Tomography-based, high-resolution modelling of mantle flow under North America: Implications for surface stress in the central and eastern US
Plate tectonics is fundamentally a 3-D process and the mantle convection stresses that drive the horizontal motions of plates also produce vertical bending stresses that generate large (km-scale) topographic undulations. The impact of these bending stresses in continental interiors is not generally recognised or well understood and yet they can provide a large (order 10 MPa) contribution to the ambient tectonic stress field. Depending on the geometrical relationship with pre-existing crustal weak zones or faults, the convection- induced surface stresses can be potentially important contributors to intraplate seismic activity, particularly in the central and eastern portions of North America. These stresses, and the associated surface undulations, evolve slowly on geological (Myr) time scales and it is therefore difficult to resolve them using space-based geodesy (e.g. GPS). We determine the impact of mantle convection on intraplate stresses in North America using a mantle flow calculation that is based on a new high-resolution tomography model that is constrained by simultaneously inverting global seismic and mantle-convection data sets (Simmons et al. 2009). The mantle flow model adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment (GIA) and convection data (Mitrovica and Forte 2004). The flow model successfully reproduces plate velocities and observations of surface gravity and topography. We find a large region of maximum compressive stress in the central portion of North America that is largely driven by viscous flow coupled to density anomalies associated with the descent of the ancient Kula-Farallon plate system. These flow calculations also show the long lived nature of the convection-driven compressive stresses under the Mississippi Valley region, extending from the southern Great Lakes to the Gulf of Mexico.
Analysis of GPS Measurements in Eastern Canada Using Principal Component Analysis
We present Global Positioning System (GPS) position time series from eastern North America that constrain the pattern and magnitude of regional crustal deformation. Initial analysis delineates consistent uplift patterns, as expected from glacial isostatic adjustment (GIA) predictions, but the associated horizontal deformation is not definitive, in part due to the short time periods for a significant number of the available stations. An eigenpattern decomposition therefore is employed in order to define a unique, finite set of deformation patterns for this continuous GPS data [Tiampo et al., 2004; Dong et al., 2006]. Similar in nature to the empirical orthogonal functions historically employed in the analysis of atmospheric and oceanographic phenomena [Preisendorfer, 1988], the method derives the eigenvalues and eigenstates from the diagonalization of the correlation matrix using a Karhunen-Loeve expansion (KLE) [Fukunaga, 1990; Tiampo et al., 2002]. This KLE technique is used to identify the important modes in both time and space for the GPS data, which potentially include such time dependent signals as plate velocities, GIA, tectonic strain, and seasonal effects. We filter both the vertical and horizontal velocity patterns on different spatiotemporal scales, in order to study the potential geophysical sources, after the removal of correlated and random noise. The results for the vertical velocities are consistent with those predicted by GIA modeling from ICE-3G [Tushingham and Peltier, 1991]. The horizontal velocity analysis allows for differentiation between several potential GIA models, and suggests that, with longer time series, this technique can be employed to remove correlated noise and improve current estimates of crustal deformation patterns and their origins.
Postglacial rebound and seismo-tectonic signatures of crustal strain rates measured by GPS in eastern Canada and northeastern U.S.
We present an update of horizontal and vertical velocities and horizontal strain rates in eastern Canada and northeastern U.S. based on high-resolution campaign Global Positioning System (GPS) data from the Canadian Base Network and continuous GPS data from a variety of sources, including provincial networks. The campaign and GPS data are rigorously combined and aligned to the ITRF2005 realization. As shown previously, the first-order signal in horizontal and vertical velocities is the ongoing postglacial rebound (glacial isostatic adjustment) from the last Ice Age. Second-order features in both velocity fields are at or within the 95% confidence interval of the measurements. By definition, strain rate estimates are noisier than the velocities from which derive. In most cases, crustal strain rates are smaller than a few nanostrain per year, with standard errors as large or larger than the mean estimates. However, some coherent patterns can be observed, such as systematic east-west to southeast-northwest shortening, that are consistent with some postglacial rebound model predictions in both style and magnitude. The sparse spatial resolution of the campaign and continuous GPS networks only allows limited focus on the seismically active zones in eastern Canada or northeastern U.S. On the basis of previous results, we test whether high seismicity regions, such as Charlevoix or Montreal, can be associated with a different strain rate signature (in style and magnitude) from regions of low or no seismicity. Preliminary results suggest that higher spatial resolution is required to derive a robust answer to this question.
