04 Faults and focal mechanism

8/7/2019 04 Faults and focal mechanism

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Faults and earthquake focal mechanism

Dr. N. Venkatanathan Page 1

INTRODUCTION

Rocks are very slowly, but continuously moving and changing shape. Under high

temperature and pressure conditions common deep within Earth, rocks can bend and flow.

In the cooler parts of Earth, rocks are colder and brittle and respond to large stresses by

fracturing. Earthquakes are the agents of brittle rock failure. A fault is a crack across, where

the rocks are balancing. They range in size from micrometers to thousands of kilometers in

length and tens of kilometers in depth. But they are generally much thinner than their

length and depth. Not only they vary in size and orientation, but they can accommodate

different styles of rock deformation, such as compression and extension. Not all faults

intersect Earth's surface, and most earthquakes do no rupture the surface. When a fault

does intersect the surface, the ground may get cracked, or raised, or lowered. The rupture

of the surface by a fault is called as a fault scarp. Identifying scarps is an important task for

assessing the seismic hazards in any region.

Fig. 1: Fence offset about 11 feet during the 1906 San Francisco California Earthquake.

Photo Credit: National Geophysical Data Center

Earthquakes and Faults

When an earthquake occurs only a part of a fault is involved in the rupture. That

area is usually outlined by the distribution of aftershocks in the sequence. Generally, when

the area of the fault that ruptures, increases, the magnitude also get increased. Although

the exact area associated with a given size earthquake varies from place to place and event

to event. It is possible make predictions for "typical" earthquakes based on the available

observations.

From the table 1, an earthquake at San Francisco, California region (April 18, 1906),

had fault length of 432 km, depth of 12 km and magnitude of Mw 7.9. In the same

California region at San Fernando Valley, an earthquake occurred on February 09, 1971 with

magnitude of Mw 6.64, had fault length of 17 km and depth of 14 km. From these two


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earthquakes it can be inferred that fault length and depth are acting as a deciding factor for

the magnitude of an earthquake.

Date LocationFault Length

(km)Depth (km)

Magnitude

(Mw)

04/18/06 San Francisco, CA 432 12 7.90

07/21/52 Kern County, CA 64 19 7.38

12/16/54 Fairview Peak, NV 50 15 7.17

12/16/54 Dixie Peak, NV 42 14 6.94

06/28/66 Parkfield, CA 35 10 6.25

02/09/71 San Fernando Valley, CA 17 14 6.64

10/28/83 Borah Peak, ID 33 20 6.93

10/18/89 Loma Prieta, CA 40 16 6.92

06/28/92 Landers, CA 62 12 7.34

Table 1: List of earthquakes with details of fault length, depth and magnitude

Magnitude Fault Dimensions (Length x Depth, in km)

4.0 1.2 x 1.2

5.0 3.3 x 3.3

6.0 10 x 10

6.5 16 x 16, 25 x 10

7.0 40 x 20, 50 x 15

7.5 140 x 15, 100 x 20, 72 x 30, 50 x 40, 45 x 45

8.0 300 x 20, 200 x 30, 150 x 40, 125 x 50

Table 2: Approximate dimensions of a fault and their corresponding magnitude

Fault Structure

A fault "zone" consists of several smaller regions defined by the style and amount of

deformation within them. The center of the fault is the most deformed and where slip

between the surrounding rocks occurs. The region can be quite small, about as wide as a

pencil is long. It is identified by the finely ground rocks called cataclasite, also called as


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fault is 0, and the dip of a vertical fault is 90. Ifyou were tunneling through a fault, the

material beneath the fault wouldbe byyour feet. The other material wouldbe hanging

above you head. The material resting on the fault iscalled the hanging wall, the material

beneath the fault iscalled the foot wall.

Fig.3: Showing method to measure the dip of a fault

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tSike

The strike is an angle used to specify the orientation of the fault and measured

clockwise from north. For example, a strike of 0 or 180 indicates a fault that is oriented in

a north-south direction. A strike of 90 or 270 indicates east-west orientedstructure.

Alwaysspecify the strike such that when you "look" in the strike direction, the faultdips to

you right. Ofcourse if the fault isperfectlyvertical you have to describe the situation as a

special case. If a faultcurves, the strike varies along the fault; such a faultdirection should

be specified with latitude and longitude.


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Fig. 4: Methodology to measure strike of a fault

Slip

Slip is to describe the direction of motion across the fault. That is, which waydid one

side of the fault move with respect to the other. The slip has two components, a

"magnitude" which tells us how far the rocks moved, and a direction.The magnitude and

direction are specifiedseparately. The magnitude ofslip issimply how far the two sides of

the fault movedrelative to one another. It's a distance usually a few centimeters forsmall

earthquakes and meters for large events. The direction ofslip is measured on the fault

surface, and like the strike anddip, it isspecified as an angle. Specifically the slipdirection is

the direction that the hanging wall movedrelative to the footwall.

