T. J. Craig

A new paradigm for large earthquakes in stable continental plate interiors

Large earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earths seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occuring when the accrual of tectonic stress localized at long-lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a pre-stressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do not apply. As a consequence, seismic hazard in SCRs is likely more spatially distributed than indicated by paleoearthquakes, current seismicity, or geodetic strain rates.

Variability in the crustal structure of the West Indian Continental Margin in the Northern Arabian Sea

ABSTRACT In this case study of the West Indian Continental Margin we present an interpretation of a volcanic margin structure based on regional mapping of high quality 2D seismic data in conjunction with regional satellite derived gravity data and selected subsidence analyses. The area shows many classic characteristics of a volcanic-type margin. Volcanic facies identified and mapped along the margin include seaward-dipping reflectors (SDRs), sub-aerial seamounts, clinoform packages interpreted as lava and volcaniclastic delta systems and thick, seismically layered sequences interpreted as volcanically derived sediment deposited in both fluvial and marine environments. The results show major variation in the overall thickness and style of volcanism across the margin both in dip and strike directions which may be related to variability in influence of the Deccan Plume in addition to localization along structural features inherited from older tectonic events. Our interpretation of rapid lateral variation in the thickness of extrusive volcanism has important implications for the distribution, preservation and hydrocarbon potential of the pre-rift sequence across the margin. The interpreted crustal structure also has a major impact on predictions of the historical and present-day heat-flow into the post-rift section. Our interpretation of the timing and distribution of volcanism is consistent with the presence of a broad region of elevated mantle potential temperatures at the time of the final break-up event on the West Indian Continental Margin, commonly attributed to the Deccan/Réunion Plume. Pre-existing structural heterogeneities appear to have played an important part in controlling the distribution of volcanism. Interpreted tectonic subsidence, based on backstripping analysis of the post-break-up interval, is also shown to be consistent with post-break-up thermal subsidence in combination with dynamic support associated with the elevated mantle temperatures into the Early Eocene.

Strain accumulation in the New Madrid and Wabash Valley seismic zones from 14 years of continuous GPS observation

The mechanical behavior—and hence earthquake potential—of faults in continental interiors is an issue of critical importance for the resultant seismic hazard, but no consensus has yet been reached on this controversial topic. The debate has focused on the central and eastern United States, in particular, the New Madrid Seismic Zone, struck by four magnitude 7 or greater earthquakes in 1811–1812, and to a lesser extent the Wabash Valley Seismic Zone just to the north. A key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults, a quantity that remains debated. Here we address this issue with an analysis of up to 14.6 years of continuous GPS data from a network of 200 sites in the central United States centered on the New Madrid and Wabash Valley seismic zones. We find that the high-quality sites in these regions show motions that are consistently within the 95% confidence limit of zero deformation. These results place an upper bound on strain accrual on faults of 0.2 mm/yr and 0.6 mm/yr in the New Madrid and Wabash Valley Seismic Zones, respectively. For the New Madrid region, where a paleoseismic record is available for the past ∼5000 years, we argue that strain accrual—if any—does not permit the 500–900 year repeat time of paleo-earthquakes observed in the Upper Mississippi Embayment. These results, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicate that either tectonic loading rates or fault properties vary with time in the New Madrid Seismic Zone and possibly plate wide.

Evidence for the release of long‐term tectonic strain stored in continental interiors through intraplate earthquakes

The occurrence of large earthquakes in stable continental interiors challenges the applicability of the classical steady-state ‘seismic cycle’ model to such regions. Here, we shed new light onto this issue using as a case study the cluster of large reverse faulting earthquakes that occurred in Fennoscandia at 11-9 ka, triggered by the removal of the ice load during the final phase of regional deglaciation. We show that these reverse-faulting earthquakes occurred at a time when the horizontal strain-rate field was extensional, which implies that these events did not release horizontal strain that was building up at the time, but compressional strain that had been accummulated and stored elastically in the lithosphere over timescales similar to or longer than a glacial cycle. We argue that the tectonically-stable continental lithosphere can store elastic strain on long timescales, the release of which may be triggered by rapid, local transient stress changes caused by surface mass redistribution, resulting in the occurrence of intermittent intraplate earthquakes.

Normal faulting sequence in the Pumqu‐Xainza Rift constrained by InSAR and teleseismic body‐wave seismology

Normal faulting earthquakes play an important role in the deformation of continents, and pose significant seismic hazard, yet important questions remain about their mechanics. We use InSAR and body-wave seismology to compute dislocation models and centroid moment solutions for four normal-faulting earthquakes (Mw 5.7–6.2) that occurred in the Pumqu-Xainza Rift (PXR), southern Tibet, a region where low-angle normal faulting has previously been inferred. We also use the fault locations and slip to investigate the correlation between earthquakes and surface topography, and to calculate stress interactions between the earthquakes. The InSAR and body-wave models give consistent focal mechanisms except for the magnitude of the 1996 event, which may be overestimated due to postseismic deformation in the long-interval interferograms. We calculate the static stress changes due to coseismic slip and find that the 1993 event was too distant to cause triggering of the later events, but that the 1998 event pair occurred in regions of increased Coulomb stress resulting from the 1996 event. All the fault planes found here dip at 40–60°, reinforcing the absence in observations for low-angle normal faulting earthquakes (dip < 30°) whose focal planes can be determined unambiguously. The fault planes of the 1993 and 1996 events are not associated with any obvious surface geomorphology, suggesting that sometimes it is unreliable to resolve the focal plane ambiguity by geomorphology, even for Mw 6.2 events. Furthermore, these events occurred outside the center of the rift, indicating that the active faulting is more distributed and over a length-scale at least 25–50 km east-west in extent, rather than confined to the 20 km width seen in the current mapped faulting and topography. These results suggest that seismic hazard in other extensional zones worldwide might also be more broadly distributed than suggested by geomorphology.

Hydrologically-driven crustal stresses and seismicity in the New Madrid Seismic Zone

The degree to which short-term non-tectonic processes, either natural and anthropogenic, influence the occurrence of earthquakes in active tectonic settings or ‘stable’ plate interiors, remains a subject of debate. Recent work in plate-boundary regions demonstrates the capacity for long-wavelength changes in continental water storage to produce observable surface deformation, induce crustal stresses and modulate seismicity rates. Here we show that a significant variation in the rate of microearthquakes in the intraplate New Madrid Seismic Zone at annual and multi-annual timescales coincides with hydrological loading in the upper Mississippi embayment. We demonstrate that this loading, which results in geodetically observed surface deformation, induces stresses within the lithosphere that, although of small amplitude, modulate the ongoing seismicity of the New Madrid region. Correspondence between surface deformation, hydrological loading and seismicity rates at both annual and multi-annual timescales indicates that seismicity variations are the direct result of elastic stresses induced by the water load.Large-scale changes in continental water storage have been shown to have an impact on seismicity. Here, the authors show that variation in the rate of microearthquakes in the New Madrid Seismic Zone coincides with hydrological loading in the Mississippi embayment at both annual and multi-annual timescales.