Physical and mechanical rock properties of a heterogeneous volcano; the case of Mount Unzen, Japan

Abstract

Abstract. Volcanoes represent one of the most critical geological settings for hazard modelling due to their propensity to both unpredictably erupt and collapse, even in times of quiescence. Volcanoes are heterogeneous at multiple scales, from porosity which is variably distributed and frequently anisotropic to strata that are laterally discontinuous and commonly pierced by fractures and faults. Due to variable and, at times, intense stress and strain conditions during and post-emplacement, volcanic rocks span an exceptionally wide range of physical and mechanical properties. Understanding the constituent materials' attributes is key to improving the interpretation of hazards posed by the diverse array of volcanic complexes. Here, we examine the spectrum of physical and mechanical properties presented by a single dome-forming eruption at a dacitic volcano, Mount Unzen (Japan) by testing a number of isotropic and anisotropic lavas in tension and compression and using monitored acoustic emission (AE) analysis. The lava dome was erupted as a series of 13 lobes between 1991–1995, and its ongoing instability means much of the volcano and its surroundings remain within an exclusion zone today. During a field campaign in 2015, we selected 4 representative blocks as the focus of this study. The core samples from each block span range in porosity from 9.14 to 42.81 %, and permeability ranges from 1.54 × 10−14 to 2.67 × 10−10 m2 (from 1065 measurements). For a given porosity, sample permeability varies by > 2 orders of magnitude is lower for macroscopically anisotropic samples than isotropic samples of similar porosity. An additional 379 permeability measurements on planar block surfaces ranged from 1.90 × 10−15 to 2.58 × 10−12 m2, with a single block having higher standard deviation and coefficient of variation than a single core. Permeability under confined conditions showed that the lowest permeability samples, whose porosity largely comprises microfractures, are most sensitive to effective pressure. The permeability measurements highlight the importance of both scale and confinement conditions in the description of permeability. The uniaxial compressive strength (UCS) ranges from 13.48 to 47.80 MPa, and tensile strength (UTS) using the Brazilian disc method ranges from 1.30 to 3.70 MPa, with crack-dominated lavas being weaker than vesicle-dominated materials of equivalent porosity. UCS is lower in saturated conditions, whilst the impact of saturation on UTS is variable. UCS is between 6.8 and 17.3 times higher than UTS, with anisotropic samples forming each end member. The Young's modulus of dry samples ranges from 4.49 to 21.59 GPa and is systematically reduced in water-saturated tests. The interrelation of porosity, UCS, UTS and Young's modulus was modelled with good replication of the data. Acceleration of monitored acoustic emission (AE) rates during deformation was assessed by fitting Poisson point process models in a Bayesian framework. An exponential acceleration model closely replicated the tensile strength tests, whilst compressive tests tended to have relatively high early rates of AEs, suggesting failure forecast may be more accurate in tensile regimes, though with shorter warning times. The Gutenberg-Richter b-value has a negative correlation with connected porosity for both UCS and UTS tests which we attribute to different stress intensities caused by differing pore networks. b-value is higher for UTS than UCS, and typically decreases (positive Δb) during tests, with the exception of cataclastic samples in compression. Δb correlates positively with connected porosity in compression, and negatively in tension. Δb using a fixed sampling length may be a more useful metric for monitoring changes in activity at volcanoes than b-value with an arbitrary starting point. Using coda wave interferometry (CWI) we identify velocity reductions during mechanical testing in compression and tension, the magnitude of which is greater in more porous samples in UTS but independent of porosity in UCS, and which scales to both b-value and Δb. Yet, saturation obscures velocity changes caused by evolving material properties, which could mask damage accrual or source migration in water-rich environments such as volcanoes. The results of this study highlight that heterogeneity and anisotropy within a single system not only add uncertainty but also have a defining role in the channelling of fluid flow and localisation of strain that dictate a volcano's hazards and the geophysical indicators we use to interpret them.

