Glossary
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EARTHQUAKE
- EARTHQUAKE
- Earthquakes are ever present phenomena which cannot be prevented nor forecast. Their impact can be mitigated by constructing seismically resistant buildings, by responsible site selection
for strategic installations (such as nuclear power plants, dams, deep ground repositories) and by informing the public and training them in effective behaviour during an earthquake.
An earthquake is caused by a sudden release of stress in the Earth’s crust, resulting in a swift movement of rock blocks along a fault. The released energy is radiated in all directions
in the form of seismic waves and after reaching the surface it causes the ground and all objects on it to shake. The strongest and most frequent earthquakes are concentrated along boundaries
of tectonic plates, approximately ten huge blocks forming the Earth’s surface. These blocks are in constant relative motion and collision. The motion is not smooth and when the blocks are
stuck, stress accumulates. This can last for hundreds of years. The bigger the finally released stress, the bigger is the resulting earthquake.
Less frequent are earthquakes accompanying volcanic activity or caused by collapsing underground cavities. - SEISMOLOGY, SEISMOLOGIST
- Seismology is the science concerned with earthquakes. Seismologists study earthquakes both through recordings of seismic waves by instruments and by analysing information about their impacts on buildings and structures, and observations of earthquakes by people, known as macroseismic effects. In the past, before instruments for recording seismic waves were introduced (approx. till 1900), the reports of felt effects and damage were the only data available for evaluation of earthquakes.
- MAIN SHOCK, AFTERSHOCK, FORESHOCK, SEISMIC SWARM
- Big earthquakes are usually followed by a number of weaker shocks from approximately the same place, called aftershocks. Sometimes a weaker earthquake may occur at first, known as a foreshock.
The strongest earthquake of a series is called the main shock. The number, size and duration of an earthquake series depends generally on the size of the main shock. The stronger it is, the longer
it takes for the region to calm down. In really big earthquakes this can take several months.
A special case is a seismic swarm, when periodical occurrences of a large number of earthquakes (up to thousands or tens of thousands) in series are observed in an area. Most of these are very weak, without a distinct strong main shock at the beginning. A typical example of a region with frequent occurrences of seismic swarms is the area around Nový Kostel in the Cheb region in West Bohemia, where most of the earthquakes felt by people in the Czech Republic are also located. - TSUNAMI
- A series of ocean waves generated by a sudden displacement of a great volume of water by a shallow offshore earthquake. Their height on the shore can rise up to several dozen metres and they can reach several kilometres inland. The most devastating tsunami waves in the last 100 years were triggered by earthquakes in Indonesia in 2004 and in Japan in 2011.
- HYPOCENTRE
- The point where the earthquake originates. Its depth is a determining factor for the intensity of shaking on the surface. Generally, shallow earthquakes (depths up to first dozen km) are more devastating but with lesser areal extent, while deep ones (up to hundreds of km) are felt at much longer distances but their impacts are less devastating.
- EPICENTRE
- The place on the Earth's surface directly above the hypocentre. Here the effects of the earthquake are usually the strongest; with growing distance they are attenuated.
- SEISMOMETER
- An instrument used to record ground motion. In the past, various forms of analogue recorders were the only possibility, nowadays digital instruments are used, which allow the detection of even very weak earthquakes and data analysis is much more precise. Seismologists commonly use seismometers which record ground velocity (velocity seismometers). For the recording of large earthquakes near the epicentre special instruments recording ground acceleration were designed (accelerometers, strong motion seismometers).
- SEISMOGRAM
- A record of ground motions by a seismometer. It used to be a roll of paper. Nowadays, with digital data, recordings from all available seismic stations can be simultaneously displayed on a computer screen. Seismologists can detect onsets of individual seismic waves and compute the location of the hypocentre and time of origin of the earthquake. The size of the earthquake, its magnitude, is computed from maximum amplitudes of recorded seismic waves.
- SEISMIC STATION
- A site where the seismometer and other instrumentation are placed. The station can be a permanent one, recording at the same site for many years, or a temporary one. Temporary seismic stations are usually installed near the epicentre after a big earthquake for detection of weaker aftershocks and their improved location. Data from seismic stations are nowadays often transmitted in real time via modern communication channels to data processing centres.
- NATIONAL SEISMIC NETWORK
- Permanent seismic stations are usually parts of national and international seismic networks. These networks are often virtual and interconnected, agencies share their data. For instance, the Czech Regional Seismic Network uses more than 20 stations in the Czech Republic and many others from neighbouring countries. Automatic data evaluation can be done in real time. Even though such interpretations are often not very precise or may even be completely false, they can give an early warning about a possibly devastating earthquake. Automatic evaluation should be manually checked and corrected by a seismologist. An early warning can be immediately sent to the respective authorities and emergency management.
- LOCAL SEISMIC NETWORK
- A network of several seismic stations situated in a relatively small area. They usually have a specific task, such as detailed seismic monitoring of a strategic site (e.g. a nuclear power plant), for detecting very weak earthquakes unrecognisable by more distant stations. This is important for understanding the seismic mode of the area and evaluating the possible occurrence of any stronger earthquakes. Other types of seismic networks monitor tremors induced by mining or by large water reservoirs. Local seismic networks in the Czech Republic monitor both national nuclear power plants, the mining district in Ostrava or the seismically active swarm area in the Cheb region in West Bohemia.
- MEASURING EARTHQUAKES
- The size of an earthquake can be described by two quantities - magnitude and intensity. Each of them measures different characteristics of an earthquake. Magnitude corresponds with the energy released in the hypocentre of the earthquake. Intensity is determined from effect of the earthquake on people, buildings and the natural environment in a certain location. There are many intensities for an earthquake depending on where you are, unlike magnitude, which is one value for each earthquake.
