Analogue Models of the Effect of Long-Term Basement Fault Movement on Volcanic Edifices

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Bull Volcanol (2009) 71:1111–1131 DOI 10.1007/s00445-009-0289-3

RESEARCH ARTICLE

Analogue models of the effect of long-term basement fault movement on volcanic edifices

Luke Wooller & Benjamin van Wyk de Vries & Emmanuelle Cecchi & Hazel Rymer

Received: 4 June 2004 /Accepted: 29 April 2009 /Published online: 16 May 2009 # Springer-Verlag 2009

Abstract Long-term fault movement under volcanoes can we graphically display the geometry of structures produced. control the edifice structure and can generate collapse The models are applied to volcanoes with known underly- events. To study faulting effects, we explore a wide range ing faults, and we demonstrate the importance of these of fault geometries and motions, from normal, through faults in determining volcanic structures and slope instabil- vertical to reverse and dip-slip to strike-slip, using simple ity. Using the knowledge of fault patterns gained from these analogue models. We explore the effect of cumulative sub- experiments, geological mapping on volcanoes can locate volcanic fault motions and find that there is a strong fault influence and unstable zones, and hence monitoring of influence on the structural evolution and potential instabil- unstable flanks could be carried out to determine the actual ity of volcanoes. The variety of fault types and geometries response to faulting in specific cases. are tested with realistically scaled displacements, demon- strating a general tendency to produce regions of instability Keywords Volcano-tectonics . Fault . Analogue modelling . parallel to fault strike, whatever the fault motion. Where Lateral collapse . Debris avalanche . Deformation there is oblique-slip faulting, the instability is always on the downthrown side and usually in the volcano flank sector facing the strike-slip sense of motion. Different positions of Introduction the fault beneath the volcano change the location, type and magnitude of the instability produced. For example, the Volcanoes have an inherent structural instability that results further the fault is from the central axis, the larger the in lateral collapse and debris avalanches, mobilising many destabilised sector. Also, with greater fault offset from cubic kilometres of material that is transported away from the central axis larger unstable volumes are generated. Such the source as a rapidly flowing mass (Siebert 1984; Siebert failures are normal to fault strike. Using simple geometric et al. 1987). Collapse events are not necessarily accompa- dimensionless numbers, such as the fault dip, degree of nied by eruptions, but may also occur at dormant or extinct oblique motion (angle of obliquity), and the fault position, centres (Siebert et al. 1987; Cecchi et al. 2004). It is important to further develop our understanding of Editorial responsibility: C. Kilburn the factors that influence and trigger volcano collapse. L. Wooller : H. Rymer These include the intrusion of shallow magmatic bodies Volcano Dynamics Group, Department of Earth Sciences, such as crypto-domes (e.g. Mount St. Helens, USA (Voight The Open University, et al. 1983; Donnadieu et al. 2001)) and dykes (e.g. Milton Keynes MK6 7AA, UK Stromboli, Italy (Tibaldi 2001, 2003)), deforming hydro- B. van Wyk de Vries (*) : E. Cecchi thermal systems (e.g. Casita Volcano, Nicaragua (van Wyk Laboratoire Magmas et Volcans, de Vries et al. 2000)), increased pore fluid pressures (Day Observatoire du Physique du Globe de Clermont, 1996) and local earthquakes (e.g. Bandai Volcano, Japan Université Blaise Pascal, 5 rue Kessler, (Mimura and Endo 1997; Glicken and Nakamura 1988)). 63038 Clermont-Ferrand, France An additional control on the direction of collapse may be e-mail: [email protected] the regional structural and lithological setting. Previous 1112 Bull Volcanol (2009) 71:1111–1131 work has suggested that collapse predominantly occurs at Belousov et al. (2005) looked at the influence of fault right angles to the regional tectonic orientation (Nakamura offset, applied to caldera boundary faults. They suggest that 1977; Siebert 1984; Francis and Wells 1988; Tibaldi 1995). the large fault offsets needed to cause volcano instability Factors influencing collapse close to the volcano include are most likely to occur at calderas and that in situations of the presence of ductile sub-strata, which allow gravitational tectonic faulting the edifice can “repair” itself between spreading of the edifice (van Wyk de Vries and Francis episodes of faulting. 1997), regional slope (Wooller et al. 2004), and active Although these earlier studies clearly show the interac- tectonic faulting beneath, or adjacent to, the edifice tion between long-term fault movement and overlying (Lagmay et al. 2000; Vidal and Merle 2000; Acoccella volcanoes, they were limited to simple fault geometries 2005). This last factor is the subject of this study. and did not investigate the transition between the various Volcanoes can serve as stress loci to concentrate or end-members of faulting (fault position, fault angle, dip-slip nucleate tectonic structures (van Wyk de Vries and Merle and strike-slip). Our study attempts to marry these separate 1996, 1998; Branquet and van Wyk de Vries 2001). This studies into a single model. This work therefore aims to leads to an increase in the fault density and fault reassess the earlier models and to examine further aspects displacement around volcanoes. Sudden movement along of the influence of basement faulting on volcano evolution these faults can destabilise the edifice and trigger a lateral and instability. collapse, leading to a debris avalanche (Siebert 1984; Siebert et al. 1987; Merle et al. 2001). Also, at longer timescales, cumulative movement along faults during the Scaling and materials growth of an overlying volcanic edifice can generate shallow faulting within the volcano (Merle et al. 2001). Firstly, it should be noted that there was no time- or rate- These fractures can define the extent of unstable sectors and dependence in the models: as they were brittle, they were potentially control both the intrusion of magmatic bodies not affected by the phenomena of creep or stress relaxation, (Lagmay et al. 2000) and the extent of hydrothermal and were ruled by the Navier-Coulomb criterion of brittle alteration (van Wyk de Vries and Francis 1997), which failure (e.g. Merle et al. 2001). As in other experiments, further destabilise the cone. slow continuous slip was achieved along the basement fault Analogue modelling methods have been extensively so that experiments were aseismic. Thus, the role of applied to studying crustal faulting (e.g. Horsfield 1977; earthquakes was not taken into account as described in McClay 1990; Dauteuil and Mart 1998). The role of faulting Merle et al. (2001). It was the geometric organisation of as an underlying control on the evolution of volcanoes has structures responding to fault movement and the volcano been studied using models by Tibaldi (1995), Merle et al. topography that was considered important. (2001), van Wyk de Vries and Merle (1996, 1998), Branquet The principle geometric variables for the volcano were and van Wyk de Vries (2001), Lagmay et al. (2000, 2003) the height (H), radius (R), layering thickness (T)in and Acoccella (2005). Lagmay et al. (2000, 2003)demon- heterogeneous (alternating layers of sand and sand / plaster) strated that underlying strike-slip faulting produces a cones, substrata thickness (B), and the fault position sigmoidal structure across the cone with two regions parallel (distance from volcano centre to the fault) (Table 1, Fig. 1). to fault strike showing instability through bulging, fracturing In addition, there were three fault displacement compo- and avalanching. These models were applied to Iriga nents: vertical throw (Fv, where positive values were Volcano, Ancestral Mount Bao (Philippines) and Mount St. normal faulting and negative reverse faulting), horizontal Helens (USA). Norini et al. (2008) applied similar models to normal displacement (Fh) and along strike displacement Nevado de Toluca (Mexico). (Fs) (Fig. 1). These described the dip of the fault plane (tan Movement along vertical (dip-slip) basement faults Fv/Fh), the opening angle in transtension (tan Fs/Fh) and centred directly below volcanic cones was also shown to the angle of obliquity (tan Fv/Fs). The latter was particu- lead to faulting, which eventually induced a strike-normal larly relevant for vertical faults. The magnitude of fault failure of the downthrown area (Vidal and Merle 2000; movement was also gained from these parameters using Merle et al. 2001). Branquet and van Wyk de Vries (2001) Pythagoras: showed that regional sub-volcanic thrusts become deflected around the edifice, leading to instabilities accommodated Fault displacement magnitude ÀÁ through shallow failures. Thrust deflection may produce 1=2 ¼ Fs2 þ Fh2 þ Fv2 ð1Þ features similar to those often associated with basal thrusting at gravitationally spreading volcanoes. Acoccella In models where the fault was offset from the cone axis, the (2005) used a simple horizontal base plate set up and found horizontal distance from the cone centre to the fault (Df), similar results. varied from zero when the cone axis was directly above the Bull Volcanol (2009) 71:1111–1131 1113

