Seismology

Introduction

Lecture #2Prepared by Dr. Amin Khalil

In this lecture

• Benioff Wadati zone• Types of Seismic Waves• Reading Seismograms• Seismic Phases• Travel time curves

Benioff-Wadati Zone

A dipping planar (flat) zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate. These earthquakes can be produced by slip along the subduction thrust fault or by slip on faults within the downgoing plate as a result of bending and extension as the plate is pulled into the mantle

Types of Seismic Waves

Seismic Waves

Body Surface

Body Waves

• Body waves is that kind of seismic waves that travel through bodies

• It is subdivided into Primary (P) and Secondary (S) waves.

• P-waves is the fastest and arrive first at the seismograph.

• S-waves arrives second with time delay depends on the distance between the source and receiver.

Properties of P-waves

• Longitudinal waves

• Highest in velocity• Can travel through

all bodies (Solids, Liquids and gases

Properties of S-waves

• Transverse waves• Lower in speed• Can travel

through solids only

Surface Waves

• Surface waves propagates along surfaces only and vanishes exponentially away from the surface.

• Lower in frequencies and velocities than the body waves.

• Less attenuated thus it dominated seismic energy at teleseismic distances

• More dangerous specially for low natural frequency structures.

Reading Seismogram

Phases nomenclature

End of lecture #2Thank you