Seismotectonics of the central part of the South Iceland Seismic Zone

ELSEVIER Tectonophysics 298 (1998) 319–335

Seismotectonics of the central part of the South Iceland Seismic Zone

Francoise Bergerat a,Ł, Agust Gudmundsson b, Jacques Angelier a, S.Th. Rognvaldsson c

a Departement de Geotectonique, ESA 7072, CNRS, Universite Pierre et Marie Curie, Bte 129, 4, place Jussieu,75252 Paris cedex 05, France

b Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norwayc Icelandic Meteorological Office, Bustadavegur 9, IS-150 Reykjavik, Iceland

Received 6 April 1998; accepted 13 July 1998

Abstract

The largest earthquakes in Iceland are associated with strike-slip faults in the South Iceland Seismic Zone (SISZ).Major destructive earthquake sequences occur in this zone at an average interval of 80 years, the last two sequences beingin 1784 and 1896. The last major single earthquake in the zone was an earthquake of magnitude 7 (M) in 1912. Theseismicity is mostly associated with a conjugate system of NNE-trending (mainly) right-lateral faults and ENE-trending(mainly) left-lateral faults, but in addition there are many NW-trending faults in the SISZ. Particular attention is paid tothe comparison between present-day fault activity (earthquake mechanisms) and Quaternary faulting (fault-slip data). Thiscomparison reveals a general similarity despite the temporal difference between these subsets independently collected.The fault populations are well exposed in the hyaloclastite mountain Vordufell, near the central part of the South IcelandSeismic Zone, where considerable fault slip occurred during the earthquake sequences of 1784 and 1896. The cumulativedisplacement on most of these faults is 1–6 m. In addition to the large-scale faults, we measured 79 minor faults(displacements of the order of centimetres) in the Vordufell Mountain. Most of the minor faults are left-lateral. Analysisof these minor faults recording palaeostresses in rocks ranging in age from 3.1 to 0.7 Ma, as well as 50 focal mechanismsfrom a swarm of present-day microearthquakes occurring beneath the Vordufell Mountain, gave consistent results in termsof seismotectonic stress (orientations of stress axes and ratios of principal stress differences). This shows that the tectonicregime remained unchanged through recent times. The results indicate, however, that there are two contrasting stressregimes in this part of the SISZ. The primary subset indicates NW–SE tension and NE–SW compression, whereas thesecondary subset indicates NW–SE compression and NE–SW tension. We propose that the primary subset is consistentwith the dominating time-averaged regional stress field in the SISZ. This dominating stress field can be largely explainedas a consequence of simple plate pull parallel with the spreading vector in South Iceland. The secondary subset and theassociated stress field, however, may be partly related to stress release and rebound and partly to dike injections in thenearby segments of the rift zone in South Iceland. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Iceland; seismotectonics; stress regimes; faults; earthquakes; transform zone

Ł Corresponding author. Fax: C33 (1) 4427 5085; E-mail: [email protected]

0040-1951/98/$ – see front matter c 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 1 9 1 - 7

320 F. Bergerat et al. / Tectonophysics 298 (1998) 319–335

1. Introduction

Most destructive earthquakes in Iceland duringthe past 1100 years since it was settled occurredin the southern lowlands, in an area referred to asthe South Iceland Seismic Zone, or SISZ (Fig. 1).Major destructive earthquake sequences occur in thisarea at intervals of 45–112 years; the last majorsequence was in 1912. The largest events in a par-ticular sequence may exceed magnitude 7 (M). Overperiods of years or decades, most of the seismicactivity is restricted to a 15–20-km-wide (N–S) and70-km-long zone centred at 64ºN (Fig. 2). Distri-bution of the main historical earthquake destructionareas, as well as recent tectonic studies in South Ice-land (Gudmundsson, 1995a) suggest, however, thatthe Holocene seismic zone is as wide as 60 km.

At the regional scale, the main fault trends areclose to N–S and NE–SW (Fig. 2). The latter trend,however, mainly corresponds to the normal faultsrelated to the major rift segments (Fig. 1). De-spite its general E–W trend, the SISZ is mainlyaffected by N–S faults (Fig. 2). The central part ofthe SISZ is partly covered by Holocene lava flows,partly by Pleistocene rocks. In the Holocene lavaflows, NNE-trending (mainly) right-lateral faults aswell as ENE-trending (mainly) left-lateral faults arepresent. These appear to be conjugate systems. Inaddition to the NNE and ENE faults, there are someNW-trending faults in the Holocene lava flows. The

Fig. 1. Location of the South Iceland Seismic Zone (SISZ). Alsoindicated are the main Holocene volcanic systems (black) ofthe rift zone, the Tjornes Fracture Zone (TFZ) transform fault,partly exposed on land in North Iceland, the ocean ridges northand south of Iceland, and the direction and magnitude of thespreading vector. The frame in South Iceland indicates locationof Fig. 2.

same strike-slip fault populations are observed in thePleistocene rocks. NNE-trending right-lateral faults,ENE-trending left-lateral faults as well as some NW-trending faults. Although NNE, ENE and even NWtrends are visible in aerial photographs (e.g., Fig. 3),the NNE-trending faults are the only ones mapped atthe regional scale in the area of the central SISZ. Allthese fault populations are well exposed in the hyalo-clastite mountain Vordufell, located near the centralpart of the South Iceland Seismic Zone (Fig. 2).Large slip occurred on some of the faults in theVordufell during the earthquake sequences of 1784and 1896, and the faults are still active (Thoroddsen,1899). During these sequences, the largest earth-quakes reached at least M D 7:1 and may havereached M D 7:5.

