Intermediate-depth Events Research Articles

Investigation on site conditions for seismic stations in Romania using H/V spectral ratio

This research evaluates the soil conditions for seismic stations situated in Romania using the horizontal-to-vertical spectral ratio (HVSR). The strong ground motion database assembled for this study consists of 179 analogue and digital strong ground motion recordings from four intermediate-depth Vrancea seismic events with . In the first step of the analysis, the influence of the earthquake magnitude and source-to-site distance on the H/V curves is evaluated. Significant influences from both the earthquake magnitude and hypocentral distance are found especially for soil class A sites. Next, a site classification method proposed in the literature is applied for each seismic station and the soil classes are compared with those obtained from borehole data and from the topographic slope method. In addition, the success and error rates of this method are computed and compared with other studies from the literature. A more in-depth analysis of the H/V results is performed using data from seismic stations in Bucharest and a comparison of the free-field and borehole H/V curves is done for three seismic stations. The results show large differences between the free-field and the borehole curves. As a conclusion, the results from this study represent an intermediary step in the evaluation of the soil conditions for seismic stations in Romania and the need to perform more detailed soil classification analysis is highly emphasized.

Vrancea earthquake early warning system: first tests to add location capabilities

Providing warning notification several seconds before dangerous earthquake waves arrive at a target site reduces the property damages and human casualties. Most earthquake early warning systems (EEWS) developed are either regional (“network based”) or on-site (“stand alone”) systems. The recent upgrade of the seismic network in Romania with high dynamic range accelerometers allows recording of moderate to large magnitude earthquakes at very close epicentral distances (less than 10–20 km). This allows an increase of the warning lead-time (the time difference between the alert notification time and the arrival time of potentially destructive waves at a given target). The seismicity of Romania is significantly affected by earthquakes produced by the Vrancea seismic source with intermediate depth events (three shocks/century with magnitude Mw greater than 7.0). In this paper we present the performance of the seismic network in rapidly locate events occurring in the Vrancea area. Since rapid location of earthquakes is the first step in issuing early warning notifications, by reducing the time of the first valid location will lead to an increase the lead-time interval. Also rapid location of events will allow a future upgrade of EEWS to cover the entire Romanian territory.

Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

Stochastic Finite Fault Modeling of Subduction Zone Earthquakes in Northeastern India

In this article, a stochastic finite fault source model is calibrated to estimate ground motion in northeastern India for intermediate depth events originating in the Indo-Burmese tectonic domain. A total of 47 three-component accelerograms from eight events with magnitudes ranging from M w 4.8–6.4 are used to estimate the input source and site parameters of the finite fault source model. Key seismic parameters such as stress drop (Δσ) and site amplification function are determined from the recorded strong motion data. The obtained stress drop of the eight recorded events lies in between 105 and 165 bars.

Aftershock seismicity of the 27 February 2010 Mw 8.8 Maule earthquake rupture zone

On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35km depth and 50–120km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50km depth and further inland at 150–160km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw=6.9) on 11 March 2010.

Spatiotemporal variations in the statistical parameters of the shear wave splitting from local events beneath Eastern Hokkaido

Statistical analysis is carried out of the distribution of the fast Swave $$(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi } )$$ , polarization azimuths, time delays between the split shear waves $$(\delta \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{t} )$$ and the dominant frequencies of P waves $$(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P )$$ from weak seismic events that hit Eastern Hokkaido during the period of 1998–2003, which also includes strong events with M > 6 and the strong Tokachi-oki earthquake (M8.0) of September 26, 2003. The analysis revealed oscillatory pattern of variations in wave parameters along Eastern Hokkaido. In the rigid crustal blocks beneath the Tokachi and Kushiro Plains, stability is observed in the fast azimuth orientation and unstable behavior of $$\delta \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{t}$$ and $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ in the interval of periods 0.5–2.5 years. In the weakened zones beneath the Hidaka Range and the Nemuro Peninsula, periodical alternation of $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi }$$ along orthogonal directions is revealed. It is found that the parameters of the waves coming from the seismic events originating in different depth intervals experience anomalous changes before strong earthquakes. These anomalies reflect depth changes in the type of deformation of the medium. Active deformation is observed beneath the Hidaka Range. Dynamic changes in beneath the stations on either side of the range (ERM, MYR), their antiphase pattern, and the discrepancies in the behavior of the parameters from the events that originate in different depths indicate that these regions act as active buffers in the redistribution of stresses in the Kuril-Japan arc-arc junction zone. The onset of instability that develops before the Tokachi-oki earthquake is marked by gradual shortening of the period of oscillations in $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi }$$ and transition to the north and east azimuths as well as by the decreasing of $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ from the intermediate-depth events starting from middle 2002, and for deep events, starting from 2000. The acceleration of the process begins in the middle of 2003 and is recognized by a noticeable increase in and a decrease in $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ beneath almost all stations.

