Received 15 January 2010, Revised 23 August 2010, Accepted 17 September 2010

Transcription

1 Indonesian Journal of Physics Vol 21 No. 4, October 2010 Seismic Moment Tensors of Padang and Jambi Events in Jambi using Three Component Local Waveforms: Identification of the Active Fault Plane Madlazim, Bagus Jaya Santosa and Widya Utama Physics Department, Faculty Mathematics and Science of ITS Jl. Arif Rahman Hakim I, Surabaya m_lazim@physics.its.ac.id Received 15 January 2010, Revised 23 August 2010, Accepted 17 September 2010 Abstract A research has been conducted to estimate earthquake source parameters occurred on 30/09/2009 with 7.5 moment magnitude and 01/10/2009 with 6.4 moment magnitude. The event on 30/09/2009 was occurred in the sea and triggered by a mechanism related to subduction plane, while the one in 01/10/2009 occurred in the ground and triggered by the activity of Semangko Fault. The data used to determine the parameters of both earthquake source are three component local waveforms that are recorded by three MY broadband stations which belong to IRIS/Malaysia (IPM, KOM and KUM) and IRIS/Singapore MS network broadband station (BTDF). Availability of the three component local waveforms data is an opportunity to conduct a detailed research for earthquake source parameters in this area. In this paper, we report a focal mechanism of both events using Discrete Wave Number method to calculate the Green function and deconvolution iteration method to invert the tensor moment from three component local waveforms. Both methods are integrated in ISOLA_GUI software. The data is inverted at 35 mhz until 70 mhz to obtain the earthquake source parameters. Source parameters of both earthquakes (seismic moment, moment magnitude, orientation and fault plane and slip length) were extracted after double couple (DC) value were obtained and the reduction variant of each events are 99.3% and 70% and also 95.3% and 73%, respectively. To identify the fault plane, the HC-plot method is used, and to determine the length and width of the fault plane and also the slip length, an empirical equation is implemented in the Coulomb software. From the analysis, we obtain the type of fault for both earthquakes having strike, dip and rake angles that represent the fault plane orientation for both faults which are 88, 70, 146 and 149, 49, and The length and width of the fault and the slip length of both events are 124 km; 33.8 km; right lateral = 1.3 m and reverse slip = 0.88 m and also km; km; right lateral = 0.59 m and reverse slip = m, respectively. Keywords: Fault plane, three component local waveforms and earthquake source parameters 1. Introduction Semangko Fault is an active fault that separates Sumatra Island into two parts, spreading out along Earthquake is a natural phenomena, in shape of Bukit Barisan mountain range, from Semangko Bay in natural shock from earth interior that propagates to the Sunda Strait until Aceh in north. The earthquakes that surface. There are three types of earthquake that is occur in Java and Sumatra are geodynamic implication commonly known. The first one is tectonic of an active deformation around Sunda (Java) trench 1). earthquake, that has close relation to fault formation, West Sumatra is the boundary of ocean slab as a direct consequence from slab collision. This type which consist of two faulting systems, which are of earthquake usually has a magnitude more than 5 strike-slip faulting system that rotates toward right Richter Scale. Vulcanic earthquake, is an earthquake direction (dextral) and interface dip-slip subduction that is related to volcano activity. This earthquake can which has larger influence 2). Slope convergence that be classified as micro to moderate earthquake, and point toward north direction from Indian and usually has a magnitude less than 4 Richter Scale. The Australian slabs is moving toward South East Asia third type is collapse that is caused by avalanche with the velocity of 60 mm/yr 3). Slab convergence is which is a minor earthquake. The magnitude of this divided into a slip parallel to the trench accomodated earthquake is very small that it does not felt in the by Sumatra fault and a perpendicular slip which is surface, appears as tremor and can only be detected by accomodated by subduction zone interface 2). Sumatra seismograph. Fault is a weak zone that can be easily fault has caused tens of earhquakes with a magnitude affected by tectonic earthquake. Large fault is also one 7 M 7.7, also several minor events, in the last of the earthquake source such as Semangko Fault that century. divides Sumatra Island. Subduction of India-Australia slab occurs at There are two zones where eaqrthquakes strike Sumatra slab boundary with velocity of about 60 the most in Sumatra: (1) slab subduction zone that is mm/yr directing toward N11 E. Oblique convergence located in West Sumatran ocean which has a potency partitioned into trench parallel to the slip is mostly of causing earthquake with a relatively big magnitude accomodated by Sumatra faulting zone whereas the and has a good chance of causing tsunami, (2) trench perpendicular to slip ismostly accomodated by Sumatra fault zone known as Semangko (Figure 1). 107

