crustal deformation studies in java (indonesia) using gps

Journal of Earthquake and Tsunami, Vol. 3, No. 2 (2009) 77–88c© World Scientific Publishing Company

CRUSTAL DEFORMATION STUDIES IN JAVA(INDONESIA) USING GPS

HASANUDDIN Z. ABIDIN∗,¶, HERI ANDREAS∗, TERUYUKI KATO†,TAKEO ITO‡, IRWAN MEILANO∗,‡, FUMIAKI KIMATA‡,

DANNY H. NATAWIDJAYA§ and HERY HARJONO§∗Institute of Technology Bandung

Jl. Ganesa 10, Bandung 40132, Indonesia†Earthquake Research Institute, The University of Tokyo

1-1, Yayoi 1, Bunkyo-ku, Tokyo 113-0032, Japan‡Research Center for Seismology, Volcanology and Disaster Mitigation

Graduate School of Environmental Studies, Nagoya UniversityNagoya 464-8602, Japan

§Indonesian Institute of Sciences (LIPI)Jl. Gatot Subroto, Jakarta, Indonesia

¶[email protected]¶[email protected]

Accepted 14 January 2009

Along the Java trench the Australian–Oceanic plate is moving and pushing onto andsubducting beneath the Java continental crust at a relative motion of about 70 mm/yrin NNE direction. This subduction-zone process imposed tectonic stresses on the fore-arc region offshore and on the land of Java, thus causing the formation of earthquakefault zones to accommodate the plate movement. Historically, several large earthquakeshappened in Java, including West Java. This research use GPS surveys method to studythe inter-seismic deformation of three active faults in West Java region (i.e. Cimandiri,Lembang and Baribis faults), and the co-seismic and post-seismic deformation relatedto the May 2006 Yogyakarta and the July 2006 South Java earthquakes.

Based on GPS surveys results it was found that the area around Cimandiri, Lembangand Baribis fault zones have the horizontal displacements of about 1 to 2 cm/yr or less.Further research is however still needed to extract the real inter-seismic deformationof the faults from those GPS-derived displacements. GPS surveys have also estimatedthat the May 2006 Yogyakarta earthquake was caused by the sinistral movement of the(Opak) fault with horizontal co-seismic deformation that generally was less than 10 cm.The post-seismic horizontal deformation of the July 2006 South Java tsunami earthquakehas also been estimated using GPS surveys data. In the first year after the earthquake(2006 to 2007), the post-seismic deformation is generally less than 5 cm; and it becomesgenerally less than 3 cm in the second year (2007 to 2008).

Keywords: Crustal deformation; Java.

1. Introduction

Along the Java trench the Australian–Oceanic plate is moving and pushing on toand subducting beneath the Java continental crust at a relative motion of about70mm/yr in NNE direction. According to Natawidjaya [2006], this subduction-zone

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78 H. Z. Abidin et al.

process imposes tectonic stresses on the fore-arc region offshore and on land of Java,thus causes the formation of earthquake fault zones to accommodate the platemovement. Historically, several large earthquakes happened in Java, as reported byNewcomb and McCann [1987].

This research uses GPS surveys method to study the activities of three activefaults in West Java region. These faults are the Cimandiri, Lembang and Baribisfaults (see Fig. 1). The study of co-seismic and post-seismic deformation related tothe May 2006 Yogyakarta and the July 2006 South Java earthquake are also the con-cern of this crustal deformation study. Several GPS campaigns have been conducted(see Table 1). GPS monitoring networks used in this study are shown in Fig. 2.

All GPS data processing were carried out using Bernese 5.0 scientific process-ing software [Beutler et al., 2007]. The IGS (International GNSS Service) pre-cise ephemeris was used, and the coordinate reference system was established byconnecting to 20 IGS stations around Indonesia, namely ALIC, BAHR, BAKO,CEDU, COCO, DARW, DGAR, GUAM, IISC, KARR, KIT3, KUNM, LHAZ,MATE, PIMO, TID2, TOW2, TSKB, WUHN, YAR2 [IGS, 2008].

Fig. 1. Active faults in the West Java region.

Table 1. GPS Surveys that have been conducted.

