Intraplate Seismicity Research Articles

Lithospheric strength and intraplate seismicity in the New Madrid seismic zone

A simple hypothesis is proposed to explain the occurrence of localized zones of tectonic deformation and seismicity within intraplate regions subjected to relatively uniform far‐field tectonic stresses. In most intraplate regions (especially continental shield areas and old oceanic basins), temperatures in the lower crust and upper mantle are quite low so that the upper mantle is cold and strong. In these regions, significant lithospheric deformation does not occur because the cumulative strength of the lithosphere far exceeds the magnitude of plate‐driving forces. If lower crust and upper mantle temperatures are relatively high, however, plate‐driving forces are largely supported by the upper crust because the lower crust and upper mantle are relatively weak. In this case, the regions can deform relatively rapidly because the cumulative strength of the lithosphere is comparable in magnitude to that of the forces acting on the lithosphere. In this paper, we apply this hypothesis to the New Madrid seismic zone and the surrounding central and eastern United States. Within the seismic zone, the heat flow appears to be slightly elevated (about 60 mW/m2) relative to the background regional value of 45 mW/m2. Calculated crustal geotherms and laboratory‐derived ductile flow laws suggest that the lower crust and upper mantle are sufficiently weak within the seismic zone that intraplate stresses are largely transmitted through the upper crust and deformation can occur at relatively rapid rates for this intraplate area. In marked contrast, in the surrounding area where the heat flow is relatively low, cumulative lithospheric strength appears to far exceed the plate‐driving force, and the tectonic stress is carried in both the crust and upper mantle. Thus the marked contrast in seismicity between the seismic zone and the surrounding area appears largely because of heat flow and whether or not the lower crust and upper mantle support an appreciable fraction of the plate‐driving forces.

Mechanical stability of the Redbank Thrust Zone, Central Australia: Dynamic and rheological implications

Deep seismic reflection profiling and other studies indicate that the Moho has been significantly displaced by intracratonic deformation on the Redbank Thrust Zone in central Australia. A two‐dimensional finite‐element model of the thick‐skinned crustal structure of the Redbank Thrust Zone shows that the gravitational stresses generated by the distribution of crustal masses are in agreement with the tectonic and seismic quiescence of the area. Rock failure is concentrated in the downthrusted, and therefore Theologically weakened, crustal segments. Conversely, the upthrusted slices of lower crust and upper mantle undergo little deformation and no failure and thus constitute a mechanically strong whole‐crustal core. The presence of north‐south oriented intraplate stresses derived from far‐field plate boundary forces does not contribute to the inferred mechanical equilibrium of the crust in central Australia. The results may have implications for the mechanical stability of intracratonic thick‐skinned crustal structures elsewhere, and for the long‐term strength of continental lithosphere in general. The models, however, do predict the possibility of intraplate seismicity in response to small changes in tectonic stress, since parts of the crust are already at or close to the failure limit.

Topographic and seismic effects of long-term coupling between the subducting and overriding plates beneath Northeast Japan

For at least five million years, the motion of the subducting Pacific plate has been steady in a direction approximately perpendicular to the strike of the island arc in northeast Japan. We have shown that the present-day topography, gravity and seismicity patterns can be modeled successfully by assuming an approximately steady tectonic compressive driving force and a plausible crustal rheology and by using a finite element modeling code which allows stick-slip faulting, erosion/deposition and time-dependent loading. In this paper we show how the strength of interplate coupling with depth and along strike is constrained by the observed topography and gravity, and we examine the correlation of coupling with observed interplate and intraplate seismicity. Topography and the gravity residual anomaly in the eastern coastal area are significantly higher in region 1 (the Kitakami range and off Sanriku) than in region 2 (off Miyagi). In order to match the observed topography and gravity, the strength of interplate coupling in region 1 is about two to four times greater than in region 2, and there is about twice as strong a coupling at greater depths. Coupling in region 1 and region 2 are considered as two extreme cases. In region 3 (the Abukuma range and off Fukushima), the coupling strength is between that of region 1 and region 2. The model predicts a higher potential for earthquakes in the crust and mantle wedge apex for the region with stronger coupling, region 1, which is consistent with the observed intraplate seismicity. Observed patterns of interplate seismicity are also consistent with differences in interplate coupling; the weaker coupling in region 2 indicates a lack of seismogenic potential which leads to a low probability for large earthquakes in that region.

