Earthquakes

What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy radiates in all directions from its source, the focus Energy moves like waves Seismographs record the event

Deformation Deformation is a general term that refers to all changes in the original form and/or size of a rock body Most crustal deformation occurs along plate margins Deformation involves Stress—Force applied to a given area

Deformation How rocks deform General characteristics of rock deformation Elastic deformation—The rock returns to nearly its original size and shape when the stress is removed Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation)

Folds During crustal deformation rocks are often bent into a series of wave-like undulations called folds Characteristics of folds Most folds result from compressional stresses which shorten and thicken the crust

Folds Common types of folds Anticline—Upfolded or arched rock layers Syncline—Downfolds or troughs of rock layers Depending on their orientation, anticlines and synclines can be described as Symmetrical, asymmetrical, or recumbent (an overturned fold)

Anticlines and Synclines Figure 6.20

Faults Faults are fractures in rocks along which appreciable displacement has taken place Sudden movements along faults are the cause of most earthquakes Classified by their relative movement which can be Horizontal, vertical, or oblique

Normal Fault Figure 6.24 A

Reverse Fault Figure 6.24 B

Strike-Slip fault Figure 6.24 D

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

Elastic rebound Mechanism for EQ’s explained by H. Reid 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 Earthquake mechanism Slips starts at the weakest point (the focus) occurs Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)

Elastic Rebound Movie in Mapview

Reid elastic rebound cartoon

Aftershocks The change in stress that follows a mainshock creates smaller earthquakes called aftershocks The aftershocks “illuminate” the that ruptured in the mainshock Red dots show location of aftershocks formed by 3 earthquakes in Missouri and Tennessee in 1811/1812

Normal Fault Quake - Nevada Strike Slip Fault Quake - Japan Strike Slip Fault Quake - California

San Andreas: An active earthquake zone San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Most segments slip every 100-200 years producing large earthquakes Some portions exhibit slow, gradual displacement known as fault creep

Fence offset by the 1906 San Francisco earthquake

Seismology Seismometers - instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

A seismograph designed to record vertical ground motion

Motion of a seismograph

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

Types of seismic waves 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-Wave Motion

S-Wave Motion

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

Earthquake focus and epicenter

Locating the epicenter of an earthquake Three seismographs 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

Graph used to find distance to epicenter

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 from 1980 to 1990

Depths of Earthquakes Earthquakes originate at depths ranging from 5 to nearly 700 kilometers Definite patterns exist Shallow focus occur along oceanic ridges Deep earthquakes occur in western Pacific east of oceanic trenches

Earthquake in subduction zones

Measuring the size of earthquakes Two measurements describe the size of an earthquake Intensity – a measure of earthquake shaking at a given location based on amount of damage 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 severity

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

Magnitudes 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

Earthquake destruction 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

Tsunamis, or seismic sea waves Destructive waves called “tidal waves” Result from “push” of fault block or undersea landslide on water In open ocean height is > 1 meter In shallow coast water wave can be > 30 meters Very destructive

Formation of a tsunami

Tsunami 1960, Hilo Hawaii

Tsunami Model, Japan Earthquake

Tsunami Model, Alaska Quake

Can earthquakes be predicted Short-range predictions Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame Research has concentrated on monitoring possible precursors – phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks

Earthquakes cannot be predicted Short-range predictions Currently, no method exists for making short-range earthquake predictions Long-range forecasts Calculates probability of a certain magnitude earthquake occurring over a given time period