Elsevier

Earth and Planetary Science Letters

Volume 482, 15 January 2018, Pages 377-387
Earth and Planetary Science Letters

Earthquake induced variations in extrusion rate: A numerical modeling approach to the 2006 eruption of Merapi Volcano (Indonesia)

https://doi.org/10.1016/j.epsl.2017.11.019Get rights and content

Highlights

  • We use a numerical conduit model to describe the phases of the 2006 eruption.

  • The addition of CO2 into the magma is a plausible cause of the peak eruption phase.

  • A nearby earthquake triggered this peak phase by causing the release of CO2.

  • CO2 increases eruption rate by reducing H2O solubility and generating overpressure.

  • Dome collapse causes variable rates by changing the pressure in the upper conduit.

Abstract

Extrusion rates during lava dome-building eruptions are variable and eruption sequences at these volcanoes generally have multiple phases. Merapi Volcano, Java, Indonesia, exemplifies this common style of activity. Merapi is one of Indonesia's most active volcanoes and during the 20th and early 21st centuries effusive activity has been characterized by long periods of very slow (<0.1 m3 s−1) extrusion rate interrupted every few years by short episodes of elevated extrusion rates (1–4 m3 s−1) lasting weeks to months. One such event occurred in May–July 2006, and previous research has identified multiple phases with different extrusion rates and styles of activity. Using input values established in the literature, we apply a 1D, isothermal, steady-state numerical model of magma ascent in a volcanic conduit to explain the variations and gain insight into corresponding conduit processes. The peak phase of the 2006 eruption occurred in the two weeks following the May 27 Mw 6.4 earthquake 50 km to the south. Previous work has suggested that the peak extrusion rates observed in early June were triggered by the earthquake through either dynamic stress-induced overpressure or the addition of CO2 due to decarbonation and gas escape from new fractures in the bedrock. We use the numerical model to test the feasibility of these proposed hypotheses and show that, in order to explain the observed change in extrusion rate, an increase of approximately 5–7 MPa in magma storage zone overpressure is required. We also find that the addition of ∼1000 ppm CO2 to some portion of the magma in the storage zone following the earthquake reduces water solubility such that gas exsolution is sufficient to generate the required overpressure. Thus, the proposed mechanism of CO2 addition is a viable explanation for the peak phase of the Merapi 2006 eruption. A time-series of extrusion rate shows a sudden increase three days following the earthquake. We explain this three-day delay by the combined time required for the effects of the earthquake and corresponding CO2 increase to develop in the magma storage system (1–2 days), and the time we calculate for the affected magma to ascend from storage zone to surface (40 h). The increased extrusion rate was sustained for 2–7 days before dissipating and returning to pre-earthquake levels. During this phase, we estimate that 3.5 million m3 DRE of magma was erupted along with 11 ktons of CO2. The final phase of the 2006 eruption was characterized by highly variable extrusion rates. We demonstrate that those changes were likely controlled by failure of the edifice that had been confining the dome to Merapi's crater and subsequent large dome collapses. The corresponding reductions in confining pressure caused increased extrusion rates that rapidly rebuilt the dome and led to further collapses, a feedback cycle that prolonged the eruption. In a more general sense, this study demonstrates that both internal changes, such as magma volatile content and overpressure, and external forces, such as edifice collapse and regional earthquakes, can affect variations in eruption intensity. Further, we also demonstrate how these external forces can initiate internal changes and how these parameters may interact with one another in a feedback scenario.

