Flank collapse triggered by intrusion: the Canarian and Cape Verde Archipelagoes

18 Pages·2000·1.82 MB·English

JournalofVolcanologyandGeothermalResearch94(cid:14)1999.323–340
www.elsevier.comrlocaterjvolgeores
Flank collapse triggered by intrusion: the Canarian and Cape
Verde Archipelagoes
Derek Elsworth a,), Simon J. Day b,c
a DepartmentofEnergyandGeo-En˝ironmentalEngineering,Pennsyl˝aniaStateUni˝ersity,Uni˝ersityPark,PA16802-5000,USA
bDepartmentofGeographyandGeology,CheltenhamandGloucesterCollegeofHigherEducation,UK
c BenfieldGreigHazardResearchCentre,Uni˝ersityCollegeLondon,London,UK
Received10May1999
Abstract
The potential to develop kilometer-scale instabilities on the flanks of intraplate volcanoes, typified by the Canary and
CapeVerdeArchipelagoes,isinvestigated.Aprimarytriggeringagentisforcedinjectionofmoderate-scaledikes,resulting
in the concurrent development of mechanical and thermal fluid pressures along the basal de´collement, and magmastatic
pressuresatthedikeinterface.Theseadditiveeffectsareshowncapableofdevelopingshallow-seatedblockinstabilitiesfor
dikethicknessesoftheorderof1m,andhorizontallengthsgreaterthanabout1km.Fordikesthatapproachorpenetratethe
surface, and are greater in length than this threshold, the destabilizing influence of the magmastatic column is significant,
andexcessporefluidpressuresmaynotbenecessarytoinitiatefailure.Thepotentiallydestabilizedblockgeometrychanges
from a flank-surface-parallelsliver for short dikes, to a deeper and less stable de´collementas dike horizontal length builds
andtheeffectsofblocklateralrestraintdiminish.Forintrusionslongerthanabout1km,thecriticalbasalde´collementdives
belowthewatertableandutilizesthecomplementarydestabilizinginfluencesofporefluidpressuresandmagma‘‘push’’at
therearblock-scarp.Inadditiontoverifyingtheplausibilityofsuprahydrostaticpressuresascapableoftriggeringfailureon
thesevolcanoes,timingoftheonsetofmaximuminstabilitymayalsobetracked.ForeventswithintheCumbreVieja(cid:14)1949.
and Fogo (cid:14)1951, 1995. pre-effusive episodes, the observation of seismic activity within the first 1 week to 4 months is
consistentwiththepredictionsofthermalandmechanicalpressurization.q1999ElsevierScienceB.V. Allrightsreserved.
Keywords:aquathermal;poromechanics;flankinstability;Fogo;CumbreVieja
1. Introduction these volcanoes vary in angle from less than 88 to
more than 208. At the lower end of this range, in
Large scale lateral collapses, with volumes of up particular, the causes of these massive failures are
to thousands of cubic kilometers, are a ubiquitous enigmatic(cid:14)Iverson,1991,1992;VoightandElsworth,
feature of oceanic island volcanoes in their most 1992. since the destabilizing gravitational stresses
active, shield-building, stageofgrowth(cid:14)e.g., Moore, generated on basal failure planes are much less than
1964; Moore and Fiske, 1969.. The overall slopes of resisting frictional forces, for reasonable values of
friction coefficients.
Models for the kinematics of preserved gravita-
)Corresponding author. Tel.: q1-814-863-1643; fax: q1-814- tionalslumpsassumedtypicalfor oceanicvolcanoes,
865-3248;E-mail:[email protected] including the Hilina slide system in Hawaii (cid:14)Swan-
0377-0273r99r$-seefrontmatterq1999ElsevierScienceB.V.Allrightsreserved.
