Seismotectonic Pattern and the Source Region of Volcanism in the Central Part of Sunda Arc

Journal of Asian Earth Sciences 25 (2005) 583–600 www.elsevier.com/locate/jaes

Seismotectonic pattern and the source region of volcanism in the central part of Sunda Arc

Alesˇ Sˇpicˇa´k*,Va´clav Hanusˇ, Jirˇ´ı Vaneˇk

Geophysical Institute of the Academy of Sciences of the Czech Republic, Bocˇnı´ II/1401, 141 31 Praha 4, Czech Republic

Received 10 February 2004; accepted 21 May 2004

Abstract

The seismotectonic pattern in the central part of the Sunda Arc (Java, Nusa Tenggara) was studied in relation to the distribution of active calc-alkaline volcanoes, using global seismological data. Hypocentral determinations of the International Seismological Centre from the period 1964–1999, as relocated by Engdahl, and Harvard Centroid Moment Tensor Solutions from the period 1976–2003 were used. The following phenomena, which could assist the location of the source region of primary magma for island arc calc-alkaline volcanism, were observed: (1) An aseismic gap without any strong teleseismically recorded earthquakes was found in the Wadati-Benioff zone of the subducting slab along the whole investigated region of the Sunda Arc, forming a continuous strip of laterally variable depth and shape, at depths between 100 and 200 km. The absence of strong earthquakes (with mbO4.0) indicates a signiﬁcant change in the mechanical properties of the subducting slab at intermediate depths. All active calc-alkaline volcanoes in the Sunda Arc are located above this gap. (2) The majority of earthquakes occurring in the lithospheric wedge of the Eurasian Plate above the subducted slab could be attributed to several deep-rooted seismically active fracture zones of regional extent. All delineated active fracture zones display a thrust tectonic regime as shown by the available fault plane solutions. (3) Clusters of earthquakes were found beneath active volcanoes of western Java, Bali and Nusa Tenggara in the lithospheric wedge above the slab and identiﬁed as seismically active columns. These clusters occur only beneath the volcanoes that are located at the outcrops of seismically active fracture zones. We interpret the earthquakes in these clusters beneath volcanoes as events induced by magma transport through the medium of the lithospheric wedge that has been subcritically pre-stressed by the process of plate convergence. (4) Beneath the volcanoes of central Java no seismically active columns were observed. The latter volcanoes are not located at the outcrop of any seismically active fracture zone. The presence of the intermediate-depth aseismic gap in the Wadati-Benioff zone and the presence of seismically active columns above the gap beneath some active volcanoes support the concept that the subducted oceanic lithosphere is the source region for the primary magma for calc-alkaline volcanoes at convergent plate margins. However, for volcanoes without any seismically active columns and fracture zones beneath them, a source region for the primary magma in the lithospheric wedge above the subduction zone cannot be excluded. q 2004 Elsevier Ltd. All rights reserved.

Keywords: Calc-alkaline volcanoes; Wadati-Benioff zone; Sunda Arc

1. Introduction Simandjuntak and Barber, 1996). During our previous investigations into the seismicity pattern of different The Sunda Arc, especially its central part, comprising convergent plate margins by means of narrow vertical Java and Nusa Tenggara, with its strong seismic as well as cross sections perpendicular to pertinent oceanic trenches, volcanic activity, represents one of the most stimulating we identiﬁed a regularly occurring intermediate-depth regions for scientists trying to understand the fundamental aseismic gap in the Wadati-Benioff zone directly situated problems of geodynamic evolution of active convergent beneath active calc-alkaline volcanoes. This led us to the plate margins (Hall, 1996; Richardson and Blundell, 1996; conclusion that the aseismic part of the subducted slab was likely to represent a partially melted medium and the site of

* Corresponding author. Tel.: C420-267-103-345; fax: C420-272-762- the main source of primary magma for active volcanoes at 546. convergent plate margins (Hanusˇ and Vaneˇk, 1976, 1978, E-mail address: [email protected] (A. Sˇpicˇa´k). 1985, 1988; Vaneˇk et al., 1987; Hanusˇ et al., 1996).

