Tectonic Moment Research Articles

Seismogenic depth and seismic coupling estimation in the transition zone between Alps, Dinarides and Pannonian Basin for the new Slovenian seismic hazard model

Abstract. Seismogenic depth and seismic coupling are important inputs into seismic hazard estimates. Although the importance of seismic coupling is often overlooked, it significantly impacts seismic hazard results. We present an estimation of upper and lower seismogenic depth and expected hypocentral depth and seismic coupling in the transition zone between the Alps, Dinarides and Pannonian Basin, characterized by a complex deformation pattern, highly variable crustal thickness, and moderate seismic hazard, supporting the development of the 2021 seismic hazard model of Slovenia. The hazard model was based on three seismic source models: area source model, fault source model and smoothed seismicity (point) source model. We estimated the lower seismogenic depth using seismological and geological data and compared them. The seismological estimate was based on two regional earthquake catalogues prepared for this study. In the area source model, estimates of lower seismogenic depth from seismological data are deeper or equal to the ones derived from geological data, except in one case. In the fault source model, we analysed each fault individually and chose seismological lower depth estimates in 12 among 89 faults as more representative. The seismogenic thickness for each individual fault source was determined for seismic coupling determination. The seismic coupling was assessed by two approaches, i.e. we chose the most trusted value from the literature, and the value determined for each fault individually by using the approach based on the updated regional fault and earthquake data sets. The final estimate of seismic coupling ranges from 0.77 to 0.38. We compared the tectonic moment rate based on long-term slip rate using different values of seismic coupling with the seismic moment rate obtained from the earthquake catalogue. The analysis is done for the whole area, as well as for the individual area zones. The analysis of N–S components of estimated slip for the largest faults in the area of west Slovenia shows that the regional geological and geodetic shortening rates are comparable. The total activity rate of three global seismic source models is compared, which gives up to a 10 % difference. Our results contribute to a better understanding of the seismic activity in the region. The presented approach for seismic coupling estimation can be applied in cases where the total slip rate is given instead of its seismic part and can be used at regional or national level. The approach is also suitable for the cross-border harmonization of the European seismic hazard modelling data.

Maximum magnitude of subduction earthquakes along the Japan-Kuril-Kamchatka trench estimated from seismic moment conservation

SUMMARYWe estimated the maximum magnitude of earthquakes in the Japan-Kuril-Kamchatka trench subduction zone with a method based on the conservation of seismic moment and the record of interplate seismicity from 1977 to 2017. The key point of this method is to base calculations on the tectonic moment rate instead of the total seismic moment rate. We modeled a seismic-moment-frequency distribution for the Japan-Kuril-Kamchatka trench on the basis of the truncated Gutenberg–Richter (G–R) law, the formula published by Utsu in 1974, the gamma distribution, and the tapered G–R law. We estimated the maximum magnitude along the Japan-Kuril-Kamchatka trench as ∼10 under the truncated G–R law and ∼11 under Utsu's formula, although the latter may be an overestimate. Therefore, the 2011 Tohoku earthquake, of moment magnitude 9.2, may not be the largest possible event in this area. The recurrence interval for magnitude 10 events based on the truncated G–R law is 4000 yr. Although these two models perform equally well in terms of Akaike Information Criterion, the range of the 95 per cent confidence level is consistently narrower for the truncated G–R law than for Utsu's formula. The estimated maximum magnitude depends not only on the model used, but also on the parameters that constitute the tectonic moment. It is essential to accumulate more seismic data and achieve more precise estimates of tectonic moment to improve estimates of maximum magnitude.

Induced Seismicity in Western Canada Linked to Tectonic Strain Rate: Implications for Regional Seismic Hazard

AbstractA rapid increase of injection‐induced earthquakes (IIE) is often linked to a higher level of seismic hazard. In this study, we compare the geodetically defined moment rate to seismicity distribution in western Canada where significant IIE are observed. The regional seismic pattern is dominated by IIE, both in number and moment, along a 150‐km wide NW‐SE band of moderate strain rate in the easternmost Cordillera and foothills. The observed rate of moment release from local earthquakes is much closer to the tectonic moment rate in the IIE‐dominated areas. We conclude that, on a regional scale, tectonic strain rate is an important control on IIE. Injection in areas with moderate tectonic strain may temporarily increase the local seismic hazard, but widespread IIE over an extended period of time may deplete the available tectonic moment and could, under the right conditions, have a limited long‐term effect of reducing regional seismic hazard.

