The preliminary results: Internal seismic velocity structure imaging beneath MountLokonRizky Firmansyah, Andri Dian Nugraha, and Kristianto Citation: AIP Conference Proceedings 1658, 050012 (2015); doi: 10.1063/1.4915051 View online: http://dx.doi.org/10.1063/1.4915051 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1658?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Preliminary result of P-wave speed tomography beneath North Sumatera region AIP Conf. Proc. 1658, 030018 (2015); 10.1063/1.4915026 Upper crustal structure beneath East Java from ambient noise tomography: A preliminary result AIP Conf. Proc. 1658, 030009 (2015); 10.1063/1.4915017 Preliminary results of characteristic seismic anisotropy beneath Sunda-Banda subduction-collision zone AIP Conf. Proc. 1658, 030006 (2015); 10.1063/1.4915014 Upper crustal structures beneath Yogyakarta imaged by ambient seismic noise tomography AIP Conf. Proc. 1554, 269 (2013); 10.1063/1.4820337 Preliminary model results for pulsar velocities AIP Conf. Proc. 366, 16 (1996); 10.1063/1.50241

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The Preliminary Results: Internal Seismic Velocity Structure Imaging beneath Mount Lokon

Rizky Firmansyah1, a), Andri Dian Nugraha2, b), Kristianto3, c)

1Geophysical Engineering, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Bandung, 40132, Indonesia.

2Global Geophysical Group, Faculty of Mining and Petroleum Engineering, Institut Teknologi Bandung, Bandung, 40132, Indonesia.

3Center for Volcanology and Geological Hazard Mitigation (CVGHM), Geological Agency, Bandung, 40122, Indonesia.

a)Corresponding author: rizkyfirmansyah@hotmail.com

b)nugraha@gf.itb.ac.id c)kris@vsi.esdm.go.id

Abstract. Historical records that before the 17th century, Mount Lokon had been dormant for approximately 400 years. In the years between 1350 and 1400, eruption ever recorded in Empung, came from Mount Lokon’s central crater. Subsequently, in 1750 to 1800, Mount Lokon continued to erupt again and caused soil damage and fall victim. After 1949, Mount Lokon dramatically increased in its frequency: the eruption interval varies between 1 – 5 years, with an average interval of 3 years and a rest interval ranged from 8 – 64 years. Then, on June 26th, 2011, standby alert set by the Center for Volcanology and Geological Hazard Mitigation. Peak activity happened on July 4th, 2011 that Mount Lokon erupted continuously until August 28th, 2011. In this study, we carefully analyzed micro-earthquakes waveform and determined hypocenter location of those events. We then conducted travel time seismic tomographic inversion using SIMULPS12 method to detemine Vp, Vs and Vp/Vs ratio structures beneath Lokon volcano in order to enhance our subsurface geological structure. During the tomographic inversion, we started from 1-D seismic velocities model obtained from VELEST33 method. Our preliminary results show low Vp, low Vs, and high Vp/Vs are observed beneath Mount Lokon-Empung which are may be associated with weak zone or hot material zones. However, in this study we used few station for recording of micro-earthquake events. So, we suggest in the future tomography study, the adding of some seismometers in order to improve ray coverage in the region is profoundly justified. Keywords: Micro-earthquake, Lokon, tomography, Vp, Vs, Vp/Vs ratio PACS: 91.40.-k

INTRODUCTION

Mount Lokon is one of five active volcanoes located in North Sulawesi on three plates boundary between Indo-Australian, Eurasia, and Pacific plate. Before the 17th century, Mount Lokon had been dormant for approximately 400 years. In the years between 1350 and 1400, eruption ever recorded in Empung, came from Mount Lokon’s central crater. Subsequently, in 1750 to 1800, Mount Lokon volcano continued to erupt again and caused soil damage and fall victims [1]. At that time, explosion product was tephra that in form of ash, sand, and lava flows.

