Seismic velocity anomalies beneath the New Hebrides Island Arc: Evidence for a detached slab in the upper mantle (original) (raw)
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Precise relocations of earthquakes and seismotectonics of the New Hebrides island arc
Journal of Geophysical Research, 1978
Earthquakes for the period 1962-1973 are relocated by the method ofjoint hypocenter determination in order to resolve better the configuration and structure of the inclined seismic zone in the New Hebrides island arc. Twelve new focal mechanism solutions are reported and, together with previously published solutions, are integrated with the new information on the spatial distribution of hypocenters. At intermediate depths the seismic zone has a uniformly steep dip of about 70 ø and exhibits no resolvable contortions or disruptions along at least 700 km of the subduction zone. The thickness of the zone, about 20 km, may be in part due to seismically active fault zones which cut across a portion of the descending lithosphere. Features associated with the anomalous central region of the arc, where the d'Entrecasteaux fracture zone is being subducted and where the islands of Santo and Malekula appear to be in positions normally occupied by the oceanic trench, include the following: (1) an inclined zone of shallow earthquakes with very much smaller dip than is found elsewhere in the arc, (2) a pronounced gap in seismic activity at depths between about 50 and 120 km, (3) evidence for features in the upper or overthrust plate with trends transverse to the arc and parallel to the east-west trends of the topographic feature being subducted, including nodal planes of shallow focal mechanism solutions, and (4) two features which appear to coincide with the downdip projection of the northern scarp of the d'Entrecasteaux fracture zone, including a well-defined boundary between two adjacent zones of plate slippage along the main plate boundary and a faultlike feature in the intermediate depth seismic zone which also has a strike parallel to the fracture zone. INTRODUCTION The New Hebrides island arc is a structural complex, almost linear, trending south-southeast from 10 ø to 22øS (see Figure 1). Volcanic and seismic activities are very high. North and west of the Santa Cruz Islands the oceanic trench bends abruptly westward. Near 23øS the trench swings east-northeast toward Fiji, along the Hunter fracture zone. The central part of the arc (between 14 ø and 18øS) is unusual. The trench disappears there, and two islands, Santo and Malekula, are located along the positions where the inner wall of the trench would be expected. In this area a prominent east-west marine fracture, the d'Entrecasteaux fracture zone, abuts the New Hebrides system. In spite of the lack of a trench in the central part, the seismicity is continuous from the north to the south. Another unusual characteristic of the central part is the presence of three belts of islands. The geology is relatively Well known [Mitchell and Warden, 1971]. Pre-mid-Miocene rocks are exposed on the western and eastern belts, while the central belt consists of Pliocene to presently active volcanoes. East of the New Hebrides island arc is the Fiji Plateau, an area 2-3 km deep with a very thin cover of sediments and very high heat flow values [Sclater et al., 1972; Macdonald et al., 1973]. Extensive studies of attenuation [Barazangi et al., 1973, 1974] and velocity structure of and near the arc [Dubois, 1971; Dubois et al., 1973; Pascal et al., 1973] have been reported. These studies delineate an area of high seismic wave attenuation and low velocities in the uppermost mantle beneath the Fiji Plateau. A similarly anomalous zone is also found in the mantle between the deep and intermediate depth earthquakes of the New Hebrides arc, and Barazangi et al. [1973] and Pascal et al. [1973] infer that the lithospheric slab descending beneath the New Hebrides arc is not continuous to the depths of the deep earthquakes. Isacks and Molnar [1971] and Johnson and Molnar [1972] studied the focal mechanisms of the large New Hebrides events which occurred through 1970. The former concluded that the data for the northern part of the arc indicate downdip extension at intermediate depths, whereas in the southern part the stress orientation within the subducted slab is variable. The latter found shallow thrust faulting north of 16øS with a consistent east-northeast slip vector but more complex results in the south. This study reports new focal mechanism solutions for events occurring through 1972 and integrates these results with previous focal mechanism data and the relocations obtained here by the joint hypocenter determination (JHD) method. In regions of significant lateral velocity variation such as island arcs, locations by standard methods, i.e., locations done independently of one another by the so-called 'single-event' technique, are subject to systematic errors that are common to the entire region as well as systematic errors that vary from event to event within the region. The latter type of error may be particularly important where the set of stations used to locate the events varies considerably from event to event, as is usually the case. Times of P wave arrivals which pass through material with anomalous velocities in the descending slab can cause significant bias when used in the locatioos. Also, errors 4957 4958 PASCAL ET AL.' SEISMOTECTONICS OF NEW HEBRIDES ARC 16½ SOLOMON 165 ø 170" E 175 ø PACIFIC OCEAN CRUZ IS. MALEKULA PENTECOST AMla FIJI PLATEAU 15 ø S % ER FI 165 ø E 175'
Structure of the upper mantle in the convex side of the New Hebrides island arc
Geophysical Journal International, 1979
S-wave arrival times from 30 earthquakes of the New Hebrides island arc are read from long-period components of the WWSS network, within the distance range 30-80". S residuals from deep earthquakes of the New Hebrides and Tonga arcs are used to correct the residuals from the lower mantle effect and station corrections. An average delay of 3 to 4 s is found. This shows the existence of a low-velocity zone for S waves in the convex side of the arc. A three-dimensional inversion scheme is used to obtain the velocity structure of this zone. Other seismological data, such as pP attenuation, confirm the results of the inversion: two zones with low shear velocity (5 to 10 per cent lower than the J-B model) are defined. The first one is located beneath the d'Entrecasteaux fracture zone at a depth range of 150 to 350 km and the second one is in the southwest of the arc at the same depth range. Furthermore the deep upper mantle north of 16" S shows a normal S velocity, while south of 16's it shows a low S velocity. A geodynamical model based on these results is proposed.