Earthquake rate and magnitude distributions of great earthquakes for use in global forecasts

We have obtained new results in the statistical analysis of global earthquake catalogues with special attention to the largest earthquakes, and we examined the statistical behaviour of earthquake rate variations. These results can serve as an input for updating our recent earthquake forecast, known as the ‘Global Earthquake Activity Rate 1’ model (GEAR1), which is based on past earthquakes and geodetic strain rates. The GEAR1 forecast is expressed as the rate density of all earthquakes above magnitude 5.8 within 70km of sea level everywhere on earth at 0.1 × 0.1 degree resolution, and it is currently being tested by the Collaboratory for Study of Earthquake Predictability. The seismic component of the present model is based on a smoothed version of the Global Centroid Moment Tensor (GCMT) catalogue from 1977 through 2013. The tectonic component is based on the Global Strain Rate Map, a ‘General Earthquake Model’ (GEM) product. The forecast was optimized to fit the GCMT data from 2005 through 2012, but it also fit well the earthquake locations from 1918 to 1976 reported in the International Seismological Centre-Global Earthquake Model (ISC-GEM) global catalogue of instrumental and pre-instrumental magnitude determinations. We have improved the recent forecast by optimizing the treatment of larger magnitudes and including a longer duration (1918–2011) ISC-GEM catalogue of large earthquakes to estimate smoothed seismicity. We revised our estimates of upper magnitude limits, described as corner magnitudes, based on the massive earthquakes since 2004 and the seismic moment conservation principle. The new corner magnitude estimates are somewhat larger than but consistent with our previous estimates. For major subduction zones we find the best estimates of corner magnitude to be in the range 8.9 to 9.6 and consistent with a uniform average of 9.35. Statistical estimates tend to grow with time as larger earthquakes occur. However, by using the moment conservation principle that equates the seismic moment rate with the tectonic moment rate inferred from geodesy and geology, we obtain a consistent estimate of the corner moment largely independent of seismic history. These evaluations confirm the above-mentioned corner magnitude value. The new estimates of corner magnitudes are important both for the forecast part based on seismicity as well as the part based on geodetic strain rates. We examine rate variations as expressed by annual earthquake numbers. Earthquakes larger than magnitude 6.5 obey the Poisson distribution. For smaller events the negative-binomial distribution fits much better because it allows for earthquake clustering.

Magnitude Limits of Subduction Zone Earthquakes

Maximum earthquake magnitude ( m x ) is a critical parameter in seismic hazard and risk analysis. However, some recent large earthquakes have shown that most of the existing methods for estimating m x are inadequate. Moreover, m x itself is ill‐defined because its meaning largely depends on the context, and it usually cannot be inferred using existing data without associating it with a time interval. In this study, we use probable maximum earthquake magnitude within a time period of interest, m p( T ), to replace m x . The term m p( T ) contains not only the information of magnitude limit but also the occurrence rate of the extreme events. We estimate m p( T ) for circum‐Pacific subduction zones using tapered Gutenberg–Richter (TGR) distributions. The estimation of the two TGR parameters, β ‐value and corner magnitude ( m c), is performed using the maximum‐likelihood method with the constraint from tectonic moment rate. To populate the TGR, the rates of smaller earthquakes are needed. We apply the Whole Earth model, a high‐resolution global estimate of the rate of m ≥5 earthquakes, to estimate these rates. The uncertainties of m p( T ) are calculated using Monte‐Carlo simulation. Our results show that most of the circum‐Pacific subduction zones can generate m ≥8.5 earthquakes over a 250‐year interval, m ≥8.8 earthquakes over a 500‐year interval, and m ≥9.0 earthquakes over a 10,000‐year interval. For the Cascadia subduction zone, we include the 10,000‐year paleoseismic record based on turbidite studies to supplement the limited instrumental earthquake data. Our results show that over a 500‐year period, m ≥8.8 earthquakes are expected in this zone; over a 1000‐year period, m ≥9.0 earthquakes are expected; and over a 10,000‐year period, m ≥9.3 earthquakes are expected.

