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Slab bending and its effect on the distributions and focal mechanisms of deep-focus earthquakes R. Myhill* * Corresponding author: [email protected] 1. SUMMARY The subduction of cold oceanic plates is often accompanied by zones of intense earthquake activity, which can extend as far as the upper-lower mantle boundary at ∼670 km depth. The factors controlling the detailed distribution of earthquakes within subduction zones have long been of interest [1], but remain poorly understood. Such factors must satisfy the requirements for earthquake generation, namely: I A source of stress, which may be generated by large-scale tectonics or local volume changes. I A rheology capable of generating earthquakes, which may or may not require the presence of particular mineral phases or reactions. Here I show that many features of earthquake distributions and focal mechanism orientations can be explained by buckling induced by resistance to subduction at the base of the upper mantle [2]. 2. TECHNIQUE Buckling has been invoked to explain the shape of the Tonga Wadati-Benioff zone (WBZ) [3], and should produce characteristic patterns of strain within subducting slabs [4]. High strain rates should be localised within the hinges of folds, with focal-mechanism B-axes parallel to the hinges. To test the hypothesis that buckling controls aspects of deep-focus seismicity, slab shapes must be accurately constrained. The Izu-Bonin, Solomons and Tonga slabs have sufficiently dense seismicity to model slab shape using a continuous curvature surface gridding algorithm [5] (Figure 1). The Engdahl, van der Hilst and Buland catalogue [6] is used to provide high quality hypocentral locations between 1960 and 2007. Double-couple moment tensor solutions are derived from the gCMT catalogue [7, 8] and rotated into the local slab reference frame. Focal mechanism similarity [9] to used to cluster earthquakes into groups. b c d f e poles to surface hinge axial plane earthquake locations best-fitting plane best-fitting surface projected locations focal mechanisms into slab along strike down dip 120˚ 120˚ 160˚ 160˚ −160˚ −160˚ −60˚ −60˚ −30˚ −30˚ 0˚ 0˚ 30˚ 30˚ 60˚ 60˚ 0 200 400 600 km Hypocentral depth North Tonga South Tonga Solomons Izu-Bonin North Tonga South Tonga Solomons Izu-Bonin a Figure 1: a) Earthquakes and study areas in the Western Pacific. b–f) A graphical representation of the WBZ surface-fitting and focal mechanism-orienting procedure. b) High quality earthquake locations are selected from the EHB catalogue [6] and converted into cartesian coordinates. c) The best-fitting plane to these locations is found. d) A surface fitting algorithm [5] is used to find the best fit surface (where the z-vector is perpendicular to the best-fitting plane). The EHB earthquake locations are projected onto this surface. e) The best-fitting great circle to the slab poles at each projected earthquake location is used to constrain fold orientations. The axial plane is found by bisecting the interlimb angle. f) The slab poles at projected EHB earthquake locations are used to rotate gCMT focal mechanisms [7, 8] into the local slab reference frame. Each earthquake is coloured according to the orientation of its focal mechanism. 3. RESULTS The Izu-Bonin slab (Figure 2) exhibits a prominent synform at 300–500 km depth, with the dip of the slab decreasing from 50–90 ◦ to 0–30 ◦ along a hinge plunging 10–15 ◦ toward the southeast. A broad, dense band of seismicity follows this hinge. Earthquakes within this band have focal mechanisms which indicate in-plane compression and hinge-parallel B-axes (orange). The subducting Solomons Sea Plate (Figure 3) is also bent, with a synform in the eastern segment and antiforms in both the eastern and western segments. Lineations of earthquakes mark the hinges of these folds. Like earthquakes in the Izu-Bonin slab, focal mechanisms have hinge-parallel B-axes. Earthquakes within the synform indicate an in-plane compressional coseismic strain field (red), while in-plane extension appears to dominate within each of the antiforms (blue). 100 200 300 400 500 600 135˚ 135˚ 140˚ 140˚ 145˚ 145˚ 150˚ 150˚ 25˚ 30˚ 35˚ A’ A −5˚ −4˚ −3˚ −2˚ −1˚ 0˚ 1˚ 2˚ 3˚ 4˚ 5˚ 100 100 200 200 300 300 400 400 500 500 600 600 700 700 A A’ 1 0 0 50 0 150 200 250 300 350 400 450 500 -50 -100 100 50 0 150 200 250 300 350 400 450 500 -50 -100 150 150 200 200 Depth (km) Depth (km) a d z’ (km) x’ (km) y’ (km) b down dip into slab along strike c −200 0 200 −200 0 200 0 1275 Figure 2: Izu-Bonin analysis. a) Map view of the slab and earthquakes. Grey contours correspond to the best-fitting surface to seismicity, defined using EHB locations (grey dots). Coloured dots represent earthquakes with gCMT solutions; short lines represent their B-axes. The slab synform is marked with a black line. b) EHB earthquake locations looking along the hinge of the fold. c) Focal mechanism types (lower hemisphere azimuthal equal-area projection), rotated into the local orientation of the slab. d) Cross section along A–A’. Grey contours correspond to the distance from the plane of section to the modelled surface. 