Ghassan Aleqabi

Using MOMA Broadband Array ScS‐S data to image smaller‐scale structures at the base of the mantle

ScS-S residuals obtained at stations of the Missouri-to-Massachusetts (MOMA) temporary broadband seismic array are used to delineate variations in seismic velocity structure above the core-mantle boundary (CMB) at scales smaller than observable with tomographic models. South American earthquakes recorded at MOMA reveal a slow-velocity anomaly that is at least as small as the limit of the resolution of ScS waves, about 300 km across. This is modeled as being within a region of fast velocities in whole-mantle models. The slow ScS-S residuals correlate well with a peak in ScS/S relative amplitudes. The small region of slow shear velocity at the CMB could be a pocket of lower mantle rock trapped beneath the descending Farallon slab, or evidence of chemical boundary layer variations.

Q Lg Distribution in the Basin and Range Province of the Western United States

Q values for the direct L g phase ( ) were estimated for event-station Q Lg paths that lie entirely within the western United States, particularly in the southern Great Basin and surrounding areas. The Qo ( at 1 Hz) values were estimated by QLg fitting synthetic spectra to observed Lg spectra using a genetic algorithm technique. We have created a tomographic image of the variations of Qo for a part of the southwestern United States. The image shows that Qo varies between about 234 and 312 at 1 Hz, with an average of 267. The lowest Qo occurs in the northwest part of the Basin and Range Province, where extensional deformation has occurred since the Mesozoic. Qo values start to increase toward the Colorado Plateau to the east and continue to gradually increase northward and decrease southward. We also find rela- tively high frequency dependence to the values, with a 1D mean of g 0.57. QLg

SLICING INTO THE EARTH

Regional arrays of seismometers provide a powerful means of mapping the details of deep-Earth structure. Our understanding of the geological processes at work within our planet depends on our ability to examine them; seismic techniques remain the best tool available. However, spatial aliasing due to the less-than-optimal distribution of global seismometers has long made it difficult to determine deep-Earth structure from teleseismic waves. The temporary deployment of portable broadband seismometers can help by providing high-resolution windows into the Earth. Patterns of global mantle convection create seismically observable features such as anisotropy at the top and bottom of the mantle, topography of upper mantle discontinuities, and heterogeneous structure at the core-mantle boundary.

Distinct crustal structure of the North American Midcontinent Rift from P wave receiver functions

Eighty-two broadband seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) collected 2.5 years of continuous seismic data in the area of the high gravity anomaly associated with the Midcontinent Rift (MCR). Over 100 high-quality teleseismic earthquakes were used for crustal P wave receiver function analysis. Our analysis reveals that the base of the sedimentary layer is shallow outside the MCR, thickens near the flanks where gravity anomalies are low, and shallows again in the MCRs center where the gravity anomalies peak. This pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1.1 Ga. Intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the MCRs mostly buried dense volcanic layers. Outside the MCR, the Moho is strong, sharp, and relatively flat, both beneath the Archean Superior Province and the Proterozoic terranes to its south. Inside the MCR, two weaker candidate Mohos are found at depths up to 25 km apart in the rifts center. The intermediate layer between these discontinuities tapers toward the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath its jog in southern Minnesota, near the Belle Plaine Fault. We interpret these results as evidence for extensive underplating as a defining characteristic of the rift, which remains continuous along the Minnesota jog, where due to its orientation, it is minimally affected by the reverse faulting that characterizes the NNE striking parts of the rift.

Source Characterization of Nevada Test Site Explosions and Western U.S. Earthquakes using Lg Waves: Implications for Regional Source Discrimination

An investigation is made of the Lg waves from 15 nuclear tests at the Nevada Test Site (NTS) and 25 shallow western United States earthquakes to obtain a scheme for discriminating between possible source mechanisms. The data were recorded at four broadband stations, operated by Lawrence Livermore National Laboratory (LLNL), which encircle the NTS. The Lg wave spectra are modeled through a genetic algorithm search to find optimal values for (1) the seismic moment ( M o ), (2) the corner frequency of the amplitude spectrum ( f c ), (3) the 1-Hz attenuation quality factors ( Q o ) for all of the paths, and (4) the frequency dependence (η) of attenuation along the paths. Because the inversion solves for the path attenuation characteristics, little a priori information is needed for the analysis. The resulting M o and f c values are used in conjunction with the m b values as seismic discriminants. The populations of earthquakes and explosions have nearly identical relationships between their moments and corner frequencies. For earthquakes, the relationship is log M o = 16.07 – 2.93 log f c . For the set of NTS explosions (using an earthquake source model), the relationship is log M o = 15.87 – 2.76 log f c . A source discriminant is found, however, through a comparison of the moments and m b values. This gives a line, log M o = 10.20 + 1.16 m b , which cleanly separates the earthquake and explosion populations, with earthquakes falling above and explosions below the line. Reliability tests examine the inverse trade-off between moments and Q o values. Noise in the Lg spectra has a limited effect on the scaling relations.