National USGS Marine Geohazards Workshop, Menlo Park, CA March 1-3, 2011

Key scientific issues for Pacific NW region in the next version of the U.S.

National Seismic Hazard Maps

Mark Petersen, Arthur Frankel, Steve Harmsen, and Gavin Hayes

U.S. Geological SurveyGolden, CO and Seattle, WA

Scientific research and information

Earthquake sources

Wave propagation (linear and nonlinear)

Fault interaction, triggering,Episodic Tremor and Slip

Earthquake monitoring,Ground-motion studies

Crustal deformation studies

Paleoseismology, LIDAR, geologic mapping

Potential field mapping

Seismic reflection, refraction, surface wave, tomography,

geotechnical, borehole studies

Applications

Seismic provisions in building codes

Seismic design

Seismic retrofit

Emergency preparedness and

management

Early warning

Land-use planning

Earthquake insurance

Products

National seismic hazard maps

Urban seismic hazard maps

Site-specific PSHA

Scenario ground motion maps

Shakemaps

Loss estimation (e.g. PAGER)

Earthquake forecasts

Synthetic seismograms

Liquefaction, landslide,and surface rupture

hazard maps

Probabilistic Seismic Hazard Methodology Example

a bEarthquake SourcesGround motion

d1

d2

d3

d4

r1

r2

r3

San Andreas fault

high seismicityzone

peak ground acceleration (pga)

Hazard curve

annu

al p

roba

bili

ty o

f ex

ceed

ing

pga

0.25g

a

M 7.6

distancepeak

gro

und

acce

lera

tion

M7.6

0.5g

Specify recurrencerates of earthquakesfor each source thatcan affect site of Interest

Time independentor time dependent

Attenuation relationstell you median ground motions that each potential earthquake will produce at site, and variability

Hazard curve:describes probabilityof having ground motions≥ a certain intensity

From National Seismic Hazard Maps to Building Codes

USGS develops national seismic

hazard maps with input from external

community

Building Seismic Safety Council

develops design procedure to apply to

hazard maps, published in 2009 NEHRP Provisions

(FEMA)

International Code Council adopts design

procedures for 2012 IBC and IRC

(also adopted in ASCE 7 2010)

Engineers choose design criteria (1997 and 2009)

• Early 2012 - Pacific NW Workshop

• Advisory Committee• Special workshops (e.g.,

turbidite data – 2010 and depth of seisogenic zone - 2011)

Merging ofUBC, SBC andBOCA codes

into IBC

Earthquake Sources Where do earthquakes occur and how often?

Source models for the National Seismic Hazard Maps

Geologic mapping, fault slip ratesEQ chronologies from paleoseismologypotential fields, seismic reflection/refraction

Earthquake catalogs(instrumental and

historical)

Crustal velocitymeasurements (GPS)

Photo from Nelson et al. (2003)

Cedar trees killed by subsidence from the 1700 Cascadia subduction zone earthquake.

From McCrory (2004))

From Zeng and ShenFrom C. Goldfinger

Photo from Brian Atwater

Core samples from turbidites in abyssal plain

Earthquake Ground motions

Western Pacific GSNStrong Ground motion

Subduction interface ground motions

BC Hydro developing new equations (forearc and backarc relations)

National Seismic Hazard Mapping Project involved in hazard analysis in many coastal areas of the U.S. and world

American Samoa

Alaska

HawaiiAlaska

Conterminous U.S.

Cascadia Subduction Zone:What are the gaps in knowledge?What are the critical research issues?

• Fault geometry and Seismogenic depths• Earthquake sizes and recurrence rates• Ground motions

1. Olympia fault (no recurrence info)

2. Tacoma fault (event 1100 years ago)

3. Other?

Crustal Sources

Tacoma fault

Olympia fault

Seattle fault

South Whidbey Is. fault

Subduction Zone Source Geometry and Seismogenic Depth

• Location of the subducting slab (seismic reflection data, low level seismicity e.g., McCrory et al.)

