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1.
Summary. A method of simultaneous reduction is presented for determining strain rates from multiple triangulation surveys where common triangulation stations have been used, but the angles of the old survey have not necessarily been reobserved. This method is applied to triangulation in the northern South Island, at the southern end of the Tonga-Kermadec-Hikurangi subduction zone. From a profile of shear strain across the Indian-Pacific plate boundary, the displacement of the Indian plate relative to the Pacific is calculated to be 54 ± 9 mm yr−1 at an azimuth of 84°± 10°, in remarkable agreement with the motion predicted by global plate tectonic models. Most of this motion occurs within a 150 km wide zone bounded on the east by the Hikurangi Trough. Within this zone the motion is partitioned: near the Hikurangi Trough no slip is occurring at the upper surface of the subducting Pacific plate (the subduction thrust) and motion is predominantly thrusting normal to the trough axis: to the west is a region of predominantly dextral strike slip faulting. This pattern is consistent with Fitch's model of oblique subduction. To the south of the profile, a change is observed in the azimuth of the faulting along a line which marks the southern extent of the subduction slab, indicating the end of the partitioned motion.  相似文献   

2.
Physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The initial mantle flow direction beneath the overriding plate can be horizontal or vertical, depending on its location with respect to the asthenospheric flow field. Imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern (horizontal or vertical, respectively). Overpressure promotes shallow dip subduction while underpressure tends to steepen the slab. Horizontal mantle flow with rates of 1–10 cm yr−1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of  ∼60°  , while the same lithospheric slab sinks vertically when no flow is imposed. Vertical drag force (due to downward mantle flow) exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere. The strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry. When the slab dip is steeper than the interplate zone, the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate. When the slab dip is shallower than the interplate zone, an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress. However, the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low.  相似文献   

3.
Shear wave splitting measurements from S arrivals of local earthquakes recorded at the Incorporated Research Institutions for Seismology (IRIS) broadband sensor SNZO are used to determine a basic anisotropic structure for the subduction zone in the Wellington region. With the use of high-frequency filters, fast anisotropic polarization ( φ ) and splitting time ( δt ) measurements typical of crustal anisotropy are evident, but the larger splitting expected from the mantle is often not resolved. The small splitting seen agrees well with the results of previous studies concerning shallow crustal anisotropy. With the use of lower-frequency filters, measurements more consistent with mantle anisotropy are made. Anisotropy of 4.4 ± 0.9 per cent with a fast polarization of 29° ± 38° is calculated for the subducting slab, from 20 to 70  km depth. Using this result in addition to the results of previous studies, a model is proposed. The model requires a frequency-dependent anisotropy of less than 1.4 per cent when measured with a period of ~2  s to be present in the sub-slab mantle.
Separate from this population, a band of events in northern Cook Strait with an 86° ± 10° fast polarization is seen. This is at about 40° from the strike of the Hikurangi margin, and suggests a source of shear strain 40° removed from that found in the majority of the region. The cause of this is probably a deformation in the subducting slab in this region, as it moves towards a greater incline to the south.  相似文献   

4.
Summary . Frictional heating in upper mantle shear flows may lead to localized thermal runaway and partial melting in the asthenosphere, but only as the result of a finite-amplitude disturbance. A rigorous two-dimensional stability analysis shows that asthenospheric shear flows are stable to small-amplitude perturbations whether such flows are supercritical (shear stress decreases with increasing plate velocity) or subcritical (shear stress increases with increasing plate velocity). Disturbances which maintain a shear stress larger than the critical value for sufficiently long will lead to runaway. The response of the asthenosphere to events which do not satisfy this criterion must be determined by a non-linear analysis. Reasonable models of flow in the asthenosphere could be driven to runaway, at a superexponential growth rate, by sudden increases in shear stress of less than 10 bar. Disturbances resulting from plate collisions may maintain large enough stresses for sufficiently long times to initiate runaways, while stress changes associated with large earthquakes probably occur too rapidly to do so.  相似文献   

5.
In this paper we present revised locations and original focal mechanisms computed for intermediate and deep earthquakes that occurred within the Southern Tyrrhenian subduction zone between 1988 and 1994, in order to improve our knowledge of the state of stress for this compressional margin. In particular, we define the stress distribution within a large portion of the descending slab, between 40 and about 450 km depth. The seismicity distribution reveals a continuous 40–50 km thick slab that abruptly increases its dip from subhorizontal in the Ionian Sea to a constant 70° dip in the Tyrrhenian. We computed focal mechanisms for events with magnitudes ranging from 2.7 and 5.7, obtaining the distribution of P - and T -axes for many events for which centroid moment tensor (CMT) solutions are not available, thus enabling the sampling of a larger depth range compared to previous studies. We define three portions of the slab characterized by different distributions of P - and T -axes. A general down-dip compression is found between 165 and 370 km depth, whereas in the upper part of the slab (40–165 km depth) the fault-plane solutions are strongly heterogeneous. Below 370 km the P -axes of the few deep events located further to the north have a shallower dip and are not aligned with the 70° dipping slab, possibly suggesting that they belong to a separated piece of subducted lithosphere. There is a good correspondence between the depth range in which the P -axes plunge closer to the slab dip (∼ 70°) and the interval characterized by the highest seismic energy release (190–370 km).  相似文献   

