scholarly journals Regional site response and uppermost mantle seismic structure of central and eastern united states

2019 ◽  
Author(s):  
◽  
Rayan Yassminh
Keyword(s):  
United States ◽  
New England ◽  
Velocity Ratio ◽  
Site Amplification ◽  
P Wave ◽  
Seismic Structure ◽  
Eastern United States ◽  
Uppermost Mantle ◽  
Pn Velocity ◽  
Mantle Temperature

This dissertation examines seismological data from regional earthquake sources in order to examine the seismological character of the crust and uppermost mantle in central and eastern United States. Firstly, site amplification of regional highfrequency Lg seismic phases is estimate ed using a Reverse-Two Station (RTS) method. RTS results show topography and sediment thickness are likely to affect amplification and both factors likely frequency-dependent. There is a negative correlation between the RTS-measured amplification and shallow shear-wave velocity. It appears that both regional topography (i.e., long-wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification. Subsequently, Pn and Sn travel time tomography is used to estimate the upper most mantle P-wave (Pn) velocity, S-wave (Sn) velocity, and the velocity ratio (VPn/VSn). In addition to velocity, effective attenuation of Sn phase (Q[superscript -1]sn) is also measured. The result shows regions of high velocity such as southern Georgia, eastern South Carolina and NMSZ and low Q[subscript Sn] values. The V[subscript Pn]/V[subscript Sn] ratio shows values higher than the average in regions such as the Mississippi Embayment, New England, and south Appalachian. V[subscript Pn]/V[subscript Sn] ratios are lower than the average in regions such as northwestern CEUS, South Georgia and eastern Texas. We estimated the uppermost mantle temperature by applying a constrained grid-search algorithm includes the observed V[subscript Sn], V[subscript Pn] and Q[subscript Sn] with the calculated velocities of specific compositional models. The uppermost mantle temperature result, [about]300-500C, beneath the northern mid-continent, and the highest temperature, 1100 C, beneath New England

Evaluation of Intensity Prediction Equations (IPEs) for Small-Magnitude Earthquakes

2021 ◽  
Author(s):  
Ganyu Teng ◽  
Jack W. Baker ◽  
David J. Wald
Keyword(s):  
United States ◽  
Ground Truth ◽  
Site Amplification ◽  
Eastern United States ◽  
Prediction Equations ◽  
Small Magnitude ◽  
Intensity Prediction ◽  
Central United States ◽  
Hazard Disaggregation

Abstract This study assesses existing intensity prediction equations (IPEs) for small unspecified magnitude (M ≤3.5) earthquakes at short hypocentral distances (Dh) and explores such earthquakes’ contribution to the felt shaking hazard. In particular, we consider IPEs by Atkinson and Wald (2007) and Atkinson et al. (2014), and evaluate their performance based on “Did You Feel It” (DYFI) reports and recorded peak ground velocities (PGVs) in the central United States. Both IPEs were developed based on DYFI reports in the central and eastern United States with moment magnitudes above Mw 3.0. DYFI reports are often used as the ground truth when evaluating and developing IPEs, but they could be less reliable when there are limited responses for small-magnitude earthquakes. We first compare the DYFI reports with intensities interpolated from recorded PGVs. Results suggest a minimal discrepancy between the two when the intensity is large enough to be felt (i.e., M >2 and Dh<15  km). We then compare intensities from 31,617 DYFI reports of 3049 earthquakes with the two IPEs. Results suggest that both the IPEs match well with observed intensities for 2.0< M <3.0 and Dh<10  km, but the IPE by Atkinson et al. (2014) matches better for larger distances. We also observe that intensities from DYFI reports attenuate faster compared with the two IPEs, especially for distances greater than 10 km. We then group DYFI reports by inferred VS30 as a proxy for site amplification effects. We observe that intensities at sites with VS30 around 300 m/s are consistently higher than at sites with VS30 around 700 m/s and are also closer to the two IPEs. Finally, we conduct hazard disaggregation for earthquakes at close distances (Dh=7.5  km) using the observed records. Results suggest that earthquakes with magnitudes below M 3.0 contribute more than 40% to the occurrence of felt shaking.

Vegetation East of the Rocky Mountains 18,000 Years Ago

1981 ◽  
Vol 15 (2) ◽  
pp. 113-125 ◽  
Author(s):  
H.E. Wright
Keyword(s):  
United States ◽  
Boreal Forest ◽  
New England ◽  
Jack Pine ◽  
Atlantic Coastal Plain ◽  
Reference Level ◽  
Eastern United States ◽  
Southern Limit ◽  
Southern Georgia ◽  
Narrow Belt

AbstractThe various lobes and segments of the southern periphery of the Laurentide ice sheet reached their maximum extension at different times between 21,000 and 14,000 yr ago, but the CLIMAP date of 18,000 yr ago is taken as a reference level to review the distribution of major vegetational formations in central and eastern United States. Tundra was apparently confined to a narrow belt peripheral to the ice margin only in the Minnesota area and from northern Pennsylvania to New England, with extensions down the crest of the Appalachian Highlands at least as far as Maryland. Some areas south of the Great Lakes may later have been marked by treeless vegetation briefly as the ice retreated. The boreal forest to the south in the central United States was dominated by spruce; the jack pine that had prevailed during previous times was apparently eliminated by the time the ice reached its maximum. In the Appalachian Highlands and the Atlantic Coastal Plain, however, jack pine occurred along with spruce, which decreased in importance southward. The southern limit of the boreal forest in the Southeast was perhaps somewhere in southern Georgia and Alabama. Oak and other temperate deciduous trees were minor components of the boreal coniferous forests especially in the southern Appalacchians, but there is no evidence yet in the southeastern states for a relic mixed mesophytic forest 18,000 yr ago similar to the rich modern deciduous forests of the region, except possibly in the Lower Mississippi Valley. The climate in much of the Southeast was apparently dry as well as cool at that time; in Florida oak/pine scrub and prairie-like openings prevailed, and all but the deepest lakes dried up.

scholarly journals Modelling Pn Wave Speeds Beneath the Central North Island, New Zealand

2021 ◽  
Author(s):  
◽  
Anya Mira Seward
Keyword(s):  
Elevated Temperatures ◽  
Least Square ◽  
P Wave ◽  
Moho Depth ◽  
Strain History ◽  
Uppermost Mantle ◽  
Wave Speeds ◽  
Partial Melt ◽  
Volcanic Region ◽  
Pn Velocity

<p>A new method of modelling Pn-wave speeds is created. The method allows the predominant wavelength features of P-wave speeds in the uppermost mantle to be modelled, as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations, using least-square collocation. A combination of National Network seismometers, local volcanic seismic monitoring networks and temporary deployments are used to collect arrival times from local events, during the period of 1990-2006. The dataset consists of approximately 11200 Pn observations from 3000 local earthquakes at 91 seismograph sites. The resulting model shows distinct variations in uppermost mantle Pn velocities. Velocities of less than 7.5 km/s are found beneath the back-arc extension region of the Central Volcanic Region, and under the Taranaki Volcanic Region, indicating the presence of water and partial melt. The region to the east shows extremely high velocities of 8.3-8.5 km/s, where the P-waves are traveling within the subducting Pacific slab. Slightly lower than normal mantle velocities of 7.8-8.1 km/s are found in the western North Island, suggesting a soft mantle. Pn anisotropy estimates throughout the North Island show predominately trench parallel fast directions, ceasing to nulls in the west. Anisotropy measurements indicate the strain history of the mantle. For the observed upper mantle Pn velocity of 7.3 km/s is one of the lowest seen in the world. Ray-tracing modelling indicate that this region extends to depths of at least 65 km, suggesting an area of elevated heat (700 - 1100 degrees C) at Moho depth. Elevated temperatures can be caused by the presence partial melt (0.4 % to 2.1 % depending on the amount of water present). Beneath the western North Island, the observed slower than normal mantle velocities, indicate a material of lowered shear modulus, susceptible to strain deformation. However, anisotropy estimations in this region, show no significant anisotropy, suggesting that this is a region of young mantle that hasn't had time to take up the signature of deformation. These observations can be explained by a detachment of the mantle lithosphere through a Rayleigh-Taylor instability more than 5 Ma.</p>

An amplitude constrained P-wave velocity profile for the upper mantle beneath the Eastern United States

1979 ◽  
Vol 69 (6) ◽  
pp. 1733-1744
Author(s):  
George A. McMechan
Keyword(s):  
United States ◽  
Velocity Profile ◽  
Wave Velocity ◽  
Upper Mantle ◽  
Epicentral Distance ◽  
P Wave ◽  
Eastern United States ◽  
Lateral Velocity ◽  
The North ◽  
P Wave Velocity

abstract A P-wave velocity profile for the upper mantle at depths between 200 and 800 km beneath Eastern United States has been constructed from a combination of data from natural and artificial sources. Data for this part of the upper mantle are scarce, particularly beyond 20° epicentral distance, because of the sparse distribution of relevant sources and stations. Nevertheless, this study is the first to use amplitude constraints in a model determination for this region, and the model that has been chosen can account for the main observed amplitude features as well as travel times. The resulting velocity profile is similar to those previously determined for the regions to the north and west, but has a broadening of velocity transitions relative to those in the western United States. Evidence is found for the existence of lateral velocity inhomogeneity within the mantle.

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