Monitoring of tectonic creep movements using TM-71 crack gauge


The EU TecNet monitoring network provides very precise measurements of tectonic creep, using a series of crack gauges known as TM-71s. More than 140 gauges are currently installed on tectonic faults within the Czech Republic, Slovakia, Slovenia and across Europe. In addition, more distal sites are located in Asia and South America. Most of the devices are situated in caves where more stable climatic conditions ensure that the effects of thermal dilation are minimised. These locations also better protect the devices against natural dangers as well as human interference.

The device

The TM-71 crack gauge can very accurately measure all relative movements of two rock blocks in 3D (up to 10-2 mm), including relative rotations. The device is very sturdy, does not require an energy supply, and can operate for many years in humid conditions - even underwater. It was designed by Dr. Košťák during the 1970s with these particular characteristics in mind. The principle of measuring is based on the moiré effect: two very fine spirals, engraved into glass plates, create specific patterns when moved relative to one another. The pattern can be converted mathematically into values and directions of movement. The readings are performed either directly on photographic paper with the pattern is illuminated using a photoflash or by using a digital camera.

Monitored sites

In the Czech Republic, we currently monitor approximately 65 sites. In addition, more than 75 sites are situated in Poland, Slovakia, Germany, Italy, Greece, Bulgaria, Slovenia, Spitsbergen, Peru and Kyrgyzstan. This vast network was constructed for the purpose of observing and measuring the slow tectonic movements known as "tectonic creep". These movements, which slowly release tectonic stresses, are one manifestation of active tectonics; more widely known are earthquakes, which occur when crustal stresses are released rapidly. Among the oldest of the sites is that of Steny in the Malá Fatra Mts. (Slovakia), which has been recording data since 1973.


Readings are usually taken once a month, which is sufficiently frequent to observe long-term tectonic creep trends. Occasionally, additional readings are taken immediately after strong local earthquakes if they occur in the vicinity of a monitored fault (M = 3-5) or after significant global seismic events (M >= 8) (e.g. Sumatra 2004, Chile 2010, or Japan 2011). The great advantage of this device is the fact that it does not require an energy source, wire, wireless network or any other connection. Only at a few sites, where higher frequency monitoring is needed, we have installed recently developed automated reading. This uses tiny cameras with direct connection to the internet. At these sites, the readings are taken every 15 minutes.
(For examples, see data from 13C Cave)

Contacts for further information

Contacts are on the next page..