Table 1 Representative values of geometric variables

Parameter Symbol Equivalent Π–number Units Model Nature Scaling ratio Volcano height H Π1 m 0.1 2000 5×10−5 Volcano radius R – m 0.3 6000 5×10−5 Layer thickness T Π2m10−3 200 5×10−5 Basal layer thickness B Π3m10−2–5×10−2 200–1000 5×10−5 Fault horizontal disp Fh Π4m10−2–10−4 2–200 5×10−5 Fault vertical disp Fv Π5m10−2–10−4 2–200 5×10−5 Fault strike-slip disp Fs Π6m10−2–10−4 2–200 5×10−5 Fault magnitude Fm – m10−2–10−4 2–200 5×10−5 Fault position Df Π7m0–0.3 0–6000 5×10−5

The type volcano is taken to be 2,000 m high with a 6,000 m radius base. The corresponding size analogue cone is 0.1 m high and 0.3 m radius. Due to these size parameters, all geometric variables have a scaling ratio of 1:50,000. The upper and lower bounds for basal layer thickness and displacement on faults (disp) are shown and are all scaled by the 1:50,000 scaling ratio. The layer thickness represents units of lava or scoria, rather than individual flows that are too small to model at the scale used here. Fm is a function of Fh, Fv and Fs

fault to positive values where the cone axis was centred on balance between cohesion (τ0) and stress (σ=ρgh). This the footwall block. ratio was the Π7 of Merle et al. (2001). Thus, for a volcano Material properties were the density (ρ), angle of internal model height of 10 cm, equivalent to a natural height of −5 friction (Φ) and the cohesion (τ0) of the brittle materials 2,000 m, the length ratio was L*=5×10 , and for model (Table 2). Gravity (g) was equal in models and nature. To density of 1,300 kgm−3, and natural density of about assure dynamic and kinematic similarity the ratios of the 2,600 kgm−3 the ratio was ρ*=0.5. As the gravity ratio g* natural and model parameters should be balanced. For the was 1, the stress ratio was σ*(ρ*g*L*)=2.5×10−5, thus the Navier-Coulomb materials, this was verified by assuring a cohesion in the model was 25,000 times smaller than in

Fig. 1 a Experimental set-up for vertical dip-slip or normal / A. Vertical / Normal faultB. Oblique-fault C. Lateral thrust faults; b Experimental set up for an oblique-slip vertical fault; c Experimental set up for laterally offset vertical dip-slip fault models. The oblique mod- Sand + plaster els are oriented at variable Sand angles (angle of obliquity=θ) from pure strike-slip faulting. The normal and reverse faults can also have a strike-slip com- Screwjack ponent; d Representation of the Motor geometric variables important in faulting experiments, as D. Geometric Parameters discussed in scaling and set-up stable side sections. The definition of the facing four sectors (facing towards, facing away towards strike-slip displacement horizontal displacement facing away, stable and Fs downthrown) used in descrip- Fs tions in the text are shown facing facing Fh towards away downthrown side vertical vertical dip-slip displacement Fv displacement Fv

Df fault dip = tan Fv / Fh angle of obliquity = Fv / Fs opening angle = tan Fh / Fs fault offset = Df / R Fault displacement magnitude = (Fs2 + Fh2 + Fv2)1/2 1114 Bull Volcanol (2009) 71:1111–1131

Table 2 Important parameters used in the scaling of the models, showing the values in models and nature and the ratios for scaling

Parameter Symbol Units Model Nature Chosen ratio Symbol

Volcano height H m 0,1 2000 5×10−5 L* Density ρ kg m−3 1300 2600 0.5 ρ* Gravity g kg m−1s−1 111 g* −1 −2 7 −6 Cohesion / stress ratio τ0 kg m s 25 10 2.5×10 τ0* Angle of internal friction Φ degrees 35 ∼35 1 Φ*

Π7 of (Merle et al. 2001) ρgh/τ0 5.2 5.2 1

The Π7 of Merle et al. (2001), is also given to show that this is the same in nature as in the model, and thus the experiments are dynamically similar to nature nature. Natural cohesions are about 107 Pa, so by scaling, Π 5 ¼ Fv=R; or the scaled normal vertical displacement the cohesion in the model should be about 25 Pa ð6Þ (Donnadieu and Merle 1998). This condition was arrived at by adding a small proportion of plaster to the sand to raise the cohesion slightly (Donnadieu and Merle 1998). Π 6 ¼ Fs=R; or the scaled along strike ðstrike slipÞ displacement Nearly pure sand has a cohesion of 0–2.5 Pa, and by adding ð7Þ plaster this is raised to up to 250 Pa at 20% plaster. The values varied from day to day according to humidity. Cohesion was checked for all experiments with the methods Π 7 ¼ Df =R; or the scaled position of the fault relative to the volcano set out in Donnadieu (2000) and generally a cohesion of about 25 Pa was encountered. We found that the cohesion ð8Þ variations did not change the structures formed. For the models to be properly scaled these dimensionless numbers should have had the same value in both nature and the model. This scaling condition was met, as they were Π Dimensionless - numbers length variables, and the length was already scaled (Tables 1 and 2). The models are simple, and because we concentrated on Note that the dip of the fault plane could be obtained by geometrical problems, we did not vary cohesion, angle of combining Π 4 and Π 5 (tan Π 5/Π 4=tan Fv/Fh). For internal friction, density, or gravity. Thus, these listed vertical faults the dip-slip/strike-slip relationship is here parameters are invariant and could be neglected in the referred to as the angle of obliquity or ‘θ’, and is the angle analysis. This left only the geometric parameters as the of deviation from pure strike-slip faulting on vertical faults, variables of interest. These were eight length parameters defined as tan Fs/Fv (or tan Π6/Π5). Hence θ=0° is pure and as there was only one dimension (length) the Buck- strike slip, and θ=85° indicates fault movement at 5° from Π ingham theorem required seven dimensionless numbers. vertical. For normal faults, the component of transtension By using volcano radius as a repeating variable these were: was obtained from combining Π 4 and Π 6 (tan Π 4/Π 6= Π 1 ¼ H=R; or the volcano slope ð2Þ tan Fh/Fs). The scaled fault displacement magnitude was found using Π 4, Π 5 and Π 6 in the Eq. (1). For this geometric study the main variables found to be important Π 2 ¼ T=R; or the characteristic thickness of layers in the cone were Π 7, fault dip and the component of transtension. ð3Þ