Studies on seismic source

During the period of 1999~2002, the Chinese seismologists made a serious of developments in the study on seismic sources including observations, experiments and theory. In the field of observation, the methods of the accuracy location of earthquake sources, the inversion of seismic moment tensor and the mechanism of earthquake source are improved and developed. A lot of important earthquake events are studied by using these methods. The rupture processes of these events are inverted and investigated combined with the local stress fields and the tectonic moment by using the measurements of surface deformation. In the fields of experiments and theory, many developments are obtained in cause of seismic formation, condition of stress and tectonics, dynamics of earthquake rupture, rock fracture and nucleation of strong earthquakes.

Perceptible earthquakes in the broad Aegean area

A probabilistic estimate of seismic hazard can be obtained from the spatial distribution, of earthquake sources, their frequency–magnitude distribution and the rate of attenuation of strong ground motion with distance. We calculate the earthquake perceptibility, i.e. the annual probability that a particular level of ground shaking will be generated by earthquakes of particular magnitude, by weighting frequency–magnitude data with the predicted felt area for a given level of ground shaking at a particular magnitude. This provides an earthquake selection criterion that can be used in the anti-seismic design of non-critical structures. We calculate the perceptibility, at a particular value of isoseismal intensity, peak ground acceleration and velocity, as a function of source magnitude and frequency for the broad Aegean area using local attenuation laws. We use frequency–magnitude distributions that were previously obtained by combining short-term catalogue data with tectonic moment rate data for 14 tectonic zones in Greece with sufficient earthquake data, and where contemporary strain rates are available from satellite data. Many of the zones show a ‘characteristic earthquake’ distribution with the most perceptible earthquake equal to the maximum magnitude earthquake, but a relatively flat perceptibility between magnitudes 6 and 7. The maximum perceptible magnitude is in the fastest-deforming region in the middle of the Aegean sea, and tends to be systematically low on the west in comparison to the east of the Aegean sea. The tectonic data strongly constrain the long-term recurrence rates and lead to low error estimates (±0.2) in the most perceptible magnitudes.

An integrated global model of present-day plate motions and plate boundary deformation

In this paper we present a global model (GSRM-1) of both horizontal velocities on the Earth's surface and horizontal strain rates for almost all deforming plate boundary zones. A model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field. In our model we perform a least-squares fit between model velocities and observed geodetic velocities, as well as between model strain rates and observed geological strain rates. Model velocities and strain rates are interpolated over a spherical Earth using bi-cubic Bessel splines. We include 3000 geodetic velocities from 50 different, mostly published, studies. Geological strain rates are obtained for central Asia only and they are inferred from Quaternary fault slip rates. For all areas where no geological information is included a priori constraints are placed on the style and direction (but not magnitude) of the model strain rate field. These constraints are taken from a seismic strain rate field inferred from (normalized) focal mechanisms of shallow earthquakes. We present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones. Generally, the strain rate tensor field is consistent with geological and seismological data. With the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs. We find that in general there is a good agreement between the present-day plate motions we obtain and long-term plate motions, but a few significant differences exist. The rotation rates for the Indian, Arabian and Nubian plates relative to Eurasia are 30, 13 and 50 per cent slower than the NUVEL-1A estimate, respectively, and the rotation rate for the Nazca Plate relative to South America is 17 per cent slower. On the other hand, Caribbean–North America motion is 76 per cent faster than the NUVEL-1A estimate. While crustal blocks in the India–Eurasia collision zone move significantly and self-consistently with respect to bounding plates, only a very small motion is predicted between the Nubian and Somalian plates. By integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in, for instance, redefining a no-net-rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation.

Maximum earthquake magnitudes in the Aegean area constrained by tectonic moment release rates

