Dr. Benedikt Halldorsson Seismicity of Iceland Dr. Benedikt Halldorsson

Earthquake Occurrence in Iceland Overview Tectonics Interplate Earthquakes Transform Zones South Iceland Seismic Zone Tjörnes Fracture Zone (North Iceland Seismic Zone) Volcanic Zones Reykjanes Peninsula Hengill Triple-junction Eastern Volcanic Zone Intraplate Earthquakes Induced earthquakes Earthquake hazard map

Iceland and Plate tectonics Horizontal motion of plates Plate tectonics is a relatively new scientific concept, introduced some 40 years ago, but it has revolutionized our understanding of the dynamic planet upon which we live. The theory has unified the study of the Earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology (the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries -- such as why earthquakes and volcanic eruptions occur in very specific areas around the world, With respect to Iceland, it lies on the Mid-Atlantic Ridge, the divergent plate boundary between the North American and Eurasian Plates. The present earth is composed of three types of tectonics; 1) CORE TECTONICS driven by energy release in the inner core, 2) PLUME TECTONICS in the mantle characterized by large-scale vertical plume movements, and PLATE TECTONICS in the upper mantle where horizontal movement of rigid plates dominates. Steve Gao

Iceland and Plume tectonics Large scale vertical mantle plume motions Hot-spots are one manifestation of plume tectonics USGS With the aid of new advances in various fields of earth science, mineral physics, simulation of mantle convection, and global seismic tomography, what is called plume tectonics has emerged. Iceland is greatly affected by this type of tectonics as well.

Tectonics of Iceland: Combination of plate and plume tectonics Surface expression of the hot-spots is manifested as an area of upwelling and hightened volcanic activity. In that sense, Iceland owes its existence to the Icelandic mantle plume. The centre of the Icelandic Mantle Plume is below the northwestern part of Vatnajokull Icecap. It is modelled as a 200-300 km wide cylindrical zone of highly viscous semi-solid material rising extremely slowly from depths of 400-700 km. The “Icelandic Mantle Plume” (Dr Dietmar Müller, University of Sydney) Lundin & Dore (2004)

Tectonics of Iceland: Combination of plate and plume tectonics Mid-Atlantic Ridge of tectonic extension between the North American and Eurasian Plates. RR=Reykjanes Ridge KR=Kolbeinsey Ridge In Iceland the interplay between the tectonic extension and mantle plume define the geodynamics Including volcanic and earthquake occurrence (Maclennam, 2001; Kaban et al., 2002) Iceland is located in the North Atlantic Ocean between Greenland and Norway. It is a landmass that is a part of a much larger entity situated at the junction of two large submarine physiographic structures, the Mid Atlantic Ridge and the Greenland-Iceland-Faroes Ridge. Iceland is a part of the oceanic crust forming the floor of the Atlantic ocean. This region is known as the Iceland Basalt Plateau, which rises more than 3 km above the surrounding the sea floor and covers about 350.000 km2. 30% of this plateau is above sea level, or approximately 100.000 km2. This is known as Iceland. Iceland is geologically very young and all of its rocks were formed within the past 25 My. The oldest exposed rocks are about 14-16 My old. The slide shows the bathymetry of the area around Iceland. The position of the Iceland plume relative to Greenland and Iceland at ages of 40, 30, 20, 10 and 0 Ma is indicated by the yellow circles. The plate boundary slowly drifts westwards with respect to the relatively stationary hot spot, resulting in successive eastwards jumps of the spreading zones. It is postulated that a new rift zone starts propagating from the center of the hotspot when the plate boundary has migrated some critical distance off it. Continuous (red) and dashed (red) lines show the active and inactive spreading axes. The active rift zones in Iceland are shown as individual fissure swarms in red colour. RR: Reykjanes Ridge; KR: Kolbeinsey Ridge; AER: Aegir Ridge; IP: Iceland Plateau; GF: Greenland, Faeroer Ridge..