- MAGNITUDE
- Magnitude is determined from the maximum amplitude in the seismogram. The relation is logarithmic, so an earthquake with magnitude greater by one generates amplitudes 10 times higher. Several types of magnitude have been defined; the most commonly used is the Richter magnitude. In the epicentral area you can usually feel an earthquake with magnitude around 2, or even less in specific cases with very shallow hypocentres. Weaker tremors are only recorded by instruments. Earthquakes with magnitude over 6 can be damaging up to a distance of 100 km from the epicentre (depending strongly on local conditions). The largest earthquake recorded so far was of magnitude 9.5 (Chile 1960).
- INTENSITY
- Intensity classifies the effects of the earthquake in a location. Several intensity scales exist. The latest and the most detailed one is the I - XII ranging EMS-98 scale,
which was developed by the international Working Group of the European Seismological Commission (ESC). Older ones used in the Czech Republic were MSK-64 (Medveděv-Sponheuer-Kárník) or MCS (Mercalli–Cancani–Sieberg).
Each intensity level is characterised by a group of effects on humans, buildings and on the environment (macroseismic effects). The highest intensity is usually observed in the vicinity of the
epicentre, it is the epicentral intensity. This value is used to describe historical earthquakes from the pre-instrumental period. The conversion between epicentral intensity and magnitude is not
easy. It depends on a number of factors, such as the depth of the hypocentre and local conditions.
Short description of the EMS-98 macroseismic scale Intensity Definition Description of effects I. not felt Not felt. II. scarcely felt Felt only by very few individual people at rest in houses. III. weak Felt indoors by a few people. People at rest feel a swaying or light trembling. IV. largely observed Felt indoors by many people, outdoors by very few. A few people are awakened. Windows, doors and dishes rattle. V. strong Felt indoors by most, outdoors by few. Many sleeping people awake. A few are frightened. Buildings tremble throughout. Hanging objects swing considerably. Small objects are shifted. Doors and windows swing open or shut. VI. slightly damaging Many people are frightened and run outdoors. Some objects fall. Many houses suffer slight non-structural damage like hair-line cracks and fall of small pieces of plaster. VII. damaging Most people are frightened and run outdoors. Furniture is shifted and objects fall from shelves in large numbers. Many well built ordinary buildings suffer moderate damage: small cracks in walls, fall of plaster, parts of chimneys fall down; older buildings may show large cracks in walls and failure of fill-in walls. VIII. heavily damaging Many people find it difficult to stand. Many houses have large cracks in walls. A few well built ordinary buildings show serious failure of walls, while weak older structures may collapse. IX. destructive General panic. Many weak constructions collapse. Even well built ordinary buildings show very heavy damage: serious failure of walls and partial structural failure. X. very destructive Many ordinary well built buildings collapse. XI. devastating Most ordinary well built buildings collapse, even some with good earthquake resistant design are destroyed. XII. completely devastating Almost all buildings are destroyed. - WHAT TO DO DURING AN EARTHQUAKE
- The Czech Republic is situated in a seismically quiet part of Europe, where felt earthquakes are quite rare and usually cause no great damage or fatalities. Nevertheless Czech citizens can
encounter stronger earthquakes when travelling abroad to many popular destinations, such as Greece, Turkey, Italy or the Balkans and to more distant ones for instance Indonesia, China, Nepal,
Japan or California.
The risk is smaller in economically strong countries, where seismically resistant buildings have recently been built. In regions with weak constructions, more vulnerable to earthquake shaking, even a relatively smaller earthquake may cause a massive disaster with many fatalities (e.g. earthquake in Iran in 2003 with magnitude 6.6 and 31 thousand casualties). Earthquake prone countries train their citizens to protect themselves during an earthquake. People all over the world practice earthquake drills annually (http://www.shakeout.org).
Earthquakes come without prediction or warning. From the initial shaking you cannot tell how long and intense it will be. The appropriate action recommended by preparedness organisations is:
1. DROP to the ground (before the earthquake drops you!)
2. Take COVER by getting under a sturdy desk or table. Cover your head and neck, stay away from windows and possible falling objects.
3. HOLD ON to your shelter and be prepared to move with it until the shaking stops.
Expect aftershocks.
Don´t panic. Do not move to other locations or outside. You are more likely to be injured if you try to move around during strong shaking. Do not use elevators.
If outdoors, move to a clear area, avoid power lines, trees, buildings.
If in a car, pull over to the side of the road, stop and set your hand brake. Stay inside the car.
If near the shore, drop, cover and hold on until the shaking stops. If severe shaking lasts twenty seconds or more, immediately evacuate to high ground as a tsunami might have been generated by the earthquake. Move inland a few km or at least 30 m above sea level. - USED ABBREVIATIONS
UTC Coordinated Universal Time (the same as GMT - Greenwich Mean Time) CET Central Europe Time = UTC + 1 hour CEST Central Europe Summer Time = UTC + 2 hours NPP Nuclear Power Plant IPE Institute of Physics of the Earth, Masaryk University, Brno SUJB State Office for Nuclear Safety IG ASCR Institute of Geophysics, Academy of Sciences of the Czech Republic EMS-98 European macroseismic scale
SEISMOLOGY, SEISMOLOGIST
MAIN SHOCK, AFTERSHOCK, FORESHOCK, SEISMIC SWARM
TSUNAMI
HYPOCENTRE
EPICENTRE
SEISMOMETER
SEISMOGRAM
SEISMIC STATION
NATIONAL SEISMIC NETWORK
LOCAL SEISMIC NETWORK
MEASURING EARTHQUAKES
MAGNITUDE
INTENSITY
WHAT TO DO DURING AN EARTHQUAKE
USED ABBREVATIONS