6.
We use data from the Chile Argentina Geophysical Experiment (CHARGE) broad-band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the South American subduction zone between 30°S and 36°S. This region contains a zone of flat slab subduction where the subducting Nazca Plate flattens at a depth of ∼100 km and extends ∼300 km eastward before continuing its descent into the mantle. We use a grid-search multiple-event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the Wadati-Benioff zone. These contours reflect slab geometries from previous studies of intermediate-depth seismicity in this region with some small but important deviations. Our hypocentres indicate that the shallowest portion of the flat slab is associated with the inferred location of the subducting Juan Fernández Ridge at 31°S and that the slab deepens both to the south and the north of this region. We have also determined first motion focal mechanisms for ∼180 of the slab earthquakes. The subhorizontal T -axis solutions for these events are almost entirely consistent with a slab pull interpretation, especially when compared to our newly inferred slab geometry. Deviations of T -axes from the direction of slab dip may be explained with a gap within the subducting slab below 150 km in the vicinity of the transition from flat to normal subducting geometry around 33°S.  相似文献   

7.
The deep seismicity of the Tyrrhenian Sea   总被引：4，自引：0，他引：4  
The study reappraises the deep seismicity of the Tyrrhenian Sea. Careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping NW, about 200 km along strike, 50 km thick, and reaching a depth of about 500 km. The zone is slightly concave to the NW at a depth of 300 km, but, contrary to many previous reports, is not tightly concave, nor are there significant spatial gaps in the seismicity, which is effectively continuous with depth. Seismicity is, however, concentrated in the depth interval 250–300 km, where the dip of the seismic zone changes from 70° (above 250 km) to a more gentle dip of 45° at greater depths. Seven fault-plane solutions are available for the largest earthquakes in this depth interval, all of them consistent with a P -axis down the dip of the seismic zone, and all of them requiring movement on faults out of the plane of the subducting slab.
Two deep earthquakes near Naples lie well outside the main zone of activity; for one of which a fault-plane solution is available that has a P -axis not aligned with the dip of the seismic zone. The tightly concave slab-geometry favoured by other reports is supported mainly by the location of these events near Naples, which we think may represent deformation in a separate, probably shallower dipping, piece of subducted lithosphere.
The lack of shallow seismicity, and particularly of thrust faulting earthquakes, at the surface projection of the Benioff zone suggests that active subduction has ceased. Estimates of the convergence rate responsible for subduction in the last 10 Myr far exceed the present convergence rate of Africa and Eurasia, suggesting that the subduction was related instead to the stretching and thinning of the crust in the Tyrrhenian Sea.  相似文献   

8.
The dynamical origin of subduction zone topography   总被引：1，自引：0，他引：1  
Summary. Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 1022 P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.  相似文献   

9.
Summary. Quartz eclogite as source rock seems capable of explaining the chemistry of calc-alkaline volcanics. This model requires partial melting of quartz eclogite in the depth range 100–200 km. Shear heating of the subducting crust is one of the most debated models. The problem may be treated with two different boundary conditions: constant strain rate or constant shear stress. In the former case an increase in temperature tends to reduce the heat production thus stabilizing the shear flow; the result is a moderate increase in temperature which will remain constant during flow. The latter case may lead to a thermal feedback instability and thus temperature and strain rate may suddenly grow to very high values. This phenomenon is termed a runaway and will be discussed in this paper using an adiabatic approximation. It is shown that for a runaway to occur the local energy density must amount to a common value independent of the rheology. In contrast to the constant strain rate case, shear heating is negligible until just before the instability occurs. When the melting point is reached shear stress will break down but the stored local energy will be set free and supply the latent heat of melting. The possibility of a runaway occurring is strongly dependent on the ambient temperature. In subduction shear zones neither shear stress nor strain rate are likely to be constant throughout, but if the former is constant or changes little on a 1–10 km scale a runaway is liable to occur at a depth of around 150 km thus possibly being the cause of calcalkaline volcanism.  相似文献   

10.
By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr−1 within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr−1 at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr−1 during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.  相似文献   

