near the trench of the Sunda subduction zone south of Java. The thrust earthquake produced a large tsunami along the southern coast of Java that resulted in more than 600 deaths and more than 75,000 people displaced. This event was a “tsunami earthquake,” meaning that the levels of high-frequency seismic radiation were relatively low for the size of the event. The earthquake was only weakly felt in regions where large tsunami run-ups occurred, and this was one likely cause for the large number of casualties. Several other notable earthquakes in the Java region have resulted in damaging tsunamis over the years. The Mw = 7.8 earthquake of 2 June …

The source mechanism of earthquake on July 2, 2013 was investigated by using moment tensor inversion. The result also compared by the field observation. Five waveform data of BMKG’s seismic network used to estimate the mechanism of earthquake, namely ; KCSI, MLSI, LASI, TPTI and SNSI. Main shock data taken during 200 seconds and filtered by using Butterworth band pass method from 0.03 to 0.05 Hz of frequency. Moment tensor inversion method is applied based on the point source assumption. Furthermore, the Green function calculated using the extended reflectivity method which modified by Kohketsu. The inversion result showed a strike-slip faulting, where the nodal plane strike/dip/rake (124/80.6/152.8) and minimum variance value 0.3285 at a depth of 6 km (centroid). It categorized as a shallow earthquake. Field observation indicated that the building orientated to the east. It can be related to the southwest of dip direction which has 152 degrees of slip. As conclusion, the Pressure (P) and Tension (T) axis described dominant compression is happen from the south which is caused by pressure of the Indo-Australian plate.

Summary The inversion of traveltimes from refraction experiments sometimes encounters the ray coverage problem. Irregular station distribution and ray shadow zones may lead to poor ray coverage in some regions of a complex underground structure. To overcome this problem, we propose a joint inversion of refraction and gravity data since gravity surveys are carried out densely and homogeneously. An interface separating homogeneous sediments and basement rocks is parametrized by a set of knot points with Lagrange interpolation. Densities are related to slownesses through experimental formulae. Model regularization is used to avoid oscillatory artefacts in the solution and to minimize the non-uniqueness of the solution. The validity of the formulation is verified using synthetic data for a valley-like basin with pseudo-random noise. The proposed method is successfully applied to actual data obtained in and around the Osaka Basin, Japan. To prevent solution instabilities owing to a lack of data beneath Osaka Bay, the interface under the bay is constrained to the depth interpretation of seismic reflection profiles. The result shows good agreement with the geological setting in this region.

The Bandung basin is a large basin in Indonesia surrounded by mountains that are associated with faults. There is the possibility of earthquakes generated by these faults shaking populated areas in the basin. The consequences will be worse because the shaking is amplified by the sediment layer of the basin. We have estimated the amplification of SH-waves generated by the Lembang fault using Haskell's method for multilayer models. The pattern of amplification is a decreasing value with increasing distance from the Lembang fault. This pattern is valid for low-frequency incident waves. For higher-frequency incident waves, the pattern looks more complicated. Fortunately, there are many areas with low amplification values. Hopefully, this result will help the local government in making decisions regarding construction planning in this region. Of course, the final objective is to reduce earthquake risks.

This study is to characterize the source mechanism of tsunamigenic earthquake based on seismic wave calculation. The source parameter used are the ratio (Θ) between the radiated seismic energy (E) and seismic moment (Mo), moment magnitude (MW), rupture duration (To) and focal mechanism. These determine the types of tsunamigenic earthquake and tsunami earthquake. We calculate the formula using the teleseismic wave signal processing with the initial phase of P wave with bandpass filter 0.001 Hz to 5 Hz. The amount of station is 84 broadband seismometer with far distance of 30° to 90°. The 2 June 1994 Banyuwangi earthquake with MW=7.8 and the 17 July 2006 Pangandaran earthquake with MW=7.7 include the criteria as a tsunami earthquake which distributed about ratio Θ=−6.1, long rupture duration To>100 s and high tsunami H>7 m. The 2 September 2009 Tasikmalaya earthquake with MW=7.2, Θ=−5.1 and To=27 s which characterized as a small tsunamigenic earthquake.

Java and Nusa Tenggara are the tectonically active of Sunda arc. This study discuss the rupture duration as a manifestation of the power of earthquake-generated tsunami. We use the teleseismic (30° - 90°) body waves with high-frequency energy Seismometer is from IRIS network as amount 206 broadband units. We applied the Butterworth high bandpass (1 - 2 Hz) filtered. The arrival and travel times started from wave phase of P - PP which based on Jeffrey Bullens table with TauP program. The results are that the June 2, 1994 Banyuwangi and the July 17, 2006 Pangandaran earthquakes identified as tsunami earthquakes with long rupture duration (To > 100 second), medium magnitude (7.6 50 second which depend on its magnitude. Those events are located far from the trench.