Relative excitation of the seismic shear waves Sn and Lg as a function of source depth and their propagation from Melanesia and Banda arcs to Australia

SUMMARY. - Seismic activity associated with the collision of the continentalpart of the Australian plate with the oceanic Melanesian arcs along Papua NewGuinea and the Banda arc provides an unusual opportunity to study the relativeexcitation of the seismic shear waves Sn and Lg. These waves are produced byearthquakes located along the arcs in the upper 200 km of the earth and arerecorded by the Australian WWSSN Stations at Charters Towers (CTA) and AliceSprings (ASP). The paths to these stations are predominantly continental. The dataclearly show that for events located at crustal depths, Lg is the predominant phaseon the records and Sn is either absent or very weak. For events deeper than about50-70 km, Sn becomes the predominant phase on the records. These observationsarc in qualitative agreement with the explanations of Sn and Lg as highermodes of surface waves, for the particle displacement amplitudes are maximumwithin the crust for Lg and maximum within the lid of the lithospheric mantlefor Sn. The data suggest that either the crustal wave guide for Lg is moreefficient than that for Sn, or that Lg is more easily excited than Sn. No clearLg is observed from shallow earthquakes when the length of the segment of thepath crossing oceanic structure is greater than about 200 km. Also, widespreadQuaternary volcanism within the « stable » area of central Papua New Guineato the south of the mobile belt does not seem to affect the efficient transmissionof high-frequency (1 Hz) shear energy.The paths from events located along the New Hebrides, Solomon, and NewBritain arcs to Australia traverse oceanic structure, and no Lg is observed fromthese paths. The inefficient propagation of Sn along these paths from bothshallow and intermediate-depth events can be explained as follows: 1) Forthe New Hebrides case, the inefficiency of Sn propagation for paths exceedingabout 20° distance is related to the relatively young age (Lower to MiddleEocene) of the suboceanic lithosphere of the Coral Sea. As proposed by CHINN,ISACKS and BARAZANGI (1979), such relatively thin lithosphere is probably not anefficient wave guide for shear energy of about 1 Hz 2). The inefficiency of Snpropagation from events located along the northern Solomon and the New Britainarcs is probably due to anomalous attenuation in the uppermost mantle beneaththe Woodlark basin and beneath southeastern Papua New Guinea. 3) Theinefficiency of Sn propagation from events deeper than about 150 km locatedin the westernmost part of the northerly-dipping Benioff zone of the NewBritain arc could be due either to structural isolation of the nearly verticaldescending segment of the plate in which the events occur, or could be due tostructural complexity of the plates in the region which interrupts the waveguide for Sn to the Australian stations. This structural feature could be theresult of the collision of the Australian plate and the New Britain arc.

Seismic response to slab rupture and variation in lithospheric structure beneath the Savu Sea, Indonesia

Variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc–continent collision in the Banda Arc are constrained by focal mechanism solutions from the CMT earthquake catalogue. In particular the slab under the western Savu Sea is unusual in that intermediate depth (70–300 km) events indicate that at this depth range the slab is largely in down-dip compression. This contrasts with the intermediate depth, down-dip tension that typifies the Sunda slab to the west and the far eastern Banda slab to the east. Down-dip compression beneath the Savu Sea reflects subduction of transitional crust of the Scott Plateau, more buoyant than the Indian Ocean crust subducting further west. In this region, enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc, and suggests that down-dip compression reflects not only more buoyant transitional crust but also a reduction in slab–wedge coupling induced by enhanced magma flux. East of the Savu Sea, the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where Australian continental crust has collided with the arc. Differences in seismic moment release around this slab window indicate asymmetric rupture, propagating to the east at a much faster rate than to the west.

