David von Seggern

Spatial resolution of acoustic imaging with the Born approximation

The practical limits on resolution of images produced by processing seismic data with a Born approximation depend on the spectrum of the source pulse, the incompleteness of the recording geometry, and the fidelity with which one can model the reference medium. These factors together prevent one from recovering a point image from data produced by a point diffractor in the subsurface. A simple imaging formula quantifies the effect of the source pulse, showing that spatial wavelet extent in an imaged volume is related to the product of the local velocity and the period of the propagating pulse. A general 3-D scheme for estimating the effect on resolution of the recording geometry is applied to various recording geometries in a homogeneous 3-D medium to show the importance of increasing the aperture of recording. A study of layered versus homogeneous media on the image resolution, lateral and horizontal, with 2-D models shows that layering, with a monotonic velocity increase, actually improves the lateral ima...

Seismic Spatial-Temporal Character and Effects of a Deep (25–30 km) Magma Intrusion below North Lake Tahoe, California–Nevada

In 2003, a magma intrusion event occurred at 25-30 km in the lower crust under the northwestern corner of Lake Tahoe, as evidenced by both an earth- quake swarm and a geodetic displacement. This study examines the seismicity as- sociated with that event and subsequent seismicity in the upper crust. HYPODD relocations showed that the deep swarm of approximately 1600 microearthquakes at the intrusion site was concentrated on a planar area with a strike of N42°W, dipping at 39° to the northeast. The largest microearthquake in this swarm was M 2.2, and an anomalously high b-value of 2.0 is seen in the recurrence-versus-magnitude plot. The swarm progressed over this plane in a somewhat irregular pattern for a period of roughly 5 months. Focal mechanisms of the deep-swarm events are highly variable and do not reflect the known regional stress field. Two months after the deep-swarm activity started, a shallow swarm of approximately 1100 microearthquakes began at 10-12-km depths in the shallow crust almost immediately above the deep swarm and continued through 2005. This swarm had a maximum M of 2.4 and a relatively high b-value of 1.5. Based on HYPODD relocations, hypocenters in this swarm are con- centrated in a narrow pipelike volume, and event depths progressed steadily upward over the more than 2 yr of observation. Focal mechanisms in this shallow swarm are more consistent with the regional stress field than those of the deep swarm. Within one focal depth horizontally of the deep swarm, postintrusion seismic activity increased significantly compared to prior years. Stress triggering from the deep magma intru- sion, although based on sub-bar stress changes in the shallow crust, is a feasible ex- planation of the observed increase.

Seismic Background Noise and Detection Threshold in the Southern Great Basin Digital Seismic Network

This study compares predicted seismic thresholds to actual thresholds for a high-resolution, three-component digital network in the southern part of the Great Basin of Nevada, centered on Yucca Mountain, the designated site of a high-level nuclear-waste geologic repository. In order to do this comparison, it was necessary to quantify carefully the statistical properties of noise, including mean, variance, and correlation, and the statistical properties of signals, including mean, variance, and site effects. It was also necessary to relate the noise levels to the long-term average (lta) and the signal levels to the short-term average (sta) used in conventional signal-detector algorithms. Combining these attributes with a random-earthquake simulator enables one to predict the seismic threshold of the network for a given source area. Initial predicted thresholds for the network were found to be approximately 0.3 to 0.4 magnitude unit less than the actual thresholds achieved by the network. There are several reasons for this, but the major reason seems to be the inability to time small signals that are automatically detected. A predicted threshold map for the network was constructed by accounting for the difference initially seen between predicted and actual thresholds. This map indicates that the network detects all events within 10 km of Yucca Mountain with three or more stations down to a threshold of roughly M L −0.5, whereas events near the fringes of the network are similarly detected down to a threshold of roughly M L +0.5. The simulations show that the network-estimated magnitude is biased upward as true magnitude approaches the threshold; this bias may be as large as roughly +0.3 for events near the threshold.

Building velocity‐depth models for 3-D depth migration

Two basic problems must be overcome when building 3-D velocity‐depth models for depth migration: (1) the mathematical representation of the model must be sufficiently flexible to adequately describe the earth’s complexities; and (2) the user interface necessary to build the depth model must be simple. The solutions to these two problems, however, seem directly opposed to each other. Typically, as the flexibility of the model representation increases, the complexity of the user interfaces also increases. Let’s examine these two problems in more detail.

Velocity Change in the Zone of a Moderate Mw 5.0 Earthquake Revealed by Autocorrelations of Ambient Noise and by Event Spectra

AbstractA moderate Mw 5.0 earthquake occurred near Mogul, Nevada (just west of Reno, Nevada), on 26 April 2008. This mainshock was surrounded by notable foreshock and aftershock sequences. Due to the long foreshock sequence, the area was well instrumented at the time of the mainshock. We investigated the foreshock and aftershock sequences for evidence of velocity changes in the structure immediate to the hypocenter and above it in the area of observed strong ground motion. Using autocorrelations of the time periods with nearly continuous foreshocks and aftershocks, we detected a nearly 1% negative change in velocity on recordings of station MOGL which was approximately over the hypocenter of the mainshock. We also observed from these recordings a shift in the spectral peaks to lower frequency following the mainshock, again indicative of a velocity decrease. Due to the different spatial sampling of the two methods, the effects could not be attributed to the same subsurface volume. However, both results are strong evidence for coseismic velocity changes accompanying an earthquake which is very much smaller than those for which previous velocity changes have been reported. We hypothesize that these changes can be observed for even smaller earthquakes, given a fortuitous placement of observing stations.