Seismicity and tectonics of the Bay of Bengal: Evidence for intraplate deformation of the northern Indian plate

Seismicity and fault plane solutions of several earthquakes in the Bay of Bengal have been studied to focus attention on the nature of intraplate deformation in the northern Indian plate. A widely dispersed diffuse seismicity provides evidence that the intraplate region of the entire Bay of Bengal and peninsular India of the Indian plate are seismically active of which the Gangetic delta and the eastern flank of the Ninety East Ridge show somewhat higher seismicity in comparison to the thickly populated epicenters at the eastern plate boundary. Predominance of thrust faulting earthquake mechanisms with consistent north-dipping fault planes and more or less NS pressure axes imply that NS compression is more prevalent in the intraplate region of the Bay of Bengal which is analogous to the Himalayan arc. This intraplate seismicity and NS compressive deformation is presumed to be the result of collisional resistance to the north moving Indian plate with the Eurasian plate at the Himalayas where subduction of the Indian lithosphere in mantle has stopped recently. As a result, to compensate the horizontal compressive force by ridge-push at the Carlsberg Ridge, the intraplate region of the northern Indian plate in continuation to the Central Indian Ocean Basin no longer remains passive due to a strong repulsive force at the collisonal plate boundary Himalaya that causes stress transmission towards south and makes the intraplate region seismically active.

Climatic changes, streamflow, and long-term forecasting of intraplate seismicity

Regions of intraplate seismicity in the eastern United States are spatially isolated areas of persistent, diffuse earthquake activity. There is no widely accepted explanation for the origin of these earthquakes. We have suggested that climate plays a key role in triggering such intraplate seismicity. Long-term increases and decreases in rainfall cause periodic regional and temporal variations in the elevation of the water table that, by pore pressure diffusion, result in small changes in fluid pressure at any given depth in the crust. In a fractured, hydraulically permeable crust, the depth of penetration of this pore pressure diffusion can be as deep as the brittle-ductile transition (15–18 km). In a seismogenic crust, stress corrosion and fatigue of rock asperities might be more important than purely mechanical effects due to small changes in hydrostatic fluid pressure; however, because any chemical effects are quasi-static, the temporal characteristics of the triggering process might ultimately be determined by the mechanical process, resulting in a “hydraulically induced” seismicity trigger that acts somewhere along paths of pore pressure diffusion. We called this model of intraplate earthquake generation “hydroseismicity”. Because streamflow is related to the regional and temporal morphology of the water table, we searched for and report here several attempts to find links between climatic changes, streamflow, and intraplate seismicity. Our results fall broadly into four categories: (1) temporal correlations of streamflow with the earthquake strain factor; (2) spectral analyses of flow and seismicity and the identification of common spectral peaks; (3) numerical modeling to estimate fluctuations in pore pressure at hypocentral depths; and (4) climatic driving mechanisms (e.g., sunspot cycles) that might substantiate a climate-earthquake link. We observe a statistically significant peak in the Fourier spectrum of surface streamflow for the seismic zones bisected by the Mississippi River, Illinois, and James River, Virginia, in the period range of 11–13 years that might be associated with sunspot activity. In addition, there is positive correlation between periods of above average values of the standard deviation of streamflow time series and periods of seismicity in the central Virginia seismic zone. Many aspects of the weather appear to be modulated by a 20-year cycle. We observe a similar periodicity (18–20 years) in seismicity in the central Virginia seismic zone. A good agreement is observed when a streamflow time series is superimposed on the record of the earthquake strain factor if a value of 50 km 2/year is assumed for crustal hydraulic diffusivity. In the central Virginia seismic zone, it is found that the number of earthquakes versus depth, ψ, is directly proportional to pressure fluctuations at the depth ψ. In addition, the fractal dimension determined from downward-continued streamflow is approximately the same as the fractal dimension of intraplate seismicity. Furthermore, using the Gutenberg-Richter relation and assuming that the earthquake data sets in the New Madrid and central Virginia seismic zones are complete for all magnitudes m ⩾ 2, the ratio of the number of earthquakes occurring per year in the New Madrid zone to the central Virginia zone is about 40. The ratio of the standard deviations of downward-continued Mississippi River streamflow (at Thebes, Illinois) to the James River streamflow is also about 40. One interpretation of this common ratio is that the number of intraplate earthquakes generated in a seismogenic crust is directly proportional to the standard deviation of vertical variations in the elevation of the water table. If the hydroseismicity hypothesis is correct, then long-term variations in streamflow can be used to forecast long-term statistical variations in intraplate seismic activity.