Experiment set-up Π 3 ¼ B=R; or the characteristic brittle basal layer thickness The model set-up (Fig. 1) was similar to that used in ð4Þ previous volcano fault models (e.g. van Wyk de Vries and Merle 1996;Lagmayetal.2000; Merle et al. 2001). The apparatus was composed of a pair of rigid baseplates, one Π 4 ¼ Fh=R; or the scaled normal horizontal fault displacement fixed and the other capable of controlled movement using ð5Þ a screwjack connected to a computer-driven step-motor, Bull Volcanol (2009) 71:1111–1131 1115 through which the rate and magnitude of fault movement direction of strike-slip movement, and two face away from could be defined. Our models only included a single the direction of strike-slip movement (Fig. 1). We use the basement fault. By rotating the screwjack angle relative to terms ‘facing towards’ and ‘facing away’ to describe these the plate a number of different fault geometries could be sectors. tested (Fig. 1b). In addition, we also tested for combinations of strike-slip and normal or reverse (dextral-normal or sinistral-reverse) Results faulting using a fault surface inclined at 60° from horizontal and angling the screwjack with 15° pitch into the plane The series of about 130 experiments allowed us to test a parallel to the fault. comprehensive range of fault geometries and cone The contact between the two plates was kept tight to positions. In the following sections we consider in turn avoid material seepage, and lubricated to prevent artificial the effects of various types of basement fault running jarring of the models. In this way we were able to examine directly beneath the summit of the volcanic cone, the long-term cumulative effect of fault movement (or including both dip- and strike-slip variations. The creep) without needing to scale for earthquake acceleration. position of the cone relative to the fault is then At the start of each experiment, the plates were aligned considered. so that the upper surfaces were parallel to give a flat rigid The fault type and fault position with respect to the cone surface. A substrata was added above the rigid base. were found to be the most important factors influencing Models of a cone sited upon a rigid base without a brittle structures. In contrast, different cone sizes, layer thick- basement were also constructed. No attempt has been made nesses and basal thicknesses were tested and were found to in this study to model weak ductile substrata similar to that have a limited effect on the structures formed on the cone. used in studies of volcano spreading (e.g. Merle and Borgia Increasing the cone size merely changed the length ratio, 1996). and allowed finer details to be observed. Adding layering to A cone representing the volcanic edifice was then added the cone had a very limited effect, with some localised to the model. Various cone types and sizes were used, and shallow instability along layer boundaries developing with the simplest material was homogenous dry sieved sand movement in some models, but in zones also affected in un- poured onto the model through a funnel. Cones of a dry layered models. sand / plaster mixture were also used. Finally, to model the Preliminary models showed that the volcano slope (Π 1) heterogeneous nature of strato-volcanoes, we constructed did not change the geometry of structures, but with slopes layered cones with sequential sand and sand / plaster layers near to the angle of repose (near the coefficient of internal representing mixed pyroclastic and lava horizons. friction) collapse structures formed earlier and led to Most experiments were recorded using digital cameras avalanches. fixed in position directly above the models and The layering in the volcano cone did not change the programmed for time-lapse photography. Photographs were geometry of faults, but did control the depth and timing of also taken from oblique angles to record particular collapses. The basal layer thickness (Π 3) was important in structures. Adding coloured markers to the model surface that the basement fault splayed out and changed angle once at the start of the experiment allowed horizontal (X-Y into the brittle medium. plane) deformation to be determined by digitising points on The magnitude of the fault displacement was impor- two images taken from a fixed position (e.g. Donnadieu et tant because different structures appear in a definite order al. 2003). In addition to this, multi-view stereo-photo during incremental deformation. The magnitude of fault geometry modelling (Cecchi et al. 2003) on several experi- offset needed to create structures was very small (< ments provided vertical (Z axis) data. This allowed us to 1 mm), which scales to 20 m in nature. Before this the chart the shape change of the cone. For some models, fault sand was deformed, but the structures were not clearly movement was terminated prior to the development of individualised. instability and landsliding in order to examine the internal The amount of fault offset expected on a volcano structures. These cones were enclosed within a thick sand will depend on the rate of fault movement, the time that cover and set by being impregnated with water, allowing the fault has to move before the volcano is modified by them to then be sectioned perpendicular to the basement erosion, or destroyed by other processes such as caldera fault strike. collapse. This time could be called the morphological It became clear that the structural pattern found on the lifetime. A rough estimate for a typical maximum rate cones could be separated into four 90° sectors. There were of fault movement is about 0.01 m per year (taken from two sectors on the downthrown side of the fault, and two on estimates given in van Wyk de Vries and Merle 1996, the upper (stable) block. Of these, two face into the 1998). The morphological lifetime of a volcano in a 1116 Bull Volcanol (2009) 71:1111–1131 strongly eroding climate could be about 20,000 years, but downthrown side producing instability leading to collapse in arid climates could be around 1,000,000 years. From perpendicular to fault strike. These structures are like those this, the largest fault offsets possible could be between developed in a sedimentary basin where a vertical basement 200 m and 1,000 m. fault creates a primary normal fault and a secondary thrust Fault patterns are likely to develop slowly on a volcanic (Horsfield 1977). On a volcano the surface outcrop of these edifice, but could appear as significant topographic features faults would be modified by the topography of the after approximately 1,000 years, equating to 10 m of overlying cone. While we did indeed find that such a basement deformation at the given displacement estimate. structure developed in response to large-displacements, The effect of faults on the stability may, however, be there were a number of additional features that warranted important even before this, as the structures begin to further inspection and resulted in a modified interpretation propagate through the edifice. of the collapse hazard posed by vertical sub-volcanic faulting (Fig. 2). One feature that was not previously described is the Vertical faulting early-stage development of a series of small normal faults that cross the summit striking perpendicular to the Our initial modelling attempted to recreate earlier studies of underlying fault. These faults formed at small displace- vertical dip-slip faulting (Vidal and Merle 2000; Merle et al. ments and continued to grow until about 1 cm displace- 2001), using a similar experimental setup and materials. It ment, or about 200 m of fault movement in nature. After was expected that a normal fault would develop first across this point the fault parallel structures predominated and the the cone, which would be followed by a thrust on the cone started to collapse normal to strike. So, initially,

