Shadow zone

Remarks

A ray with parameter $p$ turns at a radius $r$ where $r / v(r) = p$. If $r/v(r)$ jumps at some radius because the speed drops sharply with depth, a range of values of $p$ has no turning point above that radius. Those rays enter the low-speed region, bend strongly, and emerge much farther away, which leaves a gap in the distances $\Delta(p)$. Example: at the core–mantle boundary the P-wave speed falls from about $13.7$ km/s to about $8$ km/s. The direct P-waves that just miss the core emerge near $104°$, and those that enter it emerge beyond about $140°$, so the P shadow zone is roughly $104°$ to $140°$. For S-waves the outer core is liquid and has $v = 0$, so no direct S-ray crosses it, and the shadow is everything beyond about $104°$. Non-example: a station at $60°$ receives direct P-waves that turn in the lower mantle, so $60°$ is not in the P shadow zone. The definition is in terms of rays; waves diffracted along the core and waves reflected from the inner core still bring weak energy into the shadow.