Goals

David von SeggernDavid von Seggern

Joint Seismic Joint Seismic Tomography/Location Tomography/Location

Inversion in the Inversion in the Reno/Carson City Area Reno/Carson City Area

Leiph Preston & David von SeggernLeiph Preston & David von SeggernNevada Seismological LaboratoryNevada Seismological Laboratory

Community Velocity Model WorkshopCommunity Velocity Model WorkshopJanuary 14-15, 2008January 14-15, 2008


GoalsGoals Develop a tomographic velocity model in Develop a tomographic velocity model in

the Reno/Carson City area at kilometer the Reno/Carson City area at kilometer scale to roughly 15 km depth.scale to roughly 15 km depth.

Relocate NSL catalog seismicity jointly with Relocate NSL catalog seismicity jointly with tomographic imaging.tomographic imaging.

Utilize cross-correlation times of P and S Utilize cross-correlation times of P and S waves to constrain the relative hypocenters.waves to constrain the relative hypocenters.

Compare imaging results with other Compare imaging results with other available velocity results.available velocity results.


Catalog Catalog Location of Location of

EarthquakesEarthquakesUsed for Used for InversionInversion

(1990-2006)(1990-2006)



Stations used in the inversion


Catalog Magnitudes of Catalog Magnitudes of EarthquakesEarthquakes


Depth Distribution of Catalog HypocentersDepth Distribution of Catalog Hypocenters


Arrival-Time Residuals From CatalogArrival-Time Residuals From Catalog

Residuals are Residuals are computed computed with respect to with respect to 1-dimensional 1-dimensional layered model.layered model.


Catalog Catalog Locations Locations of Blasts of Blasts

UsedUsed


Summary of InputSummary of Input

> 200,000 P and S travel times> 200,000 P and S travel times > 200,000 cross-correlation times> 200,000 cross-correlation times > 14,000 earthquakes> 14,000 earthquakes 23 blasts23 blasts 71 stations71 stations


Image ParametersImage Parameters

horizontal resolution = 2 km (90 x 91 grid)horizontal resolution = 2 km (90 x 91 grid) vertical resolution = 1 km vertical resolution = 1 km 42 depths, from -5 to 36 km, relative to MSL42 depths, from -5 to 36 km, relative to MSL Vp and Vp/Vs imagesVp and Vp/Vs images


Establish Resolving CapabilityEstablish Resolving CapabilityDependence on Starting Model:We ran cases with various reasonable starting models and found no significant dependency of result on starting velocities.

Checkerboard Test:We perturb the final model, recompute travel-times, re-image the velocity model, and see if perturbations are recovered.


Checkerboard Test (Vp)Checkerboard Test (Vp)


Checkerboard Test (Vp)Checkerboard Test (Vp)

vertical exag. = 3vertical exag. = 3


Imaged Depth Section (Vp): -1 kmImaged Depth Section (Vp): -1 km


Imaged Depth Section (Vp): 2 kmImaged Depth Section (Vp): 2 km


Imaged Depth Section (Vp): 10 kmImaged Depth Section (Vp): 10 km


Imaged Cross-section(Vp): 39.55NImaged Cross-section(Vp): 39.55N

RenoRenoBasinBasin



Imaged Cross-section (Vp): -119.75WImaged Cross-section (Vp): -119.75W



Imaged Depth Section (Vp/Vs): -1 kmImaged Depth Section (Vp/Vs): -1 km


Imaged Cross-section (Vp/Vs): 39.55NImaged Cross-section (Vp/Vs): 39.55N



Imaged Cross-section (Vp/Vs): 39.25NImaged Cross-section (Vp/Vs): 39.25N



Imaged Cross-section (Vp/Vs): -119.75WImaged Cross-section (Vp/Vs): -119.75W



E-W Cross-sectionsE-W Cross-sections


N-S Cross-sectionsN-S Cross-sections


Iso-surface: 6.0 < Vp < 6.5Iso-surface: 6.0 < Vp < 6.5


How Do We Compare?How Do We Compare?

2-D 2-D RefractionRefractionLine Line TomographyTomography(Louie et al.)(Louie et al.)

3-D3-DJoint Joint InversionInversion(This Study)(This Study)


Original Catalog EpicentersOriginal Catalog Epicenters Joint Inversion EpicentersJoint Inversion Epicenters


Caveats With Seismicity RelocationCaveats With Seismicity Relocation

Many earthquakes are culled out by Many earthquakes are culled out by residual analysis and data requirements residual analysis and data requirements during inversion.during inversion.

Locations trade off with structure in joint Locations trade off with structure in joint inversion.inversion.

Nevada seismicity tends to be diffuse, with Nevada seismicity tends to be diffuse, with unclear relation to known fault traces.unclear relation to known fault traces.

Lineations, if present, need to be viewed Lineations, if present, need to be viewed by zooming plots. by zooming plots.


Example of Seismicity LineationExample of Seismicity Lineation

~ 6 - 10 km north~ 6 - 10 km northof Kings Beachof Kings Beach

Car

son

Ran

geC

arso

n R

ange


SummarySummary A volume in the region 38.5,-121.0 to 40.0,-119.0 down A volume in the region 38.5,-121.0 to 40.0,-119.0 down

to ~ 15 km depth below MSL was imaged using P and S to ~ 15 km depth below MSL was imaged using P and S wave tomography and simultaneous solution for refined wave tomography and simultaneous solution for refined hypocenters.hypocenters.

Low velocities exist east of the Sierra Nevada at shallow Low velocities exist east of the Sierra Nevada at shallow depths, coinciding with known basins, and especially low depths, coinciding with known basins, and especially low in the Reno basin. in the Reno basin.

The Sierra Nevada crest and westward have high The Sierra Nevada crest and westward have high velocities at shallow depth.velocities at shallow depth.

Anomalously high Vp/Vs material apparently exists just Anomalously high Vp/Vs material apparently exists just above the 2003 deep (25-30 km) swarm of earthquakes above the 2003 deep (25-30 km) swarm of earthquakes under north Lake Tahoe. under north Lake Tahoe.

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