Three‐component ( P , SH , and SV ) expanding spread profiles (ESP), common‐midpoint profiles, and sparse three‐dimensional P wave data were collected over an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), in southeastern Georgia. Shear wave reflections from the SBS at 10.9 s (16 km depth), and possibly from the lower crust at 18.3 s (29 km depth), were recorded but required substantial source effort (stacking) and were too weak for reliable reflectivity measurements. Reflections on the ESPs delineate a 1.5‐km‐thick Atlantic Coastal Plain section whose seismic properties (Vp=2.53 km/s, Vs=1.51 km/s, Vp/Vs= 1.67) are consistent with quartz‐rich sandstones and siltstones, sitting atop a 15‐km‐thick upper crust (Vp=6.38 km/s, Vs=3.25 km/s, Vp/Vs=1.96), which in turn overlies a 15 km‐thick lower crust of slower material (Vp=6.02 km/s, Vs≈3.26 km/s, Vp/Vs≈1.84). The velocity inversion may result from underthrusting of upper crustal rocks during suturing of Florida to North America. Amplitude‐versus‐offset analyses, combined with an earlier reflection polarity test and waveform modeling, indicate that the SBS originates from a thin (∼80–120 m), high‐impedance layer, most likely a mafic dike or tectonically emplaced ultramafic body.
ABSTRACT Precariously balanced rocks (PBRs) and other fragile geologic features have the potential to constrain the maximum intensity of earthquake ground shaking over millennia. Such constraints may be particularly useful in the eastern United States (U.S.), where few earthquake-source faults are reliably identified, and moderate earthquakes can be felt at great distances due to low seismic attenuation. We describe five PBRs in northern New York and Vermont—a region of elevated seismic hazard associated with historical seismicity. These boulders appear to be among the most fragile PBRs in the region, based on reports from hobbyists. The PBRs are glacial erratics, best evidenced by glacial striations on bedrock pedestals. The pedestals themselves are locally high knobs, often situated on regionally high topography; this setting limits soil development and indicates that any outwash deposits were likely ephemeral. As a result, PBR ages can be reliably established by the retreat of the last continental ice sheet, ∼15–13 ka. To quantify the fragility of the PBRs, we surveyed them with ground-based light detection and ranging and calculated geometric parameters from the point clouds, field observations, and seismic responses. Preliminary validation of the 2023 time-independent U.S. National Seismic Hazard Model (NSHM) shows that the existence of PBRs is generally consistent with the median site-specific hazard curves. Only the Blue Ridge Road site suggests a modest reduction in hazard. To visualize the ensemble of data, we mapped the minimum permissible distance to potential source faults around each PBR site as a function of source magnitude by using the ground-motion models from the 2023 NSHM. Viewed in this manner, our data are consistent with potential M∼6.5 earthquake-source faults in many parts of the Lake Champlain Valley and northern Adirondack Mountains. Our work illustrates a potential pathway for better constraining earthquake-source faults in regions of cryptic faults.
Near-surface thrust fault splays and antithetic backthrusts at the tips of major thrust fault systems can distribute slip across multiple shallow fault strands, complicating earthquake hazard analyses based on studies of surface faulting. The shallow expression of the fault strands forming the Seattle fault zone of Washington State shows the structural relationships and interactions between such fault strands. Paleoseismic studies document an ∼7000 yr history of earthquakes on multiple faults within the Seattle fault zone, with some backthrusts inferred to rupture in small (M ∼5.5–6.0) earthquakes at times other than during earthquakes on the main thrust faults. We interpret seismic-reflection profiles to show three main thrust faults, one of which is a blind thrust fault directly beneath downtown Seattle, and four small backthrusts within the Seattle fault zone. We then model fault slip, constrained by shallow deformation, to show that the Seattle fault forms a fault propagation fold rather than the alternatively proposed roof thrust system. Fault slip modeling shows that back-thrust ruptures driven by moderate (M ∼6.5–6.7) earthquakes on the main thrust faults are consistent with the paleoseismic data. The results indicate that paleoseismic data from the back-thrust ruptures reveal the times of moderate earthquakes on the main fault system, rather than indicating smaller (M ∼5.5–6.0) earthquakes involving only the backthrusts. Estimates of cumulative shortening during known Seattle fault zone earthquakes support the inference that the Seattle fault has been the major seismic hazard in the northern Cascadia forearc in the late Holocene.