After 1949, Mount Lokon dramatically increased in its frequency: the eruption interval varied between 1 – 5 years, with an average interval of 3 years and a rest interval ranged from 8 – 64 years [2]. Then on October 1991, a big eruption occurred. The eruption brought pyroclastic flow and end with a lava plug called with “Sumbat Lava 1991” at the bottom of the crater. Furthermore, in the period between 2000 to 2003, the volcano continuously erupted almost every year and gradually eroded Sumbat Lava 1991 that generating new hole [3]. Center for Volcanology and Geophysical Hazard Mitigation (CVGHM) recorded ash explosion with a maximum of height

4th International Symposium on Earthquake and Disaster Mitigation 2014 (ISEDM 2014)AIP Conf. Proc. 1658, 050012-1–050012-7; doi: 10.1063/1.4915051

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reached 1500 m [4] in this period.Mount Lokon volcano became dormant until December 2007 after eruption in 2003. High volcanic activity

occurred again after December 2007 with the occurrence of volcanic-tectonic A and B, below 5 events per day in average [4]. According to Kristianto et al. [5] until the beginning of 2011, the number of volcanic earthquakes fluctuates between 100 – 800 events each month. Then, on June 26th, 2011, standby alert set by the CVGHM. And peak activity happened on July 4th, 2011 that Mount Lokon continuously erupted until August 28th, 2011 [6].

To better understand the characteristics of Mount Lokon volcano, through this study, we would like to do imaging subsurface with 3-D seismic velocity tomography approach. The main aim was to see subsurface velocity variations that have been done by several other researcher since 1980s which the results were good to be applied in the volcanic area [7,8,9,10,11,12,13,14,15,16]. From its results, we can analyze the mechanism of magma to the surface [16] and the presence of magma chamber.

Data and methods

We used hypocentral data in the period of January 2009 to March 2010 from the CVGHM with five short-period seismic stations. During fifteen period, there were 733 earthquakes consists of 3387 phase of P-wave and 1871 phase of S-wave. The data was filtered again according to our research focus.

To get subsurface velocity variation accurately, the initial model and source hypocenter should be close to the actual values. Effective approach to get minimum 1-D model could be done with VELEST algorithm [17,18]. We tried several initial models to obtain minimum 1-D model. The best result then used for input data in tomographic model with SIMULPS12 algorithm.

Tomographic inversion with SIMULPS12 algorithm [19,20,21] using a combination of parameter separation [22,23] and damped least-square inversion for P-wave travel time and S-P arrival times to determine the variation of Vp and Vp/Vs velocity structure. Calculation of Vp/Vs model can be determined from the difference in arrival time of S-P, not from S-wave arrival time directly, because this method is robust to gain Vp/Vs value [24].

Determining objective damping for Vp and Vp/Vs can be obtained by a trade-off curve (Figure 1). Appropriate damping value was chosen when the data variance drop drastically with the moderate increase of variance model [24].

FIGURE 1. Trade-off curve for Vp (above) and Vp/Vs (below). Appropriate damping for Vp was 10 and Vp/Vs was 6.

RESULTS AND DISCUSSIONS

The quality of Vp and Vp/Vs tomogram can be viewed from their ray path coverage (Figure 2). The more dense of their ray path, the more it could be resolved. We plotted tomographic results with derivative weighted sum (DWS) an diagonal resolution element (DRE). Derivative weighted sum is depend on the length of ray path segments that arranged on tomographic inversion in simulps12 while derivative resolution element has a range between 0 (not resolve) to 1 (good resolution). In this study, our diagonal resolution element of Vp is between 0 – 0.401 while Vp/Vs between 0 – 0.1507. This result of small values could be caused by our limited observations per each events.

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FIGURE 2. Ray path coverage from P-wave and focus study area. In our focus study, grid was arranged densely that was 1 km spacing. There was much hypocenter data that outside in the seismic network. Red plus: 3-D grid nodes, blue lines: P-wave ray

path, blue inverted triangles: stations, open black circles: hypocenter.

Interpretation of Vp, Vs, and Vp/Vs Tomogram

Various studies in volcanic tomography revealed that high-velocities anomaly (Vp, Vs, and Vp/Vs ratio) has a strong association to any solidified magmatic rocks [11,12,14,16,19,27] and it could be a future magma intrusions [27]. While low-velocities anomaly in Vp and Vs models, but high in Vp/Vs ratio, has a close relationship with presence of fractures or weak zones and also containing hot material [9,11].

