Stress Drop Δσ Research Articles

The Gulf of Suez earthquake, 30 January 2012, northeast of Egypt

The present day tectonic deformation within Egypt is related to the regional tectonic forces of the surrounding plate boundaries including the African–Eurasian plate margin, the Red Sea plate margin and the Levant transform fault in addition to the local tectonics of Egypt. Based on the spatial distribution of earthquake epicenters in Egypt, the most seismically active areas are located in the Northern Red Sea, Gulf of Suez, Gulf of Aqaba and Mediterranean coastal zone. On 30 January 2012, a moderate earthquake of local magnitude ML 4.6 struck the northeast part of Egypt, at the entrance of Gulf of Suez. This event was felt in many areas surrounding the epicenter with low damage recorded. To understand the mechanism and the reflected irregularity of the evolved stress field concerning the Gulf of Suez area, calculation of source parameters and source mechanisms of this event are necessary. Moment tensor inversion is used to retrieve the fault plain solutions and the seismic moment by using waveforms recorded by the Egyptian National Seismic Network. The fit between observed and synthetic seismograms was computed for an elastic layered media and minimized using a least squares algorithm. The derived source mechanism indicates normal faulting along NNW–SSE trending faults, in agreement with the rifting of the northern Red Sea in its northern branches (Gulf of Suez and Gulf of Aqaba). The source parameters are estimated and the derived corner frequency f o for the P-wave spectra shows a value of 3.00 Hz. The seismic moment M o is 1.278 × 1023 Nm, the fault length r is 0.8 km, the average displacement D 0 is 0.023 m, and the value of the stress drop Δσ is 1.10 MPa. The source mechanism and source parameters imply the continuous activity of the Red Sea rift system at the Gulf of Suez opening.

Stress Drop during Earthquakes: Effect of Fault Roughness Scaling

We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents ( H R ), H ||=0.6 in the direction of slip and H ⊥=0.8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale ( λ ) as std Δσ ( λ )∝ λ H R -1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop Δσ ( r )∝ r H R -1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.

Crustal deformation associated with east Mediterranean strike–slip earthquakes: The 8 June 2008 Movri (NW Peloponnese), Greece, earthquake (M w6.4)

The 2008 mainshock ( M w = 6.4) was the first modern, strong strike–slip earthquake in the Greek mainland. The fault strikes NE–SW, dips ∼ 85°NW while the motion was right-lateral with small reverse component. Historical seismicity showed no evidence that the fault ruptured in the last 300 years. For rectangular planar fault we estimated fault dimensions from aftershock locations. Dimensions are consistent with that a buried fault was activated, lateral expansion occurred only along length and the rupture stopped at depth ∼ 20 km implying that more rupture along length was favoured. We concluded that no major asperities remained unbroken and that the aftershock activity was dominated rather by creeping mechanism than by the presence of locked patches. For M o = 4.56 × 10 25 dyn cm we calculated average slip of 76 cm and stress drop Δσ ∼ 13 bars. This Δσ is high for Greek strike–slip earthquakes, due rather to increased rigidity because of the relatively long recurrence ( Τ > 300 years) of strong earthquakes in the fault, than to high slip. Values of Δσ and Τ indicated that the fault is neither a typical strong nor a typical weak fault. Dislocation modeling of a buried fault showed uplift of ∼ 8.0 cm in Kato Achaia ( Δ ∼ 20 km) at the hanging wall of the reverse fault component. DInSAR analysis detected co-seismic motion only in Kato Achaia where interferogram fringes pattern showed vertical displacement from 3.0 to 6.0 cm. From field-surveys we estimated maximum intensity of VIII in Kato Achaia. The most important liquefaction spots were also observed there. These observations are attributable neither to surface fault-breaks nor to site effects but possibly to high ground acceleration due to the co-seismic uplift. The causal association between displacement and earthquake damage in the hanging wall described for dip–slip faults in Taiwan, Greece and elsewhere, becomes possible also for strike–slip faults with dip–slip component, as the 2008 earthquake.

