A New Semi-Continuous GPS Network and Temporary Seismic Experiment Across the Montello-Conegliano Fault System (NE-Italy)

Abstract

The Montello–Conegliano Thrust is the most remarkable structure of the Southern Alpine fault belt in the Veneto-Friuli plain, as a result of the conspicuous morphological evidence of the Montello anticline, which is associated to uplifted and deformed river terraces, diversion of the course of the Piave River, as well as vertical relative motions registered by leveling lines (Galadini et al., 2005; Burrato et al., 2008). Many papers dealt with its geometry and evolution, and the presence of several orders of Middle and Upper Pleistocene warped river terraces (Benedetti et al., 2000) in the western sector strongly suggests that the Montello–Conegliano anticline is active and driven by the underlying thrust. However, in spite of the spectacular geomorphic and geologic evidence of activity of the Montello-Conegliano Thrust, there is only little evidence on how much contractional strain is released through discrete events (i.e. earthquakes) and how much goes aseismic. Benedetti et al. (2000) hypothesized that the western part of the thrust (Montello) may have slipped three times in the past 2000 years (during the Mw 5.8 778 A.D., Mw 5.4 1268 and Mw 5.0 1859 earthquakes), yielding a mean recurrence time of about 500 years, whereas, the eastern part of the thrust (Conegliano) would be silent. The Italian seismic catalogues have very poor-quality and incomplete data for these events associated with the Montello thrust, leaving room for different interpretations, as for example the possibility that these earthquakes were generated by nearby secondary structures. In this latter case, the whole Montello–Conegliano Thrust would represent a major “silent” structure, with a recurrence interval longer than 700 years, because none of the historical earthquakes reported in the Italian Catalogues of seismicity for the past seven centuries can be convincingly referred to the Montello Source. Given the uncertainties regarding the seismic potential of this segment of the Southern Alpine fault system, we designed and realized a new GPS network across the Montello region (Fig. 1), with the goal of detecting the present-day velocity gradient pattern and develop models of the inter-seismic deformation (i.e., geometry, kinematics and coupling of the seismogenic fault). In the 2009, we started realizing a new concept of GPS experiment, called “semi-continuous”. As the name suggests, the method involves moving a set of GPS receivers around a permanently installed network of monuments, such that each station is observed some fraction of the time. In practice, a set of GPS receivers can literally remain in the field for their entire life span, thus maximizing their usage. The monuments are designed with special mounts so that the GPS antenna is forced to the same physical location at each site. This has the advantage of mitigating errors (including possible blunders) in measuring the antenna height and in centering the antenna horizontally. This also has the advantage of reducing variation in multipath bias from one occupation session to another. The period of each “session” depends on the design of the operations. At one extreme, some stations might act essentially as permanent stations (though the equipment is still highly mobile), thus providing a level of reference frame stability, and some stations may only be occupied every year or two, in order to extend or increase the density of a network’s spatial coverage. In this work we will present the motivations and tools used to develop and implement the new GPS network. During the 2010 we will integrate the existing GPS network with 10 mobile seismic stations, belonging to the INGV mobile network, with the goal of illuminate local micro-seismicity patterns that would help constraining the locked fault geometry.