Tectonophysics 430 (2007) 83–95 www.elsevier.com/locate/tecto Highland Boundary Fault Zone: Tectonic implications of the Aberfoyle earthquake sequence of 2003 ⁎ L. Ottemöller , C.W. Thomas British Geological Survey, Murchison House, West Mains Road, EH9 3LA, Edinburgh, United Kingdom Received 30 January 2006; received in revised form 6 November 2006; accepted 15 November 2006 Available online 3 January 2007 Abstract The Highland Boundary Fault Zone (HBFZ) is one of the major faulted tectonic boundaries in Great Britain. Historically, seismicity has occurred in this zone around the town of Comrie. But an earthquake sequence that occurred in 2003 near the village of Aberfoyle (ML 1.3–3.2) was the first significant activity to be recorded in the HBFZ since the installation of modern seismograph networks in the 1970s. This study describes detailed analysis of these data. The waveform signals of the events were almost identical and by applying a cross-correlation technique combined with multiple event location, the alignment of the events was found to be WSW–ENE. This alignment matches one of the nodal planes determined by joint focal mechanism analysis. The fault plane dips to the northwest, and shows oblique sinistral strike–slip with normal movement. The orientation of the event alignment matches the direction and orientation of observed features in the HBFZ. Hence, it is concluded that the WSW–ENE striking nodal plane was the causative fault that is associated with the HBFZ. The orientation of maximum compressional stress is rotated from the regional average expected due to the Mid-Atlantic ridge-push force. This rotation is possibly explained by stresses due to postglacial rebound. Smaller events in the sequence were used as empirical Green's functions and deconvolved from the larger events to determine source time functions. The corresponding corner frequencies matched results from spectral fitting, showing that the events were of relatively low stress drop. © 2006 Elsevier B.V. All rights reserved. PACS: 91.30.Bi; 91.30.Dk; 91.30.Vc Keywords: Highland Boundary Fault; Waveform correlation; Empirical Green's function; Multiple event location 1. Introduction boundaries in Scotland, defining the southeastern limit of the Grampian Highlands (Fig. 1). On land, the HBFZ The Aberfoyle earthquake sequence that occurred trace extends from Stonehaven on the east coast to the between June and September 2003 was located about Isle of Arran in the west, and it forms a major basement 3.5 km WSW of the village of Aberfoyle, where the structure northeastwards into the North Sea, where it is larger events were felt (Figs. 1 and 2). The epicentres lie interpreted to have influenced late Palaeozoic and adjacent to the surface trace of the Highland Boundary Mesozoic (24–65 Ma) rifting patterns and to coincide Fault Zone (HBFZ), one of the fundamental structural with changes in thickness of Permian and Triassic strata (Zanella et al., 2003). To the west, it continues southeast of the Kintyre Peninsula (Pharoah et al., 1996) into ⁎ Corresponding author. Tel.: +44 131 6500224. Northern Ireland, where it appears to be represented E-mail address: [email protected] (L. Ottemöller). by the Antrim–Galway Line, a prominent lineament 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.11.002 84 L. Ottemöller, C.W. Thomas / Tectonophysics 430 (2007) 83–95 Fig. 1. Tectonic overview showing the Highland Boundary Fault Zone extending from the Isle of Arran to Stonehaven, the locations of Aberfoyle, Dunoon and Comrie seismic events, and tectonic lineations. Circles show earthquake locations (ML ≥2.5). Historic seismicity (before 1970) is light grey, and instrumental seismicity (since 1970) is dark grey. The inset map shows the location in Great Britain. The box including the Aberfoyle earthquakes indicates the area shown in Fig. 2. defined on geological and geophysical grounds (Ryan In historic times, the area around Comrie has been et al., 1995). The HBFZ separates rocks in the Scottish fairly active with 33 events between 1608 and 1921; the Midland Valley from Neoproterozoie to Cambrian rocks largest event was an ML4.8 event in 1839. The mag- of the Dalradian Supergroup underlying the Highlands nitudes of historic events are determined from macro- (Fig. 2). Included within the fault zone are slivers of seismic observations and are calibrated against the Cambro–Ordovician rocks of oceanic origin assigned to instrumental local magnitude scale (Musson, 1996). the Highland Border Complex. It is a complex structure Since the installation of seismic stations in the 1970s, with a long history. Its significance as a major tectonic few earthquakes have been observed in the HBFZ. Of boundary is unquestioned, although its origins and sig- note is an earthquake that occurred on 16 September 1985 nificance as an early structure remain unclear (Dentith near Dunoon in the HBFZ at a depth of about 6.5 km with et al., 1992). Other major