The South Iceland Lowland (SIL) project, initi-ated in 1988, focuses on earthquake prediction in theSISZ (Stefansson et al., 1993). A Global Position-ing System (GPS) network was established in SouthIceland in 1986 and was remeasured in 1989 and1992 (Sigmundsson et al., 1995). Previous tectonicstudies in the South Iceland Seismic Zone (SISZ)included mapping of the 1912 earthquake fracture,located in the Holocene lava flows of the eastern partof the SISZ (Bjarnason et al., 1993) and mappingand measurements of some major (Gudmundsson,1995a) and minor (Passerini et al., 1997) faults in thezone.

The principal aim of this paper is to present a de-tailed seismotectonic study of the Vordufell Moun-tain and its surroundings (Fig. 2). This area wasselected for this study because (1) it is located inthe central part of the SISZ, (2) it has excellent out-crops that allow a detailed study of the active majorand minor faults at the surface, and (3) among theearthquakes recorded through the SIL network (Ste-fansson et al., 1993) with determined earthquakesmechanisms (Rognvaldsson and Slunga, 1993, 1994;Slunga et al., 1995), 68 shallow earthquakes from1991 to 1995 are available. We have calculated thestress tensors obtained from the fault-slip field data(Bergerat et al., 1997) and those determined from thefocal mechanisms (Angelier et al., 1996).

Our analysis includes comparisons first betweensmall-scale and large-scale faults, and second be-tween faults and earthquakes. The latter comparisonhas the potential to reveal contrasts in faulting modes

F. Bergerat et al. / Tectonophysics 298 (1998) 319–335 321

Fig. 2. Seismicity of the South Iceland Seismic Zone and location of the Vordufell Mountain where the main study was made. Alsoindicated are the outlines of the main Holocene volcanic systems (thin, subparallel lines), the associated central volcanoes (thick, roughlyelliptical areas). The dots indicate selected and accurately located earthquakes exceeding magnitude 0.5 during the year 1995. The shortlines within the seismic zone indicate some of the NNE-trending, seismogenic dextral strike-slip faults.

(e.g., because non-seismic slip also occurs on geo-logical faults), and to highlight a temporal evolutionof faulting (because geological measurements applyto faults that moved during the Quaternary, and evenpossibly during the latest Pliocene). Small-scale, orminor, faults refer to fault lengths and offsets gener-ally smaller than about 100 m and 1 m, respectively.Large-scale, or major, faults refer to longer faultswith large offsets, that can be geologically mappedat usual scales (e.g., at the scale of aerial pho-tographs, 1 : 50,000 in the Vordufell). For both thefaults and the earthquakes, attention is paid not onlyto the geometry of brittle features, but also to theirmechanical consistency. For this reason, the studyneeds to be made not only in terms of displacementsand strain, which have little potential to reveal sucha consistency, but also in terms of stresses. Note, in-cidentally, that regardless of the scale considered theamount of slip could not be measured in the field fora majority of striated faults. This difficulty precludedour attempts at reliably quantifying strain. In con-trast, it does not prevent palaeostress analysis from

being valid, provided that the usual requirementsare met (e.g., Angelier, 1994). With this basis, wecompare the tectonic stress field controlling large-scale faulting and crustal deformation in the SISZin general (Fig. 2) with that of the Vordufell area(Fig. 3).

2. Tectonic framework

The SISZ is partly located in Holocene lava flows,partly in Pleistocene rocks. The Holocene lava flowsare mainly basaltic Aa flows; Pahoehoe flows, most ofwhich are associated with shield volcanoes, are alsocommon. The main lava flow in the area is the 8000-year-old Thjorsarhraun lava flow. This lava flow cov-ers roughly 930 km2 and its volume may be as greatas 21 km3. The longest lava stream associated withthis flow is 120–140 km long; it follows the channelof the river Thjorsa (latitude 64ºN) for a considerabledistance. The earthquake fractures of the SISZ aremost easily recognized in this lava flow, particularly


Fig. 3. Aerial photograph (A) and structural pattern (B) of the Vordufell Mountain. Main faults and dikes as thick lines, lava flow andhyaloclastite slopes as dotted lines.

in the part that follows closely latitude 64ºN. North ofthis latitude, there are Pleistocene rocks where frac-tures are less easily recognized, and south of it theHolocene lava flows are either covered with thick soilor subject to wind erosion so that the seismotectonicfault traces disappear rather quickly. This is one rea-son why the N–S width of the SISZ is commonlyunderestimated and thought to be limited to the frac-ture parts that dissect the Thjorsarhraun lava flow.

The Pleistocene rocks are mostly hyaloclastites(moberg, basaltic breccias), sediments and tillites.Most of the hyaloclastites are basaltic brecciasformed in subaquatic or subglacial eruptions dur-ing periods of glaciation. Some, however, may haveformed in submarine eruptions during the waningstages of the ice periods. In contrast, one of theprincipal Pleistocene mountains in the area is largelyan erosional rather than constructional structure: this

mountain is the Vordufell (Fig. 3), which is mostlymade of hyaloclastites topped with lava flows.

