Kamchatka earthquake of 17 August 1983: A large intermediate event at the junction of the Aleutian and Kuril-Kamchatka arcs

This paper gives macroseismic and instrumental data on the 17 August 1983 Kamchatka earthquake which occurred in the center of the Kamchatka Gulf bend at a depth of 98 km with an epicentral intensity of VI–VII (MSK-64 scale), energy class 15, and magnitude MLH = 6.9. The focal mechanism represents a thrust along the inclined surface across the strike of the Kamchatka Gulf coastal line. The Primary-wave seismic moment M ₀ is 6.3 × 10¹⁹ Nm, the Rayleigh wave M ₀ is 1.6 × 10¹⁹ Nm, and the stress drop is 2.5 MPa. Copies of displacement and acceleration records are presented and the temporal and spatial distribution of the aftershocks is analyzed.     

The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications

We have studied the focal mechanisms of the 1980, 1997 and 1998 earthquakes in the Azores region from body-wave inversion of digital GDSN (Global Digital Seismograph Network) and broadband data. For the 1980 and 1998 shocks, we have obtained strike–slip faulting, with the rupture process made up of two sub-events in both shocks, with total scalar seismic moments of 1.9 × 10¹⁹ Nm (M_w = 6.8) and 1.4 × 10¹⁸ Nm (M_w = 6.0), respectively. For the 1997 shock, we have obtained a normal faulting mechanism, with the rupture process made up of three sub-events, with a total scalar seismic moment of 7.7 × 10¹⁷ Nm (M_w = 5.9). A common characteristic of these three earthquakes was the shallow focal depth, less than 10 km, in agreement with the oceanic-type crust. From the directivity function of Rayleigh (LR) waves, we have identified the NW–SE plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the SE. Slow rupture velocity, about of 1.5 km/s, has been estimated from directivity function for the 1980 and 1998 earthquakes. From spectral analysis and body-wave inversion, fault dimensions, stress drop and average slip have been estimated. Focal mechanisms of the three earthquakes we have studied, together with focal mechanisms obtained by other authors, have been used in order to obtain a seismotectonic model for the Azores region. We have found different types of behaviour present along the region. It can be divided into two zones: Zone I, from 30°W to 27°W; Zone II, from 27°W to 23°W, with a change in the seismicity and stress direction from Zone I. In Zone I, the total seismic moment tensor obtained corresponded to left-lateral strike–slip faulting with horizontal pressure and tension axes in the E–W and N–S directions, respectively. In Zone II, the total seismic moment tensor corresponded to normal faulting, with a horizontal tension axis trending NE–SW, normal to the Terceira Ridge. The stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4.4 mm/yr.     

Constraining stress magnitudes using petroleum exploration data in the Cooper–Eromanga Basins, Australia

The magnitude of the in situ stresses in the Cooper–Eromanga Basins have been determined using an extensive petroleum exploration database from over 40 years of drilling. The magnitude of the vertical stress (S_v) was calculated based on density and velocity checkshot data in 24 wells. Upper and lower bound values of the vertical stress magnitude are approximated by S_v = (14.39 × Z)^1.12 and S_v = (11.67 × Z)^1.15 functions respectively (where Z is depth in km and S_v is in MPa). Leak-off test data from the two basins constrain the lower bound estimate for the minimum horizontal stress (S_hmin) magnitude to 15.5 MPa/km. Closure pressures from a large number of minifrac tests indicate considerable scatter in the minimum horizontal stress magnitude, with values approaching the magnitude of the vertical stress in some areas. The magnitude of the maximum horizontal stress (S_Hmax) was constrained by the frictional limits to stress beyond which faulting occurs and by the presence of drilling-induced tensile fractures in some wells. The maximum horizontal stress magnitude can only be loosely constrained regionally using frictional limits, due to the variability of both the minimum horizontal stress and vertical stress estimates. However, the maximum horizontal stress and thus the full stress tensor can be better constrained at individual well locations, as demonstrated in Bulyeroo-1 and Dullingari North-8, where the necessary data (i.e. image logs, minifrac tests and density logs) are available. The stress magnitudes determined indicate a predominantly strike-slip fault stress regime (S_Hmax > S_v > S_hmin) at a depth of between 1 and 3 km in the Cooper–Eromanga Basins. However, some areas of the basin are transitional between strike-slip and reverse fault stress regimes (S_Hmax > S_v ≈ S_hmin). Large differential stresses in the Cooper–Eromanga Basins indicate a high upper crustal strength for the region, consistent with other intraplate regions. We propose that the in situ stress field in the Cooper–Eromanga Basins is a direct result of the complex interaction of tectonic stresses from the convergent plate boundaries surrounding the Indo-Australian plate that are transmitted into the center of the plate through a high-strength upper crust.     