SUMMARY Some special operators and algorithms have been introduced to the finite difference (FD) traveltime calculation to overcome numerical instability related to head waves. However, further modifications are still required for head waves travelling along an irregular interface, because the interface often generates multiple arrivals of the head waves, causing another instability in the traveltime calculation. We solve this multipathing problem by defining new 3-D operators and implementing them with illumination conditions. Podvin & Lecomte operators are adopted for 2-D models. We then apply these operators to 2- or 3-D synclinal structures for checking their feasibility.

A growing number of low-frequency passive seismic surveys over proven oil and gas fields throughout the world have shown the presence of anomaly spectra (between 2–4 Hz). These signals are known as hydrocarbon microtremors, and the HyMAS (Hydrocarbon Microtremor Analysis) method has been widely used and developed by Spectraseis as a complement method for direct hydrocarbon indicators (DHI). This method, which is completely passive and does not require seismic artificial excitation sources, is useful for optimizing well placement during exploration, appraisal, and development for the oil and gas fields. We conducted several signal attributes provided for low-frequency passive seismic data that resulted in a better correlation between the presence of hydrocarbons in the subsurface and the observed spectral anomalies. In this study, we measured the attributes for 121 stations with two proven hydrocarbon production stations among them. We mapped each attribute value and interpreted the results based on spectral and polarization attributes from the proven hydrocarbon production stations. We also included the analysis of noise identification from the investigation site. Using the results, we combined each mapped attribute and made a spatial scoring map that provided the consistency level of each attribute for the DHI. This map provides a quick look to find an area of interest for further exploration which could be used for new optimized well placement during exploration.

An increasing demand for oil and gas production undoubtedly triggered innovation in exploration studies to find new hydrocarbon reservoir. Low-frequency passive seismic method named Hy MAS (Hydrocarbon Microtremor Analysis) is a new method invented and developed recently by Spectraseis which provide a quick look to find new hydrocarbon reservoir prospect area. This method based on empirical study which investigated an increasing of spectra anomaly between 2 - 4 Hz above reservoir but missing from the measurement distant from the reservoir. This method is quite promising because it has been used as another DHI (Direct Hydrocarbon Indicator) instead of active seismic survey which has some problem when applied in sensitive biomes. Another advantage is this method is completely passive and does not require seismic artificial excitation sources. In this study, by utilizing many attributes mentioned in the latest publication of this method, we try to localize new hydrocarbon prospect area outside from the proven production field. We deployed 63 stations of measurement with two of them are located above the known reservoir production site. We measured every single attribute for each data acquired from all station and mapped it spatially for better understanding and interpretation. The analysis has been made by considering noise identification from the measurement location and controlled by the attribute values from the data acquired by two stations above the reservoir. As the result, we combined each attribute analysis and mapped it in weighted-scoring map which provide the level of consistency for every single attribute calculated in each station. Finally, the new reservoir location can be suggested by the station which has a weighted-score around the values from the two production reservoir stations. We successfully identified 5 new stations which expected to have good prospect of hydrocarbon reservoir.

The 25 October 2010 Mentawai earthquake has magnitude Mw 7.7 and shallow depth (12 km). Although the local people of Sikakap - the closed island near epicenter (78.33 km) only felt low shaking (II-III Mercally Intensity), it rised tsunami 7.4 m height causing casualties 447 people died. We suggested the phenomenon is the tsunami earthquake. Therefore we calculate the source mechanism by using W-phase method to determine its parameter. The result yield good value due to reference of GCMT solution with strike 340.3°, dip 8.3°, and slip 116.9°. The cross section diagram of hypocenter show the event located on the beginning of subduction. We obtain the values of seismic radiated energy 1.76×1014 Nm, seismic moment 5.37×1027 Nm, and its ratio is Θ = −6.1. It counted as the threshold of tsunami earthquake. The rupture process has long duration 82 s. The facts proved that the 2010 Mentawai as a tsunami earthquake.

A tsunami earthquake (Mw 7.7) occurred south of Java on July 17, 2006. There was no ground motion damage from the earthquake but there was extensive damage and loss of life from the tsunami along the south coasts of Java, with extensive damage to wooden and unreinforced masonry buildings. There was no tsunami warning system in place, so escaping the large waves depended on the reaction of people to the weak earthquake shaking. This experience emphasizes the need for tsunami warning systems around the Indian Ocean.

The bedrock depth is one of the most important parameters in seismic hazard analysis. This parameter has not been identified well in Jakarta. This study was conducted to determine the bedrock depth in Jakarta based on S-wave velocity parameters. Microtremors array method was applied in this study to obtain 1D and 2D S-wave velocity profiles. The spatial autocorrelation (SPAC) method was used to estimate dispersion curves, while S-wave velocity structure was derived by genetic algorithm. Reffering to the geological condition in the study area, microtremors array measurements were conducted for two East-West lines in the northern and southern parts of Jakarta. The result of 2D construction of S-wave velocity structure shows stratigraphy cross sections that consists of four layers, where the bedrock depths in northern Jakarta can be depicted in the range from 519 to 662 m and in the southern part in the range from 353 m to 399 m. It has a positive correlation with the local geological condition i.e the sediment thickness increases to the north.