2 108 IJP Vol. 21 No. 4, 2010 subduction zone. More detailed map of Sumatra faulting zone (SFZ) shows that Sumatra fault consist of many segments. The influence of the fault segmentation to the dimension of seismic source shows that the dimension for future seismic events is influenced by fault geometry 2). Understanding cracks caused by an active fault is the fundamental purpose that has not been achieved in earthquake science. The main reason of the slow development is the data rareness and lack of relevant analysis of how strain accumulate on the region around fault and how does fault release that accumulated strain 4). The event on 30/09/2009 occured in the sea and triggered by subduction, while the one that happened on 01/10/2009 occured in the land and triggered by Semangko Fault. Beside subduction and fault zone, earthquake can also be triggered by the increasing of Coulomb stress caused by major earthquake 5). The event on 01/10/2009 is believed to be triggered by the increase of Coulomb stress around Semangko fault 6). Based on USGS category, it is said that the intensity of 30/09/2009 event in Padang is between IV-V MMI. The Jambi earthquake on 01/10/2009 had a destructing power more than 7.6 SR whereas earthquake that occured on 30/09/2009 was felt stronger by the people around the epicenter. USGS determine that the magnitude of this earthquake is between IV-V MMI. Hypocenter, depth and the origin time of both events on 30/09/2009 and 01/10/2009 had been reported by IRIS 9), Geofon 10), BMKG (Badan Metreologi, Klimatologi dan Geofisika) 11), USGS 12) and Harvard Global CMT 13), as shown in Table 1. Table 1. Hypocenter, M w and origin time of events 2009/09/30 and 2009/10/01 Origin Time Depth Event Lat( ) Lon( ) M Source (UTC) w (Km) IRIS 2009/09/30 10:16: /10/01 01:52: Geofon 2009/09/30 10:16: /10/01 01:52: BMKG 2009/09/30 10:16: /10/01 01:52: USGS 2009/09/30 10:16: /10/01 01:52: Harvard 2009/09/30 10:16: /10/01 1:52: There are differences in values of hypocenter, magnitude moment and origin time of earthquakes that are provided by the five seismological institutes. Only two of five institutes provide CMT, which are USGS and Harvard Global CMT. Both institutes have analyzed the CMT of both events using teleseismic data (distance between epicenter and stations > 25 ). The CMT data that is provided by both seismological institutes are also significantly different. In this paper, we present three component local waveforms analysis in which the data were recorded by three MY network stations and BTDF station, with a distance of less than 10 from the epicenter of both earthquake, to predict the parameters of both earthquake sources, and to identify the fault plane of both earthquakes and determine its length, width and also its slip length. 2. Event Locations Earthquake characteristics can be known from the earthquake source parameters. Earthquake source parameters can be obtained by analyzing earthquake data. Seismic waves that originate from the earthquake source (hypocenter) are recorded by stations installed around the earthquake region. To obtain seismic wave data from both Padang and Jambi earthquakes, we used three component waveforms from the local data recorded by three IRIS/Malaysia MY network stations (IPM, KOM dan KUM) and one IRIS/Singapore MS network station as illustrated (Figure 1). The distance of each stations to the epicenter is not more than 10. Figure 1. Epicenter positions of 30/09/2009 and 01/10/2009 (blue star) events and the four stations ( IPM, KUM, KOM and BTDF) (red rectangular) The epicenter distances of 30/09/2009 events for BTDF, KOM, IPM and KUM stations are, km, km, km and km, respectively, whereas the epicenter distances for 01/10/2009 events for BTDF, KOM, IPM and KUM stations, are

