Seismic Ray Paths Part II of : Types of Seismic Waves Physics 131 M. Castner Canisius College

Bent rays Body waves do not travel in straight lines though the earth. They ‘bend’ because the fastest path from hypocenter to a specific spot on the surface of the Earth is a path deeper in the Earth.

Body waves & shadow zones R. Zimmerman, “Deep Impressions” Natural History. March 2004. P. 57

Body waves & shadow zones If the interior of the Earth were homogeneous, ray paths would be simple, as shown in drawing ‘a.’

Body waves & shadow zones R. Zimmerman, “Deep Impressions” Natural History. March 2004. P. 57

Body waves & shadow zones But P waves fail to appear in specific areas called shadow zones. This tells us that the Earth has a core made of material different from that of the mantle. See drawing ‘b.’

Body waves & shadow zones R. Zimmerman, “Deep Impressions” Natural History. March 2004. P. 57

Body waves & shadow zones S waves fail to appear completely after a certain distance from an earthquake. This tells us that the core is made of a material that does not transmit shear waves, namely a liquid. See drawing ‘c.’

Body waves & shadow zones R. Zimmerman, “Deep Impressions” Natural History. March 2004. P. 57

Bolt, 5e, Plate 6

Ray Nomenclature Body waves that reflect off the surface of the Earth are labeled by the type of wave they are. Body waves that reflect off the outer core are labeled by the type of wave with the letter c in the middle to note the core reflection.

Introduction to Seismology. Peter M. Shearer. P. 51

Earthquakes. B. Bolt. P. 128

Ray Nomenclature P waves that pass through the outer core have the letter K in the middle to signify that part of the path. P waves that reflect off the inner core are labeled by the type of wave with the letter i in the middle to note the inner core reflection.

Introduction to Seismology. Peter M. Shearer. P. 51

Here is another author’s drawing to explain the ray nomenclature.

An Introduction to Seismology, Earthquakes, and Earth Structure An Introduction to Seismology, Earthquakes, and Earth Structure. Seth Stein and Michael Wysession. P. 165

Specialized Nomenclature Lower case p and s are used for the initial primary and secondary waves from a deep focus earthquake. After the wave reflects off the surface of the Earth, it is designated with the appropriate upper case P or S.

Introduction to Seismology. Peter M. Shearer. P. 51

Multiple paths Due to reflections from discontinuities in the crust and upper mantle, plus the bending of wave paths due to increased speeds at greater depths, a point on the surface of the Earth may receive the ‘same’ seismic wave several times.

An Introduction to Seismology, Earthquakes, and Earth Structure An Introduction to Seismology, Earthquakes, and Earth Structure. Seth Stein and Michael Wysession. P. 165

Complex Ray Paths Some ray paths become very complex and are beyond the scope of this course; specifically, see the next slide.

An Introduction to Seismology, Earthquakes, and Earth Structure An Introduction to Seismology, Earthquakes, and Earth Structure. Seth Stein and Michael Wysession. P. 165

Would that all earthquakes could be read as easily as this one. Abbott. Natural Disasters, 5e. P. 93

Travel Speeds and Times Summary: Seismic waves travel at different speeds through different parts of the Earth, if they travel there at all. Arrival times, when various wave types can be expected to arrive at specific distances from the epicenter, can be plotted on a travel-time chart.

Remember: seismic waves travel at different speeds at different depths Abbott. Natural Disasters, 5e. P. 91

Abbott. Natural Disasters, 5e. P. 93 The farther a P wave has to travel, the deeper in the earth it can go, and there it travels faster. Abbott. Natural Disasters, 5e. P. 93

Travel Times As we start to read seismograms, travel-time charts will be important to us.

picture Buzz Aldrin, Apollo 11, w/ lunar seismometer Apollo 8 - Earthrise

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