The Swiss Digital Network (SDSNet)

Introduction

Brief History of the Seismic Network
Instrumentation and Communication
Site Location
Data Acquisition and Processing 

Introduction

The Seismic Network at the Swiss Seismological Service is tasked with monitoring the seismicity of Switzerland and its adjacent areas, rapid notification of the authorities and the public, and providing reliable data for earthquake research. The Network currently maintains one of the most modern and dense networks in the world, consisting of high quality stations with realtime communication to distributed processing hubs at ETHZ Zurich, where data is analysed and events are detected within seconds of their occurrence. The Seismic Network acquires data from the CHNet Swiss National Networks (both the broadband / short period SDSNet and strong motion SSMNet) and the sister Special Network and Seismo@School Network. Over 100 stations in Switzerland have continuous, realtime communication.

Accurate earthquake locations require multiple observations without large azimuthal gaps. Additionally, damaging ground motion can be felt from events which occur outside Swiss territory. Thus, the Seismic Network also monitors all available realtime stations close to Swiss territory currently operating in neighbouring countries.

Figure 1. Station Map for All Stations Continuously Monitored in Real-time at SED, including foreign stations.

Brief History of the Seismic Network

Instrumental monitoring of earthquake activity in Switzerland began at the onset of the 20th Century (complete history). Since this time, Switzerland has been at the forefront of seismic monitoring in Europe. By the 1970s, a telemetered high-gain short-period seismic network was installed, with data recorded on microfilm. By the mid 1980s, the network consisted of a dense network of high-gain sensors with radio communications digitized on a central computer system and automatically processed.

In the early 1990s, the initial National Strong Motion Network was established, with dial-up 12-16 bit triggered instrumentation. The modern Swiss Seismic Network was created in the late 1990s, when broadband sensors alongside 24-bit dataloggers replaced the short period sensors, GPS timing was introduced, and communications were upgraded to a high bandwidth, secure internet system. This new network is known as the CHNet, comprising the national broadband (SDSNet) and the national strong motion (SSMNet) networks, as well as some Special Networks (targeted local densifications based on temporary projects).

Since 2006 new installations of the Swiss Strong Motion Network (SSMNet) all have had continuous monitoring in real time with high dynamic range. Currently, high quality strong motion sensors are co-located with broadband sensors at 13 sites, and similar instrumentation is located over 30 freefield, mainly urban locations.  

Instrumentation and Communication

The realtime stations of the permanent network are shown in Figure 2.

The majority of SDSNet stations are broadband. The number of broadband, continuously monitored stations in Switzerland is 34. 29 of these stations are equipped with Streckeisen STS-2 seismometers covering a period range from 120 s to 0.02 s with nearly constant response, 4 consist of 40s to 0.02 Nanometrics Trillium sensors, and a borehole station is equipped with a 360s Guralp CMG-3T. For the majority of these stations, digitizers, communication and data acquisition systems are all supplied from Nanometrics. At GRIMS, a Quanterra Q330 is deployed, and at OTER1, a Guralp is used.

At 8 more SDSNet locations, short period Lennartz 5 and 1s sensors are deployed alongside Nanometrics dataloggers.

All SSMNet sites include a broadband 200Hz Kinemetrics EpiSensor accelerometer. At 13 SDSNet locations a strong motion sensor is co-located, though stand-alone accelerometers are deployed at over 30 sites alongside Nanometrics Taurus dataloggers.

The Nanometrics digitizers are currently comprised of 3 different Nanometrics models: HRD24, Trident and Taurus. All dataloggers used in the CHNet are synchronized by GPS. The SDSNet stations sample the seismic signals at at least 100sps, typically 120s (broadband) and 250sps (strong motion). The Trident and HRD store digitized data for up to two hours , though all other dataloggers are capable of many months of local storage.

Communications from the field to Zurich is provided by variety of independent internet based providers to prevent a single point of failure.

Site Location

Site selection for the SDSNet requires particular attention as locations with minimal ambient noise across the frequencies if interest, from 50Hz-1000s, are preferred. At many cases during the initial setup, former analogue sites continue to be used as it is important to have continuity; long term observations and relocations have minimised noise (at east at high frequencies); and the local infrastructure, like power supply and communication usually already exist. At some locations, completely new sites were selected, with the goal to obtain a better coverage of the Swiss territory.

Many stations are located in former military bunkers or caverns as excellent infrastructure exists; security and environmental protection are high; cultural noise, temperature and pressure variations are extremely low; and the stations can be located on solid rock.

For all candidate new stations, continuous seismic noise measurements were taken over a period of at least 2 weeks days. The noise spectra were compared to the low noise earth model. In the Alpine foreland where the sedimentary layers are substantially thicker than in the more alpine regions, we had to accept higher noise levels at the lower frequencies due to the oceanic microseisms as well as at higher frequencies due to higher cultural noise.

The daily performance of the network in terms of noise spectra, updated daily, are available for all continuously monitored stations in the SDSNet and SSMNet networks in the PQLX webpages.

Figure 2. Station Map for Stations Continuously Monitored at SED  

Data Acquisition and Processing

Reliable seismic monitoring can be achieved only by avoiding single points of failure. Wherever possible, we have continued to install redundant hardware and ensure alternate data flow paths.

Data acquisition from the Nanometrics Dataloggers is performed on redundant Linux PC's in Bern (for the subset of stations using the Government internet lines), as well as suite of Linux servers at 2 separate locations on the two main ETHZ campuses in Zurich. Gap filling is performed using the NAQS software. The NAQS software performs signal detection (triggers) based on STA/LTA in programmable frequency bands and event detection based on coincidence criteria of triggers.

Data from foreign stations and the Swiss stations in Greenland are typically acquired by SeedLink and merged with the NAQS streams.

Data transmission and acquisition is spread over a variety of communications providers, including the wide area network (WAN) of the Swiss federal government which is well protected from the general Internet. The advantage of using this network is always enough bandwidth for the transmission of the seismic data and network congestion is very unlikely. Using multiple providers increases the tolerance of the network to points of failure.

The data processing system consists of two physical SunFire computers which perform identical processing and hence provide redundancy. Event detection, quantification and alerting are performed on these machines, and earthquake information for local events affecting Switzerland is typically available via the website within 90s of an event origin time. Data is archived in the standard GSE2 and miniseed formats used by the SED. The high sample rate continuous waveforms are archived permanently on NAS, maintained by the Informatik Department at the ETH. This continuous archive is complete and dates back to the first acquistion of the boradband data in 1999. All data can be accessed at arclink.ethz.ch.

Swiss Data is made available in realtime to neighbouring Seismic Networks (INGV Italy, LED Germany, LGIT and Nice France, ZAMG Austria) as well as European and Global Agencies (ORFEUS, GFZ, IRIS). Event and continuous data from the SDSNet network is freely available to the community.