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sent northwestwards out into sparsely populated country. The time between wave crests for the strongest waves
was 1–3 seconds, which are the wave times most liable to cause taller buildings to collapse. But since these waves were directed out into the country where there are no high- rise buildings, the threat to downtown areas was small.
The earthquake affected an area of 4000 square kilometres, lifting the San Fernando Valley and mountains in a slight dome, with the greatest lifting of about 40 centimetres being where the fault plane comes closest to the surface.
When an earthquake occurs, the stress is relieved
in the rocks that move; but places nearby that were not previously under stress may become stressed as a result. They, in turn, may fail, causing further earthquakes. This stress is the reason for the many smaller earthquakes (called aftershocks) that are felt after a large earthquake. Aftershocks may occur for many years following a major event.
The pattern of aftershocks that occur within the hours, days and months after a large earthquake follows a logarithmic rule: every large magnitude shock will be
(Below) The pattern of earthquakes recorded over a
long period of time clearly picks out the edges of the tectonic plates. Notice that the depth of earthquakes varies across the world. In general, earthquakes are shallow where continents collide or where ocean plates pull apart. But they are much deeper where ocean plates are pushed below thick continents, as, for example, around some parts of the Pacific Ocean.
Depth of earthquake
0–69 km 70–299 km 300–800 km
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