Oceanic Intraplate Earthquakes Research Articles

Finite source properties of large strike-slip earthquakes

SUMMARY Earthquake ruptures are complex physical processes that may vary with the structure and tectonics of the region in which they occur. Characterizing the factors controlling this variability would provide fundamental constraints on the physics of earthquakes and faults. We investigate this by determining finite source properties from second moments of the stress glut for a global data set of large strike-slip earthquakes. Our approach uses a Bayesian inverse formulation with teleseismic body and surface waves, which yields a low-dimensional probabilistic description of rupture properties including the spatial deviation, directivity and temporal deviation of the source. This technique is useful for comparing events because it makes only minor geometric constraints, avoids bias due to rupture velocity parametrization and yields a full ensemble of possible solutions given the uncertainties of the data. We apply this framework to all great strike-slip earthquakes of the past three decades, and we use the resultant second moments to compare source quantities like directivity ratio, rectilinearity, average moment density and vertical deviation. We find that most strike-slip earthquakes have a large component of unilateral directivity, and many of these earthquakes show a mixture of unilateral and bilateral behaviour. We notice that oceanic intraplate earthquakes usually rupture a much larger width of the seismogenic zone than other strike-slip earthquakes, suggesting these earthquakes may often breach the expected thermal boundary for oceanic ruptures. We also use these second moments to resolve nodal plane ambiguity for the large oceanic intraplate earthquakes and find that the rupture orientation is usually unaligned with encompassing fossil fracture zones.

Anisotropic Horizontal Thermal Contraction of Young Oceanic Lithosphere Inferred From Stress Release Due To Oceanic Intraplate Earthquakes

How freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information, because it reflects the boundary condition of the oceanic lithosphere, which includes information regarding the magnitude of driving/resisting forces of plate tectonics. We investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes (OCEQs) and numerical simulations. The stress release due to OCEQs in young oceanic lithosphere (5–15 Ma) shows significant differences between the spreading directional component and the ridge‐parallel component. The extensional stress release of the ridge‐parallel component is 6 times as large as that of the spreading directional component, while the compressional stress release of the ridge‐parallel component is one seventh that of the spreading directional component. We conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge‐parallel direction can explain the observed anisotropic stress release. The result indicates that young oceanic lithosphere (5–15 Ma) barely contracts in the ridge‐parallel direction (only 0–30% of the spreading directional contraction rate), while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low‐viscosity asthenosphere. From the results, we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0.44 MPa.

Strain rate dependency of oceanic intraplate earthquake b‐values at extremely low strain rates

AbstractWe discovered a clear positive dependence of oceanic intraplate earthquake (OCEQ) b‐values on the age of the oceanic lithosphere. OCEQ b‐values in the youngest (<10 Ma) oceanic lithosphere are around 1.0, while those in middle to old (>20 Ma) oceanic lithosphere exceed 1.5, which is significantly higher than the average worldwide earthquake b‐value (around 1.0). On the other hand, the b‐value of intraplate earthquakes in the Ninety East‐Sumatra orogen, where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere, is 0.93, which is significantly lower than the OCEQ b‐value (about 1.9) with the same age (50–110 Ma). Thus, the variation in b‐values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure. We revealed a negative strain rate dependency of the b‐value at extremely low strain rates (<2 × 10−10/year), which can clearly explain the above b‐values. We propose that the OCEQ b‐value depends strongly on strain rate (either directly or indirectly) at extremely low strain rates. The high OCEQ b‐values (>1.5) in oceanic lithosphere >20 Ma old imply that future improvement in seismic observation will capture many smaller magnitude OCEQs, which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics.

