Earthquakes:Faults, Hazards, &

the Bay AreaRichard Sedlock, SJSU Geology/BAESI

408 924-5020 sedlock@geosun.sjsu.edu

http://www.baesi.org -- 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

Frequency0.1 Hz to 10 Hz (outside human sensory

range)

Types of Motion

P waves

S waves

surface waves

velocity amplitude

How does “slip” on the fault happen?

Elastic rebound

Alternative idea: Displacement pulse (“the rug”)

How do geologists.... Find faults?

How do geologists find faults?

How do geologists.... Find faults?

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)

4,000 yr

1 m.y.

80 m.y.

Consider this schematic roadcut/seacliff:

fault #1fault #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.”

This diagram applies at the

latitude of Bakersfield or

San Luis Obispo.

Red arrow: predicted

motion: 50 mm/yrBlue arrows:

subsetsof the motion that

“add up” to the predicted motion.

.

predicted at latitude of central CA

(51 ± 3 mm/yr, N32W)

San Andreas fault zone(35 ± 4 mm/yr, N34W)

Basin & Range(12 ± 1 mm/yr, N35W)

discrepancy

0

10

20

30

40

102030

velocity of PAC to W (mm/yr = km/Ma)

32°

just like on a map

North

West

23

94-8?

17-23

9>6

17 96

2-5?

17? 23?

15

7-10?1-3?

FarallonIslands

~Stockton

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 determineindividual rates.Let’s add up the slip

on faults along four paths to see

whether we’ve found the ~36

mm/yr.

Seismic Hazards and Risks

Seismic hazard: A natural process caused by an EQ

Seismic risk: The potential for casualties andproperty 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.

Surface rupture

Landslides

Dust & Disease

Tsunamis

Liquefacton & subsidence

Ground shaking

Natural Seismic Hazards

Surface RuptureEarth’s surface breaks and shif

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

riskdue to surface rupture?

ts at an active fault.

LandslidesDownslope 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?

TsunamiDestructive sea waves (NOT tidal waves)

Caused by processes that abruptly moves ocean water:

earthquake submarine volcanic eruptioncoastal/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

LiquefactionAccompanies M6+ EQs.Water escape often causes the ground surface to subside.Subsidence can damage pipes, foundations, etc.How could you evaluate the level of

riskdue to liquefaction & subsidence?

Ground ShakingDue to passage of P, S, and surface waves

Ground type controls amplitude and duration of shaking:

Bedrock shakes least, shortestMud-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 aftershockof the 1989 Loma Prieta earthquake.