Crustal Shear Wave Velocities Inferred From Love Wave Group Velocity Dispersion Between Mt. Merapi and Mt. Lawu, Central Java

Transcription

1 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 38 Crustal Shear Wave Velocities Inferred From Love Wave Group Velocity Dispersion Between Mt. Merapi and Mt. Lawu, Central Java Gatot Yuliyanto, Kirbani Sri Brotopuspito, Waluyo, Wiwit Suryanto Abstract-- Analysis of Love waves dispersion data have been carried out to determine shear wave velocities on the crustal structure between Mt. Merapi and Mt. Lawu. The record of earthquake of near S. Coast of Honsu, Japan, 5 September 2004 with magnitude 7.5 MW at seven MERAMEX seismic stations located between Mt. Merapi and Mt. Lawu have been used. The dispersion of group velocity data have been processed by using Fourier transform-based multiple-filtering frequency-time analysis developed by Kolinsky. In particular, it can be concluded that there are low velocity zones in the crustal structure between Mt, Merapi and Mt. Lawu with shear wave velocity vary to each stations. All stations have crustal shear wave velocity below 4.0 km/s and Moho discontinuity inferred between km depth. Crustal structure beneath CI1 station near the Mt. Merbabu has the highest shear wave velocity km/s. It can be inferred from the crutal shear wave velocity structure, that the low velocity zone in the region between Mt. Merapi and Mt. Lawu primarily at 4-8 km depth. Index Term-- crustal structure, low velocity, Love wave, Conrad discontinuity I. INTRODUCTION Koulakov et al [1] from their 3D structure model of the crust and uppermost mantle beneath Mt. Merapi and Mt. Lawu, Central Java, from local source seismic tomography found that there is a strong low velocity anomaly in the crust between Mt. Merapi and Mt. Lawu, and it can be interpreted as feeding area of both volcanoes. This anomaly is termed as Merapi and Lawu volcanoes anomaly (MLA). The volcanoes are located at the southern limit of the MLA. The difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, and the value of the Vp/Vs ratio inside the MLA is more than 1.9 that show a probable high content of fluids and partial melts within the crust [2]. In 2009, Koulakov [3] observed that in the crust beneath the Gatot Yuliyanto Department of Physics, Faculty of Science and Mathematics Diponegoro University, Ph.D. candidate in Physics Department, Faculty of Mathematics and Natural Science, Gadjah Mada University gatotyulianto@undip.ac.id, g_touch2001@yahoo.com Wiwit Suryanto Department of Physics, Faculty of Mathematics and Nat ural Science, Gadjah Mada University Kirbani Sri Brotopuspito Department of Physics, Faculty of Mathematics and Natural Science, Gadjah Mada University Waluyo Department of Physics, Faculty of Mathematics and Natural Science, Gadjah Mada University middle part of central Java, north to Merapi and Lawu volcanoes, there is a large and very intense anomaly with a velocity decrease of up to 30% and 35% for P and S models, respectively. Inside this anomaly E-W orientation of fast velocity takes place, probably caused by regional extension stress regime. They observed that in a vertical section there is a faster horizontal velocities inside this anomaly that might be explained by layering of sediments and/or penetration of quasi-horizontal lenses with molten magma. Luehr et al [4] identified a large low velocity body in the crust which extends down to the upper mantle beneath Central Java. The shear wave signals recorded above this anomaly are strongly attenuated compared to the neighboring areas. Active volcanoes like Merapi, Sumbing, and Lawu are located at the edge of this anomaly between high and low velocity regions. In this paper we try to model the shear wave velocities in the crust between the Merapi and Lawu volcanoes anomaly (MLA) inferred from Love wave dispersion. II. METHOD We used earthquake event which occured near south coast of Honsu, Japan, 05 September 2004, 14:57:00, latitude 33.2 N and longitude E, depth 11 km, and magnitude 7.5 MW, and data from MERAMEX stations: BI2, CI1, CJ1, CJ2, CJ3, CK4, and CK6 located between Mt. Merapi and Mt. Lawu (Fig. 2). For the group velocity analysis, the group velocity dispersion have been measured by using SVAL program developed by Kolinsky [5]. This program used the method of Fourier transform-based multiple-filtering frequency-time analysis [6], and all the standard procedures as well as the new enhancements of the modified technique are described in detail in the paper by Kolínský and Brokešová [7]. Fig. 1. Some highlighted structures mentioned in the interpretation in [3]. The dark red area is the MLA anomaly with an extremely strong amplitude. The yellow area is supposed as feeding the volcanoes in central Java. Blue circles are placed over locally higher-velocity anomalies. They probably reflect the position of magmatic intrusions frozen in volcanic channels and reservoirs. Grey areas are placed over the high-velocity anomalies in the forearc and probably mean rigid crustal blocks surrounded by folded belt s.