Introduction

Volcanic eruptions forming lava domes present a prolonged and dangerous hazard to surrounding populations. The primary hazard of these eruptions is pyroclastic density currents (PDCs) caused by collapse of an active lava dome at the summit (e.g., Voight et al., 2000). While the size and frequency of PDCs generally correlates with the extrusion rate (Nakada et al., 1999, Carr et al., 2016), and periods of increased extrusion rate can be anticipated through extensive geodetic and seismic monitoring (e.g. Surono et al., 2012, Ratdomopurbo et al., 2013), the causes of changes in extrusion rate are generally not well constrained. Numerical models of magma ascent in volcanic conduits have previously tested the effect of different parameters on extrusion rate, such that general relationships between a range of magma and conduit conditions have been well described (Melnik and Sparks, 1999, Mastin, 2002; de' Michieli Vitturi et al., 2008, de' Michieli Vitturi et al., 2010). However, these relationships can be applied to determine the cause of varying extrusion rates in real eruptions only when the system is sufficiently well-constrained to reduce the number of free parameters. Numerous eruptions at Merapi Volcano (Java, Indonesia) are well documented, making them excellent case studies to further test the role of magma properties, conduit conditions, and external forces such as earthquakes in controlling volcanic processes. Here we apply a 1D, steady-state conduit model, with inputs well-constrained for the 2006 Merapi eruption, and pair it with a detailed record of extrusion rate (Harris and Ripepe, 2007, Ratdomopurbo et al., 2013, Preece et al., 2013, Carr et al., 2016) to explain the causes of variations during the eruption sequence. We also test the feasibility of hypotheses proposed in previous works (Walter et al., 2007, Harris and Ripepe, 2007, Deegan et al., 2010, Troll et al., 2012) that suggest a regional tectonic earthquake triggered the peak phase of the eruption.

Merapi Volcano, located 30 km north of Yogyakarta in central Java (Fig. 1), is one of Indonesia's most active and dangerous volcanoes. For much of the late 19th, 20th, and early 21st centuries, activity at Merapi Volcano consisted of continued slow extrusion leading to the formation of a series of basaltic-andesite lava domes (Hammer et al., 2000, Voight et al., 2000). The long-term background extrusion rate at Merapi is approximately 0.03 m3 s−1 (Siswowidjoyo et al., 1995; rates for dense rock equivalent values, DRE = 2800 kg m−3, Costa et al., 2013). Every few years (an average of <7 yr, Ratdomopurbo et al., 2013) extrusion rates have increased to 1–4 m3 s−1 for short periods lasting a few weeks to months during which time PDCs have been most common (Siswowidjoyo et al., 1995). This pattern of low-level background activity interrupted by relatively brief periods of increased extrusion rate and PDC frequency was so persistent at Merapi that it has been described as “Merapi-type” activity and the term is used to describe similar eruptions at other volcanoes (Voight et al., 2000).

Observations and studies of the 2006 eruption have identified five phases (Global Volcanism Program, 2007, Harris and Ripepe, 2007, Ratdomopurbo et al., 2013, Preece et al., 2013, Carr et al., 2016). Phase 1 began on April 26 when lava extrusion increased above background levels (Ratdomopurbo et al., 2013), though extrusion rates were ≤1 m3 s−1 (Ratdomopurbo et al., 2013, Carr et al., 2016). Extrusion rate increased to ∼2 m3 s−1 during Phase 2, which began on May 11 with the first PDCs generated by dome collapse (Ratdomopurbo et al., 2013, Preece et al., 2013, Carr et al., 2016). The start of Phase 3 is marked by the May 27th MW 6.4 strike-slip earthquake located at a depth of 10 km approximately 50 km S of the Merapi vent (7.89°S, 110.41°E) (Fig. 1). The earthquake caused thousands of fatalities around Yogyakarta (Nakano et al., 2006). Following the earthquake, the frequency of PDCs increased (Walter et al., 2007) and the average extrusion rate rose to 3.3–3.6 m3 s−1 for a period of nearly two weeks (Harris and Ripepe, 2007, Ratdomopurbo et al., 2013, Preece et al., 2013, Carr et al., 2016). This increase was followed by a brief decrease in activity during Phase 4 (June 9–13) when extrusion rates returned to ∼1 m3 s−1 (Harris and Ripepe, 2007, Carr et al., 2016). The weight of the growing lava dome initiated a progressive failure of the southern crater wall during Phases 3 and 4 (Ratdomopurbo et al., 2013). This failure caused the primary direction of PDCs to switch from the SW down the Krasak and Boyong drainages to the S down the Gendol drainage (Charbonnier and Gertisser, 2008, Ratdomopurbo et al., 2013) (Fig. 1). The start of Phase 5 is marked by the final and complete collapse of the crater wall on June 14, leading to the largest PDCs of the 2006 event which traveled up to 7 km from the vent and caused the only two fatalities directly attributed to the eruption (Charbonnier and Gertisser, 2008). Extrusion rates were variable during Phase 5, ranging from 1 to 2 m3 s−1 (Preece et al., 2013, Carr et al., 2016). Carr et al. (2016) mark the end of Phase 5 on July 10, when the Indonesian Center of Volcanology and Geological Hazards Mitigation lowered the alert level for Merapi from 4 to 3 (on a 1–4 scale) (Global Volcanism Program, 2007).

