Frequently asked questions
References see below
- What are earthquakes and how are they generated?
- Where do earthquakes occur?
- Where do earthquakes occur in Europe?
- Where do earthquakes occur in Switzerland?
- What is a seismogram?
- What is a seismometer?
- What is a hypocenter?
- What is an epicentre?
- What is microseismicity?
- What is induced microseismicity?
- What is magnitude?
- What types of magnitudes are used?
- What can the intensity tell us about an earthquake?
- What is the instrumental intensity?
- What is the twelve-stage European Macroseismic Scale 1998 (EMS-98)
- What is the Peak Ground Acceleration (PGA)?
- What is the Peak Ground Velocity (PGV)?
- What are shake maps?
- What is a strong-motion measuring station?
- What is UTC time?
generated when tensions built up due to relative movement between rocks along
faults are suddenly released. The seismic energy from these events travels
through the Earth in the form of waves and causes the tremors observed as
Most earthquakes occur in the Earth’s crust, a 10-50km thick layer covering our planet. This layer has broken apart into oceanic and continental plates. The relative movement of these thin, rigid plates to each other causes the build-up of tensions along the boundaries, which are released in the form of earthquakes. The movement of the plates can be convergent (towards each other, e.g. Andes), divergent (away from each other, e.g. mid-ocean ridge) and/or transform (horizontal sideways movement, e.g. San-Andreas-fault). The movement at the faults is in accordance with the movement of the plates as a whole. Furthermore, earthquakes can also be linked to volcanic or magmatic activity, the displacement or emplacement of hot rocks. This type of earthquake is closely related to processes occurring along plate boundaries, where most volcanoes are situated. Earthquakes can occur within tectonic plates as well. The cause for these can be tensions transferred from the boundaries towards weak spots of the plate where they are then released; or they can be related to local tectonic movements independent of the relative movements of the plates themselves.
Earthquakes in Europe can either occur in close proximity to plate boundaries or within the plates. The most endangered areas are situated along the boundary of the African and Eurasian plate in central and especially south-southeastern Europe.
Fig.1: Border area between the African and Eurasian plates. The northwards motion of the Arabian plate, which is responsible for the movement along the Anatolian fault (in red) is shown as well (Udías und Buforn, 1991).
These plates collided approximately 65 million years ago, with the formation of the Alps as a consequence. The upper crustal part of the Adriatic micro plate (Italy), which is part of the African plate, was thrust on top of the European plate, which in turn was pushed down.
Fig.2: Subduction zone in the Alps. The African plate is shown in lilac, the Eurasian plate in red. Both plates were split horizontally within the crust (comparable to two interlocked crocodile mouths). The upper part of the African plate has since been eroded to a large extent (NFP20, 1990). The peak of the Matterhorn is a prominent example of a remnant of the African plate.
All of Switzerland was affected by the formation of this new mountain range. However, tectonically speaking Switzerland is rather quiet these days and it is being discussed at the moment if the collision is still underway at all in this area. Earthquakes, which are signs of movement within the crust, are for the most part of small to medium magnitude (ML < 5.0). The exception to this rule was the earthquake in Basel in 1356 which is classified as one of the most destructive known earthquakes north of the Alps.
The medium to high seismic activity in the eastern part of the Mediterranean is an indication that today the converging motion is absorbed primarily in that area (Fig.3). Earthquakes of medium magnitude and the two volcanoes Etna and Vesuvius are strong signs of tectonic movement in Italy as well.
Fig.3: Earthquakes in Europe with a magnitude
greater than 5 from 1973 until 2006 (USGS/NEIC-Catalogue)
has small to medium seismic activity. Areas of increased activity are the
Rheingraben near Basel, the Wallis, Graubünden, the Rheintal in the St. Gallen
area and Central Switzerland. The strongest earthquake recorded in Switzerland
since the installation of an area-wide sensor network at the SED at ETH in the
mid-70s occurred near Vaz in Graubünden in 1991. It had a magnitude of 5.2. The
strongest known earthquake overall occurred in 1356 near Basel; it had an
intensity of IX. Estimates of its magnitude vary between 6.2 and 6.9. No active
faulting is observed at the surface today, but there is evidence of deep
large-scale active faults, e.g. in the northern part of the Wallis (Maurer et
al., 1997), Fribourg (Kastrup, 2002) and Martigny (Deichmann et al., 2002). In
addition the Rheingraben is an ancient fault system where earthquakes still
occur today. It has been postulated that the large Basel earthquake is
connected to the activity in the Rheingraben (Reinach fault; Meghraoui et al.,
A seismogram is the electronic recording made by a seismometer of the ground movements associated with earthquakes. The resulting diagram is wave-shaped with varying amplitudes and wavelengths since the movement at a measuring station during an earthquake is non-linear (Fig.4). The energy released by an earthquake (strength, magnitude) as well as the location of its hypocenter can be estimated by evaluating a large number of such diagrams.
Fig.4: Seismograms of a small earthquake in Switzerland recorded by six different stations of the SED network.