Faults or Earthquake Area Time of GPS Surveys

Cimandiri Fault 1–4 Dec. 2006; 20–23 Aug. 2007; and 8–12 Aug. 2008Lembang Fault 22–24 June 2006; 09–13 Nov. 2006; 16–19 Aug. 2007; and

13–16 Aug. 2008Baribis Fault 1–4 May 2007; and 3–6 Nov. 2007Pangandaran (July 2006 South

Java earthquake)23–30 July 2006; 9–15 Aug. 2007; and 1–5 Aug. 2008

Yogyakarta (May 2006 Yogyakartaearthquake)

4–8 June 2006 and 21–26 June 2008

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Crustal Deformation Studies in Java (Indonesia) Using GPS 79

(a) (b)

(c) (d)

(e)

Fig. 2. GPS monitoring networks for studying inter-seismic deformation in Cimandiri fault (a),Lembang fault (b), Baribis fault (c); post-seismic deformation of the 2006 Yogyakarta earthquake(d); and post-seismic deformation of the 2006 South Java earthquake (e). In this picture CFZ,LFZ, BFZ and OFZ are Cimandiri, Lembang, Baribis and Opak Fault Zones, respectively.

2. Inter-Seismic Deformation of the Lembang, Cimandiriand Baribis Faults

Lembang fault is topographically and geologically remarkable and runs about10 km north of Bandung in the EW direction as shown in Fig. 1. According to

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the Directorate of Environmental Geology, the Lembang fault has an earthquakecycle of about 500 years [Pikiran Rakyat, 2004]. Relatively large population is livingaround the fault area, e.g. city of Bandung and town of Lembang. Cimandiri fault

is an active fault in Sukabumi area (West Java), which runs in the direction fromPelabuhan Ratu, passing Sukabumi, Cianjur and Padalarang as shown in Fig. 1.According to Darji et al. [1994], Cimandiri is a sinistral strike-slip fault. Severalprevious earthquakes such as Pelabuhanratu earthquake [1900], Cibadak earthquake[1973], Gandasoli earthquake [1982], Padalarang earthquake [1910], Tanjungsariearthquake [1972], Conggeang earthquake [1948], and Sukabumi earthquake [2001],occurred along the Cimandiri fault. Baribis fault is a thrust fault in the regionof Subang–Majalengka–Kuningan, in the eastern part of West Java. Its existence isfirst proposed by van Bemmelen [1949], based on the name of hilly region of Baribis,located southwest of Majalengka and west of Ciremai volcano. Baribis fault can beconsidered as an active fault, since there have been several earthquakes occurringin the region along this fault.

Inter-seismic deformation of Lembang, Cimandiri and Baribis faults are derivedfrom the position displacements as estimated from consecutive GPS surveys. If morethan two campaigns have been conducted, then the average displacement rates wereestimated. The horizontal displacements around Lembang, Cimandiri and Baribisfaults as obtained from GPS surveys are shown in Figs. 3–5. The reference frameof the displacement vectors is ITRF 2000.

Fig. 3. Average horizontal displacement rates of the area around Lembang fault, as estimatedfrom GPS surveys on June 2006, August 2007 and August 2008. The rates vary spatially between0.3 to 1.4 cm/yr.

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Fig. 4. Average horizontal displacement rates of the area around Cimandiri fault, as estimatedfrom GPS surveys on December 2006, August 2007 and August 2008. The rates vary spatiallybetween 0.5 to 1.7 cm/yr.

Fig. 5. Horizontal displacements of the area around Baribis fault, as estimated from GPS surveyson June 2007 and November 2007. The displacements vary spatially between 1.0 to 2.1 cm.

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Inter-seismic horizontal displacement rates around the Lembang fault area, asestimated from GPS surveys conducted on June 2006, August 2007 and August2008, are shown in Fig. 3. The displacement due to the Sunda block motion, withthe rate of about 2.7 cm/yr with the south-east direction [Bock et al., 2003] has beenremoved. However, the horizontal displacement rates between 0.3 to 1.4 cm/yr, arestill apparent. Since the northern and southern parts of the Lembang fault aremoving with relatively the same direction, this displacement should not be fullyrelated to the activity of the Lembang fault.