3 IJP Vol. 21 No. 4, km, km, km and km, respectively. 3. Three Component Local Waveforms Inversion and Fault Plane Determination The three components seismogram recorded by MY and MS network was inverted using the Green function which was calculated iteratively, using Wave Number Discretisation method 14). To calculate the Green function, we used 1-D velocity model (Table 2) and the hypocenters of both events from IRIS. The first six layers of all the velocity models with their parameters was obtained by applying modification to an approach introduced by Novotny, et al 15). The modification was based on earth model proposed by Santosa 16). The inversion of three component waveforms was performed using iteration deconvolution method 17,18). This method was implemented by using ISOLA software 19-21) to obtain earthquake source parameters. The inversion utilizes frequency band between 17.5mHz and 52.5mHz for 30/09/2009 events and 16.5 mhz until 42.5 mhz for 01/10/2009 events. The earthquake source parameters were used to determine the orientation, length of fault plane and width and slip length of both earthquakes. To determine the real fault plane orientation, HC-plot method was used 22). Coulomb ,23) software is used for determining the length and width of the faulting plane as well as the slip length. Table 2. 1 D velocity model that is used in three ccomponents local waveform inversion Depth Vp Vs Rho (km) (km/s) (km/s) (g/cm 3 ) Qp Qs Earthquake source parameters Earthquake source parameters are used for microzonation and seismic risk treatment 20). In this analysis, the seismic moment (M 0 ), moment magnitude (M w ), depth, orientation and fault plane width as well as slip length were determined for both events using the three component local waveforms. The source parameters can be extracted from mathematical models, if good fitting is achieved between the measured and synthetic seismogram. The searching process of the highest Double couple (DC) value and its reduction variant to obtain the best seismogram fitting is shown in Figure 3 and 4. The DC values and their reduction variants for both events are 99.3% and 70% for Padang earthquake and 95.3% and 73% for Jambi earthquake. Seismogram fitting, DC values and reduction variants are presented in Figure 5 and Figure 6. Figure 3. DC searching plot and optimal correlation as time function for 30/09/2009 events. Figure 4. DC searching plot and optimal correlation as time function for 01/10/2009 events Based on the analysis, earthquake source parameters for both earthquake events are obtained (Figure 7 and Figure 8). To identify the actual fault plane of both faulting planes, HC-plot method was used. The obtained centroid coordinates are (strike=88 ; dip=70 and depth=80 km) for 30/09/2009 event and (strike=149; dip=49 and depth=15 km) for 01/10/2009 as shown in Figure 8 and Figure 9. The hypocenter coordinate for 30/09/2009 event is obtained using IRIS data whereas for 01/10/2009 event is obtained using Geofon data (Table 1). Based on the analysis results, the 30/09/2009 event (Figure 11) has hypocenter-centroid distance of about 5.0 km. The fault plane 1 shown in Figure 8 is characterized by strike=191 ; dip=58 ; rake=24 (green) with distance of 2.76 km from the hypocenter. The fault plane 2 is characterized by strike=88 ; dip=70 ; rake=146 (red) with a distance of 1.74 km from the hypocenter. The distance of fault plane 2 (Figure 11b) to the hypocenter is closer than fault plane 1 (Figure 11a). Therefore, the real fault plane is fault plane 2 (Figure 11b).

4 110 IJP Vol. 21 No. 4, 2010 Figure 5. 3 Observed local (black) and synthetic waveforms (red) for 30/09/2009 event Figure 6. 3 Observed ocal (black) and synthetic waveforms (red) for 01/10/2009 Event

5 IJP Vol. 21 No. 4, Figure 8. Earthquake source parameters (CMT) for 30/09/2009 event Figure 9. Earthquake Source Parameters (CMT) 01/10/2009 Event (a) (b) Figure 11. The distance of fault plane 2 is closer to the hypocenter than fault plane 1

6 112 IJP Vol. 21 No. 4, 2010 The distance between hypocenter and centroid for 01/10/2009 event (Figure 12) is 7.8 km. The fault plane 1 shown in Figure 9 has geometry of strike= 149 ; dip=49 ; rake=-176 (green) with distance of 1.30 km from the hypocenter whereas fault plane 2 has geometry of strike=56 ; dip=87 ; rake=-41 (red) with a distance of 5.45 km from the hypocenter. The distance of fault plane 1 (Figure 12b) to the hypocenter is closer than fault plane 2 (Figure 12a). Therefore, the real fault plane is fault plane 1 (green). (a) (b) Figure 12. The distance of fault plane 1 is closer to the hypocenter than fault plane 2 The Coulomb software was used for determining the real length and width of the fault plane as well as the slip length. The input for the software are strike, dip and rake angle of the identified fault plane and moment magnitude. Length and width of the fault planes and the slip length of the events in 30/09/2009 and events 01/10/2009 are 124 km; 33.8 km; right lateral=1.3 m and reverse slip=0.88 m and km; km; right lat.=0.59 m and rev. slip= m. 5. Discussion Accurate hypocenter parameters and focal mechanism estimation can provide vital information regarding the earthquake strength, orientation, fault plane length and width and slip length. In this paper, we used three component local broadband waveforms that is recorded by IRIS/Malaysia, MY network stations and IRIS/MS station 5). The Station code (St), distance ( ), centroid depth(d), M 0, M w, strike (stk), dip, rake (rak), fault plane length (p) and width (l) and right lateral (rl) also reverse slip (rv_s) for each event are presented in Table 3 and 4. Comparison between hypocenters of 30/09/2009 earthquake obtained from IRIS and those estimated by this research shows the same longitude and lattitude point with 5 km difference in the earthquake source calculation (85 km and 80 km). The moment magnitude calculated by our research is 7.5 (M w ), while from IRIS is 7.9 (M w ). Parameters obtained from this research show good seismogram fitting on the three components for all stations. The size of fault block that caused the earthquake has a dimension of 124 km 33.8 km, with 1.6 m shift. Faulting type of this earthquake is reverse oblique with rake angle of 146. The origin time of this result is slightly different (+0,1 second) to the hypocenter origin time. Table 3. Earthquake source parameters of 2009/09/30 earthquake St (km) d M 0 x10 22 P (dyne-cm) M (km) w stk dip rak (km) l(km) rl(m) rv_s(m) BTD F KOM IPM KUM Table 4. Earthquake source parameters of earthquake St (km) d M 0 x10 23 P M (km) (dyne.cm) w stk dip rak (km) l(km) rl(m) rv_s(m) BTDF KOM IPM , KUM

