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Earthquakes: Faults, Hazards, & the Bay Area. Richard Sedlock, SJSU Geology/BAESI 408 924-5020 -- click on “Workshop Links”. What is an Earthquake?. A release of energy stored on a fault. What is a fault?.

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Earthquakes: Faults, Hazards, & the Bay Area

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Earthquakes:Faults, Hazards, &the Bay Area

  • Richard Sedlock, SJSU Geology/BAESI

  • 408 924-5020

  • -- click on “Workshop Links”

What is an Earthquake?

A release of energy stored on a fault

What is a fault?

A roughly planar surface where rock has broken and separated

Why does an earthquake happen?

Built-up energy exceeds frictional resistance on the fault

Earthquake Waves


0.1 Hz to 10 Hz (outside human sensory range)

Types of Motion

P waves


S waves


surface waves

How does “slip” on the fault happen?

Elastic rebound

Alternative idea: Displacement pulse (“the rug”)

Find faults?

How do geologists....

How do geologists find faults?

Find faults?

How do geologists....

How do geologists....

Determine whether a fault is active?

State of California (A-P act): An active fault is one that has slipped once in the last 11,000 years (or 2 or more times in the last 700,000 years)

Consider this schematic roadcut/seacliff:

fault #1

fault #2

fault #3

Some of the Bay Area’s active faults

Spaced-based measurements (VLBI and GPS)

show that PAC-NA motion in CA is ~50 mm/yr.

Earthquakes in California & Nevada, 1970-2003

The San Andreas is NOT “the PAC-NA plate boundary.”

Red arrow: predicted

motion: 50 mm/yr

Blue arrows: subsets

of the motion that

“add up” to the predicted motion.

This diagram applies at the latitude of Bakersfield or

San Luis Obispo.


About 36 mm/yr happens on the San Andreas in central CA, but northwest of Hollister, things are a LOT messier.










The ~36 mm/yr must be divided up on many faults. Geologists study each to determine

individual rates.






Let’s add up the slip on faults along four paths to see whether we’ve found the ~36 mm/yr.


17? 23?

Seismic Hazards and Risks

Seismic hazard: A natural process caused by an EQ

Seismic risk: The potential for casualties and

property loss due to a seismic hazard or hazards.

Example 1: Landslide in built-up Los Gatos vs.

landslide in the Sierra Nevada backcountry

Example 2: Tsunami at Los Angeles harbor vs. tsunami at the Big Sur coastline.

Natural Seismic Hazards

Surface rupture


Dust & Disease


Liquefacton & subsidence

Ground shaking

Surface Rupture

Earth’s surface breaks and shif

ts at an active fault.

Only affects construction built directly atop the fault.

Affects the smallest area of all the seismic hazards.

Includes creep and earthquakes.

How could you evaluate the level of risk

due to surface rupture?


Downslope movement of Earth materials due to gravity

Most affect soil or loose sediment, but some affect rock.

May be triggered by earthquakes or heavy rainfall.

How could you evaluate the level of risk

due to landslide?


Destructive sea waves (NOT tidal waves)

Caused by processes that abruptly moves ocean water:


submarine volcanic eruption

coastal/submarine landslide

ET impact

Time of arrival can be predicted—public warnings

How could you evaluate the level of risk

due to tsunami?

Possible tsunami

“run-up” zones


Accompanies M6+ EQs.

Water escape often causes the ground surface to subside.

Subsidence can damage pipes, foundations, etc.

How could you evaluate the level of risk

due to liquefaction & subsidence?

Ground Shaking

Due to passage of P, S, and surface waves

Ground type controls amplitude and duration of shaking:

Bedrock shakes least, shortest

Mud-rich soil shakes most, longest

How could you evaluate the level of risk

due to ground shaking?

Effect of Ground Type on Shaking

Seismograms for a

M4 aftershock

of the 1989 Loma Prieta earthquake.

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