The Shaking Earth: Earthquakes

Definitions

Normal Fault: One slab drops down (divergent)

Reverse Fault: One slab forced up (convergent)

Faulting: used to describe fault rupture, faulting causes earthquakes

Focus/Hypocentre: point where fault rupture first occurs

Epicentre: geographic point directly above the hypocentre on the Earths surface

Mainshock: earthquake, largest event; strong shaking from a major shock lasts 30-60 seconds

Aftershocks: small events after the mainshock

Foreshock: smaller events preceding the mainshock, but can only be identified after the mainshock occurs

Seismic Waves: Energy released by faulting, which moves outwards from the hypocentre

Describing Waves

Amplitude: corresponds to displacement

Wavelength: distance between successive waves

Period: time between waves (1/Frequency)

Frequency: number of waves in one second (1/Period)

Types of Seismic Waves

Body Waves: Pass through the entire Earth

Surface Waves: Only travel on Earth's outer layer

Primary Waves

Fastest Waves(short period), first to reach recording centres

Push-pull motion (Spring)

Move through liquids, solids, gases

Sudden audible jolt at epicentre

Secondary Waves

Second to reach recording station

Transverse in vertical plane motion (Shaking a rope)

Move through solids

Up & down, side to side shaking, that does damage to buildings

Love Waves

Travel only on surface of the Earth

Longer periods than body waves, meaning they can carry energy farther

Transverse on horizontal plane motion (Snake)

Can only move through solids

Long periods cause swaying of buildings, wave motion in bodies of water

Rayleigh Waves

Travel on surface of Earth, arrive last at recording stations

Backwards rotating motion (rolling boat at sea)

Moves through solids and liquids

More energy is released in the form of Rayleigh waves when the hypocentre is close to the surface

Produces vertical and horizontal shaking

How Seismic Waves Damage Buildings

Natural Frequency: Frequency at which a system vibrates naturally once it has been set into motion

Resonance: Tendency of a system to oscillate with a larger amplitude (sway more) when it is subjected to vibrations with a frequency that matches its natural frequency

All buildings have resonant frequencies depending on their height. When the frequency of seismic waves matches the resonant frequency of a building, the building resonates, and sways more violently

Short Buildings: short periods = high resonant frequencies = resonate with high frequency waves

Tall buildings: Long periods = low resonant frequencies = resonate with low frequency waves

Measuring Earthquakes

Seismographs(seismometers): instruments that record earthquake waves

Seismograms: traces(diagrams) of amplified, electronically recorded ground motion made by seismographs

Recorded amplitudes of seismic waves indicate the amount of energy released by the quake

Seismic monitoring: recording drum moves with the shaking ground, recording a pattern, called a seismogram(P waves recorded first)

Locating the Epicentre

Since P waves arrive first and S waves second, there two separate shocks. The farther from the epicentre, the longer the time delay between the two shocks(called the P-S interval)

Travel-Time graph plots the P-S gap vs. the distance from the epicentre, helping determine how far each station is from the epicentre

At least 3 seismograms from 3 different recording stations should be used to locate the epicentre

The 3 seismograms will produce three circles and the epicentre will be where the circles intersect

Classifying Earthquakes according to magnitude

Magnitude: refers to size; I.e the amount of energy stored in the fault before it ruptured

With increasing magnitude, shaking will last longer and spread outer a bigger area

Richter Scale

Measures the maximum amplitude on a seismogram and adjusts for distance from epicentre

Ranges from 1-10 and functions as a logarithmic scale(magnitude 6 is 1000 times bigger than magnitude 3)

Every one increase means energy release increases by 45 times, & spreads out over a larger area and longer time

Only for small to moderate, shallow quakes

Moment Magnitude (Mw)

Used to rank big quakes, because it takes into account:

The size of the ruptured area ion the fault plane

Amount of offset or slip movement along the fault plane

Strength of rocks on either side of fault plane

The longer the fault rupture, the greater the magnitude

High-Frequency Seismic Waves: caused by short rupture, and do more damage at the epicentre, then die out with distance

Low Frequency Seismic Waves: caused by a long rupture, and do more damage farther away, as they can travel far from epicentre

Classifying Earthquakes according to Intensity

Intensity: effects on people and buildings

Mercalli Scale: Ranges from 1-12, quantifies what people feel during an earthquake

Earthquakes only have one magnitude, but can have several intensities depending on where and when it strikes

Building in Earthquake Country

Damage to buildings occurs at magnitude 5

Soft Stories: large openings(garages, storefronts, unbraced windows) which are weak spots, because they are flexible

Fixes

Cross-braicng

Plywood reinforcement

Diagonal braces(for soft stories)

Steel jackets(for highway support columns)

Alternating layers of steel and concrete(for bridges)

Base Isolation(absorbs shaking, e.g wheels, ball bearings, rubber doughnuts)

Shear walls: designed to receive horizontal forces from floors, roofs, and trusses, and transmit them to the ground (parking garages)

Eliminating resonance: change height/shape, moving weight to lower floors, building tall flexible structures on hard foundations, or short stiff structures on soft foundations

Defining Hazard Risk

Earthquakes don't kill people, buildings do

Risk is higher in countries where no earthquake proof building codes exist

Hazard depends on

Population density

Construction Standards

Emergency preparedness

Primary vs. Secondary Effects

Primary

Ground Shaking: initial effect of earthquake

Ground Rupturing : permanent fissures/cracks 10-15m

Liquefaction: when water moves upwards, out of loose sandy soils with high moisture contents

Secondary

Landslides

Tsunamis

Fires

Droughts

Earthquake Forecasting: Precursor Events

Ground uplift/tilting:due to swelling of rocks from building pressure

Higher or lower than normal water levels in wells

Emission of Radon Gas: emissions show up in well water

Unusual Radio waves

Strange animal behaviour