7 IJP Vol. 21 No. 4, The hypocenter location obtained in this research is 5.3 km deeper than the one obtained from IRIS, while the longitude and latitude position of this research and IRIS is the same. The moment magnitude of this research and IRIS is slightly different (M w = 6.4 and 6.6). The 01/10/2009 earthquake was caused by a fault plane with a dimension of km km and a shifting of about 60 cm. The type of this earthquake fault is normal oblique with rake The origin time of this research is slightly different (+0,25 second) to the hypocenter origin time. The strike line direction of 30/09/2009 event is pointing toward the West Sumatra coast (88 ). Fault plane slope for this event is nearly perpendicular to the earth surface (dip angle = 70 ). While for 01/10/2009 event, the strike line is located alongside Sumatra island with 146. The fault plane (dip angle) for this event is slight slope (49 ). This slight slope dip is rarely met in fault block triggered by Semangko fault s activity. 6. Conclusion Earthquake parameters for both Padang and Jambi events (seismic moment, moment magnitude, orientation and fault plane width as well as slip length) were extracted after the fitting between measured and synthetic seismogram is achieved. The DC values and their reduction variants for both events are 99.3% and 70% for Padang earthquake and 95.3% and 73% for Jambi earthquake. Using HC-plot method, we can identify that the real fault plane for 30/09/2009 event that is a plane that has values of strike=88, dip=70, rake=146, while for 01/10/2009 event, the real plane that has values of strike=146, dip=49 and rake= The fault type of each event are reverse oblique and normal oblique, respectively. The fault plane length, width and slip length of 30/09/2009 event are 124 km; 33.8 km; right lateral=1.3 m and reverse slip=0.88 m, whereas those of 01/10/2009 event are km; km; right lat=0.59 m and reverse slip=-0.05 m. Acknowledgement We would like to express our gratitude to the GFZ-Postdam Geofon - Jerman and BMKG - Indonesia that gave us permission to download the waveform data recorded by IA network stations. We are indebted to thank Prof. Dr. Zahradnik and Dr. Efthimios Sokos for their guidance in understanding ISOLA-GUI and HC-plot softwares and for applying the softwares to estimate earthquake source parameters using three dimensional local waveforms ( We are deeply indebted to Prof. Dr. Toda, S., Prof. Dr. R. S. Stein, Prof. Dr. P. A. Reasenberg, Prof. Dr. J. H. Dieterich, and Prof. Dr. A. Yoshida for their support in estimating the length and width of fault plane block and also slip length using Coulomb software ( ing/ home/swf). References 1. S. Lasitha, M. Radhakrishna, and T.D. Sanu, Seismically Active deformation in the Sumatra- Java Trench-arc region: Geodynamic Implications, Current Science, 90, D. H. Natawidjaya, Neotectonics of the Sumatra Fault and Paleogeodesy of the Sumatra Subduction Zone, California Institute of Technology Pasadena, California (Thesis), K. R. Newcomb, and W.R. McCann, Seismic history and seismotectonics of the Sunda Arc, Journal of Geophysical Research, 92, , L. Prawirodirdjo, et. al., Geodetic observations of interseismic strain segmentation at the Sumatra subduction zone, Geophysical Research Letters, 24, , S. Toda, et al., Stress transferred by the MW = 6.9 Kobe, Japan, shock: Effect on aftershocks and future earthquake probabilities. J. Geophys. Res., 103, , Madlazim, B. J. Santosa, and U. Widya, Parameter-parameter Sumber Gempa Bumi Padang dan Korelasinya dengan Gempa Bumi- Gempa Bumi Berikutnya, Prosiding Seminar Nasional Fisika dan Aplikasinya, ITS, /1.117.orang.meninggal.akibat.gempa.pa dang 8. content&task=view&id=2541&itemid= odeling/home.swf M. Bouchon, A simple method to calculate Green's functions for elastic layered media, B Seismol. Soc. Am., 71, , O. Novotný, J. Zahradník, and G-A.Tselentis, North-western Turkey earthquakes and the crustal structure inferred from surface waves observed in the Corinth Gulf, Greece, B Seismol. Soc. Am., 91, , B. J. Santosa, Analyzing the seismogram of earthquakes on Sumatra-Java Subduction plane at CHTO observation station, Jurnal MIPA, 13, 25 43, M. Kikuchi and H. Kanamori, Inversion of complex body waves III, B Seismol. Soc. Am., 81, , E. Sokos and J. Zahradník, A Matlab GUI for Use with ISOLA Fortran Code, University of

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