Intraplate Areas Research Articles

Review of tectonic inversion of sedimentary basins in NE and N Brazil: Analysis of mechanisms, timing and effects on structures and relief

Most intraplate areas worldwide are under compression characterized by thrust and strike-slip deformations. The sources of the stresses in these regions are a combination of far-field (regional) stresses (e.g., ridge push, slab pull, lithospheric drag) and near-field (local) stresses (e.g., topography, lateral and vertical anisotropy of strength or elastic properties, erosion, flexural bending, overloading, buoyancy of low-density regions). These intraplate regions are sites favorable for the tectonic inversion of sedimentary basins. Here, we review the tectonic inversions of the sedimentary basins in NE and N Brazil, which comprise a significant part of the South American intraplate (the South American Platform). These regions have been under compression since the Late Cretaceous. Reports of post-rift basin inversions derived from seismic profiles, fault-slip data, and topography have been described in the Sergipe-Alagoas, Paraíba, Potiguar, Ceará, Rio do Peixe, Araripe (NE Brazil), and Solimões (N Brazil) basins. Reports of inversions in the rift phase have also been described in the Araripe Basin. The present-day stress field indicates that inversions in NE Brazil can mainly be characterized as strike-slip and thrust fault regimes, whereas in N Brazil, inversions have occurred under reverse stress regimes. The most frequent inversion structures are reactivated faults, folds, and dome structures affecting the topography and the consequent uplift of rift and post-rift sedimentary units. Inversions have led to the reactivation of continental-scale structures, such as the strike-slip dextral shearing of the Transbrasiliano lineament, which affected rift and post-rift units in the Equatorial Margin of Brazil. The small crustal thickness of these areas favors inversion, as indicated by the most recent Moho depth map of South America. The interplay between far-field and near-field stresses explains the stress pattern in NE and N Brazil, where inversions have been observed.

Correlation of paleoearthquake records at multiple sites along the southern Yangsan Fault, Korea: Insights into rupture scenarios of intraplate strike-slip earthquakes

The construction of spatiotemporal models of earthquake occurrence for intraplate areas is challenging due to the low deformation rates in these areas. In this study, we conducted paleoseismological investigations along the southern Yangsan Fault (SYF), a typical low-deformation-rate fault, on the Korean Peninsula. The SYF is distinct from the northern Yangsan Fault (NYF), and the boundary between them is located at the junction between the NNE-striking YF and another major structure, the NNW-striking Ulsan Fault (UF), which branches off from the YF. Paleoseismological trenches at four sites along the SYF indicate that this fault section has not ruptured during the Holocene, in contrast to the NYF and UF. In detail, surface ruptures along the studied section of the SYF occurred during three different time periods, as inferred from stratigraphy and radiocarbon dating: 74 to 49 ka at two sites, 39 to 35 ka at another site, and 28,000 cal yr BP (or 30 ka considering the OSL age) to 16 ka at all sites. These results suggest two alternative rupture scenarios for the timing of paleoearthquakes along the studied fault section during the Late Pleistocene: (1) full rupture along the entire studied section during each earthquake event, or (2) multiple partial ruptures along the two structurally distinguishable parts of the studied fault section, that is, the Wolsan–Miho and Inbo north–Inbo sections. We conclude that geometric discontinuities of the long-lived YF system in the Korean Peninsula intraplate region have played an important role in controlling recent spatiotemporal rupture behavior.