The 2014Mw 6.1 Bay of Bengal, Indian Ocean, Earthquake: A Possible Association with the 85° E Ridge

Oceanic intraplate earthquakes are known to occur either on active ridge-transform structures or by reactivation of their inactive counterparts, generally referred to as fossil ridges or transforms. The Indian Ocean, one of the most active oceanic intraplate regions, has generated large earthquakes associated with both these types of structures. The moderate earthquake that occurred on 21 May 2014 (M-w 6.1) in the northern Bay of Bengal followed an alternate mechanism, as it showed no clear association either with active or extinct ridge-transform structures. Its focal depth of >50 km is uncommon but not improbable, given the similar to 90 Ma age of the ocean floor with 12-km-thick overlying sediments. No tectonic features have been mapped in the near vicinity of its epicenter, the closest being the 85 degrees E ridge, located similar to 100 km to its west, hitherto regarded as seismically inactive. The few earthquakes that have occurred here in the past are clustered around its southern or northern limits, and a few are located midway, at around 10 degrees N. The 2014 earthquake, sourced close to the northern cluster, seems to be associated with a northwest-southeast-oriented fracture, located on the eastern flanks of the 85 degrees E ridge. If this causal association is possible, we believe that reactivation of fossil hotspot trails could be considered as another mechanism for oceanic intraplate seismicity.

A seismotectonic study of the 21 May 2014 Bay of Bengal intraplate earthquake: evidence of onshore-offshore tectonic linkage and fracture zone reactivation in the northern Bay of Bengal

The earthquake of 21 May 2014 (Mw 6.0) in the northern Bay of Bengal (BOB) highlights the importance of studies on intraplate earthquakes in the oceanic regime for understanding the stress state of the oceanic lithosphere. The epicenter of the earthquake is located at a water depth of 2.5 km where the sediment thickness is nearly 12 km, and it occurs at a depth of ~50 km within the upper mantle. Its location on the seismotectonic map of the region shows that the epicenter is far from the seismically active zone of the Burmese Arc in the east and low-to-moderately active seismic region of the east coast of India in the west. The fault plane solution of this earthquake indicates that it was a strike-slip event with a right-lateral sense of motion on a NW-oriented nodal plane, and it occurred on one of the NW-SE-trending fracture zones previously mapped in the BOB. Based on a compilation of long-term (1900–2011) intraplate earthquakes along with available focal mechanisms in the BOB and the adjoining east coast of India, we conclude the following: (1) the Precambrian structural trends, basin-scale faults and minor lineaments on the east coast of India are favorably reactivated in their offshore extensions up to the shelf-slope areas of the margin; (2) earthquake occurrences in the BOB region can be correlated with the fracture zone trends in the central BOB and along the Ninetyeast ridge or at the intersections of fracture zones with the subsurface trace of the 85°E ridge. The 21 May 2014 earthquake is the result of reactivation of such a NW-SE-trending fracture zone lying in the lithosphere of >100 Ma in age. Further evaluation of this event in light of the global occurrence of oceanic intraplate earthquakes in the older lithosphere (>80 Ma) suggests that such reactivation is possible in the high ambient stress state.

Apparent Slow Oceanic Transform Earthquakes Due to Source Mechanism Bias

Slow earthquakes, characterized as ones whose magnitude increases with period, are often observed occurring on oceanic transform faults (Kanamori and Stewart 1976; Okal and Stewart 1982; Stein and Pelayo 1991; Choy and Boatwright 1995; Newman and Okal 1998; Perez-Campos et al. 2003). Previous work has found that oceanic transform earthquakes often have lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This phenomenon might result from the slow events having lower stress drop. Alternatively, the effect could be due to a source mechanism bias resulting from the fact that transform earthquakes are primarily strike-slip events on steeply dipping faults. In this geometry, body waves arriving at teleseismic distances left the source near nodal planes, resulting in smaller amplitudes than for dip-slip events of the same moment. Slow earthquakes are ones that release energy more “slowly” than typical earthquakes and therefore radiate more energy at longer periods. Such earthquakes have been identified in various ways. One is from an anomalous ratio of the body wave magnitude mb , which reflects the energy release at a period of 1 s, to the surface wave magnitude MS , which reflects the energy release at a period of 20 s. Although slow earthquakes have been observed in various tectonic environments, they are especially common for oceanic transform earthquakes that have strike-slip mechanisms, as shown by analysis of a global catalog of oceanic earthquakes (Stein and Pelayo 1991). The transform earthquakes generally had lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This effect is illustrated in Figure 1A, where a transform event with a moment larger than the ridge event shown has lower mb than …