3-D velocity structure of the 2003 Bam earthquake area (SE Iran): Existence of a low-Poisson's ratio layer and its relation to heavy damage

To understand the generation mechanism of the Bam earthquake (Mw 6.6), we studied three-dimensional V_P, V_S and Poisson's ratio (σ) structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of the seismogenic region was well resolved to a depth of 14 km with significant velocity variations of up to 5%. The general pattern of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20 km along a line 4.5 km west of the known geological Bam fault; this source fault dips steeply westward and strikes a nearly north–south line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in the desert. The 3-D velocity structure shows a thick layer of high V_S and low σ (minimum: 0.20) at a depth range of 2–6 km. The deeper layer, with a thickness of about 2 km, appears to have a low V_S and high σ (maximum: 0.28) from 6 km depth beneath Bam to a depth of 9 km south of the city. The inferred increase of Poisson's ratio from 2 to 10 km in depth may be associated with a change from rigid and SiO₂-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of aftershock distribution at the depth range of 2 to 7 km coincides well with the large slip zone in the shallow thick layer of high V_S and low σ. The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Bam area suffered heavy damage.     

Upper crustal seismic structure of the Mazury complex and Mazowsze massif within East European Craton in NE Poland

The large-scale seismic experiment POLONAISE '97 (POlish Lithospheric ONsets—An International Seismic Experiment) was carried out in May 1997 in Poland, Lithuania, and Germany. Its main purpose was to investigate the structure of the crust and the uppermost mantle in the region of the Trans European Suture Zone (TESZ) that lies between the East European Craton (EEC) and the Palaeozoic Platform. This paper covers the interpretation of seismic data along the NW–SE-trending, 180-km-long profile P5 located on the EEC. The recordings were of a high quality with seismic energy clearly visible along the whole profile. We have not found waves refracted below the upper crust in first arrivals. In the NW part of the profile, we have delineated a high-velocity body with the P-wave velocity in the range of 6.5–6.75 km/s in the upper crust. It corresponds to the K trzyn anorthosite massif within the Mazury complex. The Mazowsze massif is rather uniformly characterized by P-wave velocities 5.9–6.05 and 6.2–6.35 km/s in two layers, respectively. Sufficient S-wave data were available to estimate the V_p/V_s ratio (as well as the Poisson ratio), being 1.80 (0.277) in the high-velocity body and 1.67 (0.220) in the upper crust.Apart from the 2-D model along the profile, results of 3-D modelling in the area of the P5 profile are presented. Using off-line recordings, we got P-wave velocity field up to 8 km/s below the P5 profile at the depth of about 40 km as well as horizontal extent of the high-velocity body.     

Source parameters of the Burma-India border earthquake of July 29, 1970, from body waves

The source parameters are determined for the Burma-India border earthquake of July 29, 1970, from body-wave spectra. We obtain seismic moment [ , ] · 10²⁶ dyne cm, source dimension [ ] km, radiated energy [ , −E_R (S) = 1.35] · 10²⁰ ergs and the stress drop = 11 bars.     

Seismic attenuation of coda waves in the eastern region of Cuba

Cuba's seismic attenuation had never been studied in detail. In this paper we present the results of the research on the seismic attenuation of Cuba's eastern zone based upon the information collected by the seismological Cuban network from 1998 to 2003. 581 earthquakes were selected from the Cuban catalogue to make this study. All of them, recorded by at least three seismic stations, had their epicenters located in the eastern Cuban region (19.3–22 N, 79–73 W), epicentral distances between 15 km and 213 km, their coda duration magnitudes ranging from 2 and 4.1 and their focal depths reaching up to 30 km. The seismic wave attenuation was studied using coda waves. The single scattering method proposed by Sato in 1977 was applied, the attenuation and frequency dependency for different paths and the correlation of the results with the geotectonics of the region are presented in this paper.The mean Q_c value calculated was Q_c = (64 ± 2)f^{0.84 ± 0.01}. The relatively low Q₀ and the high frequency dependency agree with the values of a region characterized by a high tectonic activity. The Q_c values of seven subregions of eastern Cuba were calculated and correlated with the geology and tectonics of the area.     