The intense seismic activity of the Sunda island arc seems to active volcanoes at different convergent plate margins be an ideal subject for the detailed study of the geometry of (Sˇpicˇa´k et al., 2004a). The study of seismicity patterns distribution of earthquake foci for finding and interpreting beneath and around 10 active calc-alkaline volcanoes from inhomogeneities in the distribution of earthquake foci in the eight different convergent plate margins demonstrated that Wadati-Benioff zone and in the overlying lithospheric an aseismic gap in the Wadati-Benioff zone occurred wedge of the Eurasian Plate. directly beneath all active volcanoes under study; however, Volcanoes at convergent plate margins are generally a seismically active column beneath individual volcanoes agreed to be related to the process of subduction (Hatherton may, or may not be present. The presence or absence of a and Dickinson, 1969; Ringwood, 1974; Maaloe and seismically active column seems to be due to local Petersen, 1981; Hawkesworth et al., 1997). However, the conditions, which cannot be determined without the site of generation of calc-alkaline magma feeding these systematic investigation of active, genetically homo- volcanoes is thought to be either in the mantle wedge above geneous, arcs. In order to understand the influence of local the slab (Plank and Langmuir, 1988; McCulloch and conditions on the relationship between the pattern of Gamble, 1991) or in the slab itself (White and Dupre´, seismicity and the source region of the magma for calc- 1986; Defant and Drummond, 1990; Edwards et al., 1993; alkaline volcanoes, the Central American arc was investi- Ryan and Langmuir, 1993). Both concepts are based gated. This investigation confirmed the presence of an predominantly on geochemical analyses and experiments intermediate-depth aseismic gap in the Wadati-Benioff zone and related modelling. beneath all Central American active volcanoes and, more- The results of our previous studies on the deep structure over, the presence of considerable seismic activity in the and seismotectonics of Sumatra (Hanusˇ et al., 1996) and on lithospheric wedge beneath all volcanoes (Sˇpicˇa´k et al., the seismicity pattern beneath and around Krakatau Volcano 2004b). (Sˇpicˇa´k et al., 2002) inspired us to apply the set of relocated ISC data (Engdahl et al., 1998) in the regions of Java and The ubiquitous occurrence of the intermediate-depth Nusa Tenggara to study the morphology and internal aseismic gap in all Wadati-Benioff zones beneath active structure of the Wadati-Benioff zone, the active tectonic calc-alkaline volcanoes and the presence of a column-like structures of the lithospheric wedge and the possible cluster of earthquakes beneath some active volcanoes led us relationship of seismic and volcanic activity. Our study on to the conclusion that the seismicity pattern could assist the Krakatau (Sˇpicˇa´k et al., 2002) pointed to an important identification of the source region of subduction-generated phenomenon—the presence of an uninterrupted column of magmas. earthquakes down to the upper boundary of the subducting The present study summarizes the seismotectonic results slab at a depth of 100 km, which indicated the brittle obtained in the central part of the Sunda island arc covering character of the mantle wedge beneath the volcano. Java and Nusa Tenggara islands between 1058E and 1208E In order to verify the presence of the intermediate-depth (Fig. 1). In this region, earthquakes are the result of aseismic gap and seismically active columns beneath active underthrusting of the Indo-Australian Plate under the calc-alkaline volcanoes, we focused our interest on selected southeastern margin of the Eurasian Plate.

Fig. 1. Distribution of shallow and intermediate-depth earthquakes (h!300 km) in the Sunda Arc. Symbols of earthquakes with ISC magnitude mb: mb%4.0, 4.0! mb%4.5, 4.5! mb%5.0, 5.0! mb%6.0, mbO6.0. The Java trench is denoted by a serrated line, central active volcanoes of volcanic domains by black-and-white triangles, boundaries of sections J3–J40, Sb1–Sb15 by thin lines (width of sections w 45 km) and the reference line for vertical sections J3–J40, Sb1–Sb15 by a dashed line. A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600 585

2. Materials and method used hypocentral determinations of the International Seismological Centre (ISC) from the period 1964–1999 The seismicity pattern of the Indonesian plate margin (Regional Catalogue of Earthquakes 1964–1999) as relo- was studied by means of global seismological data. We have cated by the procedure of Engdahl et al. (1998), denoted as

Fig. 2(a) Vertical cross sections perpendicular to the trench giving the depth distribution of earthquake foci in relation to the trench for volcanic domains Gedeh (J7, J8), Guntur (J10, J11), Slamet (J13), Merapi (J16) and Kelut (J21); the reference line (see Fig. 1) is denoted by a dashed line, axis of the Java trench by an arrow, the Wadati-Benioff zone schematically by solid parallel lines and the intermediate-depth aseismic gap by dashed lines. For position of sections and volcanic domains and for symbols of earthquakes and volcanoes see Fig. 1. (b) Vertical cross sections for volcanic domains Raung (J25, J26), Batur (J29, J30), Rinjani (J32), Tambora (J35) and Sangeang (J37, Sb1). (c) Vertical cross sections for volcanic domains Ranakah (Sb4), Paluweh (Sb6, Sb7), Lewotobi (Sb9, Sb10) and Sirung (Sb12, Sb13). 586 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

Fig. 2 (Continued)

EHB relocations. The EHB database contains events that are are not considered, because the problem of deep seismicity well-constrained teleseismically by arrival times reported to is beyond the scope of this article. ISC. Harvard Centroid Moment Tensor Solutions (HCMTS) Information on the volcanoes was taken from the were available for the period from 1976 to the beginning of Catalogue of the Active Volcanoes of the World (Neumann 2003. It should be noted that deep earthquakes (hO300 km) van Padang, 1951), Volcanoes of the World (Simkin et al., A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600 587

Fig. 2 (Continued)

1981), Smithsonian Global Volcanism Program, Volcanoes Volcanoes of the World and in individual papers (e.g. of the World, Volcanoes of Indonesia (http://www.volcano. Foden and Varne, 1980). si.edu/gvp/) and Volcanic Activity Reports (Venzke et al., To analyse seismic activity of the region, narrow 2002). The petrological character of lavas for individual vertical cross-sections perpendicular to the Java Trench volcanoes can be found in the Catalogue of the Active and to the volcanic chain (azimuth 108 from 1058Eto 588 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

1198E and 08 from 1198E to 1258E, section width 45 km) (Fig. 2a–c). It should be stressed that the Wadati-Benioff have been used (Fig. 1). This technique enabled us to zone is understood as that part of the subducting slab where distinguish between the events belonging to the Wadati- earthquakes are observed; the varying thickness of the Benioff zone and those located in the overlying wedge Wadati-Benioff zone, schematically shown by parallel lines

Fig. 3. Projection of the intermediate-depth aseismic gap (shaded area) and of earthquake foci in the Wadati-Benioff zone for Java (J1–J40) and Sumba (Sb1–Sb17) sections on the upper plane of the Wadati-Benioff zone along the Java trench; D is distance measured along the dip of the slab (dip of 458 taken for all sections). Projections of all active and Holocene volcanoes in the intermediate-depth aseismic gap are denoted by open triangles, central volcanoes of volcanic domains by black-and-white triangles. For position of sections and volcanic domains and for symbols of earthquakes and volcanoes see Fig. 1. Some of earthquake data are shown twice in sections J38–J40 and Sb1–Sb3, respectively, due to an overlap of these sections with different azimuths (108 and 08, respectively). .S A. ˇ pic ˇ a ´ ta./Junlo sa at cecs2 20)583–600 (2005) 25 Sciences Earth Asian of Journal / al. et k