Fig. 2 Results for a layered A’ cone built directly over a verti- Downthrown cal dip-slip fault at the end of side model run. a Vertical photograph illustrating the primary normal fault and the secondary fracturing; b Structures are il- Primary lustrated by a sketch and main normal destabilisation directions are fault given; c Shows the horizontal Stable 5 cm displacement vectors calculated A A B from successive images. Note the horizontal outward movement (in contrast to the vertical- only movement of the fault), and (mm) 2 the larger oblique vectors on the lower left flank, that are related -2 to the developing oblique collapse; d Z-axis (vertical) height -6 change between two successive -10 images. This contour field shows that the summit area has net height loss, while the upper Experiment 59 flank has net height gain, as the 5 cm 10 cm flank slides outwards; e Photo- C D graph of section A-A’ through a vertical dip-slip fault model, cut perpendicular to basement fault Cover for section strike that shows the major cutting only internal structures that developed at early stages within the cone. The red sand cover in this and subsequent sections is for cutting purposes only and was not present during the experiment, which was halted before full development of the thrust Throw = 200 m fault and subsequent collapse A A’ E Bull Volcanol (2009) 71:1111–1131 1117 unstable zones grew nearly parallel to the main fault strike compressional and extensional sectors producing thrust and and the normal orientated collapse structures described by normal faulted boundaries respectively (van Wyk de Vries Merle et al. (2001) only developed at large throws. and Merle 1998; Lagmay et al. 2000). This results in two The strike-parallel unstable areas were delimited by the zones of intense fracturing and instability parallel to the major normal fault. On the lower flanks, thrusting was underlying fault strike. The experiments of pure strike-slip divided into three branches separated by minor strike-slip faulting agreed with this earlier work, and replicate both the faults (Fig. 2b). Two of these defined the regions of strike- structures and regions of instability of previous studies. parallel slumping that bracketed a strike-parallel thrust. They show two sets of sigmoidal structures on opposing This situation has clear implications for collapse hazard, sides, bracketing regions of instability parallel to strike, because a small amount of fault movement is required to which act as the points of failure above narrow thrust zones. produce the conditions for strike-parallel collapse, which The sigmoidal structures form on the cone at acute angles makes such events more likely to occur than fault–normal to the basal shear zone. failures. These models therefore are consistent with earlier work (Vidal and Merle 2000; Merle et al. 2001), but define the Normal faulting early stage of structural instability. The larger strike-normal collapses must, however, be viewed as less common We tested a normal fault dipping at 60° (Fv/Fh=1.73) because the fault displacements required are considerably positioned directly beneath the cone. Pure dip-slip faulting larger than that which can be expected during the typical on an inclined (normal) fault surface will produce extension lifetime of an edifice. of the substrata (Horsfield 1977), and we would expect this to be transferred into an overlying cone as in the models of van Wyk de Vries and Merle (1996). Strike-slip faulting The normal fault in our models propagated through the cone, and a secondary antithetic normal fault formed, As a second comparison between the models described here producing a summit graben (Fig. 3). There was a and those of previous workers, the effect of pure strike-slip remarkable simplicity to this structure, with only two major fault movement directly below an overlying cone was faults developed, although there were minor extensional tested. It has previously been shown that such faulting fractures within the graben. These indicated a minor would produce a sigmoidal structure across the edifice, with component of strike-slip motion on each flank accommo-

Fig. 3 Models using layered cones built directly over a nor- Downthrown N’ Antithetic normal mal dip-slip fault dipping at 60°. a The basement fault propagates side fault through the cone forming (in plan view) an arcuate normal fault. The extension transferred from the basement fault to the cone causes an antithetic normal fault to develop that encloses a shallow summit graben within which there is a zone of frac- Superficial graben turing. b Sketch of structures A N B and instability directions. c Sec- tion N-N’ shows the primary normal fault (which propagates through the cone from the basement) and an antithetic normal fault (right) that together accommodate the extension within the cone N N’

C Downthrown 1118 Bull Volcanol (2009) 71:1111–1131 dating minor outward radial movement (Fig. 3b). Instability profile controlled by the topography. There was also a on the cone was confined to the area within the graben, second, minor thrust zone extending around the base of the with the outer sectors remaining stable and undergoing little footwall section of the cone (Fig. 4). Bulging of the flank or no deformation relative to their underlying basement. accompanied the thrusting, inducing instability perpendic- Here, the collapse hazard resulting from fault movement ular to strike. The compression on the downthrown side alone is limited to the graben and is again nearly parallel to was accompanied by a region of extension further back on the basement fault strike. the cone, creating an arcuate normal fault that cut back into We tested normal faults with dips of between 30° and the upthrown block. This fault was the line of separation 85°. Higher degrees of extension on lower-angle normal between the stable and mobile sectors of the cone, and in faults increased the extension within the cone (and hence section (Fig. 4c) was seen to connect down to the basement the width of the graben), similar to the effect seen in studies reverse fault. Small, shallow normal faults were observed of crustal faulting (Horsfield 1977). With high-angle that were mainly perpendicular to the basement reverse normal faulting there was still some extension in the cone. fault strike. There was also a small degree of extension Only with dips greater than 85° did the thrust fault seen in across the summit region at later stages in the models vertical fault experiments appear. (Fig. 4) as the mobile block began to fail.

Reverse faulting Oblique faulting

In models where a sub-volcanic reverse fault intersected the Most faults are a combination of dip-slip and strike-slip cone, the fault propagated to the surface along a curved components, giving oblique-slip motion. We tested a

Fig. 4 Model of a reverse fault dipping at 60° with pure dip-slip motion, sited directly under the Downthrown summit (Π7=0). a Plan view T’ side photograph (fault is dipping at 60° towards the bottom of the image); b structural sketch of the model. This causes an arcuate thrust fault on the footwall side and a secondary normal fault that forms directly above the basement fault. c Section T-T’ shows how the fault propagates through the cone, but is flattened out and surfaces as an arcuate thrust towards the base Stable of the downthrown side. This structure is accommodated T within the cone by a zone of A B extension and normal faulting

T’ T

5 cm C Bull Volcanol (2009) 71:1111–1131 1119 variety of oblique faults to examine the change A comparison with the pure strike-slip and dip-slip between the pure dip-slip and strike-slip end-members. models showed a simple progression between end mem- Vertical faults directly below the cone were tested with θ bers: even minor degrees of oblique motion such as θ=5° values (we reiterate that θ=tan Π6/Π5, or tan Fs/Fv) of (nearly strike-slip) and θ=86.5° (nearly vertical) produced a 5°, 10°, 15°, 30°, 45°, 60°, 75°, 80°, 85°, 86.5° and 89° significant geometric difference. This was firstly seen in a (Figs. 5 and 6). modification of the primary fault away from either arcuate