Most faults in Iceland are either normal faultsor strike-slip faults. The normal faults are mostlyassociated with the divergent plate boundary, as rep-resented by the rift zone, whereas strike-slip faultsare common in off-rift areas. Normal faults are muchmore common than strike-slip faults, but most largeearthquakes (maximum M 7.1–7.5) are associatedwith strike-slip faults. Not surprisingly, geologicalfield studies of faults as well as reviews of earth-quake focal mechanisms reveal a larger proportion ofstrike-slip faulting in the two major transform-typefault zones (the left-lateral SISZ and the right-lat-eral Tjornes fracture zone, see Fig. 1) than in mostother areas of Iceland. Most of the major (Figs. 3and 4A) and minor (Fig. 4B) faults that we ob-served in the Pleistocene formations of the Vordufell

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Fig. 4. Fractures and faults in the Vordufell area. (A) Aerial view of some major faults in the northern half of the Vordufell; faults striking N020ºE, N060ºE and N160ºE,respectively, are visible. (B) Minor faults in a hyaloclastite quarry at the northern edge of the Vordufell Mountain: set of NE–SW- to ENE–WSW-trending strike-slip faultscharacterizing a NNE–SSW compressive stress. (C) At the Seydisholar crater cone: normal fault plane striking N165ºE in Holocene scoriae.


Mountain are strike-slip in type. However, normalfaults oblique-slip or dip-slip faults are also presentin the central SISZ (e.g., Fig. 4C, showing a normalfault affecting the Holocene scoriae of a crater conenamed Seydisholar, 64º040N 20º5003000W).

The normal and strike-slip faults generally de-velop from joints in the lava pile, in particular fromcolumnar (prismatic) joints in the basaltic lava flows.During their development, tension fractures com-monly form arrays that eventually evolve into nor-mal faults or strike-slip faults. The development ofthese arrays into normal faults has been described byGudmundsson (1992); here we shall briefly describetheir evolution into strike-slip faults, as are observedin the SISZ.

The overall evolution of tension-fracture arraysinto large-scale strike-slip faults can be observed inthe Holocene lava flows of the SISZ. The N- andNNE-trending arrays of tension fractures, observed,for example, at the surface of the 8000-year-oldThjorsarhraun lava flow, are related to hidden NNE-trending right-lateral strike-slip faults, active sincethe Pleistocene, buried by the Holocene lava flows.Commonly, the fractures are arranged en echelonand there are 0.5–2 m high hills between the nearbyends of the en-echelon tension fractures of each ar-ray. These hills are either related to the shear stressesthat develop between the nearby ends of en-echelonfractures when loaded in tension perpendicular to thearray trend, or are pressure ridges related to areasof transpression generated by right-lateral slip on theburied strike-slip faults. The individual tension frac-tures of the NNE-trending arrays generally strike ina northeast direction.

In addition to these right-lateral strike-slip faultsand associated arrays of tension fractures, there areleft-lateral strike-slip faults and associated arraysin the Holocene lava flows. For example, threeof the northernmost segments of the 1912 earth-quake fractures at the farm Selsund (63º5503000N19º570W) are left-lateral arrays of tension fractures;two of these trend N51ºE and N69ºE. The mainactive fault there is right-lateral and trends approx-imately NNE–SSW. Because these NNE-trending(right-lateral) and ENE–WSW (left-lateral) featuresall indicate strike-slip and moved at the same time,they should be regarded as forming a conjugate pat-tern of faults. The ENE–WSW trend of left-lateral

segments is common in the Holocene lava flows ofthe SISZ.

The attitudes of the strike-slip faults of the SISZare most easily studied in the Pleistocene areas, suchas in the Vordufell Mountain (Fig. 3). In the Pleis-tocene areas, the strike-slip faults are steeply dippingand occur in two main populations, one in whichmost faults strike N0º–30ºE (such as for the longestfractures in Figs. 3 and 4A), the other in which mostfaults strike N60º–70ºE (such as for the shorter butmost numerous fractures in these figures). Accord-ingly, the analysis of fracture patterns in outcrops, asdiscussed below, revealed major and minor right-lat-eral faults with N to NNE trends, and large quantitiesof minor left-lateral faults with NE to ENE trends(e.g., Fig. 4B). As in the Holocene lava flows, mostNNE-trending faults are right-lateral, whereas mostENE-trending faults are left-lateral.

Estimating the amounts of displacement alongstrike-slip faults cutting through a nearly horizontallava flow pile is often difficult or impossible. How-ever, in the Vordufell Mountain, many of the strike-slip faults dissect subvertical dikes, so that the cu-mulative displacement on them could be determinedwith great accuracy. On most of the faults the hori-zontal displacement is in the range of 1–6 m. In thePleistocene rocks of the central part of the SISZ ingeneral, some strike-slip faults have cumulative dis-placements exceeding 15 m. In this part of the SISZ,the arithmetic average displacement on right-lateraland left-lateral faults is the same: 4–5 m.

3. Palaeostress regimes and fracture patterns

The fracture patterns and palaeostress regimeswere determined using (1) fault-slip data from minorfaults, mostly with displacements of the order ofcentimetres, (2) large-scale faults, mostly with dis-placements of the order of metres, (3) basaltic dikesand (4) mineral veins.

3.1. Fault-slip analysis in Vordufell

The determination of palaeostress tensors wasmade using fault-slip data collected in two selectedsites in the Vordufell Mountain (Fig. 3): one in a re-cently opened quarry (Fig. 4B) on the road between


the farms Helgastadir and Ida at the northern edge ofthe mountain (64º0504000N 20º310W), the other in theUlfsgil canyon at the southern edge of the mountain(64º030N 20º3203000W). Both sites are in basalticrocks of age 0.7–3.1 Ma. The fault-slip inversiontechnique used to compute the reduced stress tensoris based on (1) minimizing the angle between thetheoretical shear vector and the actual slip vector

Fig. 5. Comparison between the main structural directions of micro- (A, B) and macro-tectonic data (C, D). (A) Rose diagram ofright-lateral (left part) and left-lateral (right part) strike-slip faults. (B) Rose diagram of tension gashes (left part) and normal faults (rightpart). (C) Rose diagram of the major structures recognized on the aerial photographs (weighted according to length). (D) Rose diagramof dikes (left part) and major faults (right part) measured in the field. Class intervals 5º.