These apparent horizontal displacements may be due to co-seismic deformationof the Indramayu earthquake (7.5Mw, depth of 290km) that occurred on Thursday,August 09, 2007 at 12:04:58 a.m. with its epicenter located at about 115 km northof the Lembang fault [USGS, 2007]. This earthquake occurred about a week beforethe last GPS campaign in Lembang fault, i.e. 16–19 August 2007. Therefore, thedisplacements shown in Fig. 5 may also contain the initial part of post-seismicdeformation. However, more research and analysis should be performed to clarifythis hypothesis.

Related to the Cimandiri fault, the horizontal displacement rates as estimatedfrom GPS surveys conducted on December 2006, August 2007 and August 2008are shown in Fig. 4. The displacements due to the Sunda block motion have beenremoved from the solution. From Fig. 4 it can be realized that the area aroundCimandiri fault moves with different rates and directions, with the rates varyspatially between 0.5 to 1.7 cm/yr. In order to verify whether these motions aredue to the fault activity or not, more GPS surveys should be conducted.

The estimated inter-seismic horizontal displacements in the Baribis fault area asobtained from GPS surveys conducted on May and November 2007, after a removalof the Sunda block motion, is shown in Fig. 5. During this period, the displacementsvary spatially between 1.0 to 2.1 cm.

Since the northern and southern parts of the Baribis fault are moving withrelatively the same direction, this displacement should not be fully related to theactivity of Baribis fault. As in the case of the Lembang fault, these apparent hor-izontal displacements may also contain the post-seismic deformation signal of theIndramayu earthquake of 9 August 2007, with its epicenter located about 150 kmnorth-west of Baribis fault [USGS, 2007]. However, more research and analysisshould be performed to clarify this hypothesis.

It should be mentioned that in the above Figs. 3–5, the 95% error ellipses areplotted at the tip of the displacement arrows. The same scales are used for bothdisplacements and error elipses. Since they are very small, they are difficult torecognize in the figures.

3. Co-Seismic Deformation of the 2006 Yogyakarta Earthquake

The 27 May 2006 earthquake hit the Provinces of Yogyakarta and Central Javaof Indonesia at 5:54 a.m. local time, with its epicenter estimated around 25 km

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Fig. 6. Opak Fault and Dengkeng Fault; courtesy of Danny H. Natawidjaja.

south-southwest of the Indonesian city of Yogyakarta and about 10 km belowthe sea bed [USGS, 2006a]. The USGS-estimated magnitude of first earthquake isMw 6.3. Subsequently, about 750 aftershocks have been reported, with the largestintensity recorded at Mw 5.2. The 2006 Jogyakarta earthquake occurred along theactive Opak fault, which runs about NE from the Parang Tritis area to the Bantularea then continue northward to the Klaten region (see Fig. 6). In this case, theprinciple earthquake fault movement along the Opak fault might also trigger somemovements along the Dengkeng fault.

A week after the earthquake, i.e. 4 to 8 June 2006, a GPS survey was conductedon 48 GPS points belonging to the 2nd order GPS network around Jogyakartaand Central Java, that was firstly observed in 1998. GPS surveys were conductedusing 14 geodetic-type dual-frequency GPS receivers. Each point is observed forabout 8 to 10 hours, while the reference point located in the UGM BoulevardJogyakarta is observed continuously. GPS results in Fig. 7 show that horizontalco-seismic displacement around Bantul and Jogyakarta are mostly less than 10 cm,with mostly south and south-west directions (see Fig. 7). By using the Okada model[Okada, 1985] and genetic algorithm, these derived-GPS displacements are thenused to estimate the fault geometry. The estimation results indicated the existence

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Fig. 7. Co-seismic horizontal displacements of the Yogyakarta 2006 earthquake.

LEFT-LATERAL FAULT

Length : 18 kmWidthStrike

: 10 km: 48

Dip : 890

Strike slip : 0.8 mDip slip : -0.26 m

strike

dip

0

-dip

Fig. 8. The estimated fault geometry causing the 2006 Yogyakarta earthquake.

of left-lateral fault with a strike of 48◦ and a dip of 89◦ (see Fig. 8). This fault islocated slightly to the east of the Opak fault as indicated in Fig. 6.

Based on the USGS Moment Tensor Solution, the dip and strike of the faultcausing the Yogyakarta 2006 earthquake are 85◦ and 59◦; and according to theHarvard Moment Tensor Solution are 90◦ and 51◦ [USGS, 2006a]. GPS-derived dipand strike, i.e. 89◦ and 41◦, are closer to those given by the Harvard Moment TensorSolution.

