Earthquakes Last time we considered faults Today we

Earthquakes Last time we considered faults. Today we discuss the consequence of a fault’s movement 1. 2. 3. 4. Slinky Beaker, Wet Sand, Weight Ball Point Pen A seismogram

What is an earthquake? n An earthquake is the vibration of Earth produced by the rapid release of energy § Rock subjected to elastic deformation snaps back and/or breaks § Energy radiates in all directions from the break’s location, AKA source, the “focus” § Energy moves like waves as moving rocks push on their neighbors § Seismographs record the event

Anatomy of Earthquakes

n Earthquakes and faults § Earthquakes are associated with faults § Motion along faults can be explained by plate tectonics

Causes of earthquakes n Sudden release of accumulated strain energy n Creation of fracture zones at faults by rupturing rocks n Creation of new faults by rupturing rocks n Shifting of rocks at preexisting faults n Deformed rock straightens back to original shape, but offset from matching beds on the other side of the fault.

Elastic rebound 1 § Mechanism for Earthquakes – Rocks on sides of fault are deformed by tectonic forces – Rocks bend and store elastic energy – Frictional resistance holding the rocks together is overcome by tectonic forces

Elastic rebound 2 § Earthquake mechanism – Slip starts at the weakest point (the focus) – Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound) – The motion moves neighboring rocks – And so on.

Aftershocks The change in stress that follows a main shock creates smaller earthquakes called aftershocks The aftershocks “illuminate” the fault that ruptured in the main shock Red dots show location of aftershocks formed by 3 earthquakes in Missouri and Tennessee in 1811/1812 Symbols: Reelfoot Fault is a Reverse Fault, teeth point to the upthrown side, so to the hanging wall. Cottonwood Grove a Transform Fault, note the arrows pointing in the directions of movement

Normal Fault Quake - Nevada Reverse Fault Quake - Japan Divergent HW Down HW Up Convergent Transform Strike Slip Fault Quake - California

San Andreas: An active earthquake zone San Andreas is the most studied strike-slip (transform) fault system in the world n Displacement occurs along discrete segments 100 to 200 kilometers long § Most segments slip every 100 -200 years producing large earthquakes § However, some portions exhibit slow, gradual displacement known as fault creep. The rocks have low strength minerals that cannot store strain, they just crumble and smear continuously as the plates move. n

Fence offset by the 1906 San Francisco earthquake San And res Fault

Landscape Shifting, Wallace Creek San Andres Fault

San Andreas Fault

Earthquake Hazards n Four major hazards occur during earthquakes n One is well known: the collapse of buildings crushes people n Three more are less well known – Fire – Liquifaction – Tsunami

Fires caused by 1906 San Francisco Earthquake Gas mains break, fires shaken out of furnaces and fireplaces. Water mains break. Debris blocks streets. Fire Fighters cannot drive to the fire.

Liquefaction Crystals from dredge muds are arranged like pick-up-sticks. Pressure waves from the earthquake force the crystals apart. Now your house is being supported by water. Makes “quick clay”

Before we consider Tsunami n We need some background in seismology

Seismology Seismometers - instruments that record seismic waves §Formerly: Recorded the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape §Today: use motion sensors similar to those in your smart phone

A seismograph designed to record vertical ground motion The heavy mass doesn’t move much The drum moves

Lateral Movement Detector In reality, copper wire coils move around magnets, generating current which is recorded.

n Types of seismic waves § Surface waves –Complex motion, great destruction –High amplitude and low velocity –Longest periods (interval between crests) –Termed long, or L waves

Two Types of Surface Waves Most of the destruction Larger amplitude than body waves

n Types of seismic waves (continued) § Body waves – Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves § Push-pull motion § Travel thru solids, liquids & gases – Secondary (S) waves § Moves at right angles to their direction of travel § Travels only through solids

P and S waves Smaller amplitude than surface (L) waves, but faster, P arrives first

Locating the source of earthquakes Focus - the place within Earth where earthquake waves originate Epicenter – location on the surface directly above the focus n Epicenter is located using the difference in velocities of P and S waves

Earthquake focus and epicenter

Delay between P and S arrivals gives distance to epicenter Note how much bigger the surface waves (aka L waves) are. Body Waves P to S delay

Graph to find distance to epicenter Average S wave speed is 3800 km in 12 minutes Average P wave speed is 3800 km in 7 minutes

Locating the epicenter of an earthquake § Three seismographs from different observatories needed to locate an epicenter § Each station determines the time interval between the arrival of the first P wave and the first S wave at their location § A travel-time graph then determines each station’s distance to the epicenter

Locating Earthquake Epicenter

n Locating the epicenter of an earthquake § A circle with radius equal to distance to the epicenter is drawn around each station § The point where all three circles intersect is the earthquake epicenter

