Chapter 4

Chapter 4 Earthquakes / Brittle Faults - Key Points:

1. Distribution

Global: See Fig. 3.9 (P.57)

Principally, but not exclusively, related to plate boundaries of the 4 overall types. Also-old fault zones within plates eg. East Ontario / Quebec.

(i)Divergent

eg. Sea floor spreading ridges; East African Rift Valley System

(ii) Convergent

(a) Subduction zones:

eg. Major trenches (eg. Chile; Japan)

(b) Collision zones:

eg. Himalayas; Alpine Mountain System

(iii) Transform (Lateral motion)

eg. San Andreas (P.111)

North Anatolian fault, North Turkey

Earthquakes in Canada: See overheads
Lake Ontario/ Toronto Region:

Relevant to nuclear reactor sites. (See overheads).

Click here to view a diagram of Earthquake Epicenters

Click here to view a picture of the relative motions of the tectonic plates

Click here to view a diagram of the cross section of the plate tectonic model

Click here to view a picture and article on the 1999 Colombia earthquake

Click here to view a simplified tectonic map of the eastern Mediterranean

Earthquakes in Canada

200-300 earthquakes recorded per year ( see next)

10-20 p.a. are sufficiently strong and widely felt to result in public response.
8 major earthquakes this century of >R7:
- 5 in the West (2 in the Queen Charlotte Islands> R8)
- 2 in the East
- 1 in the Arctic
East has ~ 1 earthquake per decade of >R6 (not much more)
- West 2>R6.5
- Arctic 2 >R6.5

Large earthquakes in the East:

Quebec City, Quebec 1630 >R8

Cornwall, Ontario 1944 R5.9

Lake Erie 1986 R5.7

Eastern USA

New Madrid, IL 1811-12 ~R7.1-7.4

Charleston, SC 1886 ~R7.5

Click here to view a map of Canadian Seismicity

Click here to view a map of Ontario Seismicity

2. Cause

Large Scale:

Principally consequences of plate tectonic extensional, compressional and lateral movements.

Specific- Movement on faults:

Two end-member types of fault movement

(i) Sudden:

Elastic Rebound Theory

-Frictional resistance on fault plane

-Accumulation of elastic strain

-Sudden release and emission of ground shaking waves + internal P and S waves

Eg. P.73

Eg. Loma Prieta 1989 (Magnitude 7.1); oblique slip: ~2m horizontal + a ~1m vertical.

(ii) Continuous: fault creep

Faults with low frictional resistance permit continuous movement with low seismicity below a low threshold.

Eg. Hayward Fault( Berkeley)-mm/year

Clearly there is a complete gradation of behaviour between (i) and (ii)To some extent, for a given overall average strain rate, the longer the time gap, the larger the earthquake (the more stored elastic strain energy).

3. Types of faults:

In simplest terms- as given on P.72

Normal-extension

Reverse-compression

Strike slip( right or left lateral)-lateral (horizontal) motion

Click here to view a picture on different types of fault

4. Earthquake Description:

See P.74

Exact position within earth = Focus (x/y/z)

Surface position directly above: Epicentre

Damaging earthquakes- few km from surface

Maximum depth @ 700 km in subduction zones

Eg. Bolivia; June 9,1994: Magnitude 8.3 at a depth of 637km

1.Body Waves (internal):

P (Primary-arr. first): compressional waves

Transmit through liquid; ~10km /s velocity in earth

S (Secondary-arr. second): transverse or shear waves

Do not transmit through liquid; this is how the outer Fe/Ni core has been found to be liquid.

~5km/s in earth

~1/2 velocity

2. Surface (L) Waves:

Complex rolling ground motion -> can produce motion sickness if prolonged.

Two end member types: See P.75

Rayleigh Waves- complex up/down motion reflecting elliptical ground motions.
Love Waves- side to side motions- the most destructive.

5. Earthquake Location:

Triangulation

See P.77: (1) S-P arrival time difference: f (distance from focus) ->

(2) compute distances and plot; for >=3 seismic stations

Click here to view a picture on the ground motion during passage of earthquake waves

Click here to view the article "Novaya Zemlya: The quake that roared"

6. Earthquake Magnitude Measurement:

Three Scales:

Modified Mercalli(P.78):

Effect of earthquakes "in the field": from felt(I) to total panic(XII).

Eg. P.79-Northridge; plot isoseismals

Richter Scale: a surface intensity scale (1935)

Logarithmic displacement scale

Hence, R_n= 10*R_n-1

=log₁₀standard ideal ground motion in microns at a ~100 km distance from the epicentre (+corrections) e.g. Mag.4= ~1 cm

Highest recent Richter Scale Earthquake: Alaska 1964 at 8.4 � 10⁵*Hiroshima in energy. Since derived from ground motion( displacement): a function of depth, not just total magnitude.

Moment Magnitude (M):

Richter is good for surface magnitude description; but not appropriate for total energy released:

M_o*Slip*Rupture Area*Rigidity of faulted rock

Then:

Seismic energy(ERGS) is proportional to M_o

M₁ or Mw₁=2/3Log M_o-10.7

Largest measured=Chile 1960; M=9.5 (See P.80)

Earthquake	Richter Magnitude	Moment Magnitude
Chile,1960	8.3	9.5
Alaska, 1964	8.4	9.2
New Madrid, 1812	8.7 (est.)	8.1
Mexico City, 1985	8.1	8.1
San Francisco, 1906	8.3 (est.)	7.7
Loma Prieta, 1989	7.1	7.0
San Fernando, 1971	6.4	6.7
Northridge, 1994	6.4	6.7
Kobe, Japan, 1995	7.2 JMA	6.9

Click here to view a chart on earthquake magnitude and energy

Earthquake Prediction; Key points: (P.109-112)

O.K. in a very general sense; but very difficult in a specific sense in terms of specific location and time; also magnitude.

3 approaches:

Statistical

From analyzing past data for a given area, can derive statistical frequencies / probabilities: very useful for building codes

Eg. Fig.4.51. Mag.8(Richter): 1 per 1,000 years

Also: Fig. 4.52

N.B. Worldwide: expect an average of 2 Mag.8 earthquakes per year.

I hadn’t realized it was so high.

N.B. Can extend record back in past by, for example, ¹⁴C dating of disrupted marsh horizons.