Journal of Geophysical Research, 1973
Shallow earthquakes that occurred during 10 years between the New Hebrides and Fiji islands are relocated by using a digital computer. The spatial distribution of the earthquakes may outline plate boundaries in the Fiji plateau; these boundaries, however, are diffuse and could be broad zones of deformation. In the center of the plateau west of Fiji times of P and S waves traveling in the uppermost mantle indicate velocities of 7.70 and 4.30 km/ sec, respectively. Along the seismically active margins of the plateau P velocities are 7.30-7.40 km/sec. These velocities are considerably lower than P and S velocities of about 8.45 and 4.75 km/sec, respectively, of the normal oceanic basins of the Pacific plate to the north and east of the plateau. The zone of low velocity beneath the Fiji plateau and its boundaries seems to coincide, with a high seismic wave attenuation zone that exists in the uppermost mantle between the Fiji and New Hebrides islands. These observations and other geophysical and geological aspects of the Fiji plateau clearly imply that the different lithospheric plates between the two opposite-facing lithospheric consumption zones of Tonga and New Hebrides arcs were recently generated and are not part of the oceanic Pacific plate.
Bulletin of the Seismological Society of America
The P-wave velocity of the uppermost mantle and crustal thickness were determined for the central Vanuatu (formerly New Hebrides) Islands using data from a local seismograph network. Two different methods and data sets were used in this investigation. The first method used travel-time plots for earthquakes located well outside of the seismograph network. The second method used time-term analysis on sets of earthquakes. Both methods yielded a mantle velocity of 7.6 (_+0.1) km/sec and a crustal thickness of 27 (_+2) km for the region located just trenchward of the volcanic arc. Published refraction studies have demon-strated that the mantle velocity near the trench has a more typical value of 8.0 km/sec. This contrast in mantle velocities between the trench and the volcanic arc has also been reported for the Japanese and Kurile-Kamchatka Island arcs and, therefore, may be a general feature of subduction zones.
Journal of Geophysical Research, 1981
In 1977 a wide-aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southern New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subruction zone. Surprisingly, small differences (less than about 10 kin) were found between the locations of shallow-and intermediate-depth earthquakes as located by the temporary local network and by the International Seismological Centre using worldwide stations. Analysis of the P wave travel time data from the OBS station closest to the trench shows limited evidence for a high-velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by the spatial distribution of small size events located in this study. Shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blocks that are differentially uplifted along mainly NW-SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the volcanic arc. Well-determined depths of these events are between 11 and 22 km. The well-located intermediate-depth events define a 20-km-thick Benioff zone that has a dip of about 70 ø. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short-term sample of small events determined by the local network and the long-term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate-depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern New Hebrides arc where the trench begins to curve around to the east. and Barazangi, 1977; Pascal et al, 1978; Isacks et al., 1981]. In fact, the New Hebrides Benioff zone has the steepest dip known except possibly for the complex intracontinental Benioff zones beneath the Hindu Kush and Romania.
