Locating the source of earthquakesLocating the source of earthquakes

FocusFocus - the place within Earth where - the place within Earth where earthquake waves originateearthquake waves originate

EpicenterEpicenter on an earthquakeon an earthquake– location on – location on the surface directly above the focusthe surface directly above the focus Epicenter is located using the difference in Epicenter is located using the difference in velocities of P and S wavesvelocities of P and S waves

Earthquake focus and epicenterEarthquake focus and epicenter

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

Three seismographs needed to locate an epicenterThree seismographs needed to locate an epicenter

Each station determines the time interval between the Each station determines the time interval between the arrival of the first P wave and the first S wave at their arrival of the first P wave and the first S wave at their locationlocation

A travel-time graph then determines each station’s A travel-time graph then determines each station’s distance to the epicenter distance to the epicenter

On a map, draw a circle around the seismic stationOn a map, draw a circle around the seismic station

Radius of circle= distance to epicenterRadius of circle= distance to epicenter

Repeat for other seismic stations (Triangulation)Repeat for other seismic stations (Triangulation)

The three circles will meet at a point; that point is the The three circles will meet at a point; that point is the epicenterepicenter

Travel-time graph used to find distance to Travel-time graph used to find distance to epicenter epicenter

The difference inarrival time of theFirst P wave and SWave in the Example is 5 minutes

Typical seismic record. Note the time interval (about 5 minutes) between the arrival of the first P waves and the arrival of the first S waves

The records obtained from seismographs, called seismograms, provide a great dealOf information about the behavior of seismic waves

Earthquake epicenter is located using the distance Earthquake epicenter is located using the distance obtained from three seismic stationsobtained from three seismic stations

Earthquake beltsEarthquake belts95% of energy released by earthquakes 95% of energy released by earthquakes originates in narrow zones that wind originates in narrow zones that wind around the Eartharound the Earth

These zones mark of edges of tectonic These zones mark of edges of tectonic platesplates

Locations of EarthquakesLocations of Earthquakes

Most occur along tectonic plate boundaries:Most occur along tectonic plate boundaries:– Around the Pacific Ocean (Circum-Pacific belt is major Around the Pacific Ocean (Circum-Pacific belt is major

site of earthquakes) site of earthquakes) (( الهادي للمحيط المطوق الهادي الحزام للمحيط المطوق ((الحزام– Mediterranean Sea area through Iran and on the Mediterranean Sea area through Iran and on the

HimalayasHimalayas

– Asia (Indonesia, Himalayan region)Asia (Indonesia, Himalayan region)

– Mid ocean ridgesMid ocean ridges ( ( المحيطات وسط المحيطات مرتفعات وسط ((مرتفعات

Some occur far from plate edges Some occur far from plate edges

Locations of earthquakes with magnitude equal to Locations of earthquakes with magnitude equal to or greater than 5 for the period 1980 to 1990or greater than 5 for the period 1980 to 1990

Circum Pacific Belt

Oceanic ridge system

Depths of EarthquakesDepths of EarthquakesEarthquakes originate at depths ranging Earthquakes originate at depths ranging from 5 to nearly 700 kilometersfrom 5 to nearly 700 kilometers Definite patterns existDefinite patterns exist

– Shallow focus occur along oceanic ridgesShallow focus occur along oceanic ridges(less than 70 km) (less than 70 km) - Intermediate Earthquakes: 60-300 km- Intermediate Earthquakes: 60-300 km– Deep earthquakes occur in western Pacific Deep earthquakes occur in western Pacific

east of oceanic trencheseast of oceanic trenches (greater (greater than 300 km than 300 km -90 % of earthquakes are found in a depth -90 % of earthquakes are found in a depth

less than 100 kmless than 100 km

Earthquake in subduction zones Earthquake in subduction zones

Measuring the size of Measuring the size of earthquakesearthquakes

Two measurements describe the size of an Two measurements describe the size of an earthquakeearthquake

IntensityIntensity – a measure of earthquake shaking at a – a measure of earthquake shaking at a given location based on amount of damagegiven location based on amount of damage

Intensity of an earthquake depends on:Intensity of an earthquake depends on:– Distance to epicenterDistance to epicenter– Building materials and designBuilding materials and design– Type of ground material-soil, rock, etc. Type of ground material-soil, rock, etc.