PII: S0377-0273(cid:14)99.00110-9
324 D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340
sonetal.,1976;Ando,1979;Buchanan-Banks,1987; cussed below. An alternative, that could reduce
Lipman et al., 1985., are difficult to justify as the strength sufficiently, is mechanical and thermal
initial geometries of catastrophic long-runout slides, pore-fluidpressurization(cid:14)ElsworthandVoight,1995,
since motion must occur updip on landward dipping 1996; Day, 1996; Voight and Elsworth, 1997; Hu¨rli-
interfaces. Slides must initiate on primarily seaward mann et al., 1997; van Wyk de Vries and Francis,
dipping interfaces if sufficient momentum is to be 1997.. Accordingto thesemodels, instabilitymaybe
generated for long-runout avalanches. In addition to triggered by a reduction in strength on the de´colle-
the well-known occurrences of such deposits around ment through an increase in pore fluid pressures in
the Hawaiian Islands, long-runout debris avalanche the volcanic edifice.
deposits have been observed on the ocean floor Here, we consider how a relatively small volume
surroundingtheCanarianArchipelago(cid:14)Holcomband of magma, emplacedas dikes in a volcanicrift zone,
Searle, 1993; Masson, 1996; Teide-Group, 1997; can destabilize the edifice through the mechanical or
Urgeles et al., 1997., together with aborted collapse thermal pressurization of pore fluids.
structures on the flanks of El Hierro (cid:14)Day et al.,
1997., and incipient collapse structures on Cumbre
Vieja (cid:14)La Palma. and Fogo in the Cape Verde 2. Field evidence for magmagenic fluid pressures
Islands (cid:14)see Day et al., 1999a-this volume.. Indica-
tions of the importance of including pore fluid pres- These postulated pressurization mechanisms have
sure effects in analysis of the stability of these been observed. Mechanical pressurization has been
steep-sidedoceanic island volcanoes can be found in observed in a series of intrusive events at Krafla,
the patterns of flank deformation associated with Iceland,withporepressuresrapidlyreaching70mof
certain recent eruptions. These are the 1949 eruption water-head in a confined aquifer 4.3 km from the
of the Cumbre Vieja volcano, La Palma (cid:14)Canary intrusion site (cid:14)Brandsdo´ttir and Einarsson, 1979;
Islands. (cid:14)Day et al., 1999a-this volume.; the 1951 Tryggvason, 1980; Stefa´nsson, 1981.. Similarly,
eruption on Fogo (cid:14)Cape Verde Islands. (cid:14)Day et al., thermalpressurizationislikelythecauseoflong-term
1999b-thisvolume.;andpossiblyalsothe1995erup- anomalousflowfromaboreholeobservedduringthe
tion of Fogo (cid:14)Heleno da Silva et al., 1999.. eruption of Heimaey (cid:14)Bjo¨rnsson et al., 1977.. These
Flank failure is only feasible if processes are processesofgeneratingexcessfluidpressuresarenot
capable of significantly increasing destabilizing limited to mid-ocean ridge environments, with at
forces, or reducing strength of a de´collement. Alter- least one example reported for island-arc volcanics
nate mechanismsto describemechanistictriggers for (cid:14)Watanabe, 1983.. The potential to overpressurize
shallow debris avalanches include consideration of rocks within the core of a volcanic edifice appears
fluid pressurization resulting from lava accretion, clear from these observed occurrences, and it is
surface saturation by rainfall (cid:14)Iverson, 1996. or the possibletomatchresponsedirectlythroughappropri-
presence of a soft underlying basement (cid:14)Clague and ate mechanistic models of static (cid:14)Sigurdsson, 1982.
