General Information on Earthquakes by danman21

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									European Commission
DIRECTORATE GENERAL JRC
JOINT RESEARCH CENTRE
Institute for the Protection and Security of the Citizen
Technological and Economical Risk Management Unit
Integration of Information for Risk and Emergency Management Sector




General Information on Earthquakes

           Prof. Boyko Ranguelov
       Bulgarian Academy of Sciences
            Geophysical Institute


             Visiting Scientist at JRC
                    June 2003
Earthquakes: Generalities                                                    R. Boyko

EARTHQUAKES – GENERAL INFORMATION

 The earthquakes are natural disasters generated by the seismic waves emitted as a result
 of elastic rebound in case of destruction of the earth’s medium. This effect is produced
 by the stress accumulation due to the internal Earth tectonic processes generated by the
 plate movements. When the stress over go the certain limit of the strength of the
 materials building the earth crust an earthquake occur. The earthquakes are the clear
 expression of the most recent geodynamics of the planet. They are fast, immediate, non
 predictable in time, sudden and sometimes strong and very destructive. They are
 located around the plate boundaries, subduction zones, transform faults, rift zones and
 other elements described by the plate tectonic theory. The biggest structures controlling
 the earthquakes are called seismic belts. The biggest ones are the Pacific belt, the Alpo-
 Himalayan belt, etc. According the depth, the earthquakes are divided to crustal,
 intermediate and deep.
 The statistics shows that on the whole planet per year more than 20-30 cases strong and
 destructive earthquakes occurred.

 Earthquake parameters definition
 The starting point of the area where the destruction is going on is called “hypocenter”.
 Its projection on the earth surface is called “epicenter”. The main parameters of each
 earthquake are – coordinates, depth, magnitude and time of occurrence (origin time).
 All these parameter can be defined using the seismographs - devices recording the
 earthquakes. Using the seismograms (seismic records recorded by seismographs) the
 epicenter (hypocenter) coordinates, depth, magnitude and origin time can be
 determined.
 To measure the strength of an earthquake two main methods are applied - magnitude
 determination (Richter’s scale) and intensity (macroseismic scales). Both methods are
 independent and not transferable.

 Magnitude scale
  The magnitude is a value introduced by Richter and Gutenberg in 1932 and has no
  clear physical meaning. It’s just a conditional value connected with the recording
  device (seismograph – in Richter’s case – “Wood-Anderson” short period, vertical
  component), the amplitude on the seismogram (in such case – 1mm.) and the
  epicentral distance (in the Richter’s case – 100 km.).
  Than the estimated by this way magnitude is equal to 1.

   The scale is logarithmic, which means that the magnitude M can be calculated by the
   formulae:

   M= lg A/T+s(d) , where

   A – is the amplitude measured on the seismogram, T – is the respective period of this
   amplitude, and s(d) is called calibration function which depends on the epicentral
   distance and is calculated by Richter and Gutenberg for the different epicentral
   distances.



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Earthquakes: Generalities                                                     R. Boyko

   Such definition shows that the magnitude scale is opened by both ends – smallest and
   biggest magnitudes. The magnitude shows the size of the emitted seismic energy by
   the earthquake source. There are different magnitudes derived by the different seismic
   waves recorded on the seismograms and the duration of the seismic record – “body
   waves (P and S) magnitude” called Mb, “surface magnitude” called Ms, “duration
   magnitude” called Md. The strongest seismic events frequently lead to so called
   “saturation of the magnitude scale” and in such cases the Mw – called “moment
   magnitude” is used. Very often the different magnitudes calculated for one and the
   same earthquake does not coincide. There are many relationships connecting the
   different types of magnitudes.

 Intensity scales
   These scales are descriptive and describe the effects of the earthquakes on the earth
   surface and the effects generated by them. They are descriptive and have several
   degrees. Different countries use different macroseismic (intensity) scales. In the
   former eastern countries the MSC-64 scale was in use. In the USA – the scale is
   called MM (Mercali Modified). It is important to mention that a single earthquake
   have different degrees due to the effects of the earthquake on the earth environment,
   structural and building consequences and the people feelings.
   Now – the unified EMS (European Macroseismic Scale) is introducing in a whole
   common Europe.

 The location
  Earthquakes do not occur everywhere on the planet Earth. They can be expected in so
  called “seismic belts” – Pacific, Alpo-Himalayan, etc. Usually the seismic belts are
  located near and around the continental plate boundaries, where subduction,
  transform faults and rifting are expressed. Sometimes (but rare) earthquakes occur in
  the plates and are mentioned as “intraplate earthquakes”.

 The consequences
  The consequence of each earthquake depends on the effect that it produces on the
  earth surface – destruction or damages of the buildings and other structures (like
  bridges, dams, roads, railways, pipe lines, etc.), liquefaction, earth cracks, landslides,
  avalanches, tsunamis, etc. Often earthquakes generated as secondary effects: fires,
  poison and chemical pollutions, flood, etc. as a result of the combined actions of the
  earthquake waves and the conditions in which the environment exist during the
  quake.
  Liquefaction appears in soft soils with high level of underground waters.
  Landslides occur on slopes with calm or active landslides.
  Earth cracks generated in the epicentral area of strong (M>6) crustal earthquakes
  Avalanches slide in case of snow-covered mountains.
  Tsunamis are generated in case of underwater strong earthquakes.
  Fires – usually appear in houses (from gas or other fuel inflamed), in factories, in fuel
  stations, due to the electricity sparks, pipelines destruction, etc.
  Flood can be observed in case of water supply system destruction




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Earthquakes: Generalities                                                    R. Boyko

   Poison and chemical pollution – in case of destruction of the reservoirs or pipelines of
   such substances.
   Communication links disturbances – in case of communication links destructions due
   to the different reasons.
   Underground and surface collapses – in old or acting open or underground mines,
   caves, metropolitan areas.