SUMMARY Seismic moment release is usually dominated by the largest but rarest events, making the estimation of seismic hazard inherently uncertain. This uncertainty can be reduced by combining long-term tectonic deformation rates with short-term recurrence rates. Here we adopt this strategy to estimate recurrence rates and maximum magnitudes for tectonic zones in the Aegean area. We first form a merged catalogue for historical and instrumentally recorded earthquakes in the Aegean, based on a recently published catalogue for Greece and surrounding areas covering the time period 550 BC‐2000 AD, at varying degrees of completeness. The historical data are recalibrated to allow for changes in damping in seismic instruments around 1911. We divide the area up into zones that correspond to recent determinations of deformation rate from satellite data. In all zones we find that the Gutenberg‐Richter (GR) law holds at low magnitudes. We use Akaike’s information criterion to determine the best-fitting distribution at high magnitudes, and classify the resulting frequency‐magnitude distributions of the zones as critical (GR law), subcritical (gamma density distribution) or supercritical (‘characteristic’ earthquake model) where appropriate. We determine the ratio η of seismic to tectonic moment release rate. Low values of η ( 1.0). The seismic and tectonic moment release rates are then combined to constrain recurrence rates and maximum credible magnitudes (in the range 6.7‐ 7.6 m W where the results are well constrained) based on extrapolating the short-term seismic data. With current earthquake data, many of the tectonic zones show a characteristic distribution that leads to an elevated probability of magnitudes around 7, but a reduced probability of larger magnitudes above this value when compared with the GR trend. A modification of the generalized gamma distribution is suggested to account for this, based on a finite statistical second moment for the seismic moment distribution.

Constraints on the frequency-magnitude relation and maximum magnitudes in the UK from observed seismicity and glacio-isostatic recovery rates

Earthquake populations have recently been shown to have many similarities with critical-point phenomena, with fractal scaling of source sizes (energy or seismic moment) corresponding to the observed GutenberG-Richter (G-R) frequency—magnitude law holding at low magnitudes. At high magnitudes, the form of the distribution depends on the seismic moment release rate M˙ and the maximum magnitude mmax. The G-R law requires a sharp truncation at an absolute maximum magnitude for finite M˙ . In contrast, the gamma distribution has an exponential tail which allows a soft or ‘credible’ maximum to be determined by negligible contribution to the total seismic moment release. Here we apply both distributions to seismic hazard in the mainland UK and its immediate continental shelf, constrained by a mixture of instrumental, historical and neotectonic data. Tectonic moment release rates for the seismogenic part of the lithosphere are calculated from a flexural-plate model for glacio-isostatic recovery, constrained by vertical deformation rates from tide-gauge and geomorphological data. Earthquake focal mechanisms in the UK show near-vertical strike-slip faulting, with implied directions of maximum compressive stress approximately in the NNW-SSE direction, consistent with the tectonic model. Maximum magnitudes are found to be in the range 6.3–7.5 for the G-R law, or 7.0–8.2 mL for the gamma distribution, which compare with a maximum observed in the time period of interest of 6.1 mL. The upper bounds are conservative estimates, based on 100 per cent seismic release of the observed vertical neotectonic deformation. Glacio-isostatic recovery is predominantly an elastic rather than a seismic process, so the true value of mmax is likely to be nearer the lower end of the quoted range.

Seismic moment distribution revisited: II. Moment conservation principle

SUMMARY We compare the tectonic rate with earthquake data using several theoretical distributions to approximate the seismic moment‐frequency relation. We derive formulae to estimate parameters of the seismic moment distributions by comparing the earthquake occurrence rate with tectonic strain. We analyse the tectonic moment rate and the earthquake moment distribution for several types of tectonic environments, including subduction zones, plate bounding transform faults and deforming continental regions. In this analysis we use finer subdivisions of tectonic regions to infer whether earthquake size distribution properties established previously for large provinces (Flinn‐Engdahl seismic regions) could be applied to smaller zones extending over a few hundred kilometres. Although the shortness of the available earthquake catalogues makes our results less reliable, several conclusions can be drawn from the analysis. The universality of the β-parameter (the slope of the moment‐frequency relation) is reasonably well confirmed for all of these tectonic regions and their subdivisions. In each region, we estimate the strain rate or relative plate motion from a plate tectonic model, active fault data or geodetic data. According to the moment‐frequency relation, the ratio of the tectonic moment rate to the earthquake rate depends on the β value, the corner magnitude (mc) and the effective seismogenic thickness. We find that this ratio does not vary systematically with the relative plate velocity, the ratio of parallel to perpendicular plate motion, time since the last major earthquake and the length of fault zone segments established by previous earthquake history. We obtain similar results for the western United States and southeast Asia; the corner magnitude is approximately equal to that for the circum-Pacific Rim and there is no systematic dependence of the mc parameter on a geographic region or strain rate. Assuming the commonly accepted values for seismogenic thickness, elastic modulus and 100 per cent seismic coupling, the corner magnitude values for plate boundary zones and continental areas are 8.3‐8.8, i.e. similar to those values obtained by statistical analysis.