Fig. 4. System of seismically active fracture zones in the lithospheric wedge of the Sunda Arc with respective earthquake epicentres, vertical sections across individual fracture zones, and HCMTS fault plane solutions with fault plane orientation corresponding to the geometry of fracture zone; inclination of fracture zones denoted by arrows. Symbols of earthquakes and volcanoes as in Fig. 1. 589 590 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

Fig. 5. Epicentral map of earthquakes belonging to the seismically active columns (dotted elliptical boundaries) of volcanic domains and position of all active volcanoes. Symbols of earthquakes and volcanoes as in Fig. 1. in Fig. 2a–c, does not imply any change in the thickness of (Hanusˇ and Vaneˇk, 1985, 1988; Delouis et al., 1996; the slab. Hanusˇ et al., 1996; Sˇpicˇa´k et al., 2002; Vaneˇk et al., 1987, At depths between 100 and 200 km, the Wadati-Benioff 1994). The depth and thickness of the gap varies laterally zone is characterized by a region without any strong along individual arcs. The absence of strong earthquakes teleseismically recorded events. Such a gap in seismic indicates a signiﬁcant change in the mechanical properties activity is characteristic for many Wadati-Benioff zones and of the subducting slab at intermediate depths. The rock correlates spatially with active calc-alkaline volcanism medium loses its ability of brittle failure necessary for

Fig. 6(a) Distribution of earthquake foci in the seismically active column and position of active volcanoes in the map of epicentres, and in N–S and E–W vertical sections for volcanic domains: Gedeh (1—Gedeh, 2—Salak, 3—Kiaraberez-Gagah), Batur (1—Batur, 2—Agung) and Rinjani. Symbols of earthquakes and volcanoes as in Fig. 1. Fault plane solutions of HCMTS. (b) Distribution of earthquake foci in the seismically active column and position of active volcanoes in the map of epicentres, and in N–S and E–W vertical sections for volcanic domains: Tambora, Sangeang and Ranakah. (c) Distribution of earthquake foci in the seismically active column and position of active volcanoes in the map of epicentres, and in N–S and E–W vertical sections for volcanic domains: Paluweh (1—Paluweh, 2—Inielika, 3—Ebulobo, 4—Iya, 5—Kelimutu), Lewotobi (1—Lewotobi, 2—Egon, 3—Lereboleng, 4—Iliboleng) and Sirung (1- Sirung, 2—Lewotolo, 3—Iliwerung). A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600 591

Fig. 6 (Continued) generating strong earthquakes. The extent of the intermedi- neighbouring active volcanoes and available HCMTS fault ate-depth aseismic gap in the Wadati-Benioff zone is shown plane solutions are also plotted. by projections of earthquake hypocentres on the upper surface of the Wadati-Benioff zone and denoted by the shaded area in Fig. 3. 3. Delimitation of volcanic domains and parameters From the distribution of earthquake foci in the litho- of the Wadati-Benioff zone spheric wedge above the Wadati-Benioff zone, a system of seismically active fracture zones was delineated (Fig. 4) and The belt of active calc-alkaline volcanoes in Java and Nusa compared to HCMTS fault plane solutions and to tectonic Tenggara was divided by visual inspection of their distribution structures in the region investigated (Hamilton, 1979; into fourteen spatially separated domains; the domains were Simandjuntak and Barber, 1996; Malod and Mustafa named after respective central volcanoes. All volcanoes Kemal, 1996). referenced in the Smithsonian data set (http://www.volcano. As for Krakatau Volcano (Sˇpicˇa´k et al., 2002), and the si.edu/gvp/–Venzke et al., 2002) were considered. The Middle American volcanic chain (Sˇpicˇa´k et al., 2004b), a position of individual active volcanoes together with the distinctive pattern of seismicity was observed beneath the interpretation of earthquake distribution in each domain are majority of volcanoes in the form of a column-like cluster of given in Table 1, which contains vertical section numbers events, clearly separated from events in the Wadati-Benioff depicting the earthquake foci distribution beneath the zone. We term this cluster a seismically active column. respective volcano (Fig. 2a–c), parameters of the Wadati- Detailed views of earthquake distribution in seismically Benioff zone beneath volcanoes, depth extent of the active columns of the Indonesian volcanic domains (Fig. 5) intermediate-depth aseismic gap in the Wadati-Benioff zone, are shown on a map of epicentres with E–W and N–S and information on the occurrence of the seismically active vertical sections in Fig. 6a–c on which the position of column in the lithospheric wedge beneath volcanic domains. 592 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

Fig. 6 (Continued)