Re-visiting the tectonics of the oceanic intraplate earthquake of March 25, 1998 (M 8.1) in the Balleny sea, offshore Antarctica

The Antarctic plate earthquake of March 25, 1998, M 8.1, and its aftershocks, are re-visited, to evaluate the role of the associated tectonics in stress concentration due to ambient horizontal crustal motion caused by deglaciation in Antarctica. Using a numerical technique called the “Distinct Element Method”, a two-dimensional, structural model of the epicentral region is developed, and tectonically loaded for a year along the inferred direction of the motion. A spatial correlation is observed between the resulting locations of relatively high shear stresses, and those of the main shock and associated aftershocks. In particular, seismicity is concentrated at and near the intersections of the main fault, Carey and Gambier fracture zones, where the shear stresses are significantly elevated, and whose sign duplicate the inferred sense of movement along them. The modeling results suggest that stress concentrations near pre-existing, intersecting fault and fracture zones, due to a possible deglaciation induced horizontal crustal force, may have been a cause of this enigmatic earthquake.

Recent oceanic intraplate earthquake in Balleny Sea was largest ever detected

The largest oceanic intraplate earthquake ever detected and the largest event worldwide since 1994 occurred in a remote oceanic region near the Balleny Islands on March 25, 1998 (03:12:26 UT; Mw 8.2). An intraplate earthquake of this magnitude is extremely rare, especially for the Antarctic plate, which shows low seismicity.The earthquake occurred about 300 km west of the Balleny Transform between the Antarctic passive margin and the Australian‐Antarctic spreading center (Figure 1). The best double‐couple mechanism from the Harvard Centroid Moment Tensor (CMT) solution shows strike‐slip faulting with nodal planes trending north‐south and east‐west (Figure 1). The CMT solution also shows a large, compensated linear vector dipole (CLVD) component that can be interpreted as indicating northwest‐southeast extension.

Viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes

A sequence of large interplate earthquakes from 1952 to 1965 along the Aleutian arc and Kurile-Kamchatka trench released accumulated stresses along nearly the entire northern portion of the Pacific Plate boundary. The postseismic stress evolution across the northern Pacific and Arctic basins, calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10(17) pascal seconds, is consistent with triggering of oceanic intraplate earthquakes, temporal patterns in seismicity at remote plate boundaries, and space-based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30-year period after the sequence.

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.

Constraints on the Central Indian Basin thermal structure from heat flow, seismicity and bathymetry

The lithosphere beneath the Central Indian Basin is characterized by high heat flow, widespread deformation of sediment and acoustic basement, and unusually high seismicity. The high heat flow suggests that temperatures in the lithosphere may be higher than expected for its age. We explore the constraints on the temperature distribution with depth by combining heat flow, bathymetry, and seismicity data. The high heat flow (~30 mW m −2 greater than expected) constraints the near-surface temperature distribution. The bathymetric depths (on average those expected for these lithospheric ages after corrections for sediment loads) constrain the integral of the temperature with depth. Since the depths of oceanic intraplate earthquakes appear to be limited by an isotherm (~ 750°C), the maximum earthquake depth (40 km) constrains the minimum depth of deformation and the maximum temperature there. The present thermal structure is investigated by examining different models matching these three observational constraints. First, the possibility is examined that the high heat flow results from anomalous lithosphere with a basal temperature or thickness different from typical oceanic lithosphere. In this case the temperature constraints from the earthquake depths restrict the additional heat flux to less than 20 mW m −2. However, since the heat flux anomaly requires plate thicknesses substantially thinner than usually assumed, it appears that at most 10 mW m −2 can be added by this mechanism. Second, the possibility is examined that the extra flux results from reheating the bottom portion of the lithosphere to asthenospheric temperatures. Substantial reheating 30–40 km below the surface is required to match the high heat flow. Such reheating should produce $ ̃ 1 km of uplift, whereas the average basement depth is no shallower than expected for its age. Thus, significant deep lithospheric reheating cannot be widespread over the deformed region. Third, the effects of a temperature perturbation within the lithospheric column and the resulting change in surface heat flux with time are examined. A temperature perturbation at shallow depths could produce the present heat flow anomaly, but no evidence for shallow intrusion or other mechanism for such an effect exists. It is concluded that, despite the heat flow anomaly, the lack of an average bathymetric anomaly and the observation of seismicity to a depth of 40 km indicate that lithospheric temperatures in the Central Indian Basin are not significantly different from those expected for its age.