2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 39 A result of the multiple filtering processed in SVAL is a set of quasimonochromatic signals. A modulus of the analytical signal represents an envelope of the quasimonochromatic signal. Maxima of the envelopes give the dispersion curve. An envelope of quasimonochromatic signal often has several local maximas, forming several ridges in spectograms obtained by frequency-time analyses. The isometric method is used to inverse the dispersion curves in order to obtain the crustal shear wave velocity structures. This method is a fast inverse algorithm developed by Málek et al. [8] and tested by Málek et al. [9]. During the inversion the dispersion curve is computed many times and the distance between theoretical and measured dispersion points (misfit function) is minimized. The starting models (velocity with a constant step between all layers) are randomly generated in the vicinity of the initial starting model. Java sea Indian Ocean Fig. 2. The location of stations III. RESULTS The tranverse component of seismograms of BI2, CI1, CJ1, CJ2, CJ3, CK4, and CK6 stations that used in this paper are given in Fig 3, after bandpass filtered Hz and zooming. The dispersion of Love wave are not clearly shown. Amplitude of surface wave (Love wave) seem less than amplitude of body wave phase. Although the signals are recorded by using short period seismograph (Mark L4-3D), they are processed by using multiple filtering technique in SVAL. Therefore, the dispersion curve can still be obtained from the maxima of envelopes. Measured Love wave group velocity dispersion curves (fundamental mode) are presented in Fig. 4 and their inversion are ilustrated in Fig.5. It shows that CI1 and CJ3 have the highest velocity while CJ1, CJ2, CK4, CK6, and BI2, have the lowest one. All the dispersion curves have a flat region in period below 10 s, and at least two ridges at period 10 s and 20 s. CK4 and CK6 relatively have complex and unsmooth dispersion curves. Furthermore, crustal shear wave velocity structure models inverted from dispersion curves are illustrated in Fig. 6. In this paper we used nine layers over the halfspace. IV. DISCUSION AND CONCLUSION According to shear wave velocity structure on Fig. 6 we can observe that crustal structure between Mt. Merapi and Mt. Lawu has a complex structure. Moho discontinuity beneath BI2, CJ1, CJ2, CJ3, and CK4 stations is not clearly shown, however, it assumed to be located in 20 km depth. Whereas beneath CI1 is assumed to be located in 24 km depth, and CK6 stations in about 30 km depth. - Crustal shear wave velocity structure beneath BI2. There are low velocity zones between 4-12 km with has the shear wave velocity km/s, and between km depth with shear wave velocity km/s. Station BI2 located on east side flange of Mt. Merapi - Crustal shear wave velocity structure beneath CI1. The shear wave velocity structure beneath CI1 is between km/s. Below 20 km until 24 km depth the shear velocity decrease to km/s, it may be interpreted as LVZ in the upper mantle. this station was located on local high-velocity anomalies (blue circles in Fig. 1) which are apparently associated with the volcanoes [3]. - Crustal shear wave velocity structure beneath CJ1 Beneath CJ1 station there is low velocity zone between km depth, with the shear wave velocity km/s. - Crustal shear wave velocity structure beneath CJ2 Crustal structure beneath CJ2 station has two low velocity zones: between 4-8 km depth and km depth, with shear wave velocity km/s and km/s respectively. - Crustal shear wave velocity structure beneath CJ3 CJ3 has low velocity zone located in km depth with shear wave velocity km/s. - Crustal shear wave velocity structure beneath CK4 Low velocity zones beneath CK4 station located at 4-8 km depth, with shear wave velocity km/s. - Crustal shear wave velocity structure beneath CK6 Crustal structure beneath CK6 station has low velocity zones between 4-8 km depth, with shear wave velocity km/s. This station located near the Mt. Lawu. Inferred from crustal shear wave model from this research, crustal structure beneath all station have value km/s km/s. The low velocity zones between Mt. Merapi and Mt. Lawu according to [1], [2], and [3] has possibility represent the location of channels or chambers filled with frozen magmatic rocks having higher seismic velocities. In particular, it can be conclude that there are low velocity zones in the crustal structure between Mt, Merapi and Mt. Lawu with shear wave velocity vary to each stations. Moho discontinuity inferred from this inversion models have depth between km. It can be inferred from the crustal shear wave velocity structure, that the low velocity zone in the region between Mt. Merapi and Mt. Lawu is primarily at 4-8 km depth.