Array Analysis of Regional-Distance P-Coda in South Asia

Abstract We present a comprehensive study of regional distance P -coda recorded by a medium-aperture array station located in Chiang Mai, Thailand (CMAR). Using array processing techniques, we analyzed data from 955 events that occurred at distances of 13°–30° with primarily continental paths. Nearly all significant, coherent arrivals in the P -coda traveled along the great circle path with ray parameters within of the direct arrival. Most of this energy was created near the source, probably via ( L g , R g )→ P scattering. This was especially true at the larger distances of 28°–30° in which coda decay rates decreased and large-amplitude significant arrivals occurred throughout the first 30 sec of the coda. The codas from approximately 40 intermediate-depth events in our data set were generally less energetic than shallow-event coda at comparable distances. In some cases, the coherent later arrivals we observed were related to upper mantle triplications. At distances of 14°–17° we observed clear secondary arrivals related to the 410 km discontinuity. They appeared on average 2 sec later than expected relative to the first arrivals, which may be partially explained by a depressed 410 beneath Burma. We isolated the diffuse component of the P -coda by subtracting array beams from individual elements to create residual seismograms. The diffuse, residual energy was highly variable from trace to trace and became equal in amplitude to the coherent component after only 20–30 sec of lapse time. We used a high-resolution maximum-likelihood technique to compute frequency–wavenumber spectra of the residual seismograms but did not conclusively observe any deterministic P → R g scatterer locations. The majority of the diffuse energy is created quite close to CMAR and does not travel as a plane wave across the array.

Combination of Acceleration-Sensor and Broadband Velocity-Sensor Recordings for Attenuation Studies: The Case of the 8 January 2006 Kythera Intermediate-Depth Earthquake

On 8 January 2006, an intermediate-depth earthquake occurred at the western part of the Hellenic trench close to the island of Kythera (southern Greece). This is the first intermediate-depth earthquake in the broader Aegean area that has produced such an extensive set of useful recordings, as it was recorded by the main permanent seismological networks and numerous acceleration sensors operating in Greece, as well as by EGELADOS, a large-scale temporary amphibian broadband seismological network deployed in the southern Aegean area. An effort to combine all the available data (broadband velocity and acceleration sensor) was made to study the properties of ground-motion attenuation of this earth- quake. The combination of both types of data revealed interesting properties of the earthquake wave field, which would remain hidden if only one type of data was used. Moreover, the data have been used for a validation of existing peak ground-motion empirical prediction relations and the preliminary study of the very inhomogeneous attenuation pattern of the southern Aegean intermediate-depth events at both near- and far-source distances.

High-Frequency Shear-Wave Propagation across the Hellenic Subduction Zone

Abstract This study investigates shear-wave propagation across the Hellenic subduction zone using high-quality waveforms of 71 intermediate-depth events (40–160-km depth). The data have been recorded by a permanent seismic network consisting of 22 broadband, three-component stations that cover most parts of Greece. Shear waves can be efficiently transmitted with low attenuation ( Q s ∼1100) through the descending slab, which extends laterally over much of the forearc area with a northwest–southeast strike. The slab also appears to be continuous downdip in the depth range 60–160 km, even though it was found that a gap smaller than 10 km would not affect shear-wave efficiency significantly. Two highly attenuating areas have been detected, one in the central/southeast Aegean corresponding to the mantle wedge and the other in the northern Aegean, probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area. These results provide additional constraints for the geometry of the Hellenic subduction zone and the geodynamic setting of the Aegean region.