Scaling relationship between the number of aftershocks and the size of the main shock.

The logarithmic number of aftershocks which occur within one month after a large shallow earthquake in Japan is found to be proportional to the logarithm of the seismic moment of the main shock. The proportional constants for interplate and intraplate earthquakes are different, but it turns out to reflect a similar difference in scaling relations of the fault area to the seismic moment between interplate and intraplate earthquakes. Thus we can derive the fundamental relationship that the aftershock number is proportional to the fault area of the main shock. This is consistent with the hypothesis that aftershocks are generated by unbroken strong patches on the main-shack fault, if the patch density is constant. Combining this new result with the results of previous studies on aftershocks, we propose that a rate of aftershock occurrence is given by where n(t) indicates the number of aftershocks which occur t days after the main shock, S indicates the fault area of the main shock in km2, Mth is the threshold magnitude, b is the b-value of the. Gutenberg-Richter relationship, and c and p are the c- and p-values, respectively, of the modified Omori formula. The constant k amounts to 13.4 for an interplate earthquake and to 31.7-63.5 for an intraplate earthquake depending on the assumption on its fault width. Apparently the areal density of aftershocks on the fault plane is higher for an intraplate earthquake than for an interplate earthquake. Further examination of the results suggests that the areal density of the aftershocks of a continental intraplate earthquake may be systematically higher than that of an oceanic intraplate earthquake. These differences may indicate some intrinsic difference in rupturing process among a continental intraplate, an oceanic intraplate, and an interplate events.

Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting

We have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid‐Atlantic Ridge from an inversion of long‐period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15°20′, and Vema transforms. We have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width. For five of the transforms, earthquake centroid depths lie in the range 7–10 km beneath the seafloor, and the maximum depth of seismic faulting is 14–20 km. On the basis of a comparison with a simple thermal model for transform faults, this maximum depth of seismic behavior corresponds to a nominal temperature of 900° ± 100°C. In contrast, the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike‐slip mechanisms is 700° ± 100°C. The difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments. Three large earthquakes on the 15°20′ transform have shallower centroid depths of 4–5 km and a maximum depth of seismic faulting of 10 km, corresponding to a limiting temperature of 600°C. The shallower extent of seismic behavior along the 15°20′ transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among North American, South American, and African plates to its present position near the transform. The source mechanisms for all events in this study display the strike‐slip motion expected for transform fault earthquakes; slip vector azimuths agree to within 2°–3° of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike‐slip rupture, was observed for five of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone. We find no evidence to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere.

Intraplate earthquakes and the state of stress in oceanic lithosphere

The dominant sources of stress relieved in oceanic intraplate earthquakes are investigated to examine the usefulness of earthquakes as indicators of stress orientation. The primary data for this investigation are the detailed source studies of 58 of the largest of these events, performed with a body-waveform inversion technique of Nabelek (1984). The relationship between the earthquakes and the intraplate stress fields was investigated by studying, the rate of seismic moment release as a function of age, the source mechanisms and tectonic associations of larger events, and the depth-dependence of various source parameters. The results indicate that the earthquake focal mechanisms are empirically reliable indicators of stress, probably reflecting the fact that an earthquake will occur most readily on a fault plane oriented in such a way that the resolved shear stress is maximized while the normal stress across the fault, is minimized.