Source parameters of intermediate-depth Vrancea (Romania) earthquakes from empirical Green's functions modeling

Several source parameters (source dimensions, slip, particle velocity, static and dynamic stress drop) are determined for the moderate-size October 27th, 2004 (M_W = 5.8), and the large August 30th, 1986 (M_W = 7.1) and March 4th, 1977 (M_W = 7.4) Vrancea (Romania) intermediate-depth earthquakes. For this purpose, the empirical Green's functions method of Irikura [e.g. Irikura, K. (1983). Semi-Empirical Estimation of Strong Ground Motions during Large Earthquakes. Bull. Dis. Prev. Res. Inst., Kyoto Univ., 33, Part 2, No. 298, 63–104., Irikura, K. (1986). Prediction of strong acceleration motions using empirical Green's function, in Proceedings of the 7th Japan earthquake engineering symposium, 151–156., Irikura, K. (1999). Techniques for the simulation of strong ground motion and deterministic seismic hazard analysis, in Proceedings of the advanced study course seismotectonic and microzonation techniques in earthquake engineering: integrated training in earthquake risk reduction practices, Kefallinia, 453–554.] is used to generate synthetic time series from recordings of smaller events (with 4 ≤ M_W ≤ 5) in order to estimate several parameters characterizing the so-called strong motion generation area, which is defined as an extended area with homogeneous slip and rise time and, for crustal earthquakes, corresponds to an asperity of about 100 bar stress release [Miyake, H., T. Iwata and K. Irikura (2003). Source characterization for broadband ground-motion simulation: Kinematic heterogeneous source model and strong motion generation area. Bull. Seism. Soc. Am., 93, 2531–2545.] The parameters are obtained by acceleration envelope and displacement waveform inversion for the 2004 and 1986 events and MSK intensity pattern inversion for the 1977 event using a genetic algorithm. The strong motion recordings of the analyzed Vrancea earthquakes as well as the MSK intensity pattern of the 1977 earthquake can be well reproduced using relatively small strong motion generation areas, which corresponds to small asperities with high stress drops (300–1200 bar) and high particle velocities (3–5 m/s). These results imply a very efficient high-frequency radiation, which has to be taken into account for strong ground motion prediction, and indicate that the intermediate-depth Vrancea earthquakes are inherently different from crustal events.     

Magnitude Calibration for Vrancea (Romania) Subcrustal Earthquakes Using Coda Waves

The coda wave amplitude is used to calibrate the moment–magnitude scale of the Vrancea (Romania) intermediate depth earthquakes. The proportionality between seismic moment and coda wave amplitude is supposed to infer a relation connecting the magnitude, the coda amplitude and the corresponding time in coda (measured from the origin time). The data set consists of waveforms recorded by the Romanian telemetered network at several seismic stations, such as Muntele Rou (MLR), Vrâncioaia (VRI), Carcaliu (CFR) and Topalu (TLB). Our results are in good agreement with the moment–magnitude scale for Vrancea subcrustal events. The independence of the two approaches allows for the verification of the local magnitude scale for the Vrancea subcrustal earthquakes. Average relations among coda amplitude, magnitude M_L and coda duration are obtained for the above-mentioned stations.     

Lithospheric gravitational instability beneath the Southeast Carpathians

The Southeast corner of the Carpathians, known as the Vrancea region, is characterised by a cluster of strong seismicity to depths of about 200 km. The peculiar features of this seismicity make it a region of high geophysical interest. In this study we calculate the seismic strain-rate tensors for the period 1967–2007, and describe the variation of strain-rate with depth. The observed results are compared with strain-rates predicted by numerical experiments. We explore a new dynamical model for this region based on the idea of viscous flow of the lithospheric mantle permitting the development of local continental mantle downwelling beneath Vrancea, due to a Rayleigh–Taylor instability that has developed since the cessation of subduction at 11 Ma. The model simulations use a Lagrangean frame 3D finite-element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow. The finite deformation calculations of the gravitational instability of the continental lithosphere demonstrate that the Rayleigh–Taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere, both in terms of stress localisation and amplitude of strain-rates. The spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the lithosphere. The mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the Transylvanian Basin. Crustal thickening is predicted above the downwelling structure and thinning beneath the basin.     

