Wave Phases - Seismology - Lecture Slides, Slides of Geology

Download Wave Phases - Seismology - Lecture Slides and more Slides Geology in PDF only on Docsity! Wave Phases • On the global scale, waves converted from reflection or refraction with the major layers of the Earth are called phases. • PS – begins as P-wave – reflected off of the surface – converted to S-wave • ScP – Begins as S-wave – reflected off outer core – Converted to P-wave • i – Reflected off the outer core/inner core boundary • K • P-wave in the outer core • I • P-Wave in the inner core • J • S-Wave in the inner core • Test: What is SKJKP? • Why is PSKIKP not possible?Docsity.com The S-wave Shadow Zone • S-waves only reach stations that are within epicentral angles of < 103 of the epicenter. • The S-wave shadow zone is a direct consequence of the liquid outer core. • Seismologists have since discovered that P-waves reflect off of a discontinuity within the core suggesting a two-layer core. • Seismic waves are not recorded at all seismic stations world wide – Called “shadow zones” • If velocity gradually increased with depth – waves would be recorded at all stations globally • Earth’s core is made of an iron alloy based on meteorites Docsity.com Attenuation • Wave amplitudes generally decrease away from their source – Energy is spread over larger volume as the wave front expands • Attenuation: the gradual loss of intensity (amplitude) of a wave as it travels through a medium – Causes of attenuation in seismic waves: • Encountering liquids or partial melts – E.g. the low velocity zone • Encountering unconsolidated (or non-elastic) material – E.g. Sand 2 4 6 8 10 1 0 1 2 3 4 5 6 Docsity.com Refraction, Velocity and Arrival Times = Moho Discovery • Seismic waves travel faster through the mantle than the crust • In 1909, Andrija Mohorovičić discovered that waves first arriving at seismic stations within 200 km of an epicenter had an average velocity of 6 km/s • Stations > 200 km away • average wave speed 8 km/s • To explain this • nearby stations received waves that only went through the crust • far away stations received waves that travel through the mantle. • The crust mantle boundary is now called the Moho, in honor of this discovery Docsity.com Velocity of P-Waves at Depth • Mantle rock = Peridotite – Ultramafic rock, mostly olivine • In general, seismic velocity increases with depth. • In oceanic crust – low-velocity zone at ~100-200 km depth. • At this depth (pressure) and temperature – peridotite partially melts < 2% • This zone permits the movement of oceanic plates. • Below the LVZ – velocities increase until the core mantle boundary Docsity.com