Fig. 5 Experiments using a vertical oblique-slip basement fault Downthrown PrimaryPrimary thrustthrust sited directly below a side heterogeneously-layered cone. a Vertical photograph of the surface structure using a basement fault oriented at 15° from pure dip-slip (θ=75°); b Structural sketch showing the main faults and fractures and the principal direction of instability. There is a similar structural pattern to Stable pure dip-slip (Fig. 2) experi- A B ments, although both the major normal and thrust faults inherit a slightly sigmoidal pattern from the strike-slip component; c Vertical difference (Z-axis) contour map for an oblique experiment (θ=85°) that shows that the flank bulge and thrusted area are more pronounced on the downthrown side facing the sense of motion; d Plan-view image of experiment using an oblique-slip vertical basement fault angled at 10° from pure strike-slip (θ=10°); e Structural summary sketch of the model in (E). There is a small deviation in the sigmoidal structure of pure strike-slip models (van Wyk de Vries and Merle 1998, Lagmay et al. 2000), including the development of a significant dip- slip component controlling the instability pattern Downthrown side

Stable

Downthrown D side E 1120 Bull Volcanol (2009) 71:1111–1131

Fig. 6 Plan views of oblique- slip models featuring basement 5˚ 10˚ faults directly below a A B heterogeneously-layered cone. a–i The angle of obliquity (labelled on each figure) increases through the series from near strike-slip (θ=5°) towards dip-slip (θ=85°). Observe the transition from the sigmoidal pattern through to the bow- shaped normal and thrust fault combination; j shows an oblique Downthrown image (θ=45°) illustrating that instability becomes highly 15˚ 30˚ concentrated in one sector on C D the downthrown side facing the sense of motion

45˚ 60˚ EF

75˚ 80˚ G H

85˚ 45˚ IJ Bull Volcanol (2009) 71:1111–1131 1121 normal (dip-slip) or sigmoidal reverse/normal (strike-slip) fault were modified and it became broken into two curving fault. strands, one on the upthrown and one on the downthrown For the model with 86.5° obliquity the main normal fault side. These were accompanied by Riedel and P-shear zones, was slightly sigmoidal, with consequent changes to the and a dominant thrust fault in the facing towards sector of thrusting pattern on the downthrown side. For small the cone, while a secondary thrust forms on the facing away displacements, thrusting was concentrated in one sector of sector (Figs. 5d, e and 6). the downthrown side of the cone, sub-parallel to strike, but on the sector facing towards the sense of motion (Fig. 5). There was also a narrower sector of thrusting on the Normal fault with strike-slip component downthrown side, on the side facing away from the sense of fault movement. In models with a greater proportion of For models with normal faulting at 60° dip, with a dextral strike-slip motion, the faulting pattern became progressive- strike-slip component the shallow faulting pattern was also ly modified towards that seen in pure strike-slip models, modified. While apparently similar primary and antithetic with a greater sigmoidal shape. normal faults formed to those seen before, there was a With equal dip-slip and strike-slip proportions (θ=45°), slight change in the amount of extension across the cone, the concentration of instability into one sector was most with an increased proportion on the downthrown side evident (Fig. 6e, j). The downthrown side quadrant facing facing towards the sense of motion–the same direction towards to the sense of motion was strongly fractured and seen in the vertical oblique-fault experiments. There were was delimited by two normal faults. One of these divided also tension fractures within the graben that showed that the stable footwall sector from the remainder of the cone. there is deformation along the graben strike (Fig. 7a, b). There was also a narrow zone of fracturing on the downthrown facing away side, but this was mainly focussed around the single normal fault. There was no Reverse fault with strike-slip component clear instability and collapse normal to the basement fault strike, and the thrust seen in vertical dip-slip experiments For models using reverse faulting with a sinistral strike-slip did not develop. With a majority strike-slip component (θ< component the experiments also showed a deviation from 45°) both the position and kinematics of the major normal pure dip-slip movement, with the reverse fault developing

Fig. 7 Results for normal and reverse faults that have a 15° oblique component. a Plan view image of an oblique dextral- normal fault; b Sketch of the model showing the main graben and the slightly increased deformation on the side facing the sense of oblique motion; c Im- age of sinistral oblique reverse fault; d sketch of reverse fault showing slightly enhanced collapse on the side facing the strike-slip motion Stable A B

Stable C D 1122 Bull Volcanol (2009) 71:1111–1131 more rapidly on the side facing towards the sense of motion cone as a function of the position of the fault beneath the (Fig. 7c, d). This was the first region to collapse, although flank and the slope angle of the cone itself. This movement again the entire downthrown side became unstable given was accommodated by a simple thrust on the downthrown sufficient displacements. The antithetic normal fault seen in side. Other models with vertical faults (oblique- and strike- earlier reverse experiments also developed, although there slip) positioned below the cone flanks produced similar was a slight modification to the extension orientation, patterns of shallow faulting to those with a vertical dip-slip which became oblique in the summit region reflecting the fault (Fig. 9). This indicates the importance of the vertical strike-slip component (Fig. 7c, d). fault position rather than the direction of movement. Normal and thrust faults that were offset from the axis of the cone were also tested. When either type of fault was Faults offset from the volcano central axis used, similar structures to those detailed above formed, depending on both the position and type of the fault. Thus, The previous sections dealt with a cone centred directlmy normal faults and thrusts that dip towards the volcano can over the basement fault. In many examples of volcanoes produce significant instability, as a major arcuate shallow underlain by faults this is not the case, and the fault passes normal fault can form (Fig. 10). This fault is coupled to instead beneath a flank. The effect of fault offset is explored either an antithetic normal fault or a thrust depending on the in the following section. type of basement fault.

Fault position Fault patterns and geometric variables

Using the simple vertical dip-slip fault apparatus, a series We have collected the fault patterns in simplified form onto of experiments were constructed with cones positioned at graphs of degree of obliquity, fault dip and fault offset varying degrees of offset (Π7) from the basement fault. (Fig. 11). These regroup the observations shown in As we have shown, where a cone was positioned directly Figs. 2–10. over a vertical dip-slip basement fault, a normal and It can be seen that for fault dip vs. degree of obliquity thrust fault combination formed. With increased offset a (Fig. 11a) with pure strike-slip motion, the fault dip had no fundamental alteration occurred to these shallow faults. effect on structures, while on faults with dip-slip compo- When the centre of the cone was located on the nents the fault dip had a strong control. The structural upthrown plate, the normal fault cut back further across patterns changed from sigmoidal at low obliquity (strike- the cone, although the thrust still surfaced at a low level slip faulting) to bow-shaped with antithetic components at (Fig. 8). The faults formed the boundaries to an exten- pure dip –slip. The antithetic type of structure began to sional sector of the cone that encouraged a collapse dominate over the sigmoidal shape at around 45°. Howev- normal to fault strike. Such a structural configuration, er, the collapse directions changed at a much earlier consisting of an arcuate normal fault open to approxi- stage, as strong fracturing in one sector was observed at mately 110° and a basal thrust, was controlled by the very low amounts of strike-slip component (θ=85°). The position of the fault beneath the cone flanks. A series of sigmoid on the downthrown side of a fault also became experiments (Fig. 9) demonstrates how the affected the principle collapse direction at low vertical components surface area varies as a function of the fault position. (θ=5°). The fracture pattern was also dependent upon which side There is a clear domination of fault offset effects of the fault the cone was positioned. The structures over dip-slip effects (Fig. 11b). Faults centred on the described only formed when the majority of the cone volcano axis first developed two small strike-parallel was on the upthrown footwall block, as this configuration instabilities in a conjugate system (this may be a normal- de-buttresses one sector of the cone. When the cone was fault/thrust fault pair, or a graben, depending on the fault centred on the downthrown block, only minimal structural dip). However, when the fault was sited near to the modification occured. This was typically in the form of a volcano edge, and if the volcano was on the footwall small normal and thrust fault combination. block, then there was a broad zone of strike normal instability. In contrast, when the volcano stood on the headwall there was no significant instability. Instability Fault type and fault position can form only in this location if there is a strike-slip component to motion (Fig. 11c). The models described above using a vertical dip-slip fault Fault offset is dominant when compared with the produced a primary normal fault, cutting back across the degree of obliquity (Fig. 11c). When the cone is centred Bull Volcanol (2009) 71:1111–1131 1123

