Most of what we know about the structure of Earth’s interior comes from seismograms, records of ground motion from seismograph stations. Typically ground motion results from earthquakes, but sensitive instruments, called broad-band seismographs, can record a wide range of energy frequencies including human activities and even wave motion. As earthquake waves travel through the earth, their behavior changes depending on the material through which they have travelled. When seismographs from all over the world are compared and compiled scientists gain a better understanding of the earth’s internal structure.
Last fall we talked with Winfried Hanka (GeoForschungsZentrum Potsdam) via email about the GEOFON Seismic Network, a global broadband seismic network operated and maintained by GFZ since 1993. Hanka gave us a few details about GEOFON, including Greenland’s three stations at Kangerlussuq, Danmarkshavn, and Summit, which is the first long-term installation on the ice.
Polar Field Services (PFS): What does a typical GEOFON installation look like and what instrumentation is included?
Dr. Hanka: Usually a seismic station is installed underground (in vaults, tunnels or boreholes) to protect the seismometer from environmental disturbances like temperature and air pressure changes, so there is not a lot visible from outside. Normally, only some communication equipment and a GPS antenna and, in some cases, also solar panels are installed above ground. Inside the vault the main "broadband" seismometer looks a bit like a turned-around cooking pot on three feet. In addition, in earthquake-prone areas, a strong motion accelerometer - a small metal box - is screwed to the ground to record extremely intense seismic waves when the main sensor is clipped. We include some weather sensors for environmental. The electronics, either installed also in seismic vault or a in nearby separate room, include a three or six channel 24-bit digitizer, a small Linux station computer, power management system, communication and system monitoring equipment.
PFS: How do you choose GEOFON locations?
Dr. Hanka: We must always find a compromise between remoteness (low noise) and logistically easy places. Also the geology (as hard rock as possible), distance from oceans, and security play an important role.
PFS: Why is Summit Station a good place to locate a seismometer?
Dr. Hanka: Summit is a great site because of its remoteness (far away from major noise sources like oceans and human activities) and excellent logistics. Main power, satellite Internet and local support is provided all-year. It is the only seismic station operated year-round within several hundred kilometers. A problem is of course the soft and moving ice, which is not an ideal underground for installing seismometers and requires regular adjustments.
PFS:How does the Summit Station location compare to other Greenland locations in the GEOFON network?
Dr. Hanka: Both other GEOFON stations are at the coast which provides more solid ground but they experience most of the year heavy ocean generated noise. Only when the sea is frozen is it quiet there. In Summit, apart from some local disturbances in the busy summer season, the seismic noise is low year-round and provides a good basis to detect also weak seismic signals.
PFS: Please describe the Summit Station seismometer and the location in which it is housed.
Dr. Hanka: The seismometer casing is cylindrical, about 25 cm wide, and hosts three independent components oriented vertical, North-South and East-West, each with a moving mass. Measurement parameter is not the mass extension during ground shaking itself but the voltage needed to keep the mass in rest related to the moving seismometer frame. The broadband sensor is capable of resolving the full seismic spectrum from high frequency signals from local earthquakes up to earth's free oscillations and the earth tides of several hours. . . . In Summit, the sensor is simply installed on a small wooden plate directly laying on the ice floor of a 2 x 2 m underground bunker, located in about 2-3 m depth. Because more or less regular maintenance is required, the bunker needs to be kept accessible all-year-round. Besides the sensor itself, some electronic equipment (including the digitizer) is located in an aluminum box in the vault next to the sensor and is connected to the station computer in the TAWO by Ethernet cable and WLAN link.
PFS: What kind of maintenance must be performed on the seismometer each year?
Dr. Hanka: Until 2013 the broadband sensor had to be re-leveled several times a year since it was very sensitive to tilting. Last summer the original sensor was replaced by another more tolerant model. Every two years the vault needs to be renewed due to the squeezing bunker height caused by the growing snow cover.
PFS: What types of maintenance are performed by Summit staff throughout the year?
Dr. Hanka: The science techs are very helpful when the sensor needs re-leveling. Then they crawl into the vault and correct the sensor tilting using the adjusting screws. Also local trouble shooting on request helps us to identify and locate technical problems. Even exchange of electronic equipment has already been done when the need arose.
PFS:How close must an earthquake be for it to register on the Summit Station seismometer? What is the largest earthquake event you have seen on the Summit seismometer? How sensitive is the instrument to small events?
Dr. Hanka: Generally, the smaller an earthquake is, the closer it has to be to be recorded. Larger earthquakes (Magnitude > 5) are registered from all over the world. The largest two events were the Sumatra-Andaman quake in 2004 (M=9.1) and the Japan (Tohoku) earthquake in 2011 (M=9.0), both of which generated disastrous tsunamis. In Greenland also very local earthquakes are recorded but are difficult to detect. To locate and quantify an earthquake one needs detected signals from at least 4 stations and with the given station spacing around Summit only events with maybe M > 4 can be analyzed properly. We do see also smaller and more local events in the Summit recordings; however, we cannot tell precisely where and how strong they are.
PFS: How do you receive data from GEOFON seismometers? What types of data do you receive?
Dr. Hanka: We use the Summit Internet connection via satellite to transfer waveform data (three components with 20 samples-per-second) and several state-of-health channels from the local station computer. Its data and SSH ports are forwarded to the Summit camp firewall, so we can access them remotely.
PFS: How do data help to alert early warning systems? What is the protocol for communicating this information to policy-makers or those involved in emergency management?
Dr. Hanka: The closer a seismic station is located to an earthquake source the more useful is it for early warning. Global networks like GEOFON cannot contribute much in terms of earthquake early warning - for this you need a dense network in the source area - but help a lot for tsunami early warning because the tsunami wave travels much slower than the seismic waves. However, even if we cannot do much to warn the people in an earthquake affected zone ourselves, our rapid publication of derived earthquake parameters help to estimate damages and losses in such areas remotely. Therefore we publish such parameters (location, depth, magnitudes, later also source mechanism) typically 4-10 minutes after the rupture has started on our web page http://geofon.gfz-potsdam.de/eqinfo/list.php, and issue alerts by email and SMS.
PFS: GEOFON has many partnering groups - how do you integrate and coordinate with these groups? What are the main ways in which you share information?
Dr. Hanka: GEOFON is a member in a worldwide network of seismic networks. We share our data with more than 200 institutions worldwide in real-time over Internet and also offer open automated access to our permanent archive to all researchers.
PFS: In what areas do you hope to expand the network?
Dr. Hanka: Presently the network extension has more or less reached its maximum due to limited resources. Desirable would be better coverage of Africa, but also more cooperation with existing networks in South America and Asia, where a lot of stations exist, but data are presently not shared.