(stria), and (2) simultaneously making relative shear-stress magnitudes as large as possible. This methodis referred to as INVDIR(Angelier, 1990).

These two sites provided heterogeneous data setsrelated to extensional as well as strike-slip regimes.Of a total of 79 fault planes collected in the these twosites, 76% indicate primarily strike-slip (Fig. 5A) and24% primarily normal-slip (Fig. 5B). It is important


to point out that at each site the whole set couldnot be accounted for by a single tectonic stressregime. Although the analysis cannot be describedin detail, simple consideration of the large azimuthaldispersion of strike-slip faults, for both the right-and left-lateral types (Fig. 5A), allows one to detectmechanical inconsistency.

Attempts at determining a single stress tensorat each site were unsuccessful because they revealedunacceptably large misfits. The simplest tensor deter-minations that could be acceptable in terms of aver-age and individual misfits involved calculation of atleast two opposite stress regimes, each including nor-mal and strike-slip faulting. Relative chronologicalcriteria that allow distinction of stress regimes werepresent but scarce, so that separation within fault-slip data had to be made on the basis of mechanicalconsistency. Furthermore, relative chronology datawere contradictory, suggesting that contrasting stressregimes were alternating rather than following a sim-ple definite succession.

The results are summarized in Table 1 and Fig. 6.The palaeostress states were identified based on nu-merous attempts with differently composed data sub-sets. Although in some cases the number of fault slipdata is too small to allow good tensor determination(the smallest diagrams in Fig. 6), the separationsmust however be regarded significant because (1) thesenses of slip could be unambiguously determined inthe field, and (2) no combination of data could pro-vide better results in terms of individual and averagemisfits.

Table 1Palaeostress tensor computations based on fault slip data analysis

Site S F N ¦ 1 ¦ 2 ¦ 3 ý Þ RUP

Road 31 p SS 41 16; 1 112; 79 285; 11 0.4 12 33s SS 8 304; 20 114; 69 213; 4 0.7 17 41p N 9 287; 81 53; 5 143; 7 0.5 16 33s N 8 123; 79 343; 8 252; 7 0.3 6 24

Ulfsgil canyon p SS 9 199; 9 16; 81 109; 0 0.5 7 36Selfoss p N 17 293; 72 62; 11 155; 13 0.4 10 25

s SS 8 331; 1 246; 42 69; 48 0.1 9 11Seydisholar p N 30 355; 71 190; 19 98; 5 0.3 5 7

Columns, from left to right: name of measurement site, type of fault slip data subset (S), primary (p) and secondary (s), number of faultslip data (N), trend and plunge of the principal stress axes ¦ 1, ¦ 2 and ¦ 3 (in degrees), ratio ý D .¦2 � ¦3/=.¦1 � ¦3/, average anglebetween computed shear stress and observed slickenside lineation (Þ, in degrees), ratio ‘upsilon’ (RUP, ranging from 0% to 200%) of theINVDIR method (cf. Angelier, 1990). Average RUP values below 50% correspond to good fits between actual fault slip data distributionand computed shear distribution.

The analysis of the strike-slip faults in thelargest mechanically consistent subset of data in-dicates palaeostress tensors with subhorizontal ¦1

and ¦3 axes trending N15º–20ºE and N105º–110ºE,respectively (Table 1 and Fig. 6). In this pri-mary subset, 82% of the faults are left-lateral andtrend N35º–90ºE, whereas 18% are right-lateral andtrend N165º–190ºE. Simple comparison with the az-imuthal distribution (Fig. 5B) shows that a majorityof strike-slip faults is consistent with this tectonicregime. Considering these trends and the senses ofstrike-slip displacements, as well as the spatial asso-ciation between these faults, one concludes that theyform conjugate systems.

Tensor determination made with the remaining,mechanically consistent subset of strike-slip data,revealed nearly horizontal ¦1 and ¦3 axes that trendESE–WNW and NNE–SSW, respectively (Table 1and Fig. 6). We note that the direction of ¦1 forthe strike-slip faults of the primary subset describedabove coincides with the direction of ¦3 in thissecondary subset.

Normal faults occur only at the north site. Despitea limited number of data, their analysis revealed twopalaeostress tensors with nearly horizontal ¦3 axesthat trend N143ºE and N72ºE (Table 1, Fig. 6). Themain trend of normal faults is N30º–65ºE.

Palaeostress analyses thus revealed significantmechanical incompatibility between two subsets,for each of the strike-slip and normal types. At-tempts were also made at determining the degreeof mechanical compatibility between these faulting


Fig. 6. Separation of fault slip data subsets within the heterogeneous data sets from two sites of the Vordufell Mountain, anddetermination of corresponding average paleostress tensors. Stereoplots are Schmidt’s projection, lower hemisphere. White circles on thephotograph of the Vordufell Mountain indicate location of the two measurement sites, 1: Road 31 and 2: Ulfsgil canyon. At the left-top(Road 31): upper row of diagrams corresponds to the primary subset of strike-slip (on the left) and normal (on the right) faults, lowerrow corresponds to the secondary subset. At the right-top (Ulfsgil canyon): primary (on the right) and secondary (on the left) subsets ofstrike-slip faults. Fault planes shown as thin lines, slickenside lineations as small dots with single or double thin arrows (mostly normalor strike-slip, respectively). Three-, four-, and five-branched stars indicate the axes of ¦3, ¦2 and ¦1, respectively. Large arrows showdirections of compression and extension. Numerical results in Table 1. White lines on the picture underline some major faults.


