Earthquake waves

An earthquake, the most destructive of natural phenomena, consists of rapid vibratory motions of rock near the earth's surface. A single shock usually lasts no more than a few seconds, though severe quakes may last for as much as three min; even in such brief times the damage done may be immense. The rapidity of the vibrations rather than the actual displacements involved is responsible for the damage. Man-made structures are shaken to pieces if they are too rigid to follow the back-and-forth motions of the underlying rock, and landslides are common. Widespread fires frequently follow earthquakes in inhabited regions since broken water mains hinder their control. But there is one useful feature of earthquakes: by studying the waves they send оut, it is possible to infer a surprising amount about the nature of the earth’s interior.

Earthquakes

Earthquakes occur without warning. Usually the first shock is the most severe, with disturbances of lessening intensity following at intervals for days or months afterward. A major earthquake may be felt over many thousands of square kilometers, but its destructiveness is limited to a much smaller area.

The great majority of earthquakes are caused by the sudden displacement of crustal blocks along faults. A fault is the scar left by a fracture which occurred when the stresses developed within the crust became too great for the rock to support. Additional stress may accumulate to the point where further slippage takes place, and this slippage in turn sends out the shock waves that are characteristic of an earthquake. The event responsible for an earthquake typically involves an area within the crust some tens of kilometers across located within a few kilometers of the surface, but in a fair number of cases depths of up to several hundred kilome­ters below the crust have been established. The place where an earthquake originates is called its focus, and its epicenter is the point on the earth's surface directly above the focus.

Sensitive instruments called seismographs have been devised which re­spond to the vibrations of even distant earthquakes. Seismographs of different types are needed to respond to vertical and horizontal movements. A vertical seismograph and two horizontal ones, one for the north-south direction and the other for the east-west direction, are needed at each observatory. Several hundred seismological stations are in operation around the world, and the data they obtain are routinely compared and correlated. It is possible to establish from such data where the focus of a given earthquake is located and something about how much energy it has released.

Earthquake severity is usually expressed on the Richter scale, which is based upon the maximum amplitude of an earthquake's vibration. Each step of 1 on this scale represents a change in vibrational amplitude of a factor of 10 and a change in energy release of a factor of about 30; thus an earthquake of magnitude 5 produces vibrations 10 times larger than one of magnitude 4 and evolves 30 times more energy. An earthquake of magnitude 0 is barely capable of being detected, and the energy released, if it could be concentrated, is just about sufficient to blow up a tree stump. An inhabited area will suffer some damage if a magnitude 4.5 quake occurs nearby, and one of magnitude 6 or more may lead to significant destruction. The energy associated with a magnitude 6 earthquake is equivalent to that of a medium-size nuclear bomb, though its effects are different because the earthquake energy is spread out over a much wider area. The energy released in a magnitude 8.6 earthquake, the greatest that have occurred to date, is about double the energy content of the coal and oil produced each year in the entire world; the Alaska earth­quake of 1964 was of nearly this magnitude.

Of the million or so earthquakes per year strong enough to be experienced as such (that is, of magnitude 2.5 or more), only a small proportion liberate enough energy to do serious damage to man-made structures. About 15 really violent earthquakes (magnitude 7 or more) occur each year on the average, and only 9 of magnitude 8.4 to 8.6 have occurred since 1899. Regions in which severe earthquakes are comparatively frequent include the mountain chains that fringe the Pacific and a broad belt extending from the Mediterranean basin across southern Asia to China. Major earthquakes have occurred sporadically elsewhere, but by far the greatest number has been concentrated in these zones. In or near the earthquake belts lie most of the world’s active volcanoes – which is no coincidence.

Ex. 3. Answer the following questions.

1. What does an earthquake consist of?

2. How long does a shock last?

3. What is responsible for the damage?

4. What frequently follows earthquakes in inhabited regions?

5. Is there one useful feature of earthquakes?

6. Do earthquakes occur without warning?

7. What are the great majority of earthquakes caused by?

8. What is the focus of an earthquake and its epicenter?

9. What kind of instruments have been devised?

10. On what scale is earthquake severity usually expressed?

Ex. 4. Look at the first paragraph again and try to explain the following:

1. back-and-forth

2. landslide

3. water mains

Ex. 5. Look at paragraphs 2 and 3 and say which words have the same meaning as:

1. as a general rule

2. situated

3. quite a lot

Ex. 6. Look at paragraphs 2 and 3 again and try to explain the following:

1. square kilometer

2. the difference between distruction and distructiveness

3. stress

4. crust

Ex. 7. Look at paragraph 4 and say what words could replace the following:

1. as a matter of course

2. information

3. set free

4. get

5. far away

Ex. 8. Look at paragraph 5 and try to explain the following:

1. amplitude

2. magnitude

3. evolves

4. factor of 10

Ex. 9. Look at paragraph 6. Can you produce sentences that show you know the meaning of the following words as used in the text?

1. man-made

2. severe

3. fringe

4. sporadic

5. zone

6. belt

Ex. 10. Translate the text into English using the vocabulary of the Unit.

Изменения в очертаниях материков и океанов вызываются медленными движениями земной коры, которые не связаны ни с землетрясениями, ни со сбросами. Эти медленные движения земной коры связаны с проявлением внутренней энергии земного шара. Огромные пространства материков, включая и области, в которых живут люди, не остаются неподвижными, а медленно опускаются или поднимаются. На такие медленные движения земной коры указывают также коралловые рифы, которые были подняты выше уровня моря. Древнейшие уральские горы растут. В некоторых местах скорость поднятия Урала достигает 5-6 миллиметров в год. К таким выводам пришли ученые сейсмической станции «Свердловск». Станция оборудована самыми современными приборам. Около двух тысяч землетрясений, происходящих на земном шаре, ежегодно регистрируется станцией. За многими землетрясениями следуют сравнительно небольшие движения земной коры. В результате таких медленных движений коры одни гавани в Японии и других местах медленно углубляются, другие мелеют. Некоторые стали такими мелкими, что в них не могут больше заходить суда. Вековые колебания суши за время многих тысячелетий очень сильно изменяют очертания материков. При поднятии суши и увеличении ее площади многие острова могут соединяться с материком. При опускании суши и затоплении ее морем материки могут распадаться на части, что приводит к образованию новых островов. Несколько тысячелетий назад остров Великобритания был частью материка, а затем от него отделился. Многие места, где сейчас находится суша, когда-то были дном моря.

to cause to connect fault to relate to manifestation to inhabit stable conclusions to arrive at to equip to follow to enter