<i>P</i>and<i>S</i>traveltimes in the Tonga-Fiji region: A zone of low velocity in the uppermost mantle behind the Tonga Island Arc

Aggarwal, Yash; Barazangi, Muawia; Isacks, Bryan L.

doi:10.1029/jb077i032p06427

Cited by 33 publications

(6 citation statements)

References 17 publications

(2 reference statements)

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“…This structure is associated with deep seismicity and is of high contrast to model 1. From their investigation of the body-wave velocities in this region, Aggarwal, Barazangi & Isacks (1972) report extremely low velocities in the uppermost mantle. Oddly enough, these velocities are about the same as those of our ridge structure.…”

Section: Oceanic Structuresmentioning

confidence: 96%

Structural Dependence of the Apparent Initial Phase of Rayleigh Wave

Frez

Schwab

1976

Geophysical Journal International

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Summary For Rayleigh waves generated by buried point sources, we investigate the effect of incomplete knowledge of the structure beneath the epicentre on the apparent initial phase. The effect is described in terms of a critical distance between epicentre and receiver, beyond which we expect an overall error in phase velocity which does not exceed 0.04 km s−1. A quarter of this error is due to uncertainty in the subepicentral structure. The main classes of focal mechanisms are treated explicitly. For the common classes of shallow‐crustal continental, and crustal oceanic sources, the effect is unimportant. The main, deep‐crustal continental mechanisms require critical distances of 1600–2200 km, with avoidance of a 30‐s period range. The results for the common, shallow‐subcrustal mechanisms show strong effects due to structural uncertainty. For an oceanic source depth of 34 km, one must accept large critical distances of 5100–6900 km if only a 40‐s period range is to be avoided; for the smaller critical distances of 2600–2100 km, the period ranges to be avoided become quite large—up to 100 s in extent. Of the common classes of sources at depths of about 72 km, only vertical dip slip in an oceanic structure gives fairly good results. Other combinations of source class and environment result in critical distances of up to 11000 km, as well as some excessively large period ranges to be avoided. The results for the deeper, vertical dip‐slip mechanisms are better: maximum critical distances of 4600 and 2000 km for source depths of about 150 and 300 km, respectively.

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Section: Oceanic Structuresmentioning

confidence: 96%

Structural Dependence of the Apparent Initial Phase of Rayleigh Wave

Frez

Schwab

1976

Geophysical Journal International

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“…Numerous studies [e.g., Aggarwal et al, 1972;Barazangi and Isacks, 1971;Barazangi et al, 1972;Mitronovas and Isacks, 1971] Azimuthal depth phase delays could be accounted for if depth phases traveling to the west traversed the lower-velocity region while depth phases traveling to the east traversed the higher-velocity region. Although this may be a factor in the apparent depth discrepancy, we feel, for reasons which will become clearer in the next section, that most of the anomaly is related to effects of the deeper slab on the direct P waves.…”

Section: October 10 1977 Earthquak Ementioning

confidence: 99%

Large Earthquakes in the Tonga Region Associated With Subduction of the Louisville Ridge

Christensen¹,

Lay²

1988

J. Geophys. Res.

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Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.

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“…attenuation above the descending plate and behind the trench[Oliver and Isacks, 1967;Mitronovas et al, 1969]. More recently,Barazangi and Isacks [1971] have shown that the intrinsic attenuation factor Q in this wedge is even smaller than Q in the asthenosphere, with values as low as 50 Aggarwal et al [1972]. described a region of very low upper mantle velocities that was coincident with the region of very high attenuation behind the Tonga trench.…”

mentioning

confidence: 99%

Lateral variations in the structure of the Philippine Sea Plate

Seekins¹,

Teng²

1977

J. Geophys. Res.

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Multiple‐filter analysis is applied to surface waves with their paths almost entirely inside the Philippine Sea plate. The resulting dispersion data are used to study the crustal and upper mantle structures of the plate. Effort is made to examine the intraplate structural variations. Part of the Philippine Sea plate appears to have a thin plate thickness of about 30 km and is overlying a very low velocity mantle. Lateral structural variations in the plate are indicated by the dispersion data. The plate can be divided into several elongated regions, all of which are parallel to the eastern plate boundary and each having a rather distinct crustal and upper mantle structure. The active marginal basin region, located immediately to the west of the Mariana trench, probably represents an area of lithospheric generation. This is supported by the very low shear wave velocity, high heat flow, and magmatism. The Kyushu‐Palau ridge along the central portion of the plate seems to have a thicker lithosphere, in contrast to its two flank regions, the Philippine basin and the Parece Vela basin.

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PandStraveltimes in the Tonga-Fiji region: A zone of low velocity in the uppermost mantle behind the Tonga Island Arc

Cited by 33 publications

References 17 publications

Structural Dependence of the Apparent Initial Phase of Rayleigh Wave

Structural Dependence of the Apparent Initial Phase of Rayleigh Wave

Large Earthquakes in the Tonga Region Associated With Subduction of the Louisville Ridge

Lateral variations in the structure of the Philippine Sea Plate

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