Seismic refraction along a Seismic refraction along a profile across the La Bajada profile across the La Bajada

faultfaultMax MoorkampMax Moorkamp

OutlineOutline

• Seismic refraction method

• Area of study and objectives

• Data

• Models and interpretation

• Conclusions

Seismic refraction methodSeismic refraction method

• Uses refractions from interfaces to determine seismic velocities

• At long distances the first arrival travels through deep layers

• The Slope-Intersect-Method can be used to determine layer velocities and thickness

• Results can be checked by calculating theoretical travel times from models

Slope intersect methodSlope intersect method

• Determine slope and intersect of first arrivals

• Each slope and intersect corresponds to one layer

• Information from 2 different sites can be used to obtain dip of layers

Slope intersect methodSlope intersect method

Area of study and objectivesArea of study and objectives

• Obtain velocity structure in the sub-surface

• Determine position of the La Bajada fault

• Estimate fault offset from position of basalt layer

Suspected fault position

Data for Site 457Data for Site 457

Slope-Intercept ModelsSlope-Intercept Models

Sites 101 – 281Sites 161 - 341

Sites 221 - 401Sites 281 – 457

Combined ModelsCombined Models

Loose Dirt

Basalt

Sites 457 - 221 Sites 101 - 161

Santa Fe Form.

Basaltic rubble

Quart. Sed.

ConclusionsConclusions

• The La Bajada fault is probably located between seismic sites 161 and 221

• The top of the basalt is only present to the west at a depth of 80 m

• The displacement on the fault is 200 m in 4My