ALBERT

Description: We apply two different algorithms to measure surface wave phase velocity, as a function of frequency, from seismic ambient noise recorded at pairs of stations from a large European network. The two methods are based on consistent theoretical formulations, but differ in the implementation: one method involves the time-domain cross-correlation of signal recorded at different stations; the other is based on frequency-domain cross-correlation, and requires finding the zero-crossings of the real part of the cross-correlation spectrum. Furthermore, the time-domain method, as implemented here and in the literature, practically involves the important approximation that interstation distance be large compared to seismic wavelength. In both cases, cross-correlations are ensemble-averaged over a relatively long period of time (1 yr). We verify that the two algorithms give consistent results, and infer that phase velocity can be successfully measured through ensemble-averaging of seismic ambient noise, further validating earlier studies that had followed either approach. The description of our experiment and its results is accompanied by a detailed though simplifed derivation of ambient-noise theory, writing out explicitly the relationships between the surface wave Green’s function, ambient-noise cross-correlation and phase and group velocities.

Description: A controlled experiment is performed to investigate how assumptions and simplifications in the measurement and analysis of surface wave amplitudes affect inferred attenuation variations in the mantle. Synthetic seismograms are generated using a spectral-element method for 42 earthquakes, 134 receiver locations and two earth models, both of which contain 3-D elastic properties and 1-D anelastic properties. Fundamental-mode Rayleigh-wave amplitudes are measured at periods of 50, 75 and 125 s for 4749 paths. The amplitudes are measured with respect to a reference waveform based on 1-D Earth structure, and thus amplitude observations that are not equal to unity can be attributed to differences in the computation of the spectral-element and reference waveforms or to uncertainties in the amplitude measurements themselves. Calculation of earthquake source excitation in the 3-D earth model versus the 1-D earth model has a significant effect on the amplitudes, especially at shorter periods, and variations in the average amplitude for each event are well explained by the effect of Earth structure at the event location on the source excitation. The effect of local Earth structure at the receiver location on the amplitude is, for most paths, much smaller than for the source amplitude. After correcting for source and receiver effects on amplitude, the remaining signal is compared to predictions of elastic focusing effects using the great-circle ray approximation, exact ray theory (ERT) and finite-frequency theory (FFT). We find that, for the earth models we have tested, ERT provides the best fit at 50 s, and FFT is most successful at 75 and 125 s, indicating that the broad zone of surface wave sensitivity cannot be neglected for the longer periods in our experiment. The bias introduced into attenuation models by focusing effects, which is assessed by inverting the measured amplitudes for 2-D attenuation maps, is most important at high spherical-harmonic degrees. Unaccounted-for scattering of seismic energy may slightly (〈5 per cent) raise average global attenuation values at short periods but has no detectable effect at longer periods. The findings of this study also provide a set of guidelines for handling source, receiver and focusing effects that can be applied to surface wave amplitudes measured for the real Earth.

Description: We calculate two-station phase measurements using single-station measurements made on USArray Transportable Array data for surface waves at periods from 25 to 100 s. The phase measurements are inverted for baseline Love and Rayleigh wave phase velocity maps on a 0.5° 0.5° grid. We make estimates of the arrival angle for each event at each station using a mini array method similar to beamforming, and apply this information to correct the geometry of the two-station measurements. These corrected measurements are inverted for an additional set of phase velocity maps. Arrival angles range from 0° to ±15°, and the associated corrections result in local changes of up to 4 per cent in the final phase velocity maps. We select our preferred models on the basis of the internal consistency of the measurements, finding that the arrival-angle corrections improve the two-station phase measurements, but that Love wave arrival-angle estimates may be contaminated by overtone interference. Our preferred models compare favourably with recent studies of the phase velocity of the Western United States. The corrected Rayleigh wave models achieve greater variance reduction than the baseline Rayleigh wave models, and the baseline Love wave models, which are more difficult to obtain, are robust and could be used in conjunction with the Rayleigh wave models to constrain radially anisotropic earth structure.