Fig. 8 Results of experiment featuring a vertical dip-slip fault Downthrown positioned below the flanks of a side O heterogeneously-layered cone (Π7=Df/R=0.6). The fault causes extension across a large cone sector, with an arcuate normal fault with an opening angle of up to 110° and a thrust on the downthrown side. a End-of-experiment vertical photograph; b The structural summary; c Horizontal deformation vectors that show the major strike normal movement; Stable d Z-axis difference contour map (scale in m) that shows move- O’ ment related to both the normal A B (depression near the summit) and the lower thrust fault. These structures would lead to insta- (m) bility and potentially collapse normal to the strike of the underlying fault; e The line of section O-O’ shows the normal and thrust fault combination

C D

O’ O

on the footwall, strike normal collapses occurred. These above, there was a strike-slip component in the degree of were largest when the fault was about halfway between obliquity. the cone edge and the volcano axis (Π7=Df/R=0.5). When the cone is directly over the fault (Π7=0), either sigmoidal or bowed antithetic faults developed depending Discussion on the degree of obliquity. The antithetic faults were, as stated before, normal / thrust pairs for a near vertical fault, The experiments demonstrate that structural modifications otherwisetheyformedagraben.Inbothcasesthesmall occur within a simple cone in response to basement faulting. strike-parallel instabilities developed for central faults, but Faults encourage fluid flow and hence hydrothermal alteration strike-normal collapses developed for faults offset towards and further weakening of the edifice, or may provide the the front (Π7=+ve). For cones set on the hanging wall pathway of least resistance for magma intrusion, as demon- (Π7=-ve), there was little instability, unless, as stated strated in previous analogue experiments (Lagmay et al. 1124 Bull Volcanol (2009) 71:1111–1131

Fig. 9 Images of experiments with offset faults in dip-slip and AB strike-slip contexts. a–d Experi- ments where the position of the cone above a vertical dip-slip fault is varied between the upthrown side (A, B) to it is on the downthrown side (C, D). The photographs show the changing shallow structures produced on the cones in response to the position of the fault. Large-scale instability and collapse perpendicular to fault strike occurs when the cone is on the upthrown side; when the cone is positioned on the downthrown C D block there is only a limited sector of shallow faulting and no significant instability. e Plan image showing the effect of a dextral strike-slip basement fault cutting one flank of a heterogeneous cone; f oblique image of the model in (D). The faulting induces a shallow normal fault to form, which leads to instability normal to strike

E F

2000). Shallow faults can therefore define the sector most al. 2001), but the instability parallel to the fault strike with likely to undergo failure during the volcano’s lifetime. The smaller displacements identified here places it more firmly depositional processes which govern the surface morphology in the context of alternative fault geometries. While larger of volcanoes as they grow will act to conceal the finer details collapses normal to strike above vertical faults are certainly of shallow faulting (Borgia and van Wyk de Vries 2004; possible, they are likely to be preceded by smaller strike- Norini and Lagmay 2005). Taking into consideration the parallel events. surrounding structures, however, detailed knowledge of the The pattern of shallow fault structures that develop with kinematics of underlying faults allows studies on the edifice increasing obliquity towards pure strike-slip supports the to target specific areas. strike-parallel collapse model, providing a more complete While earlier studies indicated only fault-normal col- picture of the transition from dip-slip to strike-slip faulting lapse above vertical faults and fault-parallel collapse above (Fig. 11). The direction of collapse is constrained where strike-slip faults, we have demonstrated a more complex there is even a few degrees of obliquity, a factor which has situation (Fig. 11). The structural pattern and instability obvious implications for hazard assessments. While there produced on an overlying edifice by vertical faulting agrees remains the potential for collapses of both the opposing in part with earlier work (Vidal and Merle 2000; Merle et sector and, with sufficient displacement, normal to strike on Bull Volcanol (2009) 71:1111–1131 1125

Fig. 10 a Plan-view image of cone placed on a normal fault footwall; b plan-view image of a cone placed on the hangingwall normal fault; c Vertical image of a cone placed on a reverse fault hangingwall; d image of cone placed on the footwall of a reverse fault. These show the 60 change in the cone’s response to basement faulting as a function of the fault position and kinematics 60

the downthrown side, the zone which is placed into a fault is sited beneath a peripheral flank, where it is able to extension by the combination of dip-slip and strike-slip destabilise a large portion of the edifice. The resulting faulting has the highest concentration of fracturing. highly arcuate (110°) normal edifice fault and basal thrust For a cone positioned above a normal fault the extension produces the conditions suitable for large-volume slope induces a shallow normal fault, similar to that seen in failures that would leave a distinctively open collapse scar. vertical dip-slip models. However, a fundamental difference This feature could be confused with the open scar collapses occurs in the secondary structures on the downthrown side. generated by failure of underlying weak strata at gravita- Here the thrust fault seen in vertical fault models is replaced tionally spreading volcanoes (van Wyk de Vries and Francis by an antithetic normal fault to accommodate the extension, 1997). As a result, care should be taken when reviewing forming a shallow graben. Such normal antithetic faults collapse scars to test for both underlying structures and were seen at fault dips of up to 85°. There is also a small weak substrata that could be involved in collapse. amount of movement lateral to the fault strike, which The specific internal structures that develop in encourages fault parallel collapse within this graben. response to basement faulting represent pathways of Sub-volcano thrust and reverse faults will propagate least resistance. These become regions of increased through the edifice to break the surface on arcuate traces, permeability and fluid flow, concentrating hydrothermal and in doing so they place a significant proportion of the alteration, and thereby representing the weakened sector cone under extension, as seen by Branquet and van Wyk de most likely to undergo slope failure. Faulting may also Vries (2001). This leads to a normal fault and an overall control the internal state of stress within the volcano, surface structural pattern very similar to that seen on cones which in turn can influence magma intrusions. Flank built above vertical faults, and can create a broad zone of faulting may produce a stress regime and fracturing that instability above the thrust. Significant instability directed could control the intrusion of cryptodomes (e.g. Lagmay normal to thrust fault strike is most likely to develop when et al. 2000) or dyke systems (e.g. Tibaldi 2003). Both of 1126 Bull Volcanol (2009) 71:1111–1131