MagnitudeMagnitude – estimates the amount of energy – estimates the amount of energy released by the earthquakereleased by the earthquake

Intensity scalesIntensity scales Modified Mercalli Intensity ScaleModified Mercalli Intensity Scale was was developed using California buildings as its developed using California buildings as its standardstandard

Ranges from I (felt by only a few) to XII Ranges from I (felt by only a few) to XII (total destruction)(total destruction)

Magnitude scalesMagnitude scales Richter magnitudeRichter magnitude - concept introduced by Charles Richter - concept introduced by Charles Richter in 1935in 1935 Richter scaleRichter scale

– Measures earthquake magnitudeMeasures earthquake magnitude– Determined by measuring the amplitude of the largest Determined by measuring the amplitude of the largest

waves on the the seismogramwaves on the the seismogram– A logarithmic scaleA logarithmic scale– Each number on the Richter Scale is tens times greater Each number on the Richter Scale is tens times greater

in wave amplitude. The wave amplitude of an in wave amplitude. The wave amplitude of an earthquake of “magnitude 6”is 10 times more than the earthquake of “magnitude 6”is 10 times more than the amplitude of a “ magnitude 5 earthquake”amplitude of a “ magnitude 5 earthquake”

– Each number on the Richter Scale involves an energy Each number on the Richter Scale involves an energy release about 32 times as great. Thus, an earthquake release about 32 times as great. Thus, an earthquake with a magnitude of 6.5 releases 32 times more energy with a magnitude of 6.5 releases 32 times more energy than one with a magnitude of 5.5than one with a magnitude of 5.5

Magnitudes scalesMagnitudes scalesMoment magnitudeMoment magnitude was developed was developed because Richter magnitude does not because Richter magnitude does not closely estimate the size of very large closely estimate the size of very large earthquakesearthquakes

– Derived from the amount of Derived from the amount of displacement that occurs along a fault displacement that occurs along a fault and the area of the fault that slipsand the area of the fault that slips

Earthquake destructionEarthquake destruction

Amount of structural damage depends onAmount of structural damage depends on Intensity and duration of vibrationsIntensity and duration of vibrations

Nature of the material upon which the Nature of the material upon which the structure rests (hard rock good, soft bad)structure rests (hard rock good, soft bad)

Design of the structureDesign of the structure

Results of EarthquakesResults of Earthquakes

Vibration of the ground-Causes damage to structuresVibration of the ground-Causes damage to structures

Thixotropic sediment causes ground to flow. Thixotropic is a Thixotropic sediment causes ground to flow. Thixotropic is a term used to refer to a solid that flows when vibrated or joltedterm used to refer to a solid that flows when vibrated or jolted

Tsunami-seismic sea wavesTsunami-seismic sea waves

FiresFires

LandslidesLandslides

Well water levels fluctuate; 1964 Alaska caused wells in SE Well water levels fluctuate; 1964 Alaska caused wells in SE US to fluctuate by 3.5 meterUS to fluctuate by 3.5 meter

TsunamisTsunamis, or seismic sea waves, or seismic sea waves

Earthquakes under the oceanEarthquakes under the ocean

Destructive waves called “tidal waves”Destructive waves called “tidal waves”

Result from “push” of fault block or Result from “push” of fault block or under sea landslide on waterunder sea landslide on water

In open ocean height is > 1 meterIn open ocean height is > 1 meter

In shallow coast water wave can be > 30 In shallow coast water wave can be > 30 metersmeters

Very destructiveVery destructive

Formation of a tsunami by displacement of Formation of a tsunami by displacement of the ocean floorthe ocean floor

The speed of a wave moving across the ocean correlateswith the ocean depth.

Tsunami 1960, HawaiiTsunami 1960, Hawaii

Can earthquakes be predictedCan earthquakes be predicted

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

Earthquakes cannot be predictedEarthquakes cannot be predicted

Short-range predictionsShort-range predictions Currently, no method exists for making Currently, no method exists for making short-range earthquake predictions short-range earthquake predictions

Long-range forecasts Long-range forecasts Calculates probability of a certain Calculates probability of a certain magnitude earthquake occurring over a magnitude earthquake occurring over a given time periodgiven time period

Probability of a major earthquake from 1988 to 2018 on the San Andreas Fault

Major historical earthquakesMajor historical earthquakes

1811 and 1812-Three large earthquakes, the largest 1811 and 1812-Three large earthquakes, the largest in U.S. history occurred near New Madrid, in U.S. history occurred near New Madrid, Missouri (7.9)Missouri (7.9)

1886 Charleston, South Carolina (7.9)1886 Charleston, South Carolina (7.9)

1906 San Francisco, CA (7.8)1906 San Francisco, CA (7.8)

1964 Alaska (9.2). Strongest earthquake to hit US 1964 Alaska (9.2). Strongest earthquake to hit US this centurythis century

1989 San Francisco, California (Loma Prieta) (6.1)1989 San Francisco, California (Loma Prieta) (6.1)

Importance of EarthquakesImportance of Earthquakes

Earthquake waves provide information about Earthquake waves provide information about the internal structure and composition of earththe internal structure and composition of earth

Earthquakes are indictors of present-day Earthquakes are indictors of present-day tectonic activity or earth movementstectonic activity or earth movements