faults converge in the ML3.3. Redmayne and Musson (1987) suggested that left- Aberfoyle area. The principal ones include the Loch lateral strike–slip movement with a small normal Tay Fault Zone and the related Duke's Pass and Loch component had occurred on a fault striking WSW–ENE Ard faults (Fig. 2). These faults are essentially strike– and dipping south. A composite focal mechanism for an slip structures that were active in mid-Palaeozoic times earthquake swarm in the Kintail area of NW Scotland in (Treagus, 1991) and upon which there has been strike– 1974 similarly showed left-lateral movement on the SW– slip of the order of kilometres and dip slips of the order NE striking Strathconon fault (Assumpção, 1981). The of hundreds of metres. It is likely that the Loch Tay to Arran earthquake of 1999 also showed the same style of Loch Ard fault system represents a strike–slip transfer faulting (Bott et al., 1999). Earthquake swarms are system that accommodated SSW to SW directed relatively common in Great Britain, examples include sinistral movement within the Grampian Highlands. Comrie (1788–1801, 1839–46), Glenalmond (1970–72), L. Ottemöller, C.W. Thomas / Tectonophysics 430 (2007) 83–95 85 Fig. 2. Simplified and schematic geological map of the Aberfoyle district, showing the principal fault systems. Fault planes within the HBFZ are not seen in outcrop. Evidence from the zone elsewhere along its length indicates that it is a steeply NW-dipping structure. Other faults are near-vertical where seen in surface outcrop. From geological surveys in the district and from other regional data, the Loch Tay – Duke's Pass – Loch Ard fault system is known to have a significant strike–slip displacement of the order of kilometres, in common with other similarly orientated faults in the Scottish Grampian Highlands (Treagus, 1991). Doune (1997), Blackford (1997–98, 2000–01), Constan- techniques, a joint focal mechanism and spectral source tine (1981, 1986, 1992–4), John Stonbridge (mid 1980s), parameters. Based on these results we give a tectonic Dumfries (1991, 1999), Manchester (2002) and Eskdale- interpretation. muir (2003). The regional stress pattern in Great Britain shows 2. Tectonics of the Highland Boundary Fault Zone predominantly horizontal compression in the NNW– SSE direction (Gölke and Coblentz, 1996; Main et al., The origin and earliest history of the HBFZ are 1999; Baptie, 2002) due to ridge-push forces from the obscure, but available geological evidence indicates Mid-Atlantic ridge. Stresses arising from postglacial that the Midland Valley and Highlands terrains were rebound, with present maximum uplift rates of 2 mm/ juxtaposed across the HBFZ by late Silurian times year in the Scottish Highlands (Shennan, 1989) are also (443–417 Ma) (Haughton et al., 1990)withina considered to cause earthquakes in Scotland (Main sinistral–transpressional, strike–slip regime. Subse- et al., 1999; Stewart et al., 2000; Firth and Stewart, quently, periodic movements from the late Silurian to 2000; Muir-Wood, 2000). It is disputed whether the Early Carboniferous (354–290 Ma) focussed on the ridge-push or glacial rebound is the dominant force HBFZ and related fracture systems considerably causing earthquakes in NW-Europe (Stein et al., 1989; influenced the tectonic development of, and sedimen- Gregersen, 1992; Stewart et al., 2000; Fejerskov and tation within, the Midland Valley. Although the Lindholm, 2000; Fjeldskaar et al., 2000; Hicks et al., geometry and history of fault movement within the 2000b). HBFZ are complex, in general the gross sense of Following a review of the tectonic history of the movement was reverse across apparently high angle, HBFZ, we describe our analysis of the seismic record- north-west dipping fault planes. Surface exposures ings of the 2003 earthquake sequence. We determine along the length of the fault zone indicate steeply precise relative locations using multiple event location dipping faults, but the nature of the HBFZ at depth is 86 L. Ottemöller, C.W. Thomas / Tectonophysics 430 (2007) 83–95 unclear. Gravity anomaly data from the vicinity of the (Paterson et al., 1990). However, most faults and frac- HBFZ were interpreted by Dentith et al. (1992) to show tures affecting Lower Carboniferous and older strata in that the fundamental bounding structure may be a low- this area have the NE and NW trends that are developed angle, northerly-dipping thrust fault, supporting the widely in the Midland Valley. Such structures are com- suggestion by Bluck (1984) that the present-day line of mon in the vicinity of the HBF, particularly in the the fault is due to later thrusting in the Late Devonian Aberfoyle district; these fractures are probably of (417–354 Ma) and Early Carboniferous. However, Variscan age or younger. Most are major joints with strike–slip movement is recorded locally by horizontal little displacement across them, but some are faults with slickensides (Anderson, 1951).