On the recurrence time of earthquakes: insight from Vrancea (Romania) intermediate-depth events

SUMMARY The statistics of earthquake inter-occurrence times in broad areas have been reported to obey universal scaling laws (e.g. by Bak et al. and Corral). We investigate the recurrence times of intermediate-depth Vrancea earthquakes (depth >60km), taking advantage of the scarcity of aftershocks for deeper events. We use a complete earthquake catalogue (M ≥ 2.8) spanning from 1974 to 2002. The analysis of the probability density distribution of the recurrence times for the entire catalogue reveals a power-law regime at small scales. In order better to understand this scaling, we also analyse declustered catalogues, from which the distinct aftershock sequences following the 1977, 1986 and 1990 major earthquakes have been removed, and a time period without large events. For these data sets, the recurrence times follow an exponential distribution at all scales. This observation suggests that deviations from the exponential model ‐ characteristic of the scaling laws recently proposed ‐ result primarily from short-term aftershock clustering.

Waveform inversion of surface wave data: test of a new tool for systematic investigation of upper mantle structures

SUMMARY In most tomographic inversion of surface wave data, the long-period seismograms are first interpreted in terms of dispersion and/or attenuation curves before performing an inversion in terms of laterally and depth-varying properties. An alternative to this approach is to perform a direct waveform inversion or, as in Cara & Levěque, (1987), to use another set of secondary observables built up from the seismograms. In this paper, we systematically test with actual Rayleigh wave records this latter technique by considering laterally homogeneous models. We use for this purpose a set of recent long-period digital records from the Geoscope station Dumont d'Urville, Antarctica, for surficial events in the south Indian Ocean and the southeast Pacific Ocean, and for intermediate-depth events in the Vanuatu and Kermadec trenches. In addition to the obvious advantages of being able to deal with situations where overtones are present in the seismogram, it is found that the waveform inversion procedure allows us to reach a better depth resolution than in classical inversion of dispersion curves, even when only the fundamental mode is present in the seismogram. Quite good resolution is obtained to depth as large as 300km for S velocity when using surficial events located at a few thousand kilometres from the station, while classical surface wave studies do not allow us to resolve S velocity at depth larger than 150 km for these events. When intermediate-depth events are used at distances of about 5000 km, the presence of overtones in the signal allows us to get resolution to depth as large as 600 km for S velocity. Poorer resolution is obtained in both situations for the attenuation factor. S velocity appears furthermore to be more robust than attenuation at depth where good resolution is achieved. Due to the great sensitivity of surface wave amplitude to departure from the assumption of lateral homogeneity, more sophisticated direct modelling would be required to get more confidence in the inverted attenuation models.

Modern evaluation of the seismic hazard for the Romanian Plain

A new technique is developed to perform a deterministic hazard analysis in terms of physical parameters, i.e. displacements, velocities, maximum accelerations, and civil engineering specific ones, design accelerations. The method is adapted for the specific features of the Romanian Plain seismicity including the latest theoretical results in both seismotectonic and prediction fields, and data acquisitions. The deterministic analysis is performed computing free surface synthetic seismograms corresponding to different seismic sources and structural models, for crustal and intermediate-depth events separately. The DGA (design ground acceleration) greatest values and the NS and EW components resultant for the velocities and horizontal displacements are considered as a measure for seismic hazard level.

The 8 January 2006 Earthquake (Mw 6.7) Offshore Kythira Island, Southern Greece: Seismological, Strong-motion, and Macroseismic Observations of an Intermediate-depth Event

On 8 January 2006 at 11:34 GMT (13:34 local time), a strong earthquake with a moment magnitude of 6.7 occurred in southern Greece, off the eastern coast of the island of Kythira. The epicentral coordinates as estimated by the European Mediterranean Seismological Centre (EMSC-CSEM, http://emsc-csem.org were 36.31°N, 23.24°E, and the focal depth was 60 km. The shock was felt in a spatially extended area that covered Greece, Italy, Turkey, Egypt, Cyprus, Israel, Syria, Jordan, and Lebanon. Despite the large magnitude of the earthquake, the reported damage was not extensive mainly due to the intermediate focal depth. No casualties were reported and the structural damage to buildings was limited to the islands of Kythira and Antikythira and to the city of Chania, western Crete. Furthermore, landslides and/or rockfalls were reported only at the village of Mitata (Kythira island), where the main square and the road were damaged due to a landslide. Intermediate-depth earthquakes in this region are related to the southern Aegean subduction zone, which is referred to as the Hellenic arc (figure 1). The deformation that takes place along this arc is caused by the motion and collision of the southern part of the Eurasian plate with the African plate at a rate of 40–60 mm/year. The basic picture of the regional kinematics has not changed much since the early work of McKenzie (1972, 1978) but has been studied in more detail recently using GPS data (LePichon et al. 1995; McClusky et al. 2000; Nyst and Thatcher 2004, among others). The continuation of the Hellenic arc is interrupted by active fault zones with a perpendicular orientation relative to the boundary of the two plates (Lyberis et al. 1982). One of these structures exists in the vicinity of Kythira island and during the 20th century produced five …

