Introduction
On Surface Wave Observations in Ambient Noise
This array was part of the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) project [Kohler et al. Maps of the (A) amplitude, A and (C) phase travel time, of the incoming wave fronts , and maps of (B) amplitude, A and (D) phase travel time, of the outgoing wave fronts.
On Site Amplification of Surface Waves
In this paper, we use the ambient noise cross-correlations of the Long Beach group with the Helmholtz wavefront tracking approach of Lin et al. Forsyth (2008), Softening in the upper mantle beneath Southern California: The physical state of the lithosphere and asthenosphere, J.
Offshore Southern California Lithospheric Velocity Structure
Abstract
A new shear wave velocity model offshore southern California is presented that images plate boundary deformation including thickening and thinning of the crust and mantle lithosphere at the westernmost edge of the North American continent. A transition to the present-day slip regime between the Pacific and North American Plates resulted in extensive deformation and uplift of the now > 200 km wide continental shelf.
Introduction
2015] and others observe a number of seismogenic fault zones in the area, as slip on the Pacific–North American boundary is widely distributed across southern California. The Southern California Earthquake Center (SCEC) has compiled Community Velocity Models (CVMs) (e.g., [Shaw et al., 2015]), with the goal of providing a reliable base for ground motion simulations, earthquake localization studies, and deeper tomography.
Tectonic Background
Based on the array aperture and the distance between stations, we image the crust and upper mantle, from oceanic lithosphere far west of the Patton Escarpment through the Inner and Outer Borderland and through continental Southern California. Their observations provide some of the first direct constraints on the depths of the Moho and lithosphere-asthenosphere boundary beneath the Borderland, as well as on the general lithospheric structure west of the Patton Escarpment.
Data and Methods
- Signal Preprocessing and Cross Correlation
- Inversion for 2D Maps at each Period
- Inversion for Shear Wave Velocity with Depth
In contrast, we find that for the 5–9 s period range of noise cross-correlations, applying the corrections reduces the strength of the fundamental surface state observation relative to the first harmonic. The stronger sensitivity of the first overtone highlights the importance of including it (where available) in an oceanic environment.
Results and Discussion
Panels C-F show four sections through the 3D model. profile A-A') and crustal thinning in the outer and inner boundary zone south of the western transverse chains (Fig. 2.10: B-B', C-C', D-D'). For example, it is not part of an accretionary prism that lies adjacent to the bulge, as the western boundary of the prism is marked by the Patton Slope.
Acknowledgments and Data
Unfortunately, many of the commonly accepted signal processing techniques such as time domain normalization or spectral prewhitening [Bensen et al., 2007]. However, none of the chapters fully explain the derivation of the equation used, and even in the original work of Lin et al.
Site Amplification, Attenuation and Scattering from Noise
Abstract
Here we show that correlation amplitudes of environmental noise from the Long Beach array can be used to directly determine frequency-dependent site gain factors. Finally, we produce site gain maps across Long Beach at frequencies and 2.0 Hz.
Introduction
Unfortunately, such averages do not describe the complex and frequency dependent patterns of wave propagation and thus often do not provide realistic estimates of the lateral variability of ground motion amplitudes [ Graves et al ., 2010 ]. We compare our site amplification results with the phase velocity observations of Lin et al.
Theoretical Background
Thus, the cross-correlation wavefield of the incoming noise (negative delay) will increase in amplitude as it passes through the source location, since the focus of the hyperbola in the stationary phase moves with the wavefield. On the other hand, in the stationary phase, the focus of the hyperbola is fixed at the location of the center station for the outgoing wavefield (positive delay), and therefore the wavefield amplitude will not be affected by the source.
Data and Methods
We observe a strong south-north trend in the amplitudes, as the signal energy is strongest from near the coastline to the south (with low SNR stations removed). Only measurements are selected for which the signal-to-noise ratio (SNR, defined as peak amplitude divided by the root-mean-square of the trace) is above a cutoff value: SNR > 8 at 0.67 Hz and 1 Hz, and SNR > 4 at 2 Hz.
Results
For example, a sharper contrast of amplitudes across the Newport-Inglewood fault at 0.67 Hz and 1.0 Hz suggests that the depth and shape of the structure both play a role. Specifically, we find a sharp contrast over the Newport-Inglewood Fault, and generally higher amplitudes to the southwest of the city.
Acknowledgments and Data
These terms are derived from direct observations and are well suited to validate the simulations or modeling that may traditionally be used to estimate such amplifications. Complex interactions of waves traveling through the very heterogeneous shallow crust will undoubtedly amplify seismic amplitudes, and direct observations of these effects are the first step toward improving future seismic hazard estimates.
Yang (2013), Extracting surface wave attenuation from seismic noise using correlation of the correlation coda, J. Finally, the surface wave attenuation maps presented here provide some of the highest resolution and most complete coverage of the crust in the continental United States.
