Seismic Activity in the Transantarctic Mountains - Results from a Broadband Array Deployment

Seismic events have been recorded in the Transantarctic Mountains using a temporary 10-station broadband seismometer array. About 50% of a total of 160 events were located to the north of the array, in the vicinity of the David Glacier, inland from the Drygalski Ice Tongue, while 40% were located near the transition between the Mulock glacier and the Ross Ice Shelf, just south of the Skelton glacier. The northern events are roughly aligned in a ESE-WNW direction, sub-parallel to faults which are postulated to lie near to the events, and close to the postulated David glacier lineament. The events may be associated with deepseated faulting along the lineament, although alternative source mechanisms involving ice movement are also possible. *Corresponding author(s.bannister@gns.cri.nz) INTRODUCTION The Transantarctic mountain range (TAM, Fig.1) is an impressive morphological feature which marks the border of the East Antarctic craton and the western Ross Sea region, reaching elevations of over 4000 metres and extending along the entire length of the West Antarctic Rift System (Wilson & Finn, 1996). Most current models postulate that the TAM represent a rift shoulder which formed in response to rift-related processes in the West Antarctic Rift System. Basement rocks in the TAM consist of early Paleozoic to a late PreCambrian metamorphic-plutonic complex. Overlying the basement is a sub-horizontal layered sequence of Devonian-Jurassic sedimentary rocks ranging up to 3500m in thickness. To the east and immediately adjacent to the TAM is the Victoria Land Basin (VLB, Fig.1), which was originally defined by marine seismic reflection data. Cooper et al. (1987) interpreted up to 14 km of stratified rock with block-faulted basement structure, typical of rift basins, and a thickness of at least 45 km of Cenozoic sediments. The VLB is inferred to be underlain by rifted continental crust, based on the presence of normal faulting (Cooper et al.,1987), thin Fig 1 The western Ross Sea region of Antarctica. Temporary seismic stations are shown as open triangles. Permanent seismograph stations at Vanda (VNDA), Scott Base (SBA) and Terra Nova Bay (TNV) are shown as solid squares. Seismic events located in this study are shown as stars. Over 60% of the events were located in the vicinity of the David glacier, and most of the remainder to the south, near Mulock glacier. Grey shading indicates elevation (in metres); VLB = Victoria Land Basin; TAM = Transantarctic Mountains. Terra Antartica Proof S. Bannister & B.L. N Kennett 42 (17-22 km) crust (Trehu et al., 1989; O’Connell & Stepp, 1993; Cooper et al.,1987), relatively high heat flow, and the chemistry of alkaline lava. Within the VLB lies the Terror Rift, a zone involving recent (Neogene and younger) faulting as well as magmatic activity. Present-day alkaline volcanism is represented by Mt Erebus, located on Ross Island (Fig. 1) in the Ross Archipelago (LeMasurier, 1990), at the southern end of the Terror Rift. Onshore, Cenozoic tectonic faulting has been inferred in the Terra Nova Bay region (Salvini et al.,1997; Mazzarini et al.,1997; Salvini and Storti,1999), as well as in southern Victoria Land (Wilson,1999; Jones,1996). Although seismicity levels in Antarctica are regarded as low, active seismicity has been recorded in the western Ross Sea region and in the Transantarctic Mountains (TAM, Fig. 1) by several temporary array studies ( .g. Cimini et al., 1995; Privitera et al., 1992) and by permanent stations (Reading,1999). In this study we further examine the seismic activity in the TAM region, using digital data recorded by a temporary broadband seismometer array. SEISMIC ARRAY DEPLOYMENT AND DATA ANALYSIS During the 1999-2000 austral summer 10 broadband seismometer stations were deployed in the central Transantarctic Mountains (Fig.1), primarily to investigate the seismicity in the region, but also to provide data for crustal and upper mantle structure studies. The seismometer array, with an average station spacing of 20-25 km, straddled the Transantarctic Mountains opposite Ross island, with array stations near the edge of the plateau ( e.g. station AN08, Fig. 1) as well as over the Victoria Land Basin (station AN01, Fig. 1). Each array station incorporated either a Guralp 40T or 3ESP seismometer, with an accompanying digital Orion recorder encased in a heavily insulated site-box, and with site power provided by solar panels. Seismic data were also recorded by the permanent seismograph station VNDA (Fig. 1), which is located down a borehole in the Bull Pass region, the station SBA, located near Scott Base, and by the Italian broadband seismograph station TNV (Fig. 1), which is located at Terra Nova Bay (Morelli et al., 1994). Event trigger algorithms were run through all of the broadband array data following data retrieval, as well as on the data from VNDA station, to search for, and associate, seismic events. Event hypocentres were subsequently determined using the Hypoellipse algorithm of Lahr (1999) based on arrival times picked from data from VNDA, TNV and the broadband array. One hundred and sixty seismic events were located, of which about 50 had horizontal position error estimates less than 20 km. Arrival time data from the TNV station were especially useful for constraining event locations. The distance of the events from the broadband array and VNDA station precludes accurate information on event depth. All event magnitudes were less than M L3.5.