Earthquake Faults - UC Berkeley Seismology Labseismo.berkeley.edu/~horst/Lect3.pdf · ·...

http://seismo.berkeley.edu/~horst/summer2016.html


31 May 16

HH Horst Rademacher

Lect 3: Earthquake Faults

Summer Session 2016

Class organization

Revised syllabus now on website

Most important change:

June 14 is now fixed as date

for field trip to Hayward Fault

Class organization

first homework assignement mix of short essays and multiple choice will be posted on website this afternoon Due date: Thursday, 2 June

No late work will be accepted unless prior approval

No electronic submissions unless prior approval

Office hours: Tuesday 3-4 pm (after class)

Where: 213 McCone Hall or BSL Conference Room (2nd floor McCone)

What is an earthquake?

Scientific answer:

When the ground breaks along a fault

Common/laymen’s answer:

When the ground shakes where I am

Both answers are correct! Two different aspects of a seismic event

Seismic waves

Seismic source

Recap from last lecture I

Recap from last lecture II

Earthquakes around the world occur in a distinct pattern

Depth in km

Recap from last lecture III

The cause is Plate Tectonics

Divergent • Established mid-ocean ridge • New rift zone, new ocean

Transform • Along mid-ocean ridges • Across continents

Convergent • Ocean – continent • Ocean – ocean • Continent – continent

Three types of plate – plate interactions

Recap from last lecture IV

EQ Faults

Suggested Reading:

Chapters 3 and 4



HH Any

Questions?

Plate Reconstruction II

Stress and Strain

Comparable to growth speed of fingernails

= 4 inch/year

Largest EQ occur, where plate speed is fastest

Stress and Strain

What happens when you constantly push or pull on an rock?

CA Hwy 14 near Palmdale

Limestone / Marble near Kings Canyon NP

It gets folded or contorted

One option:

Stress and Strain

What happens when you constantly push or pull on an rock?

It brakes or fractures =

Earthquake

Another option:

Important: breakage not random, along preexisting stress planes =

Faults

Faults

Focus = Hypocenter

Faults

Faults, Earthquakes and Plate Tectonics

Fault: Boundary Line between two plates Accumulates mechanical stress from tectonic movement Releases stress in Earthquake – plates slip past each other Over thousands and millions of years: Accumulated slip same as average tectonic movement

1. Normal

2. Reverse

3. Strike-slip

4. Oblique

Dip-slip faults

Mixed Dip-slip & Strike-slip

Four Types of Faults Faults

Assume a dipping surface along lake shore

surface of the lake makes a horizontal line along the surface = the strike line

the angle between the strike line and north is the strike angle (measured clockwise from north)

the dip angle is the angle between a vertical line down the face of the surface and horizontal The rake is the angle along which a fault slips: 0O in purely horizontal strike slip faults 90O in pure normal or thrust faults . 0

Faults: The Details Nomenclature of fault features:

dip, strike

and rake



HH Any

Questions?

Faults: The Details

movement purely along

the dip

Hanging Wall & Footwall

Faults: The Details Nomenclature of fault features:

Normal Faults b

efo

re

aft

er

Extensional environments

• Subsidence, basin formation

“Normal” because

hanging wall is sliding down

foot wall stable

Faults: The Details

Normal Faults Faults: The Details

Examples:

Reverse Faults also called Thrust Faults

be

fore

a

fte

r

Compressional environments

• Mountain building

Faults: The Details

“Reverse” because

hanging wall is sliding up

foot wall stable

Reverse Faults Faults: The Details

Example:

1999 Chi-Chi Earthquake, Taiwan

Faults: The Details

movement purely along

the strike

Strike-slip Faults b

efo

re

aft

er

Translational environments

e.g San Andreas and Hayward Faults

• Right- and left-lateral

Faults: The Details

Strike-slip Faults Faults: The Details

Direction?

Left lateral

Right lateral

Oblique slip Faults b

efo

re

aft

er

Both strike-slip and dip-slip

Faults: The Details

Oblique slip Faults Faults: The Details

Here: right lateral and normal

Guatemala

What kind of faulting? Faults: The Details

Right lateral strike slip

Normal or Reverse??

How are Faults and Plate Tectonics

connected?

Divergent Convergent Transform

Normal Reverse Strike Slip



HH Any

Questions?

Divergent

How are Faults and Plate Tectonics

connected?

