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          Interior
          Earth's
          of
          Probes
          as
          Waves
          Seismic
              a  V,>Vs>Vp>V   Refraction   WN)   “eiope”      v   So   ee                               High   Apparent   Velocity        —s  Wavefronts  ===   —  RaypathS      that   ae  Models  and  travel-time  graphs  of direct  and  critically  refracted  rays  from  horizontal  interfaces  separating   7.24  FIGURE   ts  in   ae  b)  Series of  a  events  on  travel-time graph.  Fairly  thick  layers  result  in  distinct   a)  velocity  with  depth.  increase  ‘   2   sie  eit



              of   from   Earth,         the         as   the       of    of   the   the     of
             regions   the               and   and rigidity   used   be   graph   indicate   thin  layers,   inter-   traveled   inverse   function   arrivals   depth.   function   with   travel-time   from   velocity   repre-   sur-  curved   effects   accord-   sta-   raypaths   measure-   (inverse   slope   180°)   rays   infinite   the   of   Law

             different   distributions   of  regions   (Vp   Vs)   and   (k)   can   travel-time   that   very   velocity  higher   ray   the   3.34).  The   a   thus   path. The   first   with   a   as   horizontally)   the   distance   signifies   the   Fig.   in   7.24   Earth’s   the   plotted   recording   to   linear   because   velocity   inverse   the   =   (A   graph;   the  center   to   Snell’s


             traverse   density   of   state   velocities   modulus  bulk   Chapter  4   in   waves.  A   slopes  with   are  many,   deeper,   the  velocity   3.33,   (Figs.   is  graph   its   of   velocity   described   (travel   line  segments,   with   slope   thus   graphs   waves,   illustrate   7.25   are  times   epicenter   3   would  follow   curved,   apparent   0°)   ~   (A   Earth   the   travel-time   the   to   way   according




             S-waves   and   along   with   physical   Chapter   3:   k+ 4/3   p           shear-wave   Earth,   the   developed   body   segments   When   there   angles   for   to  relates   layer   travel-time   deepest   part   increasing   be   can   out   discrete   in   at-‘@ny   point   travel-time   earthquake   Fig.   in   depth.  Travel   earthquake   :   waves   be  would   how  Notice   epicenter   side   of   the   on   all   outward,


             P-   velocities,   the   in                  and   the   equations.   theory   of  earthquake   7.24a).   steeper   emergence   refracting   the   the   indicating   velocities   bottom   of   decreasing   curve   point;   and   paths   of   models   with   the   body.   graph   the   Near   opposite   zero   of   downward,   bend
         Seismology   which   at   seismic   interpret   to   discussed   Ver           compressional   within  point   the   Earthquakes   refraction   travel   the   refraciing   depth   (Fig.   emerge   at   of   critically   segments  on   encountered   on   curve.   thin   that   7.24a,b,  waves   7.24c).  Instead   gradually   of   slope   the   deepest   its   The  models   travel   The  account.   velocity   from  measured   velocity,   travel-time   surface.  curved   distan



        Earthquake   the  speeds   These   used   be   equations   the   known   a   for   from   for   The  Case   analyze   layers  shows   with   rays   angle  Each   of   top   the   for  any  of  the   line   wave   convex-upward   may   so   be   Figs.   (Fig.   curve,   of   the   encountered   at   Surface’   longer   For   into   increasing   constant   had   resulting   Earth’s   angular   velocity.   resulting   thus   gradually   Taypaths   the


             suggests   interior.   can   the   if   determined   Layer   to   velocity   refracted   7.24b).   the   at   the   As   in   depth   z   smooth   inverse   Earth   Earth.   taken   as   well   distance   surface.   Earth   The   with   true   is   velocity   7.25b).

        7        data,   to             words,   are   (p)   be   Curves   point   horizontal   in   a   on   layers   V(z).   at   a   The   wave   be   as   angular   the   along   changes   the   vertically,   velocity
        Chapter   distances   Earth’s   gravity   according   and:   other   density   (p.)   can   Flat   Starting   increase   critically   (Fig.   faces   horizontally   slope  of  the   velocity   converge   The   depth,   velocity   is  graph   source.   the   Curved   flat   a   must   curvature   to   on   the   If   7.25a).   are  ments   slope)   represents   emerge   apparent   the   If   (Fig.


        206                             In       Travel-Time   a   for   the       that   sent   face   ing   tions   (Fig.   Earth
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