Search

The moment magnitude (MW)~7 earthquake that struck Charleston, South Carolina, on 31 August 1886 is the largest historical earthquake in the United States east of the Appalachian Mountains. The fault(s) that ruptured during this earthquake has never been conclusively identified, and conflicting fault models have been proposed. Here we interpret reprocessed seismic reflection profiles, reprocessed legacy aeromagnetic data, and newly collected ground penetrating radar (GPR) profiles to delineate faults deforming the Cretaceous and younger Atlantic Coastal Plain (ACP) strata in the epicentral area of the 1886 earthquake. The data show evidence for faults folding or vertically displacing ACP strata, including apparent displacements of near-surface strata (upper ~20 m). Aeromagnetic data show several northeast (NE)-trending lineaments, two of which correlate with faults and folds with vertical displacements as great as 55 m on the seismic reflection and radar profiles. ACP strata show only minor thickness changes across these structures, indicating that much of the displacement postdates the shallowest well-imaged ACP strata of Eocene age. Faults imaged on the seismic reflection profiles appear on GPR profiles to displace the erosional surface at the top of the upper Eocene to Oligocene Cooper Group, including where railroad tracks were bent during the 1886 earthquake. Some faults coincide with changes in river trends, bifurcations of river channels, and unusual river meanders that could be related to recent fault motion. In contrast to our interpreted NE fault trends, earthquake locations and some focal mechanisms in the modern seismic zone have been interpreted as defining a nearly north-striking, west-dipping zone of aftershocks from the 1886 earthquake. The relationship between the modern seismicity and the faults we image is therefore enigmatic. However, multiple faults in the area clearly have been active since the Eocene and deform strata in the upper 20 m, providing potential targets for field-based geologic investigations. [ABSTRACT FROM AUTHOR]

In this contribution, we propose a model of Quaternary seismicity in the Outer Western Carpathians. The model presents results of analysis of fractured clasts performed in 268 exposures of Quaternary fluvial gravels within the ca. 5,000 km2 large segment of the Outer Western Carpathians comprising: Outer Carpathian nappes, the innermost part of the Carpathian Foredeep, and two intramontane basins – the Orava-Nowy Targ Basin and the Nowy Sącz Basin. Moreover, to precise stratigraphic position of the host gravels we performed OSL dating in 16 exposures and 14C dating in 4 exposures. Outside the intramontane basins, seismic damage in Quaternary strata is largely focused along all major overthrusts and some of the large-scale strike-slip and normal faults, whereas, within the intramontane basins, the damage is dispersed within the whole basins. Our results show that seismic damage within Quaternary strata in the Outer Western Carpathians results both from co-seismic fault rupture and from seismic shaking. During Quaternary times, the Outer Western Carpathians have been affected by earthquakes since the Pleistocene until Present. Seismic damage in Quaternary strata results from light (Mw5-) earthquakes with the exception of hanging limbs of the normal faults bordering the Orava-Nowy Targ Intramontane Basin which have been affected also by moderate (Mw5+) earthquakes. Our model can be significant for understanding seismic hazards in low-seismicity thrust-and-fold belts. In the studied case, this is important as the densely populated intramontane basins show the largest seismic damage. • Quaternary seismicity studied in Western Outer Carpathians. • The area endured earthquakes since Pleistocene times until Present. • Seismic damage largely concentrated in intramontane basins. • Damage outside basins focused on overthrusts and faults. • Light earthquakes (M4.7-M5) main cause of seismic damage in studied area. [ABSTRACT FROM AUTHOR]

In their comments Bonini et al. argue that our seismotectonic interpretation of the Emilia 2012 seismic sequence does not agree with observations, and follow three lines of arguments to support their statement. These concern the structural interpretation of seismic reflection profiles, the relationship between seismogenic sources and seismicity patterns, and the fit of inferred fault geometry to InSAR observations. These lines of arguments are mostly repeating what has been previously presented by the same authors, and none of them, as discussed in detail in our reply, presents a strong case against our structural interpretation, that, we are convinced, does not conflict with the available data. The two adjacent rupture surfaces outlined by accurately relocated aftershocks are an indication of the presence of two different active fault planes. Interpretation of seismic profiles supports seismological observation and indicates the occurrence of relevant along-strike changes in structural style. These pieces of information have been integrated to build a new seismotectonic interpretation for the area of the Emilia 2012 seismic sequence. Analysis of geodetic data from the area of the Emilia earthquakes has produced very different models of the fault planes; unlike what has been stated by Bonini et al., who see a difficult fit to InSAR data for the fault planes we have identified, the most recent results are consistent with our interpretation that see a steep fault in the upper 8–10 km under the Mirandola anticline. We point out that the geological structures in the subsurface of the Ferrara Arc do change along strike, and the attempt of Bonini et al. to explain both the May 20 and May 29 sequences using a single cross section is not appropriate. [ABSTRACT FROM AUTHOR]

