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Icelandic Strong Icelandic Strong-Motion Array (ICEARRAY

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Icelandic Strong Icelandic Strong-Motion Array (ICEARRAY Icelandic StrongStrong--motionmotion Array (ICEARRAY) Benedikt Halldórsson and Ragnar Sigbjörnsson Earthquake Engineering Research Centre, University of Iceland, IS-800 Selfoss, Iceland Abstract The ICEARRAY, designed in order to maximise its versatility and usefulness for seismological and earthquake engineering applications, consists of 14 stations in an area of 1.23 km 2 inside the town of Hveragerdi. The The tectonically unique and populated South Iceland Seismic Zone (SISZ) has been the location of numerous array has an aperture of 1.9 km and a minimum interstation distance of 50 m. The ICEARRAY geometry is destructive earthquakes in the past. Its capability to produce earthquakes that may exceed magnitude 7 is a shown in Fig. 4 (top left) where the stations are represented by dots and a triangle, the latter indicating a constant threat to lifelines in the region, such as pipelines, electric transmission systems, dams and bridges. recording station commonly shared with the IceSMN. The recording system at each ICEARRAY station consists The Icelandic strong-motion array (ICEARRAY), is the first small-aperture strong-motion array in Iceland and of a single CUSP-3Clp strong-motion accelerograph unit manufactured by Canterbury Seismic Instruments Ltd. has been installed in the western part of the SISZ for the purpose of: 1) monitoring future significant events in The units are equipped with triaxial low-noise (70 micro-g rms) Micro-Electro-Mechanical (MEM) the region; 2) quantifying spatial variability of strong-motion over short distances; and 3) shedding light on accelerometers with a large dynamic range (± 2.5 g) and a wide frequency passband (0-80 Hz at 200 Hz earthquake source processes. ICEARRAY is equipped with CUSP-3Clp three-component digital accelerographs sampling frequency). Each unit possesses a continuous GPS timing system, acts in triggered mode and saves with GPS timing system and a perpetual connection to the Internet via wireless GPRS. The CUSPs are low cost, data files directly to solid state memory. The units draw their power from the mains supply of the respective but have a high-dynamic range and possess a very low noise floor (70 micro-g), thus additionally rendering building, although a backup battery ensures functionality for up to a week in case of power failures. The the array useful for studying the weak-to-strong motion transitions over a wide magnitude range. We communications system consists of standard TCP/IP protocols (https, sftp, rsync, email etc.), wirelessly and optimize its recording efficiency via a novel real-time common-triggering scheme. The number of array continuously connected using GPRS data modems. This system provides reliable connectivity for remote stations and their arrangement were based on an optimization of the shape of the corresponding array maintenance, control, and for automatic transfer of instrument recordings to a central server immediately transfer function (ATF). The optimal ICEARRAY configuration comprises 14 stations, has an aperture of 1.9 km, after recording an event. The latter is especially useful as it provides near real-time access to the complete and a minimum interelement distance of 50 m, and possesses a near-azimuthally independent ATF with a data set after an earthquake. The array is monitored by a dedicated CUSP-HUB that runs a novel common- sharp main lobe, negligible sidelobes and a wavenumber range of 1.5-24 rad/km. Accordingly and for array triggering scheme that ensures maximum recording efficiency across the array and at the same time processing purposes, the ICEARRAY has the intended capabilities of capturing seismic waves in the frequency minimizes efforts on behalf of an analyst reviewing the strong-motion data range of 1-20 Hz. At 15:45 UTC on 29 May 2008 a Mw6.3 earthquake occurred in the western part of the SISZ. During the earthquake the ICEARRAY produced high-quality extreme near-fault recordings on 11 stations. The recordings are characterized by high intensity strong motion of short duration with a prominent long-period energy along the strike-normal and strike-parallel directions. In both cases it is associated with permanent tectonic displacement. Moreover, the ICEARRAY has recorded over 1000 aftershocks of the Ölfus earthquake. Introduction Seismicity in Iceland is related to the Mid-Atlantic plate boundary crossing the island. Based on written documentation covering roughly the last 1000 years, it can be deduced that a damaging earthquake or damaging earthquake series strike roughly twice every century on average. Over the last 25 years, four earthquakes of magnitudes 6-6½ have struck in the South Iceland Seismic Zone (SISZ). The earthquakes were recorded on the Icelandic strong motion network (ICESMN), which is operated by the Earthquake Engineering Research Centre of the University of Iceland. The last earthquake was also recorded on the ICEARRAY, a new small-aperture strong-motion array