A seismometer is an instrument with which the motion of the ground at
a location can be recorded. Modern seismometers are highly sensitive
electromechanical devices which can register ground motion on the
order of nanometers (a millionth part of a millimeter). Earlier
mechanical instruments were called seismographs. The display of the
recorded motion on a time-axis is called seismogram.
The hypocenter describes the position of the seismic focus and is
specified by its depth, longitude and latitude.
The epicentre is the vertical projection of the hypocenter onto the Earth’s surface. It is specified by its longitude and latitude.
Microseismicity describes the occurrence and frequency of small local earthquakes (micro quakes). These are normally so weak that they cannot be felt on the surface and can only be recorded with highly sensitive instruments. Many quakes are in fact so weak that they are hidden in the station-specific background noise and can thus not be distinguished as signals.
The term induced microseismicity refers to small earthquakes caused by human activities. They can be a consequence of the development of geothermic reservoirs, oil or gas fields. The underlying cause is the inflow or outflow of fluids. In geothermic reservoirs for example the water pumped into the ground has to find its way through the rock. It accomplishes this by opening existing faults and clefts. The new fluids in these cracks decrease friction between the adjacent rocks and sudden displacements can occur. The multitude of such events increases the permeability. The events are generally so small that they are not felt on the surface. However, there are sporadic cases where the movement is large enough for an event to be noticed on the surface as a jerky ground movement or a loud bang.
The magnitude of an event is a physical quantity describing the energy released during an earthquake as a logarithmic value. An earthquake of magnitude 6 is thirty times stronger than an earthquake of magnitude 5 and 900 times stronger than an earthquake of magnitude 4. There are multiple ways to determine the magnitude. They are not directly related, but in some way or another they all measure the amplitude of a ground motion (velocity or acceleration) at different distances and in different frequency bands. The classical approach used here is called local magnitude and was developed by Richter (hence the term Richter scale). Earthquakes can be felt on the surface above a magnitude of 2.5-3.
The local magnitude ML is determined for earthquakes that occur relatively close to the recording stations, normally for distances up to a few hundred kilometres between the station and the earthquake. The first magnitude scale, the “Richter scale” developed in 1935 by Richter, is such a local earthquake magnitude. Even today earthquakes are commonly classified in units on the Richter scale.
The body wave magnitude mb is normally used for earthquakes which occurred more than 2000km away from a sensor station. The mb-estimation for such far earthquakes can be done relatively quickly because mb is determined from the amplitudes of the P-waves. The P-waves are compression waves which are transmitted through the Earth’s interior and constitute the first signal that is recorded of every earthquake. For larger earthquakes (stronger than mb=6) mb is “saturated”, meaning that the mb value will not increase much past this threshold even when an earthquake was significantly stronger. Such strong earthquakes, which occur anywhere between 50-100 times per year worldwide, require different scales in order to accurately calculate their true magnitude.
The magnitude MS is calculated from surface wave data. Surface waves run along the Earth’s surface with a much smaller velocity than the P-waves – 3-4 km/s compared to 8-14 km/s for P-waves. Thus one has to wait for longer at a far away station for the surface waves to arrive and MS cannot be calculated quite as fast as mb. Depending on the distance it can take 1 to 2 hours for the waves to arrive compared to a maximum of 20 minutes (New Zealand – Switzerland) for P-waves. The surface waves used for the calculation of MS have a period of around 20 seconds compared to about 1 second for the calculation ob mb from P-waves. Saturation is only an issue for very strong earthquakes above MS=8. The slow spreading of the surface waves is the reason why seismologists cannot estimate immediately after an earthquake if it was “just” strong (magnitude larger than 6) or potentially catastrophic.
Earthquakes which occur close to the surface (within the topmost 30 kilometres) create a large amount of surface waves compared to earthquakes of the same magnitude which occurred deeper within the Earth – the deepest earthquakes take place in subduction zones in up to 700km depth. The difference is caused by the way surface waves are created. Shallow earthquakes are more likely to cause large-scale damage simply due to their proximity to the surface. A comparison of MS and mb values can thus be used to estimate the potential damage; if the MS value is large compared to the mb value the earthquake probably occurred in a relatively shallow region and more damage has to be expected. The MS to mb ratio is also used to distinguish earthquakes from (nuclear) explosions. The latter have a smaller source volume than an earthquake of similar magnitude. In addition explosions cause less shear waves which are mainly responsible for the formation of surface waves. It follows that the MS values are typically much smaller for explosions. The mb/MS ratio is thus a good criterion for separating shallow earthquakes from explosions – if the value is high an explosion is the likely cause of the event.
The moment magnitude Mw is the only type of magnitude that is directly related to the physical processes and parameters at the hypocenter. Mw was derived from the seismic moment M0 based on theoretical studies. M0 is the product of the size of the fault times the average displacement at that fault times the shear strength of the rock. In principle Mw does not suffer from the problem of saturation since M0 considers the complete fault. There are various ways to determine Mw. Often synthetic seismograms are matched to the observations, either by looking at the wave forms or by comparing spectral amplitudes. The required effort is somewhat larger than the simple measurement of seismogram amplitudes (for the estimation of ML, mb, MS). For strong earthquakes the moment magnitude is available a few hours after the event.