Thermal stress and the spacings of transform faults

Bathymetric charts are used with satellite altimeter profiles to locate major ridge‐transform intersections along five spreading ridges. The ridges are the Mid‐Atlantic Ridge, the East Pacific Rise, the Chile Rise, the Pacific‐Antarctic Rise, and the Southeast Indian Ridge. Analysis of these data show spacings between transform faults W increase linearly with spreading rate ν (W/ν = 6.28 m.y.). This linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate ν. The traction‐free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults. A two‐dimensional thermoelastic solution for the in‐plane stress shows the largest stress component is tensional and parallel to the ridge. Stresses are zero at the ridge and increase as (age)½ to a maximum value at an age of W/4ν. All stress components are small for ages greater than W/ν. When the transform spacing is large compared with the spreading rate (W/ν > 100 m.y.) thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 Ma. The observed maximum ratio of transform spacing to spreading rate (W/ν = 10 m.y.) results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 Ma. Thus transform faults relieve most of the thermal stress. Model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge. Moreover, oceanic intraplate earthquakes rarely occur in lithosphere older than 30 Ma as predicted by the model. These and other geophysical observations confirm Turcotte's hypothesis that transform faults are thermal contraction cracks.

Mechanisms and depths of Atlantic transform earthquakes

We have studied 40 earthquakes on seven major transforms along the Mid‐Atlantic Ridge. The majority show the expected transform‐parallel strike‐slip motion on steeply dipping fault planes. Some “anomalous” mechanisms, suggesting either unusually shallow dip or a component of dip‐slip motion, occur near ridge‐transform intersections. The vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform. Centroid depths, determined by waveform inversion, are limited to a shallow zone corresponding approximately to the area above the 400°C isotherm of the thermal structure expected from lithospheric cooling. The depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600°C isotherm. Assuming that the areas above the 400°C or 600°C isotherms fail seismically, moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion, suggesting that some aseismic slip occurs if the time period sampled is representative. In contrast, on the Vema Transform the inferred seismic slip is greater than predicted unless faulting extends to the 650°C isotherm. Alternatively, the Vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history. In contrast to transform events, centroids for oceanic intraplate earthquakes extend to approximately the 750°C isotherm. This result is intriguing if depths reflect lithospheric strength as a function of depth. Transforms have much higher strain rates than intraplate areas, so if both had the same rheology the limiting isotherm for transforms should be higher. Alternatively, if a weaker rheology were appropriate, the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra‐plate regions. Transforms may thus be either weaker and/or have higher temperatures than expected. In the latter model the required high temperatures, perhaps due to intrusion at depth, predict measurable heat flow anomalies. The second possibility, unusual rheological weakness, is reminiscent of the unexplained weakness of the San Andreas fault, where heat flow data exclude anomalously high temperatures.

Earthquake source mechanisms from body-waveform inversion and intraplate tectonics in the northern Indian Ocean

Double-couple point-source parameters for 11 of the largest intraplate earthquakes in the northern Indian Ocean during the last 20 y were determined from a formal inversion of long-period P and SH waveforms. Nine of the events have centroid depths at least 17 km below the seafloor, well into the upper mantle; two have centroid depths as great as 39 km. Using the source mechanisms of these earthquakes, we distinguish two major intraplate tectonic provinces in the northern Indian Ocean. To the west of the Ninetyeast Ridge, in the southern Bay of Bengal, intraplate earthquakes have thrust-faulting mechanisms with P axes oriented N-S. The centroid depths of these earthquakes range from 27 to 39 km below the seafloor. Lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle, while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding. In the immediate vicinity of the Ninetyeast Ridge, earthquakes display strike-slip mechanisms with left-lateral motion on planes parallel to the ridge. This type of faulting occurs from at least 10°S to the northern end of the Ninetyeast Ridge near 10°N, where the ridge meets the Sunda Arc. Seismic activity diminishes to the east of the Ninetyeast Ridge, but is also characterized by strike-slip faulting. Despite these variations in deformational style, the inferred orientation of greatest compressive stress in the northern Indian Ocean displays a consistent long-wavelength pattern over a large portion of the Indian plate, varying smoothly from nearly N-S in the Bay of Bengal to NW-SE in the northeastern Indian Ocean. This plate-wide stress pattern and the high level of intraplate seismicity in the northern Indian Ocean are likely the results of substantial resistance, along the Himalayan continental collision zone, to the continued northward motion of the western portion of the Indian plate. Oceanic intraplate earthquakes in other regions, where the level of deviatoric stress associated with the long-wavelength part of the stress field is likely to be smaller, need not be comparably reliable indicators of the plate-wide stress field.