Multifractal and Chaotic Analysis of Vrancea (Romania) Intermediate-depth Earthquakes: Investigation of the Temporal Distribution of Events

The Vrancea seismic region contains an isolated cluster of events beneath the Carpathian Arc Bend in Romania, dipping to about 200 km depth. Seismic activity mainly occurs at intermediate depths (h > 60 km). The main goal of the paper is to perform an in-depth, complex analysis of the occurrence times of these intermediate-depth events. We also try to show the versatility of the methods used to characterize different aspects of the seismicity evolution and to offer a user-friendly software toolbox to do most of the related computations. The earthquake catalog used in this study spans from 1974 to 2002 and includes only the intermediate-depth events. In the first part of the paper, we analyze the multifractal characteristics of the temporal distribution of earthquakes. The study reveals two distinct scaling regimes. At small scales we found a clear nonhomogeneous, multifractal pattern, while at large scales the temporal distribution of events shows a monofractal, and close to Poissonian (random), behavior. The multifractal behavior at small scales (minutes-hours) is shown to be clearly an effect of the “short” aftershock sequences that occurred after some major Vrancea earthquakes. In the second part of the paper we analyze whether our temporal series shows a persistent (or anti-persistent) long-term behavior, by using the Detrended Fluctuation Analysis (DFA) method. The results suggest that the analyzed temporal series of Vrancea earthquakes is a non-correlated process. In part three of the paper we seek to determine whether the dynamics of our earthquake system (described by the occurrence time of Vrancea earthquakes) is deterministically chaotic, deriving from a rather simple evolution law, or whether it is stochastic and is generated by a system that possesses many degrees of freedom. The results suggest that our signal is stochastic (probably does not possess an attractor). The limited time-span of the catalog and the analysis performed in this paper cannot rule out the emergence of an interesting, quasi-deterministic and low-dimensional structure in the case of major Vrancea earthquakes.

Intra-plate seismicity in the subducting Philippine Sea Plate, southwest Japan: magnitude–depth correlations

Recent studies of intra-plate seismicity have focused on detailed earthquake locations and potential hypotheses for intermediate depth events. Three main competing hypotheses for intra-plate events are: (1) transformational faulting; (2) shear melt instabilities; and (3) dehydration reactions. Recent work, using precise hypocenters and thermal-petrological models, indicates that the spatial distribution of intra-plate events coincides with dehydration reactions. These studies have mainly focused on the location of events and the proposed reactions with little attention given to the magnitude distribution of events. Presented here is a magnitude–depth correlation associated with intra-plate events for southwest Japan. Earthquake hypocenters were obtained from relocating a data set consisting of Japan Meteorological Agency (JMA) seismicity data and Japan University Network Earthquake Catalog (JUNEC). Data from 1985 to 1996 were combined and relocated using a standard location routine (HypoInverse) that allows regional variations in velocity models and station corrections. HypoInverse results show a well defined plane of seismicity associated with the subducting Philippine Sea Plate. The data were further relocated using a double-difference relocation algorithm (HypoDD). HypoDD results show a magnitude–depth correlation for Kii peninsula and northern Kyushu. Histogram plots indicate that events near the plate interface were generally magnitude 3 or less, whereas most large events were located within the lower crust or possibly the subducting oceanic mantle. This observation also appears to hold true for intra-plate events beneath Tohoku (Japan trench) and Hokkaido (Kuriles), but is not clearly observed beneath the Tokai region. A dehydration reaction model can well explain the magnitude–depth correlation.

