Earthquakes

Solid Earth SeismologyTheory, Practice, and Application to the Earth's Interior

GEOL 595 SP/L

Text Book: Introduction to SeismologyBy Peter M. Shearer

What is Seismology ?

From the Greek root word: “seismos” to shake

Seismic Waves: elastic wave energy that propagates in solid or fluid materials.

Seismology: the study of earthquakes and the elastic waves generated by the rupture.


Seismic Waves: elastic wave energy that propagates in solid or fluid materials.

- energy creation (earthquakes, nukes, bombs, eruptions, sonic booms)

- energy transmission (propagation, absorption, scattering, reflection, diffraction)

- energy recording (seismometry, computer storage, data transmission, etc)

- societal (e.g., forecasting earthquakes, tsunami, seismic)


Seismology bridges:

Geology + Geophysics + Earth Science

Seismic imaging at depth can verify whether geologicalobservations (e.g. faults, plate boundaries,ancient sutures, granitic outcrops) on the surface continue to deeper depths in the crust or mantle.

Earthquake aftershocks can help us map faults and plate boundaries at depth in the crust and mantle.

Measurements of seismic anisotropy can indicate whether mineralogical alignment occurs during plate movement or shear.

Volcano seismology measure earthquake patterns that may indicate magma rising through a volcano chamber

Mount St Helen'sMount St Helen's

* Erupted 1980, Washington State* Local earthquakes felt 2 months before* Evacuations saved 1000's of people* 57 people died including Inn keeper Harry Truman, a photographer, a geologist, and many animals.

movie

Earthquakes can tell us about Magma Flow beneath Volcanoes

- An eruption event at Mt St Helen's in September, 2005- Can be seen in the earthquake record by deep events of magma flow

The map on the right shows historic earthquake activity near the epicenter (star) from 1990 to present.

As shown on the cross section, earthquakes are shallow (orange dots) at the Japan Trench and increase to 300 km depth (blue dots) towards the west as the Pacific Plate dives deeper beneath Japan.

Images courtesy of the US Geological Survey

Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPANMagnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPANFriday, March 11, 2011 at 05:46:23 UTC Friday, March 11, 2011 at 05:46:23 UTC

Seismicity Cross Section across the subduction zone showing the relationship between color and earthquake depth.

Northridge Earthquake, 1994

San Fernando, 1971San Fernando, 1971

Northridge, 1994

Seismic image of a blind thrust fault at depth

Seismology and Society

Study of Seismology, seismic waves, and earthquakesAlso aid in hazard analysis for communities in earthquake country.

- Earthquake risk

- Earthquake Engineering

- Tsunami hazard analysis


- January 17, 1994, 4:30 am- Magnitude = 6.9- Blind thrust fault- Difficult to find or predict- Duration 10-20 seconds

CSUN Parking Structure

Many Active Faults in Southern California

And more offshore! How close do you live to a fault ? Los Angelino's need to know and need your help to prepare!

L.A. mayor's proposals for concrete, wood quake retrofits

Los Angeles Times Monday, Jan 26, 2015

The mayor's proposals for mandatory retrofits target two of the riskiest types of structures built in Los Angeles before 1980: brittle concrete buildings and multi-story wooden buildings supported by weak columns on the ground floor.lRelated L.A. mayor calls for mandatory earthquake retrofitting for thousands of buildings

L.A. mayor's proposals for concrete, wood quake retrofit

The mayor's proposals for mandatory retrofits target two of the riskiest types of structures built in Los Angeles before 1980: brittle concrete buildings and multi-story wooden buildings supported by weak columns on the ground floor.lRelated L.A. mayor calls for mandatory earthquake retrofitting for thousands of buildings

Concrete Buildings:Retrofit deadline: 30 years

Cost of retrofit: Varies. Could be more than $1 million for a 10-story buildingAffected buildings: About 1,500 need further study to determine if retrofit is needed

Vulnerabilities: Many don't have enough steel reinforVulnerabilities: Many don't have enough steel reinforcement to hold columns in place during shaking. The collapse of two concrete office towers killed 133 people in a 2011 quake in New Zealand.