You might find a magnitude designated only as “M” in one of our lists; this means that the seismic observatory which estimated this particular magnitude has not specified which type they used. Oftentimes this concerns observations made by the US Geological Survey. In these cases one assumes that a type of magnitude which is not saturated has been used; for strong earthquakes these magnitudes are often of the Mw type.
The intensity describes the strength of an earthquake based on the extent of the destruction (buildings, landscape) and the subjective perception of an observer. The intensity of an earthquake is location-specific and is defined by the magnitude, the distance to the hypocenter and the geology of the subsurface. The European Macroseismic Scale 1998 (EMS-98) uses numbers between I (not felt) to XII (complete destruction) for the individual classification of earthquakes. Nowadays determining the intensity can be done instrumentally as well by looking at the peak ground movement velocities and accelerations at a station, among other considerations.
The instrumental intensity describes the shaking of the ground due to an earthquake. It is calculated from the peak ground acceleration and velocity amplitudes recorded at the measuring stations. In this way an overview of the distribution of ground movements in an area can be attained much quicker than by questioning the affected population and by damage evaluation, which are needed in the classical approach to creating an intensity map. The values can be displayed as dots on a map and/or they can be connected to form contours (ShakeMaps). As a simpler alternative to this shake maps can be created, which only contain the peak ground velocities and accelerations.
Click on the picture for an enlarged version
Earthquake waves cause a two-way movement of the ground in horizontal (parallel and perpendicular to the direction of wave propagation) and vertical direction. How big and how fast (acceleration and velocity) this deviation is depends on various factors: The length of the fault (magnitude), the distance between the station and the fault and the geology of the subsurface. The latter can have a large influence on the ground acceleration and wave form, which makes it very important to know about even slight changes in the subsurface below the station. It follows that the values of ground acceleration show a high variation even within small areas. This is especially true for medium to large-size earthquakes and interpolating the values is thus difficult. Generally the acceleration will decrease the further away we are from the fault. For the calculation of the PGA one considers only horizontal ground movements. The acceleration is given as a fraction or multiple of the Earth’s gravitational acceleration, g=9.81 ms−2. For small earthquakes (magnitude below 3) humans will mostly feel the acceleration (Wu et al., 2003). The values can be displayed on PGA or PGV maps, also referred to as shake maps (main source: Earth and Space Sciences, University of Washington).
The PGV is measured in cm/s. For its calculation one uses the horizontal component of the ground movement, just like in the calculation for PGA. For medium and strong earthquakes the resulting velocity pattern mimics the geometry of the fault; the highest velocities occur close to the fault and in the direction of the spreading. The nature of the subsurface has an influence on the velocities as well, but this factor is much less important here than it is for the acceleration. Large-scale damage and damage to elastic structures are usually correlated with high velocities. The largest PGV ever measured was 183 cm/s. For small earthquakes (magnitude below 3) humans will mostly feel the acceleration (Wu et al., 2003). The values can be displayed on PGA or PGV maps, also referred to as shake maps (main source: Earth and Space Sciences, University of Washington).
The peak horizontal ground movement during an earthquake can be displayed on so-called shake maps. They either refer to the peak ground velocity or the peak ground acceleration. For small earthquakes (magnitude below 3) humans will mostly feel the acceleration (Wu et al., 2003).
These measuring stations are designed specifically to record large ground movements which cannot be recorded without distortion by “normal” seismometers due to technical constraints (overdrive).
The Universal Time Coordinated (UTC) is the internationally valid world time. The universal time UT follows the variations of the Earth’s rotation by adjusting the length of the time unit; the UTC on the other hand achieves the same by introducing intercalary seconds, leading to the time units always being the same as the SI units used by the international atomic clock.
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Haering, M.O., 2004, Haering Geoprojekt
Hanks, T.H., Kanamori, H., 1979. A moment magnitude scale. JGR, 84, B5, pp 2348-2350
Kastrup, U., Erdbeben. Ein Szenario als Teil des Szenarien- und Expertenpool Risikoanalyse Schweiz, ETH Zürich, pp 8, 2002
Kastrup, U., Seismotectonics and Stress Field Variations in Switzerland, Dissertation ETH Zürich, 2002
Maurer, H.R., Burkhard, M., Deichmann, N., Green, A.G., Active tectonism in the Central Alps; contrasting stress regimes north and south of the Rhone Valley. Terra Nova, 9; 2, 91?94, 1997
Meghraoui, M., Delouis, B., Ferry, M., Giardini, D., Huggenberger, P., Spottke, I., Granet, M., Active Normal Faulting in the Upper Rhine Graben and Paleoseismic Identification of the 1356 Basel Earthquake, Science, 293: 2070-2073, 2001
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Schweizerischer Erdbebendienst, ETH Zürich
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Wu, Yih-Min, Teng, Ta-liang, Shin, Tzay-Chyn, and Hsiao, Nai-Chi, 2003. Relationship between Peak Ground Acceleration, Peak Ground Velocity, and Intensity in Taiwan, BSSA; February 2003; v. 93; no. 1; p. 386-396