Velocity field in Asia inferred from Quaternary fault slip rates and Global Positioning System observations

We perform a joint inversion of Quaternary strain rates and 238 Global Positioning System (GPS) velocities in Asia for a self‐consistent velocity field. The reference frames for all geodetic velocity observations are determined in our inversion procedure. India (IN) moves relative to Eurasia (EU) about a pole of rotation at (29.78°N, 7.51°E, 0.353° Myr−1), which yields a velocity along the Himalaya within India that is ∼73–76% of the magnitude of the IN‐EU NUVEL‐1A velocity and a vector azimuth that is 8–10° clockwise of NUVEL‐1A IN‐EU vector azimuth. Relative to Eurasia, south China moves at 9–11 mm/yr in the direction 110–120° with a pole position (64.84°N, 156.74°E, 0.12° Myr−1). Amurian block motion has a pole position in a similar location but at a slower rate (64.61°N, 158.23°E, 0.077° Myr−1) and most of the Amurian‐Eurasia motion is accommodated by extension across Lake Baikal. Tarim Basin moves relative to Eurasia about a pole of rotation at (39.24°N, 98.2°E, −0.539° Myr−1) and ∼16–18 mm/yr of shortening is accommodated across the west central Tien Shan. There is distributed E‐W extension throughout both southern and north central Tibet. Within southern Tibet, between the longitudes of 77°E to 92°E, the deformation field accommodates ∼16–19 mm/yr of E‐W extension. We compare predicted seismic moment rates with those observed in this century in Asia. Total observed seismic moment rates within the entire area of central and east Asia (2.2×107 km2) in this century are 2.26±0.7×1020 N m yr−1 as compared with a predicted total rate of 2.03±0.066×1020 N m yr−1. Comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long‐term seismic moment rates from a catalog of limited duration (94 years). An observation period of ∼10,000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates, inferred from the joint inversion of GPS and Quaternary rates of strain. We show that in general, a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude (mb ≥ 5.0, for the period January 1, 1965, to January 1, 1999).

Statistical physics, seismogenesis, and seismic hazard

The scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics. This has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon, involving locally nonlinear dynamics, with simplified rheologies exhibiting instability or avalanche‐type behavior, in a material composed of a large number of discrete elements. In particular, it has been suggested that earthquakes are an example of a “self‐organized critical phenomenon” analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit. In this stationary state of marginal stability the distribution of avalanche energies is a power law, equivalent to the Gutenberg‐Richter frequency‐magnitude law, and the behavior is relatively insensitive to the details of the dynamics. Here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during (1) dynamic slip on a preexisting fault, (2) fault growth, and (3) fault nucleation. The individual physical models share some generic features, such as a dynamic energy flux applied by tectonic loading at a constant strain rate, strong local interactions, and fluctuations generated either dynamically or by fixed material heterogeneity, but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution. However, all exhibit critical or near‐critical behavior, with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes, including the Gutenberg‐Richter law. Some of the results are sensitive to the details of the dynamics and hence are not strict examples of self‐organized criticality. Nevertheless, the results of these different physical models share some generic statistical properties similar to the “universal” behavior seen in a wide variety of critical phenomena, with significant implications for practical problems in probabilistic seismic hazard evaluation. In particular, the notion of self‐organized criticality (or near‐criticality) gives a scientific rationale for the a priori assumption of “stationarity” used as a first step in the prediction of the future level of hazard. The Gutenberg‐Richter law (a power law in energy or seismic moment) is found to apply only within a finite scale range, both in model and natural seismicity. Accordingly, the frequency‐magnitude distribution can be generalized to a gamma distribution in energy or seismic moment (a power law, with an exponential tail). This allows extrapolations of the frequency‐magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates. The answers to other questions raised are less clear, for example, the effect of the a priori assumption of a Poisson process in a system with strong local interactions, and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons. The results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors. However, there remains no consensus, on both theoretical and practical grounds, on the possibility or otherwise of reliable intermediate‐term earthquake prediction.

