FIGURE 5.6 Left two panels show map view of the Northridge mainshock and aftershocks as filled octagons, with seismic stations as triangles. (a) Vertical component velocity seismograms of the initial P waves of the Northridge mainshock. SMF is a high-gain record that quickly clips, while the other traces are artificially clipped to simulate limited dynamic range recording and to show that the onset of the first P wave at this scale is abrupt. (b) Low-gain recordings of the initial P waves of the Northridge mainshock at the same sites. In each case the arrow indicates the first arriving waves shown in the upper panel. The onset, though abrupt, remains weak until about 0.5 second into the mainshock. Similar behavior is observed before other earthquakes. (c) The duration of the weak initial onset, or seismic nucleation phase, for earthquakes from several studies indicates that the seismic nucleation phase (vertical axis) scales with the seismic moment (horizontal axis) of the mainshock. SOURCE: G.C. Beroza and W.L. Ellsworth, Properties of the seismic nucleation phase, Tectonophysics, 261, 209-227, 1996. With permission from Elsevier Science. (c)

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