types. Results with acceptable misfits were obtained,suggesting that separation between strike-slip andnormal faulting modes may be mechanically unnec-essary. Two subsets were thus obtained, each subsetcontaining strike-slip and normal faults. Within theentire data set of the Vordufell Mountain, 77% offault slips form a ‘primary subset’, consistent witha roughly ESE–WNW-trending ¦3 axis (uppermostrow of stereoplots in Fig. 6), whereas 23% form a‘secondary subset’, consistent with a roughly NE–SW-trending ¦3 axis (second row of stereoplots inFig. 6). Strike-slip faulting prevails in both thesesubsets. Furthermore, 84% of the strike-slip faults inthe primary subset are left-lateral.

The above results from the Vordufell indicate thatthe dominating stress field in the central part of theSouth Iceland Seismic Zone favours strike-slip fault-ing and has the ¦3 axis horizontal and trending ap-proximately WNW–ESE to NW–SE; the ¦Hmax axistrends approximately NNE–SSW to NE–SW. Thisconclusion is strongly supported by the NNE–SSW-to NE–SW-trending arrays of mineral veins (most ofwhich are pure tension fractures) and by the generalNE trend of the large-scale normal faults in South Ice-land. We emphasize, however, that in addition to thisdominating stress field, there is a contrasting, but sec-ondary, stress field characterized by approximatelyESE–WNW compression and NNE–SSW tension.

Fig. 7. Determination of average palaeostress tensors in Upper Pleistocene (Ingolfsjall, Selfoss) and post-glacial (Seydisholar) formations.P D primary fault slip data subset; S D secondary subset. Other explanations as for Fig. 5.

3.2. Fault-slip analysis in the surrounding area

For comparison, we collected fault-slip data atseveral sites in the vicinity of the Vordufell Moun-tain. These sites are in Late Pleistocene to Holocenerocks, so that the observed brittle deformation ofthese rocks has a much narrower time span thanthat of the rocks in the Vordufell Mountain. Herewe present two examples of typical sites. Onesite is in a quarry in basaltic rocks at the south-ern edge of the Ingolfsfjall Mountain near Self-oss (63º580N 21º0402000W, 25 km southwest of theVordufell). The age of the rocks in this quarry isLate Pleistocene (less than 0.7 Ma). Two fault sub-sets were recognized. The largest subset includesN25º–70ºE-trending normal faults and indicates asubhorizontal ¦3 axis trending N155ºE. A smallersubset is less constrained, and includes only N0º–20ºE-trending left-lateral strike-slip faults (Fig. 7,Ingolfsfjall), with a trend of extension being approx-imately ENE–WSW (Table 1).

The second site is in a quarry located 15 km westof the Vordufell in a Holocene scoria crater cone(Fig. 4C). It shows normal faults with oblique-slipstriae (Fig. 7, Seydisholar) indicating a palaeostresstensor with horizontal ¦3 axis trending N98ºE (Ta-ble 1). These results show that the stress field inthe Late Pleistocene and Holocene rocks is the same


as that observed in the Early Pleistocene VordufellMountain.

3.3. Fracture patterns

We conducted a detailed study of the dikes andlarge-scale faults in the Vordufell Mountain (Fig. 3).The data set includes 48 basaltic dikes and 21 nor-mal, reverse and strike-slip faults (Fig. 5D). Mostdikes were presumably emplaced in the western seg-ment of the rift zone in South Iceland (Fig. 1) andsubsequently drifted out of it because of plate move-ments. The dikes are thus generally of an age similarto that of the Vordufell Mountain. Many of theNE-trending normal faults are also generated withinthe western rift-zone segment, but some of thesehave been reactivated in subsequent sequences in theSISZ. Also, all the faults with a major strike-slipcomponent must clearly be generated, or reactivated,outside the rift-zone segment and are thus youngerthan, and cut through, the dikes of the VordufellMountain.

The dikes are generally thin (most are less than0.5 m thick) and show a relatively wide spreadingin strike and dip (Fig. 5D). As regards thicknessand, in particular, attitude, the dikes of the Vor-dufell Mountain form a swarm that is more similarto a typical sheet swarm rather than a typical re-gional dike swarm. The swarms of inclined sheetsare generated inside central volcanoes and their atti-tudes and thicknesses are controlled by a local stressfield associated with the shallow source chamber ofthat volcano, whereas the regional dikes are mostlysubvertical and subparallel, much thicker than thesheets, and their emplacement is controlled by theregional stress field of the rift zone (Gudmundsson,1995b).

The explanation for this attitude of the dikesin Vordufell is thus that they are controlled by alocal stress field, partly generated by the mass ofpillow lava at the bottom of the mountain. Thispillow-lava mass has very different elastic propertiesfrom those of the surrounding hyaloclastites and thusacts as a dome-shaped ‘inclusion’ which modifiesthe regional stress field of the rift zone into a localstress field similar to that surrounding the upper halfof a spherical magma chamber. Thus, while there isa dominating NE trend of the dikes (Fig. 5D), in a

direction parallel with the rift-zone segment in SouthIceland, the overall attitude of the dikes reflects alocal stress field associated with the formation of theVordufell Mountain itself.

