Jan 15, 2016

Effect of Velocity Models on the Accuracy of Earthquake Hypocenters. Sudipta Sarkar, Youshun Sun, M. Nafi Toks ö z Earth Resources Laboratory Massachusetts Institute of Technology USA. Big Picture. Earthquake hypocenters could be in error by tens of kilometers - PowerPoint PPT Presentation

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Effect of Velocity Models on Effect of Velocity Models on the Accuracy of Earthquake the Accuracy of Earthquake

HypocentersHypocenters

Sudipta Sarkar, Youshun Sun, M. Nafi ToksSudipta Sarkar, Youshun Sun, M. Nafi Toksöözz

Earth Resources LaboratoryEarth Resources Laboratory

Massachusetts Institute of TechnologyMassachusetts Institute of Technology

USAUSA

Big PictureBig Picture

Earthquake hypocenters could be in Earthquake hypocenters could be in error by tens of kilometerserror by tens of kilometers

Several factors contribute to errorsSeveral factors contribute to errors– Poor station coveragePoor station coverage

– Phase reading errorsPhase reading errors

– Poor knowledge of velocityPoor knowledge of velocity

– Lateral variations of velocityLateral variations of velocity

OutlineOutlineEarthquake location errors due to errors in Earthquake location errors due to errors in velocity models: Examples -velocity models: Examples -

I.I. Local scaleLocal scale

II.II. Regional scaleRegional scale

Improving location accuracy by using a 3D Improving location accuracy by using a 3D velocity modelvelocity model

China caseChina case

A practical approach to improving location A practical approach to improving location accuracyaccuracy

Source Specific Station correctionSource Specific Station correction

Applicability to the Gulf regionApplicability to the Gulf region

ConclusionsConclusions

Velocity Model and Depth of EventsVelocity Model and Depth of Events

0

500

1000

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2500

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3500

4000

0 1000 2000 3000 4000 5000 6000 7000

Vp (m/s)

Dep

th (m

)

Vel1

0

500

1000

1500

2000

2500

3000

3500

4000

0 1000 2000 3000 4000 5000 6000 7000

Vp (m/s)

Dep

th (m

) Vel1

Vel2

Velocity Model and Depth of EventsVelocity Model and Depth of Events

Velocity Model and Depth of EventsVelocity Model and Depth of Events

0

500

1000

1500

2000

2500

3000

3500

4000

0 1000 2000 3000 4000 5000 6000 7000

Vp (m/s)

Dep

th (m

) Vel1

Vel2

Vel3

Depth Error in Local Events Due to Depth Error in Local Events Due to Incorrect Velocity ModelIncorrect Velocity Model

Vel #1 Vel #2 Vel #3

1D1D(km)(km)

AK135AK135(km)(km)

11 Events 11 Events (Explosions)(Explosions)

1010 2020

EarthquakeEarthquake 1212 2020

Location Errors Due to Lateral Location Errors Due to Lateral Variation of VelocityVariation of Velocity

Our experience in CHINA

3D3D(km)(km)

1D1D(km)(km)

AK135AK135(km)(km)

11 Events 11 Events (Explosions)(Explosions)

11 1010 2020

EarthquakeEarthquake 3.53.5 1212 2020

Location Errors Reduced by Location Errors Reduced by Introducing a 3D ModelIntroducing a 3D Model

Our experience in CHINA

Event Relocation Using 3D Travel Event Relocation Using 3D Travel Times: A Regional Study in ChinaTimes: A Regional Study in China

MethodMethod

Determine 3-D, regional velocity Determine 3-D, regional velocity structure in crust and upper mantle structure in crust and upper mantle (tomography)(tomography)

Use 3-D velocity / travel time grids to Use 3-D velocity / travel time grids to locate eventslocate events

A Regional Study in ChinaA Regional Study in China

A Regional Study in ChinaA Regional Study in China

A Regional Study in ChinaA Regional Study in China

3D3D(km)(km)

1D1D(km)(km)

AK135AK135(km)(km)

11 Events 11 Events (Explosions)(Explosions)

11 1010 2020

EarthquakeEarthquake 3.53.5 1212 2020

Location Errors Reduced by Location Errors Reduced by Introducing a 3D ModelIntroducing a 3D Model

Our experience in CHINA

Station Distribution and Events in Station Distribution and Events in the Gulfthe Gulf

Prospects of 3D Tomography and Prospects of 3D Tomography and Event Location in the GulfEvent Location in the Gulf

A Practical ApproachA Practical ApproachDoing true 3D locations is Doing true 3D locations is computationally expensive and most computationally expensive and most often not possible!often not possible!

A practical solution to this problem – A practical solution to this problem – use use Source Specific Station Corrections.Source Specific Station Corrections.

refij

Dijij ttttSC 3

ProcedureProcedureUse 3-D velocity models to calculate Use 3-D velocity models to calculate travel times from all potential source travel times from all potential source regions to all stationsregions to all stations

Determine source specific station Determine source specific station corrections relative to reference model corrections relative to reference model used for locationsused for locations

Determine an initial locationDetermine an initial location

Apply the station correction to times at Apply the station correction to times at each station specific for the source each station specific for the source regionregion

Relocate the eventRelocate the event

ConclusionsConclusionsLocal and regional seismic networks are Local and regional seismic networks are improving rapidlyimproving rapidly

Accurate event locations with these networks Accurate event locations with these networks require accurate 3D velocity modelsrequire accurate 3D velocity models

Such velocity models can be derived through Such velocity models can be derived through regional seismic tomographyregional seismic tomography

3D travel times can be implemented through 3D travel times can be implemented through source specific travel time correctionssource specific travel time corrections

New networks in the Gulf region provides an New networks in the Gulf region provides an opportunity for 3D velocity models and more opportunity for 3D velocity models and more accurate locationsaccurate locations

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