4. Intermediate-depth aseismic gap also supported by results of England et al. (2004) that in the Wadati-Benioff zone the location of arc volcanoes is controlled by a process that depends critically on temperature. Vertical sections in Fig. 2a–c confirm the presence of a The lateral variation and extent of the intermediate-depth region without strong intermediate-depth earthquakes with aseismic gap along the volcanic chain are shown by its mbO4.0 in the Wadati-Benioff zone beneath all active calc- projection on the plane of the upper boundary of the Wadati- alkaline volcanoes of the Indonesian volcanic chain. The Benioff zone (dip 458, strike 108 in the region of Java and 08 aseismic gap in the Wadati-Benioff zone was found at first in the region of Nusa Tenggara) and denoted by the shaded in 1976 in the Andean subduction zone (Hanusˇ and Vaneˇk, area in Fig. 3, which confirms that the thickness and depth of 1976, 1978) and later confirmed in several active subduction the intermediate-depth aseismic gap substantially varies zones (Hanusˇ and Vaneˇk, 1985; Vaneˇk et al., 1987, 1994; under individual volcanic domains. Fig. 3 shows the lateral Delouis et al., 1996; Hanusˇ et al., 1996). It is difficult to course of the intermediate-depth aseismic gap with several interpret this aseismic gap in the Wadati-Benioff zone abrupt offsets in its position, namely at J4 to J5, J10 to J11, beneath volcanic chain from seismic observations only. This J16 to J17 and J30 to J31. The analysis of causes of such phenomenon should be caused by a change in mechanical behaviour is beyond the scope of this article. properties of the rock medium, rather than by a difference in The projections of all active (A) and Holocene (H) stress conditions or heterogeneity in the slab during volcanoes of the Sunda Arc from the Smithsonian data set subduction. The rock medium at intermediate depths loses (http://www.volcano.si.edu/gvp/- Venzke et al., 2002) are its ability of brittle failure necessary for generating strong also plotted from W to E in Java sections J3–35 and in earthquakes, and may be interpreted as a partially melted Sumba sections Sb1–13 of Fig. 3: Krakatau (A) in J 3, domain in the subducted slab. The idea of partial melting is Radjabasa (A) in J4, Danau Complex (H) and Karang (H) in Table 1 Volcanic domains in the central part of the Sunda Arc (Java, Nusa Tenggara) and parameters of the Wadati-Benioff zone beneath them

Volcano name and coordi- Associated volcanoes Section no (ﬁg. no) WBZa beneath the volcanic domain IDAGb SACc nates Dip (8) Azimuth (8) Max. depth (km) Thickness (km) Depth (km) Gedeh (6.788S, 106.988E) Salak (6.728S, 106.738E) J7, J8 (Fig. 2a) 40 10 180 70 80–100 Yes Kiaraberez-Gagak (6.738S, 106. 658E)

Guntur (7.138S, 107.838E) Tangkubanparahu (6.778S, 107. J10, J11 (Fig. 2a) 45 10 270 70 100–170 No S A. ˇ

608E) pic ˇ a Papandayan (7.328S, 107.738E) ´ Galunggung (7.258S, 108.058E) 583–600 (2005) 25 Sciences Earth Asian of Journal / al. et k Cereme (6.898S, 108.408E) Slamet (7.248S, 109.218E) J13 (Fig. 2a) 45 10 250 55 150–230 No Merapi (7.548S, 110.448E) Dieng (7.208S, 109.928E) J16 (Fig. 2a) 40 10 220 50 150–220 No Sundoro (7.308S, 109.998E) Sumbing (7.388S, 110.068E) Merbabu (7.458S, 110.438E) Kelut (7.938S, 112.318E) Arjuno-Welirang (7.728S, 112.588E) J21 (Fig. 2a) 40 10 190 70 100–140 No Semeru (8.118S, 112.928E) Bromo (7.948S, 112.958E) Lamongan (8.008S, 113.348E) Raung (8.128S, 114.04E8) Ijen (8.068S. 114.248E) J25, J26 (Fig. 2b) 35 10 170 60 80–120 No Batur (8.248S, 115.388E) Agung (8.348S, 115.518E) J29, J30 (Fig. 2b) 35 10 170 60 80–100 Yes Rinjani (8.428S, 116.478E) J32 (Fig. 2b) 45 10 270 60 75–160 Yes Tambora (8.258S, 118.008E) J35 (Fig. 2b) 35 10 225 75 75–100 Yes Sangeang (8.188S, 119.068E) J37, Sb1 (Fig. 2b) 45 0 280 60 110–150 Yes Ranakah (8.628S, 120.528E) Sb4 (Fig. 2c) 35 0 140 55 80–110 Yes Paluweh (8.328S, 121.718E) Inielika (8.738S, 120.988E) Sb6, Sb7 (Fig. 2c) 45 0 210 75 75–125 Yes Ebulobo (8.818S, 121.188E) Iya (8.888S, 121.638E) Kelimutu (8.768S, 121.838E) Lewotobi (8.538S, 122.788E) Egon (8.708S, 122.508E) Sb9, Sb10 (Fig. 2c) 40 0 280 60 110–165 Yes Lereboleng (8.408S, 122.808E) Iliboleng (8.348S, 123.268E) Sirung (8.518S, 124.158E) Lewotolo (8.278S, 123.508E Sb12, Sb13 (Fig. 2c) 45 0 310 65–90 135–170 Yes Iliwerung (8.548S, 123.598E)

a WBZ—Wadati-Benioff zone. b IDAG—intermediate-depth aseismic gap in the Wadati-Benioff zone. c SAC—seismically active column in the lithospheric wedge beneath volcano. 593 594 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

