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Figure 3. Peak ground acceleration and return period.

the urgent supporting measures and before the beginning of masonry repair, while the second one occurred during the preparation of masonry for grouting by stitching of cracks and local deep re-pointing. After three and four months from the end of the first grouting period, during which grouting was implemented up to the springing level of groin vaults, two more weak earthquakes were recorded.

4 SEISMIC HAZARD EVALUATION

The selection of the technical specifications of the accelerometers that are installed on the monument and on soil surface has to be based on the quantitative estimation ofthe acceleration ofthe ground motion, which is assessed by seismic hazard analysis. It was evaluated that the annual mean peak horizontal acceleration with mean frequency of two events per year is 0.059 g, while the maximum expected peak acceleration on soil surface is 0.80 g, as shown in Figure 3. Considering the standard deviation of the used sample earthquake data and in order to have a high signal to noise ratio, the noise level was selected to be 0.001 g.

5 SEISMIC MONITORING SYSTEM

A digital dynamic monitoring system has been installed to record the response of the core of the monument. Accelerometers and displacement meters are used to measure acceleration and relative displacement at characteristic locations. These locations were selected in order to avoid local vibrations of individual masonry units during earthquakes, taking into consideration the existing cracking patterns.

The accelerometers were placed at five characteristic levels (a) on ground level free field; (b) on the cornice where the central arches and the squinches are springing; (c) on the base of the drum; (d) on the base of dome (top of the drum). The displacement transducers were placed to record the relative displacement between north and south as well as east and west internal walls of the central part of the church.

Figure 4. Control room.
Figure 5. Accelerometers located at the Dome's base.

The monitoring system is based on National Instruments PXI and SCXI units that are controlled by a Labview application. All signals are collected by a central unit. A special program was developed by NTUA to control the unit, to digitize data using pre-trigger and post-trigger events and a voting system for the initialisation of the recording. The resolution of A/D converter is 16-bits, the sampling frequency is 200 Hz. Antialising Butterworth filters with a cut-off frequency 100 Hz are connected, the clock of the system is synchronised by a GPS, while there is a DC power supply that acquire the uninterrupted function of the system for two days. The acquisition system is placed in a control room as shown in Figure 4. Data are saved locally and can be transmitted on line to the analysis centre at NTUA, while it is capable to call the users when a seismic event is captured.

Fourteen accelerometers (Bernhard, B. 2002) and eight displacement transducers were installed to record acceleration and relative displacement respectively. The selection of the sensors was based on the Seismic Hazard. The maximum acceleration that

Figure 6. Accelerometers located at the Drum's Base.
Figure 7. Accelerometers located at the Cornice and Ground.
Figure 8. Displacement transducers.

can be recorded is 2 g, while the noise of the system - accelerometers-filters-A/D converter is 0.001 g. In Figures 5 to 8 the instrumentation setup is presented.

Table 2. Seismic events.

Recorded event

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