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part of California that geologists know as the Transverse Ranges. The famous San Andreas Fault runs to the east of the city, and the main fault system close to Los Angeles is called the Oak Ridge Fault System.
The Northridge earthquake struck Los Angeles at 4.30 a.m. on 17th January 1994. The epicentre of the earthquake at Northridge, just north of Los Angeles, experienced a shock of magnitude 6.7. The effect of
the shock was to cause devastating damage rather
than huge loss of life, and so was typical of the way
in which earthquakes affect modern industrial cities. Fifty- seven people were killed by the earthquake, and 9000 were injured, a very small number compared to the area population of over fifteen million people. But at the same time, the amount of destruction was large.
This was only a moderate-sized earthquake whose ground-shaking lasted between 10 and 20 seconds.
The main effects were to cause buildings to collapse, highway bridges to fall down, and water, gas and electricity supplies to be cut.
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Northridge
(Above and below) The Northridge earthquake produced quite different kinds of tremors even over a short distance, as the seismograms below show.
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(Below) The landscape of the western Los Angeles Basin.
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A Los Angeles
The initial rupture of the ground that caused the Northridge earthquake lay about 17.5 kilometres below the surface. The rupture then extended itself, like a
zip being undone, growing northwestwards at about
3 kilometres per second for 8 seconds. It also spread upwards, so that when it stopped, the end of the rupture zone was just over 5 kilometres below the surface. This, like the other earthquakes in the
past in this area, occurred on a
‘blind’ thrust fault, meaning that
it was a dead-end spur, not part
of a continuous fault system.
In fact, major shocks came from three distinct places on the rupture zone, producing overlapping shock waves.
The waves were larger in the direction of the rupture,
which in this case meant that the most severe waves were
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