Topography of Iceland About half of Iceland's land area, which is of recent volcanic origin, consists of a mountainous lava desert (highest elevation 2,119 m (6,952 ft) above sea level) and other wasteland shown in the brown and yellow brown colors.. The lowlands are shown in green, but that is just the color on the map – it does not necessarily indicate green vegetation. Eleven percent is covered by three large glaciers shown in grayish white. Twenty percent of the land is used for grazing, and only 1% is cultivated. It is one of the most sparsely populated countries in the world with 320000 population currently. Björnsson & Palsson (2008)

Volcanic Systems and Earthquake Epicentres in Iceland Red dots = earthquake epicentres 1994-2005 Volcanic systems Black circles = central volcanoes Yellow regions = fissure swarms White regions = glaciers Most of the seismicity of Iceland is in some way related to the mid-Atlantic plate boundary that crosses the country. The relatively simple tectonic picture of a mid-oceanic plate boundary with spreading centers and transform faults, however, does not readily apply to Icelandic tectonics. In Iceland, the plate boundary is superimposed on a large hot spot with a presumed deep root in the mantle resulting in a partial melting layer that underlies most of the island. The excessive hot spot volcanism is the driving force of the volcanic activity in Iceland and is responsible for building up the Icelandic lava pile and therewith, for Iceland’s existence. At least one ridge jump has occurred in North Iceland, and in South Iceland a second ridge jump seems to be presently in progress. The Western Volcanic Zone (35-10%) is being replaced by the Eastern Volcanic Zone (65-90%), which seems to be propagating southwestwards, away from the center of the hot spot in east Central Iceland. In response to this unstable situation, complex fracture zones have developed in the north (the Tjörnes Fracture Zone, TFZ) and south (the South Iceland Seismic Zone, SISZ) which connect the eastward displaced rift zones in Iceland to the spreading centers of the Mid-Atlantic Ridge to the north and south of the island. Differential movements are taken up by these fracture zones. Because they are young and the strain field that causes them is unstable, complex series of faults are formed that are not oriented parallel to the spreading direction The Icelandic mantle plume supplies magma to Iceland’s volcanic Systems. Volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of the plates being pulled apart which is associated with the mid-ocean ridges, and the magma dynamics of the Iceland Mantle Plume. Most systems are 40–150 km long, 5–20 km wide, and develop a central volcano. They supply magma to all eruptions in Iceland. Eruptions on the fissure swarms are fed directly from the mantle while the central volcanoes have a shallower source of magma, characterized by a magma chamber. A geothermal field, silicic volcanism and a caldera structure are frequently associated with the central volcano. Iceland is one of the most active volcanic regions in the world, with eruptions occurring on average roughly every three years (in the 20th century there were 39 volcanic eruptions on and around Iceland). About a third of the basaltic lavas erupted in recorded history have been produced by Icelandic eruptions. Notable eruptions have included that of Eldgjá, a fissure of Katla, in 934 (the world's largest basaltic eruption ever witnessed), Laki in 1783 (the world's second largest), and several eruptions beneath ice caps, which have generated devastating glacial bursts, most recently in 2010 after the eruption of Eyjafjallajökull. Guðmundsson (2001)

Present-Day Geodynamics of Iceland Average horizontal velocities from GPS measurements Green=NS-component Red = West component Blue= East component Defines the Present-Day Rift Axis of the Mid-Atlantic Ridge in Iceland As the seismicity is associated with the present day rift axis, its determination is important. Average horizontal velocities derived from GPS measurements 1993 and 2004. Shown are north-south components (green) and east-west components as well as the resulting velocity vectors (blue or red). This clearly depicts the characteristic features of the accepted view on tectonism in Iceland, indicating the borderline between the North American and Eurasian Plates in Iceland. This borderline is marked by the Northern Volcanic Zone, with the spreading axis extending northward from Vatnajokull Glacier, and by the Eastern Volcanic Zone South of Vatnajokull Glacier. Then the borderline crosses the South Iceland Lowland and passes along the Reykjanes Peninsula. For the first time the strain rate tensor is calculated for the entire country. Previously it had only been done for very limited areas. Northward overall motion of about 2.5 cm/year. Western part is moving west, eastern part to the east. Based on data from the National Land Survey of Iceland [Sigurdsson, 2006]. Sigbjörnsson et al., 2006