MODELLING OF THE GROUND MOTION AT RUSSE SITE (NE BULGARIA) DUE TO THE VRANCEA EARTHQUAKES

An approach, capable of synthesising strong ground motion from a basic understanding of fault mechanism and of seismic wave propagation in the Earth, is applied to model the seismic input at a set of 25 sites along a chosen profile at Russe, NE Bulgaria, due to two intermediate-depth Vrancea events (August 30, 1986, M ω=7.2, and May 30, 1990, M ω=6.9). Accordingly to our results, once a strong ground motion parameter has been selected to characterise the ground motion, it is necessary to investigate the relationships between its values and the features of the earthquake source, the path to the site and the nature of the site. Therefore, a proper seismic hazard assessment requires an appro-priate parametric study to define the different ground shaking scenarios corresponding to the relevant seismogenic zones affecting the given site. Site response assessment is provided simultaneously in frequency and space domains, and thus the applied procedure differs from the traditional engineering approach that discusses the site as a single point. The applied procedure can be efficiently used to estimate the ground motion for different purposes like microzonation, urban planning, retrofitting or insurance of the built environment.

Guided waves propagating in subducted oceanic crust

We use guided waves traveling updip along the surface of the Nazca slab to image subducted oceanic crust at the Chile‐Peru subduction zone. Observed P onsets of intermediate depth events near 21°S in northern Chile reveal waveguide behavior: with growing focal depth, low‐frequency energy (<2 Hz) becomes more and more dominant, and higher frequencies arrive delayed, sometimes resembling two distinct phases. To explain the observations, we employ two‐dimensional finite difference (FD) simulations of complete P‐SV wave propagation along an updip profile of the subduction zone. The FD calculations shed some light on several basic issues regarding crustal waveguides. The development of guided waves dependent on event focal depth is simulated. Further, we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle. Simulations also yield constraints on source locations relative to the low‐velocity structure. Finally, the frequency content of P onsets is used to constrain the thickness of the waveguide. The results indicate that a structure of <4.5 km average width and 7% low‐velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km. The layer is interpreted as the unaltered lower part of the subducted oceanic crust, suggesting that complete eclogite transformation in the Chile‐Peru subduction zone is unlikely to take place until beyond the volcanic front.

Oblique Plate Convergence in the Indo-Burma (Myanmar) Subduction Region

— The Indo-Burma (Myanmar) subduction boundary is highly oblique to the direction of relative velocity of the Indian tectonic plate with respect to the Eurasian plate. The area includes features of active subduction zones such as a Wadati-Benioff zone of earthquakes, a magmatic arc, thrust and fold belts. It also has features of oblique subduction such as: an arc-parallel strike-slip fault (Sagaing Fault) that takes up a large fraction of the northward component of motion and a buttress (the Mishmi block) that resists the motion of the fore-arc sliver. In this paper, I have examined the seismicity, slip vectors and principal axes of the focal mechanisms of the earthquakes to look for features of active subduction zones and for evidence of slip partitioning as observed in other subduction zones. The data set consists of Harvard CMT solutions of 89 earthquakes (1977–1999 with 4.8≰M w ≰7.2 and depths between 3–140 km). Most of these events are shallow and intermediate depth events occurring within the Indian plate subducting eastward beneath the Indo-Burman ranges. Some shallow events within the fore-arc region have arc-parallel Paxes, reflecting buttressing of the fore-arc sliver at its leading edge. Some of the shallowest events have nearly E-W oriented P axes which might account for recent folding and thrusting. Examination of earthquake slip vectors in this region shows that the slip vector azimuths of earthquakes in the region between 20°–26°N are rotated towards the trench normal, which is an indication of partial partitioning of the oblique convergence. It is seen that all aspects of seismicity, including the paucity of shallow underthrusting earthquakes and the orientation of P axes, are consistent with oblique convergence. The conclusions of this paper are consistent with recent geological studies and interpretations such as the coexistence of eastward subduction, volcanic activity and transcurrent movement through mid-Miocene to Quaternary period.