Tectonic stress within the New Madrid seismic zone

Refraction data indicate a significant high‐density rift pillow beneath the New Madrid seismic zone. We present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present‐day stress field, and we consider the implications for intraplate seismicity. These models were run for a loading time of 100 m.y. to account for relaxation and transfer of stress since the last reactivation of the rift in the mid‐Mesozoic. Results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes. The model predicts a maximum compression of 30–40 MPa above the rift pillow in the center of the rift axis. If the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10°–30° in the direction of SHmax (maximum horizontal compression) near the rift axis, the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces. The addition of the local stress associated with the rift pillow, however, results in an approximately 30% reduction in the resolved maximum horizontal shear stress. Thus, while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure, reduction in the resolved shear stress requires a separate mechanism for strength reduction. Results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m.y. and may still play a role in present‐day deformation and seismicity of the New Madrid seismic zone. Local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones.

Stress perturbation associated with the Amazonas and other ancient continental rifts

The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0–2.6 km depth) breakout data and deep (20–45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ∼75° rotation relative to a regional E‐W compressive stress direction in the South American plate. Elastic two‐dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift‐normal compressional stresses between 60 and 120 MPa, with values of 80–100 MPa probably most representative of the overall structure. The observed ∼75° stress rotation constrains the ratio of the regional horizontal stress difference to the rift‐normal compressive stress to be between 0.25 and 1.0, suggesting that this rift‐normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift‐normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike‐slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift‐normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust‐faulting stress regime in the vicinity of the Amazonas rift. A rift‐normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike‐slip faults parallel to the axis of the rift, although this results in a 20–40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift‐normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

Thermomechanical structure of European continental lithosphere; constraints from rheological profiles and EET estimates

SUMMARY The EET (equivalent elastic thickness) of the lithosphere is a measure of the integrated lithospheric strength. It is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere. We present a comparison of EET estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the European and Eurasian continental lithosphere. We discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for Precambrian segments of Europe's lithosphere, Variscan Europe and the Alpine collision belt. This analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western Europe and Asian parts of Eurasia. The EET estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of EET derived from flexural studies of sedimentary basins, forelands and orogenic belts. These rheological profiles suggest weakening of the major parts of the European and Eurasian continental lithosphere by decoupling of the crustal and upper-mantle parts- A comparison with the seismicity-depth distribution for some selected sites suggests that the intra-plate seismicity is essentially restricted to the upper crustal parts of Europe's lithosphere, providing additional support to the notion of the decoupled lithosphere. The presence of intra-plate stress fields can explain a significant part of the observed variations in EET estimates within individual thermotectonic age groups. A comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of EET inferred from the rheological response of basins and orogens at more moderate levels of intra-plate stress.