Empirical Measurements of Tectonic Moment Release In Nuclear Explosions From Teleseismic Surface Waves and Body Waves

SUMMARY A method of source-parameter estimation based principally on observations of teleseismic long-period surface waves is developed and applied to 71 underground nuclear explosions at the Balapan (Shagan River) test site in Kazakhstan. First, we quantify the observed radiation patterns of Love and Rayleigh waves, noting that such data place only three linear constraints on the five parameters of the standard point source model for each explosion. The three constraints are the strengths of the isotropic, sin2@, and cos2Q components of the observed pattern (where @ is azimuth). The five source parameters are the isotropic moment M,; the doublecouple moment Adt,; and the strike, dip and rake angles, (QS, 6, A). These last four parameters characterize the inferred faulting (tectonic release). Surface waves which propagate from different seismic events in a geographically tight cluster to a distant station are similar in shape, and for very shallow sources the propagation operator can be described by a single wave shape, a differential-propagation delay, and a single amplitude factor containing the effects of attenuation, scattering, focusing and geometrical spreading. The inverse problem is formulated which iteratively solves simultaneously for the path-amplitude factors and the three radiation-pattern parameters for all events using observed seismograms as the empirical wave shapes. We fit 1110 relative measurements of Rayleigh- and Love-wave amplitudes to determine path-amplitude factors to 29 globally distributed stations and radiationpattern parameters for the 71 explosions. Second, we present a new method for interpreting such radiation patterns. In previous studies, two of the five source parameters have typically been fixed by making the very restricted assumption of pure dip-slip faulting at 45 dip. The assumption leads to estimates of strike that are mostly in the north-west-south-east direction at Balapan, and to estimates of M, that correlate very poorly with estimates based upon short-period signals (P, Lg). However, now that excellent yield estimates have become available, we can avoid the assumption of pure dip-slip faulting. Instead we can estimate MI from P and Lg-wave data, thus adding a fourth constraint to help in interpreting the teleseismic surface-wave amplitudes. The outcome is a method for estimating the strike as of faulting associated with tectonic release for each explosion, and for estimating the two combinations M,, sin 6 cos A and $Mo sin 26 sin A, which, respectively, are the strike-slip and dip-slip components of the tectonic release. The strike directions of inferred faulting at Balapan are found to lie in the range 90-120, which conforms better with limited geological information on faulting in the region than does the trend that has long been inferred from assuming pure dip-slip faulting. We find that without additional information about individual explosions, we are not able to provide a high-precision isotropic moment or yield estimate from teleseismic surface waves.

Seismotectonics and the earthquake frequency-magnitude distribution in the Aegean area

SUMMARY A study of Aegean seismotectonics and the resulting frequency–magnitude distribution on a broad scale is undertaken, using the tectonic model of Le Pichon & Angelier. This implies a tectonic moment release rate due to the spreading of the Aegean of 17 ± 8 times 1018 N m-1 yr-1 over the past 13 Ma, if stretching is due mainly to a series of normal faults dipping at about 45° in a seismogenic crust 10–20 km deep. The moment-magnitude relation from an instrumental magnitude catalogue is log M0=A+Bm, with A= 10.970 ± 0.294, B= 1.206 in SI units. This is established from 50 events of magnitude Ms>5.3 whose scalar seismic moments M0 have been evaluated in a homogeneous manner for the region by spectral analysis of surface waves at 30–60 s period. By contrast published body-wave analyses, from waveform modelling at 20 s period and moment tensor analyses seem to produce seismic moments which are lower by a factor of two or so on average. This moment–magnitude relation implies a seismogenic moment release rate due to stretching in the Aegean of 14 times 1018 N m-1 yr-1 during the time period 1918–81 and 17 times 1018 N m-1 yr-1 for the period 1964–81, within a factor of two or so uncertainty. This implies that the energy release in the brittle crust due to Aegean spreading is substantially seismic within the stated accuracy, and is also relatively stationary even when compared with the tectonic moment release rates. A similar comparison using the same tectonic model shows that the subducting slab is sinking beneath the Hellenic arc almost completely aseismically, and that processes such as near vertical slab pull and confined thermal expansion could explain most of the observed seismic energy release by internal deformation of the slab. The frequency–magnitude statistics for the area of active extension has a characteristic earthquake of magnitude 7Ms, corresponding approximately to fault depths of 10–14 km for an aspect ratio of 2 and a strain drop of 10-4. This is close to the observed seismogenic depth (10–20 km) for well-determined earthquakes in the Aegean area.