• Shallow dip – few large earthquakes

• Determine location of coseismic rupture extent (e.g., episodic tremor and slip (ETS) events dissipate 80-100% stress accumulation below 25 km (Chapman and Melborne, 2009), thermal models of 350° isotherm (Fleuck et al., 1997); geodetic models (McCaffrey et al, 2007), Chile earthquake extends down 55 km (Hayes, 2009).

Using Slab1.0 to Infer Seismogenic Width

Seismogenic width is measured betweeninferred shallow and deep limits, along the 3D SZ geometry (rather than assuming linear geometries from gCMT dips, which show ~10o bias toward steeper dips, thus causing an underestimation of Sw).

Slide from Gavin Hayes

Improvements Offered From Regional Data

Adding double-difference relocated EQ data set from Fuis et al., 2008.Slide from Gavin Hayes

1. Chris Goldfinger (SSA 2010): 19 events define full ruptures indicating a northern Cascadia margin Holocene recurrence rate of ~500 years; 41 events define a Holocene recurrence for the southern Cascadia margin of ~240 years, currently we apply a 500 year recurrence for full ruptures and additional smaller ruptures. We may want to separate the northern and southern and also to compare off-shore and onshore recurrence.

New Research on Cascadia Subduction Source Recurrence

Research Priorities determined at Workshop

• Looking at more onshore sites for evidence of M8’s

• More core locations for turbidites (Hydrate Ridge to Rogue)

• Tracking turbidites with chirp data• Alternative correlation possibilities• How much ground motion does it take to trigger

turbidites? Can M7’s do it?• More research into segmentation using uplift

and GPS data

Issues for the 2013 National Seismic Hazard Maps in Pacific NW

1. Cascadia subduction zone source modela. Recurrence for southern segments? Northern segments?b. Clustered behavior?c. Geometry (seismicity)? Location of seismogenic layer?

2. Cascadia subduction zone ground motion modelsa. New data (Chile, Japan, Indonesia)b. Other published relations (Kanno et al., Gregor et al.)c. Adjustments for longer distances to current equations

3. Crustal faultsa. Yakima fold and thrust belt (published slip rates?)b. Olympia (evidence of recurrence?)c. Tacoma (relationship to Seattle fault?)d. Little Salmon (does this rupture simultaneously with Cascadia)

4. Geodetic models5. Update earthquake catalog

• What do historic large earthquakes teach us about future earthquakes (sizes, locations)? Do these earthquake rupture in one event or clustered? Can we segment faults? Are segment boundaries persistent? What sizes of earthquakes can we expect on a structure?

• What is the ground shaking from interface, inslab, deep, and outer rise earthquakes?

• Why do these types of earthquakes occur in some areas and not others? (e.g., absence of deep earthquakes beneath Oregon, outer rise events?)

• Can we determine seismogenic sections of subduction zones using ETS events, heat flow, seismicity?

• Can Accretionary Wedge Sources rupture in large earthquakes? (M 7.6 - New Hebrides in wedge)

• Can we quantify strike-slip faults offshore for use in PSHA (e.g., S. CA – San Clemente)

Conclusions: Questions we need to answer for updating the hazard maps

Wish List; Improving Slab1.0

i) More active seismic lines which imageshallow SZ geometry.

ii) Point measurements of SZ depth - e.g. Receiver Functions.

iii) Regional earthquake catalogs; preferably relocated.

iv) Independent measurements of SZ coupling - e.g. models derived from GPS data.

Slide from Gavin Hayes

Am

lia F

ract

ure

Zo

ne

Freymueller, et al. 2008

Analyzing patterns in moment release can tell is vital information about earthquake cycles.

Do background rates vary leading up to, or following mega-thrust earthquakes?

What do areas of low moment release mean? High hazard, or low?

Do moment release rates correlate with oceanic plate structure, upper plate structure, etc? What causes such along-strike variability?

Using Slab1.0 to Infer Moment-Release Rate

Slide from Gavin Hayes

1. New recurrence rates on Cascadia – how many additional

M 8 earthquakes should we include in the model?

2. Potential time-dependent (renewal and cluster models)

branches in logic tree

3. Geodetic block models and strain-rate models in addition

to geologic models.

4. New ground motion prediction equations on subduction

zone and crustal faults

Conclusions

Logic tree applied to National Seismic Hazard Maps 2008