Denlinger, 1994.. Although plausible, the first set of and migrating (cid:14)Elsworth and Voight, 1992. intru-
triggers are rejected by Iverson (cid:14)1996. as insuffi- sions.
cientlystrong.Thepresenceofanextensiveunderly- Paleo-evidence for aquathermal pressurization by
ing magmatic plumbing system appears plausible, dikes is present in the exhumed oceanic island
although it is applied (cid:14)Clague and Denlinger, 1994. Seamount Series in the Barranco de Las Angustias,
to the landward-dipping geometry of the Hilina La Palma (cid:14)Staudigel, 1997.. These rocks were up-
(cid:14)Kilauea. slide, and therefore is classified as kine- lifted above sea level and deeply incised both before
matically incapable of developing into an avalanche. and after the growth of overlying subaerial volcanic
Thismechanismalsoimpliesthatdeformationshould rocks, resulting in exposures of pillow lavas cut by
continue between eruptions; although this is the case an intense dike swarm. Both intrusive and extrusive
in Hawaii, Fogo and Cumbre Vieja show little evi- rocks were affected by contemporaneous hydrother-
dence of deformation in inter-eruptive periods, but malactivity.Pipe-likezonesofhydrothermalmineral
significant deformation in eruptive periods, as dis- (cid:14)mainly zeolite.-cemented breccia along dike mar-
D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340 325
Fig.1.(cid:14)a.IrregularwesternmarginofN–S-trendingaphyricdikewithintrusivehornstructureandirregularbrecciazone(cid:14)extendingtoright
ofpicture.atcontactincentreoffieldofview.Hostrockisolder,alteredolivine–phyricbasalticdikerock.Thicknessofmainbrecciabody
ca. 30 cm: pen in centre of field of view is ca. 15 cm long. (cid:14)b. Detail of main breccia zone showing zeolitic veining of host rock and
brecciatedmassofstronglychilledaphyricbasaltdikerock.Noteirregular,sharply-definedchilledmarginatleftsideofbrecciatedrock
mass.
326 D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340
gins result, typically at jogs and breached bridges in sures; their potential occurrence and influence are
the dikes (cid:14)Fig. 1a.. The breccias cut both the host examined in the following.
rocks and the broad, well-defined chilled margins
(cid:14)Fig. 1b. of the dikes. The brecciated dike rock
3. Collapse mechanisms
contains a finely-divided hyaloclastitic matrix in ad-
dition to the hydrothermalmineralization,suggesting Both mechanical and thermal fluid pressurization
that the brecciated zones formed soon after (cid:14)but not mechanismsmaybequantified.Mechanicalbehavior
during. dikeemplacement.Theexplosivebrecciation is represented by a migrating volumetric dislocation
is driven by thermal fluid pressurization. (cid:14)Cleary, 1977, 1978; Rudnicki, 1981; Elsworth,
Although the example shown in Fig. 1 is from a 1991., representing either the laterally or vertically
submarinevolcanicsequence,similareffectsarepos- migrating dike geometries illustrated in Fig. 2a, b.
sible below the water table in subaerial volcanoes. These represent observed intrusive modes for dike
The geometry of the water table has been at least propagation during the shield-building phase. The
partly determined in a number of modern oceanic development of lateral, rift-parallel intrusion within
island volcanoes, including Kilauea in Hawaii the volcanic pile is well documented for Kilauea
(cid:14)Thomas et al., 1983; Thomas, 1987., Tenerife volcano (cid:14)Klein et al., 1987., where the asymmetric
(cid:14)Anonymous, 1991. and El Hierro (cid:14)Navarro and lateral stress regime favors this form (cid:14)Rubin and
Soler, 1994. in the Canary Islands, Fogo in the Cape Pollard, 1987.. Vertical modes of propagation are
Verde islands (cid:14)Bjelm and Svensson, 1982; Martins, reported for mid-oceanic ridge volcanoes in west
1988; Descloitres et al., 1995. and the Piton de la (cid:14)Gudmundsson et al., 1992. and central Iceland
FournaisevolcanoonReunion(cid:14)Coudrayetal.,1990.. (cid:14)Brandsdo´ttir and Einarsson, 1979; Tryggvason,
The water table typically rises gradually from sea 1980., in island-arc volcanoes (cid:14)e.g., Mt. Unzen,
level at the coast, is moderately seaward-dipping Nakada et al., 1997. and for intraplate volcanoes in
between the coast and the major volcanic rift zones the Cumbre Vieja (cid:14)La Palma. (cid:14)Day et al., 1999a-this
(cid:14)which form the main topographic ridges. and cen- volume. and on Fogo (cid:14)Day, 1996.. Dike intrusion
tral summit regions, and rises sharply within the within predefined rifts in the upper 1 or 2 km may
main intrusion swarms to between 50% and 90% of concurrently generate large ‘‘uplift’’ pore pressures
the topographic elevation; water is perched and and lateral tractions from magma ‘‘push;’’ these
trapped between the dikes in the intrusion swarms. effects are additive.