Main tectonic structures and earthquake epicentres Data obtained in the last decades have greatly improved our knowledge of their volcanotectonic environment; as a result, the geometry of the plume is better constrained, and the crust, previously considered thin (;10 km), is now modeled as thick (;20– 40 km). Additionally, the seismicity is now better constrained. On the basis of the present day rift axis, the location of the volcanic systems in Iceland and locations and focal mechanisms of earthquake epicentres we can classify roughly two types of seismotectonic regions in Iceland: The extensional rift zones, and the transverse fracture zones. The volcanic zones coincide largely with the exensional rift zones where both volcanic eruptions and earthquakes occur. The transverse fracture zones coincide with areas where no volcanic eruptions occur but strong-earthquakes take place. The location of the volcanic zones indicates a complex history of volcanic and tectonic activity in Iceland. Depending on the location of the volcanic systems, their activity either decreases or increases faulting in the two main seismic zones. Through historical times since the settlement of the country in the ninth century, seismic activity has been observed and reported in all seismic zones and moderate to strong earthquakes have occurred. Very strong earthquakes of the order encountered at the Pacific plate boundaries, however, are unlikely to take place. Possibly the strongest earthquake in historic times is the 1784 earthquake in the SISZ, which caused severe damage to farmhouses in the area and three people were killed. Based on the extension of the damage zone with comparison with instrumental magnitudes of more recent events, its local surface wave magnitude has been assessed to be 7.2. Through ancient annals and chronicles, the earthquake history of Iceland is reasonably well established. From the eighteenth century and onwards the records are quite reliable with good description of damage zones and destruction of farmhouses. The first instrumental records were obtained during the 1896 earthquakes in the South Iceland Lowlands (SISZ). The most recent activity was manifested by the Year-2000 earthquakes in the SISZ, and the earthquakes in 2008 in the SISZ. Additionally, the largest earthquake sequence over the last 30 years in the Tjornes Fracture Zone is currently taking place in the North, with the largest earthquake reaching local magnitude 5.6 but all of them occurring offshore.

Simplified Geological Map of Iceland This is a simplified geological map of Iceland showing historical and Holocene lava ows, glaciers, and the main chronologically-defined units. The Historical lavas are shown in red, the prehistoric in pink. During the eleven centuries of settlement in Iceland earthquake and volcanic activity has repeatedly affected the population, directly and indirectly, and sometimes with extreme severity. Eruptions and events directly related to volcanic and geothermal activity commonly occur and their consequences range from direct impact of incandescent tephra or lava to jökulhlaups and contamination of air, water and crops. Guðmundsson et al (2008)

Population Density in Iceland For the most part Iceland is sparsely populated with no permanent settlements in the interior highlands. Population clusters mainly occur along the coast, with about 70% of the 320 thousand inhabitants living in the greater Reykjavík area and along the southern shore of Faxaói Bay in southwest Iceland This map shows the population density of Iceland by the black circles. Guðmundsson et al (2008)

The Icelandic Strong-motion Network It is in the context of the previously mentioned characteristics of the respective locations of the seismic zones, the respective earthquake hazard, and the elements that create risk, i.e. the location of the population and its infrastructure, along with special important structures that we shall discuss the seismicity in a more detailed manner. The Icelandic strong motion network provides the framework in which we discuss the seismicity of Iceland. The ICESMN purpose is to monitor earthquake strong-motions in the seismic zones and in population centres affected by earthquake ground motions, and the measurements are used for managing and reducing seismic risk. The Icelandic Strong-motion Network is owned and operated by the Earthquake Engineering Research Center of the University of Iceland. It has been in operation and constant expansion over the last three decades. During this time it has recorded all significant earthquakes in Iceland. Currently it consists of 22 instruments in houses and public buildings 3 instrumentation systems in dams; 5 instrumentation systems in hydroelectric power plants 2 instrumentation systems in office buildings 2 instrumentation systems in seismically isolated bridges TWO dense small-aperture urban strong-motion free-field and structural arrays The ICEARRAY I – free field array (11 instruments) The ICEARRAY II – free field and structural array (8 instruments) The spacing between stations in the SISZ is 5-10 km typically and it is the densest earthquake monitoring network in the country. Its density is due to the fact that the seismic zone is completely onshore

The South Iceland Seismic Zone Strong-motion stations in South Iceland Monitor ground movements and building response in the region The South Iceland Seismic Zone (SISZ) Most destructive earthquakes in Iceland have occurred within the South Iceland Lowlands, which currently is one of the most populated regions in Iceland. The ICESMN stations in the region are classified into The earthquake zone extends about 70 km in the E-W direction with almost perfect alignment of earthquakes in a 5-10 km broad band from the Hengill and Ölfus region in the West, closest to Reykjavík, towards Rangárvellir in the east. No major E-W striking faults can be found and the destruction zones of individual earthquakes tend to be elongated in a N-S direction. Major earthquake sequences have affected the sparsely populated farmlands through historical times with recurrence intervals between 45 to 112 years.

