Results from a 1500 m deep, three-level downhole seismometer array: Site response, low Q values, and f_(max)

A three-level downhole array is being operated in a 1500-m-deep borehole within the seismically active Newport-Inglewood fault zone, Los Angeles basin. The array consists of three three-component 4.5 Hz seismometers deployed at the surface, and at 420 and 1500 m depth. An M = 2.8 earthquake that occurred 0.9 km away from the array at a depth of 5.3 km on 31 July 1986 generated rays traveling almost vertically up the downhole array. The P- and S-wave pulse shapes show increasing pulse rise time with decreasing depth, and the initial pulse slope is less steep at the surface than at 1500 m. The average value of t_s/t_p between 1500 and 420 m depth is 1.7 and between 420 and 0 m is 3.4. A near-surface site response results in amplification on the P wave by a factor of four and S waves by a factor of nine. These data indicate a near-surface Q_α of 44 ± 13 for rays traveling almost vertically. In the case of S waves, most of the high frequency content of the waveform beyond ∼ 10 Hz observed at 1500 m depth is lost through attenuation before the waveform reaches 420 m depth. The average Q_β is 25 ± 10 between 1500 and 420 m depth and 108 ± 36 between 420 and 0 m depth. The spectra of the S waves observed at 420 and 0 m of the downward reflected S phases may overestimate Q_β, because they are limited to a narrow band between 5 and 10 Hz and affected by the near-surface amplification. A Q_c of 160 ± 30 at 6 Hz was determined from the decay rate of the coda waves at all three depths. The corner frequency as determined from displacement spectra may be higher (f_c ∼ 10 Hz) at 1500 m depth than at (f_c ∼ 7 Hz) 420 and 0 m depth. Similarly, f_(max) significantly decreases as the waveforms travel toward the earth's surface, indicating that f_(max) is affected by near-surface attenuation. Beyond f_c, the average slopes of the spectra falloff of P-wave spectra is ∼f^(−2) at 1500 m depth and ∼ f^(−3) at the surface

Images of Crust Beneath Southern California Will Aid Study of Earthquakes and Their Effects

The Whittier Narrows earthquake of 1987 and the Northridge earthquake of 1991 highlighted the earthquake hazards associated with buried faults in the Los Angeles region. A more thorough knowledge of the subsurface structure of southern California is needed to reveal these and other buried faults and to aid us in understanding how the earthquake-producing machinery works in this region

Understanding earthquake hazards in southern California - the "LARSE" project - working toward a safer future for Los Angeles

The Los Angeles region is underlain by a network of active faults, including many that are deep and do not break the Earth’s surface. These hidden faults include the previously unknown one responsible for the devastating January 1994 Northridge earthquake, the costliest quake in U.S. history. So that structures can be built or strengthened to withstand the quakes that are certain in the future, the Los Angeles Region Seismic Experiment (LARSE) is locating hidden earthquake hazards beneath the region to help scientists determine where the strongest shaking will occur

Geophysics

Includes bibliographical references and index.Print version record.Front Cover; Geophysics; Copyright Page; Contents; Preface; Chapter 1. Elastic Properties of Rocks and Minerals; 1. Introduction; 2. Ultrasonic Techniques; 3. Brillouin Spectroscopy; 4. Thermal Diffuse Scattering; References; Chapter 2. Laboratory Measurement of Internal Friction in Rocks and Minerals at Seismic Frequencies; 1. Introduction; 2. Characterization of Nonelastic Behavior of Solids; 3. Experimental Methods and Associated Problems; 4. Conclusions; References; Chapter 3. Measurement of Rock Deformation at High Temperatures; 1. Introduction; 2. Deformation Apparatus; ReferencesChapter 4. Diffusion Measurements: Experimental Methods1. Introduction; 2. Theory; 3. Solutions to Fick's Second Law; 4. Experimental Methods; 5. Analytical Methods; 6. Summary; References; Chapter 5. Rock Fracture and Frictional Sliding; 1. Introduction; 2. Single-Crack Propagation; 3. Double Torsion Technique; 4. Double Cantilever Beam Technique; 5. Notched Bending Beam Technique; 6. In Situ Measurements; 7. Acoustic Emissions; 8. Frictional Sliding; 9. Permeability; References; Chapter 6. Shock Wave Techniques for Geophysics and Planetary Physics; 1. Introduction2. Impedance Match Solutions3. Shock-Induced Dynamic Yielding and Phase Transitions; 4. Shock Wave Velocity Measurements; 5. Release Isentrope Experiments; 6. Measurement of Sound Speed behind the Shock Front; 7. Shock and Postshock Temperatures; References; Chapter 7. The Multianvil Press; 1. Introduction; 2. Design and Construction of Multianvil Presses; 3. Pressure Cell-Sample Assemblies; 4. Pressure Calibration and Accuracy; 5. Conclusions: Experiments with Multianvil Systems; References; Chapter 8. Thermal Conductivity of Rocks and Minerals; 1. Thermal Conductivity2. Radiative Thermal ConductivityReferences; Chapter 9. Experimental Methods in Rock Magnetism and Paleomagnetism; 1. Introduction; 2. Fundamental Concepts in Rock Magnetism and Paleomagnetism; 3. Experimental Methods of Paleomagnetism; 4. Rock Magnetism; 5. Concluding Comments; References; Index; Contents of Volume 24, Part BElsevie