Introduction to Seismology

Ali O. Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Lecture 1Overview

Dr. Ali O. Oncel: Building 26, Room:319

(oncel@kfupm.edu.sa)

Sunday . 2:00-4:00 PM room 319

Tuesday. 2:00-4:00 PM room 319

Scheduled Office Hours

Class Website

An introduction to Seismology, Earthquakes, and Earth Structure by Stein and Wysession

GEOP 204: Final Exam 20%Mid Term Exam 20%Term Project 15%Class presentation 15%Quiz 10%Homework 10%Class Participation 10%

Recommended Text and Grading

Homework is due 2 weeks after it is assigned Homework is due 2 weeks after it is assigned Late homework will not be accepted one week after the deadline. Late homework will be penalized: 80% (1 day late),60% (2 day late), 40% (3 day late), 20 % (4 day late) and 10% (5-7 day late).

Homework

What is seismology?

To study the generation, propagation and recording of elastic waves in the earth and other celestial “outer space” bodies.

Japanese Folklore: Earthquakes caused by giant Namazu.

Seismological Funding

•Scientific Results• Understanding the Earth’s deep interior

• Helping the establishment of plate tectonics

•Social and Political Applications • Disaster mitigations • Monitor WMD

Background of Seismology

•Seismology - study of seismic waves, earthquakes ‘EQ’, which are used to image earth structure •Passive seismology uses EQ source for crustal to whole earth scale studies•Active seismology uses artificial sources for environmental, engineering, exploration studies, crust and upper mantle scale studies•Theoretical seismology means mostly slanted towards Earthquake seismology A large part of this course concerned with study of seismic waves common to both passive and active methods

Quantitative seismology is ~100 years old

In early 1800s, theory of elastic waves (body and surface waves) developed by Poisson, and Rayleigh, etc.

First electromagnetic seismometers (basis for all modern seismometers) developed in early 1900s

The first seismoscope was built in 132 AD The first mechanical seismometer was invented in 1870’s

Background of Seismology

What can seismology provide us on earth’s structure?

VelocitiesDensityAttenuationAnisotropyBoundaries

The Shape of the Earth

Mechanical Layering

LithosphereThe lithosphere (derived from Greek, it means “rock sphere”) consists of the crust and the uppermost mantle directly beneath the crust. It is a cold, strong layer and is generally around 50-100 km thick.

AsthenosphereThe Asthenosphere (asthenes is Greek for “weak”, so Asthenosphere is the “weak sphere”) is the region of the mantle directly below the lithosphere. In contrast with the lithosphere, the Asthenosphere is hot and relatively fluid; it will flow over long periods of time. In some sense, the lithosphere “slides” on the Asthenosphere.

Mechanical Layering

Internal structure from Seismology?Seismology and Body Wave Travel Times: Remote Sensing of Earth’s InteriorWhen large earthquakes happen, the properties of the Earth such as major discontinuities:

Moho separating Crust and Mantle which is inferred from the velocity discontinuity by Mohorovicic in 1909. Earth’s core inferred in 1906, based on absence of body waves at source-receiver offsets beyond 100° by Oldham from Indian Geological Survey.Solid inner core by Lehman was discovered in 1936.

Such kind of information on Earth’s interior which are defined from the seismic concepts as:

a) Reflections, refractions, and scattering of Seismic waves b) Deduction in changes of seismic velocities versus depth c) Seismic imaging (tomography), Seismic anisotropy

•Travel times of waves versus offset (travel time tables) published by Zoeppritz, Gutenberg and Jeffreys and Bullen in 1907, 1914 and 1940

•Travel times used to derive earth’s average velocity versus depth structure over 50 years ago

•Gradual increase of velocity with depth causes seismic waves to refract along curved ray paths that eventually return to the surface

•Beneath this, velocity increases gradually with depth as predicted by pressure/temperature effects on rocks of uniform composition.

•Earth divided into crust, mantle and outer core

Crust typically 6 km thick under oceans, 30-50km under continents

•Mantle shows rapid velocity increase between 300-700km termed “transition zone” due to several mineralogical phase changes, e.g., at 410 and 660km.

•CMB is sharp interface between solid mantle and fluid outer core

Core-Mantle Boundary (CMB)P-wave velocity drops from 14 to 8 km/sS-wave velocity drops from 7 to 0 km/s

At inner core boundary (ICB), core becomes solid as a results of phase change in iron.

Within outer core velocity increases gradually consistent with well mixed fluid

Standard reference model established in 1981 by Dziewonsky and Anderson called PREM (Preliminary reference Earth Model), which is accepted as a Current standard by IASPEI91, AK135.

Depth Variation of P- and S-wave

b) An elastic material returns to its original shape and volume when deforming stress is removed

Elastic Behavior

a) The deformation of a material (strain) results from a force per unit area (stress) acting on the material.

a) During ductile deformation particles remain connected and flow

b) Brittle deformation results in the development of fractures

Style of Deformation

P and S not seen

P/S shadow zones

• What shadow zone suggest you to think about?•Drop of speed at a boundary: mantle/core•Outer core is liquid•Inner core solid - consider later

S-wave

shadow zone

Can you read this?

Seismologists “read” seismograms to learn about the earth.

Seismic Body Waves

Seismic Body Waves

Seismic Surface Waves

Seismic Surface Waves

Tomography/Travel Time Curves

Bolt, 1976

Pacific Super-plume

Courtesy of B. Romanowicz

Courtesy of Van Heijst et al.

African Super-Plume

Seismic refraction profile recorded in ocean basin, Sheriff and Geldart, 1995

S – Reflections from bottom of water

Refraction Interpretation

DD - Direct Arrival

P

P - Lower Crustal Refraction within thecrust and from the Moho

S

Fowler, 1990

Seismic Refraction

Interpretation

Reflection Interpretation

Transform Fault

Basement

Bowen and White, 1986

A migrated seismic-reflection profile across the Vema Transform Fault on the Mid-Atlantic Ridge at 6°N, approximately midway between the African and South American plates.

P-Velocity S-Velocity Density

Velocity/Density Versus Depth