 The prediction
  It is well known fact that the recent science cannot predict the exact time of the strong
  earthquakes. This is due to the so-called, no uniqueness of the precursors. It means
  that no mutual connection between the observed before earthquakes anomalies in the
  different geophysical fields and the occurrence of the earthquakes. Very often the
  anomalies have been observed and no earthquake occurred, and vice versa. No
  anomalies observed and the earthquakes occurred. This is valid for so called short-
  term prediction. In the long-term cases the places and the maximum expected
  magnitudes are known and a long-term prediction can be done. There are two main
  scientific hypotheses about the long-term prediction – so called “seismic cycle” and
  so called “seismic episodes”. In first case (developed mainly by the Russian and
  American specialists) the no exact (but quasi) periodicity exist in the strong
  earthquake occurrence. According the second hypothesis mainly developed by the
  China specialists the strong earthquakes occurred like clusters that are random in time
  and no cycle can be observed during the written history of the mankind. So, in some
  particular cases the long-term prediction (about tens of years) is possible (some cases
  of successful long term predictions have been reported in the former USSR, USA and
  Japan).

 The prevention
  If the prediction is not possible, only several preventive measures can be done.
  The                                  antiseismic                             measures.
  They are applicable to the buildings and constructions increasing their resistance to
  the seismic loading during the earthquakes. Usually these measures are defined in the
  codes and rules in the countries prone to the earthquakes. The constructive measures
  include strengthening the basements, walls and roofs. Sometimes so-called “seismic
  isolation” is applied. The method performs special constructive devices incorporated
  in the building construction able to decrease the seismic loading, by dumping or
  absorption of the seismic waves energy.
  The correct urban planning is also a simple measure to decrease the consequences of
  the earthquakes. For this purpose special seismic microzonation is performed to areas
  prone to the strong earthquakes.

   Another preventive measures are connected with the education of the people - what to
   do to decrease the personal risk – so called protective measures.

 The protective measures
  Usually the instructions about the people behavior for destructive earthquakes are
  general, never more specific due to the general view that the affections are the same.



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Earthquakes: Generalities                                                     R. Boyko

   It must be underlined that non-destructive earthquakes have in general psychological
   effects – panic, scare, etc.
   The destructive earthquakes affected local area. They are strongly felt and produced
   significant effects and heavy destructions in the near epicentral vicinity. Of course the
   distances where the destruction can be observed depend on the magnitude of the
   earthquake but the destructive effects rarely covers radius more than 100 km. In
   dependence of intensity the effects can be rather bad.
   So the protection measures in case of destructive earthquake (the worst case) are as
   follows:

     Before the quake (seismically calm period):

     Full information about the seismic environment has to be disseminating to the
     population, decision-makers and Civil Defense. It means – expected seismic
     influence from all seismic sources around, possible effects and destructions.
     Personal attention to the possible expected situations – at home, at the office, on the
     streets and in public places in case of a strong earthquake occurrence. Brain training
     of the activities intended for performance. Find the most safe places at home, office,
     etc. Spend sometime for a scenario in your mind “how to act if strong earthquake
     starts now”.
     Tight upgrading of all heavy facilities at home and in the office. This protects our
     life security. Don’t put heavy furniture near to the doors at home!
     Special attention paid to the vessels containing inflammables, poisons and other
     dangerous liquids and/or explosives.
     Preparation of a personal “food and water” package containing: food, water, most
     necessary medicines, documents, money.

     During the Earth’s tremors

     Don’t panic!!!
     Stay at the safest place in the building (you must justify it in the earlier stage). Do
     not stay near the windows or other weak places.
     If you can leave the house for 3-5 seconds – do it as fast as you can. This situation
     can exists if you are at the first floor and near the doors. If you’re at the higher
     building – stay inside and protect yourself if the things start to fall. Stay under
     heavy furniture (beds, big tables, etc.)
     In a public place – cinema, restaurant, theater, etc. do not panic. Protect yourself if
     possible. Go out fast but not in panic. Children, women and disabled - first.
     Don’t use lifts
     After the vibrations stop – go out. Expect new tremors.
     If you are in the transport – it must stop and than act as in case of a public place.
     On the streets – move to the free spaces – far from the walls, electric lines, high
     buildings.




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Earthquakes: Generalities                                                      R. Boyko

     After the quake:
     Left your place, switch off the gas, electricity and water supply and take your “foot
     and water” package.
     Give first aid the affected people. Mark places where you are sure have some buried
     people.
     Wait the instructions of the Civil Defense and other authorities. Do not evacuate
     yourself. It can create difficulties afterward searching you.
     It is good if you have preliminary justified place where the information of all
     relatives and familiar people can be collected.
     Do not use your cars, telephones and mobiles after the shock. It can create
     difficulties for the rescue teams.
     Follow the instructions of the Civil Defense and other authorities about the help –
     food, water, tents, etc.
     Keep the hygiene at a high level. The possibilities of the epidemics increased
     rapidly.
     For the destructions after the quake and rescue operations – ask the local authorities
     about the reimbursement and the financial help.
     Write all you see and thing is interesting. It might be useful afterwards.

     The rescue operations
     The rescue operations are effective during the first several days after the quake.
     That’s why the fast rescue actions are most important. The local people and local
     structures of the Civil Defense authorities provide the first help. They are doing first
     rescue operations and marked the bigger collapsed structures. The communications
     with the central authorities of Civil Defense are essential, for the better
     coordination. The world statistics shows that the effectiveness decrease sharply with
     the time…The most successful are the operations preformed to the next three days
     after the quake. The next days’ survivors are exceptional.




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