MODELING OF SUBSURFACE FAULT FOR STRONG MOTION PREDICTION INFERRED FROM SHORT ACTIVE FAULT OBSERVED ON GROUND SURFACE

Short active faults observed on the ground surface have been supposed to indicate longer subsurface faults in the seismogenic layer. However, no concrete procedures have been proposed so far for modeling the subsurface faults for predicting strong ground motions. Hence, we surveyed several important research papers, and proposed a procedure for modeling the subsurface faults. Here, the seismic moment M0 of 7.5×1025 dyne•cm was assumed as the minimum size of the source for the case that any short active faults were observed on the surface. The modeling procedure was based on the asperity model, that was adopted by the Headquaters of Earthquake Research Promotion (2005) in Japan. Five main fault parameters of the area of the entire fault S, the averaged stress drop Δσ, the area of the asperities Sasp, the stress drop on the asperities Δσasp, and the short-period level A were determined by the two theoretical relationships and the three empirical relationships among the six main fault parameters including the seismic moment M0.

Site Amplification, Qs, and Source Parameterization in Guwahati Region from Seismic and Geotechnical Analysis

As devastating earthquakes continue to inflict widespread destruction to life and property and hinder the development of urban areas, the technical and scientific problems of seismic hazard assessment and risk-related issues warrant urgent attention. In this regard, seismologists play a key role in defining the source parameters of earthquakes and estimating the associated site-specific ground motion amplification commonly termed site response. The relevant analyses are usually performed using either recorded ground motion for a handful of seismic events of moderate to large magnitude or by synthesizing stochastic processes based on the observational data. The hazard projection is aimed at a great earthquake or a maximum earthquake or maximum credible earthquake on probabilistically/deterministically defined terms for a seismic zone under investigation. Seismic sources generally are characterized by well-defined physical parameters such as corner frequency f c , seismic moment M , and stress drop Δσ, derived directly from waveform data, i.e. , strong ground motion records for events with significant magnitude. On the other hand, site amplification of ground motion is attributed to either the geomorphological features that produce scattering, focusing, or defocusing of incident energy or thick alluvium-filled terrain that causes reverberations due to trapped energy. The potentially severe consequences of this phenomenon have been demonstrated in the damage patterns of several earthquakes, such as the 1985 Michoacan, Mexico, earthquake (Singh et al. 1988), the 1988 Armenian earthquake (Borcherdt et al. 1989), and the 1989 Loma Prieta earthquake (Hough et al. 1990; Borcherdt and Glassmoyer 1992). Another aspect of ground motion analysis or synthesis is the attenuation of seismic waves along the propagation path connecting the earthquake source and the recording site (observatory). The attenuation may be attributed to degradation in the elastic properties such as shear and compressional moduli and the scattering of seismic waves caused …

Determination of source parameters of strong earthquakes of Kamchatka, the Kurile Islands, and Japan from aftershock sequences

Aftershock sequences of some strong earthquakes of Kamchatka, the Kurile Islands, and Japan are examined. Such source parameters as the length L, along-dip width W, motion on fault D, and stress drop Δσ are determined from the aftershock sequences considered. The values of these parameters were obtained by the formal estimation of linear source parameters (lower bound estimates) and visually (upper bound estimates). The correlation dependences of the obtained parameters on the surface wave (MS) and seismic moment (MW) magnitudes are calculated.

A study on the focal parameters of Shidian M=5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian MS=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 1010–1016 N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×105–20×105Pa. Through the statistical analyses of variation of corner frequency fc and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.

強震記録に基づく北海道周辺のやや深発地震の高振動数成分の励起特性

Generation of the high-frequency wave for intermediate-depth earthquakes in and around Hokkaido region is estimated from the source spectra in the frequency range from 1 to 10Hz. The average stress drop Δσ that is calculated from the high-frequency flat level of acceleration source spectra is about 30MPa for the down-dip-extension type earthquakes while it is about 8MPa for the subduction type earthquakes. Variation of Ads of the down-dip-extension type earthquakes is meaningfully smaller than that of the subduction type earthquakes. As for the down-dip-extension type earthquakes, Aaof large earthquakes coincides with the upper level of those of small earthquakes.