Source parameters of the ML 4.1 earthquake of November 08, 2006, southeast Beni-Suef, northern Egypt

In the early morning hours on Wednesday November 08, 2006 at 04:32:10(GMT) a small earthquake of M_L 4.1 has occurred at southeast Beni-Suef, approximately 160 km SEE of Cairo, northern Egypt. The quake has been felt as far as Cairo and its surroundings while no casualties were reported. The instrumental epicentre is located at 28.57°N and 31.55°E. Seismic moment is 1.76 E14 Nm, corresponding to a moment magnitude M_w 3.5. Following a Brune model, the source radius is 0.3 km with an average dislocation of 1.8 cm and a 2.4 MPa stress drop. The source mechanism from a first motion fault plane solution shows a left-lateral strike-slip mechanism with a minor dip-slip component along fault NNW striking at 161°, dipping 52° to the west and rake −5°. Trend and plunging of the maximum and minimum principle axes P/T are 125°, 28°, 21°, and 23°, respectively. A comparison with the mechanism of the October, 1999 event shows similarities in faulting type and orientation of nodal planes.Eight small earthquakes (3.0  M_L < 5.0) were also recorded by the Egyptian National Seismological Network (ENSN) from the same region. We estimate the source parameters and fault mechanism solutions (FMS) for these earthquakes using displacement spectra and P-wave polarities, respectively. The obtained source parameters including seismic moments of 4.9 × 10¹²–5.04 × 10¹⁵ Nm, stress drops of 0.2–4.9 MPa and relative displacement of 0.1–9.1 cm. The azimuths of T-axes determined from FMS are oriented in NNE–SSW direction. This direction is consistent with the present-day stress field in Egypt and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS measurements.     

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes (m_b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc.Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Shear wave velocity and attenuation structure for the shallow crust of the southern Korean peninsula from short period Rayleigh waves

We analyzed the short period Rayleigh waves from the first crustal-scale seismic refraction experiment in the Korean peninsula, KCRUST2002, to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the Korean peninsula and to examine if this can be related to the surface geology of the study area. The experiment was conducted with two large explosive sources along a 300-km long profile in 2002. The seismic traces, recorded on 170 vertical-component, 2-Hz portable seismometers, show distinct Rayleigh waves in the period range between 0.2 s and 1.2 s, which are easily recognizable up to 30–60 km from the sources. The seismic profiles, which traverse three tectonic regions (Gyeonggi massif, Okcheon fold belt and Yeongnam massif), were divided into five subsections based on tectonic boundaries as well as lithology. Group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method, respectively, were inverted for the shear wave models. We obtained shear wave velocity models up to a depth of 1.0 km. Overall, the shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by  0.4 km/s in the shallowmost 0.2 km and by 0.2 km/s at depths below 0.2 km. Attenuation coefficients, determined from the decay of the fundamental mode Rayleigh waves, were used to obtain the shear wave attenuation structures for three subsections (one for each of the three different tectonic regions). We obtained an average value of Q_β^− 1 in the upper 0.5 km for each subsection. Q_β^− 1 for the Okcheon fold belt ( 0.026) is approximately three times larger than Q_β^− 1 for the massif areas ( 0.008). The low shear wave velocity in the Okcheon fold belt is consistent with the high attenuation in this region.     

2-D seismic tomographic and ray tracing modelling of the crustal structure across the Sudetes Mountains basing on SUDETES 2003 experiment data

The seismic data obtained during SUDETES 2003 experiment are analysed, and detailed crustal structure for profiles S02, S03 and S06 is presented using three different 2-D techniques: (1) “smooth” tomography of refracted waves travel times, (2) ray tracing of reflected and refracted waves, and (3) joint velocity and depth of reflector tomographic inversion. In spite of different interpretation techniques used, the models of the crustal structure show common characteristic features. The low velocity (V_p < 4 km/s) sedimentary layer was documented in the northeastern part of the study area. The topmost basement has in general a velocity of 5.8–6.0 km/s, and velocities at ca. 20 km depth are 6.15–6.25 km/s. The strong reflecting boundaries were found at 20–23 and 25–28 km depth with a velocity contrast about 0.4 km/s, and the highest velocities in the lowermost crust are 6.8–7.2 km/s. In general, the crust of the Bohemian Massif is slightly thicker (33–35 km) than in the northern part of the area. Velocities beneath Moho are relatively low, of 7.95 km/s. On the basis of well recorded reflected waves, mantle reflectors were discovered in the depth interval ca. 40–70 km. Apart of new results for the geology and tectonics of the area, some conclusion could be made about different techniques used. In the 2-D case the “clasical” ray tracing method with using all correlated phases gives the most adequate model of the structure, because of full, manual control of the model creation. The “smooth” first arrival travel times tomography, although very fast, is not satisfactory enough to describe the complex structure. So, the best candidate in 3-D case seems to be travel time tomography for both refracted and reflected waves in multi-layers models.     

Three-dimensional P-wave velocity image under the Carpathian arc

An inversion of P-wave travel time residuals from selected earthquakes in the distance range 30°–98° to two seismic station networks was used to model P-wave velocity anomalies down to 250 km depth. In the first inversion experiment a region between 43.5°–47.5°N and 21°–29°E was modelled, using 35 seismic stations, while in the second one a region between 44°–47°N and 25°–29°E was modelled, using 19 seismic stations. The 4-layer block model of the first inversion offers 19% reduction in residual variance, while the 5-layer block model of the second one offers 26% reduction, the rest being explained by noise and smaller scale heterogeneities. The obtained velocity anomalies correlate remarkably well with the gravity anomalies and with the tectonic model for the Vrancea region of Fuchs et al. (1979).     

Source parameters of the 2001 Mw 7.7 Bhuj earthquake,Gujarat, India,aftershock sequence

We present the estimated source parameters from SH-wave spectral modeling of selected 463 aftershocks (2002–06) of the 26 January 2001 Bhuj earthquake, the well-recorded largest continental intraplate earthquake. The estimated seismic moment (M_o), corner frequency (f_c), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 1.7 to 5.6 range from 3.55×10¹¹ to 2.84×10¹⁷ N-m, 1.3 to 11.83 Hz, 107 to 1515 m and 0.13 to 26.7 MPa, respectively, while the errors in f_c and Δσ are found to be 1.1 Hz and 1.1 MPa, respectively. We also notice that the near surface attenuation factor (k) values vary from 0.02 to 0.03. Our estimates reveal that the stress drop values show more scatter (Mo^{0.5 to 1} is proportional to Δσ) toward the larger M_o values (≥10^14.5 N-m), while they show a more systematic nature (Mo³ is proportional to Δσ) for smaller M_o values (<10^14.5 N-m), which can be explained as a consequence of a nearly constant rupture radius for smaller aftershocks in the region. The large stress drops (= 10 MPa) associated with events on the north Wagad fault (at 15–30 km depth) and Gedi fault (at 3–15 km depth) can be attributed to the large stress developed at hypocentral depths as a result of high fluid pressure and the presence of mafic intrusive bodies beneath these two fault zones.     

Seismic velocities of granulite-facies xenoliths from Central Ireland: Implications for lower crustal composition and anisotropy