Tectonic geomorphology of the Ottawa-Bonnechere Graben, Eastern Canada: implications for regional uplift and intraplate seismicity

In intraplate areas where regional tectonic strain is accommodated by the reactivation of pre-existing structures, the level of seismic hazard associated with faults may be underestimated due to the poor surface expression of faults, scattered earthquake distribution, and long earthquake recurrence intervals. The cause(s) of seismicity in eastern Canada remains unresolved. This is partially because surface expressions of faults have been eroded during glacial and deglacial periods and in the Ottawa-Bonnechere Graben (OBG) are undetectable until a seismic event. Morphotectonic analysis has been widely applied to assess relative tectonic activity in various geological settings. To establish whether active uplift is occurring and to investigate the spatial distribution of relative uplift rates, 131 bedrock drainage basins in OBG were analysed. The aim of this was to (1) test the applicability of geomorphic indices for quantifying active deformation, (2) quantify the spatiotemporal distribution of relative uplift rates, and (3) explore the implications for faulting mechanisms, deformation styles, and ultimately regional seismic hazard. We measured valley floor width-to-height ratio ( Vf), basin elongation ratio ( Re), basin hypsometric integral, and normalized channel steepness index ( ksn). The results show that high relative uplift rates exist in all the six bedrock escarpment sections investigated, suggesting that they are presumably related with regional broad wavelength uplift (epeirogeny) caused by a complex interaction between far-field tectonic stress and glacial isostatic adjustment (GIA). Our analysis showed that Vf, Re, and ksn reveals no considerable spatial differences in high relative uplift rates, consistent with the Canadian Base Network GPS uplift rates. Highlights Geomorphic indices are applied in a low relief and low-medium seismicity area. Geomorphic indices reveal a broad wavelength regional uplift and show relative tectonic activity in the OBG. Relatively high geomorphic indices values provide longer term deformation history compared to global positioning system (GPS) records. Landscape formation and river incision is driven by a complex interaction between GIA and far-field tectonic stresses. Relative uplift rate results show similar patterns to published GPS uplift rates. Possible link between relative tectonic activity and current seismicity.

TRACES OF SEISMIC ACTIVITY IN THE LAKE BOTTOM SEDIMENTS OF EASTERN FENNOSCANDIA

This paper presents the results of a study on traces of ancient earthquakes in the lake bottom sediments of eastern Fennoscandia (Kola region and Karelia). Based on the sediment cores from six lake basins located within large active structures of the region, consideration is being given to the most typical features of sedimentation affected by neotectonic movements of the Earth’s crust. The data presented here provide lithological-stratigraphical, paleobotanical and chronometrical evidence for lakes. The paper causes and mechanisms of formation of seismic structures in bottom sediments and their distinction from deformations of another origin. Generalized earthquake history in provided an opportunity to organize the existing information on the time of paleoearthquake manifestations and distinguish three active periods in paleoseismicity. These are Late Glaciation-Early Holocene marked by the most rapid uplift after the retreat of the last ice sheet (13500–8100 cal yr BP), Middle Holocene (6800–6600 cal yr BP), and Late Holocene (3100–200 cal yr BP). The results of this study showed that the number and intensity of seismic events changed at different stages of sedimentation. Since the Younger Dryas, fault zones (or their segments) have been repeatedly activated. Combined with the data on paleoseismicity in adjacent parts of Fennoscandia, our studies indicate an irregular pattern of the Late Pleistocene-Holocene seismic activity in stable areas formerly covered by glaciers. The obtained data make further adjustments to the neogeodynamic and seismic estimates of intraplate areas.

Development a local magnitude scale for South Egypt

A local magnitude (ML) scale for the southern Egypt region was developed using 3435 maximum amplitudes of 343 high-quality records of earthquakes recorded by the Egyptian National Seismic Network (ENSN). The analysis is started by the relocation of the recorded earthquakes, and then instrument responses were deconvolved from the recording and simulated on a standard Wood–Anderson torsion seismograph. Zero-to-peak amplitude of Sg or Lg waves in the two horizontal components is measured on each simulated Wood–Anderson seismograph. Events that were recorded by at least four stations and located at hypocentral distances between 10 and 1000 km and with a magnitude range of 1–4.5 are selected for preforming the analysis. The maximum recorded hypocentral depth was 40 km. The normalized amplitude decay shows two transition distances at 90 km, and 175 km yielding three segments. The trilinear regression model was adopted to retrieve three formulae that describe the local magnitude as follows:ML = log (A) + 1.4223*log(R) −0.0014*(R) +0.301 for R < 90.ML = log (A) +1.3786*log(R) −0.0014*(R) +0.379 for 90 < R < 175.ML = log (A) +3.1369*log(R) −0.0014*(R) −3.57 for R > 175.Where A is the ground displacement amplitude (mm) on the horizontal component, R is the hypocentral distance (km). The station corrections were also determined for 49 different geological sites using the regression model and producing values ranging between −0.4 and 0.7. The residual of magnitude values are reduced by 46% when using station corrections. The newly derived scale for South Egypt is well correlated with scales determined for South California and those for other intraplate areas, such as the Ethiopian Plateau and central US.