J5, Kiaraberes-Gagak (A) in J6, Perbakti (H) and Salak (A) Thus it may be concluded that the aseismic gap, in our in J7, Gedeh (A) in J8, Patuha (H), Tangkubanparahu (A), interpretation a partially melted part of the subducted slab, Wayang-Windu (H) and Malabar (H) in J9, Kendang (H), is likely to represent the source region of primary magma Papandayan (A), Kamojang (H), Guntur (A), Tampomas for active calc-alkaline volcanoes. (H), Galunggung (A), Talagabodas (H) and Karaha (H) in J10, Cereme (A) in J11, Slamet (A) in J13, Dieng Complex (A), Sundoro (A) and Sumbing (A) in J15, Ungaran (H), 5. Seismically active fracture zones Telomoyo (H), Merbabu (A) and Merapi (A) in J16, Lawu (H) and Wilis (H) in J18, Kelut (A) and Kawi-Butak (H) in The distribution of earthquake foci at convergent plate J21, Arjuno-Welirang (A), Penanggungan (H), Malang margins reveals that the contemporary tectonic processes Plain (H) and Semeru (A) in J22, Tengger Caldera (A) are also manifested by relatively high seismicity in the and Lamongan (A) in J23, Iyang-Argapura (H) and Luru (H) lithospheric wedge above the Wadati-Benioff zone (Hanusˇ in J24, Raung (A) in J25, Ijen (A) and Baluran (H) in J26, and Vaneˇk, 1977–78, 1979, 1984, 1987; Hanusˇ et al., 1996). Bratan (H) in J28, Batur (A) and Agung (A) in J29, Rinjani It appears that the foci of these earthquakes are not (A) in J32, Tambora (A) in J35, Sangeang Api (A) in Sumba distributed randomly, but instead show a tendency to Sb1, Sano Wai (H) in Sb3, Poco Leok (H) and Ranakah (A) occur in well-separated linear zones. These zones can be in Sb4, Inierie (H) and Inielika (A) in Sb5, Ebulobo (A) in interpreted as a system of deep seismically active fractures Sb6, Iya (A), Paluweh (A), Sukaria Caldera (H) and Ndete induced and/or activated by the process of subduction. Napu (H) in Sb7, Kelimutu (A) in Sb8, Egon (A) in Sb9, In the region under study ten seismically active fracture Ilimuda (H), Lewotobi (A), Leroboleng (A) and Riang zones were delineated (Fig. 4, Table 2). Their traces were Kotang (H) in Sb10, Iliboleng (A) and Ililabalekan (H) in determined by the shallowest earthquakes associated with Sb11, Lewotolo (A), Tara Batu (A) and Iliwerung (A) in the individual fracture zones. The position of the trace of Sb12, Sirung (A) in Sb13. each fracture zone is given in geographical co-ordinates. Previous authors dealing with the structure and mor- The length and width of the fracture zone are measured on phology of the Wadati-Benioff zone did not mention this the surface. The dip is measured from the horizontal plane, remarkable phenomenon. This is probably due to the lateral and the thickness is measured perpendicular to the dip. The depth variations of the intermediate-depth aseismic gap. We azimuth is measured in the clockwise direction from the have veriﬁed that an intermediate-depth aseismic gap in the north. The maximum depth is given by the deepest Wadati-Benioff zone does not usually appear if the vertical earthquake associated with the fracture zone. Number of cross-sections perpendicular to the trench are broader than earthquakes and their magnitude range are added. Available 50–100 km. The intermediate-depth aseismic gap has thus HCMTS solutions for earthquakes located in the fracture escaped the attention of investigators studying the distri- zone are also mentioned; their numbering follows the bution of earthquake foci in the Wadati-Benioff zone who scheme of the HCMTS database. The fracture zones are used broader vertical sections (100 km and more), see labelled with names of localities or geomorphological Barazangi and Isacks (1976, 1979a,b), Cardwell and Isacks features situated in their traces. (1978), Engdahl (1981), Hatherton and Dickinson (1969), Fault plane solutions shown in Fig. 4 offer fault planes Isacks and Barazangi (1981), Jacob et al. (1981), Newcomb consistent with the geometry of fracture zones determined and McCann (1987), Sacks (1984), Scho¨ffel and Das (1999). from the distribution of earthquake hypocentres; no focal Another factor that prevents the observation of a mechanisms of this type are available for fracture zones K1 ubiquitous intermediate-depth aseismic gap in the Wadati- and L1. The presence of seismically active fracture zones of Benioff zones seems to be the inappropriate projection of regional extent in the lithospheric wedge above the slab earthquake foci. The epicentral maps, as well as longitudi- probably reﬂects the response of the overriding plate to the nal sections parallel to the trenches projected onto the process of subduction. Similar seismically active fracture vertical plane are not suitable for visualization of a gap in zones are observed also at other convergent margins, e.g. seismicity inside the subducted slab. The only projection of Mexico, where fracture zones are the sites for shallow earthquake foci that makes possible the recognition of the disastrous earthquakes (Vaneˇk et al., 2000) and in South gap is a projection on the inclined plane representing the America, where fracture zones serve as feeding channels of upper surface of the slab (Fig. 3). hydrothermal solutions for hypogene ore deposits (Hanusˇ The regular spatial relationship of the aseismic gap in the et al., 2000). Wadati-Benioff zone with the chain of active volcanoes, Fig. 4 shows the non-uniform distribution of seismic forming the Indonesian archipelago, represents the domi- activity in the lithospheric wedge (mentioned also in nant factor governing the distribution of volcanism at this Scho¨ffel and Das, 1999). The seismic activity in western convergent plate margin. There is no active calc-alkaline Java between 1058E and 1098E (fracture zones K1, L1, L2), volcano in the region under study that could not probably connected with the bend of the arc between be correlated with an intermediate-depth aseismic gap, Sumatra and Java, is followed by a region with very low lying immediately beneath it in the Wadati-Benioff zone. seismic activity between 1098E and 1148E; the only Table 2 Seismically active fracture zones in the lithospheric wedge of Java and Nusa Tenggara

Fracture zone (F.Z.) Position of the F.Z. Azimuth (8) Length (km) Width (km) Thickness Dip (8) Max. depth No. of events mb range (km) (km) Pelabuhan Ratu F.Z. (K1) 6.328S, 106.908E–6.768S, 60 95 40 35 60 to SE 55 12 4.4–5.1 106.168E 6.628S, 107.068E–7.068S, 106.368E The fracture zone contains the Pelabuhan Ratu fault (Schlu¨ter et al., 2002). Bawean F.Z. (K2) 5.008S, 113.208E–7.208S, 50 355 105 85 50 to SE 55 12 4.2–5.3 110.808E

5.708S, 113.808E–7.848S, S A.