11.
Summary. This paper explores the middle ground between complex thermally-coupled viscous flow models and simple corner flow models of island arc environments. The calculation retains the density-driven nature of convection and relaxes the geometrical constraints of corner flow, yet still provides semianalytical solutions for velocity and stress. A novel aspect of the procedure is its allowance for a coupled elastic lithosphere on top of a Newtonian viscous mantle. Initially, simple box-like density drivers illustrate how vertical and horizontal forces are transmitted through the mantle and how the lithosphere responds by trench formation. The flexural strength of the lithosphere spatially broadens the surface topography and gravity anomalies relative to the functional form of the vertical flow stresses applied to the plate base. I find that drivers in the form of inclined subducting slabs cannot induce self-driven parallel flow; however, the necessary flow can be provided by supplying a basal drag of 1–5 MPa to the mantle from the oceanic lithosphere. These basal drag forces create regional lithospheric stress and they should be quantifiable through seismic observations of the neutral surface. The existence of a shallow elevated phase transition is suggested in two slab models of 300 km length where a maximum excess density of 0.2 g cm−3 was needed to generate an acceptable mantle flow. A North New Hebrides subduction model which satisfies flow requirements and reproduces general features of topography and gravity contains a high shear stress zone (75 MPa) around the upper slab surface to a depth of 150 km and a deviatoric tensional stress in the back arc to a depth of 70 km. The lithospheric stress state of this model suggests that slab detachment is possible through whole plate fracture.  相似文献   

12.
Destructive upper crustal earthquakes in Central America are often located between active volcanic centres—a geometric relationship that we study using finite element Coulomb failure stress (CFS) models that incorporate the rheologically heterogeneous nature of the volcanic arc. Volcanoes are simulated as mechanically weak zones within a stronger crust. We find that deformation of the volcanic centres within a regional stress field dominated by dextral shear causes stress increases in surrounding crust, with a maximum CFS change between neighbouring volcanoes. This increase in CFS enhances the probability of fault slip on arc-normal faults that are located between volcanic centres; for example, the Tiscapa fault, which ruptured during the 1972 December 13,   M s   6.2 Managua earthquake. The amount of stress increase due to long-term (100 yr) volcano shearing is on the order of 0.1–0.6 bars, similar to values estimated for subduction zone earthquakes.  相似文献   

13.
Summary. An inversion of ISC travel-time data from selected earthquakes in the distance range 30°-90° to 53 stations in Central Europe has been used to model velocity down to 600 km depth. The model explains 0.1–0.2s of the residuals, as for other array studies, leaving 0.5 s unexplained as noise. The uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology. This may be due to deep-seated thermal anomalies or, in some areas, to delays introduced by passage of the rays through sedimentary cover. The deeper anomalies are smaller and unrelated to those in the lithosphere, which suggests that the asthenosphere is decoupled from the rigid lithosphere. The structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity. The models show some suggestion of a high velocity slab trending from east to west beneath the Alps.  相似文献   

14.
Summary. Bulletins of the International Seismological Centre (ISC) show very large residuals, up to 15 s early, for arrivals from events in the Tonga–Kermadec subduction zone to the New Zealand network of seismometers. The very early arrivals are confined to events south of about 22°S, and shallower than about 350 km. The waveforms show two distinct phases: an early, emergent, first phase with energy in the high-frequency band 2–10 Hz, and a distinct second phase, containing lower frequency energy, arriving at about the time predicted by JB tables.
The residuals are attributed to propagation through the cold, subducted lithosphere, which has a seismic velocity 5 per cent faster, on average, than normal. Ray tracing shows that the ray paths lie very close to the slab for events south of 22°S, but pass well beneath the slab for events further north, corresponding to the change in residual pattern. This characteristic of the ray paths is due to the curved shape of the seismic zone, and in particular to the bend in the zone where the Louisville ridge intersects the trench at 25°S.
The residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36°S. The very high-frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high Q , and to transmit 2–10 Hz energy with little attenuation.
The absence of low-frequency energy in the first phase is due to the narrowness of the high-velocity slab, which transmits only short-wavelength waves. The second phase, which contains low frequencies, is identified as a P -wave travelling beneath the subducted slab in normal mantle. There is no need to invoke any special structures, such as low-velocity waveguides or reflectors, to explain any of the observations. The S -wave arrivals show similar effects.  相似文献   

15.
We describe results of an active-source seismology experiment across the Chilean subduction zone at 38.2°S. The seismic sections clearly show the subducted Nazca plate with varying reflectivity. Below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2–5 km thick, highly reflective region, which we interpret as the subduction channel, that is, a zone of subducted material with a velocity gradient with respect to the upper and lower plate. Further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 Valdivia hypocentre. The plate interface itself can be traced further down to depths of 50–60 km below the Central Valley. We observe strong reflectivity at the plate interface as well as in the continental mantle wedge. The sections also show a segmented forearc crust in the overriding South American plate. Major features in the accretionary wedge, such as the Lanalhue fault zone, can be identified. At the eastern end of the profile a bright west-dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc.  相似文献   