Seismicity in the South Iceland Seismic Zone This slide shows the locations of microearthquakes in the SISZ over the last two decades as black dots. The North-south lines denote the approximate locations of historical strong earthquakes over the last 300 years, with those associated with the earthquake sequence in 2008 as red dots. Plate divergence in the southern part of Iceland is accommodated by two subparaliel rift zones, the Western and the Eastern Volcanic Zones. The gap between them is bridged in the south by a transform zone, the South Iceland Seismic Zone, which takes up the tr~sfo~ motion between the Reykjanes Ridge and the Eastern Volcanic Zone The South Iceland Seismic Zone is completely onshore and in which strong-earthquakes occur on parallel N–S trending, near vertical, dextral strike-slip faults. This mechanism is sometimes referred to as bookshelf faulting. The overall leftlateral transform motion along the zone thus appears to be accommodated by right-lateral faulting on many parallel, transverse faults and counterclockwise rotation of the blocks between them. On the basis of the historical catalogue, strong earthquakes in the Zone can be grouped according to their origin and type into three different categories. The relatively strong trigger earthquakes of the major sequences are placed in the first category. They occur in the eastern part of the zone, which seems to be the starting point of these sequences. The trigger earthquakes, which are the largest magnitude earthquakes to occur, are followed by a sequence of smaller earthquakes, which take place all over the Zone in a matter of days, weeks or months. Some sequences have lasted up to two even several years. The major sequences in this category occurred in 1294, 1339, 1389-91, 1630-33, 1732-34, 1784, 1896, and then the 2000 earthquakes with its two strong earthquakes four days apart, along with the 2008 earthquake. The large interval including the fifteenth century may be due to poor historical records from that time of pest epidemic, diseases and chaos in the Country, the socalled historical gap, rather than scarcity of earthquakes. The second category classifies single earthquakes, which have occurred without any noteworthy aftershocks in the eastern part of the Zone. The third category classifies similar single events in the western part, which are generally smaller. [Einarsson, et al.] Stefansson et al. (2006)

The Tjörnes Fracture Zone (North Iceland Seismic Zone) Strong-motion stations in North Iceland The Tjörnes Fracture Zone is largely offshore The North Iceland Seismic Zone (NISZ) The Northern transform zone is usually called the Tj¨ornes Fracture Zone (TFZ) and is of a completely different geometry.