Patterns of seismicity associated with asperities in the Central New Hebrides Island Arc
Journal of Geophysical Research, 1986
In the central part of the New Hebrides Island Arc (Efate-Malekula region, 16"S-18.6'S) a sequence of moderate sized earthquakes, their aftershock sequences, and other clusters of small earthquakes together form an intricate but coherent time-space pattern that probably reveals a major asperity complex along the interplate boundary of the subduction zone. This pattern is determined by study of data from a local network. The sequences, including one event with magnitude MW 7.1, eight events with magnitudes M w between 5.8 and 6.3, and nearly 13,000 smaller events, occurred during the six year period 1978-1984. The seismicity is very unevenly distributed in space: a sharply defined east-west line near 17.2"s separates the very active Efate region to the south, where nearly 10,500 earthquakes occurred, from the less active Malekula region to the north, where less than 2,500 earthquakes occurred during the six year period. In the Efate region several spatial patterns are highlighted. First, the seismic regimes of the updip and the downdip part of the interplate boundary are different. The updip part is characterized by a 'Now at Departmknt of Marine, Earth and Atmospheric Sciences, The shallow seismicity of the New Hebrides island arc stands out among the world's convergent plate boundaries in
Analysis of earthquakes in the distance range 40–70° and inferred lower mantle structure
Physics of the Earth and Planetary Interiors, 1982
. Analysis of earthquakes in the distance range 40-70°and inferred lower mantle structure. Phys. Earth Planet. Inter., 28: 242-250. About fifty earthquakes in the distance range 40-70°and azimuthal range 45-120°from the Celebes, Philippines, Mariana and Kurile Island regions, and recorded at Gauribidanur seismic array in southern India, were used in the present study. Measurements on slowness and apparent azimuths were made on the first 30 s of the short period P-wave trains using an adaptive processing technique. Analysis of this data set has revealed no strong evidence for any triplications in the travel-time curve over the ranges in question. The P-wave velocity increases continuously with an almost uniform gradient below 1000 km depth range and is in very close agreement with the JB model. Almost all the observed slowness values of the events were anomalously low and consistent suggesting that they are caused by some azimuthal dependent structure near the array.
Journal of Geophysical Research, 1993
On oceanic transforms, most earthquakes are expected to occur on the principal transform displacement zone (PTDZ) and to have strike-slip mechanisms consistent with transform-parallel motion. We conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the Harvard centroid moment tensor catalogue and the bulletin of the International Seismological Centre, respectively. Events with unusual mechanisms occur on several transforms. We have determined the source mechanisms and centroid depths of 10 such earthquakes on the St. Paul's, Marathon, Owen, Heezen, Tharp, Menard, and Rivera transforms from inversions of long-period body waveforms. Relative locations of earthquakes along these transforms have been determined with a multiple-event relocation technique. Much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the PTDZ or the presence of large structural features that may influence slip on the fault. Reverse-faulting earthquakes occur at a compressional bend in the Owen transform in the area of Mount Error and at the St. Paul's transform near St. Peter's and St. Paul's Rocks. A normal-faulting earthquake on the Heezen transform is located at the edge of a pull-apart basin marking an extensional offset of the fault. Normal-faulting earthquakes along the Tharp, Menard, and Rivera transforms may also be related to extensional offsets. Some events with unusual mechanisms occur outside of the transform fault zone, however, and do not appear to be related to fault zone geometry. For instance, earthquakes with mechanisms indicating reverse-faulting on ridgeparallel fault planes are located near the ridge-transform intersections of the St. Paul's and the Marathon transforms. Possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion, differential lithospheric cooling, and the development of a zone of weakness along the fault zone, but we do not find strong evidence to conf'mn the influence of these processes. 1985; Sibson, 1985, 1986; Nabelek et al., 1987; Barka and Kadinsky-Cade, 1988; Saucier et al., 1992] have shown that bends or offsets in the fault can strongly affect the state of stress and 1Now at pattern of earthquake faulting near such features. Second, there is increasing evidence that at least some oceanic and continental transforms act as weak zones relative to the adjacent lithosphere and that the stress state near the fault departs from classical theory [Zoback et al., 1987; Mount and Suppe, 1987; Wilcock et al., 1990]. In this paper, we present new information from a study of oceanic transform earthquakes with unusual mechanisms or locations confirming that a vertical strike-slip fault along the PTDZ is not always an adequate model for transform geometry and slip. Recent studies of individual oceanic transforms with highresolution bathymetric mapping, side-scan sonar imaging, and observations from submersibles have revealed complex fault geometries and structures within the transform domain [Fox and Gallo, 1984, 1986]. Extensional or compressional jogs (bends or offsets) in the PTDZ have been documented along several transforms [e.g., Macdonald et al., 1979, 1986; Lonsdale, 1986; Gallo et al., 1986; Fornari et al., 1989]. Microearthquake experiments conducted with ocean bottom seismometers have indicated patterns of seismicity and fault plane solutions consistent with the presence of extensional relay zones along the Rivera [Prothero and Reid, 1982] and Orozco [Trdhu and Solomon, 1983] transforms. Large earthquakes occurring near compressional fault jogs on the Kane and Vema transforms in the Atlantic have been shown to consist of primary strike-slip events and secondary events with reverse faulting mechanisms [Bergman and Solomon, 1988]. Engeln et al. [1986] also noted eight Atlantic transform earthquakes with apparently shallowly dipping fault planes or a dip-slip component of displacement, but independent body waveform inversions have shown that two of these events are normal-faulting earthquakes occurring on the nearby ridge segment and one is a strike-slip earthquake on a steeply dipping plane with a smaller reverse-faulting precursor [Huang et al., 1986; Bergman and Solomon, 1988]. 