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 Boundary Plates move towards one another Oceanic 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 Boundary Plates move away from one another Oceanic 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 Boundary Plates slide past one another Oceanic 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 plate boundary offshore) California Strike Slip boundary the San Andreas Fault

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

Oops – pattern Not quite what we expected!

California OK But why: None in Cascadia Some east of Mississippi!

Why?

Hey! What about Alaska, Hawaii and Puerto Rico?

Quantifying Earthquake Size

Size, a tricky buisness … What is a big person?

a tall person, with height in meters a heavy person, with weight in kilograms a rich person, with fortune in dollars an influential person, with influence in % of population impacted

Richter: an earthquake is big when the ground shakes a lot

Earthquake Magnitude

An earthquake’s size is defined to be Magnitude 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 km Its Magnitude 5 if it causes 36 microns at 100 km And so forth Note 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 moment Tend to cause Earthquakes with high magnitude With a factor of 30 increase in moment causing a one-unit incease in magnitude (the line in the graph) But the relationship is not exact. Much variability is observed (the dots in the graph)

Getting Back to Plate Boundaries …

“Maximum” size of earthquake Divergent mid-ocean ridge: magnitude 5 continental rift: magnitude 7 Strike Slip oceanic fracture zone: magnitude 7 continental fracture zone: magnitude 8 Convergent collisional mountain belt: magnitude 8.5

subduction 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: World Earthquakes in 2001 Magnitude range 8.0-9.9

7.0-7.9

6.0-6.9

5.0-5.9

4.0-4.9

number 1 14 127 1199 8143

1341 earthquakes with magnitude greater 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

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?

This graph for 2001 data shown in a previous slide

Why do Earthquake Cause Damage ?

“Earthquakes don’t kill people … … buildings kill people” Prof. Chris Scholz Columbia University

Types of Earthquake Hazard

Ground Shaking building and other structures collapse Landslides shaking causes collapse of hills Tsunamis shaking causes ocean-crossing waves coastal areas experience very rapid flooding

Ground Shaking

Quantified by ground acceleration units: meters per second squared or percent of gravity (g=9.8 m/s 2 ) 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 after Aerial photos of damage Cause by tsunami from Dec. 26, 2004 Sumatra Andaman Island Earthquake.