Although the large-scale faults measured in theVordufell have a general NE trend, three subgroupscan be distinguished (Fig. 5D). There is a group withan ENE trend, peaking at N60º–70ºE, a group witha NE trend, peaking at N30º–40ºE, and a group,including only a couple of faults, with a NNE trend.In addition to these faults, there are NW-trendinglineaments (Fig. 3), but these were not recognizedas large-scale faults in the field. All the faults dipsteeply, between 60º and 90º (Fig. 5D). The mea-sured displacement of the faults ranges from 1 m to17 m, but all except one have displacements between1 and 7 m (Fig. 5D). The measured displacementon most of the faults is dip slip. Many, however,show indications of a strike-slip component, but ex-cept where the faults dissect subvertical dikes thereis lack of suitable markers to measure the strike-slipdisplacement.

There are two faults, however, that are clearlyright-lateral strike-slip, and two that are clearly left-lateral strike-slip. The strike and displacements ofthe right-lateral faults are N53ºE (1.5 m) and N35ºE(5.2 m), whereas that of the left-lateral faults areN63ºE (2.5 m) and N69ºE (5.5 m). These data, al-though few, indicate that the left-lateral faults aremore easterly trending than the right-lateral faults, inagreement with data obtained from the Holocene partof the SISZ (Gudmundsson, 1995a). Comparing thetrends of the major and minor strike-slip faults, theyare quite similar for the left-lateral faults, whereasminor dextral faults are not numerous enough toallow reliable comparison of trends (Fig. 5A).

3.4. Chronology

The dikes are generally the oldest tectonic el-ements in this central part of the SISZ. Some ofthe NE-trending normal faults are very likely of thesame age, and generated inside the western rift-zonesegment, as the dikes. Chronological relationshipsamong the large-scale and minor faults are, however,rarely found in the field. Only seven minor faultplanes in the sites at Vordufell showed two genera-tions of striations or cross-cutting relationships. Of


these planes, six belong to the primary subset andindicate that the first motion was extension or dip(normal) slip, whereas the second motion was strikeslip. The succession of normal slip followed by strikeslip has also been observed in the eastern part of theSISZ (at the mountain Burfell) by Passerini et al.(1990). Nevertheless, data from the SISZ as a whole(Passerini et al., 1997) do not permit definite con-clusions to be drawn as regards the chronology ofdip-slip and strike-slip faulting, and it is likely thatthe associated stress regimes alternated through time.

Similarly, we could not establish a chronologicalrelationship between the primary and secondary sub-sets of minor faults. Our field studies, as well as theearthquake focal mechanisms discussed below, indi-cate that the faults of the secondary subset are notonly less abundant but also more scattered in strikethan those of the primary subset. Consequently, thesecondary subset, and the associated stress regime,is, although important, subordinate to the primarysubset and its associated stress regime.

4. Stress regimes indicated by earthquakemechanisms

In order to see how the palaeostress regime deter-mined above fitted with the current stress field in thearea, a data set of 68 earthquakes occurring in thearea of the Vordufell Mountain (64º030N to 64º070N

Fig. 8. Application of the right dihedra method to the 1991–1995 earthquakes of the Vordufell area (from Angelier et al., 1996). (A)primary subset, (B) secondary subset. Graphic expression of the results: the pattern with horizontal hatches corresponds to compatibilitydomain for extension, whereas the pattern with vertical hatches corresponds to compatibility domain for compression; the hatchedpatterns are denser in high compatibility domains. Black arrows indicate directions of compression and extension. Barycentres of thecompatibility domains are considered as average axes (triangle for extension, pentagon for compression, square for intermediate axis).

and 20º300W to 20º360W) from 1991 to 1995was used. These earthquakes were recorded by theSIL network (Stefansson et al., 1993). 18 data result-ing from quarry blasts were rejected and the remain-ing 50 focal mechanisms of natural shallow earth-quakes were analyzed. For these 50 earthquakes,magnitudes are smaller than 3 and 28 magnitudesfall in the range 0–1. In this paper, we simply presentthe results of this analysis in terms of stress regimes(Figs. 8 and 9). Although the 50 earthquake mecha-nisms are not plotted in the conventional way in thispaper, the upper couple of stereoplots in Fig. 9 showsfor each double-couple mechanism the orientation ofthe nodal plane retained, as well as the correspond-ing slip vector and sense of motion (so that both theother nodal plane, perpendicular to the slip vector,and the nature of the dihedra, pressure or tension,are easily inferred). Detailed information concerningthe earthquake location, the quality of double-couplesolutions and the choices among nodal planes hasalready been given (see tables in Angelier et al.,1996).

The variety of these focal mechanisms shows thatthe whole set of earthquake focal mechanisms can-not correspond to a single tectonic stress regime.Within the range of acceptable misfits, two mainsubsets of data and related stress regimes account forthe whole set of mechanisms. The P- and T-dihedramethod (Angelier and Mechler, 1977) was used firstin order to constrain the main directions of compres-


Fig. 9. Application of the inverse methods INVDIR (analytical direct inversion method) and R4DT (4-D search method) to the fault planesolutions of the 1991–1995 earthquakes of the Vordufell area (from Angelier et al., 1996, modified). (A) primary subset, (B) secondarysubset. (a) Fault planes, (b) INVDIR method, (c) R4DT method. Legend of diagrams as for Fig. 5. Numerical results in Table 2.

sion and tension (Fig. 8), because it does not requireany choice among the nodal planes for each datum.The main subset was found to be consistent withNE–SW-trending compression and NW–SE tension,whereas the smaller subset indicated NW–SE com-pression and NE–SW tension (Table 2).

