Deep Moho Research Articles

Modeling ground-motion attenuation in eastern North America

In this study, we have utilized our understanding of the physics of elastic wave propagation to simulate ground motions within about 200 km from earthquakes in eastern North America. Detailed crustal velocity structure models were developed for sources located in the Grenville and Appalachian provinces. The former consists of a single gradient in the crust, while the latter contains a mid-crustal discontinuity and a deep Moho. Semi-empirical synthetic seismograms were computed by convolving the generalized ray response of the structures with an empirical source function obtained from a recording close to the source. Peak ground motions derived from the synthetic seismograms adequately interpolate the sparse empirical data, while providing information on the wave propagation phenomena that define ground-motion attenuation. They suggest that peak ground-motion amplitudes are controlled by (in order of increasing range) the upgoing, direct S-wave, the diving S-wave, post-critical reflections from the mid-crustal or Moho discontinuities, and finally, L g.

Crustal thickness under the Gulf of St. Lawrence, northern Appalachians, from gravity and deep seismic data

We have determined crustal thickness in the Gulf of St. Lawrence, an area that corresponds to an offset of the main northern Appalachians units. A "complete" Bouguer anomaly was calculated from recent depth-to-basement compilations and sediment densities from well data. The Moho surface was obtained by inverting the Bouguer anomaly, assuming a single density contrast at depth, and using an average depth provided by deep reflection seismic data. The resulting crustal model shows a Moho depth of 42–44 km beneath the Grenville Craton, north of the Appalachian deformation front. South of this front, the depth to Moho displays a pronounced thinning of the crust beneath the Carboniferous Magdalen Basin. This is in striking contrast to the deep seismic data, which give a Moho depth of about 43 km. The modelling of the Bouguer anomaly in the Magdalen Basin, taking into account the seismic reflection and refraction data, reconciles these different results and suggests that a 43 km deep Moho beneath the basin is associated with a lower crustal layer about 13 km thick, with high velocity (7.35 km/s) and density (3.05 g/cm3). The Bouguer anomaly suggests that the lateral extent of this high-density layer is confined roughly to the Magdalen Basin. We suggest that this layer is due to mantle underplating of the crust as a result of the Carboniferous-age formation of the Magdalen Basin, and that it is not a feature related to the early to middle Paleozoic development of the Appalachian Orogen.

Crustal structure and surface zonation of the Canadian Appalachians: implications of deep seismic reflection data

In 1986, 1181 km of marine seismic reflection data was collected to 18–20 s of two-way traveltime in the Gulf of St. Lawrence area. The seismic profiles sample all major surface tectono-stratigraphic zones of the Canadian Appalachians. They complement the 1984 deep reflection survey northeast of Newfoundland. Together, the seismic profiles reveal the regional three-dimensional geometry of the orogen.Three lower crustal blocks are distinguished on the seismic data. They are referred to as the Grenville, Central, and Avalon blocks, from west to east. The Grenville block is wedge shaped in section, and its subsurface edge follows the form of the Appalachian structural front. The Grenville block abuts the Central block at mid-crustal to mantle depths. The Avalon block meets the Central block at a steep junction that penetrates the entire crust.Consistent differences in the seismic character of the Moho help identify boundaries of the deep crustal blocks. The Moho signature varies from uniform over extended distances to irregular with abrupt depth changes. In places the Moho is offset by steep reflections that cut the lower crust and upper mantle. In other places, the change in Moho elevation is gradual, with lower crustal reflections following its form. In all three blocks the crust is generally highly reflective, with no distinction between a transparent upper crust and reflective lower crust.In general, Carboniferous and Mesozoic basins crossed by the seismic profiles overlie thinner crust. However, a deep Moho is found at some places beneath the Carboniferous Magdalen Basin.The Grenville block belongs to the Grenville Craton; the Humber Zone is thrust over its dipping southwestern edge. The Dunnage Zone is allochthonous above the opposing Grenville and Central blocks. The Gander Zone may be the surface expression of the Central block or may be allochthonous itself. There is a spatial analogy between the Avalon block and the Avalon Zone. Our profile across the Meguma Zone is too short to seismically distinguish this zone from the Avalon Zone.

Lithospheric structure in southern Sweden—results from FENNOLORA

The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Småland-Värmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0–150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a transition zone of limited lateral extent, less than a few tens of kilometers. We suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the Småland-Värmland Granite Belt and the Svecofennides.

THE COMMON FEATURES OF DEEP STRUCTURES IN SOME LARGE-EARTHQUAKE AREAS OF THE NORTH CHINA PLAIN AND THEIR IMPLICATION FOR EARTHQUAKE SITE PREDICTION

This work deals with the deep structure of the earth's crust and the uppermost mantle as revealed by a method using P-SV converted waves from earthquakes in Beijing-Tianjin-Tangshan and some large-earthquake areas of the North China Plain. During the last ten years a series of seismic profiles have been completed in the studied region with a total length of about 2, 000 km. The comparison of deep structure with the distribution of earthquakes indicates that most strong earthquakes are located at the boundary of fault-blocks and especially at the intersection of deep-seated fault zones where the depth of deep boundaries changes discontinuously. Some common features of the deep structure have been revealed in Tangshan and other large-earthquake areas of the North China Plain. These include shallow crystalline basement, deep Moho discontinuity, thick crystalline crust, and intersection of deep-seated faults with different trends. By way of structural comparison, some areas with deep structures similar to those of large-earthquake areas have been established in the northern part of the North China Plain, which could be considered the areas with potential risk for strong earthquakes.

Crustal structure and variability of the Himalayan border of Tibet

Reversed critical wide angle reflection profiles of the crust–mantle boundary south of the Yarlung Zangbo suture in Tibet reveal a deep (70 km) Moho extending north from the High Himalayas, whilst to the south the Moho is 15 km higher. Even further south, reflections from 35 km depth may correspond to the Moho of peninsular India.

A Review of the Investigations into Earth's Crustal Structure from the Dispersion of Seismic Surface Waves

Investigations into continental crustal structure and upper mantle structures of the continents and oceans from the observation of seismic surface waves dispersion are reviewed in this second paper.Part II : Continental Crustal Structure.Up-to-date method, e. g., to determine the crustal structure through the careful fitting of theoretical dispersion curves basing upon the multi-layered crustal models with observed data are quite active in U. S. A. In this continent, Rayleigh waves dispersion due to nuclear explosion under the ground are well utilized. Higher mode surface waves have also been observed and studied. Geographical condition also enables U. S. seismologists to observe phase velocity dispersion over various regions in this continent.In Asia continent, deep Moho in and around Tibet Plateau is evident from the dispersion of Love Waves.Boundary of Antarctic continent is a matter of interesting. This problem, however, has not yet been fully solved from the dispersion of surface waves.Variation of Moho depth beneath Japan has been estimated from phase velocity of both Love and Rayleigh Waves.