Amplification and Attenuation across USArray using Ambient
Abstract
We use cross-correlation of ambient noise to make observations of surface wave amplification and attenuation for shorter periods (8 – 32 seconds) than can be observed with traditional teleseismic earthquake sources alone. These amplification and attenuation observations are sensitive to crustal materials in ways other than traveltime observations and can be used to better constrain temperature or density variations.
Introduction
Furthermore, such processing affects the sensitivity nuclei of an NCF [i.e. Fichtner et al., 2016], making it difficult to correctly attribute the effect of such processing without full knowledge of the background noise field. This was the basis of similar work using ambient noise signals from a very dense array in Long Beach, CA to recover surface wave amplification effects at frequencies down to 1-2 Hz [i.e. Bowden et al., 2015].
Methods
- Ambient Noise Cross Correlations
- Wavefront Tracking Across a Given Subarray
Here, we use observations of and that are specifically for a single period of the vertical component of the fundamental mode Rayleigh waves. The vector field of the maximum amplification magnitude and direction is attributed to the 2 ∙ τ/ term in Eq.
Results and Discussion
- Amplification Maps
- Amplification comparison to other models
- Attenuation Maps
- Attenuation Comparisons
- Conclusions
Throughout all periods, the Yellowstone hotspot is prevalent as a region of high attenuation, associated with high temperatures in the magma chamber at depth (Huang et al., 2015). There are some differences, notably that our observations show high attenuation in the Colorado Plateau that is not present in the Bao et al [2016] maps.
Acknowledgments and Data
Gibbs (1976), Effects of local geologic conditions in the San Francisco Bay region on ground motions and the intensities of 1906. Schuster (1995), Causes of low-frequency ground motion amplification in the Salt Lake Basin: the case of the vertically incident P wave, Geophys.
Earthquake Ground Motion Amplification for Surface Waves
Abstract
Surface waves resulting from earthquakes are known to cause extensive damage, especially to larger structures such as skyscrapers and bridges. Applying this framework to the amplification in the Los Angeles Basin, we find that peak ground accelerations for certain large regional earthquakes are underpredicted if surface waves are not properly accounted for, and that the frequency of the strongest ground motion amplification is significantly can differ.
Introduction
Here we show that surface waves have a unique and distinct frequency-dependent response to known geological structure and that this amplification can be calculated analytically in a manner similar to current hazard practices. Contrary to both of these expectations, we show that the application of analytical theory originally developed for long-period surface waves can easily be applied to shorter-period ground motions relevant to earthquake hazards.
Analytic Description
Although this relation has usually only been applied to very long-period surface waves (e.g., more than 24 seconds by Lin et al.), the same physics applies to high-frequency surface waves as long as the velocity structure changes sufficiently without lateral problems [Tromp and Dahlen , 1992]. This description of local relative site amplification does not and should not include terms for path effects; anything that can affect the amplitude due to the path of a beam such as focusing , attenuation, basin lateral resonance [e.g., Bard and Bouchon , 1985 ] or a conversion of wave types to sharp boundaries [e.g., Liu and Heaton , 1984 ; Field , 1996 ] is not described here, this formulation has nothing to do with the excitation of surface waves either.
Simple Basin Example
Although both wave types are amplified, there is a significant quantitative difference for the surface waves entering this basin compared to the shear waves. Furthermore, although surface waves are often ignored at higher frequencies, it is these higher frequencies for which surface waves are most amplified.
Application to a Southern California Velocity Model
For this idealized model, surface wave amplification is about 50% stronger than and with a peak frequency twice that of the vertically incident shear wave (see Fig. 5.1c). Our division between shear wave arrivals and surface wave arrivals is based only on visual inspection of the waveform at the hard-rock site, and some of the PGA peaks may be ambiguous or close to the boundary.
Discussion and Conclusions
In each panel, the Peak Ground Acceleration (PGA) is identified separately for shear waves and surface waves, and the ratio of this PGA to the hard rock location, PASC, is indicated. At frequencies of 0.3-0.7 Hz, surface waves are amplified 2-3 times more than shear waves, relative to the hard rock location, again consistent with the predictions of the SCEC CVM in Figure 2.
Acknowledgments and Data
Cases where surface waves ultimately contribute to the highest levels of ground motion may explain some of the epistemic variability observed in strong motion catalogs, since correlations with local geology or other proxies do not have the same relationship between surface waves and body waves. For all these reasons, scientists and engineers must be aware that a single definition of "site amplification" may be insufficient to describe both surface waves and body waves in terms of both frequency and amplitude.
Stewart (2006), Evaluating the effectiveness of theoretical 1D amplification factors for earthquake ground motion prediction, Bull. The causal, positive-lag, outgoing wavefield is sensitive to sources only on the virtual-source side of the two stations.
