Earthquakes Bill Menke September 16, 2005. Summary What is an earthquake? Why do earthquakes occur?...

Earthquakes

Bill Menke

September 16, 2005

Summary

• What is an earthquake?

• Why do earthquakes occur?

• How is size quantified?

• Where do earthquakes occur?

• How frequently do earthquakes occur?

• How do earthquakes cause damage?

What is an earthquake ?

An earthquake is the shaking of the ground that is caused by sudden slip on a geological fault.

Why do Earthquakes Occur ?

Forces in the earth slowly build up to where they exceed the factors impeding fault motion, causing sudden slip on the fault.

Both friction and unbroken rocks are factors impeding fault motion.

Sudden slip causes earthquakes. Slow, steady slip (=creep), which can sometimes occur on faults, does not.

Analogy: breaking a twig

Step 1: Slowly bend twig. Energy, provided by your muscles, is stored in twig, because it is elastic.

Step 2: Keep bending (=straining). Eventually the force (=stress) is great enough to cause cracking (=faulting).

Step 3. The twig cracks. The stored elastic energy is used up by 1) tearing molecules apart (90%) and 2) making the cracking sound (vibrations, the earthquake)

Example: San Andreas fault

From the air, the fault really does look more-or-less line a line

Step 1

Step 2

Step 3

It really happens …

Forces are highest on Plate Boundaries

If one plate in moving in one direction …

And the other plate in moving in another direction …

Then the boundary between the two plates will be experiencing lots of force (=stress)

The Type of Plate Boundary Matters

Lithosphere – brittle part of earth – earthquakes happen here

Asthenosphere – viscous part of earth – no earthquakes

Convergent Plate Boundary

Convergent BoundaryPlates move towards one anotherOceanic Lithosphere: Subduction Zone (shown here)Continents: Collisional mountain belt (e.g. Himalayas)Extremely Large Earthquakes

(since lithosphere old, cold and thick)

Divergent Plate Boundary

Divergent BoundaryPlates move away from one anotherOceanic Lithosphere: Mid-Ocean Ridge (shown here)Continents: Rift Zone (e.g. East African Rift Valley)Smallish Earthquakes

(since lithosphere young, hot and thin)

Strike-Slip Plate Boundary

Strike Slip BoundaryPlates slide past one anotherOceanic Lithosphere: fracture zone (shown here)Continents: (e.g. San Andreas Faults)Intermediate Size Earthquakes

(since lithosphere intermediate in age, temperature and thickness)

What about the US ?

West Coast – Plate Boundary East Coast – No plate Boundary

Oregon and Washington:Convergent Plate Boundary,the Cascadia Subduction Zone

(but note divergent plateboundary offshore)

CaliforniaStrike Slip boundarythe San Andreas Fault

In the US, Where Have the Big Damaging Earthquakes Been ?

Oops – patternNot quite whatwe expected!

California OKBut why:

None in Cascadia

Some east ofMississippi!

Why?

Hey! What aboutAlaska, Hawaii andPuerto Rico?

Quantifying Earthquake Size

Size, a tricky buisness …

What is a big person?a tall person, with height in metersa heavy person, with weight in kilogramsa rich person, with fortune in dollarsan influential person, with influence in

% of population impacted

Richter: an earthquake is bigwhen the ground shakes a lot

Earthquake Magnitude

An earthquake’s size is defined to beMagnitude 3 on the Richter Scale

if it causes 0.36 microns of ground shaking at points 100 km distant from the fault

Its Magnitude 4 if it causes 3.6 microns at 100 kmIts Magnitude 5 if it causes 36 microns at 100 kmAnd so forthNote that an increase of 1 magnitude unit corresponds to a factor of

ten increase in ground shaking … the scale is logarithmic

So magnitude quantifies the amount of shaking caused by a fault, without saying anything particular about the physical size of the fault or the amount of slip on it …

But what about quantifying the size of the faulting ?

(Note the phrase “size of the faulting”, which is not the same as the “size of the fault”, since it must account both for the “size of the fault” (that is, its length and width), and the amount of slip that occurred.

Sumatra-Andaman Island Earthquake of Dec 26, 2004. The faulting was 15 meters of slip on a fault 1000 km long and 200 km wide.

Seismic Moment

Seismic Moment = Fault area times fault slip times rock rigidity

Faulting with high momentTend to causeEarthquakes with high

magnitude

With a factor of 30 increasein moment causinga one-unit inceasein magnitude (theline in the graph)

But the relationship is notexact. Muchvariability isobserved (the dotsin the graph)

Getting Back to Plate Boundaries …

“Maximum” size of earthquakeDivergent

mid-ocean ridge: magnitude 5continental rift: magnitude 7

Strike Slipoceanic fracture zone: magnitude 7continental fracture zone: magnitude 8

Convergentcollisional mountain belt: magnitude 8.5subduction zone: magnitude 9

Getting back to the US

Subduction zones are BIG sources of hazard !

• There are three subduction zones near the United States

• Aleutian Subduction Zone, in western Alaska. Magnitude 9.2 earthquake in 1964.

• Puerto Rico Subduction Zone. Magnitude 8.1 in 1946 near the Domincan Republic.

Cascadia

• The Cascadia Subduction zone (western Oregon and Washington) is capable of a magnitude 9 earthquake (although none have occurred there since the European settlement of that area in the early 1800’s)

• But on January 26, 1700 a large tsunami hit Japan. It was probably from a magnitude 9 earthquake on Cascadia.

How frequently do earthquakes occur?

There are many more small earthquake than large ones:

Magnitude range

number

8.0-9.9 1

7.0-7.9 14

6.0-6.9 127

5.0-5.9 1199

4.0-4.9 8143

World Earthquakes in 2001

1341 earthquakes with magnitudegreater than or equal to 5.0 in 2001 !

I’ve picked the lower limit of magnitude 5 because earthquakes that are smaller rarely cause significant damage.

Fortunately, most of these earthquakes occurred beneath the sea floor or in sparsely inhabited regions. Nevertheless, 23534 people died.

My Motto

There’s always the next earthquake …

The rate of earthquakes is fairly constant with time …

Gutenberg-Richter Statistics

This graph for 2001 data shown in a previous slide

The cumulative number of earthquakes per unit time (a year in this case) with a magnitude greater than or equal to a given value is highly predictable.

Can the red line be extrapolated to predict the rate of occurrence of the very largest earthquakes? Does a magnitude 9 really occur once every ten years, as the dotted red line would predict?

Why do Earthquake Cause Damage ?

“Earthquakes don’t killpeople …

… buildings kill people”

Prof. Chris ScholzColumbia University

Types of Earthquake Hazard

Ground Shakingbuilding and other structures collapse

Landslidesshaking causes collapse of hills

Tsunamisshaking causes ocean-crossing wavescoastal areas experience very rapid flooding

Ground Shaking

Quantified by ground acceleration

units: meters per second squared

or

percent of gravity (g=9.8 m/s2)

An ground shaking of 10% g is big enough to do significant damage, especially if it includes horizontal motions.

1994 Northridge Earthquake

maximum shaking exceeded 66% g (red) over a wide

area

but note that acceleration

decreases rapidly with distance

Landslide induced by 1994 Northridge Earthquake blocks Highway

Before and afterAerial photos of damageCause by tsunami fromDec. 26, 2004 Sumatra-Andaman IslandEarthquake.