Despite jointing, dikes are generally impermeable, The kinematics of the block is defined in Fig. 2c
relativetothehostvolcanicrocks;theydominatethe with the underlying detachment surface sloping sea-
hydrology. On El Hierro, a single 6-m-thick dike ward at angle a, shallower than the edifice surface,
impounds at least 70 m of water head (cid:14)Soler, 1997.. b, and with the water table rising from sea level at
Dike swarms and intensely altered volcanic se- an inclination, u. The crest of the rear scarp of the
quences in the walls of old, infilled collapse struc- block is at elevation, h , above sea level, with this
s
tures may combine to produce very high perched used as the principal length scale in normalizing
water tables such as that beneath the Cha das other parameters. The ‘‘daylighting’’ toe of the fail-
Caldeiras in Fogo (cid:14)Bjelm and Svensson, 1982; Mar- ing block emerges at depth, a, below sea level. A
tins, 1988; Descloitres et al., 1995.. A related phe- dike of thickness, w, is intruded along the rear
nomenonoccurs where active rift zones occur on the boundary of a potentially unstable flank block, of
flanks of older, higher structures and partially block width, d, and at intrusion rate, U. Stability is influ-
the downslope flow within the edifice, such as the encedbythedike(cid:14)horizontal.length, d ,andheight,
m
east rift of Kilauea, where water levels are 400 m h ,thatcontactsthemobileblockrearshowninFig.
m
higher than those on the southern side (cid:14)Thomas, 2a and b. This geometric terminology is used
1987; Jackson and Kauahikaua, 1987.. throughout, with thickness reserved to define the
The presence of a largely saturated edifice is a shortest dike dimension, w. Raw parameter magni-
necessary, but insufficient, precondition for the me- tudes used in the analysis follow those of Table 1 in
chanical and aquathermal generation of pore pres- Elsworth and Voight (cid:14)1995..
D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340 327
Fig.2.Intrusiongeometrieswithinavolcanoflank,illustrating(cid:14)a.laterallymobileand(cid:14)b.verticallymobiledikes,injectedatemplacement
velocity,U,andofthickness,w.Effectivedikehorizontallength,d maybelessthanthewidthofthedestabilizedblock,d .Dikedepthis
m D
definedrelativetotheupperscarpsurfaceasafreeboardheight(cid:14)heightfromdikecresttogroundsurface., h ,leavinganeffectivemagma
f
columnacting against the rear boundaryof the block. From the emplacementgeometry, the equivalentforces acting on a potentialslide
block may be determined. These include uplift due to groundwaterpressures, F , magmastaticloading at the rear block scarp, F , and
ps m
destabilizingupliftforcesduetomechanically, F ,andthermally, F ,inducedporefluidpressures.