Significance: The Provo River Bridge is a significant example of a Pratt through-truss bridge and is assumed to be built all of wrought iron. The bridge surveyed was originally one of three spans built in 1884 by the Union Bridge Company of Athens, Pennsylvania. Each of the spans was 165.5 feet in length and the three were designed to carry a narrow gauge railroad across the Green River in Utah. Sixteen years later the railroad converted to standard gauge and dismantled the three span Green River Bridge. The railroad then engaged the Louisville Bridge and Iron Company to shorten the dismantled spans to 82 feet and widen each to accommodate standard gauge tracks. One of these modified trusses was installed across the Price River near Wellington, Utah, in 1901 and then moved again in 1919 to its present site across the Provo River at the mouth of Provo Canyon.
Unprocessed Field note material exists for this structure: FN-7
Survey number: HAER UT-14
Building/structure dates: 1884 Initial Construction
Building/structure dates: 1901 Subsequent Work
Building/structure dates: 1919 Subsequent Work
Building/structure dates: 1969 Subsequent Work
The New Madrid seismic zone (NMSZ) is the most active seismic zone in the United States east of the Rocky Mountains. Frequency‐magnitude relationships show that an earthquake magnitude of 6.3 or greater can be expected in this area about once every 100 years, on average [ Johnston and Nava , 1985]. During the winter of 1811–1812, three earthquakes of moment magnitude near 8 devastated New Madrid, Mo., and the surrounding region. Today, if an earthquake in the magnitude 6–7 range were to strike, in addition to loss of life and injuries, residents would suffer about $3.6 billion in immediate property losses [ Algermissen , 1991] and feel the pinch of a long‐term economic impact.
Studies for urban hazard or resource assessment often take place in densely populated areas characterized by considerable cultural noise. These site conditions can severely compromise seismic reflection data quality. We have collected vibroseis and hammer (weight drop) seismic reflection data in a range of geologic conditions to image stratigraphy and structures in the upper one km along regional highways, city streets, and power line access roads. In addition to the challenges of safety and outreach, acquisition efforts along busy streets and highways often encounter poor receiver coupling and largeamplitude coherent noise from traffic and power lines. Although higher quality seismic reflection data may be obtained by simply choosing alternate sites with less cultural noise, modifications to the acquisition and processing steps can minimize the effects of cultural noise and poor coupling where profiling is most relevant. Flagging crews, flyers and public announcements assist with outreach and safety concerns, and the local news media are often enthusiastic about publicizing geologic studies. Recording long-record vibroseis data reduces the effects of noise by itself, but data quality can be further optimized by recording uncorrelated, unstacked data and applying precorrelation amplitude adjustments and filters. Recording individual hammer shots likewise allows gains or mutes to normalize or remove traffic noise prior to vertical stacking. Large numbers of receiver channels allow attenuation of random noise and velocity filtering to remove coherent noise. Because ground roll and normal moveout (NMO) corrections minimize near-surface coverage, asymmetric source-receiver geometry allows for additional near-surface fold while muting large amplitude ground roll and NMO stretch. Source and geophone coupling on road shoulders can degrade signal quality due to variable materials and topography, but these problems are often addressed with static corrections. Our experience is that high-quality seismic data can be obtained in noisy urban areas, but many recorded channels and a careful attention to acquisition and processing procedures can significantly improve the results.
A small reflection seismic experiment with an explosive source was conducted in southeastern Georgia to determine the polarity of an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), which was found at a depth of approximately 16 km during earlier COCORP profiling in the region. In addition to being very bright, the SBS is notable for being unusually flat and horizontal for about half of its 4 km length. As these characteristics are similar to those of fluid‐caused reflections at shallow depths, it has been suggested that the SBS may be caused by in situ midcrustal fluids. If caused by fluid enclosed in fracture porosity in solid rock, the reflection would be expected to exhibit a negative polarity from the top of the porous zone. The vibroseis data, however, gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal. The new experiment consisted of four dynamite shots, each recorded at three receiver stations by Seismic Group Recorders (SGR) borrowed from Amoco Production Company. Comparison of the dynamite records with geophone polarity tests indicate that the SBS is characterized by a positive reflection coefficient at its top. This result itself does not negate the fluid hypothesis—a fluid‐fluid interface could cause the positive reflection as well as the ‘flat‐spot’ nature of the reflector. Waveform modeling shows that the SBS is caused by two or more thinly‐spaced reflectors. A relatively high‐impedance layer about 120 m thick provided a good match to the observed dynamite data, but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary (0.7 versus 1.0). A ∼150 m thick porous zone model also provided a relatively good fit to the observed dynamite data, but in this model the polarity test requires that the initial SBS reflection (of positive polarity) be caused by a fluid‐fluid interface within the porous zone and that the top of the porous zone be relatively non‐reflective. Though these observations do not preclude the fluid hypothesis, they certainly make the high‐impedance model the simpler of the two alternatives presented here. The additional constraints imposed by the modeling also suggest that the SBS is more complex than these simple, two‐interface models. In either of these cases wavelet tuning contributes in part to the unusually large amplitude of the SBS reflection.
