Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography

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Abstract

Within the KRAKMON project for multiparameter monitoring of Anak Krakatau volcano (Indonesia), a network of temporary stations was installed on the islands of the Krakatau complex as well as in the surrounding areas of the Sunda Strait, Sumatra and Java. The network was operated from June 2005 until January 2006. More than 700 local events were recorded during this experiment, and travel times from these events were used to perform a tomographic inversion for P and S velocities and for the Vp/Vs ratio. In this study, special attention was paid to the validation of the computed model based on different tests, such as inversion of independent data subsets and synthetic modeling. Although the network configuration and the distribution of the events are not favorable for high-quality tomographic imaging, we have obtained some important and robust features which give information about sources of volcanic activity in the Krakatau complex. The most interesting feature of this study is a zone of high Vp/Vs ratio beneath the Krakatau complex. At depths down to 4 km depth we observe anticorrelation of higher P- and lower S-velocities that leads to Vp/Vs ratio higher than 2. This is a probable indicator of the presence of partially molten and/or with high fluid content material with a composition corresponding to deeper layers. It is important that the anomaly of high Vp/Vs ratio beneath the Krakatau complex appears to be separated in two parts at a depth of 5–6 km. This fits to results of geobarometric analysis that presume the existence of several levels of magma chambers beneath Anak Krakatau.

Graphical abstract

Highlights

► The distribution of P and S velocity anomalies beneath Krakatau volcano is derived from local earthquake tomography. ► Beneath volcanic complex we observe high Vp/Vs ratio (up to 2.2) which is probably related to high fluid/melt content. ► Our results show an existence of multiple magma storage regions beneath volcano, which fits with geobarometric analysis data.

Introduction

Krakatau volcano is one of the most dangerous volcanoes in the world and thus attracts vital interest from researchers of different disciplines. Geographically located in Indonesia, between Sumatra and Java, it is a part of the Sunda Arc (Fig. 1). Along this arc, the plate boundary between Eurasia and the Indian-Australian plate hosts around 100 active volcanoes [Simkin and Siebert, 1994, Hilton and Craig, 1989]. In the area of Java and Sumatra, the northward subduction of the Indian oceanic plate under the Sunda block occurs at a rate of about 6.8–7.2 cm/yr [DeMets et al., 1990]. The age of the subducted plate in this segment of the Sunda Arc is about 80–100 Ma [Müller et al., 1997], one of the oldest values observed for the oceanic crust in the Indian Ocean.

The angle of subduction changes from near perpendicular (13°) in front of Java to oblique (55°) in front of Sumatra (Jarrard, 1986). The Sumatran rotation has resulted in extension, as reported in Harjono et al. (1991) and associated thinning of the crust to ~ 20 km in the Sunda Strait, as compared to 25–30 km in Sumatra and west Java (Nishimura and Harjono, 1992) (Fig. 1). The contemporary Krakatau volcanic complex consists of four islands; Rakata, Sertung, Panjang and Anak Krakatau (Fig. 1) and it is part of a NNW-SSE trending lineament of Quaternary volcanic edifices which lies approximately parallel to the Java trench. Sertung and Panjang islands are remnants of a caldera formed after an earlier explosive eruption, recorded in local Javanese folk stories of the Book of Kings, or ‘Pararaton’ (Judd, 1889, Nishimura et al., 1986, Camus et al., 1987) and describes heavy rains of stone in the year 338 Saka (416 AD). While there is no direct evidence for an eruption of this size at that time, it could be a mistaken date for another eruption described for 535 AD (Wohletz, 2000). In addition to this, there is also evidence for an even older large ignimbrite eruption attributed to 60,000 BC (Ninkovich, 1979), further indicating the cyclic behavior of the Krakatau eruptions.

Rakata is the part of Krakatau Island which remained after the catastrophic 1883 eruption. Anak Krakatau (“the child of Krakatau”) appeared in 1927. Today, Anak Krakatau is a typical cinder cone with an approximate radius of 2 km. It rises up to 315 m above sea level and shows ongoing moderate activity, having grown at an average rate of ~ 8 cm/week over the last 80 years.

The 1883 eruptive events which began in May, 1883, and concluded on August 27th 1883 with the catastrophic explosion that almost completely destroyed the island and gave rise to a tsunami of 30–40 m in height (e.g. Yokoyama, 1981). It killed more than 36,000 people in the coastal areas of the Indian Ocean. The pyroclastic surge and its deposits on the islands NW of Krakatau and the mainland is discussed extensively in Carey et al. (1996). About 20 km3 of volcanic material was erupted during the 1883 eruption (e.g., Self and Rampino, 1981, Simkin and Siebert, 1994), and the ash column reached 30 km altitude. The explosions could be heard in a radius of 4000 km, and the air wave reverberated around the earth about seven times (Harkrider, 1967).