Epicenter located using three seismographs

Earthquake Belts 95% of energy released by earthquakes originates in narrow zones that wind around the Earth §These zones mark of edges of tectonic plates

Locations of earthquakes 80% of seismic energy around from 1980 to 1990 Pacific Rim Broad bands are subduction zone earthquakes, narrow are MOR

Depths of Earthquakes originate at depths ranging from 5 to nearly 700 kilometers Definite patterns exist – Shallow focus occur between mid-ocean ridges – Deep earthquakes occur in Pacific landward of oceanic trenches – Central continent (intraplate) earthquakes are of various causes. Some causes still uncertain. Devastating earthquakes occur less than 60 kilometers because cold rock is more elastic, and transmits waves better than warmer rocks below

Earthquake Depth and Plate Tectonic Setting Weakest are the divergent zone earthquakes Strongest Here Strongest, with worst Subduction Zones discovered by Benioff Tsunamis, at entrance to subduction zones

Earthquakes in subduction zones Recent example, 9. 0 Christmas 2004 Earthquake and Tsunami, Sumatra

Earthquakes at Divergent Boundaries - Iceland A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR

Measuring the size of earthquakes n Two measurements describe the size of an earthquake § Intensity – a measure of earthquake shaking at a given location based on amount of damage to buildings. § Magnitude – estimates the amount of energy released by the earthquake.

Intensity scales § Modified Mercalli Intensity Scale was developed using California buildings as its standard § Drawback is that destruction may not be true measure of earthquakes actual energy

Earthquake destruction n Amount of structural damage depends on § Intensity and duration of vibrations § Nature of the material upon which the structure rests (hard rock good, soft bad) § Design of the structure

Magnitude scales § Richter magnitude - concept introduced by Charles Richter in 1935 § Richter scale – Based on amplitude of largest seismic wave recorded – LOG 10 SCALE Each unit of Richter magnitude corresponds to 10 X increase in wave amplitude and 32 X increase in Energy

Magnitude scales § Moment magnitude was developed because Richter magnitude does not closely estimate the size of very large earthquakes – Derived from the amount of displacement that occurs along a fault and the area of the fault that slips

Tsunamis, or seismic sea waves § Incorrectly called “tidal waves” § Result from “push” of underwater fault or undersea landslide § In open ocean wave height is < 1 meter § In shallow coast water wave can be > 30 meters (more than about 98 feet) § Very destructive

Formation of a tsunami 3. Deep water Wave 1 meter above surface, very fast 2. Wave as deep as water 1. Water pushed up SNAP 4. In shallows wave rears up, and slows down.

The Hawaiian Islands vulnerable. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

Tsunami 1960, Hilo Hawaii

Tsunami Model, Japan Earthquake

Tsunami Model, Alaska Quake

Earthquake prediction n Long-range forecasts § Calculates probability of a certain magnitude earthquake occurring over a given time period n Short-range predictions § Ongoing research, presently not much success

Long Term Predictions Seismic Gaps

Long Term Predictions n Strain Energy - accumulates uniformly - release irregularly n Some locked by friction “Seismic gaps” – Prime candidates for major earthquake n Some release energy continuouslycreep – No major earthquakes there

Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

Seismic Gap along Himalayas USGS web page on October 8 2005 magnitude 7. 6 in Pakistan

Can earthquakes be predicted? Short Term, Not very well n Short-range predictions § Goal: provide warning location & magnitude within a narrow time frame § Research on precursors due breaking prior slip. Breaking of rock lets the fault slip § Breaks called fracture zones. The thickness of the fracture zone is proportional to the length of the fault. § Breaking causes volume increase (dilation) and uplift in the rocks. § Dilatancy causes many measurable changes

Short-Term Earthquake Prediction Dilatancy of Highly Stressed Rocks BOX OF ROCKS 58

Investigating Earth’s Interior n Earthquakes help us understand Earth’s Interior Structure. We use: n Speed changes in different materials due changes rigidity, density, elasticity n Reflections from layers with different properties n Attenuation of Shear Waves in fluids n Direction changes (Refraction)

Result: 3 Major Layers of Earth 60

Shallow Components of Earth !

Seismic-wave velocities are faster in the upper mantle Mohorovičić discontinuity Croatian seismologist Andrija Mohorovičić Velocity increases w depth, waves bend back to surface. Waves that travel via mantle arrive sooner at far destinations

Wave Velocities Crust slow Mohorovičić discontinuity Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast

Mineralogy of Earth’s Layers This slide for graduate students. http: //pubs. usgs. gov/gip/interior/

The S-Wave Shadow Zone Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a large shadow for S waves covering everything past 103 degrees away from the source.

The P-Wave Shadow Zone Discovery of the solid inner core P-waves pass ing through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid. Inge Lehmann Behavior of waves through center reveals Earth’s Interior

End of Earthquakes