Intraplate seismicity in the Atlantic Ocean Basin: a teleseismic catalog

A comprehensive compilation and reassessment of intraplate seismicity in the Atlantic Ocean Basin is presented for the years 1918 through 1990. Of the 846 earthquakes whose original bulletin epicenters were located in Atlantic Ocean crust and were at least 2° from active plate boundaries, 289 were found to be reliably intraplate. A significant number (237) of the original epicenters were actually mislocated interplate earthquakes that located to active plate boundaries upon relocation, or whose 95% confidence locations determined through Monte Carlo simulations intersected seismically active boundaries. Many of these erroneous locations, particularly in regions surrounding the Mid-Atlantic Ridge, frequently appear in global seismicity maps. The resulting reliable catalog of intraplate seismicity is not randomly distributed but shows significant clustering in space and time. Much of the seismicity is associated with the locations of hot spots, such as the Bermuda, Canary, and Cape Verde Islands. Seismicity is also clustered around other tectonic features, such as the diffuse plate boundary between North America and South America, and the fossil spreading ridge between Greenland and North America. While we did not examine intraplate earthquakes within 2° of the Mid-Atlantic Ridge and some of our unreliable near-ridge earthquakes may be truly intraplate, we do not find an indication within our “reliable” data set of an increase in intraplate seismicity with increased proximity to the Mid-Atlantic Ridge beyond our 2° limit.

Common cyclicities in the seismicity and water level fluctuations at the charlevoix seismic zone on the St Lawrence River, Quebec, Canada

Intraplate seismicity has no widely accepted explanation for its origin. The hydroseismicity hypothesis suggests that natural increases in hydraulic head, caused by transient increases in water table elevation, can be transmitted to hypocentral depths (10–25 km) in a fractured, prestressed, near-failure crust, and, along with long term hydrolytic weakening of rocks, contribute to the triggering of earthquakes. In this study, the temporal characteristics of seismicity and water level fluctuations at the Charlevoix seismic zone on the Saint Lawrence River in Quebec, Canada, are determined using Fourier spectral analysis and are investigated for possible temporal relationships, as predicted by hydroseismicity, using group delay analysis. Fourier spectral analysis of the temporal release of seismic energy indicates the presence of short term cyclicities of periods of 13–14 yr, along with longer term periods of approx. 55–70 yr. Spectral analysis of the water level data indicates cyclicities with periods of 1, 23, 14 and 8 years. Group delay analysis of the 14 yr periodicities in the water level and seismicity time series yields a time lag of 2.3–2.6 yr, with the water levels leading the seismicity time series. Crustal diffusivity values estimated from the group delay analysis range between 0.3 and 2.7 m 2/s, for a depth interval from 7 to 20 km, which is within the range of published values of diffusivities for the crust (0.1–100 m 2/sec). The present study suggests that common cyclicities are present in both the water levels and seismicity in the Charlevoix region. Such cyclicities are an essential element of the hydroseismicity hypothesis for the diffusion of surficial fluid pressure variations to hypocentral depths. Furthermore, the temporal relationship between the cyclicities observed allows for a possible causal relationship between repetitive mechanical effects of pore pressure transients and seismicity in the Charlevoix region.

EARTHQUAKE RECURRENCE IN KUWAIT INDUCED BY OIL AND GAS EXTRACTION

A magnitude 4.7 earthquake which occurred on June 2, 1993 in the vicinity of the Minagish/Umm Gudair oilfields in Kuwait was a complete surprise. It was felt over an area of about 2,500 sq. km, and the main shock was followed two days later by an aftershock with a magnitude of 3.2. Five earthquakes are known to have occurred between 1975 and 1977 in this area, but there had been no previously reported seismic activity. In Kuwait, seismic hazards could result from excessive fluid withdrawal, fluid injection, intraplate seismicity within the Arabian Plate, or from the effects of large earthquakes along the Zagros Belt. Sinkholes have been reported to form as a consequence of fluid‐withdrawal related subsidence. The spatial correlation between recent, moderate‐sized earthquakes and operating oilfields suggests that the current seismicity may be related to oil production. It is proposed that a permanent seismic network in Kuwait be established, to monitor and evalute seismic hazards, and predict damaging earthquake events