The mechanical behaviour and failure modes of volcanic rocks: a review

“…The ratio of wet to dry uniaxial compressive strength is 1.09 and 0.96 for the two basalts (basalt 1 and basalt 2, respectively), 0.92 for the trachyandesite, 0.87 for the block-and-ash flow, and 0.56, 0.93, and 1.07 for the three andesites (Kumamoto, andesite 1, and andesite 2, respectively) (Table 1). The ratios calculated for the dacites from Kendrick et al (2021) are 1.04, 0.81, 0.97, 0.60, and 0.82 (for UNZ1, UNZ9a, UNZ9b, UNZ13, and UN14, respectively; see Kendrick et al 2021).…”

“…Only one wet experiment was performed on each of the dacites from Mt Unzen volcano. Because sample-tosample variability was high for these dacites (Kendrick et al 2021), these data may invite overinterpretation. More experiments are now required to better understand water-weakening in lavas.…”

“…Also included in Fig. 8a are recent data for dacite from Mt Unzen volcano (Japan; Kendrick et al 2021). The ratio of wet to dry uniaxial compressive strength is 1.09 and 0.96 for the two basalts (basalt 1 and basalt 2, respectively), 0.92 for the trachyandesite, 0.87 for the block-and-ash flow, and 0.56, 0.93, and 1.07 for the three andesites (Kumamoto, andesite 1, and andesite 2, respectively) (Table 1).…”

“…Fig.5Uniaxial compressive strength (UCS) as a function of porosity for volcanic rocks (n = 870; these data are provided in a Microsoft Excel© spreadsheet that accompanies this contribution as Supplementary Material). Data from: this study,Behre (1929),Price and Jones (1982),Price (1983),Erdoğan (1986),Erguvanlı et al (1989),Ayday and Göktan (1990),Martin et al (1994),Okubo and Chu (1994),Schultz and Li (1995),Topal and Doyuran (1997), Al-Harthi et al (1999), Topal and Sözmen (2003), Török et al (2004), Apuani et al (2005), Yassaghi et al (2005), Jackson et al (2005), Tuncay (2009), Erguler and Ulusay (2009), Zhu et al (2011, 2016), Heap et al (2012), Yavuz (2012), Karakuş and Akatay (2013), Kendrick et al (2013a), Heap et al (2014a), Pola et al (2014), Siratovich et al (2014), Heidari et al (2014), Wyering et al (2014), Çelik et al (2014),Karaman and Kesimal (2015),Celik and Ergül (2015),Schaefer et al (2015),Heap et al (2015aHeap et al ( , 2015c,Montanaro et al (2016),Zhu et al (2016),Marmoni et al (2017),Bubeck et al (2017),Heap et al (2018aHeap et al ( , 2018b, Coats et al (2018), Mordensky et al (2018), Lavallée et al (2019), Harnett et al (2019), Heap et al (2020b), Ryan et al (2020), Kennedy et al (2020), Di Muro et al (2021), andKendrick et al (2021) …”

“…Fig.25Tensile strength as a function of porosity for volcanic rocks (n = 277; these data are provided in a Microsoft Excel© spreadsheet that accompanies this contribution as Supplementary Material). Data from:Tuğrul and Gürpinar (1997), Gupta and Rao (2000), Chen et al (2004), Ersoy and Atici (2007), Kılıç and Teymen (2008), Nara et al (2010b), Kahraman and Yeken (2010), Graue et al (2011), Lavallée et al (2012a), Heap et al (2012), Wedekind et al (2013), Karakuş and Akatay (2013), Hashiba and Fukui (2015), Siratovich et al (2015), Fener and Ince (2015), Ündül and Er (2017), Yavuz et al (2017), Lamb et al (2017), Malik et al (2017), Aldeeky and Al Hattamleh (2018), Zorn et al (2018), Hornby et al (2019), Harnett et al (2019),Moon and Yang (2020),Yasar and Komurlu (2020),Kendrick et al (2021) …”