Seismicity in the Tjörnes Fracture Zone Recent strong historical earthquakes (Green stars) Microearthquake epicentres 1994-2012 (Black dots) Volcanic systems Central volcanoes (Black circles) Fissure swarms (red shaded regions) Rift axis (red dashed lines) and direction of rifting 7.0 This slide shows a map of the North Iceland Seismic Zone, the Tjornes Fracture Zone, in which black dots denote microearthquakes from 1994-2012, and green stars denote strong, historical earthquakes. The red dashed lines denote schematically the rift axis and the arrows the direction of plate spreading. The North Iceland Seismic Zone is a broad region of faulting and seismic activity, which connects the submarine Kolbeinsey Ridge and the Northern volcanic zone in North Iceland. Earthquake epicentres are scattered throughout the region, which is about 80 km wide from north to south and 150 km long. The seismicity is too diffuse to be associated with one fault or a simple plate boundary. Instead, the transform motion appears to be taken up by a series of parallel NW-striking faults or seismic zones. The seismic character of the zone is complex and cannot be associated with a single fault or plate boundary. Studies of recent earthquakes show that a considerable part of the seismicity is associated with possibly three parallel WNW trending lineaments, which originate in the volcanic belt. (1) The first lineament is located northeast of the HFF, the Gr´ımsey Oblique Rift (GOR), that both connect the NVZ to the Kolbeinsey Ridge (KR). The Kopasker earthquake occurred in 1976 The Grimsey seismic lineament runs slightly north of Grimsey and joins the Krafla fissure swarm in Axarfjörður. It has no clear trace in the topography. Bathymetry data shows four NS-striking volcanic systems exist along the GOR, with left-stepping, en-echelon, fissure swarms which have been subjected to right-lateral strike-slip motion. Strong- to large earthquakes on the GOR, however, seem to be associated with both left lateral strike-slip faulting along NNW and NNE-striking faults whereas normal events generally have a NNE strike, perpendicular to the spreading direction (2) The second zone is about 40 km south of the first one, and is well defined by the seismicity near its western end. It is the the 100-km-long HFF and which runs NWN from Husavik across Flatey and the mouth of Eyjafjör›ur The HFF fault system is more akin to oceanic transform faults. It has an overall strike of N65W and can be traced offshore across the TFZ as a segmented WNW trending transform fault. The 1872 magnitude 6.5 earthquake caused widespread damage in Husavik, Flatey and Flateyjardalur. (3) The third speculative seismic lineament also runs NWN from Krafla across Eyjafjör›ur near Dalvik and to the mouth of Skagafjör›ur. Finally, a magnitude 7 earthquake occurred in the mouth of Skagafjör›ur on the third speculative lineament in 1963. As is the case with most historic earthquakes in the NISZ, the epicentre was off the coast in the ocean and the land intensity therefore low. Recent studies have shown that at present the the HFF accommodates 1/3 of the total plate motion which means that the GOR must account for 2/3. A notable change in seismicity was observed in the region following the rifting episode. The rifting episode clearly triggered the Kópasker earthquake (magnitude 6.4) on the GOR while apparently putting most of the HFF into a stress shadow. As a result, the southeastern part of the HFF fault lacks microseismicity, particularly since the Krafla rifting-episode 1975–1984. The rifting episode likely released accumulated stress on the southeastern part of the HFF Depending on the location of the volcanic systems, their activity either decreases or increases faulting in the two main seismic zones. From this, we can infer that emplacement of the feeder-dike to the largest historical eruption in Iceland (that of Laki in 1783) increased shear stress in the South Iceland Seismic Zone and almost certainly triggered the largest (M;7.1 in 1784) historical earthquake in Iceland. The Krafla rifting episode (1975 - 1984) was a major stress-changing event in northern Iceland. It consisted of a series of about 20 dike intrusions, originating from the Krafla central volcano in the Northern Volcanic Zone, causing a cumulative spreading of about 8 - 9 meters at maximum and 3.5 meters in average along a 70-80 km part of the Krafla rift segment. 6.2 7.0 6.5 6.5 7.0 6.3 6.5 6.2 IMO (2012)

Ongoing earthquake sequence since 2012 in the Tjörnes Fracture Zone Activity on the two major structures of the TFZ, and the extension ridge Largest events so far are M5.5 and M5.6 Measured by the new strong-motion array in Husavik, North Iceland IMO (2013)

Seismicity in the Reykjanes Peninsula and the Hengill Triple Junction Narrow seismic zone with shallow focus earthquakes Normal faulting Hengill Triple junction between the Reykjanes Volcanic Zone, The Western Volcanic Zone, and the South Iceland Seismic Zone. Seismicity of the Hengill volcanic system, normal faulting and seismicity associated with geothermal activity South Iceland Seismic Zone Reykjanes Peninsula The Reykjanes Peninsula is a zone of high seismicity and recent volcanism that forms a transition between the Reykjanes Ridge to the west and the Western Volcanic Zone and the South Iceland Seismic Zone to the east. The plate boundary can be defined as a narrow seismic zone that enters Iceland near the tip of Reykjanes (Fig. 3) and runs along the peninsula. Detailed studies by Klein et al. (1973, 1977) show that the seismic zone is less than 2 km wide in most places. The earthquakes mostly occur at a depth of l-5 km and are not located on any one particular fault. The seismicity seems to be caused by deformation of a brittle crust above a deeper seated. aseismic deformation zone. Small-scale structures and seismic lineations can be resolved within the zone, striking obliquely or even transversely to the main zone. Normal faulting is the most common faulting mechanism. Further to the east in this region strike-slip faulting becomes more prominent. In the Hengill volcano in Southwest Iceland, many events involve a non-double-couple mechanism. The seismicity is interpreted as the result of extensional failure and heat extraction from a cooling magma chamber.