111.408E ˇ pic

Fault plane solution B062102B indicates strike-slip motion and B082602E reverse mechanism with strong strike-slip component; both solutions have fault plane orientation corresponding to the geometry of the ˇ a ´ fracture zone. The NE part of the fracture zone is situated in the Bawean Arch between the Pati Trough and East Florence Basin (Simandjuntak and Barber, 1996), the seismic activity being concentrated inthe 583–600 (2005) 25 Sciences Earth Asian of Journal / al. et k axes of the latter three tectonic structures. West Flores F.Z. (K3) 7.708S, 120.968E–9.188S, 30 190 55 45 60 to NW 65 22 4.5–6.2 120.108E 7.968S, 121.388E–9.448S, 120.528E Fault plane solutions C073189A, B012300A, B012300C, B080901F indicate reverse mechanism with fault plane orientation corresponding to the geometry of fracture zone; solutions B080682B, B030190C, B071999C show strike-slip mechanism. East Flores F.Z. (K4) 5.948S, 123.738E–8.908S, 30 385 70 60 60 to NW 70 43 4.0–5.8 122.008E 6.288S, 124.308E–9.248S, 122.568E Fault plane solutions B051993B, B073198D indicate reverse mechanism with fault plane orientation corresponding to the geometry of the fracture zone; solutions B122582A, B052195A, B061795A show strike- slip mechanism. Alor F.Z. (K5) 7.808S, 124.608E–9.248S, 30 185 50 45 60 to NW 60 41 4.0–6.2 123.788E 8.048S, 125.008E–9.488S, 124.188E Fault plane solutions B070491C, C070491F, B051994D, B052094B indicate reverse mechanism with fault plane orientation corresponding to the geometry of fracture zone; solution C112687A shows left-lateral strike-slip mechanism, B080282A points to normal mechanism. The strike of the fracture zone corresponds to the strike of two unnamed left-lateral strike-slip faults (Simandjuntak and Barber, 1996) bordering the island of Alor (compare fault plane solution C112687A). Pulau Deli F.Z. (L1) 6.628S, 105.928E–7.408S, 120 165 75 50 35 to NE 55 25 4.4–5.1 107.248E 7.268S, 105.588E–8.048S, 106.888E Fault plane solution B121286E indicates strike-slip mechanism, solution B071399D points to reverse faulting and B071200B shows normal faulting; the latter three solutions do not correspond to the geometry of fracture zone. Bandung F.Z. (L2) 6.468S, 107.808E–7.248S, 125 155 30 25 70 to NE 40 5 4.6–5.6 108.968E 6.108S, 109.648E–7.468S, 108.808E Fault plane solutions B070690A, B020492A, B092400A indicate reverse mechanism with fault plane orientation corresponding to the geometry of fracture zone. The outcrop of the fracture zone may correspond to the Citandui Fault (Simandjuntak and Barber, 1996). (continued on next page) 595 596 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

). exception is the Bawean fracture zone K2 comprising earthquakes with strike-slip mechanism. From longitude 114.58E eastward, intensive seismic activity connected with range

b fracture zones M2, M3, M4, K3, K4, K5 is observed. The m stress ﬁeld in the lithospheric wedge beneath Bali and Nusa Tenggara islands is probably strongly inﬂuenced by the continental collision in the zone of convergence between Australia and the Banda arc near Timor (Simandjuntak and Barber, 1996). No. of events Simandjuntak and Barber, 1996 The delimitation of seismically active fracture zones performed in this study seems to indicate that only two preferred orientations from the whole set of existing faults

e orientation corresponding to the geometry of are activated by the regional tectonic stress ﬁeld. The orientation corresponding to the geometry of the sm, solutions C073189A, B082493C show reverse

(km) seismically active fracture zones are oriented either oblique he Flores Thrust (

ali Basin and the E part to the outcrop of the W part of to the plate boundary (oriented in SW–NE or SE–NW

he fracture zone coincides with the position and southward directions—K and L fracture zones) or along the plate boundary (M fracture zones). The latter are inclined ) Max. depth 8 southwards, i.e. against the subducting oceanic plate. Both

Dip ( types of seismically active fracture zones are activated in a thrust tectonic regime with fault planes corresponding to the strike of the respective fault zone as documented by available focal mechanisms (Fig. 4). This is in agreement with tectonic stress analysis carried out by Slancova´ et al.

(km) (2000). Such a tectonic regime is a consequence of the regional tectonic stress initiated by convergent movement of lithospheric plates. Focal mechanisms of strike-slip type are also observed; they occur only in those parts of fracture zones where seismically active columns beneath active volcanoes were found—excepting the Bawean fracture zone K2, where earthquakes of strike-slip mechanism occur about 300 km N from the volcanic chain.

). 6. Seismically active columns beneath active volcanoes ) Length (km) Width (km) Thickness 8 The detailed investigation into the distribution of earthquake foci in the lithospheric wedge of the Indonesian

). convergent margin revealed the presence of clusters of 90 180 50 45 60 to S 50 19 4.0–6.1 90 285 75 65 50 to S 85 98 4.0–6.7 90 600 110 65 35 to Searthquakes 90 in the 85 continental 4.0–6.1 wedge beneath nine of 14 active volcanic domains investigated in the present study. S, S, S, S, S, S, 8 8 8 8 8 8 These clusters, which we denote as seismically active columns, may represent an important phenomenon con- E–7.10 E–7.55 E–7.64 E–8.38 E–7.12 E–8.10 tributing to the solution of the problem of source region and 8 8 8 8 8 8 mode of transportation of primary magmas for active calc- E E E E E E alkaline volcanoes at convergent plate margins. For the ﬁrst 8 8 8 8 8 8 S, 125.24 S, 125.24 S, 120.38 S, 120.38 S, 114.58 S, 114.58 8 8 8 8 8 8 time, a seismically active column was observed beneath the ˇ ˇ´