16.
Signature of remnant slabs in the North Pacific from P-wave tomography   总被引：1，自引：0，他引：1  
A 3-D ray-tracing technique was used in a global tomographic inversion in order to obtain tomographic images of the North Pacific. The data reported by the Geophysical Survey of Russia (1955–1997) were used together with the catalogues of the International Seismological Center (1964–1991) and the US Geological Survey National Earthquake Information Center (1991–1998), and the recompiled catalogue was reprocessed. The final data set, used for following the inversion, contained 523 430 summary ray paths. The whole of the Earth's mantle was parametrized by cells of 2° × 2° and 19 layers. The large and sparse system of observation equations was solved using an iterative LSQR algorithm.
A subhorizontal high-velocity anomaly is revealed just above the 660 km discontinuity beneath the Aleutian subduction zone. This high-velocity feature is observed at latitudes of up to ~70°N and is interpreted as a remnant of the subducted Kula plate, which disappeared through ridge subduction at about 48 Ma. A further positive velocity perturbation feature can be identified beneath the Chukotka peninsula and Okhotsk Sea, extending from ~300 to ~660 km depth and then either extending further down to ~800 km (Chukotka) or deflecting along the 660 km discontinuity (Okhotsk Sea). This high-velocity anomaly is interpreted as a remnant slab of the Okhotsk plate accreted to Siberia at ~55 Ma.  相似文献   

17.
We image the Hikurangi subduction zone using receiver functions derived from teleseismic earthquakes. Migrated receiver functions show a northwest dipping low shear wave feature down to 60 km depth, which we associate with the crust of the subducted Pacific Plate. Receiver functions (RF) at several stations also show a pair of negative and positive polarity phases with associated conversion depths of ∼20–26 km, where the subducted Pacific Plate is at a depth of ∼40–50 km beneath the overlying Australian Plate. RF inversion solutions model these phases with a thin low S -wave velocity zone less than 4 km thick, and an S -wave velocity contrast of more than ∼0.5 km s−1 with the overlying crust. We interpret this phase pair as representing fluids near the base of the lower crust of the Australian Plate, directly overlying the forearc mantle wedge.  相似文献   

18.
An analysis of the Zihuatanejo, Mexico, earthquake of 1994 December 10 ( M = 6.6), based on teleseismic and near-source data, shows that it was a normal-faulting, intermediate-depth ( H = 50 ± 5 km) event. It was located about 30 km inland, within the subducted Cocos plate. The preferred fault plane has an azimuth of 130°, a dip of 79° and a rake of −86°. The rupture consisted of two subevents which were separated in time by about 2 s, with the second subevent occurring downdip of the first. The measured stress drop was relatively high, requiring a Δσ of about a kilobar to explain the high-frequency level of the near-source spectra. A rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below Zihuatanejo, based on the depth and the rupture extent of this event, is 40 km.
This event and the Oaxaca earthquake of 1931 January 15 ( M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, low-angle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event.  相似文献   

19.
Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s–1 at 0–10 km depths to 3.86 km s–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.  相似文献   

20.
Joint inversion of active and passive seismic data in Central Java   总被引：2，自引：0，他引：2  
Seismic and volcanic activities in Central Java, Indonesia, the area of interest of this study, are directly or indirectly related to the subduction of the Indo-Australian plate. In the framework of the MERapi AMphibious EXperiments (MERAMEX), a network consisting of about 130 seismographic stations was installed onshore and offshore in Central Java and operated for more than 150 days. In addition, 3-D active seismic experiments were carried out offshore. In this paper, we present the results of processing combined active and passive seismic data, which contain traveltimes from 292 local earthquakes and additional airgun shots along three offshore profiles. The inversion was performed using the updated LOTOS-06 code that allows processing for active and passive source data. The joint inversion of the active and passive data set considerably improves the resolution of the upper crust, especially in the offshore area in comparison to only passive data. The inversion results are verified using a series of synthetic tests. The resulting images show an exceptionally strong low-velocity anomaly (−30 per cent) in the backarc crust northward of the active volcanoes. In the upper mantle beneath the volcanoes, we observe a low-velocity anomaly inclined towards the slab, which probably reflects the paths of fluids and partially melted materials in the mantle wedge. The crust in the forearc appears to be strongly heterogeneous. The onshore part consists of two high-velocity blocks separated by a narrow low-velocity anomaly, which can be interpreted as a weakened contact zone between two rigid crustal bodies. The recent Java M w= 6.3 earthquake (2006/05/26-UTC) occurred at the lower edge of this zone. Its focal strike slip mechanism is consistent with the orientation of this contact.  相似文献   

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