Andri Dian Nugraha

Preliminary results of teleseismic double-difference relocation of earthquakes around Indonesian archipelago region

Indonesian archipelago region is located in active tectonic setting and high seismicity zone. During the last decade, Indonesian was experienced with destructive major earthquakes causing damage and victims. The information of precise earthquake location parameters are very important in partular for earthquake early warning to the society and for advance seismic studies. In this study, we attempted to improve hypocenter location compiled by BMKG for time periods of April, 2009 up to June, 2014 for about 22,000 earthquake events around Indonesian region. For the firts time, we applied teleseismic double-difference relocation algorithm (teletomoDD) to improve hypocenter region in Indonesia region combining regional and teleseismic stations. Hypocenter relocation was performed utilizing local, regional, and teleseismic P-wave arrival time data. Our relocation result show that travel-time RMS errors were greatly reduced compared to the BMKG catalog. Seismicity at shallower depth (less than 50 km) shows signif...

Hypocenter relocation using a fast grid search method and a 3-D seismic velocity model for the Sumatra region

Determination of earthquake hypocenter in Indonesia conducted by the Meteorological, Climatological, and Geophysical Agency (MCGA) has still used a 1-D seismic velocity model. In this research, we have applied a Fast Grid Search (FGM) method and a 3-D velocity model resulting from tomographic imaging to relocate earthquakes in the Sumatran region. The data were taken from the MCGA data catalog from 2009 to 2011 comprising of subduction zone and on land fault earthquakes with magnitude greater than 4 Mw. Our preliminary results show some significant changes in the depths of the relocated earthquakes which are in general deeper than the depths of hypocenters from the MCGA data catalog. The residual times resulting from the relocation process are smaller than those prior to the relocation. Encouraged by these results, we will continue to conduct hypocenter relocation for all events from the MCGA data catalog periodically in order to produce a new data catalog with good quality. We hope that the new data catalog will be useful for further studies.

Study of seismicity around Toba area based on relocation hypocenter result from BMKG catalogue

Toba area has complex tectonic setting attracting many earth scientists to study and understand tectonic and geological process or setting. The area is affected by oblique subduction zone, Renun Sumatran fault sub segment and some volcanoes that are near it. The earthquake catalogue provided by BMKG from April, 2009 to December, 2011 must be relocated firstly to get the precise hypocenter. We used catalogue data of P and S phase or P phase only and double-difference method to relocate the earthquakes. The results show hypocenter position enhancement that can be interpreted tectonically. The earthquakes after relocation relating to the Sumatran fault, subduction zone, volcanoes and seismic activities beneath Toba caldera can be mapped clearly. The relocated hypocenters in this study are very important to provide information for seismic hazard assessment and disaster mitigation study.

Unexpected earthquake of June 25th, 2015 in Madiun, East Java

An earthquake with magnitude 4.2 struck Madiun and its vicinity on June 25, 2015. According to Indonesian Meteorology, Climatology, and Geophysics Agency (BMKG), the earthquake occurred at 10:35:29 GMT+7 and was located in 7.73° S, 111.69 ° E, with a depth of 10 km. At least 57 houses suffered from light to medium damages. We reprocessed earthquake waveform data to obtain an accurate hypocenter location. We manually picked P- and S-waves arrival times from 12 seismic stations in the eastern part of Java. Earthquake location was determined by using Hypoellipse code that employs a single event determination method. Our inversion is able to resolve the fix-depth and shows that the earthquake occurred at 10:35:27.6 GMT+7 and was located in 7.6305° S, 111.7529 ° E with 14.81 km focus depth. Our location depicts a smaller travel time residual compared to that based on the BMKG result. Focal mechanism of the earthquake was determined by using HASH code. We used first arrival polarity of 9 seismic records with az...

Improve earthquake hypocenter using adaptive simulated annealing inversion in regional tectonic, volcano tectonic, and geothermal observation

Observation of earthquakes is routinely used widely in tectonic activity observation, and also in local scale such as volcano tectonic and geothermal activity observation. It is necessary for determining the location of precise hypocenter which the process involves finding a hypocenter location that has minimum error between the observed and the calculated travel times. When solving this nonlinear inverse problem, simulated annealing inversion method can be applied to such global optimization problems, which the convergence of its solution is independent of the initial model. In this study, we developed own program codeby applying adaptive simulated annealing inversion in Matlab environment. We applied this method to determine earthquake hypocenter using several data cases which are regional tectonic, volcano tectonic, and geothermal field. The travel times were calculated using ray tracing shooting method. We then compared its results with the results using Geiger’s method to analyze its reliability. Our...

Preliminary result of earthquake hypocenter determination using hypoellipse around western Java region

West Java region is located in high seismicity and active tectonic setting region influenced by subducting Indo-Australian plate beneath Eurasia plate. There are several on land active faults namely Cimandiri fault, Lembang fault, and Baribis fault in the region. MCGA earthquake data catalog from 2009 to 2014 shows the earthquakes were located not only around active faults but also relatively far away from the active faults in West Java region. In this study, we determined the earthquake location through re-picking of P-and S-wave arrival times recording by MCGA network. Earthquake location was determined by using Hypoellipse code that employs a single event determination method. We then relocated the events using hypocenter double-difference method. We also have been conducting focal mechanism analysis to estimate the type of fault slip. Our preliminary results show generally the epicenter location were distributed around the on land active fault which have the focus depth less than 30 km. For ongoing an...

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.

Analysis of spatiotemporal variation in b-value for the Sunda arc using high precision earthquake location

The Sunda arc is one of the most active tectonic regions, which has a complex tectonic setting due to different tectonic regimes and subduction geometry along this arc. We analyzed variation in b-value for this region in order to obtain better information regarding the state of stress in this region. For the first step, we relocated earthquake hypocenters taken from the BMKG catalog for the period 2009 – 2015 by employing a teleseismic double-difference (DD) relocation method and using a 3D velocity model. There are 10,440 earthquakes that were successfully relocated with greatly reduced residual errors. Based on its tectonic feature and earthquake distribution, we divided the study area into 8 regions, i.e. northern Sumatra, central Sumatra, southern Sumatra, Sunda strait, western Java, eastern Java, lesser Sunda islands, and Sunda-Banda transition zone. For b-value analysis we combined the BMKG catalog with the International Seismological Centre (ISC) catalog from 2006 to 2009 to obtain a longer time pe...

Preliminary result of P-wave speed tomography beneath North Sumatera region

The structure of P-wave speed beneath the North Sumatra region was determined using P-wave arrival times compiled by MCGA from time periods of January 2009 to December 2012 combining with PASSCAL data for February to May 1995. In total, there are 2,246 local earthquake events with 10,666 P-wave phases from 63 stations seismic around the study area. Ray tracing to estimate travel time from source to receiver in this study by applying pseudo-bending method while the damped LSQR method was used for the tomographic inversion. Based on assessment of ray coverage, earthquakes and stations distribution, horizontal grid nodes was set up of 30×30 km2 for inside the study area and 80×80 km2 for outside the study area. The tomographic inversion results show low Vp anomaly beneath Toba caldera complex region and around the Sumatra Fault Zones (SFZ). These features are consistent with previous study. The low Vp anomaly beneath Toba caldera complex are observed around Mt. Pusuk Bukit at depths of 5 km down to 100 km. T...

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.