16,187 16,188 WOLFE ET AL.: UNUSUAL OCEANIC TRANSFORM EARTHQUAKES Studies of continental strike-slip faults have illustrated the importance of fault geometry on the pattern of faulting. Jogs in strike-slip faults are known to produce uplift or subsidence along the fault, depending of whether the sense of the jog is compressional or extensional [Sylvester, 1988; Bilham and King, 1989; Anderson, 1990]. Earthquake rupture can be stopped by both compressional and extensional jogs [Segall and Pollard, 1980; King and Nabelek, 1985; Sibson, 1985, 1986]. Numerical models of strike-slip faults that are offset or contain bends show that such geometry can considerably alter the state of stress [Segall and Pollard, 1980; Saucier et al., 1992]. For instance, the thrust faulting component to the mechanism of the Loma Prieta earthquake has been attributed to the event' s location at a local bend in the San Andreas fault in the Santa Cruz Mountains [McNally et al., 1989]. In the North China Basin [Nabelek et al., 1987] and along strike-slip faults in Turkey [Barka and Kadinsky-Cade, 1988] earthquake mechanisms and locations in conjunction with geologic information indicate normal faulting at extensional jogs and reverse faulting at compressional jogs. The orientation of stresses in central California, as inferred from borehole breakout data, off-fault focal mechanisms, and the trends of active reverse faults and thrust-related anticlines along the fault system, indicate that ol is nearly perpendicular to the San Andreas fault within only a few kilometers of the fault zone [Zoback et al., 1987; Mount and Suppe, 1987; Jones, 1988]. Such an orientation differs from that of the stress field farther (-100 km) from the fault. The lack of a heat flow anomaly across the San Andreas fault requires that shear stresses acting on the fault plane be low, less than about 20 MPa [Brune et al., 1969; Lachenbruch and Sass, 1980]. These results, and a small predicted component of convergence between the Pacific and North American plates, have led to the suggestion that plate motion along the San Andreas is decoupled by a weak fault zone into a low-stress, strike-slip component along the fault and a high-stress, compressional component off the fault [Zoback et al., 1987; Mount and Suppe, 1987]. The orthogonality of ridge-transform plate boundaries suggests that oceanic transforms are also comparatively weak. A perpendicular ridge-transform-ridge configuration minimizes the energy dissipated along the plate boundary if the transform is a zone of weakness, i.e., if stresses resisting plate separation along the ridge axis are larger than the shear stresses along the transform [Lachenbruch and Thompson, 1972; Froidevaux, 1973; Stein, 1978]. Curvature of the ridge axis neovolcanic zone toward the transform fault as the ridge-transform intersection is approached is also consistent with ridge axis stresses being several times larger than shear stresses on the transform [Phipps Morgan and Parmentier, 1984]. Fracture zone bathymetry and magnetic anomalies indicate that oceanic transforms are zones of weakness that adjust to changes in plate motion and can be deformed by compression or extension [Menard and Atwater, 1968]. In a microearthquake experiment along the active transform portion of the Kane Fracture Zone, Wilcock et al. [ 1990] observed that while
The depths of the deepest deep earthquakes
Journal of Geophysical Research, 1985
The maximum depth of seismic activity is a fundamental observation providing a constraint on models of mantle dynamics. Although most recent investigations of mantle seismicity and dynamics state that seismicity extends to "about 700 km," focal depths of 720 km have been reported for large earthquakes, as well as depths greater than 800 km for small events. We have examined focal depths of events in several catalogs and conclude that (1) the deepest reliable focal depths are at about 670-680 km, and events with focal depths beneath 600 km occur in several widely separated geographic regions, (2) observations supporting focal depths exceeding 680 km are usually few, poor, or inconsistent, (3) the reduction of seismic activity beneath about 650 km is quite abrupt. However, we cannot determine whether activity stops completely at 670-680 km, or the maximum size of events decreases gradually beneath 650 km, with magnitudes of 6.5-7.0 occasionally occurring at 650 km, magnitudes of 5.5-6.0 at 680 km, 4.0-4.5 at 695 km, etc. Several models can explain the absence of seismic activity in the lower mantle. Two models are consistent with the abrupt termination of activity at about 650-680 km in widely separated areas: a barrier which resists penetration by the subducting lithosphere or a phase transition which modifies the properties of the subducted material as it goes into the lower mantle.
Large intermediate-depth earthquakes and the subduction process
Physics of The Earth and Planetary Interiors, 1988
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 (mB >= 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 mB >= 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.