Wooden Buildings:Cost of retrofit: About $60,000 to $130,000 for a modest-sized apartment building

Affected buildings: About 17,000 need further study to determine if retrofit is needed

Vulnerabilities: These types of buildings have collapsed during both the 1989 Loma Prieta and 1994 Northridge earthquakes. Sixteen people died when the Northridge Meadows apartment complex pancaked.

ALBACORE ProjectCaltech, UCLA, CSUN

Offshore faults, seismicity, tsunami risk analysis, bathymetry Body wave, surface wave tomography, ambient noise, SKS splitting Recorded passing of Honshu earthquake tsunami on seafloor


* CSUN Parking Structure (near G3 and G4 parking lot today)* Built in 1991, three years before the earthquake.


Earthquake damage to freeways, buildings, cars, gas lines.

This tsunami propagation forecast model shows the forecast maximum tsunami wave height (in cm). Interference of water waves with seafloor bathymetry affects the wave trajectory, and reflections. Travel up a continental shelf raises wave height quickly..


Projected travel times for the arrival of the tsunami waves across the Pacific.

Nearby the earthquake there are only minutes to evacuate. However, in many other regions there is advance warning.

A tsunami map shows projected travel times for the Pacific Ocean. This map indicates forecasted times only, not that a wave traveling those distances has actually been observed.. NOAA


Magnitude 7.3 Earthquake near El Salvador

Earthquake Mg 7.3 (Aug 26, 2012) Offshore Elsalvador (9:30 pm California time) Tsunami warning was put into effect for Costa Rica, Nicaragua, El Salvador, Honduras, Guatemala, Panama and Mexico.


The underlying physics is simple:

F = ma

V = d / tVelocity,

Force,

But assumptions can quickly become complex when consideringrealistic sources, Earth structure that changes with depth and pressure,and receiver instrumentation.

Requires more sophisticated math and computation.

Earthquake Source Theory

Quantification of the earthquake rupture process

Zhang Heng was a celebrated astronomer of ancient China. An inventor of the seismoscope and armillary sphere, he made great contributions to the development of astronomy in ancient China. 132 AD, Eastern Han Dynasty

Early Detectors of Ground Movement

Seismology and Plate Tectonics

Earthquakes generally occur at plate boundaries. Thus plate tectonics is intimately linked with earthquakes and seismology

Seismology and Plate Tectonics

Most of the largest earthquakes occur at collisional plate boundaries.

Planetary Seismology ?

What kind of living would a seismologist make on another planet ?

What is the plausibility of earthquakes on other planets, on the moon ?

- Is there active plate tectonics on other planets ?

Seismic waves inside the Earth

How does earthquake energy travel through the Earth's interior ?

Movie: Wysession

Seismic Tomography: from Seismic rays in the Earth

Many seismic stations placed across a region of interest Create Images of “velocity” or seismic structure

US Arraywww.iris.edu

Earth's Interior Structure

Seismology is the chief method of determining the interiorStructure of the Earth.

Indirect methods of studying the Earth's Interior

Can we just go to the center of the Earth ?

• Deep interior of the Earth must be studied indirectly– Direct access only to crustal rocks and

small upper mantle fragments brought up by volcanic eruptions or slapped

onto continents by subducting oceanic plates

– Deepest drill hole reached about 12 km, but did not reach the mantle

• Geophysics is the branch of geology that studies the interior of the Earth

SE Germany – 10 km drill hole

Indirect Study of the Earth's Interior - Geophysics

- Seismic Waves- Gravity- Heat Flow- Magnetic Field

• Crust and upper mantle together form the lithosphere, the brittle outer shell of the Earth that makes up the tectonic plates

– Lithosphere averages 70 km thick beneath oceans and 125-250 km thick beneath continents

The Mantle Lithosphere

• Beneath the lithosphere, seismic wave speeds abruptly decrease in a plastic (ductile) low-velocity zone called the asthenosphere

• Are low seismic velocities caused by partial melt, water, density?