Volcanic Earthquakes in Iceland Rifting structures mostly aseismic except during eruptions Spatial clustering of epicenters – central volcanoes Primary classes of seismicity of the volcanic zones Rifting earthquakes Inflation earthquakes Deflation earthquakes Intrusion tremors Eruption tremors Relatively smaller magnitudes than in the transform zones The seismicity of the volcanic zones is characterized by spatial clustering of epicenters. Most clusters coincide with central volcanoes. Rifting structures such as fissure swarms and normal faults are mostly aseismic except during episodes of rifting and magmatism.

Intraplate Earthquakes in Iceland Very rare events Primarily two cases Earthquakes in west Iceland Borgarfjordur events of 1974 Largest magnitude was 6 No apparent single fault Normal faulting Earthquakes on the insular shelf off Eastern Iceland Most located near shelf-edge

Induced Seismicity due to Fluid Injection Hellisheidi Geothermal Power Plant, Southwest Iceland Epicenters of induced earthquakes due to fluid injection at Hellisheiði Power Plant (45 days) Station locations of the ICEARRAY I in Hveragerði, 11 km away from the Power Plant The induced seismicity culminated in two ML 3.8 earthquakes on 15 October 2011 Worldwide, induced earthquakes associated with geothermal projects have raised public concern on several occasions even though the seismicity has rarely had adverse physical effects Public concern about the number and magnitude of induced earthquakes in some cases delayed and/or threatened cancellation of geothermal projects European Case Study Out of 41 case studies of induced seismicity in Enhanced Geothermal Fields, earthquakes were not felt or not reported in 50% of cases Largest earthquakes were ML 3.5 Icelandic Cases until now Induced earthquakes have been very small (< ML 2.0) in the past Two ML 3.8 induced earthquakes occurred on 15 October 2011 as a result of fluid injection at the Hellisheiði Power Plant. Halldorsson et al. (2012)

Earthquake Hazard in Iceland 10% probability in 50 years of exceeding the specified Peak Ground Acceleration The seismicity of Iceland has been classified and parametrizised and the historical catalogue extended by the generation of a synthetic catalogue of future earthquake events in order to prevent historical bias and improve statistical robustness. Based on Olafsson and Sigbjörnsson´s new attenuation model for seismic waves, the peak accelerations for the whole country have been calculated for a selected subset of the extended catalogue based on Brune’s source spectrum. Thus a fairly stable earthquake zoning map for the whole country has been produced with 475-year peak accelerations. The maps form the basis of earthquake resistant design in Iceland in accordance with Eurocode 8. We are here! (Solnes, Sigbjörnsson & Elíasson, 2004)

Summary Iceland is one of the most active countries in the world in terms of seismicity and volcanism Its seismicity is caused by a complex interaction between the tectonics and volcanism of Iceland The two having different geodynamics and manifestation The largest earthquakes in Iceland occur in the two transform zones in the south and north, respectively Style of faulting depends on the development of the fault lineament The SISZ exhibits “bookshelf tectonics” The SISZ constitutes a unique natural field laboratory for the study of earthquakes, their strong-motions and their effects on the built environment The mid-Atlantic plate boundary in Iceland is expressed by a series of seismic and volcanic zones. The structure of the plate boundary is strongly influenced by the Iceland hot spot. The relative motion of the Mid-Atlantic Ridge with respect to the hot spot leads to ridge jumps, propagating rifts and other complexities. Most large earthquakes in Iceland occur within two transform zones that connect the presently active Northern and Eastern Volcanic Zones to the ridges offshore. In the south the South Iceland Seismic Zone is marked by a lo-15 km wide, E-trending epicentral belt. The large earthquakes occur by faulting on N-S striking right-lateral faults. The left-lateral transform motion along the zone thus appears to be taken up by slip on numerous parallel faults by counterclockwise rotation of the blocks between them (bookshelf tectonics).