pm pt
3.1. Mechanical pressurization permeability, k, and hydraulic diffusivity, c, of the
host rock, or through the surrogate nondimensional
Mechanically induced fluid pressures are con- groupings of intrusion velocity, U , and dike thick-
D
trolled by intrusion rate, U, dike thickness, w, fluid ness, w . The non-dimensional fluid pressure gener-
D
328 D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340
ated by mechanical pressurization, Pm, is controlled rameter ranges of A and D are selected from the
D D
as (cid:14)Elsworth and Voight, 1995.: material properties given in Table 1 of Elsworth and
PmsFFwU ,w ,geometryoffailuresurfacex. (cid:14)1. Voight (cid:14)1995. and used in the evaluation of flank
D D D
instability. The one-dimensional geometry of De-
Nondimensional parameters are defined as: laney (cid:14)1982. is used for the specific geometries of
2p(cid:14) pyp . k Uh Fig. 2a and b. Intrusion-induced fluid pressures are
Pms s ; U s s ;
D Uw m D 2c summed over the base of the delineated block. For
the vertically propagating dike, the one-dimensional
m wc
w s (cid:14)2. pressure profile is integrated along the de´collement
D k pgwh2s and applied uniformly parallel to strike. For the
where pyp is the fluid overpressure relative to laterally propagating dike, the pressure profile is
s
ambient pressure, p , and m and g represent the integrated over the full de´collement (cid:14)Fig. 2a..
s w
dynamic viscosity and unit weight of the fluid.
The model for mechanical pressurization directly 3.3. Stability model
followsthatdescribedinElsworthandVoight(cid:14)1995.
except the boundaries of the finite dike are approxi-
Flank stability is determined by isolating a poten-
mated. This is achieved by assuming an infinite dike
tially unstable block as a simple free-body (cid:14)Fig. 2c..
to be intruded where the summation for fluid pres-
Uplift fluid forces are evaluated, and used to resolve
sures is taken only over the effective dike horizontal
thetranslationalequilibriumoftheblockwheremag-
length, d , and height, h (cid:14)Fig. 2.. This approxima-
m m mastatic pressures are applied at the block rear. The
tion is reasonable, given the bounds on error in
factor of safety, F, represents the ratio of forces
estimating mechanical properties.
resisting translational failure to those promoting fail-
ure. Where frictional resistance, alone, is assumed,
3.2. Thermal pressurization
the factor of safety may be divided by the friction
coefficientfortheflankmaterial,tanf,wherevalues
Thermally induced pore fluid pressures (cid:14)Fig. 2.
of Frtanf in the range 1–0.6 represent incipient
are modulated by the differential magma tempera-
failure for rocks of apparent frictional strengths typi-
ture, DT, bulk skeletal modulus, K , and thermal
b cal to flank environments (cid:14)458-f-608..
expansioncoefficient, a, withmigrationratesofthe
t
pressure pulse controlled by thermal, k, and fluid
diffusivities, c. These parameters may be arranged
into two unique nondimensional groupings, A and 4. Stability evaluation
D
D, and a diffusive time, t , to define thermal pres-
D
surization, Pt, following intrusion of a feeder dike.
D Flank stability is evaluated for both vertically and
Correspondingly:
laterally intruded dikes; mechanical and thermal ef-
Pt sFFwA ,D,t ,geometryoffailuresurfacex fects are decoupled. Mechanical effects are typically
D D D
(cid:14)3. short-lived, of the order of days (cid:14)Elsworth and
Voight,1992.,andthermaleffectsaremoreenduring
with individual parameters defined as: and widespread. Since their mutually reinforcing ef-
(cid:14) pyp . AK D (kn fectsarenotevaluated,slopeswillbelessstablethan
Pt s s ; A s b ; Ds ; calculated. A slope inclination of 248 and groundwa-
D K aDT D g h c
b t w s ter table inclination of 128 are used in the analyses.
4kt
t s . (cid:14)4.
D h2s 4.1. Vertical dike propagation
Porosity of the intruded host, n, and time follow-
ing emplacement, t, control behavior through the The effects of pore fluid pressurization are exam-
definition of these nondimensional parameters. Pa- ined for the geometry of Fig. 2b. The dike propa-
D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340 329
gates verticallywith locationdefinedby the depth of below the surface, h , scale directly to kilometers.
f
the dike front below the ground surface, h . Mag- Ranges of dimensionless dike thicknesses, w , rep-
f D
mastatic pressures act over the portion of the dike in resent minimum (cid:14)w -10y1. and maximum (cid:14)w )
D D
contact with the block rear, to a height of h , with 102. reasonable mechanical events for dike thick-
m
the upper limit of magma pressure at the dike crest nesses of the order of 1 m.
