S42A-01
Determination of Fault Plane and Rupture Direction of the April 18, 2008 Earthquake, Mt. Carmel, Illinois
We located a large number of aftershocks to determine the fault plane of the April 18, 2008, Illinois earthquake.Those aftershocks were detected by a sliding-window cross correlation (SCC) technique that we developed in this study. We applied this technique to continuous waveforms recorded by the Cooperative New Madrid Seismic Network stations. It detected 86 aftershocks down to magnitude 0.8 in the two-week time window following the mainshock, which is twice more than the number of aftershocks reported by the seismic network. Most aftershocks happened within 24 hours of the mainshock. We then relocated all events by the double-difference relocation algorithm. Accurate differential P- and S-wave arrival times were obtained by waveform cross correlation. After relocation, all events are located in a SW-NE line which delineates an N40E oriented strike-slip fault. The fault is nearly vertical down to ~20 km. To determine the direction of mainshock rupture propagation, we used waveforms of a small magnitude aftershock as the empirical Green's functions to estimate source time function of the mainshock. Results show that the rupture propagated nearly horizontally to the north in the fault plane oriented in N30E, consistent with the fault plane determined by earthquakes locations.
S42A-02
A Synthesis of the Crustal and Upper Mantle Structure of the Central US
The seismic structure of the crust and upper mantle of the central US plays a significant role in determining the spatial distribution of damaging earthquake strong ground motions. In addition, anomalies in the deep structure have been hypothesized to correlate with the locations of earthquakes. Such anomalies include paleo-rifts, sutures between accreted terranes, and upper-crustal zones postulated to contain fluid-filled cracks. In view of these important physical consequences, numerous seismological studies have been undertaken in recent years in the central US. The most basic observation is that the crust is relatively thick (40-50 km), with a high average crustal velocity (6.4-6.6 km/s) that is indicative of a dense, mafic lower crustal composition. Measured upper mantle velocities (Pn) are 8.0-8.2 km/s, values that are greater than or equal to the North American continental average of 8.0 km/s. Such high Pn velocities are indicative of a cooler upper mantle, as compared with the western US. Although there are fewer S-wave crustal measurements (n=125) than P-wave measurements (n=750), the available data clearly show relatively high average crustal S-wave velocities (>3.7 km/s) as compared with the western US (<3.4 km/s). These observations are consistent with the higher measured Qp and Qs values in the central US as compared with the western US. The most pronounced lateral variations in crustal structure are found when crossing rift grabens, such as the Mississippi Embayment, and suture zones, such as the Grenville Front. Local earthquake tomographic inversions, as well as 3D inversions for seismic attenuation have been interpreted to indicate that the New Madrid seismic zone, for example, is characterized by localized, upper crustal fluid-filled cracks that may be related to on-going earthquake activity.
S42A-03
A Tectonic Model for the Midcontinent U.S. Lithosphere Based on Structural Analyses of Mesoproterozoic Through Cenozoic Deformation
Insights into the tectonic fabric of the midcontinent U.S. lithosphere are provided by structural investigations of exposed basement and its supra-crustal sedimentary cover sequences. Mesoproterozoic basement rocks of the St. Francois terrane possess an orthogonal pattern of vertical NW- and NE-trending strike-slip fault zones. The NW trend dominates Mesoproterozoic deformation and is inherent from an older fabric that controlled the location of Mesoproterozoic igneous activity. Two of these NW-trending zones appear to have from 60 to 75 km and 30 to 75 km of accumulative left slip. Pre-Late Cambrian vertical, right-lateral, strike-slip faulting on NW-trending structures in the St. Francois terrane, emplacement of dominantly NE-trending, 1.33 Ga mafic dikes, and uplift and erosion of ~2 to 4 km of rocks represents the assembly and breakup of Rodinia in the rock record of the midcontinent basement. Re-activation of NE-trending structures in the Late Cambrian resulted in formation of the Reelfoot rift and was accompanied by re-activation of vertical NW- trending structures with left-lateral displacement. Faulting in the Paleozoic, Mesozoic, and Cenozoic cover sequences document re-activation of both vertical trends as far-field strike-slip faults during the Acadian, Taconic, Ouachita, Alleghany, and Laramide orogenies. Step overs from one strike-slip fault strand to another during these orogenies produced local uplift along restraining bends and subsidence in pull-apart grabens and basins. The New Madrid seismic zone and other sites of Quaternary deformation in the midcontinent also are attributed to re-activation of inherited vertical fabric. In summary, a tectonic model of the midcontinent lithosphere is best portrayed as consisting of an orthogonal mosaic of vertical zones of shear that presumably penetrate the crust and upper mantle, and are therefore long lived and prone to reactivation under lithospheric stresses. Much worldwide intraplate seismicity is attributed to reactivation of ancient rift structures, however not all seismicity fits this interpretation. We suggest that in the midcontinent, U.S., some seismicity can be attributed to reactivation of vertical strike-slip fault zones that are not associated with any rift. Furthermore, some seismically active old rift structures in the midcontinent may be inherently related to even older strike-slip fault systems.
S42A-04 INVITED
Mapping Sedimentary Basins Across Canada Using Receiver Function Analysis
Receiver function studies are being applied within several sedimentary basins across Canada to map basin geometry and sediment thickness. Teleseismic receiver functions are ideal, in many ways, for this type of study. They provide site-specific information, constraints on the shear wave velocity, interface geometry, and they can be used to identify structures from the near-surface to mantle depths. Although the frequency content of teleseismic waveforms limits the resolution, high-frequency receiver functions can resolve layers as thin as 1-2 km. In the Western Canada Sedimentary Basin (near Edmonton, Alberta), receiver functions were used to image the 2.5-km-thick sedimentary package at this site and a low-velocity zone in the upper crust. Currently, a deployment of 10 broadband seismic stations in Atlantic Canada is targeting the Paleozoic and Carboniferous sedimentary basins in the Gulf of St. Lawrence. Significant variations in the arrival time of the continental Moho phase along a north-south transect indicate crustal thickness variations, with the earliest Moho arrivals being in the central part of the basin, indicating a thinner crust here. The direct arrivals in the receiver functions at stations in the southern part of the gulf are broad, indicating notable thicknesses of near-surface sediments. These observations are consistent with what is known about the regional geology and the centrally-located Maritimes Basin. These initial observations, and other geophysical data for the region, will feed into interpretations of crustal structure developed through numerical modelling.In the Nechako Basin of central BC, receiver functions from seven broadband seismograph stations are being used to map sediments in this basin that are overlain by volcanic basalts. These receiver function show clear evidence for shallow dipping low-velocity layers. Constrains on the basin geometry and sediment thickness will be used to improve assessments of oil and gas potential in this region.
S42A-05
Rayleigh Wave Tomography in the Lower Great Lakes Region
The Grenville orogen located at the eastern edge of the Canadian Shield has been relatively quiescent since it was generated by a continental-continental collision about 1Ga. To better understand the mechanisms of the Grenville orogen and its interaction with the Canadian Shield, we construct a high resolution shear wave model in southeast Ontario in Canada and northeast New York in the United States from Rayleigh waves that are recorded at 38 broadband seismic stations. We use 40 events with the magnitude larger than 5.5 in this study. Rayleigh wave phase velocities at the periods of 30-150 s have been obtained by applying the two- plane-wave inversion method. The average phase velocity in the study area varies from 3.63 km/s at period of 20 s to 4.37 km /s at 143 s. These values are generally higher than phase velocities from the global model. High phase velocity anomalies are continuously imaged beneath the northern part of the Grenville province and towards northwest to the Canadian Shield from 77 s to143 s, suggesting a fast, presumably cold and strong lithosphere. At short periods of 30 s to 45 s, low phase velocity anomalies locate beneath the Ontario Lake, which correspond to thick crust imaged from receiver functions. We will develop a 3D shear wave model from the inversion of the 2D phase velocities and discuss its tectonic implications.
S42A-06
A revised seismotectonic framework for the Charleston, South Carolina earthquakes
The Middleton Place Summerville Seismic Zone (MPSSZ) located about 30 km NW of Charleston is the location of the destructive 1886 earthquake and the current seismicity. Instrumentally located seismicity in MPSSZ between ~1974 and 2004 was relocated using a double-difference logarithm. The first motions at various stations were used to delineate a system of causative faults. The location and style of faulting of these faults was constrained and compared with a wide variety of geological, geophysical and geomorphological data on a common GIS database. The results of this exercise led to the delineation of a revised seismotectonic framework associated with MPSSZ. This framework consists of ~50 km long, ~N30°E striking, NW dipping Woodstock fault associated with right-lateral-oblique strike-slip motion, with a ~6 km long antidilational compressional left step near Middleton Place, dividing it into Woodstock North and South faults. Three ~NW-SE striking, inward dipping reverse faults were recognized within this step. They are the steeply NE dipping Sawmill Branch fault zone and Lincolnville fault, and the shallowly SW dipping Charleston fault. Currently most of the seismicity is occurring on the ~3km wide Sawmill Branch fault zone Seismicity occurs in response to a compressional stress field oriented ~N60°E.
S42A-07
High-resolution Vp and Vs Structure of the Post-Paleozoic Sediments in the Upper Mississippi Embayment, Central USA
Site response, sedimentary basin geometry, earthquake induced strong ground motion, distribution of earthquake hypocenters, and geometry and characteristic feature of active faults are among the most essential elements for seismic hazard assessment in the upper Mississippi Embayment. However, these elements cannot be successfully evaluated without reliable Vp and Vs information for the embayment sediments. A total of 20 sites in the northern Mississippi Embayment were selected for field experiment to explore detail Vp and Vs structural information. At each site, a seismic reflection/refraction line has been conducted using a seismic source that generates both P- and S-waves. For sites not near any regional seismic network stations, a temporary portable broadband seismic station has been installed for a few weeks to record local earthquakes. Seismic data from the reflection/refraction line and from the recorded local earthquakes are analyzed to explore the Vp and Vs structures of the post Paleozoic sediments beneath each site. The Vp and Vs structures for the sediments beneath adjacent sites are then compared to explore lateral velocity structural variations. Preliminary results reveal that seismic velocities and lithologic features of the sediments in the embayment are characterized by extremely low seismic velocity, especially Vs, near surface and by very significant lateral and vertical variations. The sediments in the Upper Mississippi Embayment cannot be described simply by any 1-D homogeneous horizontally layered velocity model, typically obtained from surface wave analysis and from long seismic refraction profiles. Thus, earthquake locations as well as seismic hazard assessment cannot be properly determined without more detail 3-D Vp and Vs structural models for the sediments in the Upper Mississippi Embayment.
S42A-08
Community Velocity Model for the New Madrid Region, Central U.S.
In 1811-1812, a series of three major earthquakes struck the Central United States in the New Madrid Seismic Zone. Having magnitudes near 7.5 and being located within the relatively stable interior of the North American Continent, these events produced widespread strong shaking. If these events were to occur today, there would be substantial devastation to people, buildings and transportation and communication infrastructure. To better understand this threat and in preparation for the upcoming bicentennial, the US Geological Survey is planning to produce and support sophisticated numerical simulations of earthquake rupture and seismic wave propagation due to a repeat of these events. To kick off this effort, the USGS began the development and construction of a community seismic velocity model for use in these numerical simulations. We have collected existing research regarding the p- and s-wave velocities, impedance contrasts and densities of the lithosphere in the New Madrid region and synthesized these results into a single model that can be used in earthquake simulations. We have identified areas of missing or incomplete information for further study. The region covers an area of approximately 600,000 km2 from Little Rock, Arkansas across to Nashville Tennessee, up to St Louis, Missouri. The model has currently been gridded at 3 km lateral resolution and from 5-m resolution near the surface to 10-km resolution at 100 km depth. Less but still substantial uncertainty exists for the Mississippi Embayment where a majority of the research has been done. Newer regional models such as those by van der Lee and others and Liang and Langston have improved regional resolution beyond a 1-dimensional model, but for ground motion simulations, greater resolution outside the Embayment is desired.