150 150 200 100 200 100 200 100 150 150 250 148˚ 148˚ 150˚ 150˚ 152˚ 152˚ 154˚ 154˚ 156˚ 156˚ 158˚ 158˚ −8˚ −6˚ −4˚ −2˚ 0˚ Depth (km) Depth (km) A A’ B B’ 300 300 400 400 500 B B’ A A’ 200 500 200 200 300 400 500 600 700 200 300 400 500 600 700 −2˚ −1˚ 0˚ 1˚ 2˚ −2˚ −1˚ 0˚ 1˚ 2˚ ? ? down dip into slab along strike a d e f z’ (km) c x’ (km) y’ (km) −200 0 200 −150 0 150 0 546 b z’ (km) x’ (km) −200 0 200 −150 0 150 y’ (km) 0 636 Figure 3: Solomons analysis. a) Map view of the slab and earthquakes. The western and eastern halves of the slab are analysed separately. Components are as described in Figure 2. Slab folds are marked with black lines and synform and antiform symbols b,c) EHB earthquake locations looking along the modelled fold hinges. d) Focal mechanism types (lower hemisphere azimuthal equal-area projection), rotated into the local orientation of the slab. e,f) Cross sections along the lines A–A’ and B–B’. Grey contours correspond to the distance from the line of section to the best-fitting surface to seismicity. 178˚ 178˚ 180˚ 180˚ −178˚ −178˚ −176˚ −176˚ −174˚ −174˚ −26˚ −24˚ −22˚ A’ A 0˚ 200 200 400 400 600 600 Depth (km) Depth (km) B B’ down dip into slab along strike e 200 a c b d main WBZ 300 400 500 600 700 −2˚ 0˚ 2˚ 200 200 400 400 600 600 Depth (km) Depth (km) A A’ compression WBZ tension B B’ 0˚ 600 600 "outboard" B B’ -200 -300 -400 -500 -400 -500 -200 -300 -400 -500 -400 -500 -100 -100 Figure 4: South Tonga analysis. a) Map view of the slab and earthquakes, as described in Figure 3. b,c,d) Cross sections along the lines A–A’ and B–B’. A cartoon interpretation is provided in (b). e) Focal mechanism types. 40 40 20 0 -20 -40 -60 40 40 20 0 -20 -40 -60 view plunge: 45˚ A 500 600 700 500 600 700 500 600 700 -100 0 100 d b c h f g B B’ A’ A B B’ A’ Depth (km) z’ (km) x’ (km) −1˚ 1˚ 0˚ 0˚ −1˚ 1˚ 0˚ -50 50 −180˚ −179˚ −178˚ −177˚ −17˚ −18˚ −19˚ e A’ A B’ B 450 500 550 600 650 450 500 550 600 650 −180˚ −179˚ −178˚ −177˚ −17˚ −18˚ −19˚ A’ A a B’ B 600 700 600 700 Figure 5: North Tonga analysis, with two different slab models. a–d) Results using a slab model that disregards ’outboard’ events. e–h) Same as (a–d), but the shape approximating the surface to seismicity has been manually adjusted accounting for the presence of a fold in the slab along the band of densest seismicity. The hinge of this fold is marked as a dashed black line. Individual subfigures are as described in Figure 2. The cross sections along line B–B’ (c,g) have different plunges; (c) is a vertical cross section, while (g) is perpendicular to the hinge line of the inferred fold in the slab. The modelled slab surface is shown as a light grey line. The southern part of the Tonga slab (Figure 4) is the site of a prominent antiform-synform pair at 22–26 ◦ S [3]. Bands of seismicity mark both of these, with in-plane compression (red, orange) dominating within the synform and hinge-parallel extension (blue) in the antiform. The preceding analyses can help the interpretation of seismicity in north Tonga (Figure 5). The stress-guide hypothesis [1] is inconsistent with the observed focal mechanism orientations if the slab is planar (Figure 5a–d). In contrast, the presence of a synform hinge plunging to the southeast (Figure 5e–h) can explain both the distribution of seismicity and the range of focal mechanisms in the deep slab. 4. DISCUSSION I The hypothesis that buckling governs the characteristics of deep-focus seismicity is supported by focusing of co-seismic strain along fold hinges (Figure 6). The dense bands of earthquakes associated with folds in the Tonga and Izu-Bonin slabs are major contributors to the broad composite peak in global depth distributions at 500–600 km [10]. 