Dynamic slab segmentation due to brittle-ductile damage in the outer rise.

Subduction is the major plate driving force, and the strength of the subducting plate controls many aspects of the thermochemical evolution of Earth. Each subducting plate experiences intense normal faulting1-9 during bending that accommodates the transition from horizontal to downwards motion at the outer rise at trenches. Here we investigate the consequences of this bending-induced plate damage using numerical subduction models in which both brittle and ductile deformation, including grain damage, are tracked and coupled self-consistently. Pervasive slab weakening and pronounced segmentation can occur at the outer-rise region owing to the strong feedback between brittle and ductile damage localization. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs10, the development of large-offset normal faults6,7 near trenches, the occurrence of segmented seismic velocity anomalies11 and distinct interfaces imaged within subducted slabs12,13, and the appearance of deep, localized intraplate areas of reduced effective viscosity14 observed at trenches. Furthermore, brittle-viscously damaged slabs show a tendency for detachment at elevated mantle temperatures. Given Earth's planetary cooling history15, this implies that intermittent subduction with frequent slab break-off episodes16 may have been characteristic for Earth until more recent times than previously suggested17.

Multi-fault segments in the Pernambuco Lineament, Brazil: Implications for fault architecture in intraplate areas

Here we report the intraplate seismicity in the Pernambuco Lineaments (PeL), a Neoproterozoic continuous and vertical ductile shear zone, 700 km long and 5 km wide ~600 Ma in intraplate Brazil. We observed one fault segment in the PeL, here defined as the São Caetano segment, to assess (1) the seismogenic reactivation of the ductile Precambrian basement fabric and (2) the correlation between seismological and structural data derived from satellite imagery to understand fault reactivation, and (3) the identification of a complex multi-fault zone architecture revealed by our seismological observations. In the São Caetano Fault segment, the overall spatial distribution of the seismic clusters follows approximately an E-W direction with a 64° dip towards the N. However, the observed multi-fault architecture revealed by the hypocentral locations exhibits different clusters that do not form a single fault structure. Hence, composite solutions are not necessarily the only representative focal mechanism of an entire fault zone. The inversion of the stress tensor using the focal mechanism from the present study is consistent with S1 nearly vertical and S3 in the E-W direction. This indicates that the interplay between the stress field, inherited structures, and seismicity can be more complex and vary at the local scale of 10s of kilometers. In this intraplate region, we show that the stress field is variable, with long-wavelength variations overprinted by more local and subtle changes, which has profound implications for intraplate seismic hazard mapping since models to estimate hazard rely on simplified assumptions of the fault geometry.

Soft-sediment deformation in the Sergipe-Alagoas Basin, Brazil: Implications for paleoseismicity in intraplate areas