Regional Ground-Motion Scaling in Central Europe

Regressions of 2700 horizontal-component broadband seismograms from 213 seismic events recorded by the German Regional Seismic Network (67 earthquakes and 146 large mining explosions and rockbursts) are carried out to study the scaling relationships of high-frequency S -wave motion for central Europe. At a set of sampling frequencies, regressions were performed on the logarithms of the peak amplitudes of narrow bandpass-filtered seismograms, as well as on the logarithms of the Fourier components of the velocity spectra. At a fixed frequency f , these values are written as \[\mathrm{AMP}(f,{ }r)=\mathrm{EXC}(f,{ }r_{\mathrm{ref}})+\mathrm{SITE}(f)+D(r,{ }r_{\mathrm{ref}},{ }f).\] EXC( f, r ref ) is the excitation at an arbitrary reference hypocentral distance, r ref , SITE( f ) is a site term, and D ( r, r ref , f ) describes the crustal attenuation in the region. The crustal propagation term, empirically estimated in the (0.5–16.0 Hz) frequency band and (40–600 km) distance range, is modeled using a complex geometrical spreading function and a frequency-dependent crustal Q . We suggest \[Q(f)=Q_{0}{\cdot}\left(\frac{f}{f_{\mathrm{ref}}}\right)^{{\eta}},{ }\begin{array}{ccc}Q_{0}&=&400\\{\eta}&=&0.42\\f_{\mathrm{ref}}&=&1.0{ }\mathrm{Hz}\end{array}\] and a log-log quadrilinear geometrical spreading. A factor exp(-πκ 0 f ) is used to fit the high-frequency roll-off of the inverted excitation terms. Since we deal with two different kinds of sources (explosions-rockbursts and earthquakes), we use \[{\kappa}_{0}=\begin{array}{l}0.08{ }\mathrm{sec}{ }(\mathrm{for explosions});\\0.05{ }\mathrm{sec}{ }(\mathrm{for earthquakes}).\end{array}\] The same Brune spectral model, characterized by a stress drop Δσ = 30 bars, is used to fit both earthquakes and explosive excitation terms. A regression on the effective duration of the ground motion following the S -wave onset is also carried out. In central Europe, duration is observed to be almost frequency-independent. This property might be explained in terms of a self-similar distribution of crustal scatterers.

Ground-Motion Scaling in the Region of the 1997 Umbria-Marche Earthquake (Italy)

We used broadband waveforms collected at short hypocentral distances ( r ≤ 40 km) during the Umbria-Marche (Italy) seismic sequence of September–November, 1997, in order to calculate the scaling relationships for the ground motion within the meizoseismal area, in the 0.5–16.0 Hz frequency band. Data were collected by a 10-station portable seismic network deployed by the Istituto Nazionale di Geofisica (Rome, Italy) shortly after the occurrence of the first mainshock of the sequence, on 26 September 1997. Among the thousands of events recorded, we selected 142 earthquakes characterized by good signal-to-noise ratios at all frequencies, and by the absence of multiple shocks within the time window spanned by each recording. The data set of the selected waveforms was made of 2030 horizontal-component seismograms. The logarithm of the peak values of narrow bandpass-filtered versions of the velocity time histories are modeled at each frequency as \[AMP(f,r)=EXC(f,r_{\mathrm{ref}})+SITE(f)+D(r,r_{\mathrm{ref}},f).\] EXC ( f,r ref ) is the excitation term at an arbitrary reference hypocentral distance, r ref ; SITE ( f ) is a site term. The empirical attenuation functional, D ( r,r ref , f ), represents an estimate of the average crustal response for the region, at the hypocentral distance r , at the frequency f . It is modeled by using the following functional form: \[D(r,r_{\mathrm{ref}},f)=\mathrm{log}{ }g(r)-\mathrm{log}{ }g(r_{\mathrm{ref}})-\frac{{\pi}f(r-r_{\mathrm{ref}})}{{\beta}Q_{0}(f{/}f_{\mathrm{ref}})^{{\eta}}};{ }(f_{\mathrm{ref}}=1.0{ }\mathrm{Hz},{ }r_{\mathrm{ref}}=10{ }\mathrm{km}).\] g ( r ) = r -1 is the body-wave geometrical spreading function; β = 3.5 km/sec is the shear-wave velocity in the crust. Due to the constraints applied to the system prior to the regressions, the excitation term represents the expected peak ground motion at the reference distance, as it would be observed at a site representative of the average site response of the network. The random vibration theory (RVT) is used to obtain a theoretical prediction of the attenuation functional. For reproducing D ( r,r ref , f ) we use the crustal attenuation parameter \[Q(f)=130(f{/}f_{\mathrm{ref}})^{0.10}\] obtained by Malagnini et al. (2000) from the analysis of a regional data set representative of the entire Apennines, in the (0.24–5.0 Hz) band. Two parameters are used to predict shapes and levels of the seismic spectra, the stress drop Δσ, and a high-frequency attenuation parameter κ 0 . The values used to reproduce the observed velocity spectra are \[{\Delta}{\sigma}=200{ }\mathrm{bars};{ }{\kappa}_{0}=0.04{ }\mathrm{sec}.\] The indicated stress drop was estimated in this region by Castro et al. (2000), on recordings of the largest shock of the Umbria-Marche sequence.