0 100 200 300 400 500 600 700 Depth (km) 0 20 40 60 80 100 Present Future Past Deep hinge zone Other Number of earthquakes Direction and magnitude of maximum compressional strain rate Seismogenic zone a b Figure 6: An interpretation of the earthquake behaviour in the Izu-Bonin WBZ. a) Cartoon cross section of the slab, and its deformation through time. The central sketch shows the slab at the present day. Lines represent the principal direction and magnitude of shortening. Note the high strain rates associated with the bend in the slab. The dashed line represents the boundary between in-plane compression and extension. b) The depth distribution of earthquakes. Red bars represent the earthquakes in a cylinder with a radius of 50 km marking the band of seismicity associated with the hinge zone. I Buckling explains the alignment of focal mechanism principal axes parallel and perpendicular to fold hinges. The rarity of ‘down-dip’ P-axes [11] is largely the result of hinges not being horizontal. I The predominance of one focal mechanism type in the Izu-Bonin and Solomons slabs suggests that the surface of minimum strain within fold hinges lies below the WBZ. In the Tonga slab, the sequence of mechanisms from hinge-perpendicular compression in the fold core to hinge-parallel extension is consistent with roughly equal amounts of bending strain and hinge-perpendicular shortening. Bending could also explain the presence of previously identified double seismic zones in the Tonga slab [12]. I Rotation and translation of fold limbs can explain the low levels of co-seismic strain relative to the levels required to confine a thickening but non-bending slab to the upper mantle [13]. I The narrow bands of earthquakes in the relatively warm Solomons slab are similar to lineations in the South America slab [14] and beneath New Zealand [15], indicating that buckling may be responsible for earthquakes in other slabs. Buckling may trigger seismicity in slabs which are otherwise too warm to deform seismically. 5. CONCLUSIONS I Large folds with localised hinge zones are observed in several slabs that have reached the upper-lower mantle boundary. I These folds are the result of ongoing buckling, which accommodates significant convergence between slabs and the lower mantle. I Buckling has a dominant influence on deep-focus earthquake distributions and their focal mechanisms beneath the western Pacific. REFERENCES [1] Isacks, B. & Molnar, P. Mantle earthquake mechanisms and the sinking of the lithosphere. Nature 223, 1121–1124 (1969). [2] Myhill, R. Slab buckling and its effect on the distributions and focal mechanisms of deep-focus earthquakes. Geophysical Journal International 192, in press (2013). [3] Giardini, D. & Woodhouse, J. H. Deep seismicity and modes of deformation in Tonga subduction zone. Nature 307, 505–509 (1984). [4] Houseman, G. A. & Gubbins, D. Deformation of subducted oceanic lithosphere. Geophysical Journal International 131, 535–551 (1998). [5] Smith, W. H. F. & Wessel, P. Gridding with continuous curvature splines in tension. Geophysics 55, 293–305 (1990). [6] Engdahl, E. R., van der Hilst, R. & Buland, R. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bulletin of the Seismological Society of America 88, 722–743 (1998). [7] Dziewonski, A. M., Chou, T. A. & Woodhouse, J. H. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. Journal of Geophysical Research 86, 2825–2852 (1981). [8] Ekstr ¨ om, G., Nettles, M. & Dziewo ´ nski, A.. The global CMT project 2004–2010: Centroid-moment tensors for 13,017 earthquakes. Physics of the Earth And Planetary Interiors 200-201, 1–9 (2012). [9] Kagan, Y. Y. 3-D rotation of double-couple earthquake sources. Geophysical Journal International 106, 709–716 (1991). [10] Sykes, L. R. The seismicity and deep structure of island arcs. Journal of Geophysical Research 71, 2981–3006 (1966). [11] Apperson, K. D. & Frohlich, C. The relationship between Wadati-Benioff zone geometry and P, T and B axes of intermediate and deep focus earthquakes. Journal of Geophysical Research 921, 13821–13831 (1987). [12] Wiens, D. A., McGuire, J. J. & Shore, P. J. Evidence for transformational faulting from a deep double seismic zone in Tonga. Nature 364, 790–793 (1993). [13] Holt, W. E. Flow fields within the Tonga slab determined from the moment tensors of deep earthquakes. Geophysical Research Letters 22, 989–992 (1995). [14] Lundgren, P. & Giardini, D. Isolated deep earthquakes and the fate of subduction in the mantle. Journal of Geophysical Research 99, 15833–15842 (1994). [15] Boddington, T., Parkin, C. J., & Gubbins, D. Isolated deep earthquakes beneath the North Island of New Zealand. Geophysical Journal International 158, 972–982 (2004). [16] Wessel, P. & Smith, W. H. F. New, improved version of generic mapping tools released. EOS Transactions 79, 579–579 (1998). ACKNOWLEDGMENTS The author would like to thank Professors Dan McKenzie and Keith Priestley for their supervision and many interesting discussions. This work was financially supported by a Natural Environment Research Council studentship and the Magdalene College Kingsley Bye-Fellowship in Earth Sciences. Figures were created using the Generic Mapping Tools [16]. AGU Fall Meeting 2012, Poster DI23A-2377 Now at the Bayerisches Geoinstitut, Universit ¨ at Bayreuth, 95440 Bayreuth, Germany