Several studies have cited soft-sediment deformation as evidence of fault reactivation and paleoseismicity in intraplate coastal settings. However, a consensus has not been reached on the extent of paleoseismic activity in several regions and the link with continental-scale faults. This research aims to determine the origin of the deformation in soft-sediments, both in the Neogene Barreiras Formation and in Pleistocene sediments of the Sergipe-Alagoas Basin, on the continental margin of northeastern Brazil. The region shows a historical record since 1808 of earthquakes with maximum modified Mercalli intensity of VII and a 40-year instrumental record of events with maximum body-wave magnitude (mb) of 5.2. The brevity of these records leads to large uncertainties in estimates of fault slip rate, recurrence, and seismic hazard. Geologic studies of seismically induced soft-sediment deformation structures and their relationship to seismically active faults offer the opportunity to reduce these uncertainties. We present detailed descriptions of the deformation structures with emphasis on morphology, ductile or brittle character, size, and distribution of the features. In addition, we integrate geologic data with seismic reflection data, thus allowing for correlation between the normal faults at the seismic scale and the soft-sediment deformation structures at the outcrop scale. Faults associated with seismites at the outcrop scale exhibit the same strike/dip and kinematics and are spatially related to the major faults identified at the seismic-section scale. Regarding ductile features, the most remarkable are convoluted folds associated with a deformed muddy layer, giant sinking structures, and complex folds. Concerning brittle structures, the most striking are clastic dikes, mixed layers, autoclastic breccias, and fault breccias. The alternating records in the Neogene sediments indicate clustered seismic activity, combined with tectonic quiescence periods. In Pleistocene sediments, the size, frequency, lateral continuity, and spatial distribution of tens of kilometers of the deformation structures indicate a period of frequent earthquakes with body-wave magnitudes equal to or greater than 6. The recognition of seismic activity and brittle reactivation of preexisting faults during the Neogene-Quaternary has far-reaching implications for seismic hazards in intraplate settings.

Intraplate earthquakes in the Potiguar Basin, Brazil: Evidence for superposition of local and regional stresses and implications for moderate-size earthquake occurrence

The understanding of earthquake occurrence in intraplate areas has been one of the most challenging tasks in Seismology. The attenuation of seismic waves in these areas is lower compared to border plate regions. Therefore, even moderate magnitude earthquakes can represent a great hazard. In this case, assessing the stress field in the intraplate areas is important to better understanding the earthquake generating mechanisms. We analyzed 241 earthquakes of the 2017 seismic sequence in the NW part of the Potiguar Basin and estimated the stress regime. Seismically defined faults were determined for four high-precision relocated NW-SE- and NE-SW-trending clusters. Faulting intersection acting in one of these clusters was representative for suggesting an intersection model for intraplate earthquakes to explain why moderate size earthquakes occur in the study region. Another cluster presented a seismically defined fault vertically segmented with a clear spatiotemporal evolution and occurrence of almost entire activity in a very short period, which suggest energy build-up and rapid release between its fault segments. We calculated the composite focal mechanism for three clusters, with predominant strike-slip faulting with a sinistral movement. The inversion of focal mechanisms reinforces evidence for a superposition of local and regional stresses, with major compressional axis subparallel to the continental equatorial margin.

Local rift and intraplate seismicity reveal shallow crustal fluid-related activity and sub-crustal faulting

Seismicity delineating the mid-ocean ridge system in the Panama Basin is mostly concentrated along transform offsets. Most intraplate areas show little-to-no seismic activity. Here we analyze passive recordings from a short-term deployment of 75 ocean-bottom seismographs (OBS) at the Costa Rica Rift (CRR) and across its southern flank along a flowline to Ocean Drilling Program (ODP) Hole 504B. The OBS array recorded 1061 events along the CRR and the Ecuador Fracture Zone, and 127 intraplate events around ODP Hole 504B. The seismic activity along the CRR occurred in clusters, with some events associated with an axial magma lens (AML) located at 3–3.5 km depth below seafloor (bsf), and four events exhibiting normal and reverse faulting focal mechanisms. These deep events are followed by the largest event cluster whose epicenters connect the AML to the seafloor, at a location where a hydrothermal plume was previously reported. During the same period, another event cluster occurs close to the seabed. This spatio-temporal pattern suggests that deeper events close to the AML, which might be related to thermal stresses or stress perturbations due to a volume change in the AML, trigger fluid-related seismicity within the shallower hydrothermal system. The easternmost extent of the seismicity along the CRR corresponds to an overlapping spreading center (OSC). At depth, the seismicity around the OSC is focused beneath one limb, but is spread over a larger area closer to the surface. The focal mechanisms calculated for OSC events show both normal and reverse motions, and might reflect complex stress or fault orientations. Intraplate seismicity around ODP Hole 504B shows both isolated and clustered events from the seafloor to ∼15–25 km bsf. Some of this seismicity is likely associated with the reactivation of east-west trending normal faults under high pore pressure and thermoelastic stresses conditions. Deep seismicity (i.e. between 15 and 20 km bsf) occurs in small, well-defined clusters close to ODP Hole 504B, at the theoretical depth of the brittle-to-plastic transition for a ∼6.9 Ma oceanic lithosphere. Our results show the importance of short-term OBS deployments for improving our understanding of mid-ocean ridge seismicity and hydrothermal processes, as well as intraplate seismicity and deformation mechanisms.