Previous research has suggested that the peak extrusion rate during Phase 3 is related to the occurrence of the May 27 earthquake (Walter et al., 2007, Harris and Ripepe, 2007, Deegan et al., 2010, Troll et al., 2012). While the static stress change caused by the earthquake (∼3 kPa, Walter et al., 2007) was not sufficient to increase extrusion rate to the observed values, the dynamic stress change caused by passing seismic waves likely played a role (Walter et al., 2007). The effect of dynamic stress change is further supported by Harris and Ripepe (2007), who identify an increase in extrusion rate at both Merapi and Semeru Volcano (280 km to the east) in the days following the earthquake. Both Harris and Ripepe (2007) and Walter et al. (2007) suggest that dynamic stress may cause increased vesiculation and promote bubble growth in the magma which leads to increased pressure and buoyancy in the magma and therefore higher extrusion rates (Manga and Brodsky, 2006).

Deegan et al. (2010) and Troll et al. (2012) propose an alternative explanation for increased activity after the earthquake. The shallow crust beneath Merapi consists primarily of carbonate rocks (to a depth of ∼10 km; Deegan et al., 2010, Costa et al., 2013), and they suggest that the earthquake fractured bedrock beneath the volcano leading to the addition of CO2 into the magma through decarbonation of limestone. Troll et al. (2012) note that, as evidenced by carbon isotope measurements, the crustal contribution to CO2 emissions at Merapi summit fumaroles increased following the earthquake. Deegan et al. (2010) performed experiments on carbonate–magma interactions under conditions representing the Merapi system at a depth of ∼15 km. The experiments showed that under such conditions the primary process of decarbonation is dissociation of the carbonate into CaO and CO2 and that this process occurred very rapidly. For example, the assimilation of a 9.75 mg sample of carbonate into 41.93 mg of magma required only ∼5 min (Deegan et al., 2010). Deegan et al. (2010) also note that at shallower depths the decarbonation process is likely to occur even more rapidly than it did in their experiments. Both Deegan et al. (2010) and Troll et al. (2012) suggest that stress changes and shaking as a result of the earthquake may have fractured the limestone bedrock beneath Merapi. The fracturing of the bedrock could cause both the release of trapped crustal CO2 pockets and an increase in the surface area of carbonate available to react with the magma, and thus lead to increased CO2 in the Merapi magmatic system at mid- to upper crustal depths (0–10 km, Deegan et al., 2010, Costa et al., 2013). This process has the potential to increase the total volatile content and reduce the solubility of water in the magma, leading to increases in the magma bubble volume fraction and pressure, thus promoting higher extrusion rates (Deegan et al., 2010, Troll et al., 2012).

Numerous reports describe increased activity at Merapi following earthquakes throughout its history (Voight et al., 2000), although it is unclear whether this activity includes increases in extrusion rate or simply increases in PDC frequency due to shaking-induced dome collapse. For example, Voight et al. (2000) note that activity at Merapi was observed to increase following a tectonic earthquake in 1943. However, since the advent of modern monitoring equipment, the only clear volcanic response at Merapi following an earthquake, aside from the 2006 event, occurred during a period of low activity in 2001, when summit fumarole temperature increased following a MW 6.3 earthquake at ∼130 km depth with an epicenter 50 km SW of Merapi (Walter et al., 2007). Many more eruptions have occurred at Merapi in the past two centuries with no preceding or concurrent tectonic earthquake activity (Voight et al., 2000). During these eruptions, changes in the intensity of activity are most commonly related to large dome collapses (Voight et al., 2000) and/or variable magma supply from depth (e.g. Preece et al., 2013).