Earthquake swarm in Steigen, northern Norway: an unusual example of intraplate seismicity

ABSTRACTOn the first of January 1992, unusual earthquake activity started in Steigen, northern Norway. By 31 December, 1992, a total of 207 events had been recorded in several pulses. Among these, several events with magnitudes larger than 3.0 (Mc) were also felt by the people. The epicentral distribution of these events seems to suggest that they all are concentrated in a small region (approximately 10 km in diameter) in the Brennvika Bay (67.8°N, 14.9°E), between Leinesfjorden in the north and Nordfolda in the south. The distribution of events in time is characteristic of a swarm in which no dear evidence of a mainshock–aftershock sequence was observed. Macroseismic intensity surveys as well as synthetic modelling indicate shallow focal depths in the range 5–8 km. Based on a composite focal mechanism, indicating an oblique‐slip fault striking NE with a normal component dipping NW, combined with an observed NE trend in the epicentres and the focal depths, these events correlate with a previously proposed fault zone along Nordfolda. The NE trend has been dominant in the area throughout geological time, starting from the Caledonian Orogeny to the post‐Caledonian basin developments, as well as the post‐glacial and neotectonic structural features. At present, the deep‐seated structural anomaly underneath the Lofoten Ridge seems to be the major factor controlling the structural trends in the area. The activity along the Nordfolda fault zone could be related to this large‐scale structure, although the causes of the stresses that would create the seismic activity are not very clear, as there are several possible stress‐generating mechanisms involved, such as post‐glacial uplift of the land masses, the ‘ridge‐push’ effect from Mohn's Ridge, and vertical stresses generated by the sediment loading in the adjacent basins.

Intraplate earthquakes in the southwest Pacific Ocean Basin and the seismotectonics of the southern Tasman Sea

An examination of 311 intraplate earthquakes in the Australian plate portion of the Pacific Ocean basin reported from 1918 to 1990 reveals that only 113 events are reliably intraplate, with most of the rest relocating to active trenches and transforms. The non‐random distribution of the reliably intraplate events gives insight into the tectonic stresses present. The central Tasman Sea is mostly aseismic except for a swarm of activity at the predicted site of the Tasmantid hot spot. To the north, the broad regions of the Coral Sea, South Fiji Basin and Lord Howe Rise show very little intraplate seismicity, yet the narrow Norfolk Ridge and Three Kings Rise, caught between the double convergence of the New Hebrides and Tonga subduction zones, support many more earthquakes.High levels of intraplate seismicity in the southern Tasman Sea adjacent to the Macquarie Ridge Complex (MRC) indicate that this region may be undergoing internal deformation due to the unusual nature of the Australia‐Pacific plate boundary. Additional support exists in the form of intraplate focal mechanisms similar to those at the plate boundary and a set of parallel gravity rolls which are observed in recent geoid maps. Some aftershocks of the Mw = 8.2 Macquarie Ridge earthquake of 1989 occurred in a fracture zone west of the Macquarie Ridge Complex [Das, 1992], but we have found several earthquakes from as early as 1924 which relocate to this feature, suggesting that its reactivation may be more significant than previously thought. This reactivation of a fossil fracture zone may be the result of the increasing amount of oblique convergence between the Australia and Pacific plates at the Macquarie Ridge Complex, formerly a spreading center, and the stresses associated with subducting recently formed Australian ocean crust beneath the older Pacific plate.

Source parameters of two rift-associated intraplate earthquakes in peninsular India: the Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970

The Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970 are two major intraplate earthquakes associated with paleorift zones in the stable continental region of peninsular India. These two events shook very large portions of the peninsula, and casualties were reported for the Broach earthquake. Ancient rifts have recently been identified as a major seismogenic tectonic feature for intraplate seismicity in stable continental regions. P-wave first motions and waveforms of long-period teleseismic P and SH waves have been modeled to constrain the focal mechanisms and to determine the source parameters for the earthquakes. Previous investigators proposed multiple fault-plane solutions for each of the events. In this paper, some of the early solutions are eliminated and the best source mechanism is derived for each earthquake. For the Bhadrachalam earthquake, the focal mechanism determined is predominantly strike-slip (strike = 245°, dip = 72°, rake = 358°). The strike direction of the NE-SW-trending nodal plane is in good agreement with the trend of the observed isoseismals and is probably the fault plane. The seismic moment is 4.59 × 10 24 dyne-cm and the moment magnitude M w is determined to be 5.7. The focal depth and the static stress drop are estimated to be 10 km and 17 bar, respectively. The strike-slip motion represents a reactivated fault motion under the present-day stress field. For the Broach earthquake, the focal mechanism determined is reverse faulting with a strike-slip component (strike = 273°, dip = 58°, rake = 130°). The direction of the E-W-striking nodal plane is in good agreement with the mapped Narmada fault, dislocation direction of fissures, and the trend of the isoseismals. The seismic moment and moment magnitude are determined to be 1.39 × 10 24 dyne-cm and 5.4, respectivety. The focal depth and the stress drop are estimated to be 11 km and 9 bar, respectively. The reverse motion of this event represents an inverted slip or tectonic inversion within a rift zone under the present-day compressional stress field. Some rift-associated earthquakes are characterized by large multiple shocks with significant strike-slip motions. The two Indian earthquakes, however, occurred as regular main shock-aftershock sequences-and represent a different mode of strain release. These two earthquakes also represent events with low stress drops and relatively long recurrence intervals. The interpretation of this last observation is uncertain and needs more study in the future.

Intraplate stresses along a section of the Kavali-Udipi DSS profile in the south Indian shield induced by density heterogeneities and topography

Occurrence of intraplate seismicity has been attributed to several causes. The perturbation in the local stress regime, either due to local strength weakening of rock mass or surface and subsurface loading, is considered as a plausible mechanism of intraplate earthquake occurrences. The present work is aimed at analysing the state of stress in a part of south Indian shield induced by existing topography and undulations in the subsurface interfaces. The stresses are computed along a section of well studied Kavali-Udipi profile. The general nature of the stresses is compressive in the upper lithosphere except for a small region of extensional regime at both the ends of the profile. The magnitude of these stresses in comparison to the plate tectonic stresses shows that it also forms a significant component of the prevailing stresses in the region. The computed results also show a lateral variation in the stresses along the profile section. Thus, the role of stresses due to density heterogeneities and topography should be taken into consideration in explaining the microseismicity of the region.

Seismic seafloor observations: a study of ‘anomalous’ intraplate seismicity in the northeastern Indian Ocean

Present-day intraplate tectonic activity in the northeast Indian Ocean, which manifests itself in high seismicity, is unusual for oceanic lithosphere plates. The contradictory points of the various tectonic hypotheses proposed to explain this ‘anomalous’ seismicity may be due to sparsity of ocean intraplate earthquakes of M > 4.5, which are recorded by means of networks of land-based permanent seismic stations. Seismological observations on the ocean floor by means of ocean bottom seismographs may provide additional and important data. Some results of such observations carried out in the Central Indian Basin simultaneously with deep seismic sounding surveys are analysed in this paper. About 118 seismic events were recorded at a site near 3°45′S and 80°E for a period of 7.3 days, and 24 events were recorded at a site near 0°50′S and 83°53′E for a period of 3.3 days. Magnitudes of these events range approximately from 2.8 to 4.0. Estimates of epicentre locations based on the Jeffreys-Bullen travel-time table show that over 90% of the earthquakes recorded at the first site were related to the Chagos Trough, during a period when no event was recorded there by land-based seismic stations. The most probable reason for these events is the Chagos Bank earthquake of 30 November 1983 ( M s = 7.6), which caused many aftershocks extending eastward from the main shock. Thus the high aftershock activity east of the Chagos Bank earthquake revealed by ocean bottom seismic observations complements the data on epicentres of larger earthquakes, which in turn provide the scope to extend the known seismicity of the Chagos Bank further east. In general, our results are in accordance with the most popular tectonic model of the Central Indian Ocean, in which a diffuse plate boundary divides the Australian plate from the Indo-Arabian or the Indian plate, initiating a new subduction zone.