V_p and V_s values have been measured experimentally and calculated for granulite-facies lower crustal xenoliths from central Ireland close to the Caledonian Iapetus suture zone. The xenoliths are predominantly foliated and lineated metapelitic (garnet–sillimanite–K-feldspar) granulites. Their metapelitic composition is unusual compared with the mostly mafic composition of lower crustal xenoliths world-wide. Based on thermobarometry, the metapelitic xenoliths were entrained from depths of c. 20–25 ± 3.5 km and rare mafic granulites from depths of 31–33 ± 3.4 km. The xenoliths were emplaced during Lower Carboniferous volcanism and are considered to represent samples of the present day lower crust.V_p values for the metapelitic granulites range between 6.26 and 7.99 km s^{− 1} with a mean value of 7.09 ± 0.4 km s^{− 1}. Psammite and granitic orthogneiss samples have calculated V_p values of 6.51 and 6.23 km s^{− 1}, respectively. V_s values for the metapelites are between 3.86 and 4.34 km s^{− 1}, with a mean value of 4.1 ± 0.15 km s^{− 1}. The psammite and orthogneiss have calculated V_s values of 3.95 and 3.97 km s^{− 1}, respectively.The measured seismic velocities correlate with density and with modal mineralogy, especially the high content of sillimanite and garnet. V_p anisotropy is between 0.15% and 13.97%, and a clear compositional control is evident, mainly in relation to sillimanite abundance. Overall V_s anisotropy ranges from 1% to 11%. Poisson's ratio (σ) lies between 0.25 and 0.35 for the metapelitic granulites, mainly reflecting a high V_p value due to abundant sillimanite in the sample with the highest σ. Anisotropy is probably a function of deformation associated with the closure of the Iapetus ocean in the Silurian as well as later extension in the Devonian. The orientation of the bulk strain ellipsoid in the lower crust is difficult to constrain, but lineation is likely to be NE–SW, given the strike-slip nature of the late Caledonian and subsequent Acadian deformation.When corrected for present-day lower crustal temperature, the experimentally determined V_p values correspond well with velocities from the ICSSP, COOLE I and VARNET seismic refraction lines. Near the xenolith localities, the COOLE I line displays two lower crustal layers with in situ V_p values of 6.85–6.9 and 6.9–8.0 km s^{− 1}, respectively. The upper (lower velocity) layer corresponds well with the metapelitic granulite xenoliths while the lower (higher velocity) layer matches that of the basic granulite xenoliths, though their metamorphic pressures suggest derivation from depths corresponding to the present-day upper mantle.     

Static and dynamic fault parameters of the Saitama earthquake of July 1, 1968

The source mechanism of the Saitama earthquake (36.07°N,139.40°E, M_s = 5.4) of July 1, 1968, is studied on the basis of P-wave first motion, aftershock, long-period surface-wave data and low-magnification long-period seismograms recorded in the nearfield. A precise location of the aftershocks is made using P and S—P time data obtained by a micro-earthquake observatory network. The synthetic near-field seismograms based on the Haskell model are directly compared with the observed near-field seismograms for wave form and amplitude to determine the dynamic fault parameters. The results obtained are as follows: source geometry, reverse dip slip with considerable right-lateral strike-slip component; dip direction, N6°E; dip angle 30°; fault dimension, 10 × 6 km²; rupture velocity, 3.4 km/sec in the direction S30°E; average dislocation, 92 cm; average dislocation velocity, 92 cm/sec; seismic moment, 1.9 · 10²⁵ dyn-cm; stress drop, 100 bar. The effective stress is about the same as the stress drop. For major earthquakes in the Japanese Islands, the dislocation velocity, .D, is found to be proportional to the stress drop, σ. This relation can be expressed by .D - (β/μ)σ, where β is the shear velocity and μ is the rigidity. This result has an importance in engineering seismology because the stress drop scales the seismic motion in the vicinity of an earthquake fault.     

Density distribution functions and correlations of earthquake parameters

The statistical analysis of the source parameters of 9 earthquake sets of different types (aftershocks, scattered events, swarms) and of different seismic regions shows that the density distribution function (ddf) of the linear dimension l of a fault is represented by a negative power law, as well as the ddf of the static stress drop σ and of the scalar seismic moment M_o. It is then suggested, and tentatively verified, that also the ddf of the root mean square ground acceleration, defined as a function of l and σ, may be represented by a negative power law and that, at least in the cases examined, it scales like the ddf of σ. It is seen that the variability of the static stress drop is significant from one region to another, as is well known, but it seems remarkable also in the same seismic region (in particular in California, σ varies by several orders of magnitude) and in the different sets of events of a given region (as observed again for California). It is hypothesized that a correlation, although weak, between the stress drop and the linear dimension of a fault exists and the analyses seem not to contradict that σ may be a decreasing function of l. Finally, it is suggested that the seismicity of a region may be represented two-dimensionally as a function of the ddf of the stress drop and of the linear dimension of a fault instead of the classic b and b_o values.