Fig. 11 Graphical illustration of A Strike-slip Dip-slip B Backside Centre Front the fault patterns created in the 1 analogue experiments, the areas reverse reverse where instability occurs and the fault fault 2 control of the main geometrical normal normal parameters. a Degree of obliq- 30˚ uity vs. Fault Dip. b Fault Offset 30˚ vs. Fault Dip, c Degree of Fault dip Fault dip Obliquity vs. Fault Offset. Note that the degree of obliquity (θ)is 1 1 1 1 the proportion of strike-slip to 60˚ 1 60˚ 2 2 dip-slip motion given by θ=tan 2 2 Π4/Π5=tan Fv/Fs. Note that the 1 1 1 1 1 1 1 fault dip is characterised by tan 2 Π5/Π4=tan Fv/Fh 90˚ 2 2 2 2 2 2 1 90˚ 0˚ 5˚ 10˚ 20˚ 45˚85˚ 90˚ -1 0 1 Fault offset C Strike-slip Dip-slip Front 1 Key

Fault offset edifice outline 2 2 main fault (normal / thrust) 1 1 1 1 1 1 1 Centre 2 0 antithetic / minor fault 2 2 2 2 2 1 unstable zone Strike-normal collapse 1 2 collapse sequence Backside -1 0˚ 5˚ 10˚ 20˚ 85˚ 90˚

these processes can lead to edifice instability, and any lateral collapses, as witnessed both by separate deposits and control by the underlying tectonic setting should be the remnants of avalanche calderas. There is a major considered. It is also likely that basement faulting exerts lineament trend, interpreted to be sub-surface faults, an influence on volcanic structures over a variety of scales striking NW-SE across the summit and marked by a from the small, such as movement along fissures beneath 700 m-wide “weak zone” of altered exposures. While at hornitos (e.g. Nordestino, Mount Etna 1970) or spatter least two large collapses, including the 1888 event, were cones (e.g. Izu-Oshima, Japan; (Sumner 1998)), to the directed almost normal to this major NW-SE fault axis, large scale, such as the volcano-tectonic features seen there have been a series of smaller collapses directed cutting the Colima Volcanic Complex (Garduño-Monroy parallel to this fault (e.g. Biwazawa deposit on Fig. 12a). Of et al. 1998). Shallow faulting is more dependent upon the the six documented collapses at Bandai Volcano, four have mechanical properties of the cone and its topography than been directed sub-parallel to the shallow fault zone on its size. produced by tectonic faulting, while the two largest were normal to this. The headwalls of these larger events cut through this sector, which was presumably weakened by Applications intense hydrothermal activity. The collapse record is that expected for a cone sited directly above a vertical fault Bandai Volcano, northern Japan, (Fig. 12) underwent a (Fig. 2) where large failures normal to fault strike are the well-documented collapse in 1888 which was preceded by latest events, preceded by initial instability confined to little, if any, precursory activity and resulted in 461 deaths zones nearly parallel to the basement fault. as nearby villages were inundated by a debris avalanche. The Colima Volcanic Complex, Mexico (Fig. 12)is This event may have been triggered by a combination of directly underlain by the southwest–northeast striking hydrothermal alteration, elevated internal pore pressures Tamazula Fault (Garduño-Monroy et al. 1998). The edifice and local seismicity (Mimura and Endo 1997). Further has suffered repeated slope failure (e.g. Luhr and work on the evolution of Bandai Volcano (e.g. Mimura and Prestegaard 1988;StoopesandSheridan1992), and debris Endo 1997) has shown that there have been at least six avalanche and debris flow deposits cover a large area Bull Volcanol (2009) 71:1111–1131 1127

02 km A B N

o 37 40’N Nevado de Colima

Volcan de 1600 Colima 1300

37o 35’N 1000

700

1888 lahar deposit 1888 DAD Biwazawa DAD (2.5 ka) 0 10 km Okinajima DAD (<40 ka) A 140o 5’E

Fig. 12 a Simplified geological map of Bandai Volcano, Japan, with shallow graben (yellow lines) that developed in response to movement avalanche calderas and debris avalanche deposits (DAD) as mapped on an underlying basement fault. Avalanche calderas (thick red lines) by Yamamoto et al. (1999). The white arrows show the alignment of and two debris avalanche deposits (blue=Nevado de Colima, after the “weak zone”, which marks a shallow fault zone. b Digital Capra and Macias (2002); green=Volcán de Colima, after Cortes- Elevation Model of the Colima Volcanic Complex, showing the Cortes (2002)) are marked. At=Atenquique town extendingtothePacificcoastline120kmtothesouthwest. been documented within the south-western sector of the Collapse of an earlier Volcán de Colima cone towards the graben (Cortes-Cortes 2002). Collapses of ancestral south was associated with the Tamazula Fault by Volcán de Colima cones have also been directed south- Garduño-Monroy et al. (1998), using the earlier models wards (Komorowski et al. 1997), underlining the addi- of collapse perpendicular to σ3 (Nakamura 1977;Siebert tional control of local topography which dips sharply to 1984). These deposits have since been shown (e.g. the south. Ground deformation studies (Wooller 2004) Komorowski et al. 1997;Cortes-Cortes2002)tohave show that subsidence of the Atenquique-Alseseca Graben originated from multiple collapses. Additionally, no is an ongoing process at Colima. explanation was offered for the earlier strike-parallel Further examples of basement fault and volcano inter- Nevado de Colima failure (Fig. 12b). Comparison of actions are readily available. For example, another Mexican shallow faulting and collapses at Colima, in relation to the volcano proposed to be affected by faulting is Nevado de analogue modelling described above (e.g. Fig. 3), show Toluca, where a well-preserved edifice graben is clearly the influence of basement faulting. When applied to the seen (Norini et al. 2008). A further good example of fracture patterns evident across the edifice (Fig. 12b), the basement faulting in an Andean volcano is that of Ollague, modelling results suggest the underlying fault is respon- Chile (Vezzoli et al. 2008). In this case the faults cut the sible for the Atenquique-Alseseca Graben (Garduño- lower edifice leading to strike-normal collapse. The large Monroy et al. 1998) which crosses the edifice. The composite Tungurahua Volcano, Ecuador (Fig. 13a) has concentration of fracturing which would be expected undergone at least two major lateral collapses directed to within the shallow graben would encourage and direct the west (Barberi et al. 1988). The most recent of these the development of hydrothermal alteration and potential (2,955 years BP, Hall et al. 1999) was connected to magma intrusions, both of which have been connected cryptodome intrusion into the upper western flanks. This with previous failures. The graben controls the collapse, with a volume of 8 km3 and a run-out length of 18,500 years BP collapse of eastern Nevado de Colima 21 km, left a 110°-wide avalanche caldera that has been (Stoopes and Sheridan 1992; Capra and Macias 2002). associated with regional faults trending NNE-SSW and More recent (3,600 years BP) deposits, originating from NNW-SSE (Hall et al. 1999) bordering the western flank of the site of the present Volcán de Colima cone and the edifice (Fig. 13b). These faults may have controlled the overlying at least two earlier avalanche deposits, have direction and extent of the failures at Tungurahua Volcano 1128 Bull Volcanol (2009) 71:1111–1131