A geophysical investigation of the 2018 Lake Muir earthquake sequence: reactivated Precambrian structures controlling modern seismicity

Seismicity in the intraplate southwest of Western Australia is poorly understood, despite evidence for potentially damaging earthquakes of magnitude > M6. Identifying stress-focusing geological structures near significant earthquake sequences assists in understanding why these earthquakes occur in seemingly random locations across a region of more than 250 000 km2. On 16 September 2018, an ML5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an ML5.4 aftershock. The main earthquake formed a mainly north-trending fault scarp ∼5 km in length and with a maximum vertical displacement of ∼40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of ML5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major ∼ east–west-trending faults segmenting eight distinct geological domains, as well as a network of less prominent northwest-trending faults, one of which aligns with the southern segment of the scarp. Surface faulting, surface deformation and earthquake focal mechanism studies suggest movements on north- and northeast-trending structures. The main shock, the aftershocks, surface faulting and changes in InSAR-derived surface elevation all occur in a region bounded to the south by a prominent northwest-trending fault and to the north by a west-northwest-trending domain-bounding structure. Thus, we interpret the north-trending thrust fault associated with the main Lake Muir event as due to local stress concentration of the regional east–west stress field at the intersection of these structures. Further, we propose that a particularly large west-northwest-trending structure may be broadly focusing stress in the Lake Muir area. These findings encourage similar studies to be undertaken in other areas of Australia’s southwest to further the current understanding of seismic release in the region. KEY POINTS A large ML5.7 earthquake occurred in southwest Western Australia in September 2018, damaging infrastructure and forming a 40 cm-high thrust fault scarp. An aeromagnetic interpretation of the Lake Muir region revealed that the new scarp aligned with pre-existing faults that were reactivated. Local and regional structures likely acted as regional stress focusers in the Lake Muir area. Identifying regional stress-focusing structures is important for the purpose of understanding seismic risk in intraplate areas such as Western Australia.

Seismic Intensity Attenuation for Intraplate Earthquakes in Brazil with the Re‐Evaluation of Historical Seismicity

ABSTRACTIntensity evaluation attempts to quantify a qualitative description of the effects observed in an earthquake ranked in terms of how strong it was felt and the amount of damage. Despite its qualitative nature, intensity data are essential to study both historical and recent earthquakes such as in earthquake hazard studies and in which dense accelerometric data are missing. The magnitude of historical earthquakes in Brazil has been determined with relations involving magnitude and felt area. Intensity attenuation equations (decay of intensity as a function of magnitude, depth, and distance) determined in other regions of the world are intrinsically dependent on the crustal tectonic characteristics and cannot be applied to Brazil without proper evaluation. We determined an intensity attenuation equation using 20 earthquakes in Brazil and neighboring intraplate areas, in the magnitude range mb 3.5–6.2 and hypocentral distances up to 720 km.The best attenuation model was I=0.995M−1.505logR−0.00116R+2.08 (standard deviation = 0.778), in which M is the magnitude (mb) and R is the hypocentral distance (in kilometers). The uncertainties of the estimated magnitudes using this equation are around mb±0.72 for events with 30 or more intensity points.We tested the new equation with intensity data from two important events: the 1939 offshore earthquake (mb 6) in southern Brazil and the 2018 Venezuela earthquake (mb 6.9). The fit was reasonable for both earthquakes, with a root mean square residual of 1.0 and 1.1 intensity units, respectively.We also re‐evaluated the 1861 earthquake in southeast Brazil. The new epicenter was 23.1° S 44.7° W with a magnitude of mb 5. The epicenter was located onshore, but the estimated uncertainties include a large offshore region. Given that, the location of the 1861 event remains uncertain.