A comparison of epicenter relocation techniques: a case study for the Juan de Fuca plate

The ‘master event relocation’ and ‘hypocentroid’ techniques are compared in an effort to determine their relative performance in evaluating the location of epicenters in a region hampered with recording station location bias and a poorly understood seismic velocity structure. The study areas chosen are the Explorer and Gorda plates, two small lithospheric subplates of the oceanic Juan de Fuca plate, which show a high degree of apparent intraplate seismicity. Previous studies have indicated systematic eastward errors in the location of epicenters due to the lack of recording stations to the west. Both techniques produce similar results in defining plate boundary vs. intraplate seismicity; however, noteworthy differences exist. The selection of a well-constrained master event has a clear effect on the alignment of relocated epicenters with plate boundaries. An event on the Gorda plate located by ocean bottom seismometer allows for a relocation of epicenters about the Gorda Rise. Using a more widely recorded master event on the Explorer plate produces parallelism of the events with the plate boundaries but an additional uniform shift in the relocated epicenters is required to align seismicity with the boundaries. Location of the master event, with respect to tectonic features of the plate, appears to affect the results, probably owing to differences in lithospheric conditions and their effect on wave travel paths through the plate. A comparison of results of the two techniques on intraplate seismicity on the Gorda plate shows opposing direction vectors for the relocations. The hypocentroid technique is less affected by changing conditions within the plate as events are clustered according to location. The relocations are still affected by the station location bias, as the events are relocated in relation to the weighted center of the event cluster. The technique produces greater definition of the trends of the relocated epicenters; however, in this situation an additional shift of the epicenters is required for alignment with the boundaries.

On the strength of oceanic fracture zones and their influence on the intraplate stress field

We use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere. If fracture zones are weaker than the surrounding lithosphere, they should be sites of concentrated intraplate seismicity, and the lithospheric stress field should be perturbed in their vicinity. The 77 earthquakes selected for the study have well‐determined focal mechanisms and epicenters in regions where fracture zones are well mapped. We have searched for dependence of faulting style, fault orientation, or principal stress direction on the distance from the nearest fracture zone. If fracture zones were generally weaker than the surrounding lithosphere, one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone; we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner. There is a slight tendency for earthquakes to occur near fracture zones, and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress. These patterns are not demonstrable at high significance, however. If oceanic transform fault zones are weak relative to the surrounding lithosphere, these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere.

Thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere

Observations of plate flexure beneath seamounts and at deep‐sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age. The mechanical thickness (H) seems to follow the depth to a particular isotherm in the cooling plate model. A detailed review of available estimates of elastic thickness (Te) shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones. Whereas Te beneath seamounts approximately follows the 400°C isotherm, H at trenches follows the depth to the 700°–800°C isotherms. The cause of this bimodal distribution is not fully understood. Thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones, and their distribution with depth is consistent with intraplate seismicity. This paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength. The results indicate that thermal bending stresses will bias the moment‐curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount (positive curvature) but relatively stronger at a trench (negative curvature). This systematic bias appears to be in qualitative agreement with published Te estimates. For old lithosphere the theory predicts a diminishing amount of bias. The general lack of sufficient Te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the Te versus age relationship. I conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement, bias introduced by thermal stresses, and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading.

Stress magnitude estimates from earthquakes in oceanic plate interiors

We propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms. Using a depth‐dependent rheology, we show it is possible to estimate the differential stress (σ1–σ3), averaged over some reference lithospheric thickness. The resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis. We apply the method to the Central Indian Ocean, where intraplate seismicity is high. From well‐studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals. This result is consistent with the high level of stress that was found from numerical model calculations by Cloetingh and Wortel (1985, 1986). From the few intraplate events in the Pacific plate, we also estimate differential stresses in this area.