A B N Avalanche caldera C Debris avalanche deposits Older debris avalanche VolcanicsV Recent volcanics

Bakening Volcano Fault-controlled valley

Chambo Tungurahua Lake Ecuador0 0 5 km 0 5 km

Fig. 13 a Shaded relief image of Tungurahua Volcano, Ecuador; b image of Bakening Volcano, Kamchatka. The fault which cuts beneath Geological sketch map of Tungurahua, showing the position of the the lower eastern flank controlled a large lateral collapse around 8 Ka, basement fault line west of the edifice, the avalanche caldera and and subsequent smaller volume failures have been linked to debris avalanche deposits directed towards this structure (adapted reactivation of this fault line (Dirksen 2003) from Hall et al. 1999); c Greyscale LANDSAT ETM+panchromatic in a manner similar to the offset fault models (Fig. 8). Examples of volcano collapse in Italy that may be Further examples of fault-influenced volcanic structures in connected to underlying tectonic faulting include Vesuvius– Ecuador include Guagua Pinchua (Barberi et al. 1992), Monte Somma (Milia et al. 2003), Roccomonfina (de Rita Reventador (Belloni and Morris 1991), El Soche (Tibaldi and Giordano 1996) and Stromboli (Tibaldi 2001), while in and Ferrari 1992) and Sangay (Monzier et al. 1999). Indonesia van Bemmelen (1949) records numerous Javan Galeras Volcano and Nevado de Tolima (both in Columbia, volcanoes that developed on, and have been modified by, Thouret et al. 1995; Rovida and Tibaldi 2003), El Misti and regional fault lines. Finally, collapses at both Mount Meru, Ubinas Volcano (Peru) are all intersected by regional faults Tanzania (Wilkinson et al. 1986) and Pico de Orixaba, and have undergone collapses (Merle et al. 2001). Mexico (Concha-Dimas and Watters 2003), have a possible Bakening Volcano (Kamchatka) is an example of connection to underlying faults striking parallel to the fault-influenced instability. A major normal fault on the collapse orientation. east side of the edifice (Fig. 13c) controlled a lateral While this is by no means an exhaustive list, these collapse some 8,000 years BP, since when there has been examples do show that many volcanic edifices have been no significant volcanic activity (Melekestsev et al. 1999). influenced by gradual, cumulative fault movement through- Subsequent smaller failures from the hydrothermally- out their evolution. The effects of this are pervasive, altered amphitheatre walls correspond to periods of creating distinctive patterns of shallow faulting across the increased movement along the underlying fault (Dirksen edifice and influencing both the internal structure of the 2003), suggesting that in this example fault movement cones and their instability. controls both the form and the timing of the collapse. In the case of Bakening, it is likely that periods of increased fault movement changed the internal stress state of the Conclusions already breached eastern flank, encouraging further fracturing, hydrothermal fluid circulation and ultimately slope This study has demonstrated an intimate relationship failures. Further examples within Kamchatka include between long-term fault movement and the structure and Kizimen Volcano (Melekestsev et al. 1995), which has stability of overlying volcanic edifices. It is not just the been modified by a fault cutting its northwest flank to case that basement faults are affected by volcanoes (van control instability in this direction, and Shiveluch Volcano Wyk de Vries and Merle 1996, 1998; Branquet and van (Belousov et al. 1999) which has suffered multiple Wyk de Vries 2001), but also vice versa. Short-term southwards-oriented slope failures. activity along faults can cause strong ground motion Bull Volcanol (2009) 71:1111–1131 1129 capable of destabilising an edifice, but this is by no means small collapses at Nevado de Toluca have also been the only impact of basement tectonics. Over the life time channelled within the summit graben (Norini et al. of a typical volcano, from the first eruption to the removal 2008). by erosion of the edifice, basement faults may undergo The general model of fault-influenced volcano evolution movement totalling several hundred metres. The models and instability covers a broad spectrum of fault types. There have demonstrated that this motion is transferred into the still, however, remain a number of variables which warrant edifice, producing structures dependent primarily upon the examination. These include a more detailed study of the type and position of the fault. transition between vertical and dipping (normal or thrust) The previously accepted simple model of lateral col- fault geometries, further investigation of the role of lapse, either normal or parallel to the strike of dip-slip or hydrothermal alteration within the cone and the possibility strike-slip faults respectively, now needs revision. Although of weakened sub-strata that could undergo gravitational with sufficient displacements on dip-slip faults collapse can spreading. The intrusion of magmatic bodies before, during occur normal to strike, smaller displacements also lead to or after faulting should also be tested. Additional features structural modifications and the development of instability include multiple faults, either active synchronously or parallel to strike, and, although smaller in volume, these consecutively, together with models utilising more realistic events have a higher frequency. cones and surrounding topography. It is also important to A normal fault directly below a cone will propagate further test this work by comparison with natural examples through the edifice and produce a shallow graben, which and especially to monitor ground deformation to obtain will constrain instability to be nearly parallel to the strike kinematic information of the volcanoes response to fault (Fig. 11). Even a small amount of strike-slip motion on a movement. fault (a small degree of obliquity) will place a further control on the direction of potential instability, concentrat- Acknowledgements LW was supported by N.E.R.C. studentship, ing it within one sector sub-parallel to the strike, and facing grant number NER/S/A/2000/03505, and by an Open University towards the sense of motion. This sector, placed under Research Development Fund fellowship. The work was also supported extension as a result of the dip-slip and strike-slip by a French INSU PNRN grant. This study was performed in the ‘ components of basement faulting, is the portion most framework of UNESCO-IUGS-IGCP Project 455, Effects of basement structural and stratigraphic heritages on volcano behaviour and affected by shallow faulting. While this feature is clearly implications for human activities’. shown by the models, there is also a narrower sector of fracturing concentrated on the other, downthrown side. For a sufficient dip-slip component (θ≥45°), and with References high displacements, larger volume strike-normal collapses can be expected to develop eventually. Such strike-normal Acoccella V (2005) Modes of sector collapse of volcanic cones: collapses are most common when the fault crosses below insights from analogue experiments. J Geophys Res 110:B02205 the lower to mid-flank of the edifice. In this case, fault Barberi F, Coltelli M, Ferrara G, Innocenti F, Navarro JM, Santacroce movement at the periphery of a volcanic edifice has the R (1988) Plio-quaternary volcanism in Ecuador. Geol Mag 125:1–14 effect of de-buttressing the cone, allowing instability to Barberi F, Ghigliotti M, Macedonio G, Orellana H, Pareschi MT, Rosi develop. It is thus important to realise that faulting at the M (1992) Volcanic hazard assessment of Guagua Pichincha periphery of a volcano may have an affect across most of (Ecuador) based on past behaviour and numerical models. J – the edifice. 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