In vertical sliced of 0 km East – West direction and 0 km North – South direction (see Figure 3b and 3f) there were higher Vp and Vp/Vs velocity rather than Vs model. It is a common feature in volcanic area because Vp velocity is influenced by both bulk and shear modulus, yet Vs is only affected by shear modulus. One of the factors that may caused shear modulus dropped is the presence of cracks and pores that contain fluids and melts [16]. If the total number of cracks and pores are high, this factor does not have much effect to P-wave, but it is able enough to reduce S-wave velocity [16]. We interpreted this features as a conduit for transporting fluids and melts to the surface.

On the other hand, There were low-velocity anomaly in Vp and Vs model, but higher in Vp/Vs ratio, located in shallow depth of eastern Mount Empung (see Figure 3c) and cross-section C – C’ on East – West direction (Figure 3g). This features may associated with partial melting or magma chamber [27].

Then, on cross-section D – D’ at East – West direction (Figure 3h), in shallow depth (up to 1 km) there were a low-velocity anomaly in Vp and Vp/Vs model while Vs was higher. It was because there was accumulated volcanic products that can reduce Vp velocity [12,14].

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(a) (b)

(c) (d)

(e) (f)

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(g) (h)

FIGURE 3. Vp, Vs, and Vp/Vs model with derivative weighted sum resolution in South – North direction (a, b, c, d) and West – East direction (e, f, g, h). Reference point (x = 0 and y = 0) is on 1.36˚ N, 124.8˚ E. Vp and Vs model were in perturbation while

Vp/Vs was in absolute value. White circles: hypocenter, inverted triangles: stations, triangles: mountains, black dashed lines: derivative weight sum resolution.

(a) (b)

FIGURE 4. (a) Schematic model of conduit containing partial melt and shallow magma chamber below Tompaluan crater and (b) schematic model of accumulated volcanic products below Mount Empung.

Our tomography results showed a consistent finding with Suparman [28]. In his geomagnetic study, he found that

there were a high-susceptibility body below Tompaluan Crater surrounding with low-susceptibility body. A lower one was interpreted as a magmatic intrusion that have been weakened [28]. Another study from Suparman [29] about focal mechanism, revealed that the source of shallow magma chamber was in 1 – 2 km below Mount Lokon – Empung.

CONCLUSION

From our tomography results, we concluded that: 1. The best tomography results was near seismic stations with a maximum depth until 3 km. 2. Vp velocity model had a better resolution than others, because there is only one three-component station.

Thus, S-wave observation becoming less and so that the resolution was poor.

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3. There were high-velocity anomaly, either Vp and Vp/Vs ratio, while Vs was lower located in C – C’ West – East and North – South direction that may represent as a conduit for transporting fluids and melts to the surface.

4. Low-velocitiy anomaly in Vp and Vs model, but higher in Vp/Vs ratio, located in shallow depth of eastern Mount Empung (cross-section C – C’ and D –D’ on the North – South direction) and cross-section C – C’ on East – West direction, are associated with partial melting or magma chamber shallow magma chamber as feeding the activity of Mount Lokon – Empung volcano.

ACKNOWLEDGMENTS

We would like to thank Center for Volcanology and Geological Hazard Mitigation (CVGHM) for the data provided in this study.

REFERENCES

1. K. Kusumadinata, Data Dasar Gunung Api Indonesia (Direktorat Vulkanologi, Bandung, 1979), pp. 661-669. 2. E. Kriswati, I. Meilano, Suhartaman, Y. Suparman, H.Z. Abidin, and T. Sinaga, Indonesian Journal of Geology

7, (abstract in English), No.4, 199-209 (2012). 3. S. R. Wittiri, Letusan G. Lokon 1991 (Direktorat Vulkanologi, Bandung, 1991). 4. PVMBG, Laporan Peringatan Dini Bahaya Gunung Api G. Lokon, Sulawesi Utara (Pusat Vulkanologi dan

Mitigasi Bencana Geologi, Bandung, 2010). 5. Kristianto, H. Gunawan, N. Haerani, I. Mulyana, A. Basuki, S. Primulyana, and F.U. Bina, Jurnal Lingkungan

dan Bencana Geologi 3 (in Indonesia), No. 3, 151-168 (2012). 6. PVMBG, Evaluasi Aktivitas G. Lokon pada status Awas, Juli 2011 (Pusat Vulkanologi dan Mitigasi Bencana