Simandjuntak and Barber, 1996; Hamilton, 1979 Krakatau Volcano (Spicak et al., 2002) and also found 126.84 7.55 126.84 122.98 8.38 122.98 120.16 8.10 120.16 beneath all active volcanic domains of Central America (Sˇpicˇa´k et al., 2004b). From six volcanic domains of Java the presence of a )

Simandjuntak and Barber, 1996; Hamilton, 1979 seismically active column was observed only for Gedeh; no seismically active column was found beneath Guntur, Slamet, Merapi, Kelut or Raung (compare Fig. 2a and b). continued For the region of Nusa Tenggara the presence of a seismically active column was observed for all eight Table 2 ( Fracture zone (F.Z.) Position of the F.Z. Azimuth ( Wetar Thrust F.Z. (M4) 7.10 Fault plane solutions C052191A, B022703A indicate reverse mechanism with fault plane orientation corresponding to the geometry of fracture zone. T mechanism with fault plane orientation pointing to the geometry of fracture zones K3, K4. The outcrop of the fracture zone corresponds to the E part ofinclination t of the Wetar Thrust ( fracture zone, solutions B080682B, B122582A, B030190C, B071891A, B121792F, B012093D, B052195A, B061795A, B092702A point to strike-slip mechani the Flores Thrust ( Flores Basin F.Z. (M3) 7.64 Fault plane solutions C030376A, B122378A, B042782D, M121292B, B121292L B030293A, B082695H, B120798E indicate reverse mechanism with fault plane Bali Basin F.Z. (M2)Fault plane solutions 7.12 C071476A, B052179E,fracture B053079A, zone, C102079B, solutions C121779B, B042277C, B072483C, B030397E, B022099D B032188A, point C100602F to indicate strike-slip reverse mechanism. mechanism The with W part fault of plan the fracture zone corresponds to the axis of the B volcanic domains. The distribution of earthquakes A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600 597

Table 3 similar to that observed in the seismically active column of Seismically active columns beneath individual volcanic domains Tambora, and B111102B, B012303A to that observed in

Volcanic No. of mb range Max. depth Related FZ the seismically active columns of Paluweh and Lewotobi; domain events (km) solution B032188A corresponds to reverse mechanism Gedeh 22 4.0–5.1 55 K1, L1 typical for fracture zone M2. Batur 20 4.0–6.1 90 M2 Rinjani 12 4.0–5.2 90 M2 6.6. Ranakah Tambora 29 4.0–5.4 80 M2 Sangeang 10 4.5–5.4 50 M2 Ranakah 17 4.5–6.0 65 K3, M3 All events are located in West Flores fracture zone K3, Paluweh 48 4.0–6.7 85 M3 some earthquakes also belong to Flores Basin fracture zone Lewotobi 41 4.0–5.0 70 M3, K4 M3. Fault plane solutions B080682B, B071999C point to a Sirung 34 4.0–5.8 60 K5 strike-slip mechanism, similar to that observed in the seismically active column of Tambora; solution B030190C belonging to seismically active columns is documented by shows a strike-slip mechanism, similar to that observed in an epicentral map (Fig. 5) and by N–S and E–W vertical the seismically active columns of Paluweh and Lewotobi. sections of each seismically active column (Fig. 6a–c) Solution B071999C corresponds to a reverse mechanism together with the position of individual active volcanoes. typical for fracture zone K3. The available HCMTS fault plane solutions are also given. The features of individual seismically active columns are 6.7. Paluweh given in Table 3 and in the following paragraphs: All events are located in Flores Basin fracture zone M3. 6.1. Gedeh This volcanic domain is characterized by many strong earthquakes, several of them being followed by aftershocks. All events are located in Pelabuhan Ratu fracture zone The largest aftershock sequence started with the main shock K1 and Pulau Deli fracture zone L1. Individual fault plane at 1992, 12, 12 (8.508S, 121.838E, 33 km, mbZ6.7); 10 solutions point to different earthquake mechanism: strike- events were located beneath volcanoes Paluweh, Kelimutu slip (B121286E), reverse (B071399D), and normal and Iya, further 30 events of this sequence belong to the (B071200B) faulting, respectively. Lewotobi volcanic domain. The following earthquakes were followed by aftershocks: 1978, 12, 23 (8.268S, 121.338E, 6.2. Batur 40 km, mbZ5.8, three events), 1982, 4, 27 (8.328S, 121.428E, 35 km, mbZ5.3, three events), 1991, 7, 18 All events are located in Bali Basin fracture zone M2. (8.168S, 121.588E, 34 km, mbZ5.4, two events), 1993, 1, Fault plane solutions C071476A, B052179E, B053079A, 20 (8.318S, 121.368E, 45 km, mbZ5.8, ﬁve events), 1995, 8, C102079B, C121779B indicate reverse mechanism typical 26 (8.308S, 121.478E, 40 km, mbZ5.5, four events). for fracture zone M2. Solutions C030376A, B122378A, B042782D, M121292B, B121292L, B082695H, B120798E show reverse mechan- 6.3. Rinjani isms typical for fracture zone M3, solutions B071891A, B012093D show a strike-slip mechanism similar, to that Almost all events are located in Bali Basin fracture zone observed in the seismically active columns of Sangeang and M2. Fault plane solution B010600B indicates normal Lewotobi, B121792F points to a strike-slip mechanism, mechanism not corresponding to the earthquake distribution similar to that observed in the seismically active column of in fracture zone M2. Tambora, and two solutions B051993B, B082493C show reverse mechanisms with fault plane orientations pointing to 6.4. Tambora the geometry of fracture zones K3, K4.