Some features in the landscape are the direct result of earthquake and tectonic deformation. The recognition and analysis of these geomorphological features allows the identification of regions subjected to past earthquake activity or tectonic deformation, which can be directly applied in many areas of engineering geology. However, some regions lack evident geomorphological features despite being subjected to recurrent earthquakes and/or ongoing tectonism. Drainage networks are very sensitive to changes along their flow path, so the characteristics of the drainage can be used to infer areas with 'hidden' evidence of past earthquake activity or tectonic deformation. Two examples from contrasting tectonic environments (i.e., Australia and New Zealand) are used to illustrate the application and effectiveness of drainage analysis. [ABSTRACT FROM AUTHOR]

Abstract: Norway and its continental margin, in spite of being located in an intraplate environment, is of great interest from a Neotectonic perspective. Influence on neotectonics stems both from the fact that this is a recently deglaciated region in which significant rebound is still occurring, and from its proximity both to a rifted continental margin and to failed rift zones. Passive continental margins and rift-related structures are the most seismically active ones within ‘stable’ continental regions, with a number of well-documented earthquakes of magnitude 7 and higher, as observed around the globe. Indeed, the disparity in seismicity between such regions and plate boundary regions is greater in terms of return times than in terms of maximum magnitude. A recent detailed assessment of neotectonics in Fennoscandia has shown strong bedrock-related evidence for several large magnitude (M 7+) earthquakes in the northern areas (Lapland), while few, if any, similar observations have been made farther south. A possible explanation for this is that postglacial earthquakes have been occurring on hidden thrusts in the south. In fact, several major structures have recently been mapped and shown to have capabilities to accommodate M 6.5–7 earthquakes without breaking the surface. Given that the largest historically known earthquake in Fennoscandia is only of the order of M∼5.8 it is likely, therefore, that the potentially largest magnitude in this region may be well above this. [Copyright &y& Elsevier]

An earthquake, probably generated on the southern Whidbey Island fault zone, caused 1-2 m of ground-surface uplift on central Whidbey Island ∼2800-3200 yr ago. The cause of the uplift is a fold that grew coseismically above a blind fault that was the earthquake source. Both the fault and the fold at the fault's tip are imaged on multichannel seismic refection profiles in Puget Sound immediately east of the central Whidbey Island site. Uplift is documented through contrasting histories of relative sea level at two coastal marshes on either side of the fault. Late Holocene shallow-crustal earthquakes of Mw = 6.5-7 pose substantial seismic hazard to the northern Puget Lowland. [ABSTRACT FROM AUTHOR]

This report opens a new window on Quaternary tectonics of the Outer Western Carpathians. We discuss results of fractured clasts analysis in Quaternary fluvial gravels within a 4.500 km2 large area comprising a part of the Outer Carpathians including Nowy Sącz Intramontane Basin (NSIB) as well as in the innermost part of the Carpathian Foredeep. Quaternary gravels bearing fractured clasts are widespread within the area, we observed them in 180 exposures. In the Carpathian Foredeep and in the Outer Carpathians excluding the NSIB, the exposures of gravels bearing fractured clasts are located largely close to (<2 km) all major overthrusts and close to (<0.75 km) the cross-fold faults of the Skawa River Fault System (SRFS). The clast-cutting fractures are mostly joints attesting to widespread Quaternary tectonic activity of the faults. Moreover, the density of occurrence of gravels bearing fractured clasts is considerably higher within the NSIB than in other parts of the study area. Furthermore, our results shed new light on the seismic hazard in the study area. In our interpretation, the clast-cutting fractures are damage resulting from coseismic rupturing of underlying bedrock faults or from regional seismic shaking. We conclude that in the study area the seismic hazard posed by natural seismicity can matter only in case of planning there the erection of critical structures like nuclear power plants, nuclear waste repositories or gas storage facilities. On the other hand, we infer that the appreciable seismic hazard is near the cross-fold faults of the SRFS, due to the location of Świnna Poręba Artificial Lake there, which can lead to triggering significant seismicity in the area. Furthermore, we suggest that fractured clasts can serve worldwide as providers of information on paleoseismicity and can be useful for mapping blind faults. • In the study area Quaternary fractured clasts occur largely along major faults. • Clast-fracturing results from rupturing of faults or regional seismic shaking. • An occurrence of fractured clasts is useful for mapping blind faults. • Fractured clasts can serve as providers of information on paleoseismicity. [ABSTRACT FROM AUTHOR]