Source modeling of the Hsingtai, China earthquakes of March 1966

The Hsingtai, China earthquakes of March 1966 were a series of destructive earthquakes associated with the Shu-lu graben. Five strong shocks of M s ≥ 6 occurred within a period of less than a month, the largest of which was M s 7.2. Body and surface waves over the period range from several to 100 s have been modeled for the four largest events using synthetic seismograms in the time domain and spectral analysis in the frequency domain. Data from ground deformation, local geology, regional seismic network, and teleseismic joint epicenter determination have also been used to constrain the source model and the rupture process. The fault mechanism of the Hsingtai sequence was mainly strike-slip with a small component of normal dip-slip. The strikes of the four largest shocks range from ∼ N26° to 30°E, approximately along strike of the major faults of the Shu-lu graben and the aftershock distribution. The source mechanisms can be explained with a NNW-SSE extensional stress and a NEE-SWW compressional stress acting in the area. The major shocks all had focal depths ∼ 10 km. The four largest shocks in the sequence were characterized by a relatively simple and smooth dislocation time history. The durations of the far-field source time functions ranged from 3.5 to 5 s, while the rise times were all ∼ 1 s. The seismic moments of the four largest earthquakes ranged from 1.43 × 10 25 to 1.51 × 10 26 dyne cm −1. The fault sizes of the four events were very close. Assuming circular faults, the diameters of the four events were determined to be between 10 and 14 km. Stress drops varied from ∼ 52 to 194 bars. A trend of increasing stress drop with earthquake size was observed. A survey of stress drop determinations for 15 major intraplate earthquakes shows that on the average the magnitude of stress drop of oceanic intraplate earthquakes and passive continental margin events is higher (∼ 200 to several hundred bars) than that of continental intraplate earthquakes (∼ 100 bars or less).

Age dependence of oceanic intraplate seismicity and implications for lithospheric evolution

We have determined depths for 16 oceanic intraplate earthquakes using body wave modeling. A data set composed of these and 11 other well‐constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700°–800°C isotherm. The maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion. The maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics. Oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events, presumably because oceanic lithosphere is stronger at high temperatures. Both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age. The cause of this trend is unclear: possibilities include nonrandom location of preexisting weak zones with respect to the age provinces, fault strengthening with age, or a decrease in the stress level. A decrease in intraplate stress with age is opposite that expected from “ridge push” stresses, which increase with age, but may result either from thermoelastic stresses which are greatest in young lithosphere or near‐ridge stress concentrations due to local heterogeneities in the spreading process.

Isostasy and the ambient state of stress in the oceanic lithosphere

Two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium. The first is that the stress is hydrostatic below some depth, the level of compensation, and the second is that above that depth there are no vertical shear stresses. In the case of a planar two‐dimensional mid‐ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge, but to determine that component an additional assumption is required. It is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid‐ocean ridges, the deviatoric stress in the oceanic lithosphere is a minimum. The unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid‐ocean ridges with a magnitude in old ocean basins of a few hundred bars, in agreement with oceanic intraplate earthquake mechanisms.At any given distance from the ridge the stress is maximum at the seafloor, and it decreases with depth at about the same rate as the cooling of the lithosphere, so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans.