Large Intermediate-depth Earthquakes Research Articles

Upper mantle melt caused by a subducted slab in the Indian-Eurasian continental subduction zone

A low-velocity layer atop the mantle transition zone has been extensively observed worldwide. In subduction zones, this layer is widely explained as partial melting triggered by slab subduction on a regional or global scale. However, direct observational evidence is still absent, and the response of the layer to slab subduction is not well known. Here, we image the seismic velocity around the mantle transition zone by matching synthetic and observed triplicated seismic P and sP waveforms in the Indian–Eurasian continental subduction zone. Our observations reveal a laterally varied low-velocity layer atop the mantle transition zone beneath the Hindu Kush, where a subducted slab extends to the mantle transition zone. The geometric morphology of the low-velocity layer documents that it is a partially molten layer induced by the subducted slab on a regional scale. Interestingly, our observations also support that the layer has a low viscosity. The decreased viscosity may facilitate slab motion in the deep mantle, contributing to slab stretching, tearing and break-off and its resultant rare recurring large intermediate-depth earthquakes in an intracontinental setting.

Self-reactivated rupture during the 2019 Mw = 8 northern Peru intraslab earthquake

The 2019/05/26 Northern Peru earthquake (Mw=8) is a major intermediate-depth earthquake that occurred close to the eastern edge of the Nazca slab flat area. We analyze its rupture process using high-frequency back-projection and seismo-geodetic broadband inversion. The latter approach shows that the earthquake propagated with almost purely normal faulting along the 60° eastward dipping plane. Both imaging techniques provide a very consistent image of the peculiar space-time rupture process of this earthquake: its 60-second long rupture is characterized both by a main northward propagation (resulting in a rupture extent of almost 200 km in this direction) and by a reactivation phase of the hypocentral area, particularly active 35 s to 50 s after origin time. Given the depth of this earthquake (125-140 km), the reactivation time window coincides with the arrival time of the surface-reflected elastic wavefield. Computed values of the dynamic Coulomb stresses associated with this wavefield are of the order of ten to several tens of kPa, in a range of values where dynamic triggering has already been observed. The reactivation phase of the Peru earthquake may thus originate from fault areas that were brought close to rupture by the initial rupture front before being triggered by stress increments provided by the reflected wavefield. Source time function complexity observed for other large intermediate-depth earthquakes further suggests that such a mechanism is not an isolated case.

Anomalously low aftershock productivity of the 2019 MW 8.0 energetic intermediate-depth faulting beneath Peru

The 26 May 2019 Peru (MW 8.0) earthquake struck within the nearly-horizontal underthrust Nazca plate at depths from ∼110 to 150 km below the upper Amazon, near a steep bend in the plate where it plunges down to a deep earthquake zone. Little prior seismicity occurred in this region, but large intraslab events with similar normal-faulting mechanisms have occurred to the west. The event is situated in a similar slab position to the 2017 Puebla-Morelos, Mexico earthquake, but the remote location resulted in limited loss of life and damage. Back-projection imaging and finite-fault inversion based on teleseismic data suggest a brittle and energetic rupture process with unilateral expansion northward over a 170-km-long zone at a rupture speed, Vr ∼3 km/s, with three normal-faulting patches of up to ∼4.5 m slip. Despite the mainshock size, it produced only three M4.0+ aftershocks within 300 km (one nearby); the aftershock productivity of the 2019 Peru earthquake is very low even among all large intermediate-depth earthquakes, but similar to that for other large Peruvian intraslab events. Comparison of aftershock productivity of the Peru event with that of global large earthquakes in various tectonic settings suggests that the low aftershock productivity can largely be attributed to regionally homogeneous faulting systems and relatively uniform stress state in the flat Peru slab.

Sources of uncertainties and artifacts in back-projection results

Summary Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. Mw > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1D or 3D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyze corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artifacts at their bounce points on the earth's surface, with these artifacts located far away from the epicenter if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple sub-events with varying focal mechanisms, BP tends to image sub-events emanating large amplitude coherent waveforms, while missing sub-events whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3D source-side velocity structures. The 3D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artifacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations, and 3D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3D source-side velocity structures, should be conducted to understand the uncertainties and artifacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics.

Faulting of natural serpentinite: Implications for intermediate-depth seismicity

The seismic potential of serpentinites at high pressure was investigated via deformation experiments on cored natural serpentinite samples, during which micro-seismicity was monitored by recording Acoustic Emissions (AEs). Deformation was performed at pressures of 3–5 GPa, using a Deformation-DIA device, and over a wide range of temperatures, both within and outside antigorite's stability field. Below 400 °C, serpentinite deformation involves “silent” semi-brittle mechanisms, even in cases where strain localization is observed. At high temperature (i.e., above 600 °C), despite conditions propitious to dehydration embrittlement (i.e., fast strain rates and reaction kinetics), joint deformation and dehydration lead to ductile shear, without generation of AEs. Brittle behavior was observed in a narrow temperature window ca. 500 °C. In this latter case, AEs are consistently observed upon faulting and extremely sharp strain localization is observed in recovered samples. The resulting microstructures are consistent with the inverse ductile-to-brittle transition proposed by Proctor and Hirth (2016) in antigorite. This may therefore be a source of seismicity in subducting slabs at mantle pressures and temperatures from 500 to 600 °C. However, the acoustic signal observed here is orders of magnitude weaker than what is obtained at low PT conditions with brittle failure, consistently with low radiation efficiency of serpentinite faulting (Prieto et al., 2013) and suggests that other mechanisms are responsible for large intermediate-depth earthquakes. In fact, the present results are in line with a recent study (Ferrand et al., 2017), that suggests that intermediate earthquakes are likely induced by mechanical instabilities due to dehydration in partly hydrated peridotites.

The Mw7.9 2014 intraplate intermediate-depth Rat Islands earthquake and its relation to regional tectonics

The 2014 Mw7.9 Rat Islands intermediate-depth (∼107 km) earthquake is studied using seismic observations at teleseismic distances. The nodal plane with a steep-dip angle (87°) and a strike of 308° inferred from the Global CMT solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the Joint Hypocenter Determination method. Its slip history is constrained using broadband P and SH waveforms and long-period (3–6 mHz) seismic waves in vertical and transverse components. The rupture process associated with this plane shows a rupture propagating at ∼2 km/s. The rupture essentially breaks one large slip patch (peak slip amplitude of 3.5 m) located 30 km west of the hypocenter. The size of this slip patch extends from 80 to 140 km depth, which would be the largest depth extent of earthquake slip ever documented. We show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600–650 °C isotherm. In addition, the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab. Our results further indicate that, along this segment of Aleutian Arc, the intermediate-depth earthquakes might contribute to the westward transporting motion of Rat block. Lastly, the slip distribution of this earthquake as well as that of two other well studied Mw>7.5 intermediate-depth earthquakes suggests that the coldest portion of the subducting slab, outlined by the double Wadati–Benioff zone, is seismic during large intermediate-depth earthquakes. Such a result is important for estimating the seismic hazard of future large intermediate-depth earthquakes.

Megathrust asperities and clusters of slab dehydration identified by spatiotemporal characterization of seismicity below the Andean margin

The space–time–size distributions of seismicity during the last four decades (1973–2010) are used to identify megathrust asperities, the degree of plate coupling (hence potential future large earthquakes) and slab dehydration below the Andean margin (15°–48°S). This seismicity displays globally a typical magnitude distribution with a b value of 1.04 ± 0.024, for a magnitude of completeness of 4.5 and without considering aftershock sequences. However, this unitary b value of the entire catalogue masks large variations (0.6–1.7) in space and time, interpreted as related to different tectonics contexts or different states of the seismic cycle. The study has been performed for different regions (upper continental plate, upper and lower parts of the subducting oceanic plate) with respect to a 3-D slab geometry model all along the Andean margin. Low b values (0.6–0.8) are observed in areas that were ruptured by large Mw≥ 7.5 megathrust earthquakes. A temporal analysis of these features show that b values where low before these earthquakes and remain low after the relaxation of the aftershock phase, suggesting that they are related to time-persistent asperities within the seismogenic zone of the megathrust. Interestingly, a persistently low b value was observed from 2001 to the end 2009 within the southern limit of the Maule Mw = 8.8 earthquake, 2010 February 27. Our results also identify other low b asperities below the Chilean margin that have not ruptured during the last 40 yr and are places of potentially large megathrust earthquakes in the future. In contrast, large b values (1.1–1.7) are observed at seismicity clusters occurring inside the subducted slab beneath the Central Volcanic Zone (CVZ) of the Andes and below the forearc near the subduction point of aseismic ridges and fracture zones. Clusters below the CVZ are spatially related to relative large intermediate-depth earthquakes (like the Mw = 7.8 Tarapaca 2005 earthquake) and may reveal extensive thermally-driven dehydration of the oceanic lithosphere that favours wet mantle melting and therefore magmagenesis feeding the volcanic chain.

Localization of T-wave energy on land revealed by a dense seismic network in Japan

SUMMARY We have investigated the wave propagation characteristics of a seismic T wave on land in northeastern Japan using the seismograms recorded by a dense seismic network. The analysed T wave is a unique one that was generated by a large intermediate-depth earthquake beneath the network that backpropagated from the Emperor Seamount Chain in the northern Pacific. The T wave has a predominant frequency of 1.5 Hz, which is lower than the previously reported values. Spatiotemporal variation of an rms envelope bandpass filtered with a passband of 1–2 Hz demonstrates multiple arrivals of the T wave and inhomogeneous distribution of amplitude. Judging from the spatial pattern of first arrival times and the polarization characteristics, such as the relatively large dip angle and high planarity, we regard the seismic T wave as a Rayleigh wave. A possible conversion area, estimated by a method similar to the source-scanning algorithm, is located to the northeast of Honshu Island and may have a dimension of ∼100 km. The most prominent feature of the spatiotemporal variation of the rms amplitude is the localization of T-wave energy in the northeastern part of Honshu Island. This localization can be attributed either to the close proximity of the conversion area to land or to the large extent of the area. However, these factors are only supplementary because they cannot explain a sharp contrast in amplitude between the two areas with comparable distance but different azimuth from the conversion point. An additional and essential factor in explaining the observation is the non-isotropic radiation from the conversion points. Since the conversion occurred on the continental slope, the maximum amplitude of converted energy is expected to appear either in the propagation direction of incoming acoustic waves or in the steepest direction of the local topography. These factors, in particular a convex shape of bathymetry, probably caused the focusing of converted energy into the northeastern part of Honshu Island.

Relocation of large intermediate-depth earthquakes in the Vrancea region, Romania, since 1934 and a seismic gap

AbstractWe relocatedM6 or larger intermediate-depth earthquakes in Vrancea, Romania that have occurred since 1934. To determine their absolute hypocenters accurately, we combined them with recent (from January 1996 to November 2003) smaller earthquakes recorded by the Romanian local network and then relocated all of them simultaneously using a modified joint hypocenter determination method. Using the data on recent small earthquakes we first obtained an estimate of the dimensions of the active seismic region of intermediate-depth earthquakes: length, 85 km; along a NE-SW direction; width, 20 km; depth 60-160 km. We then determined that the 1940 (M7.7), 1977 (M7.5) and 1990 (M6.9) major earthquakes were located near the NE edge of the seismic region, while the 1986 (M7.2) was located inside the seismic region. The focal depths of the 1940, 1977, 1986 and 1990 major earthquakes were calculated to be 124, 98, 135 and 84 km, respectively and their source areas to range from 60 to 140 km in depth. The only unbroken area is at a depth of 140–160 km, which we current consider to be a seismic gap. Based on the regularity of past large earthquakes, we propose that the nextM7 intermediate-depth earthquake can be expected to occur in Vrancea at a depth interval of 140–160 km sometime early in this century.

Different stress directions in the aftershock focal mechanisms of the Kushiro-Oki earthquake of Jan. 15, 1993, SE Hokkaido, Japan, and horizontal rupture in the double seismic zone

On 15 January, 1993, a large intermediate-depth earthquake with a magnitude of M=7.8, shook the eastern part of the island of Hokkaido, Japan, close to the northeastern Japan trench and to the Kuril trench; this earthquake is now referred to as the Kushiro-Oki earthquake. The main shock of the Kushiro-Oki earthquake showed a down-dip extension mechanism having horizontal and vertical nodal planes. The hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting Pacific plate. After the mainshock, more than 300 aftershocks were recorded by three seismograph networks. From these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks. The aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate; it did not, however, reach the upper plane. We estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone. Thirty-eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock, but 25 aftershock mechanisms had almost opposite directions to those of the mainshock. The hypocenters of these aftershocks were spatially mixed. This suggests that the stress state is strongly inhomogeneous in the aftershock region, and that horizontal movement occurs by the down-dip extensional stress which is stored in the region between two planes of the deep seismic zone. The remanent down-dip extensional stress was released as the consistent group of aftershocks. High aftershock activity may be attributed to the inhomogeneity of the material in the region. The previous four intermediate-depth earthquakes beneath Hokkaido Island as well as aftershock distributions of two of them have been examined for comparison with the Kushiro-Oki earthquake. The mainshocks of the four large intermediate-depth earthquakes are also located on the lower plane with a similar mechanism to that of the Kushiro-Oki earthquake. The aftershock regions of two of these earthquakes are also distributed horizontally. They indicate dominantly horizontal movement, with down-dip extension mechanisms at an intermediate depth in the subducting Pacific plate beneath Hokkaido Island.

Numerical Simulation of Displacement and Stress Fields Associated with the 1993 Kushiro-oki, Japan, Earthquake

—We constructed a three-dimensional finite element model to simulate coseismic and postseismic displacement and stress fields associated with the 1993 Kushiro-oki earthquake, which was a very large intermediate-depth earthquake that occurred within the subducted Pacific plate at a depth of 107 km beneath the southeastern part of Hokkaido, Japan. Taking the configuration of the subducted Pacific plate into account, we constructed a realistic model with lateral heterogeneity of viscoelastic structure. We assigned a variable slip distribution to the fault plane, which was obtained from inversion analysis of near-field seismic waveforms. The result shows that elastic deformation associated with the faulting reflects the assigned inhomogeneous slip distribution on the fault plane near the fault region, while it does not reflect the distribution on the free surface of the model. The calculated postseismic deformation does not reflect the slip distribution, but shows symmetric spatial patterns concerning the dipping direction of the fault both near the fault region and on the model surface. For the next 20 years following the earthquake, the amount of the calculated deformation is a fraction of the coseismic deformation. The calculated coeseismic deformation is large just above and below the fault plane, reaching 1 m, while the postseismic deformation is dominant near the upper and lower material boundaries between the subducted plate and the surrounding asthenosphere. The spatial distribution of maximum shear stress near the fault plane corresponds to the assigned slip distribution, amounting to 32 MPa. The directions of principal stress-change axes represent reverse fault type in the SSE region of the fault, whereas normal fault type is dominant in the NNW region with the exception of some asymmetrical spatial patterns of the principal stress-change axes on the fault due to the inhomogeneous slip distribution. Time variations both in the amount and the directions of stresses are minor, suggesting that the coseismic state of the stress would remain unchanged for two decades after the event.

Transient electric variations associated with large intermediate-depth earthquakes in South Aegean

A telluric station was installed on Crete in the South Aegean in order to investigate the existence of transient electric variations before large near intermediate-depth earthquakes.During the experiment, two transient electric variations (not correlated with geomagnetic variations) were recorded and associated with two time isolated intermediate-depth earthquakes with local magnitude ML 5.8 and 4.8, respectively.Finally a qualitative discussion on the form of the electric signal, based on the charge dislocation model, is presented.

Estimation of Ground Motions in the Valley of Mexico from Normal-Faulting, Intermediate-Depth Earthquakes in the Subducted Cocos Plate

The Valley of Mexico is exposed to seismic risk from normal-faulting, large intermediate-depth earthquakes. We explore two approaches to estimate future ground motions from such events at CU, a hill-zone site in the valley. In the first we obtain parameters of an ω2 seismic source model and determine amplification of seismic waves due to local site effects at CU. This permits estimation of Fourier spectrum of expected ground motion at CU from postulated earthquakes. We find that the S-waves suffer an amplification of 2.5 between 0.2 to 3.0 Hz. This amplification is similar to that observed from deep teleseismic events but differs from that obtained from shallow coastal events. In the second approach the available recordings at CU are used as empirical Green's functions (EGF) to synthesize motions from future large earthquakes. This approach is very powerful if the smaller event is truly an empirical Green's function for the postulated earthquake.

Forecasting large intermediate-depth earthquakes in the South Aegean

Statistical considerations and periodic patterns of seismicity supply evidence that hazardous shock(s) of M s ⋍ 6.8–8.0 must be expected in the next few years at depths of ∼ 80–100 km of the South Aegean Benioff zone. Space-time characteristics indicate that the most likely location of the next event is one of the two earthquake nests at the central and eastern earthquake sections of the zone. From the history and a lithospheric model of the region, the most probable locations of maximum intensity are along the convex arc side.

Large intermediate-depth earthquakes and the subduction process

This study provides an overview of intermediate-depth earthquake phenomena, placing emphasis on the larger, tectonically significant events, and exploring the relation of intermediate-depth earthquakes to shallower seismicity. Especially, we examine whether intermediate-depth events reflect the state of interplate coupling at subduction zones, and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes. Historic record of large intraplate earthquakes (m_B ≥ 7.0) in this century shows that the New Hebrides and Tonga subduction zones have the largest number of large intraplate events. Regions associated with bends in the subducted lithosphere also have many large events (e.g. Altiplano and New Ireland). We compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with M > 6 and depth between 40 and 200 km. The final catalog includes 335 events with 47 new focal mechanisms, and is probably complete for earthquakes with m_B ≥ 6.5. For events with M ≥ 6.5, nearly 48% of the events had no aftershocks and only 15% of the events had more than five aftershocks within one week of the mainshock. Events with more than ten aftershocks are located in regions associated with bends in the subducted slab. Focal mechanism solutions for intermediate-depth earthquakes with M > 6.8 can be grouped into four categories: (1) Normal-fault events (44%), and (2) reverse-fault events (33%), both with a strike nearly parallel to the trench axis. (3) Normal or reverse-fault events with a strike significantly oblique to the trench axis (10%), and (4) tear-faulting events (13%). The focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down-dip extensional stress prevails in a gently dipping plate. In contrast, along decoupled subduction zones great normal-fault earthquakes occur at shallow depths (e.g., the 1977 Sumbawa earthquake in Indonesia). Type 2 events, with strike subparallel to the subduction zone, and most of them with a near vertical tension axis, occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km. The increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and, consequently, the change in focal mechanism from type 1 to type 2 events. Events of type 3 occur where the trench axis bends sharply causing horizontal (parallel to the trench strike) extensional or compressional intraplate stress. Type 4 are hinge-faulting events. For strongly coupled zones we observed temporal changes of intermediate-depth earthquake activity associated with the occurrence of a large underthrusting event. After the occurrence of a large underthrusting event, the stress axis orientation of intermediate-depth earthquakes changes from down-dip tensional to down-dip compressional (e.g., 1960 Chile, 1974 Peru, 1982 Tonga and 1952 Kamchatka earthquakes), or the number of large intermediate events decreases for a few years (e.g., 1964 Alaska and 1985 Valparaiso earthquakes). We conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate-depth seismicity, spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate-depth events.

