Source: NPS Source: USGS

Earthquakes

Seismic Hazard

Source: USGS

California Tectonics: Present

Source: USGS

California Tectonics: Past

http://www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif

What Is An Earthquake?An earthquake is the vibration of Earth caused by a rapidrelease of energy in the form of waves

http://geography.sierra.cc.ca.us/booth/California/1_lithosphere/earthquakes.htm

Types of Earthquake Waves Surface waves

• Complex motion • Slowest velocity of all waves

Body waves 1. Primary (P) waves

• Push-pull (compressional) motion • Travel through solids, liquids, and gases• Greatest velocity of all earthquake

waves 2. Secondary (S) waves

• "Shake" motion • Travel only through solids • Slower velocity than P waves

Seismic Waves

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P-waves

S-waves

A seismogram records wave amplitude vs. time

http://ww2.lafayette.edu/~malincol/Geol120/seismogram.GIF

Locating an Epicenter

1. Measure the difference in arrival of p and s waves.

Step 2 of locating the epicenter

• 2. Use a graph to determine the distance from the monitoring station to the epicenter.

Steps 3, 4 & 5 of Locating the Epicenter3. Draw a circle, centered around the monitoring

station with a radius equal to the distance from the epicenter.

4. Repeat two more times.

5. The point where all three circles intersect is the location of the Epicenter.

Measuring The Size of An Earthquake

• Intensity – determined by effects on people, structures, and the environment

• Magnitude – measures energy released at the source of the earthquake.

Magnitude From Amplitude

http://www.earthquakes.bgs.ac.uk/earthquakes/education/eq_booklet/dia_richter_scale.jpg

Intensity Witness observations

I Felt by very few people; barely noticeable

II Felt by a few people, especially on upper floors.

III Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake. Hanging objects swing.

IV Felt by many indoors, by few outdoors. May give the impression of a heavy truck passing by.

V Felt by almost everyone, some people awakened. Small objects move. Trees and poles may shake.

VI Felt by everyone. Difficult to stand. Some heavy items of furniture move, plaster falls. Slight damage to chimneys possible.

VII Slight to moderate damage in well-built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall.

VIII Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse.

IX Considerable damage to specially built structures, buildings shifted off foundations. Noticeable cracks in ground. Wholesale destruction. Landslides.

X Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction.

XI Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.

XII Total damage. Waves seen on ground. Objects thrown up into air.

Abbreviated Modified Mercalli Intensity Scale

Did You Feel It?

http://earthquake.usgs.gov/eqcenter/dyfi.php

Earthquake Hazards

• Ground shaking• Liquefaction• Ground Rupture• Landslides• Tsunamis• Fire

Shakinghttp://www.abag.ca.gov/bayarea/eqmaps/gif99/bayas06m.gif

Liquefaction

http://www.tulane.edu/~sanelson/images/liquefaction.gif

http://cee.uiuc.edu/sstl/education/liquefaction/Pictures/APTS.jpg

Niigata Earthquake, Japan, 1964: Tilting of apartment buildings

Asian Earthquake and Tsunami of 2004What Happened and Why?

Magnitude 9 earthquake

Largest earthquake since the 9.2 magnitude Alaskan earthquake (1964)

Energy release equivalent of 23,000 atom bombs such as the one that destroyed Hiroshima

Why did it occur here?

Wind-generated Waves vs. Tsunamis

Earthquakes and Tsunamis

What, besides earthquakes, can cause a tsunami?

Tsunami Risk in California

• California is at risk of tsunamis that are generated locally and from a distance.

• 1964: 10 killed and 35 injured when tsunami from the 1964 Alaska earthquake reached Crescent City (Oregon/CA border).

• A Cascadia earthquake could bring a tsunami to Northern CA in ~ 15 minutes.

Short-term Prediction

Precursors: May or may not occur• Uplift• Foreshocks• Anomalous animal behavior• Changes in water levels in wells• Release of radon gas• Changes in velocity of P waves

Long-Range Forcasting

• Based on knowledge of when and where past earthquakes have occurred.Paleoseismology – record of past offsets and

recurrence interval

Seismic gaps: a segment of an active geologic fault or subduction zone that has not slipped in a very long time; they are often considered susceptible to future strong earthquakes.

Source: USGS

Forecasting (probability) vs. prediction

Bay Area EQ Probabilities

Hayward – Rodgers Creek Faults have the highest probabilities

Paleoseismology - the study of prehistoric earthquakes. 

http://www.tulane.edu/~sanelson/images/paleosseis.gif

http://pubs.usgs.gov/fs/1999/fs152-99/images/faults.jpg

M7 1868 Hayward earthquake

• 30 fatalities, 5 in San Francisco (12th most lethal US earthquake)

• $350,000 (>$5-100M in 2007 dollars) in damage in San Francisco alone

• Extensive damage in San Leandro, Hayward, and Fremont (total population less than 2000)

• Bay Area population was 260,000 (it is now 27 times larger)Source: USGS

1868 HaywardEarthquake (“The Great

SF Earthquake”)

Source: USGS

38°

37.5°

-121°

Earthquake of M > 6.8 on the Hayward Fault?

A major earthquake today on the Hayward fault

• would impact more than 5 million people and

• Cause estimated total economic losses to residential and commercial properties would likely exceed $165 billion.

• Other factors, such as fire, damage to infrastructure and related disruption would substantially increase the loss

http://www.usgs.gov/newsroom/article.asp?ID=1899&from=rss