EQ Faults:

Spreading about 1 inch/yr

Can one see faults? EQ Faults:


Photo: Horst Rademacher

San- Andreas-

Fault

near

Palm Springs

Faults

Can one see faults?

Faults Can one see faults?

San Andreas Fault in Carrizo Plain

Can one see faults?



Faults


San Andreas Fault – Wallace Creek Central California

Wallace Creek: San Andreas Fault Faults:

Off-set curbs in Hollister along Calaveras Fault


1971

Hayward Fault at Rose Street in Hayward



Fault step overs = jumps to left or right

San Andreas Fault Along Hwy 25 S of Hollister



HH Any

Questions?

Locked vs. creeping faults

CREEPING along

LOCKED until the

next earthquake…

Fault behavior

SEISMIC

Earthquakes

Stick-Slip

ASEISMIC = Creep

Flanks constantly move

Free Slipping

Spectrum of fault behavior

Even along creeping faults, the reality is somewhere in between.

Fault behavior

Hayward Fault Hybrid Fault

Source: Estelle Chaussard, SUNY Buffalo

Hayward Fault Hybrid Fault

Faults

Locked faults cause Earthquake Cycles

Calaveras Fault in Hollister

San Andreas Fault along Hwy 25

Fault behavior Creeping Faults

Rose Street,

Hayward, CA 1971

1974

1993

2005

Steady creep

approx. 1 inch/year

No earthquakes


10 mm/yr - 2.5 mm/yr? = ???

Long Term Buildup

(Plate Tectonics) Amount left for

Earthquake Steady slip - =

makes earthquakes smaller


Creep relieves

strain



HH Any

Questions?

Fault behavior Difference between Creep and an Earthquake?

Aseismic Creep: Accumulated strain gets released continuously at low rate –

no or very little seismic hazard

Earthquake: Sudden release of large amount of accumulated strain –

large seismic hazard

Fault behavior

How does an Earthquake work?

LOCKED until

the next

earthquake…

1. A Fault has to be locked

Fault behavior

2. Stress builds up as tectonic plates move

past one another

3. Friction along the fault

prevents slip, Instead: elastic deformation

4. Stress exceeds rupture strength,

fault slips …earthquake!


Harry F. Reid 1859-1944

Johns Hopkins U.

Elastic rebound

theory

Fault behavior

Elastic Rebound also explains

Earthquake Cycles

Fault behavior



HH Any

Questions?

Slip

Fault behavior


How large is the coseismic slip?

EQ M Slip [m]

South Napa (2014) 6 1

Coalinga (1983) 6.5 0.9

Loma Prieta (1989) 7.1 1.6 ± 0.3

San Francisco (1906)

7.9 6.1 - 7.8

Maule (Chile) 2010 8.8 19

Tohoku (2011) 9 >50

Alaska (1964) 9.2 >30

Chile (1960) 9.5 90 ± 4.2

Fault behavior

Coseismic Slip

Fault behavior

Coseismic Slip 1m at depth a few cm on surface

South Napa EQ 24 Aug 14, M=6.0

Fault behavior

Tohoku EQ 11 Mar 11, M=9.0



HH Any

Questions?

1. Normal

2. Reverse

3. Strike-slip

4. Oblique

Dip-slip faults

Mixed Dip-slip & Strike-slip

Four Types of Faults Things to remember I

One can see faults!


Things to remember II

SEISMIC

Earthquakes

Stick-Slip

ASEISMIC = Creep

Flanks constantly move

Free Slipping

Spectrum of fault behavior

Even along creeping faults, the reality is somewhere in between.

Things to remember III

10 mm/yr - 2.5 mm/yr? = ???

Long Term Buildup

(Plate Tectonics) Amount left for

Earthquake Steady slip - =

makes earthquakes smaller

Creeping Faults

Creep relieves

strain

Things to remember IV

Class organization

first homework assignement will be posted on website this afternoon Due date: Thursday, 2 June

No late work will be accepted unless prior approval

No electronic submissions unless prior approval

Thursday:

Earthquakes in the US Man made EQ

Seismicity report: look at past week’s seismicity: discuss interesting events, EQ in the news, etc.

For California events: http://earthquakes.berkeley.edu

For global events: http://ds.iris.edu/seismon/ http://earthquake.usgs.gov/earthquakes/map/

Any volunteers?

http://ds.iris.edu/seismon/



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