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4. Post-Seismic Deformation of the 2006 South Java Earthquake

The July 17th 2006 South Java earthquake (Mb6.1, Ms7.7 and Mw7.7, Harvardglobal CMT) occurred about 200 km south of western Java Island (see Fig. 7). Itoccurred on Monday, July 17, 2006, at 3:19:28 p.m. local time (8:19:28 UTC). TheUSGS recorded at least 22 aftershocks south of Java ranging between magnitude4.6 and 6.1. Two largest aftershocks measured 6.0 and 6.1 Mw [USGS, 2006b].

The July 17th 2006 South Java earthquake (Mb6.1, Ms7.7 and Mw7.7, Harvardglobal CMT) occurred about 200 km south of western Java Island (see Fig. 9). Itoccurred on Monday, July 17, 2006, at 3:19:28 p.m. local time (8:19:28 UTC). TheUSGS recorded at least 22 aftershocks south of Java ranging between magnitude4.6 and 6.1. Two largest aftershocks measured 6.0 and 6.1 Mw [USGS, 2006b].

In order to study the post-seismic deformation of the earthquake, three GPS sur-veys were conducted on July 2006, August 2007 and August 2008, by the researchersfrom ITB, LIPI, Nagoya University and Tokyo University. About thirty GPS pointsalong approximately 200 km coastline were observed.

The observation length at each site is more than 18 hours. The dual-frequencygeodetic type receivers were used for the observation. The GPS data were processedusing Bernese 5.0 GPS processing software [Beutler et al., 2007]. In this case, theITRF 2005 was used as a reference frame, and 23 IGS stations were involved in theprocessing along with all observation sites.

Fig. 9. Earthquake location; after [USGS, 2006b].

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Fig. 10. Post-seismic horizontal deformation of the 2006 South Java earthquake, as estimatedfrom GPS surveys on July 2006 and August 2007. The displacements are in general less than 5 cm.

Fig. 11. Post-seismic horizontal deformation of the 2006 South Java earthquake, as estimatedfrom GPS surveys on August 2007 and August 2008. The displacements are in general lessthan 3 cm.

The estimated post-seismic horizontal deformation is shown in Figs. 10 and 11.It can be seen that the magnitude of post-seismic deformation between July 2006and August 2007 is in general less than 5 cm, with increasing magnitudes towardthe west direction, i.e. decreasing distances toward the epicenter. The post-seismic

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deformation is getting smaller with time, and between August 2007 and August2008, its magnitudes are in general less than 3 cm. The physical reasons on the spa-tial variation in post-seismic deformation direction as indicated in Figs. 10 and 11are still under investigation.

5. Closing Remarks

Based on GPS survey results it was found that the area around Cimandiri, Lembangand Baribis fault zones have the horizontal displacements of about 1 to 2 cm/yr.Further research is however still needed to extract the real inter-seismic deforma-tion of the faults from those GPS-derived displacements. GPS surveys have alsoestimated that the May 2006 Yogyakarta earthquake was caused by the sinistralmovement of the (Opak) fault with horizontal co-seismic deformation that was gen-erally less than 10 cm. The post-seismic horizontal deformation of the July 2006South Java tsunami earthquake has also been estimated using GPS surveys data.In the first year after the earthquake (2006 to 2007), the post-seismic deformationis generally less than 5 cm; and it becomes generally less than 3 cm in the secondyear (2007 to 2008).

Considering the vulnerability of Java Island toward the future earthquakes andits relatively dense population, these crustal deformation studies using GPS sur-veys will be continued. In the future, data from newly established continuous GPSstations will be incorporated in the analysis, and possibility of InSAR-based defor-mation study [Massonnet and Feigl, 1998] will also be explored and implemented.

References

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Bock, Y., Prawirodirdjo, L., Genrich, J. F., Stevens, C. W., McCaffrey, R., Subarya, C.,Puntodewo, S. S. O. and Calais, E. [2003] “Crustal motion in Indonesia from globalpositioning system measurements,” Journal of Geophysical Research, 108(B8), 2367,doi:10.1029/2001JB000324.

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Massonnet, D. and Feigl, K.L. [1998] “Radar interferometry and its application to changesin the Earth’s surface,” 36(4), 441–500.

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crustal deformation studies in java (indonesia) using gps

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