1.

If the hanging wall moves to the right, the slipdirection is 0 (LeftLateral

Strike Slip).

2.If it moves up, the slip angle is 90 (Reverse).3.If it moves to the left, the slip angle is 180 (RightLateral Strike Slip).4.If it movesdown, the slip angle is 270 or-90 (Normal).

Fig. 5: Methods to measure slip of a fault


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GeometTic Classificationof fa

Ults

Hanging wall movementdetermines the geometricclassification of faulting. Hanging

wall movementscan be classified into "dip-slip" and "strike-slip. Dip-slip movement occurs

when the hanging wall movedpredominantly up ordown relative to the footwall. If the

motion wasdown, the fault iscalled a normal fault, if the movement was up; the fault is

called a reverse fault.

Downward movement is "normal" because we normally would expect the hanging

wall to slide downward along the foot wall because of the pull ofgravity.Moving the

hanging wall up an inclined faultrequires work to overcome friction on the fault and the

downwardpull ofgravity.

1.The hanging wall moves horizontally, it's a strike-slip earthquake.2.If the hanging wall moves to the left, the earthquake iscalledright-lateral. If

objects move to right, it's a right-lateral fault.

3.If it moves to the right, it'scalled a left-lateral fault. If objects on the otherside of the fault, move to left, it's a left-lateral fault.

When the hanging wall motion is neitherdominantlyvertical nor horizontal, the motion is

called oblique-slip. Although oblique faulting isn't unusual, it is lesscommon than the

normal, reverse, andstrike-slip movement.

Fig. 6: Different types of faults


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Faults and Forces

The style of faulting is an indicator of rock deformation and reflects the type of

forces pushing or pulling on the region. Near Earth's surface, the orientations of these forces

are usually oriented such that one is vertical and the other two are horizontal (NS and EW).

The precise direction of the horizontal forces varies from place to place as does the size of

each force. The style of faulting that is a reflection of the relative size of the different forces

V in particular is the relative size of the vertical to the horizontal forces. There are three

cases to consider, the vertical force can be the smallest, the largest, or the intermediate.

1.If the vertical force is the largest, normal faulting.2.If it is the smallest, reverse faulting.3.If the vertical force is the intermediate force, strike W slip faulting.

Normal faulting is indicative of a region that is stretching, and on the continents,

normal faulting usually occurs in regions with relatively high elevation such as plateaus.

Reverse faulting reflects compressive forces squeezing a region and they are

common in uplifting mountain ranges and along the coast of many regions.

Strike X slip faulting indicates neither extension nor compression, but identifies regions

where rocks are sliding past each other.

Fault Type: Normal Faulting Reverse Faulting Transform Faulting

Deformation Style: Extension Compression Translation

ForceY

rientation:Vertical Force Is

Largest

Vertical Force Is

Smallest

Vertical Force Is

Intermediate

Table 3: Showing deformation style and force orientation for different types of faults

Earthquake Focal Mechanis

s - Beach Ball Representation

Specific set of symbols to identify faulting geometry on maps. The symbols are called

earthquake focal mechanisms or sometimes "seismic beach balls". A focal mechanism is a

graphical summary of the strike, dip, and slip directions. An earthquake focal mechanism is

a proa

ection of the intersection of the fault surface and an imaginary lower hemisphere

surrounding the center of the rupture. The intersection between the fault "plane" and the


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sphere is a curve. The focal mechanism shows the view of the hemisphere from directly

above.

Fig. 7: Schematic diagram of a focal mechanism

The orientation of a plane (i.e. the strike and dip) can be represented usingb

ust one

curve. To include information on the slip, use two planes and shade opposite quadrants of

the hemisphere. The ability to represent slip is that one cannot identify which of the two

planes on the focal mechanism is the fault without additional information. Infos on location

and trend of aftershocks is important.

Fault Choice 1 Fault Choice 2

Strike(deg) 95.0 231.0

Dip(deg) 30.0 67.0

Slip(deg) 129.7 69.7

Table 4: Representation of a fault geometry in terms of numerical values


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Fault Types Cartographic

Depiction

Earthquake Source

Mechanisms

ThrustFault

Normal Fault

Strike-Slip

Fault

Oblique

Thrust

(Combinationof thrust and

strike-slip

faulting)

Fig. 8: Diagrammatic beach ball representation and cartographic representation of different

types of faults

04 Faults and focal mechanism

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