強震記録から評価した短周期震源スペクトルの地域性

We examine the excitation strengths of source spectra in high-frequency range, and discuss their regional variation. Target events are subduction earthquakes off Fukushima Prefecture, intermediate depth earthquakes in the Kanto region, and shallow earthquakes in the southern Kanto region in Japan. The analyzed data are S-wave portions of 672 accelerograms from 40 events observed at 27 stations in the southern Tohoku and the Kanto region. Source spectra are evaluated by simultaneously separating source, propagation path, and local site effects using inversion technique in spectral domain [IWATA and IRIKURA (1986)]. The magnitude of these events ranges from 3.8 to 7.0, and hypocentral distance from 20 to 200km. First of all, the relationship between the seismic moment M0 and the amplitude of source spectra in high-frequency range MHF is examined. MHF is evaluated by taking logarithmic average of acceleration source spectrum from 1 to 10Hz. MHF is nearly proportional to M1/30, which is expected from the scaling of source spectra based on ω-2 model. MHF shows regional variation even if M0 is nearly equal. To quantitatively estimate the amplitude difference in high-frequency range, BRUNE stress drop Δσ [BRUNE (1970, 1971)] is evaluated for each event using M0 and MHF. Average Δσ for shallow earthquakes in the southern Kanto, subduction earthquakes off Fukushima Prefecture, and intermediate depth earthquakes in the Kanto region is about 100 bars, 200 bars, and 370 bars, respectively. Taking into account the relation that MHF is proportional to Δσ2/3, intermediate depth earthquakes excite about 2.4 times larger amplitude than that for shallow earthquakes in high-frequency range. Thus, deeper events generally show large Δσ in comparison with shallower events, suggesting that the focal depth is a key parameter to control the excitation strengths of source spectra in high-frequency range.

Estimation of seismic stress drop from the peak velocity of ground motion

According to the source dislocation model suggested by Brune(1970), the authors have calculated the displacement spectra of S-wave and source parameters of the Heqing Ms 5.3 earthquake sequence, using the digital data of this sequence obtained in the Western Yunnan Earthquake Prediction Experimental Field (WYEPEF). Based on this calculation we have studied the dependence of the peak velocity (rv) of ground motion on the seismic stress drop Δσ. From the seismic scaling law we obtained (rv) ∞Δσ2/3, thus the three formulae of calculating seismic stress drop Δσ using the peak velocity parameters can be derived: lg(rv)=d1+1/3lgM0+2/lgΔσ lg(rv)=d2+1/3ML+2/3lgΔσ=-1.0+1.5lg(rv). Assuming that the average stress drop Δσ=3.0×106 Pa for great and small earthquakes, then the constants d1= −3.88 and d2=−0.38 are determined by the observational data of the Heqing Ms5.3 sequence. Results of the source parameters for this sequence show that the seismic moment M0 is between 1011 N · m and 1015 N · m, the rupture radius a of the source is between 200 m and 600 m, the stress drop Δσ is between 0.1 MPa and 10 MPa and the average stress drop Δσ=3.7 MPa calculated from the peak velocity parameter of the ground motion. Δσ values measured from these scaling relations are basically in agreement with the results given by Brune’s method (1970). Results of this study show that the dependence of the ground motion peak velocity parameter (rv) on the stress drop Δσ is even stronger than that on the seismic moment M0.