Multiple numerical models of magma ascent in volcanic conduits of varying complexity have been developed over the past decades to aid the understanding of both effusive and explosive volcanic eruptions. Many of these models treat the rising magma as a laminar two-phase flow of exsolved volatiles and melt that includes the liquid, crystals, and dissolved volatiles (e.g. Jaupart and Allègre, 1991, Woods and Koyaguchi, 1994, Melnik and Sparks, 1999, Mastin, 2002; Costa, 2005; de' Michieli Vitturi et al., 2008; La Spina et al., 2015). Magma rise is driven by the pressure gradient between the storage region and the vent while being resisted by gravity and viscous forces. The viscosity of the magma is controlled by magma composition, exsolved and dissolved volatile content, temperature, and the crystal content of the melt, and can change dramatically with variations in any of these parameters (Lejeune and Richet, 1995, Hess and Dingwell, 1996). Jaupart and Allègre (1991) and Woods and Koyaguchi (1994) showed that lateral degassing through the conduit walls is an important process for determining whether an eruption is explosive or effusive. Enhanced lateral degassing may promote effusive eruptions, or the formation of dense plugs at the vent along with cyclic effusive or explosive activity (Diller et al., 2006, Clarke et al., 2007; de' Michieli Vitturi et al., 2008, de' Michieli Vitturi et al., 2010). Overpressure in a magma storage zone or chamber exerts significant control on magma ascent rates and the intensity of activity observed at the vent (Woods and Koyaguchi, 1994). A lava dome at the vent decreases the pressure gradient and consequently decreases mass flow rates (Jaupart and Allègre, 1991, Woods and Koyaguchi, 1994, Sparks, 1997, Melnik and Sparks, 1999).

Section snippets

Magma ascent model

The model we present here is derived from the theory of thermodynamically compatible systems (Romenski et al., 2010; de' Michieli Vitturi et al., 2011; La Spina et al., 2015, La Spina et al., 2017) and applies to 1D, two-phase and multicomponent, steady-state flow. It can be accessed at https://github.com/demichie/MAMMA. The model accounts for the effects of the main processes controlling magma rise in a vertical conduit, such as degassing and crystallization and the associated rheological

Results

We initiate our model runs for Phases 1 and 2 with no overpressure and a dome height of 150 m, consistent with observations of low extrusion rates beneath a complex of older domes from previous eruptions. Under these conditions, a 2000 m conduit length, 15 m conduit radius, temperature of 950 °C, 0.5 initial crystal volume fraction, 2.5 wt% volatiles results in a volume flow rate of 1.4 m3 s−1 (DRE), which is within the range of observed extrusion rates for Phases 1 and 2 of 1.0–1.9 m3 s−1

Discussion

We successfully tested the numerical model presented here for the Merapi system by matching output to each phase of the eruption using input values constrained by published literature values. Our model results compare well with independent estimates of extrusion and ascent rate during the 2006 effusive eruption of Merapi Volcano based on observational (Walter et al., 2007, Ratdomopurbo et al., 2013), thermal remote sensing (Harris and Ripepe, 2007, Carr et al., 2016), and petrologic data (

Conclusions

In this study we aim to better understand drivers that lead to variable intensity during dome-forming volcanic eruptions. To achieve this goal, we use a 1D steady-state isothermal numerical model of magma ascent in a volcanic conduit to explain variable extrusion rates observed during the 2006 effusive eruption of Merapi Volcano. The peak phase of the eruption followed a MW 6.4 tectonic earthquake ∼50 km away, and previous work has suggested two hypotheses for how the earthquake may have

Acknowledgments

Author BC was supported in part by summer Ph.D. student research fellowships awarded by the School of Earth and Space Exploration and funded by a Graduate College University Block Grant at Arizona State University. We thank Giuseppe La Spina, an anonymous reviewer, and Tamsin Mather for insightful comments that greatly improved the manuscript.

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