in the SISZ. The networks performed well during these earthquakes, and it can be stated that most of the relevant data had been collected, pre-processed and stored in a database by a central computer within an hour. The data obtained include both far-field and near-field records. The data is disseminated via the Internet Site for European Strong-motion Data (www.isesd.hi.is) (a) (b) Figure 3 – North -south trending alignment of the seismicity distribution for the duration of 29 May - 29 June, 2008 , (circles, from SIL) in South-west Iceland indicates the location of the causative faults (approximated by the dashed lines) of the 15:45 UTC 29 May 2008 earthquake. The inset picture bottom right shows Iceland with respect to the Mid-Atlantic Ridge (line), the Tjörnes Fracture Zone (TFZ) in the north and South Iceland Seismic Zone (SISZ) in the south are shown by the dashed outlined areas. The solid rectangle indicates the area shown in the main picture. The inset picture at top left shows the locations of the recording stations of the ICEARRAY against the street layout in the village of Hveragerdi. Note the proximity of the fault to the ICEARRAY. The triangles are recording sites of the Icelandic Strong Motion Network. ICEARRAY strong-motion recordings during the Mw 6.3 Ölfus earthquake At 15:45 UTC on 29 May 2008 a M 6.3 earthquake occurred in the western part of the SISZ. The first motion Figure 1 – (a) Overview of main tectonic structures and earthquake epicentres: The thick lines outline the plate margin, the grey w areas indicate volcanic zones; SISZ is the South Iceland Seismic Zone; TFZ is the Tjörnes Fracture Zone; WIFZ is the West Iceland originated approximately 6.5km east–southeast of Hveragerdi on what aftershocks appear to identify as an Fracture Zone; open circles denote earthquake epicentres4; the filled circles show epicentres of events recorded by the Icelandic almost 10 km long north–south trending fault (Fig. 3). However, most aftershocks outlined another almost 20 Strong-motion Network, where the light-gray ones refer to the South Iceland earthquake sequence in June 2000; the average km long north–south trending fault less than 2 km from the town of Hveragerdi. During the earthquake the motion of the Eurasian Plate and North American Plate is indicated by the bold arrows. (b) The Icelandic Strong-motion Network, ICEARRAY produced high-quality recordings on 11 stations. The recordings are characterized by strong motion where the markers indicate free field stations (circles), bridges (inverted triangles), buildings (squares), earth fill dam (triangles), of short duration and high intensity, manifested by the geometric mean of horizontal peak ground power plant (triangles pointing right); Height above sea level is indicated using the following color code: white coastal areas 0-200 acceleration ranging between 0.44 and 0.87 g, seen in Figures 4 and 5. Moreover, a prominent long-period m; gray 200-600 m; dark gray > 600 m. The white areas inland are glaciers. Note that the distance between the meridians at 63° parallel is approx. 50.5 km. The EERC is located in Selfoss, which is the location of a structurally monitored building (see the square near-fault velocity pulse is observed both along the strike-normal and strike-parallel directions. In both cases outside of Reykjavik). it is associated with permanent tectonic displacement. Moreover, the ICEARRAY has recorded over 1000 aftershocks of the Ölfus earthquake. The ICEARRAY continues to monitor the South Iceland Seismic Zone. ICEARRAY small-aperture strong-motion array The ICESMN has been augmented through the deployment of the Icelandic Strong-motion Array (ICEARRAY), the first of its kind in Iceland. Its purpose is the monitoring of significant earthquakes in the SISZ, establishing quantitative estimates of the spatial variability of their strong ground motion, and shedding light on earthquake source processes. Additionally, the ICEARRAY provides an opportunity to investigate the transitional characteristics of weak-to-strong motion. ICEARRAY is located in the town of Hveragerdi, in the Ölfus District of South Iceland, due to its proximity to destructive earthquakes in the past, the relatively high populationdensity of the western SISZ and for collocatingthe array with engineering structures. ICEARRAY was designed via optimization of its array transfer function (ATF). The ATF describes the sensitivity and resolution of an array for seismic signals with different frequency contents and slownesses. It is the single most relevant characteristic of an array and holds information on all its aspects. The ATF is defined as Figure 4 – East-west (strike-normal) component of recorded Figure 5– North-south (strike-parallel) component of recorded where k is the two-dimensional wavenumber vector and k0 is the true wavenumber vector of a acceleration on the ICEARRAY during the Ölfus earthquake acceleration on the ICEARRAY during the Ölfus earthquake monochromatic plane wave that traverses across the array. The number of array stations is M and r0 is the vector of station locations in the x-y plane.
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