The June 9 Bolivia and March 9 Fiji deep earthquakes of 1994: I. Source processes

We examined the source characteristics of the 1994 deep earthquakes of Bolivia (June 9) and Fiji (March 9) over a period range of 1–1000 s using moment tensor and body wave analyses. Moment tensor inversions from 100–1000 s reveal that these two earthquakes were essentially point sources at very long period with insignificant isotropic and non‐double couple components with scalar moments Mo = 2.56 × 1021 Nm for the Bolivia and Mo = 0.26 × 1021 Nm for the Fiji earthquakes. Body wave analysis for the Bolivia event reveals a 45 s duration, a nearly linear E‐NE 1 km/s rupture on the subhorizontal focal mechanism plane giving a stress drop Δσ = 114 MPa. For the Fiji earthquake we find a change in focal mechanism and a linear 3 km/s rupture plunging shallowly to the north on the steep dipping nodal plane with a duration of at least 20 s and Δσ = 14 MPa. Both ruptures propagated parallel their null (σ2) axis and suggests that their width is controlled by the thickness of the seismogenic volume in deep slabs.

The Shikotan Earthquake of October 4, 1994: Lithospheric earthquake

A large Mw=8.2 earthquake occurred off Shikotan Is., one of the Kurile Is., on October 4, 1994. We inverted 32 body‐wave records to determine the rupture pattern using an iterative deconvolution method. The mechanisms of the subevents were allowed to vary during rupture. The source parameters obtained are: the location of the initial break = (43.48°N, 147.40°E); the centroid depth = 56 km; (strike, dip, rake) = (49°, 75°, 125°) for the total source; the seismic moment Mo = 2.6×1021 Nm (Mw=8.2); source time duration T = 42 s; the average rupture velocity ν = 2.5 km/s. We also determined the mechanism using long‐period Love and Rayleigh waves from 14 stations. The solution for a finite source distributed over a depth range from 0 to 90 km is (strike, dip, rake) = (54°, 76°, 129°) with Mo = 2.3×1021 Nm, in good agreement with that from body waves. Referring to the extent of the aftershock area and the subevent distribution, we estimated the fault area S = 120 × 60 km², the average slip D=5.6m, and the stress drop Δσ=11 MPa. We computed synthetic waveforms as well as static displacements using either the steep or the low‐angle plane as the fault plane, and found that the steep‐dip fault model fits the data better. Our result (the mechanism, large centroid depth, high stress drop) strongly suggests that the 1994 Shikotan earthquake is a lithospheric earthquake: an intra‐plate event that ruptures through a substantial part of the subducting oceanic lithosphere. This type of lithospheric earthquake is relatively common.

Source characteristics of the 1992 Nicaragua tsunami earthquake inferred from teleseismic body waves

We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment is M o = 3.4 × 1020 Nm, which is consistent with the value determined from long-period surface waves, M o = 3.7 × 1020 Nm. The average rise time of dislocation is determined to be τ ≈ 10s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault width W = 50 km, we obtained the dislocation D = 1.3 m. From τ and D the dislocation velocity is D = D/τ ≈ 0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop Δσ = 1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2μE s /M o , where μ and E s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than Δσ. The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.

Evidence for the variation of stress drop between normal and thrust faulting earthquakes in Italy

The horizontal components of 45 strong motion accelerograms recorded in Friuli, Italy, from May to September 1976, have been analyzed in order to estimate the stress drop of eight thrust faulting earthquakes. Two different stress parameters, the Brune stress drop Δσ and the apparent stress σa, have been considered. The Friuli main shock has a strong stress drop (Δσ ≈ 800 bars), while the aftershocks have stress drops which range from 250 to 400 bars. Five accelerograms written by the Montenegro, Yugoslavia, earthquake of April 1979 have also been analyzed. This thrust faulting earthquake had a stress drop of Δσ ≈ 900 bars. The stress drops of these thrust faulting earthquakes are compared to the stress drops determined by Rovelli et al. (1988a) for nine normal faulting earthquakes which occurred in the Apennines region from 1979 to 1984. The radiated energies and apparent stresses have also been calculated for the set of Apennine earthquakes. The thrust faulting earthquakes in the Alpine‐Dynaride region exhibit higher values for both the Brune and the apparent stress than the normal faulting earthquakes in the Apennines region. Excluding the Friuli and Montenegro main shocks, the Brune stress drops of the thrust faults are approximately 3 times those of the normal faults, while the apparent stresses are twice as large. This difference is statistically significant at a confidence level of 95%. The same difference also emerges from the scaling of the peak ground motions recorded in these two regions, suggesting an enhanced severity of the seismic input in the Alpine‐Dynaride environment compared with the Apennines.

