Michael L. Begnaud

Velocity structure from forward modeling of the eastern ridge‐transform intersection area of the Clipperton Fracture Zone, East Pacific Rise

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to “fill in” the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

SALSA3D: A Tomographic Model of Compressional Wave Slowness in the Earth’s Mantle for Improved Travel‐Time Prediction and Travel‐Time Prediction Uncertainty

Abstract The task of monitoring the Earth for nuclear explosions relies heavily on seismic data to detect, locate, and characterize suspected nuclear tests. Motivated by the need to locate suspected explosions as accurately and precisely as possible, we developed a tomographic model of the compressional wave slowness in the Earth’s mantle with primary focus on the accuracy and precision of travel‐time predictions for P and Pn ray paths through the model. Path‐dependent travel‐time prediction uncertainties are obtained by computing the full 3D model covariance matrix and then integrating slowness variance and covariance along ray paths from source to receiver. Path‐dependent travel‐time prediction uncertainties reflect the amount of seismic data that was used in tomography with very low values for paths represented by abundant data in the tomographic data set and very high values for paths through portions of the model that were poorly sampled by the tomography data set. The pattern of travel‐time prediction uncertainty is a direct result of the off‐diagonal terms of the model covariance matrix and underscores the importance of incorporating the full model covariance matrix in the determination of travel‐time prediction uncertainty. The computed pattern of uncertainty differs significantly from that of 1D distance‐dependent travel‐time uncertainties computed using traditional methods, which are only appropriate for use with travel times computed through 1D velocity models.

Constraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation for the 2009 North Korea Nuclear TestConstraints of Crustal Heterogeneity and Q(f) from Regional (<4 Hz) Wave Propagation

We carried out 3D finite-difference (FD) simulations (< 4 Hz) of regional wave propagation for the 2009 North Korea nuclear explosion and compared the synthetics with instrument-corrected records at stations INCN and TJN in South Korea. The source is an isotropic explosion with a moment magnitude of 4.1. Synthetics computed in the relatively smooth Sandia/Los Alamos National Laboratory SALSA3D (SAndia LoS Alamos 3D) velocity model significantly overpredict Rayleigh-wave amplitudes by more than an order of magnitude while underpredicting coda amplitudes. The addition to SALSA3D of a von Karman distribution of smallscale heterogeneities with correlation lengths of ∼1000 m, a Hurst number of 0.1, and a horizontal-to-vertical anisotropy of ∼5 produces synthetics in general agreement with the data. The best fits are obtained from models with a gradient in the strength of the velocity and density perturbations and strong scattering (10%) limited to the top 7.5–10 km of the crust. Deeper scattering tends to decrease the initial P-wave amplitudes to levels much below those for the data, a critical result for methods discriminating between explosive and earthquake sources. In particular, the amplitude at the onset of Pn can be affected by as little as 2% small-scale heterogeneity in the lower crust and upper mantle. Simulations including a constant Q of 200 (INCN) to 350 (TJN) below 1 Hz and a power-law Q f formulation at higher frequencies, with an exponent of 0.3, generate synthetics in best agreement with the data. In our simulations, very limited scattering contribution from the near-source area accumulates along the regional path. Electronic Supplement: Description of the source time function used in the simulations, additional waveform comparisons (stations INCN and TJN), snapshots of wave propagation, and vertical cross sections and horizontal slices of the SAndia LoS Alamos 3D (SALSA3D) model with and without small-scale heterogeneities. Introduction The primary aim of nuclear-explosion monitoring is to be able to detect nuclear explosions, which includes discrimination between seismic records generated by different sources. The seismic signature in the records used in explosion monitoring is a combination of source, path, and local site effects. The origin of the source generally provides a constraint on the relative amount of P-, S-, and surface-wave phases included in the recorded waves. Although explosive sources typically give rise to records with a relatively large ratio of P-to-S waves, details of such trends depend on the length and character of the crustal path encountered by the waves. In many cases, the closest available records are obtained from stations several hundred kilometers from the source, typically causing the addition of severe path effects to the recorded seismic waves. These path effects are generated by a combination of (frequency-dependent) anelastic attenuation, crustal scattering, focusing, and multipathing. Unless detailed modeling can be carried out to sufficiently high frequencies, path effects can lead to misinterpretation of the origin of the wavetrains. It is therefore imperative to be able to replicate the general characteristics of the seismic records from regional wave propagation. Conventionally, regional modeling of crustal wave propagation has been carried out using either 1D or 2D approximations, due to the large computational cost associated with fully 3D deterministic models. However, recent advances in available supercomputing resources have facilitated full-waveform simulations in 3D velocity models for 1369 Bulletin of the Seismological Society of America, Vol. 108, No. 3A, pp. 1369–1383, June 2018, doi: 10.1785/0120170195

Identifying and Correcting Timing Errors at Seismic Stations in and around Iran

A fundamental component of seismic research is the use of phase arrival times, which are central to event location, Earth model development, and phase identification, as well as derived products. Hence, the accuracy of arrival times is crucial. However, errors in the timing of seismic waveforms and the arrival times based on them may go unidentified by the end user, particularly when seismic data are shared between different organizations. Here, we present a method used to analyze travel‐time residuals for stations in and around Iran to identify time periods that are likely to contain station timing problems. For the 14 stations with the strongest evidence of timing errors lasting one month or longer, timing corrections are proposed to address the problematic time periods. Two additional stations are identified with incorrect locations in the International Registry of Seismograph Stations, and one is found to have erroneously reported arrival times in 2011.