Physical Processes in Earth and Environmental Sciences Phần 7

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Physical Processes in Earth and Environmental Sciences Phần 7LEED-Ch-04.qxd 11/26/05 14:08 Page 190 190 Chapter 4 4.17.6 Earthquakes and strain slip). The remainder dissipates over weeks or months by aftershocks as smaller and smaller roughness elements on the fault plane shear past each other until all the strain In the introduction to this chapter we noted that energy is released. If the fault responsible breaks the earthquakes were marked by release of seismic energy Earth’s surface then the coseismic deformation is that along shear fracture planes. This energy is released partly measured along the exposed fault scarp whose length may as heat and partly as the elastic energy associated with rock reach tens to several hundreds of kilometers. compression and extension. In the elastic rebound theory Different types of faults give rise to characteristic first of faults and earthquakes the strain associated with tec- motions of P-waves and it is this feature that nowadays tonic plate motion gradually accumulates in specific zones. enables the type of faulting responsible for an earthquake to The strain is measurable using various surveying be analyzed remotely from seismograms, a technique known techniques, from classic theodolite field surveys to as fault-plane solution. Previously it was left to field surveys to satellite-based geodesy. In fact, the earliest discovery of determine this, often a lengthy or sometimes impossible task. what we may call preseismic strain was made during inves- The first arrivals in question are those up or down peaks tigations into the causes of the San Francisco earthquake measured initially as the first P-wave curves on the seismo- of 1906, when comparisons were made of surveys docu- gram record (Fig. 4.142). It is the regional differences in the menting c.3 m of preearthquake deformation across the nature of these records caused by the systematic variation of San Andreas strike-slip fault. We have already featured the compression and tension over the volume of rock affected by results of modern satellite-based GPS studies in decipher- the deformation that enables the type of faulting to be deter- ing ongoing regional plate deformation in the Aegean area mined. This is best illustrated by a strike-slip fault where of the Mediterranean (Section 2.4). All such geodetical compression and tension cause alternate zones of up (posi- studies depend upon the elastic model of steady accumu- tive) or down (negative) wave motion respectively as a first lating seismic strain and displacement. But then suddenly arrival wave at different places with respect to the orientation the rupture point (Section 3.15) is exceeded and the of the fault plane responsible (Fig. 4.142). When plotted on strained rock fractures in proportionate or equivalent mag- a conventional lower hemisphere stereonet (Cookie 19), nitude to the preseismic strain. This coseismic deformation with shading illustrating compression, the patterns involved represents the major par ...