defined by the lithostat. Maximum fluid pressures The destabilizing effects are important only for
are truncated at lithostatic. dikes greater than about 1 km in length; below this
threshold, side restraint stabilizes the block. As the
4.1.1. Mechanical pressurization dike migrateshigher in the edifice (cid:14)h “0., stability
f
The destabilizing effect of a vertically upwelling diminishes. For a dike horizontallength of 1 km, the
dike is illustrated in Fig. 3 for the geometry defined dike crest must approach the surface to initiate fail-
previously. For the 1-km-high rear scarp examined ure, with pore pressures providing an important
here, the magnitudes of dimensionless block width, destabilizingeffect.Forlongerdikes,approaching10
d , effective dike height, h , and freeboard height kmin length(cid:14)d ., porefluidpressurizationdoesnot
D m m
(cid:14)height from dike crest to ground surface. of magma appear necessary to initiate failure, although the
Fig.3.Theeffectsofdikehorizontallength, d of(cid:14)a.10,(cid:14)b.1,and(cid:14)c.0.1kmonflankstabilityfora1-km-highflankcrestdissectedbya
m
verticallyupwellingdikeattherear,for10y1-w -102.Mechanicalfluidpressurizationeffectsonly.Dikecrestdepthbelowtheground
D
surface, h ,asdefinedinFig.2(cid:14)c..Alllengthsarenormalizedbyvolcanoheightabovesealevel, h .
f s
330 D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340
presence of this effect may generate instability be- sure effects felt for dikes as deep as 2 or 3 km (cid:14)Fig.
fore magma ascent is complete; magmastatic forces 4a.. Porepressuresdonotincreasesubstantiallywith
are the dominant effect. For the longest intrusions, ascent, and stability remains constant until the intru-
pore pressure magnitudes may reach lithostatic mag- sion approaches the surface, and the destabilizing
nitudes, but with the minimum effect (cid:14)d s10y1. effect of ‘‘magma push’’ is felt. The destabilizing
m
inducing inconsequential fluid pressurization. For effects of pore fluid pressurization and ‘‘magma
reasonable magnitudes of edifice strengths, flanks push’’are of the sameorder, andworkadditively,as
will be stable in the absence of ‘‘magma push’’ and apparent in Fig. 4a.
fluid pressurization, as apparent from Fig. 3c. With dike ascent, the inclination of the critical
Failure geometry also changes with dike length, basalplanerotatesfromnearslope-parallel(cid:14)Fig.4b.,
thickness and location within the edifice. These ef- where neither pore pressure or ‘‘magma push’’ ef-
fectsare illustratedin Fig. 4 for an ascendingdikeat fects are large, to a more shallow inclination, close
different ascent depths, h . The factor of safety to the inclination of the groundwater surface, as the
f
reduces as the dike ascends (cid:14)Fig. 4a.; thicker dikes intrusion approaches the surface. The ‘‘daylighting’’
have the largest destabilizing effect, with pore pres- toe moves up the submarine slope to close to sea
Fig.4.Variationof(cid:14)a.factorofsafety,(cid:14)b.criticalfailureplaneinclination(cid:14)degrees.,(cid:14)c.locationofthe‘‘daylighting’’toewkmx,and(cid:14)d.
criticalblockwidthwithmagmadepthbelowsurface, hrh .Freeboardheightscorrespond, h ,correspondto0through4km.Resultsare
f s f
formechanicalpressurizationeffectsandforthestandardflankgeometrywitha1-kmscarpheightabovesealevel.Selectedhorizontaldike
lengthsare1and10km.Alllengthsarenormalizedbyvolcanoheightabovesealevel, h .