In terms of stress consistency, the use of theP- and T-dihedra method is appropriate for setsof double-couple earthquake mechanisms becausethe choices among nodal planes are by definitionunnecessary. However, by using this method there issome loss of information in that one deals only withthe orientations of the stress axes and there is noassumption made that the ratio of stress differences,ý D .¦2�¦3/=.¦1�¦3/, is unique (Angelier, 1994).

For this reason, numerical inverse methods werealso used in order to compute the average stresstensor which best fits the fault plane solutions. Themethods adopted were (1) the analytical direct inver-sion method used for fault slip data, INVDIR (An-gelier, 1990), and (2) the 4-D search method, R4DT(Angelier, 1984). The latter method involves simpleminimization of the angle between the computedshear and actual slip vectors by numerical means.

Either method, however, requires a choice betweenthe two possible nodal planes of each mechanism.

Three criteria were used in order to choose be-

Table 2Results of stress determinations from focal mechanisms ofearthquakes

Method S N ¦ 1 ¦ 2 ¦ 3 ý Þ RUP

P–T–D p 38 237; 30 52; 60 146; 2INVDIR p 38 242; 17 35; 71 150; 8 0.4 10 33R4DT p 38 239; 14 27; 73 147; 8 0.4 10 32P–T–D s 12 140; 2 45; 67 231; 23INVDIR s 12 117; 27 331; 58 215; 15 0.7 16 43R4DT s 12 133; 6 27; 69 225; 20 0.7 15 41

Columns contain from left to right: method used, right-dihedra(P–T–D), INVDIR and R4DT (Angelier and Mechler, 1977; An-gelier, 1984, 1990), type of the fault plane solution subsets (S),primary (p) or secondary (s), number of fault plane solution (N),trend and plunge of principal stress axes ¦ 1, ¦ 2 and ¦ 3 (in de-grees), ratioý D .¦2�¦3/=.¦1�¦3/, average angle between com-puted shear stress and observed slickenside lineation (Þ, in de-grees), ratio ‘upsilon’ (RUP, ranges from 0% to 200%) of the IN-VDIR method (cf. Angelier, 1990). Note that the axes given fromP–T–D method are not principal stress tensor axes but describeonly barycentres of the compatibility domains for ¦ 1 and ¦ 3.


tween the nodal planes (Angelier et al., 1996): (1) thebest shear-slip fit within the frame of average stresstensor determinations, (2) the comparison betweenthe orientations of the nodal planes and those of thefaults, fractures and other weakness planes identifiedin the field and from aerial photographs, and (3)the comparison with fault mechanisms observed inoutcrops. Note that the first criterion implies inter-action between the choice of nodal planes and thestress tensor inversion, whereas the second and thirdcriteria imply consideration of geologically possibleorientations of weakness planes and of fault slip.

Combining these three criteria resulted in the fi-nal selection of the nodal planes. The results interms of stress regimes are listed in Table 2. Theyare clearly significant for the main subset. For thesmaller subset, the misfits are large despite its smallsize; this indicates inhomogeneity. Contrary to theprimary subset, this secondary subset is mainly com-posed of nearly horizontal and nearly vertical nodalplanes, which suggests that horizontal contacts andvertical columnar joints in the basaltic rock werereactivated. For several planes and slip vectors theorientations are identical in both the subsets, thesole difference being the opposite sense of motion(Fig. 9). As a consequence, it is likely that fault orfracture surfaces normally active under the primarystress regime have undergone reactivation with slipreversal under the secondary stress regime.

The primary subset includes 30 strike-slip, 4 re-verse and 4 normal focal mechanisms. It is consistentwith NE–SW compression and NW–SE tension, inagreement with left-lateral slip on E–W-trendingfaults and right-lateral slip on N–S-trending faults(Fig. 9A). The secondary subset includes 8 strike-slip, 1 reverse and 3 normal focal mechanisms, in-dicating NW–SE compression and NE–SW tension(Fig. 9B). As for the fault-slip data analysis, the maindifference between the subsets results from a kindof permutation of the stress axes. Other differencesbetween subsets, especially in terms of location (in-cluding depth) and magnitudes, are few and theirsignificance is limited because of the small size ofthe Vordufell data set (50 natural earthquakes). Al-though this set is large enough to allow reliable deter-mination of tectonic regimes, it is not large enoughand the magnitudes of earthquakes are too small toallow obtention of significant results in terms of scal-

ing comparison with the fault data. The two oppositetectonic regimes, dominantly strike-slip in type, in-volve nearly perpendicular extensions trending NW–SE (primary) and NE–SW (secondary). This contrastbetween two subsets is the most striking property ofthe Vordufell earthquake mechanisms (Angelier etal., 1996). The average directions of the maximumand minimum stress axes are N60ºE and N150ºE,respectively, for the primary subset, and N120ºE andN 40ºE for the secondary subset (Table 2).

5. Discussion and conclusion

The structural pattern of the Vordufell Mountain,as regards the main tectonic features (major normal

Fig. 10. Schematic, kinematic behaviour of the SISZ. Majorand minor faults in the SISZ are mostly N–S right-lateral, NE–SW normal and ENE–WSW left-lateral. These fault populationsbelong to the primary subset and are in kinematic agreement witha general E–W left-lateral behaviour of the SISZ. However, thesecondary subset of faults and focal mechanisms of earthquakesshow that the controlling stress field of the SISZ is more complexthan this kinematic representation indicates.


