The time variation of crustal velocities in tectonic regions is most reasonably attributed to stressinduced variations in crack porosity. The decrease in V•,/Vs before arthquakes is due primarily to a large decrease in Vp. This supports the Nur dilatancy hypothesis but not the effective stress hypothesis. New data from the San Fernando region verify the Vp drop, show that this drop cannot be en...

Seismologists, like mathematicians, are accustomed to dividing their subject into a pure and an applied branch. The desirability of such division is underlined by the manner in which non-seismologists frequently approach the subject. I t is too often taken for granted that, because an earthquake is a motion of the ground, seismology must be chiefly concerned with the detailed analysis of that m...

range and the location of artificial reflectors or intermediate values at these points. A similar fictitious sources inherent in discrete wave economy can be obtained using the itrerative reflection and transmission coefficient relations number techniques. The number and spacing of wave numbers in wave number integration schemes are of Kennett. Kamel and Felsen (1981) presented a hybrid determi...

The history of seismology has been traced since man first reacted literarily to the phenomena of earthquakes and volcanoes, some 4000 yr ago. Twentysix centuries ago man began the quest for natural causes of earthquakes. The dawn of modern seismology broke immediately after the Lisbon earthquake of 1755 with the pioneering studies of John Bevis (1757) and John Miehell (1761). It reached its pin...