The Asthenosphere

Subduction Zones in North America

Subducting Farallon slab is imaged through seismic tomography extending to at least 2000 km depth

Farallon reaches this depth somewhere beyond the east coast of North America

Grand et al., 2001.

Earth's resources

Oil explorationActive sourceReflection seismology

Gasoline contaminationEnvironmental compliance

The Earth's Natural Resources

• Energy• Water• Raw Materials

• Energy

1. Oil2. Natural Gas3. Coal

Fossil Fuels

4. Solar5. Wind6. Dams

New technologies used aboard offshore platforms in the Gulf of Mexico can recover oil and gas from rock reservoirs below very deep waters. [Larry Lee Photography/CORBIS.]

Oil Resources

New find in GOM(Jack No. 2 test well)

• up to 3-15 billion barrels of oil

• US consumption 20 million barrels/day (that's ~7.3 billion barrels/year)

• 5 months - 2 years

• reservoir is 8 km under sea level

Oil Resources: New Find in Gulf of Mexico! (Will this solve all our problems ?)

Oil Consumption in U.S.• U.S. Oil consumption in 2007 was 20.68 million barrels per day!

• This is 3 times as much as China, 8 times Germany, 10 times France,12 times England,12 times Iran,28 times Venezuala,usage of oil.

• We are the biggest oil consumer in the world.

Oil Peaks – 1960's

In the 1850's oil mainly used for kerosene lamps – remaining was burned or disgarded.

Current oil production rates peakedin 1960's but U.S. consumption ratesincrease dramatically above this.

Don't panic! -Try to conserve energy -Seek alternative energy sources

Petroleum Reserves• As petroleum prices rise, alternate

petroleum sources, such as heavy crude, oil shale and oil sand, will be increasingly exploited

– Heavy crude is dense, viscous petroleum

– Oil shale is black or brown shale with high solid organic matter content from which oil can be extracted by distillation

– Oil sands (or tar sands) are asphalt-cemented sand or sandstone deposits

Seismologists must image anticlines, petroleum traps, liquid reservoirs, partial melt along grain boundaries.

Depiction of a three-dimensional seismic survey collected from airguns dragged behind a ship. Sounds waves reflect off the seafloor and deeper sediment layers. The colors show the layers of sediments beneath the seafloor, some trapping oil and natural gas. [ (b) Courtesy of Satoil, Veritas, and BP.]

Petroleum Exploration and Production

Exploration Geophysics

CSUN students (Robin Sehler) in the field in south Africa Learn geophysical exploration techniques\ Map and locate palladium veins in crustal rocks GPS, active source seismology, GPR, gravity, Magneto tellurics

( Robin Sehler, Carol Zamora, Gabrielle Zamora, Cristo Ramirez, Melissa NunleyJasmyn Nolasco, Brittany Huerta, Christian Mirreles)

The Seismogram

A seismogram is made up of a series of signals only one of which is the earthquake.

Earthquake source Scalar Explosion• Rupture along a plane (e.g. shear or vertical slip)

Medium wave travels through Receiver characteristics

Stein and Wysession (figure 1.1-1)

Earthquake Sources and Seismometer Receivers

Seismology: What is Known an Unknown

What we know for sure!- Vp speed (fraction of a percent)- Average radial velocity of Earth Known very well fo 50 years!- Earthquake locations Mapped daily by GSN- EQ radiation patterns “

What we are still studying:- Physics of earthquake rupture largely unknown.- Damping by inner core Only approximate– Anisotropy in the inner core ? Still trying to find the J phase!

