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followed by ten shocks that are a tenth as powerful, a hundred that are a hundredth as powerful, and a thousand that are a thousandth as powerful.
Patterns of earthquakes
It is now possible to monitor the pattern of earthquakes across the world. The maps of earthquakes clearly show that in any month, for example, there are some small and some big earthquakes. The maps also show that there are some places where earthquakes are common, and others where they are rare.
The map on page 15 shows that more large earthquakes occur around the edges of Asia than anywhere else, but that there are also important clusters along the western coast of the Americas. On the European continental mainland there are very few. To the east of Asia the pattern of earthquakes follows a series of curves, or arcs, each bulging to the east.
Scientists now know that the most common places for earthquakes to form are at the boundaries of the dozen or so large slabs of the earth’s crust known as the tectonic plates. (For a full description
(Below) The shocks associated with
the Northridge earthquake are shown on this diagram.
An earthquake is a rupture process that expands from an initial point on a fault plane called the hypocentre. The Northridge earthquake ruptured the fault plane for about 8 seconds and
had an average slip across the plane of about 1 metre. The rupture extended across the plane northwesterly from the hypocentre at about 3 km/s. However, some parts of the plane exhibited little or no movement, and some parts slipped more than 3 metres.
The red circles show the locations of the thousands of small aftershocks that followed the initial rupture. The presence of the older faults (blue lines) across the path of the Northridge rupture may have stopped the initial rupture from going any further.
There were a thousand aftershocks each day in the first month following the rupture in January 1994, and even by January 1996 there were still a few each day. Of the aftershocks, nine were greater than magnitude 5.0, and 42 had a magnitude of more than 4.0.
0
–5
–10
San Fernando Valley
Active fault line
S
Northridge
Finish of rupture
Santa Susana Mountains
Santa Clara Valley N
Old fault lines
–15
Start of
–20
rupture
0 5 10 15 20 25 30 35 40 Distance (kilometres)
Depth (kilometres)