Earthquakes And Earth’s Structure

Earth’s Structure View of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?

Origin of the Earth Meteors and Asteroids bombarded the Earth Gravitational compression Our early Earth grew from a barrage of extraterrestrial impacts, increasing in mass over time. In the early stages of planetary accretion, the Earth was much less compact that it is today. The accretionary process led to an even greater gravitational attraction. Gravitational energy converted into heat. Radioactive elements are unstable, and over time become more stable. Radioactive decay processes releases heat. U-238 to Pb-206. Initial accretion led to a homogeneous sphere that eventually became density stratified with the heavier iron and nickel sinking to the center of the earth, and the lighter silicates (rocky) floated to the top. Density Stratified planet

Earth’s Interior Core Mantle Crust dense Iron and Nickel Inner Core - solid Outer Core - liquid Mantle Less dense than core Iron and Magnesium silicates Mostly solid Upper mantle is partially molten Core – composed of two parts. The core is very dense and composed mostly of Iron and Nickel. The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form. The outer core is liquid iron and nickel. Again, it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core. Mantle is composed of Iron and Magnesium silicates (Si + O). It is mostly solid, but the uppermost part of the mantle is partially molten (slush – where you do have liquid and solid mixed). Now, this is very important in plate tectonics. Crust –This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid. If you can imagine an apple. It would have the thickness of the skin of an apple. There is two types of crust – continental crust and oceanic crust. Important notes: Thin crust + uppermost mantle is rigid. This forms the “lithospheric plates”. partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth. Crust Outermost layer Very thin and rigid Continental – granite Density = 2.8 g/cm3 Oceanic – basalt Density = 3.0 g/cm3

Evidence of Internal Structure Density calculate density of Earth Speculate on probable compositions Basalt Granite Meteorites Use composition and age to determine composition and age of Earth By calculation, the Earth’s density is 5.8 gm/cm3. In the Sierras, composed mostly of granite, represents the thin part of Earth’s interior, i.e. skin of an apple. If you calculate the Earth’s surface using granite, it comes to 2.8 gm/cm3. What is this telling us? There are heavier rocks in the Earth. We know the age of the earth is 4.6 Ga. People who study meteorites determined their age to be about 4.5 Ga – same as Earth. Meteorites are composed of Iron and Nickel. It is hypothesized the Earth’s interior is composed of the same elements. P-waves travel 4x faster than S-waves and also travel through different material. These also give us some clue on the properties of the Earth’s interior. Seismic waves Travel times and direction give indication of internal structure of Earth

Types of Seismic Waves P waves S waves Primary waves Push and pull movement Travel fastest (~ 6 km/sec) Travel thru solids and liquids S waves Secondary waves Move side-to-side Slower (~ 4 km/sec) Travel thru solids only

Seismic Waves Through Earth Scientists use waves generated by earthquakes to determine the Earth’s interior. that the outer core of the earth is liquid. Earthquakes generate P-waves and S-waves within the earth. Shadows occur on the opposite side of the earth from the earthquake epicenter because the outer core reflects S-waves, and bends P-waves. S-waves are reflected because they cannot travel through liquids, and they cast a larger shadow than the bent P-waves. Geologists and seismologists determined the size of the outer core by using the 154-degree arc of the S-wave shadow and measurements taken on the surface of the earth.

Parts of an Earthquake Focus Epicenter place within earth where EQ originate Epicenter Location on surface directly above focus Energy radiates in all directions from the focus Energy is in the form of waves – “seismic waves”

Seismic Waves Locating the Epicenter Seismograph Instruments around the world record EQ Records Earth movement by stationary mass on rotating drum Seismogram Locate an epicenter Determine magnitude

Seismogram Help determine the epicenter Measure the distance between P- and S-waves This is the time difference in arrival times

Locating the Epicenter Plot the time difference on y-axis (time interval) Trace plot to blue line to determine distance to epicenter

Locating an Epicenter Triangulation Measure that distance around the seismic station The epicenter may be located anywhere on that line A minimum of 3 seismic stations are needed to determine the epicenter

Earthquake Strength Richter Scale A measure of the strength of an earthquake or the amount of energy released

Determining Magnitude Measure amplitude of the strongest wave Amplitude is the height on paper Plot distance between p- and s-wave Plot amplitude Connect plots to determine magnitude

Earthquake Intensity Modified Mercalli Scale Effects of an EQ vary from place to place There is only 1 magnitude, but intensity can vary Subjective scale

Intensity Map – Loma Prieta 1989

Oakland, 1989 The Yanks and Mets are currently playing in the fisrt subway World Series since 1956. However, 11 years ago, the bay-side cities of Oakland and San Francisco played in a World Series shocker (sorry). The 7.1 magnitude Loma Prieta earthquake occurred on October 17, 1989. This powerful quake, the strongest in San Francisco since 1906, struck just before the third game of the series. Across the bay area, the quake was responsible for 62 deaths, and damage estimates neared the 5 billion dollar mark. A number of lives were lost when elevated highways, such as the Cypress viaduct of Interstate HIghway 880 in Oakland (pictured above), collapsed or "pancaked." H. G. Wilshire of the U.S.G.S took this remarkable photograph. In addition, the shaker triggered a 4 foot tsunami in Monterey Bay. Parts of San Francisco were dark for three days, and the World Series didn't resume until October 27. For the record, the Oakland As swept the San Francisco Giants in the 89' Series, 4 games to 0. The part of the Cypress freeway structure in Oakland, California, that stood on soft mud (dashed red line) collapsed in the 1989 Loma Prieta earthquake, killing 42 people. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. Seismograms (upper right) show that the shaking was especially severe in the soft mud. (Photograph by Lloyd S. Cluff) Oakland is farther from epicenter, but scored a 9 on Intensity Scale

You are now a Seismologist!! Virtual Earthquake Introduction: http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeExecute/htm Virtual Earthquake: http://sciencecourseware.org