All events are located in Bali Basin fracture zone M2. 6.8. Lewotobi Fault plane solutions B030397E, B022099D indicate strike- slip mechanism, occurring also in seismically active All events are located in Flores Basin fracture zone M3 columns of the following domains, which differs from and East Flores fracture zone K4. The seismically active earthquake reverse mechanism C100602F characteristic for column includes 30 events from the eastern part of the 1992 fracture zone M2. aftershock sequence, with the main shock in the Paluweh domain. Fault plane solutions B122582A, B061795A, 6.5. Sangeang B052195A, B092702A indicate strike-slip mechanisms similar to those observed in the seismically active column All events are located in Bali Basin fracture zone M2. of Paluweh and solution B030293A corresponds to reverse Three earthquakes have strike-slip mechanism: B042277C mechanism typical for fracture zone M3. 598 A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600

6.9. Sirung (Java, Nusa Tenggara) enabled the delimitation of: (1) a continuous zone with no moderate-to-strong earthquake Almost all events are located in Alor fracture zone K5. activity in the Wadati-Benioff zone of the subducting slab; Fault plane solution B080282A indicates a normal mech- (2) seismically active fracture zones in the lithospheric anism, and solution C112687A corresponds to a strike-slip wedge in the Eurasian Plate above the slab; (3) clusters of mechanism similar to that observed in the seismically active earthquakes (seismically active columns) in the lithospheric columns of Paluweh and Lewotobi. wedge beneath volcanoes of western Java, Bali and Nusa Tenggara. Beneath the volcanoes of central Java no 6.10. Relationship between seismically active fracture seismically active columns were observed. The correlation zones and volcanoes of these phenomena with each other and with the position of active volcanoes of the Sunda Arc allowed us to formulate From the point of view of moderate to strong earthquake the following conclusions: occurrence, the volcanic domains of Java and Nusa Tenggara can be divided into two types: those without any teleseismi- 1. The depth distribution of earthquake foci in the Wadati- cally recorded seismicity in their neighbourhood and those Benioff zone of the slab subducting below Java and Nusa with distinct seismically active columns beneath them. It Tenggara is not homogeneous. We have observed the O follows from Fig. 4 that seismically active columns in the absence of earthquakes with mb 4.0 in the depth range lithospheric wedge occur only beneath those active volcanoes 100–200 km and have identified such a phenomenon as an that are located at the outcrops of deep-rooted seismically intermediate-depth aseismic gap. The depth and the width active fracture zones of regional extent. Because seismically of the gap varies laterally. We interpret this gap as a active columns form the most seismically active parts of partially melted domain in the slab in which conditions for fracture zones, it can be supposed that magma penetrating the generating strong earthquakes are not fulfilled. The lithospheric wedge from the region of its origin to the position of the gap is spatially related to the chain of volcanoes at the Earth’s surface is responsible for local active volcanoes at the surface above the gap. There are no increase of seismicity in the fracture zones. The mechanism of active calc-alkaline volcanoes in our region of interest such a process can be compared to the process of seismicity that could not be correlated with an intermediate-depth induced by man-made fluid injections: stresses generated by aseismic gap. magma transport interact with tectonic stress caused by the 2. Earthquakes in the lithospheric wedge above the slab can convergent movement of lithospheric plates and trigger be attributed to several seismically active fracture zones moderate to strong earthquakes in the subcritically prestressed of regional extent. The position of earthquake foci of medium weakened by fracture zones. Such an interaction can events in the wedge determines the geometry of the generate a strong earthquake only in portions of continental fracture zones (their strike, dip, width, thickness and wedge where large fracture zones with faults in favourable extent in depth). All fracture zones in the region of study orientations relative to the regional tectonic stress occur. On occur in a thrust tectonic regime as documented by the the contrary, where volcanoes are located above a relatively available focal mechanisms. The parameters of one of the unfractured lithospheric wedge (Guntur, Slamet, Merapi, nodal planes offered by fault plane solutions usually Kelut, Raung) they are not associated with any teleseismically correspond to the strike and dip of the respective fracture recorded earthquakes in the underlying lithospheric wedge. zone, determined from the distribution of earthquake foci. The occurrence of earthquakes with strike-slip mechan- Fault plane solutions of earthquakes belonging to the isms in seismically active columns that do not fit the set of fracture zones systematically differ from those located in earthquakes with reverse mechanism typical for all deli- the Wadati-Benioff zone. neated fracture zones, shows that magma may generate 3. Clusters of earthquakes were found in the lithospheric earthquakes also along steep faults oriented perpendicular to wedge beneath nine of the total offourteen recently active the plate margin. It should be stressed that earthquakes volcanic domains of the region. We term such clusters occurring in the seismically active columns under volcanoes seismically active columns. Their extent in depth varies reflect a completely different process from the high- from several tens of km to 100 km. The seismically active frequency earthquakes, low-frequency earthquakes, columns can be found only beneath volcanoes located on explosion earthquakes, and volcanic tremors, commonly the outcrops of the above mentioned seismically active associated with shallow subsurface magmatic processes and fracture zones. These seismically active fracture zones recorded only by local seismic networks. might represent the major migration paths for the calc- alkaline magma to active volcanoes. We interpret the earthquakes occurring in the seismically active columns 7. Conclusions as events induced by magma transport from the depth to the Earth’s surface through a medium that is subcritically Detailed analysis of the spatial distribution of earthquake prestressed by the process of lithospheric plate conver- foci carried out in the central part of the Sunda Arc gence and weakened by extensive fracture zones. A. Sˇpicˇa´k et al. / Journal of Asian Earth Sciences 25 (2005) 583–600 599

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