Earthquake Sources and Seismometer Receivers

To understand the seismogram observed at a seismometer We must understand

Earthquake sources How the medium behaves, deforms, relaxes The response characterists of the seismometer instrument How seismic waves are generated and travel


- January 17, 1994, 4:30 am- Magnitude = 6.9- Blind thrust fault- Difficult to find or predict- Duration 10-20 seconds

CSUN Parking Structure


* CSUN Parking Structure (near G3 and G4 parking lot today)* Built in 1991, three years before the earthquake.


Damage to freeways, buildings, cars, gas lines.

Historic Earthquakes 1994 Northridge - 1.8 miles from CSUN - Shaking 40 seconds - Damaged all 53 CSUN buildings - Damaged 300 other schools - Lower story buildings collapsed - 4 interstate hwy's closed for months (Golden State, Santa Monica fwys) - 15000 people live in tents for days - 10000 without water, elect, gas

- Arid climate did not cause liquifaction of soils – shaking minimized (compared to 1964 Alaska and 1989 Loma Prieta).

- Landslides in Santa Susana, Santa Monica, San Gabriel Mtns blocked roads and traffic, damaged water lines and homes in Palisades

- Sylmar – Olive View Hospital – rebuilt from 1971 to code stayed intact.

- Aftershocks 5.9 1 min after main shock, 5.6 11 hrs after.

Earthquakes

1989, Loma Prieta earthquake, Mw = 7.2

Historic Earthquakes

1906 San Francisco - 280 miles of displacement - Shaking ~1 minute - Damaged water mains, fires spread and caused many deaths (3000).


1989 Loma Prieta, Mw = 7.2 - Shaking for 15 seconds - Death toll 63


1964 southern Alaska (Mw = 9.2) - Shaking for 3 minutes - Rupture 350,000 square miles - Death toll from quake 15 (remote area) - Tsunami, landslides 100 more

2002 inland Alaska – Denali Fault - Propagated east 7000 miles/hr - Offset streams, glaciers, landslides - Trans-Alaska Oil Pipeline no serious damaged - pre-engineered


Historic Earthquakes2004 Sumatra – Andaman E.Q. - Mw = 9.3 - Second largest recorded on Earth - Major damage in Sumatra - Tsunami damage spread far to Indonesia, Thailand, Sri Lanka, India, East Africa - Death toll 220,000 from force of tsunami wave - Tsunami movie

Japan’s Kashiwazaki-Kariwa facility is the world’s largest nuclear power plant, with seven reactors and a total generating capacity exceeding 8200 megawatts. It was damaged by a powerful earthquake (magnitude 6.8) that struck the region on July 16, 2007. The plant was shut down and required extensive repairs. [STR/AFP/Getty Images.]

Harnessing Nuclear Energy

Petroleum Recovery

• Oil fields are regions underlain by one or more oil pools

– Largest in U.S. are in Texas and Alaska

• Oil and natural gas are removed through wells drilled down into an oil trap within a reservoir rock

• Negative environmental effects resulting from oil recovery and transport include oil spills, brine contamination of surface water, and ground subsidence

Exploration Geophysics

CSUN students (Robin Sehler) in the field in south Africa Learn geophysical exploration techniques\ Map and locate palladium veins in crustal rocks GPS, active source seismology, GPR, gravity, Magneto tellurics

( Robin Sehler, Carol Zamora, Gabrielle Zamora, Cristo Ramirez, Melissa NunleyJasmyn Nolasco, Brittany Huerta, Christian Mirreles)

Exploration Geophysics: Metals and Ores• Metal ores - naturally occurring

materials that can be profitably mined

• Whether or not a mineral deposit is an ore depends on chemical composition, the percent extractable metal, and current market value of the metal

• Metallic ore deposits originate from crystal settling in igneous intrusions, hydrothermal fluids cooling in pores and factures, chemical precipitation in water, or sedimentation in rivers (placers)

Earthquakes

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