1

Lecture 10 – Faults and Earthquakes

Causes of Earthquakes

There are basically three types:-VOLCANICTECTONICCOLLAPSE

(Also man-made due to nuclear explosions)

2

Tectonic Earthquakes

Most earthquakes occur at plate margins due totension, compression or shearing forces.

Rocks at plate margins are in constant motionand are being pushed, pulled, bent, twisted andfolded.

Inevitably at some point they must break or crack to produce FAULTS!!

[Note – some rocks break much more easily than others]

What is a Fault?

• A fault is a break or fracture between two blocks of rocks in response to stress.

• Three types of stresses produce faults1) Tension2) Compression3) Shear

• One block has moved relative to the other block.• The surface along which the blocks move is

called a fault plane.

3

There is a chicken and egg relationship between faults and earthquakes

1) It was initially thought that earthquakes caused faulting (but then what caused the earthquake?)

2) It was later realized that faulting produced the earthquakes.

Thus earthquakes may occur because:-a) Rocks are initially broken to produce a fault.b) Movement or re-activation of an already existing

fault.

[Faults may therefore be thought of as “fossil” relictsof previous earthquakes]

Relationship between faulting and an earthquake

4

Folding and Faulting

COMPRESSION – causes rocks to fold and ultimatelybreak to produce a fault.TENSION – causes rocks to stretch and also break toproduce a fault.

TENSION

COMPRESSION

Faulting also occurs in response to shearing stresses

Relationship of stressesto types of faulting

Example of foldedrocks that have brokento produce a fault

5

Some basic terminology

STRIKE – a horizontal line along the direction of a fault plane.

DIP – the angle between the fault plane and thehorizontal.

(see chapter 3 pages 62-67 in book)

More terminology

Hanging wall – is the blockabove the fault plane.

Foot wall is the block belowthe fault plane.

Hanging wall

Foot wall

Terminology comes from mining

6

Normal Faults

Normal faults are producedby tensional forces.The upper block (hanging wall) moves down relativeto the lower block (foot wall).

Two examplesof normal faults

Fault plane

Fault plane

7

Two scarps produced bynormal faulting.

Horsts and Grabens

A series of parallel normal faults produceshorsts and grabens

GRABEN - valley formed asone block drops down.

HORST - hill formed as theother block moves up.

8

Reverse Faults

Reverse faults are producedby compressive forces.The upper block (hanging wall) moves up relativeto the lower block (foot wall).

Small reverse faultassociated with foldedrocks

Another small reversefault

Fault plane

9

A larger reversefault

Close-up of a reversefault plane.

Note the two completelydifferent rock-types.

Thrust Faults

A thrust (or overthrust) fault is a low-angle reverse fault,again resulting from compressive forces.

Thrust faults are very common in highly folded mountainbelts (the example is from Glacier National Park).

ChiefMountain

10

A thrust fault may look a bit like an unconformity in thatcompletely different rocks are separated by a planesurface. With a thrust fault, however, the older rockshave been pushed up on top!

Fault plane

Older rocks

Younger rocks

Transform Faults(Strike-Slip Faults)

Transform faults move horizontally in response to shearing stresses.They are also called strike-slip faults because the movement is along strike.

11

Strike-slip fault in a field

Two examples of strike-slip movement along transformfaults (note lack of vertical movement).

12

Transform Faults

An example in a field of lettuce.

An aerial view of amuch larger example.

The San Andreas Fault

A very large example of atransform fault involving over200 km of offset.

13

Summary of Fault Movements

Why is it unlikely that one would find all thesefaults in the same place?

Different faults result from different stresses

More Complex Examples

In this example there has been BOTH horizontaland vertical movement along the fault plane.

14

In these two examples there has been both horizontaland vertical movement along the fault plane.