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Active Tectonics of the Adriatic Region (Central Mediterranean)

M. Battaglia, M. H. Murray. R. Bürgmann D. Zuliani, A. Michelini, CRS (Italy)

Introduction

The Adriatic region can be considered an ideal natural laboratory for studying the kinematics and dynamics of micro-plate tectonics because of the wide variety of tectonic processes encompassed. These include continental collision, subduction of continental and oceanic lithosphere, major continental faults such as the Periadriatic Line, and a variety of smaller scale processes associated with African-Eurasian plate interaction. It is widely accepted that the Adriatic block was originally an African promontory that was indented into the European lithosphere by the convergence of the African and Eurasian plates. However, the present-day kinematics of the Adriatic block are more uncertain: is it still closely connected with the African continent or does it behave as an independent microplate? The distribution of seismicity in the Adriatic region is distinguished by very low seismicity in the plate interior compared to intense activity in the high topographic belts that border the region in the SW, NW and NE. One of the strongest clusters of seismic activity is in the greater Friuli region, located at the convergent NE edge of the Adriatic region, where the Ms=6.5 1976 Friuli earthquake caused widespread damage.

Geodetic monitoring in the Friuli Seismic Zone

The Friuli Regional Deformation network (FReDNet) of continuously operating Global Positioning System (GPS) receivers monitors crustal deformation along northeast boundary of the Adriatic microplate, which in the Friuli region is thought to be on the order of 5 mm/yr north-south convergence. FReDNet is operated by the Centro Ricerche Sismologiche (CRS) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS). The principal goals of the FReDNet program are to determine the distribution of deformation in this region, to estimate interseismic strain accumulation on its active faults to better assess seismic hazards, to monitor hazardous faults for emergency response management, and to provide infrastructure for geodetic data management and processing. FReDNet will also be part of a proposed larger program of geodetic monitoring of the Adriatic microplate that includes repeating episodic measurements of geodetic points (e.g., points of the IGM95 geodetic network).

In June 2002, Maurizio Battaglia and Mark Murray visited the CRS in Udine, located in the central Friuli region, to assist in site reconnaissance, permitting, and installation of the first FReDNet stations, and to initiate data acquisition, quality checking, archiving, and analysis. Most of the stations will be colocated with broadband seismometers maintained by the OGS, allowing us to employ some of the procedures developed at the BSL to integrate geodetic and seismic data from the BARD and BDSN networks. In April 2002, David Zuliani of the CRS visited the BSL to better study these procedures.

The CRS purchased 6 Ashtech Micro-Z GPS receivers with chokering antennas for the initial deployment of the new continuous stations. We inspected 5 sites with acceptable sky visibility for GPS observations. Four of the sites are colocated with existing seismic stations (ZOU, LSR, CAE, and MPRI, Figure 26.1), all within the southern Alpine foothills in north Friuli. The southern Friuli region is overlain by Quaternary sediment deposits that are not suitable for GPS monuments needing long-term stability. However, at the Medea site a block of Jurassic sandstones has been uplifted, providing a more tectonically stable setting for the GPS monument. Except for ZOU, additional permitting was required at all the sites. Currently, we have obtained permits for all the sites except LSR, although the fees requested for the CAE site were deemed to prohibitive. We are now pursuing an alternative site at Faloria (AFL), colocated with a seismic station in the Dolomites to the west of Friuli. A sixth station will be located near the main OGS campus in Trieste for demonstration purposes.

Figure 26.1: Existing and planned GPS continuous stations of the FReDNet in the Fruili region, northeastern Italy. Stations IESO is near Venice, TRI is near Trieste and the border with Slovenia, and ZOU is near the Austrian border. Red stars, GPS stations. Triangles, CRS seismic stations monitoring the Friuli seismic zone. Epicenter of the Ms=6.5 1976 Friuli earthquake is near the station BOO.
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In June 2002, we installed the GPS station ZOUF at Zouf Plan, colocated with the ZOU seismic station (Figure 26.2). A 1-m concrete pier was attached to surface bedrock using epoxied metal rods, and the chokering antenna was attached to the monument using a SCIGN antenna adapter and covered with a SCIGN dome. The antenna cable was trenched to a nearby building housing the seismic data loggers and GPS receiver. Cellular modems are used to telemeter the data once per day, using SHARC/EGADS to control the data retrieval. A second station (UDIN) was installed on the roof of a CRS building in Udine for testing purposes. In July 2002, the monument at Monte Prat (MPRI) was installed (Figure 26.3). The CRS is currently resolving problems telemetering the data from this station.

We installed the GAMIT/GLOBK GPS analysis package on a Linux computer at CRS, and initiated automatic processing of the data from ZOUF and UDIN, in combination with other EUREF continuous stations. Preliminary analysis shows short-term repeatabilities in the horizontal components of 2 mm in the north and 4 mm in the east. The higher east repeatabilities are mostly likely due to poor phase ambiguity resolution between the FReDNet and EUREF sites, which are over 100 km distant. These results should improve as the other FReDNet stations are installed, providing short distances that can be more easily resolved.

In the coming year, the CRS plans to finish the installation of the 6 initial stations and make the data publicly available, and we will continue to assist them in processing the observations in combination with other European stations in order to obtain initial estimates of the rate of convergence across the Friuli Seismic Zone.

Acknowledgements

We thank the staff of the CRS/OGS for their abundant assistance and hospitality during the station installations, particularly Alberto Michelini, David Zuliani, Giorgio Duri, Fausto Ponton, Paolo Di Bartolomeo, Gianni Bressan, Aladino Govoni, and Pier Luigi Bragato. This project is partially supported by the Centro Ricerche Sismologiche (CRS) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy.

Figure 26.2: GPS station at Zouf Plan (ZOUF), colocated with seismic station ZOU. The 1-m high concrete pier with GPS antenna and dome mounted on top are on the right. Solar panels and radio antenna mast for the seismic station are on the left.
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Figure 26.3: Installation of the GPS monument at MPRI. Concrete pier is tied to near surface bedrock using epoxied steel bars. A compass attached to a wooden rod is here being used to orient the antenna adapter to north.
GPS monument at MPRI

Figure: Preliminary position estimates of ZOUF relative to a European fixed reference frame defined by several IGS stations. Short-term repeatabilities are typical for unresolved phase ambiguity solutions.
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