The bimodal nature of the Krakatau complex, with extended periods of basaltic and/or basaltic-andesitic eruptions culminating in colossal caldera forming ignimbrite eruptions before the cycle recommences at the basalt stage, was discussed by Van Bemmelen (1949), and has since been strengthened by findings of other authors (Camus et al., 1987, Mandeville et al., 1996a, Mandeville et al., 1996b).

Currently, Anak Krakatau generally erupt feldspar-rich basaltic andesites with silica contents of ~ 53–56 wt.% (Dahren, 2010), suggesting that their source is already slightly evolved. This may imply that residual magma from the 1883 eruption may be involved or that rapid differentiation of basaltic magmas from a deeper source has occurred.

The location of the Krakatau volcanic complex in the Sunda Strait is characterized by anomalously strong and frequent volcanic activity, as compared to other parts of the Sunda Arc. This can be explained by a hypothesis that the magmatism in the Krakatau area is not merely caused by subduction processes, but might also be due to extensional tectonism in the Sunda Strait (Nishimura and Harjono, 1992), attributed to the rotation of Sumatra in relation to Java. In Sukadana, ~ 100 km north of Krakatau along the same volcanic lineament, the influence of the rifting is manifested as an 0.8–1.2 Ma old MORB-type basalt is found (Nishimura and Harjono, 1992). Further research into the depth structures beneath the Krakatau complex may give valuable information for the prognoses of future catastrophic eruptions at Krakatau.

Despite global interest in the activity of Krakatau, a relatively few geophysical and petrological studies were performed in the Krakatau archipelago. Investigations of local seismicity and S-wave attenuation by Harjono et al. (1989) found evidence for multiple levels of magma storage beneath Krakatau, likely located at depths of about 9 and 22 km. The characteristics of the calderas formed by the ~ 535 AD and 1883 eruptions, as well as of feeding channels, were studied by gravity modeling in Deplus et al. (1995). These authors found a large flat-bottomed 240 m deep depression to the SW of Anak Krakatau, interpreted as a caldera caused by previous eruptions. Seismic tomography has never been applied to Krakatau on a local scale, however.

A multidisciplinary project, KRAKMON, was initiated by researchers from BGR (Germany) in 2005 (Hoffmann-Rothe et al., 2006), including various research approaches such as: measurements of local seismicity (Ibs-von Seht, 2008), electromagnetics, deformation analysis using GPS, ground temperatures, meteorological parameters, sea level, chemical and physical parameters of fumarole gasses, and optical monitoring. Here we consider the seismological data collected from local seismicity to derive a 3-D seismic model beneath the Krakatau complex.

Section snippets

Data and algorithm

The seismic network deployed at Krakatau and in surrounding areas is shown in Fig. 2. Within the project 14 seismic onshore stations were installed. The majority of them were located on the central volcano and surrounding islands. Some stations were located on the coasts of Java and Sumatra, and also on remote islands in the Sunda Strait. No ocean bottom stations were used in this project and the network installation was limited to the onshore areas. The network configuration appears to be

Results and verification

We performed a number of inversions of real data to explore the influence of different parameters and starting models on the results. This is especially important since the considered dataset is based on a rather poor geometric configuration of the network. First of all, we concluded that 1-D optimization at a preliminary step did not provide satisfactory results. After performing a number of trials, we understood that the resulting 1-D models were strongly dependent on starting distributions

Distribution of seismicity

The seismic events located in the study area (Fig. 2) can be separated into three groups. The first group contains deep focused events with a depth of between 100 and 160 km (purple area in Fig. 2A). These events are lying beneath the volcanic arc and are probably caused by phase transitions in the subducting slab that lead to an active release of fluids into the mantle wedge (e.g. Hacker et al., 2003). Ascent of these fluids leads to decreasing melting temperature above the slab and gives rise

Conclusions

In this study, we present a first tomographic model with the distribution of P and S velocities and Vp/Vs ratio using local seismicity data. Based on the results, we found indications for a multilayered structure of a magma chamber system beneath the Krakatau complex which seems to be in accordance with petrological studies (mineral-melt equilibria). Although the data distribution was not favorable for a tomographic inversion, we presented a number of tests which reveal the resolution capacity

Acknowledgments

The KRAKMON project was funded by the German Federal Ministry of Education and Research (BMBF) within the framework of the GEOTECHNOLOGIEN program (FKZ03G0578A). Part of the equipment for the station network was kindly made available by the geophysical instrument pool of the GFZ Potsdam. The work of Ivan Koulakov and Kairly Jaxybulatov is supported by the Helmholtz Society and RFBR Joint Research Project 09-05-91321-SIG a, Multidisciplinary Projects SB RAS #21, and Project ONZ RAS #7.4. Dahren

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