A Parametric Analysis of Fault Reactivation in the New Madrid Seismic Zone: The Role of Pore Fluid Overpressure

AbstractEarthquakes in Stable Continental Regions (SCR), where localized present‐day tectonic loading is negligible, remain difficult to explain. The New Madrid Seismic Zone (NMSZ) is a type‐locale for such events, with four M&gt;7 events in 1811–1812 and a regional seismicity that continues to this day. Here, we seek to determine the most favorable conditions for fault reactivation in such a context using 33 earthquakes in the 2.1–4.7 magnitude range with well‐determined focal mechanisms while accounting for the vertical gradient of differential stress with depth. To do so, we developed a Mohr‐Coulomb‐based parametric analysis of fault reactivation that allows us to vary the orientation of the principal stresses, the shape ratio of the stress tensor, the fault friction coefficient, the pore fluid overpressure, and the gradient of differential stress with depth. Doing so, we are able to determine the lowest stress perturbation conditions required for fault reactivation. Our results show that the reactivation of the faults studied here requires pore fluid overpressure, unless their friction coefficient is 0.4 or less. We argue that such weak faults are unlikely and favor a triggering mechanism via deep fluids, possibly upwelling from the upper mantle where a low‐velocity seismic anomaly could indicate their presence. This mechanism, documented in other intraplate areas, does not require local tectonic stress or strain accumulation to explain seismicity in active intraplate regions, where elastic strain is drawn from a prestressed crust.

A Nontectonic Origin for the Present‐Day Stress Field in the Paris Basin (France)

AbstractThe large‐scale stress patterns observed in intraplate areas are often considered to result from far‐field boundary forces that drive plate tectonics. However, no present‐day deformation has been detected in the French Paris Basin, yet significant deviatoric stresses are measured in limestone formations observed above soft argillite layers encountered in this region. Further, the pore pressure measured in the argillite is larger than that measured in the surrounding permeable zones. These observations suggest a presently active source of stress in this sedimentary system. We propose that this stress field is not related to tectonics but only to gravity acting on the series of viscoelastic orthotropic geomaterials that fill up the basin. This viscoelastic response is linked to pressure solution effects activated by pore pressure transients related to climatic variations. These pressure transients develop in the fracture system that affects some of the geomaterials and imply a time‐dependant deformation field, with time constants related to those of climatic variations. This model outlines the influence of time‐dependent material properties on the present‐day stress and pore pressure fields that prevail at various depths. It may be considered as a possible loading mechanism for the analysis of intraplate seismicity.

Cenozoic intraplate tectonics in Central Patagonia: Record of main Andean phases in a weak upper plate