Time-variable Deformation in the New Madrid Seismic Zone
A new analysis of GPS measurements across the New Madrid Seismic Zone (NMSZ) in the North American
midcontinent shows that deformation accumulates at a rate indistinguishable from zero and less than 0.2
mm/yr. Residual velocities relative to the rigid interior of North America are all below their uncertainties at 95%
confidence and a simulation shows that they can be explained as non-tectonic artifacts. At steady-state, a
(maximum) rate of 0.2 mm/yr implies a (minimum) repeat time of 10,000 years for low M7 earthquakes, in
contrast with the 500-900 year repeat time of paleo-earthquakes. Strain in the NMSZ is therefore currently
accumulating too slowly to account for seismicity over the past ~5,000 years, hence excluding steady-state fault
behavior. In addition, geological observations show that large earthquakes and significant motions on the
Reelfoot fault started most recently in the Holocene, indicating that the NMSZ area as a whole may be
experiencing a transient burst of seismic activity. Models proposed so far to explain how large earthquakes
may repeat with little far-field straining all require an ad-hoc weak zone under the NMSZ, a feature that is not
corroborated by independent geophysical observations. Here, we investigate a model where stress changes
are caused by the Quaternary denudation/sedimentation history of the Mississippi valley. We show that flexural
stresses are sufficent to trigger earthquakes in a continental crust at failure equilibrium. The first large event
weakens the main fault and may bring neighboring faults to failure (or very close). Subsequently, the resulting
viscoelastic relaxation leads to failure again on the main fault (lower strength threshold) and on neighboring
faults. In the absence of significant far-field loading, this process can only maintain seismic activity for a few
Since New Madrid's not Moving... A Complex System View of Midcontinental Seismicity and Hazards
Successive GPS studies over the past eighteen years within the New Madrid Seismic Zone show no detectable motion to steadily increasing precision - currently 0.2 mm/yr. The NMSZ is thus deforming too slowly - if at all - to account for large earthquakes in the area over the past ~5,000 years. Hence the recent cluster of large magnitude events does not reflect long-term fault behavior. More generally, the GPS data together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicate the need for a different view on midcontinental earthquakes. Traditionally, intraplate seismic zones have been treated like slowly deforming (less than 2 mm/yr) plate boundaries. We expected steady deformation focused primarily in narrow zones like the New Madrid system, such that the past rates and locations shown by geology and the earthquake record would be consistent with present deformation shown by geodesy, and predict future seismicity. It now seems more useful to view mid- continent earthquakes as migrating, episodic, and clustered. Instead of occurring quasi-periodically along a single major fault system, they migrate over many faults in a large area. A given fault will be active for several earthquake cycles, and then become dormant as other faults become active. Because deformation can be steady for a while then shift, the past earthquake history can be poor predictor of the future, and seismic hazard assessment based on the recent earthquake record can overestimate risks in regions of recent large earthquakes and underestimate them where seismicity has been quiescent. In this scenario the currently active parts of the New Madrid System are the presently most active one of many faults, the recent seismicity are primarily aftershocks of the large past events, and the lack of present deformation suggests that the recent cluster of earthquakes may have ended. It is thus useful to view midcontinent earthquakes as a classic complex system controlled by interactions between faults. Although an individual fault taken in isolation acts in a simple quasi-period fashion, the network of interacting faults gives complex variability in space and time. Initial numerical modeling shows that fault interactions can give rise to such variability without local or time-variable loading, either of which can provide further variability. Hence a complex system view seems likely to lead to an improved understanding of midcontinental tectonics, the resulting earthquakes, and the hazards they pose.
Partitioning of Seismicity in the Charlevoix Seismic Zone, Quebec: a 3D Stress Model.
The Charlevoix seismic zone in the St. Lawrence valley of Quebec is historically the most active in eastern Canada, both as a site of repeated but infrequent large events, as well as continuous low-level background seismicity. The structurally complex region comprises a series of subparallel steeply dipping rift faults formed during the opening of the Iapetus Ocean, superimposed by a 350 Ma meteorite impact structure, resulting in a large heavily faulted volume. The seismic zone is approximately 85 by 30 km, and runs through the crater with the long axis parallel to the St. Lawrence River. Most of the large events (M > 5) localize outside the crater and are consistent with slip along the rift faults, whereas the background seismicity tends to occur within the volume of rock bounded by the rift faults rather than on the faults themselves, with a large concentration beneath the impact structure. This high level of seismicity and its geometrical partitioning is unusual, given that large impact structures are typically aseismic. Although no relationship between observed seismicity and the major structural features has yet been proposed, the patterns of seismicity are strongly suggestive of an underlying mechanism between the two. The interaction between rift and crater faults is explored with a three dimensional numerical model constructed using the stress analysis code FLAC-3D. The rift faults are represented by dipping discontinuities that are assigned frictional strength parameters. A bowl-shaped elastic volume of reduced modulus represents the heavily fractured impact structure. Initially set to lithostatic, deviatoric stresses slowly build up from boundary displacements to evolve the system in a manner similar to tectonic loading. The effect of fault strength on the state of stress in the crater and surrounding area is tested. Furthermore, the influence of the crater on the distribution and rate of slip is also observed by monitoring shear displacements on the fault surfaces as the model evolves. Preliminary results indicate that there is a corresponding increase in the deviatoric stress in the region of the crater bounded by the faults, as well as the region directly below it, when the rift faults are weakened. This increase in deviatoric stress results in the decrease of stability of optimally oriented faults, and may explain the localization of low-level seismicity in these areas. In addition, monitoring of slip distribution along the rift faults throughout the evolution of the model shows that large slip events tend to localize just outside the perimeter of the crater, consistent with the locations of large earthquakes. Analysis of the slip vector of these events provides focal mechanisms with P-axis oblique to the applied regional stress field, which are similar in style to the 1925 (M 6.2) and 1979 (M 5.0) events. The models show a remarkable correlation with observations, suggesting that the distribution of both large and small events in the Charlevoix seismic zone may be explained by the combined effects of the crater and relatively low strength rift faults, in altering the contemporary stress field to one that enhances fault slip.
