Tomographic P‐wave velocity images of the Loma Prieta Earthquake asperity

Tomographic inversion is applied to delay times from local earthquakes to image 3‐D velocity variations surrounding the main rupture of the 1989 Loma Prieta earthquake. The 55×45 square km region is represented by blocks of 1 km per side laterally and by 8 layers of varying thickness to 18 km depth. High quality P‐wave arrival times recorded on the USGS CALNET array from 549 crustal earthquakes with depths of O to 25 km were used as sources. Preliminary results several velocity variations (5–12%) that correlate with specific characteristics of the 1989 rupture. These include prominent high‐velocity anomalies near the mainshock hypocenter and prominent low‐velocity anomalies where the dip of the San Andreas fault appears to change significantly. The termination of prominent low velocity features existing primarily in the hanging wall to depths of 7–9 km, correlates with the top of the rupture zone. High‐velocity variations along the fault dominate where aftershock activity is high. The high velocity anomaly located at depth along the fault is interpreted as imaging the asperity on which the Loma Prieta earthquake occurred

Reshaping spectrum estimates by removing periodic noise: Application to seismic spectral ratios

An automated method for removing line spectrum elements embedded in colored spectra is presented. Since smooth spectrum estimates are desired, line spectra tend to smear out over an effective smoothing window. This introduces a bias in spectrum estimation that seriously degrades signal‐to‐noise ratios, spectral deconvolution or any other operation where spectrum shape is important. Multi‐taper analysis provides a simple algorithmic solution including a method of determining where spectral peaks are both significant with high power. The method is completely general, and examples include estimation of signal‐to‐noise ratio at the 1990 high frequency array, Pinyon Flat, CA. A comparison of noise spectra line segments and signal spectra line spectra reveals similarities associated with instrument noise and shallow resonances stimulated by incoming seismic signals. Identification and removal of resonances can provide a better means of estimating background noise spectra for modeling earthquake source spectra and path effects associated with attenuation

Cartesian parametrization of anisotropic traveltime tomography

A new method for inverting P-wave traveltimes for seismic anisotropy on a local scale is presented and tested. In this analysis, direction-dependent seismic velocity is represented by a second- or fourth-order Cartesian tensor, which is shown to be equivalent to decomposing a velocity surface using a basis set of Cartesian products of unit vectors. The new inversion method for P- and S-wave anisotropy from traveltime data is based on the tensor decomposition. The formulation is formally derived from a Taylor series expansion of a continuously extended, 3-D velocity function originally defined on the surface of the unit sphere. This approach allows us to solve a linear inversion instead of the standard non-linear method. The resultant, linearized, fourth-order traveltime equation is similar to a previous fourth-order result (Chapman and Pratt 1992), although our representation offers a natural second-order simplification. Conventional isotropic traveltime tomography is a special case of our tensorial representation of velocities. P-wave velocity can be represented by a second-order tensor (matrix) as a first approximation, although S-wave traveltime tomography is intrinsically fourth order because of S-wave solution duality. Differences between isotropic and anisotropic parameterizations are investigated when velocity is represented by a matrix A. The trade-off between isotropy and anisotropy in practical tomography, which differs from the fundamental deficiency of anisotropic traveltime tomography (Mochizuki 1997), is shown to be ~ 1; that is, their effects are of the same order. We conclude that anisotropic considerations may be important in velocity inversions where ray coverage is less than optimal. On the other hand, when the ray directional coverage is complete and balanced, effects of anisotropy sum to zero and the isotropic part gives the result obtained from inverting for isotropic variations of velocity alone. Synthetic test data sets are inverted, demonstrating the effectiveness of the new inversion approach. When ray coverage is fairly complete, original anisotropy is well recovered, even with random noise introduced, although anisotropy ambiguities arise where ray coverage is limited. Random noise was found to be less important than ray directional coverage in anisotropic inversions

Tomographic images of P wave velocity variation at Parkfield, California

Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations near Parkfield, California. The 25 × 20 square km region is represented by nearly cubic blocks of 0.5 km per side. Arrival times of P waves from 551 local earthquakes, with depths of 0 to 15 km, were used as sources producing 3135 rays covering the target region. The data were recorded on low-noise downhole seismographs. The results of the inversion show correlation with some of the local geological and geophysical features. The correlation of higher-velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower-velocity, less competent rocks

A Comparison of the Ocean Microbarom Recorded on the Ground and in the Stratosphere

The, ocean microbarom is an acoustic signal generated via nonlinear interaction of ocean surface waves. It can propagate for thousands of kilometers and represent a significant infrasonic noise source for ground infrasound stations across the globe. However, wind noise often compromises detections at ground stations. Furthermore, the microbarom may travel in elevated acoustic ducts that do not transmit enough energy for detections on ground stations. Here the presence of the ocean microbarom on two high-altitude balloon flights is investigated. A spectral peak consistent with the microbarom was observed on sensors in the stratosphere but not on those deployed on the ground near the flight path of the balloon. This is probably due to an elevated acoustic duct and/or a superior signal-to-noise ratio in the stratosphere. Thus, microbarom activity quantified solely with ground-based sensors may underestimate the occurrence of the phenomenon. However, high levels of interference from flight system electronics and/other other payloads may have obscured other microbarom episodes during the balloon deployments

Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data

High precision P and S wave travel times for 2104 microearthquakes with focus <6 km are used in a non-linear inversion to derive high-resolution three-dimensional compressional and shear velocity structures at the Coso Geothermal Area in eastern California. Block size for the inversion is 0.2 km horizontally and 0.5 km vertically and inversions are investigated in the upper 5 km of the geothermal area. Spatial resolution, calculated by synthetic modeling of a cross model at critical locations, is estimated to be 0.35 km for Vp and 0.5 km for Vs. Model uncertainties are estimated by a jackknife approach and simulation of random and associated picking errors. Low-velocity zones for both P and S waves are identified at geothermal production depths (1-3 km). A large, low Vp (-6%) zone is found at depth 2-2.5 km 2 km southwest of Sugarloaf Mountain where high attenuation has been previously reported. However, a general high-Vp zone is seen under Coso Hot Springs with a slightly low Vs zone, which is characteristic of fluid saturation. The overall distributions of Vp and Vs perturbations do not correlate. An isolated high-Vs (+9%) feature, about 2 km in diameter, is unambiguously seen 2 km due west of Sugarloaf extending from surface to depth. This feature is surrounded by a circular, low-Fs belt of ∼1 km width. The surrounding belt is probably the cracked, high-porosity reservoir/conduit of geothermal fluid flow. In the 2 km southwest Sugarloaf region, we found low Vp and high Vs at geothermal production depths from 1 to 2.5 km. Combined with attenuation results, this may represent a hot, fluid-depleted center of magmatic activity

Sound produced by the rapidly inflating Santiaguito lava dome, Guatemala

Vertical inflation of the Caliente lava dome at Santiaguito (Guatemala) occurs coincidentally with the onset of explosive eruptions and produces infrasound that is generally peaked between 0.5 and 2 Hz with amplitude of up to 5 Pa (at ∼1 km from vent). Inflation of up to ∼1 m progresses rapidly (within ∼2 s) and can encompass the entire surface of the ∼200-m-diameter dome. We use particle image velocimetry to quantify the time history of dome uplift and demonstrate that deflection of a volcano's solid surface can generate infrasound waves with amplitudes of a few Pa. The volumetric displacement history is used to model linear volumetric acoustic sources, both compact and of finite extent. Synthetic waveforms match recorded infrasound in terms of timing and frequency content. Amplitude fit is very good for a station located 3.3 km from the vent and less good for stations shielded by near vent topography

Tomographic imaging of local earthquake delay times for three- dimensional velocity variation in western Washington

The Puget Sound region of Western Washington is represented by nearly cubic blocks of 5 km per side. P-wave arrival time observations from 4387 crustal earthquakes, with depths of 0 to 40 km, were used as sources producing 36 865 rays covering the target region. A conjugate gradient method (LSQR) is used to invert the large, sparse system of equations. To diminish the effects of noisy data, the Laplacian is constrained to be zero within horizontal layers. The resolution is estimated by calculating impulse responses at blocks of interest and estimates of standard errors are calculated by the jackknife statistical procedure. Results of the inversion are correlated with some known geologic features and independent geophysical measurements. High P-wave velocities along the eastern flank of the Olympic Peninsula are interpreted to reflect the subsurface extension of Crescent terrane. Low velocities beneath the Puget Sound further to the east are inferred to reflect thick sediment accumulations. The Crescent terrane appears to extend beneath Puget Sound, consistent with its interpretation as a major accretionary unit. In the southern Puget Sound basin, high velocity anomalies at depths of 10-20 km are interpreted as Crescent terrane and are correlated with a region of low seismicity. Near Mt. Rainier, high velocity anomalies may reflect buried plutons

Three-dimensional attenuation tomography at Loma Prieta: inversion of t<> \ for Q

Three-dimensional Q-1 variations in the aftershock region of Loma Prieta are derived by tomographic inversion. Low Q is observed near the surface and Q generally increases with depth. The southwest side of the San Andreas fault exhibits lower Q than does the northeast side and this feature apparently extends to approximately 7 km depth. The fault zone, as determined by the dipping plane of aftershock activity, is characterized by slightly higher Qp and lower Qs, compared to regions immediately adjacent to the fault. These correlate with high-velocity anomalies associated with seismicity at depth

Finite-difference time-domain modeling of transient infrasonic wavefields excited by volcanic explosions

Numerical modeling of waveform diffractions along the rim of a volcano vent shows high correlation to observed explosion signals at Karymsky Volcano, Kamchatka, Russia. The finite difference modeling assumed a gaussian source time function and an axisymmetric geometry. A clear demonstration of the significant distortion of infrasonic wavefronts was caused by diffraction at the vent rim edge. Data collected at Karymsky in 1997 and 1998 were compared to synthetic waveforms and variations of vent geometry were determined via grid search. Karymsky exhibited a wide range of variation in infrasonic waveforms, well explained by the diffraction, and modeled as changing vent geometry. Rim diffraction of volcanic infrasound is shown to be significant and must be accounted for when interpreting source physics from acoustic observations