3 vel. (km/s) BI2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 40 CI1 CJ1 CJ2 CJ3 CK4 CK6 Fig. 3. Tranverse component of seismograms that used in this paper after bandpass filtered Hz and zooming. The dispersion of Love wave are not clearly shown. 4 Love wave group velocity dispersion curve BI2 CI1 CJ1 CJ2 CJ3 CK4 CK6 Fig. 4. Measured Love wave group velocity dispersion curves.

4 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 41 BI2 CI1 CJ1 CJ2 CJ3 CK4 CK6 Fig. 5. Inversion of measured Love wave group velocity dispersion curves from Fig 4.

5 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 42 BI2 CI1 CJ1 CJ2 CJ3 CK4 CK6 Fig. 6. Velocity structure models inferred from inversion of Love wave group velocity dispersion in Fig 5

6 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06 43 ACKNOWLEDGEMENT The authors would like to acknowledge MERAMEX and Geophyics Study Program Gadjah Mada University for seismogram data, and Mr. Kolinsky for SVAL. REFERENCES [1] Koulakov I, M. Bohm, G. Asch, B.G. Lühr, A. Manzanares D structure of the crust and uppermost mantle beneath Merapi and Lawu volcanoes, Central Java, from local source seismic tomography. Geophysical Research Abstracts, Vol. 8, [2] Koulakov I., M. Bohm,G. Asch, B.G. Luehr,A. Manzanares, K. S. Brotopuspito, Pak Fauzi, M. A. Purbawinata, N. T. Puspito, A. Ratdomopurbo, H. Kopp, W. Rabbel, and E. Shevkunova P and S velocity structure of the crust and the upper mantle beneath Central Java from local tomography inversion. Journal of geophysical research, vol. 112, b08310 [3] Koulakov I., A. Jakovlev, B.G. Luehr Anisotropic structure beneath Central Java from local earthquake tomography. G 3 Research Letter Volume 10, Number 2, 24 February [4] Luehr, B.G., I. Koulakov, W. Rabbel, J. Zschau, A. Ratdomopurbo,S.B., Kirbani, Fauzi, and D. P. Sahara Fluid ascent and magma storage beneath Gunung Merapi. Geophysical Research Abstracts. Vol. 13, EGU , [5] Kolínský, P., 2004: Surface wave dispersion curves of Eurasian earthquakes: the SVAL Program, Acta Geodyn. Geomater., Vol. 1, No. 2 (134), [6] Dziewonski, A., Bloch, S. and Landisman, M.: 1969, A technique for the analysis of transient seismic signals, Bull. Seism. Soc. Am., 59, [7] Kolínský, P. and Brokešová, J.: 2007, The Western Bohemia uppermost crust shear wave velocities from Love wave dispersion, Journal of Seismology, 11, [8] Málek J, Horálek J, Janský J (2005) One-dimensional qp-wave velocity model of the upper crust for the West Bohemia/Vogtland Earthquake Swarm Region. Stud Geophys Geod 49: [9] Málek J, R užek B, Kolᡠr P (2007) Isometric method: efficient tool for solving non-linear inverse problems. Stud Geophys Geod 51: