A 70‐km‐long seismic reflection profile in western Michigan provides new insight into the nature, distribution, and structure of the Keweenawan Supergroup volcanic and overlying sedimentary rocks and the controversial Keweenaw fault along the southern boundary of the Midcontinent Rift System in the Lake Superior basin. Interpretation of the 5‐s reflection data constrained by surface geology, magnetics, and gravity modeling shows that volcanic rocks which cropout north of the Keweenaw fault dip northerly to depths of the order of 17 km. South of the fault, volcanic rocks overlain by ∼2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the South Range. The volcanic pile within the basin thickens rapidly to the north of the Keweenaw fault, suggesting that the volcanics were deposted in an extensional fault‐bounded basin. Clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event. The thickness of the volcanic‐filled basin implies that the upper crust was almost completely broken during the rifting event. The available evidence is interpreted to show the Keweenaw fault as a moderate‐ to high‐angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence. There is no evidence from the seismic profiling for major faulting (except for the Keweenaw fault), intrusions, or folding of the Keweenawan Supergroup in this region.
Scalar magnetic anomaly data from MAGSAT, reduced to vertical polarization and long wavelength pass filtered free air gravity anomaly data of South America and the Caribbean are compared to major crustal features. The continental shields generally are more magnetic than adjacent basins, oceans and orogenic belts. In contrast, the major aulacogens are characterized by negative anomalies. Spherical earth magnetic modeling of the Amazon River and Takatu aulacogens in northeastern South America indicates a less magnetic crust associated with the aulacogens. Spherical earth modeling of both positive gravity and negative magnetic anomalies observed over the Mississippi Embayment indicate the presence of a nonmagnetic zone of high density material within the lower crust associated with the aulacogen. The MAGSAT scalar magnetic anomaly data and available free air gravity anomalies over Euro-Africa indicate several similar relationships.
A 100‐km‐long record section of NTS explosions recorded in the eastern Snake River Plains lpar;7°<Δ<8°) shows the cusp of critical refractions from the steepened P velocity gradient at the bottom of the upper mantle LVZ. Synthetic seismograms calculated with a modified reflectivity program have been used to derive a regional velocity model of the upper mantle beneath the eastern Great Basin. The model suggests that observed very weak P n arrivals are due to a slight negative velocity gradient below the Moho and that no high velocity mantle lid exists in this region.
Combine the bold landscape of northern New Mexico (Figure 1) with a unique educational program that blends teaching and research as a partnership among universities, industry, and federal laboratories and you have SAGE (Summer of Applied Geophysical Experience). SAGE was conceived from a vision that something special would come from pooling the resources and talents of diverse groups. It enables undergraduate and graduate students from large and small schools alike to share the excitement of hands-on, modern field geophysical research and learning. Much more than a summer geophysics field camp, SAGE is an immersion in geophysics, an educational experience that many students say is the most satisfying in their lives. Students participate in every phase of the field program: They collect data with modern equipment; they process, model, and interpret the data with workstations and PCs; and they present their results in both oral and written form. The program is not just about using equipment; it's about unde...
Abstract Based on retrospective modeling of earthquakes from the southern California earthquake catalog, along with previously published evaluations from the New Madrid Seismic Zone, the modified time-to-failure method may be used as an intermediate-term earthquake prediction technique for locating and predicting the size and time of a future mainshock. Modeling previous mainshocks for hypothesis development indicates that the method predicts the actual magnitude of the mainshock to within approximately ±0.5 magnitude units. The error associated with the time-of-failure is approximately ±1.1 years assuming the last precursory event is known. When the last event in the precursory sequence is not known, the predicted magnitude remains similar, but the predicted time will require refinement as additional events are added, with time, to the sequence. The mainshock location can also be identified within a circular region with a radius on the order of tens of kilometers. Criteria are provided for defining acceleration sequences and mainshock locations. The criteria reduce the number of false predictions but also eliminate some mainshocks from our evaluation. Mainshocks as small as magnitude 5.5, occurring between 1980 and 1995, were evaluated from the Southern California earthquake Catalog (SCC). The results were used in association with previous studies to develop a method that can be used for practical (future prediction) applications. The modified time-to-failure method was used to search the SCC for future mainshocks occurring after 17 August 1998. One region satisfied all the criteria and may be modeled by the modified time-to-failure method. The region likely to have a mainshock is a 65-km-radius area centered at 31.43° N, 115.47° W (northern Baja California, Mexico). The predicted magnitude is 6.36, ±0.55, and the predicted time of failure is 1998.565 (7/25/98), ±1.127 years. The addition of future precursory events will allow refinement of the predicted values.
Three refraction traveltime tomography methods were applied to a 6.4 km crooked-line seismic profile acquired in the dry riverbed of the Rio de Truches, located in the Espanola Basin in northern New Mexico. The methods are based on ray-tracing, Fresnel volumes, and the adjoint-state. Each method was able to provide refraction statics for reflection processing that sufficiently accounted for changes in near-surface velocity and/or thickness. The purpose of this paper is to describe, compare, and contrast these tomographic methods and detail their application to a crooked-line survey. Velocity models and static solutions from each method are quantified by comparison of stacks, to which refraction statics have been applied, with a stack using delay-time refraction statics serving as a baseline. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 204A (Anaheim Convention Center) Presentation Type: Oral
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