Lithospheric Deformation in Northwest Europe: a Comparison of Seismicity, Geodetic and Geologic Information
Evaluating the maximal magnitude and the recurrence of large earthquakes depends on where and how the strain is released in the lithosphere. Therefore, to characterize the long term seismic activity in northwest Europe, we evaluated and compared the strain accumulated by the known seismic activity with that observed in the recent geological records. In this part of Europe, surface movements are very weak and geodetic data can only provide an evaluation on the maximal value of the present horizontal strain rates as around 5 10-10 yr-1. Nevertheless, this information is important in the discussion of seismicity models. The evaluation of the scalar seismic moment release during historical times suggests that in western and northern Europe, M ≥ 6.5 - 7.0 earthquakes should be very rare, and the seismic strain is relaxed by numerous moderate earthquakes with magnitude between 5.5 and 6.0. The earthquake moment release in NWE during the historical period (the last 700 years) is of the order of 1016 N.m/yr. Considering a seismogenic layer average thickness of 10 km, this is equivalent to an average strain rate of 10-10 yr- 1, which corresponds roughly to 20% of the possible maximal geodetic strain rate. This is also an order of magnitude higher than the strain deduced from the cumulative seismic moment during the last 100 years, which suggests a deficit of the present short term earthquake activity. An evaluation of the moment release by the active faults in the Lower Rhine Embayment for the last 10,000 years provides also a value around 1016 N.m/yr. This suggests that, on the long term and in average, the Lower Rhine Embayment relaxes most of the strain in this part of intraplate Europe, even if, at short term scale, local strain release can occur in other parts of the region. Repeated absolute gravity measurements across the Belgian Ardenne and the Roer Graben and in Oostende, on the Belgian coast, suggest that the whole region is presently in subsidence with a rate of 1-2 mm/yr, in agreement with the most recent published model of the glacial isostatic adjustment (GIA). The GIA models, validated by geodetic data in the most uplifted area of the Fennoscandian GIA, suggest a NNE-SSW compressive strain in northwest Europe, which contradicts the strain deduced from the earthquake fault-plane solutions and the geological observations in the Lower Rhine Embayment. This questions the possible relationships between earthquake activity and GIA.
Sumatra Megathrust Earthquakes Trigger Intraplate Seismicity in the Indo-Australian Oceanic Lithosphere
The present-day intraplate deformation between India and Australia started 9 Myrs ago. In the Central Indian Basin (CIB), this deformation is recorded in the thick sediments of the Bengal fan. The equatorial, dense E-W thrust fault network in this region is the result of a massive reverse reactivation of normal faults at the onset of deformation. The Wharton Basin (WB), separated from the CIB by the NinetyEast Ridge (NyR), shows a contrasting style of deformation with mainly left-lateral strike-slip seismicity. The WB finite deformation and seismicity also involve pre-existing faults, in this case the N-S paleo-transforms of the fossile Wharton spreading-ridge system. The oceanic plate seismicity after the December 2004 Aceh subduction earthquake shows strike-slip events with a clear intraplate P-axis. No thrust faults are detected. This indicates short-term reactivation of the transform faults near the trench. Spatial and temporal distribution of intraplate erthquakes, as well as their anomalous moment release suggests triggering by the Aceh megathrust earthquake, which appears to have acted as an "accelerator" for the oceanic intraplate deformation. In this study, we use Coulomb stress static variations to confirm our seismicity observations. We first assume that the reactivated transform and the neoformed thrust fault plane families are present in the oceanic lithosphere. We then compute the coseismic stresses in the vicinity of the trench from the Aceh and Nias earthquakes slip distributions. Finally, we derive the normal and shear stresses on the fault planes. The results show that the strike-slip events are all favored by the subduction earthquakes coseismic stresses. They also show that the normal fault earthquakes at oceanic bulges are supported by the modeled coseismic stresses, except offshore Myanmar. The particularly interesting result is that all the possible neoformed thrust faults perpendicular to the intraplate P-axis are inhibited by the same coseismic stresses. This suggests that the style of intraplate deformation favored near the Sumatra Trench in the short-term by subduction earthquakes is the same than the long-term style. Under the effect of northward slab pull forces, Australia tries to detach from its Indian "brake" along the WB's N-S transform faults.