s
D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340 331
level (cid:14)Fig. 4c. as the dike reaches above the ground- mechanical and thermal parameters that yield alter-
water table. Where pore fluid pressurization or nately the minimum and maximum thermal destabi-
‘‘magma push’’ are significant destabilizing agents, lization.
the critical block width is close to the dike length Forthemaximumlengthdike,thebehaviorclosely
(cid:14)Fig. 4d.. For more stable configurations, unaffected follows that of mechanical pressurization with zero
by these agents, the critical failure geometry is the freeboard height, h ; pore fluid pressurization and
f
maximum considered block length, d s100, as ap- ‘‘magma push’’ are both significant. These agents
D
parent in Fig. 4d. areespeciallyimportantatlatertimes,oftheorderof
weekstomonths,butmayalsoinfluencebehavioron
4.1.2. Thermal pressurization shorter time scales. Again, the threshold for destabi-
The destabilizing influence of thermally induced lization is a dike of 1 km length, although even for
pore fluid pressureson the flank geometryof Fig. 2a this critical length, the maximumthermal parameters
are examined for intrusion lengths, d , of 10, 1 and are required; the minimum thermal parameters show
m
0.1 km in Fig. 5. Stability is referenced to time no effect on stability. For the maximum thermal
followinginstantaneousemplacement,withrangesof effect, destabilization increases monotonically with
Fig.5.Theeffectsofdikehorizontallengths,d of10,1,and0.1kmonflankstabilityfora1-km-highflankcrestdissectedbyavertically
m
upwellingdikeattherear.Thermalfluidpressurizationeffectsonly.Dikeisintrudedtogroundsurfaceattimets0,withalltimesinhours
whx,dayswdx,andyearswyx,relativetothisinstantaneousemplacement.Nominaldikelengthsfora1-kmelevationrearscarpare(cid:14)a.10km,
(cid:14)b.1km,and(cid:14)c.0.1km.
332 D.Elsworth,S.J.DayrJournalofVolcanologyandGeothermalResearch94(1999)323–340
time due to the continuous thermal output of the orientations of the de´collement that dip landward (cid:14)a
dike. isnegative.,inFig.7.Formechanicalpressurization,
Fluid pressurization conditioned by the maximum only magma upwelling close to the surface is suffi-
thermal effect is an important destabilizing compo- cient to initiate failure at depth. This identifies the
nent; instability progresses with time (cid:14)Fig. 6a.. For predominant influence of the magma column in in-
the minimum thermal parameters, behavior is con- ducing slip. For the particular dike length chosen in
trolled by the invariant ‘‘magma push’’ geometry, Fig. 7a mechanical pressures are necessary, in addi-
resulting in shallow dipping critical failure surfaces tion to the ‘‘magma push’’, to precipitate slip, at
(cid:14)Fig. 6b. ‘‘daylighting’’ (cid:14)Fig. 6c. at about 750 m depth. Similarly, where thermal pressurization is
below sea level. However, these geometries are not considered(cid:14)Fig.7b.,deepslipisalsopossible.Mini-
close to failure for any reasonable choice of strength mum stability, in these cases, is at the maximum
parameters (cid:14)Fig. 6c.. dike length.
Although updip failure cannot precipitate long-
4.1.3. Deep failure and seismic instability runout slides for the deep geometry, it may clearly
Deepseatedfailuremayalsoresultfrommechani- result in deep seated faulting and earthquakes, as is
calorthermalpressurizationeffects,asillustratedfor for example, characteristic of the Hilina fault system
Fig.6.Variationof(cid:14)a.factorofsafety,(cid:14)b.criticalfailureplaneinclination(cid:14)degrees.,(cid:14)c.locationofthe‘‘daylighting’’toewkmx,and(cid:14)d.
criticalblockwidthwithtimefollowingmagmaemplacement, t. Resultsareforthermalpressurizationeffectsandforthestandardflank
geometry with a 1-km scarp height above sea level. Selected intrusion lengths are 1 and 10 km. All lengths are normalized by volcano
heightabovesealevel, h .
s