Geologi, Bandung, 2011). 7. C. H. Thurber, Science 223, 165-167 (1984). 8. J. R. Evans and J. J. Zucca, J. Geophys. Res. 93, No. B12, 15016-15036 (1988). 9. J. M. Lees and R.S. Crosson, J. Geophys. Res. 94, No. B5, 5716-5728 (1989). 10. H. M. Benz, B. A. Chouet., P. B. Dawson, J. C. Lahr, R. A. Page, and J. A. Hole, J. Geophys. Res. 101, No. B4,

8111-8128 (1996). 11. M. Laigle, A. Hirn, M. Sapin, J. C. Lepine, J. Diaz, J. Gallart, and R. Nicolich, J. Geophys. Res. 105, No. B9,

21633-21646. (2000). 12. S. Tanaka et al., Geophys. Res. Lett. 29, No. 10 (doi:10.1029/2002GL014983), 1420 (2002). 13. A. D. Jolly, S.C. Moran, S.R. Mc Nutt, and D.B. Stone, J. Volcanol. Geotherm 159

(doi:10.1016/j.jvolgeores.2006.06.022), 326-342 (2006). 14. P. Berger, J. L. Got, C. V. Gonzalez, V. Monteiller, J. Volcanol. Geotherm. 200

(doi:10.1016/j.jvolgeores.2010.12.016), 234-244 (2011). 15. A. D. Nugraha, S. Widiyantoro, A. Gunawan, and G. Suantika, J. Math. Fund. Sci 45, No. 1, 17-28. (2013). 16. P. Y. Kuznetsov and I. Yu Koulakov, J. Volcanol. Geotherm. Res 276, 10-21 (2014). 17. E. Kissling, Rev. Geophys. 26, No. 4, 659-698 (1998). 18. E. Kissling, W. L. Ellsworth, D. Eberhart-Phillips, and U. Kradolfer, J. Geophys. Res. 99, No. B10,

19635-19646 (1994). 19. C. H. Thurber, J. Geophys. Res. 88, 8226-8236 (1983). 20. C. H. Thurber, “Local Earthquake Tomography: velocities and Vp/Vs - theory” in Seismic Tomography:

Theory and Practice, edited by H.M. Iyer and K. Hirahara (Chapman & Hall, London, 1993), pp. 563-583. 21. D. Eberhart-Phillips, “Local Earthquake Tomography: earthquake source regions” in Seismic Tomography:

Theory and Practice, edited by H.M. Iyer and K. Hirahara (Chapman & Hall, London, 1993), pp. 613-643. 22. G. L. Pavlis and J. R. Booker, J. Geophys. Res., 4801-4810 (1980). 23. C. Spences and D. Gubbins, Geophys. J. R. Astron. Soc. 63, 95-116 (1980). 24. D. Eberhart-Phillips, Bull. Seismol. Soc. Am. 76, No. 4, 1025-1052. (1986). 25. J. R. Evans, D. Eberhart-Phillips, and C. H. Thurber, User's Manual For SIMULPS12 For Imaging Vp and

Vp/Vs: A Derivative of the "Thurber" Tomographic Inversion SIMUL3 For Local Earthquakes and Explosions (U.S. Geological Survey, California, 1994).

26. M. Palo, J. M. Ibanez, M. Cisneros, M. Breton, E. Del Pezzo, E. Ocana, J. Orozco-Rojas and A.M. Posadas,

050012-6 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

167.205.22.104 On: Wed, 29 Apr 2015 01:03:43

Geophys. J. Int. 177, 1383-1398 (2009). 27. S. Sherburn, R. S. White and M. Chadwik, Geophys. J. Int. 166, 957-969 (2006). 28. Y. Suparman, Penyelidikan Geomagnet Gunung Lokon (Pusat Vulkanologi dan Mitigasi Bencana Geologi,

Bandung, 2007). 29. Y. Suparman, M. Iguchi, M. Hendrasto, Kristianto, “Comparison of Focal Mechanisms and Source Parameters

of Volcano-tectonic Earthquakes between Active and Normal Periods at Lokon Volcano, North Sulawesi, Indonesia,” in Laporan dan Kajian Vulkanisme (Pusat Vulkanologi dan Mitigasi Bencana Geologi, Bandung, 2012), pp. 47-64.

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