An alternative view of the Bayesian probabilistic prediction of strong shocks in the Hellenic Arc

A Bayesian discrete distribution, as developed by Ferraes (1985), is applied to predict the inter-arrival times for strong shocks in the Hellenic Arc on the basis of nine samples of shocks with seismotectonic locations very different from those used by Ferraes. The results suggest an alternative view of the Bayesian probabilistic prediction of strong earthquakes in the Hellenic Arc, and can be summed up as follows: 1. (a) Maximum final Bayesian probabilities of various inter-arrival times in a given seismotectonic segment are very dependent on the data set used and particularly on its time length. 2. (b) When using this method to determine the time intervals during which large shocks are to be expected in the Western and Eastern Hellenic arcs, it is very difficult to estimate intervals of less than a decade. The determination of the occurrence time, even in the long-term sense, remains the major problem in the prediction of these shocks. 3. (c) Bayesian probabilities in conjunction with seismicity observations indicate that large intermediate depth earthquakes in the Hellenic Arc are long overdue. Shocks of this sort can be expected to occur in the next few years. It is also pointed out that although Bayesian-type predictions may be useful for engineering purposes, they are not a suitable basis for making specific predictions or taking special precautions.

Interplate coupling and temporal variation of mechanisms of intermediate-depth earthquakes in Chile

Abstract We investigated the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large under-thrusting earthquakes in Chile. Focal mechanisms were determined for three large events (1 March 1934: M = 7.1, d = 120 km; 20 April 1949: M = 7.3, d = 70 km; and 8 May 1971: MW = 7.2, d = 150 km) which occurred down-dip of the great 1960 Chilean earthquake (MW = 9.5) rupture zone. The 1971 event is down-dip compressional: θ (strike) = 12°, δ (dip) = 80°, and λ (rake) = 100°. The 1949 earthquake focal mechanisms is θ = 350°, δ = 70°, and λ = −130°. The data available for the 1934 event are consistent with a down-dip tensional mechanism. Thus, the two events which occurred prior to the great 1960 Chilean earthquake are down-dip tensional. Published fault plane solutions of large intermediate-depth earthquakes (28 March 1965 and 7 November 1981) which occurred down-dip of the Valparaiso earthquakes of 1971 (MW = 7.8) and 1985 (MW = 8.0) are also down-dip tensional. These results suggest that before a major thrust earthquake, the interplate boundary is strongly coupled, and the subducted slab is under tension at intermediate depths; after the occurrence of an interplate thrust event, the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the down-going slab. This interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer-rise events are due to changes of interplate coupling.

Focal process of the South Sandwich Islands earthquake of May 26, 1964

A source study is made, primarily on the basis of long-period surface-wave data, on a large intermediate-depth earthquake that occurred in the South Sandwich Islands region on May 26, 1964. Long-period multiple Love and Rayleigh waves (G3 and R3) are obtained for a period range 100 to 400 s from the long-period records of WWSSN stations around the source. All the seismograms are equalized to a uniform propagation distance of 5 2 π. These observed seismograms are compared with synthetic seismograms for wave form, radiation pattern and asymmetry to estimate various source parameters. Wave forms of Rayleigh waves are very sensitive to depth, so that the source depth is determined to be 120 ± 20 km. A double-couple force system explains the observed data of surface and body waves. The aftershock hypocentres are confined to a remarkably thin zone which elongates northward from the origin of the main shock. The geometry of the aftershock area fits the geometry of one of the nodal planes of the double-couple solution. This strongly suggests that the earthquake occurred in the form of a shear faulting. This large-scale faulting is presumably due to the extensional force exerted by a sharp bending of the sinking lithosphere. The following source parameters are determined: force system, double-couple; source-time function, step function; fault plane, dip direction N 70° W, dip angle 48°; auxiliary plane, dip direction N 55° E, dip angle 57°; rupture length, 60 to 100 km; rupture velocity, 2 to 3 km/s towards the north; seismic moment, 6.2 × 10 27 dyne · cm; average slip dislocation, 3.5 m; stress drop, 120 b; strain drop, 1.7 × 10 −4; released strain energy (residual strain is assumed to be zero), 5.3 × 10 23 erg. In these calculations, the fault area and the rigidity are assumed to be 2.5 × 10 3 km 2 and 0.7 × 10 12 dyne/cm 2, respectively. The stress drop, 120 b, is about 100 b higher than stress drops for large shallow earthquakes. This may suggest that an effective cohesive strength of the mantle material increases with depth.