Earthquake Case Study

Discussion Have you ever felt an earthquake?

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.

Example: San Andreas fault

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

Step 1

Step 2

Step 3

Discussion What factor promote big earthquakes?

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)

Convergent Plate Boundary Biggest Earthquakes collisional mountain belt: “up to” magnitude 8.5 subduction zone: “up to” magnitude 9.5

Divergent Plate Boundary Smallish Earthquakes Divergent mid-ocean ridge: “up to” magnitude 5 continental rift: “up to” magnitude 7

oceanic fracture zone: “up to” magnitude 7 continental fracture zone: “up to” magnitude 8 Strike-Slip Plate Boundary intermediate

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

Discussion Should we give up on the Richter Scale, and switch to something non-logarithmic ? For example, something that directly measures fault size? e.g. fault area  fault slip

Discussion There are three subduction zones near the United States What are they? Which one is the riskiest?

Aleutian Subduction Zone, in western Alaska. Magnitude 9.2 earthquake in Puerto Rico Subduction Zone. Magnitude 8.1 in 1946 near the Dominican Republic. 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 World Earthquakes in 2001

Discussion If there are only 15 earthquakes per year in the world with magnitude  7 How fequent are they given region? What are the implications in terms of education?

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, people died.

My Motto There’s always the next earthquake …

Why do Earthquake Cause Damage ? “Earthquakes don’t kill people … … buildings kill people” Prof. Chris Scholz Columbia University

A building that ‘pancaked’ during an earthquake

School collapses all near fault

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

Discussion What are “risk factors” for each of these: Building and other structures collapse Landslides Tsunamis

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.

Earthquake Predictibility, Forcasting and Early Warning

Summary Are long-term predictions of earthquakes possible? Are short short-term (or intermediate term) predictions of earthquakes possible? Can specific earthquakes be forecast? Are a few seconds or minutes of Early Warning useful. Is rapid assessment useful?

1.Most earthquakes are on plate boundaries 2.Plate motions are very constant over long periods of time 3.Faults at plate-boundaries are long term features 4.Long-term fault slip rate of faults are fairly constant 5.Segments of faults seem to rupture time and time again in similar earthquakes 6.Earthquake occurs when loading exceeds strength Why might long-term predictions be possible ?

Long-term Predictability Most earthquakes are on plate boundaries

Almost no new faults Faults grow slowly A big earthquake on a fault tends to increase the length of the fault The bigger the fault, the bigger the earthquake that can occur

Strategy: map the faults to determine where earthquakes will occur (but look for evidence of recent motion, make sure it’s a recently active fault)

Problem: deeply buried faults, such as blind thrusts (especially if they have few small earthquakes) (example fault that caused 1994 Northridge Earthquake) (But now we know it’s there!)

Long-term Predictability Faults segmentation: characteristic large earthquakes

Segmentation in Japan

Long-term Predictability Earthquake occurs when loading exceeds strength time, years loading Maximum load Loading rate correlates With plate-tectonic motions Eq Now: where Are we in the Loading cycle ?

Results of this kind of analysis Is a prediction of likelihood of a large earthquake on each segment of each fault Assuming: long-term loading rates determined by GPS and/or geological studies and closeness to failure based on when last large event occurred

Detectable changes in fault behavior as it approaches failure Examples: Foreshocks – small earthquakes that occur before the big one – short term Seismicity rate changes – increase in rate of moderate earthquakes prior to the big one – intermediate term Why might short-term or intermediate- term predictions be possible ?

Foreshock little one before the Big One In California, foreshocks occur less than 5 days before about half of the large earthquakes. For these reasons, the California Office of Emergency Services issues an advisory of an increased likelihood of a major earthquake within the next 5 days following moderate-sized earthquakes.

Discussion What can you do with a prediction of an earthquake ? Especially if it has low skill

Short term heightened emergency preparedness curtain endangered activities evacuate people Intermediate term redirect preparedness funds re-site future construction

Early Warning - or every second counts -

How long do you have ? Strong ground motion sensors city 50 km distant At 2 km/s shear wave velocity Is 25 seconds Minus 10 seconds to Detect strong motion at a Few stations near fault Is … 10 km fault 50 km

15 seconds

50 km 100 km But say the damaging effects extend to 100 km … … There may be a lot of people & structure in the >15 second warning region area > 15 second warning < 15 second warning

For this to have any hope of working you must plaster the earth with sensors capable of detecting strong ground motion and immediately sending that information to a processing and distribution center

Seismic Intensity Stations in Japan

So little time is available that both the announcement of impending strong shaking and the response must be fully automated

Discussion How much are you willing to trust automation? And to do what?

What can you do in 15 seconds ? Shut down delicate or dangerous equipment Have people dive for shelter (?)

Just knowing where the strong shaking occurred can help in formulating an emergency response Strong ground motions after the 1995 Kobe, Japan earthquake