Contraction in intraplate areas is still poorly understood relative to similar deformation at plate margins. In order to contribute to its comprehension, we study the Patagonian broken foreland (PBF) in South America whose evolution remains controversial. Time constraints of tectonic events and structural characterization of this belt are limited. Also, major causes of strain location in this orogen far from the plate margin are enigmatic. To unravel tectonic events, we studied the Cenozoic sedimentary record of the central sector of the Patagonian broken foreland (San Bernardo fold and thrust belt, 44°30′S-46°S) and the Andes (Meseta de Chalia, 46°S) following an approach involving growth-strata detection, U-Pb geochronology and structural modeling. Additionally, we elaborate a high resolution analysis of the effective elastic thickness (Te) to examine the relation between intraplate contraction location and variations in lithospheric strength. The occurrence of Eocene growth-strata (~44–40Ma) suggests that contraction in the Andes and the Patagonian broken foreland was linked to the Incaic phase. Detection of synextensional deposits suggests that the broken foreland collapsed partially during Oligocene to early Miocene. During middle Miocene times, the Quechua contractional phase produced folding of Neogene volcanic rocks and olistostrome deposition at ~17Ma. Finally, the presented Te map shows that intraplate contraction related to Andean phases localized preferentially along weak lithospheric zones (Te<15km). Hence, the observed strain distribution in the PBF appears to be controlled by lateral variations in the lithospheric strength. Variations in this parameter could be related to thermo-mechanical weakening produced by intraplate rifting in Paleozoic-Mesozoic times.

Crustal Structure of the Tengchong Intra-plate Volcanic Area

We here provide an overview of our current understanding of the crustal structure of Tengchong in southwest China, a key intra-plate volcanic area along the Himalayan geothermal belt. Given that there is hitherto a lack of information about the near-surface structure of intra-plate volcanic areas, we present the first seismic reflection and velocity constraints on the shallow crust between intra-plate volcanoes. Our near-surface seismic images reveal the existence of dome-shaped seismic reflectors (DSRs) in the shallow crust between intra-plate volcanic clusters in Tengchong. The two DSRs are both ~2 km wide, and the shallowest parts of the DSRs are found at the depth of 200–300 m. The velocity model shows that the shallow low-velocity layer (<4 km/s) is anomalously thick (~1 km) in the region where the DSRs are observed. The presence of DSRs indicates significant levels of intra-plate magmatism beneath the along-axis gap separating two volcano clusters. Along-axis gaps between volcano clusters are therefore not necessarily an indicator of lower levels of magmatism. The seismic images obtained in this technically challenging area for controlled-source seismology allow us to conclude that shallow crustal structures are crucial for understanding the along-axis variations of magmatism and hydrothermal activities in intra-plate volcanic areas.

Aftershock Imaging with Dense Arrays (AIDA) Following the Mw 4.0 Waterboro Earthquake of 16 October 2012 Maine, U.S.A.

Abstract Arrays of highly portable, exploration style seismographs were deployed following the 16 October 2012 M w 4.0 Waterboro, Maine, earthquake with the goal of improving hypocenter locations and source mechanisms of aftershocks, lowering the detection threshold for such aftershocks, and using the aftershocks as sources to image subsurface structure in the hypocentral volume. Based on the Aftershock Imaging with Dense Arrays (AIDA) experience following the Mineral, Virginia, earthquake of August 2011, the Maine deployment consisted of three complementary arrays totaling 110 stations: one array was for event detection and location, another for structural imaging, and a third for measuring regional attenuation. Although the arrays operated from day 3 through day 10 following the mainshock, only one aftershock was detected by the 43 stations that had been deployed at the time of the event. This aftershock was located at a focal depth of 6.9±0.15 km, with a horizontal uncertainty of ∼0.15 km. Although the strike‐slip focal mechanism for this aftershock differs from the thrust mechanism of the mainshock, both share an east–west maximum stress direction. Recordings of the aftershock from the high‐density portion of the deployment show possible upper‐ and lower‐crustal reflections that we attempt to image using vertical seismic profiling methods. However, the lack of redundant sources and the limited areal extent of the imaging arrays hinders any useful interpretation of the nature of the crustal reflections. Based on comparison with the Virginia experiment, in which numerous aftershocks were recorded, and from examining the signal‐to‐noise ratio of the single aftershock recorded, we believe the AIDA Maine deployment should have detected any aftershocks greater than magnitude m b of approximately −2.5. That only one aftershock was detected supports the observation that earthquakes in some intraplate areas are notable for their lack of aftershock activity down to this level.