Description: We develop an algorithm for the detection and location of seismic sources using intermediate-period (35-150 sec) surface waves recorded on a global array of stations. Continuous vertical seismic waveforms from the global network are collected and a 4 degrees X4 degrees global grid of target locations is defined. For each target location and each station, a surface-wave propagation operator is deconvolved from the seismogram to restore any source pulse present. The envelope of the seismogram is calculated and cross correlated with a theoretical source-pulse shape. The resulting waveforms are stacked to improve signal-to-noise characteristics, and the quality, strength, and timing of the potential detection are determined. When a successful event detection is made, a finer grid is applied to locate the event with greater precision. We apply the algorithm systematically for the period 1993-2003 and catalog the events. Approximately 2000 events are detected and located for each year and 98% of shallow events in the Harvard Centroid Moment Tensor (CMT) catalog are detected and located by the new algorithm. A comparison of 9482 events common to the two catalogs allows the detection strength to be calibrated against the CMT moment magnitude. All detected events have estimated moment magnitudes M (sub w) 〉4.5. In each year, approximately 100 events not listed in other global seismicity catalogs are detected and located. Many of these events lie along the ridge-transform plate boundaries in the Southern Hemisphere and may be regular earthquakes that have gone undetected because of poor station coverage. Other events, located in areas where global and regional networks provide good coverage, are potentially anomalous and may have escaped detection as a result of their unusual source properties.

Description: We present an efficient and accurate method for calculating local nonlinear crustal effects on the eigenfrequency of free oscillations of the Earth. We implement the method in the calculation of synthetic normal-mode seismograms and demonstrate that neglecting the nonlinear crustal effects in the tomographic waveform inversion leads to velocity models that appreciably overestimate fast velocity anomalies beneath orogenic belts and cratons at 250-400 km depth.

Description: We examine the effect of overtone interference on fundamental-mode Love-wave phase measurements made using single-station and array-based techniques at 25-100 s periods. For single-station teleseismic measurements on USArray Transportable Array data, the contamination effects are small, less than 1% of the path-averaged phase velocity, consistent with previous studies. Single-station amplitude measurements provide complementary constraints on the interference pattern. For array-based measurements on the same data set, contamination effects are much larger: up to approximately 10% of the phase velocity for two-station measurements and up to approximately 20% for mini-array measurements. The interference pattern for single-station measurements from shallow earthquakes can largely be explained by interactions between only two modes, the fundamental mode and the first higher mode. This interpretation is confirmed using measurements on both mode-summation synthetic waveforms for a 1D Earth model and synthetic waveforms calculated using SPECFEM3D Globe and a 3D Earth model. Array-based phase measurements are calculated from differences of the single-station phase delays, and we demonstrate that the overtone interference pattern for array-based measurements can be approximated using gradients of the single-station interference pattern with distance. This relationship can lead to an overall bias to higher phase velocities when combined with common quality selection and data-reduction procedures for array measurements. Our results indicate that array-based Love-wave phase measurements must be carefully scrutinized for overtone contamination and suggest the possibility of new approaches for measuring overtone phase velocities.

Description: We built a device for translating a GPS antenna on a positioning table to simulate the ground motions caused by an earthquake. The earthquake simulator is accurate to better than 0.1 mm in position, and provides the "ground truth" displacements for assessing the technique of high-rate GPS. We found that the root-mean-square error of the 1-Hz GPS position estimates over the 15-min duration of the simulated seismic event was 2.5 mm, with approximately 96% of the observations in error by less than 5 mm, and is independent of GPS antenna motion. The error spectrum of the GPS estimates is approximately flicker noise, with a 50% decorrelation time for the position error of approx.1.6 s. We that, for the particular event simulated, the spectrum of dependent error in the GPS measurements. surface deformations exceeds the GPS error spectrum within a finite band. More studies are required to determine whether a generally optimal bandwidth exists for a target group of seismic events.