Mode of stress release within a subducting slab of lithosphere: implication of source mechanism of deep and intermediate-depth earthquakes

We inverted the long-period P-wave seismograms of 28 deep and intermediate-depth earthquakes including 16 Tonga events to their sources. The results are incorporated with our previous results for 18 deep and intermediate-depth earthquakes to obtain a general picture of slab earthquakes. A large event with seismic moment Mo > 5 × 10 27 dyn cm tends to be a multiplet that consists of several subevents. A small event with Mo < 5 × 10 26 dyn cm remains a singlet. Stress drop Δσ of the largest subevent is 800 bar on average and significantly higher than Δσ(∼ 200 bar) of the largest subevent for interplate thrust earthquakes. Stress drop tends to scatter for smaller earthquakes beyond the resolution limit of our analysis. Deep and intermediate-depth shocks in the Tonga region possess higher Δσ, indicating a significant regionality in the mode of stress release within a subducting slab. Fault planes have, in general, preferred orientation according to the focal depth, indicating alignment of weak planes within the slab. The slab is therefore not simply elongated or shortened under down-dip extension or compression but rather rotates by bulk simple shear. We suggest that this bulk shear is a dominant mechanism for the deformation of a subducting plate, including its bending and unbending near deep-sea trenches.

Characteristics of serrated flow in commercial Cu–Be–Co alloy

Serrated yielding in a Cu–l·8Be–0·3Co (wt-%) alloy has been studied as a function of strain rate and temperature. Serration characteristics, e.g. type, stress drop δσ, frequency ν, vacancy concentrations–strain exponent m, mobile dislocation density–strain exponent β, and activation energy Qm for serrated yielding, have been determined. Strain rate sensitivity of flow stress δσ/∆ ln ɛ and activation volume V parameters have also been derived through strain rate change tests. The results have been interpreted in the light of the static strain aging model of the Portevin–Le Chatelier effect.MST/540

Characteristics of serrated flow in commercial Cu–Be–Co alloy

Serrated yielding in a Cu–l·8Be–0·3Co (wt-%) alloy has been studied as a function of strain rate and temperature. Serration characteristics, e.g. type, stress drop δσ, frequency ν, vacancy concentrations–strain exponent m, mobile dislocation density–strain exponent β, and activation energy Qm for serrated yielding, have been determined. Strain rate sensitivity of flow stress δσ/∆ ln ɛ and activation volume V parameters have also been derived through strain rate change tests. The results have been interpreted in the light of the static strain aging model of the Portevin–Le Chatelier effect.MST/540

Source retrieval for mantle earthquakes by iterative deconvolution of long-period P-waves

Long-period P-waves of eighteen deep and intermediate-depth earthquakes in a moment range 1×10 26−3× 10 28 dyn · cm are analysed by the method of Kikuchi and Fukao (this issue) to iteratively derive point dislocations on a plane. The fault plane is defined as the plane of point dislocations that gives the best fit between the synthetic and observed waveforms. For dip-slip type earthquakes the fault plane is found to dip more steeply than the auxiliary plane. In general, point dislocations tend to cluster into one or a few subevents. We characterize the largest subevent by its maximum moment rate ( M 0) m and its half duration τ from which the associated stress drop Δσ can be roughly estimated. For earthquakes shallower than 400 km, half duration τ is approximately constant, 5 s, so that stress drop Δσ increases from < 100 to > 1000 bar as ( M 0) m increases. The value of τ for the great Banda Sea earthquake of 1963 ( h = 100 km) is exceptionally large (τ~10 s, for which Δσ ~ 700 bar). For earthquakes deeper than 400 km, τ increases from 3 s to 9 s as ( M 0) m increases so that Δσ is approximately constant, 700 bar. Stress drop so obtained approximates to an overall stress drop with a possible exception of the several largest earthquakes. The distribution of point dislocations and the moment rate function indicate that rupture most often expands in a fan-shaped fault area and that a large part of the rupture front ceases propagation in a coherent manner. A sharp contrast in stress drop behavior across a depth of 400 km may be related to the olivine-spinel transition within the downgoing slab of lithosphere.