Seismic Networks

The ocean bottom seismometer with hydrophone deployed on the flat top of the Marsili submarine volcano (790 m deep) by the Gibilmanna OBS Lab (CNT-INGV) from 12th to 21st July, 2006, recorded more than 1000 transient seismic signals. Nineteen of these signals were associated with tectonic earthquakes: 1 teleseismic, 8 regional (located by INGV) and 10 small local seismic events (non located earthquakes). The regional events were used to determine sensor orientation. By comparing the signals recorded with typical volcanic seismic activity, we were able to group all the other signals into three categories: 817 volcano-tectonic type B (VT-B) events, 159 occurrences of high frequency tremor (HFT) and 32 short duration events (SDE). Small-magnitude VT-B swarms, having a frequency band of 2-6 Hz and a mean length of about 30 s, were almost all recorded during the first 7 days. During the last 2 days, the OBS/H mainly recorded HFT events with frequencies of over 40 Hz and of a few minutes in length. Signals that have similar features in frequency and time domain are generally associated with hydrothermal activity. During the last two days a signal was recorded that had a frequency content similar to that of VT-B events was recorded. It will be referred to as continuous volcanic tremor (CVT). The SDE signals, characterized by a quasi-monochromatic waveform and having an exponential decaying envelope, may have been generated by oscillations of resonant bodies excited by magmatic or hydrothermal activity. By applying polarization and parametric spectral analyses, we inferred that the VT-B were probably multi P-phase events having shallow sources that were situated in narrow azimuthal windows in relation to the positions of the OBS/H. The parametric spectral analysis of the SDE signals allowed us to determine their dominant complex frequencies with high accuracy; these frequencies are distributed in two distinct clusters on the complex plane.

The strong motion network of the National Institute for Research and Development in Construction, Urban Planning and Sustainable Territorial Development "URBAN-INCERC" was established in Romania in 1967. Its paramount utility for a seismic country was proved when the unique recording of March 4, 1977 earthquake was obtained in INCERC Bucharest station.
At present, the network consists of approximately 55 accelerometers, located in 45 localities in Romania. Of these, 11 accelerometers are located in Bucharest and 44 are distributed all over the country, particularly in the extra-Carpathian area, which is the most exposed to the Vrancea seismic source.
The goals of the network and of the pertaining infrastructure are manifold, aiming not only at ground motion recording, but also, in a larger perspective, at the improvement of the security and resilience of building stock to earthquakes and extreme actions. In this respect, a system for monitoring and displaying recorded ground motion data in real time is envisaged, in conjunction with data provided by SHM devices from instrumented buildings.
Recently, a plan for real-time transmission of the recorded seismic data was developed in collaboration with the Romanian Special Telecommunication Service. Accordingly, starting from November 15th, 2015, 32 seismic stations, of which 4 in Bucharest and 28 distributed throughout the country, were connected to real-time data transmission. This is provided both for stations located in free field-type conditions and for monitored building structures.
The goals for the near future are the extended integration and collaboration within major European infrastructures in the field, as the European Plate Observing System, EPOS. This implies further standardization of instrumentation and operational procedures, establishment of permanent communication channels for data exchange, upgrading of obsolete equipment, increasing the number of real-time stations and of seismic stations in general and development of the communication, IT and software infrastructure.

The number and quality of seismic stations and networks in Europe continually improves, nevertheless there is always scope to optimize their performance. In this EGU2015 SM1.2/GI1.5 session we welcomed contributions from all aspects of seismic network installation, operation and management. This includes site selection; equipment testing and installation; planning and implementing communication paths; policies for redundancy in data acquisition, processing and archiving; and integration of different data sets including GPS and OBS.