Testing atmospheric and tidal earthquake triggering at Mt. Hochstaufen, Germany

Seismicity closely related to hydrological impacts has been observed in several locations worldwide, particularly in intraplate areas where tectonic stressing rates are small. The triggering mechanism is usually explained by a poroelastic response of the seismogenic crust to surface water flux, leading to pore pressure changes at depth. To explain the earthquake triggering in response of those small stress changes, however, the crust has to be near a critical state in which other transient processes might be significant. One of the prominent examples is at Mt. Hochstaufen in SE Germany, where seismicity is known to vary seasonally. A previous analysis showed that the seismicity in 2002 was highly correlated with model forecasts based on fluid diffusion and rate‐ and state‐dependent frictional nucleation. Here we revisit this case by accounting additionally for poroelastic effects, as well as for thermoelastic and tidal stresses. We also test whether the model can explain the observations of the subsequent 8 years between 2003 and 2010. Our analysis confirms that rainfall is the dominant driving force in this region. The model not only fits the year 2002 activity very well but also provides with the same parameters a reasonable fit to the subsequent period, with a probability gain of about 4 per event in comparison to a time‐independent Poisson model.

A Local Magnitude Scale ML for the United Kingdom

We have developed a new local magnitude scale ML for the United Kingdom (UK) to replace the Hutton and Boore (1987) scale developed for southern California, which has been used in the UK until now. The new UK scale is developed from 1482 observations of 85 earthquakes on 50 stations located across the British Isles and Ireland. Most of the observations are from epicentral distances of less than 600 km and only few from greater distances up to 900 km. The distance range of the scale is, therefore, 0–600 km. The amplitude observations were used to invert for the parameters defining distance dependence in the ML scale and station corrections. Synthetic tests showed that the inversion was robust. The new ML scale for the UK is given by ML=logA+0.95logR+0.00183R−1.76, in which A is horizontal‐component ground displacement amplitude in nanometers. The amplitudes are measured on traces that are filtered to simulate the Wood–Anderson seismograph. R is the hypocentral distance (in km). The UK scale is intermediate between scales determined for California and those of other intraplate areas such as Norway or the northeastern United States. The absolute station corrections found are all less than 0.5. The scale derived for the UK helps to reduce the overall variance of the mean magnitude estimates by 30%. Much of this improvement is due to the use of station corrections. Applying the UK scale to the database of recorded earthquakes results in a reduction of magnitude for earthquakes above ML 2 and a slight increase in magnitude for earthquakes below ML 2. The biggest change to the ML computation is likely to be for small earthquakes with few amplitude readings, where the use of station corrections makes a significant difference.

Upper crustal earthquake swarms in São Caetano: Reactivation of the Pernambuco shear zone and trending branches in intraplate Brazil

Seismogenic fault reactivation of continental-scale structures has been observed in a few intraplate areas, but its cause is still a matter of debate. The objective of the present study is to analyze two seismic swarms that occurred along the EW-trending Pernambuco ductile shear zone and in a NE-trending branch, in 2007 and 2010 in São Caetano County, Northeastern Brazil. We studied both epicentral areas using a nine- and a seven-station network during 180 and 54 days, respectively. The results indicate that the 2007 swarm correspond to a right-lateral, strike–slip fault with a normal component of slip (strike 74°, dip 60°, and rake − 145°) and the 2010 swarm corresponds to a normal fault (strike 265°, dip 79°, and rake − 91°). The former reactivated a NE-trending branch, whereas the latter reactivated the main E-W-trending mylonitic belt of the Pernambuco shear zone. These results are consistent with seismogenic reactivation of this major structure, generated by the present-day EW-trending compression and NS-trending extension, as observed by previous studies. This shear zone was reactivated as rift faults in the Cretaceous during the South America–Africa breakup. However, our study confirms that the basement fabric such as continental-scale ductile shear zones, show evidence of crustal weakness outside areas of previous rifting, and it reveals the potential for large earthquakes along dormant rift segments associated with major basement shear belts.