Fig. 11. The two stress regimes of the SISZ. One is associated with the main subset of faults, the other with the secondary subset offaults. Local stress permutations of extreme axes play a major role in the SISZ, resulting in conflicting mechanisms for the present-dayshallow earthquakes as well as for the recent fault movements.

and strike-slip faults, dikes), the microtectonic data(primary subset), and the focal mechanisms of earth-quakes (primary subset), is of a Riedel type (Riedel,1929). These geometrical relationships are summa-rized in Fig. 10. Therefore this mountain is mainlycharacterized by extension features (normal faults)trending roughly NE–SW and bisecting an importantconjugate strike-slip fault pattern. Such an arrange-ment, as recognized in the Vordufell, is typical ofmajor E–W left-lateral shear zones (Fig. 10). Signif-icantly, at the regional scale, the main behaviour ofthe SISZ corresponds kinematically to a shear zoneof this type.

One of the basic conclusions of this study is thatthe development of seismogenic faults in the centralpart of the South Iceland Seismic Zone is controlledby two contrasting stress regimes (Fig. 11). In oneregime the general trend of the maximum compres-

sive stress is NE–SW, in the other regime it is NW–SE (Angelier et al., 1996). The results suggest fur-thermore that the current stress regimes, as indicatedby the present-day focal mechanisms, are identicalwith the palaeostress regimes, as indicated by ouranalysis of large-scale faults and minor faults. Thus,these stress regimes have been responsible for theformation of most seismogenic faults in this area andare likely to control the future development of thesefaults and the associated seismic hazard. Any viabletectonic model of the SISZ must therefore accountfor both these stress regimes.

Our data indicate that the stress regime withthe maximum horizontal compressive stress trendingNE–SW represents the dominating time-averaged re-gional stress field in the central part of the SISZ. Thisconclusion is in agreement with the results of a moregeneral study on the current seismicity in the SISZ


as a whole. From a study of 605 earthquakes of mag-nitude 0–2 during the period July 1991 to February1992, Stefansson et al. (1993) concluded that themean trend of the maximum horizontal compressivestress was roughly N50ºE. The data of Stefansson etal. (1993) indicated primarily strike-slip faulting inthe central part of the SISZ but more normal faultingnear the rift-zone segments at the west and east endof the SISZ. These results are in agreement with theconclusion that the stress field in the Vordufell areafavours mainly strike-slip faulting.

In addition to being present in our data, the pri-mary stress regime is recorded in recent studies ofborehole breakouts in the central part of the SISZ.The studied borehole is located at the farm Nefsholt,15 km south of the Vordufell Mountain.This bore-hole was drilled in 1977 and is 1100 m deep. Thebreakouts occurred at depths of 780–870 m, 955–985 m and 1060 m. These breakouts indicate thatthe maximum horizontal compressive stress in thehost rock of the borehole at these depths is aroundN30º–36ºE (F. Roth, pers. commun.), modifying apreliminary interpretation (Roth, 1997). This trendcoincides roughly with the trend of the maximumcompressive stress inferred for the primary stressregime in our study, suggesting that this drillhole islocated in an area where the primary stress regime iscurrently dominating.

The general state of stress in the vicinity of diver-gent plate boundaries such as the rift-zone segmentsin South Iceland is largely the consequence of twocontrasting processes: plate pull and plate push. Platepull is the process resulting in crustal dilation or ex-tension in a direction parallel with the spreadingvector of the associated plates. A simple plate-pullmodel can largely account for the dominating re-gional stress field in the SISZ (Gudmundsson andBrynjolfsson, 1993; Gudmundsson, 1995a). How-ever, such a model cannot account for the othermain, but secondary, stress regime in the SISZ.

There is no clear chronological relationship be-tween the two main stress regimes in the Vordufellarea and probably not at other locations in the SISZ.On the contrary, it is likely that these two regimesinterchange at irregular intervals in different parts ofthe SISZ, as has been found elsewhere in Iceland(Passerini et al., 1997). This means that while thedominating stress regime controls seismogenic fault-

ing in some (main) parts of the SISZ, the secondarystress regime controls faulting in other parts. Duringthe general development of the SISZ, however, allparts of it become subject to both regimes but at dif-ferent times. It is thus not surprising that our tectonicanalysis in the 3.1–0.7 Ma old rocks of the VordufellMountain revealed the presence of the dominating aswell as the secondary stress regimes.

Although the existence of the secondary stressregime is clear, its origin is less so. In general,the stress field in the central part of the SISZ ischaracterized by stress permutations between ¦1 and¦2 (change from normal to strike-slip regime) andbetween ¦1 and ¦3 within the same regime (the di-rections of tension and compression, respectively,being perpendicular). The first type of permutation.¦1=¦2/ is commonly observed in Iceland (Berg-erat et al., 1988, 1990; Passerini et al., 1990, 1991,1997; Gudmundsson et al., 1992). The second typeof permutation pattern .¦1=¦3/ reflects an inhomo-geneous tectonic stress field. We propose that thesecondary stress regime may be partly due to stressrelease and rebound (Angelier et al., 1996) and partlyto dike injection in the adjacent rift-zone segmentsin South Iceland (Gudmundsson and Brynjolfsson,1993; Gudmundsson, 1995b).

Acknowledgements

This work was supported by a grant from theEuropean Commission (contract ENV4-CT96-0252)and by the French–Icelandic scientific and culturalcollaboration programme (Iceland Ministry of Ed-ucation and Culture and the French Ministere desAffaires Etrangeres). The authors thank the IcelandicGeodetic Survey for permission to publish Fig. 3Aand Fig. 4A, as well as Dr. J.-P.Burg, Jolly and A.Braathen for helpful and constructive comments.

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Seismotectonics of the central part of the South Iceland Seismic Zone

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