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.
Discussion What factor promote big earthquakes?
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 BoundaryBiggest Earthquakes collisional mountain belt: “up to” magnitude 8.5 subduction zone: “up to” magnitude 9.5
Divergent Plate BoundarySmallish Earthquakes Divergent mid-ocean ridge: “up to” magnitude 5 continental rift: “up to” magnitude 7
Strike-Slip Plate Boundaryintermediate oceanic fracture zone: “up to” magnitude 7 continental fracture zone: “up to” magnitude 8
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
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 1964. • 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: 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 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 …
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
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/s2) An ground shaking of 10% g is big enough to do significant damage, especially if it includes horizontal motions.
1994 Northridge Earthquakemaximum shaking exceeded 66% g (red) over a wide areabut note that acceleration decreases rapidly with distance
Before and after Aerial photos of damage Cause by tsunami from Dec. 26, 2004 Sumatra- Andaman Island Earthquake.
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?
Why might long-term predictions be possible ? Most earthquakes are on plate boundaries Plate motions are very constant over long periods of time Faults at plate-boundaries are long term features Long-term fault slip rate of faults are fairly constant Segments of faults seem to rupture time and time again in similar earthquakes Earthquake occurs when loading exceeds strength
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
Loading rate correlates With plate-tectonic motions Maximum load Long-term Predictability Earthquake occurs when loading exceeds strength loading Eq Eq Now: where Are we in the Loading cycle ? time, years
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