Shear-wave splitting near Guam

Physics of the Earth and Planetary Interiors, 72(1992)211—219 Elsevier Science Publishers B.V., Amsterdam

211

Shear-wave splitting near Guam Jiakang Xie Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA (Received 22 January 1992; accepted 10 February 1992)

ABSTRACT Xie, J., 1992. Shear-wave splitting near Guam. Phys. Earth Planet. Inter., 72: 211—219. Polarities of shear waves from intermediate-focus events underneath Guam are studied. For records from a group of ten events, shear-wave splitting with faster-arriving E—W components are observed. This event group occurred within, or above, one geographic portion of the Wadati—Benioff zone, with depths ranging between 57 and 148 km. Ray tracing calculations were performed for 3-D and 1-D velocity models constructed for the region to determine expected S-wave polarities and ray patterns, as well as their sensitivities to variations in velocity structure. These were used to infer the probable existence of intrinsic anisotropy at depth and to determine the location and magnitude of anisotropy which can explain the observed shear-wave splitting. The most probable location of the anisotropy is beneath the crust and above, or partially within, the subducting slab. Assuming a maximum depth range of 10—120 km for the location of the anisotropy, its amount is about 1%, which may be viewed as a lower bound. Plausible causes of the anisotropy include mantle flow and thin, sheet-like channels filled with lava, or water vapor migrating upward from the subducting slab.

1. Introduction Shear-wave splitting from intermediate to deep focus earthquakes has been observed in a few regions in the world, where volcanic mountain chains or island arcs are underlain by subducting slabs. These include a region near Honshu, Japan (Ando et al., 1983), Tonga (Bowman and Ando, 1987) and Bucaramanga, Colombia (Shih et al., 1991). For all these observations, seismic anisotropy in the mantle wedges above the subducting slabs was proposed to be a probable cause of the shear-wave splitting. The anisotropy was interpreted in terms of upper-mantle flow associated with slab subduction or with back-arc basin opening, or in terms of stress-induced cracks containing magma. These interpretations were generally ambiguous because of limited ray cover-

Correspondence to: J. Xie, Department of Earth and Atmospheric Sciences, St. Louis University, 3507 Laclede Avenue, St. Louis, MO 63103, USA. 0031-9201/92/$05.00 © 1992

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age, as well as the scatter and local variations in the direction of polarization and in the magnitude of shear-wave splitting. In this paper, I report and discuss an observation of shear-wave splitting for Station GUMO on Guam, a volcanic island in the Mariana arc (Fig. 1).

2. Regional seismicity and a geometrical model for the Benioff zone Fig. 2 shows the regional seismicity in an area of 800 km by 800 km centered at 144.8662°E, 13.1878°N,the location of Station GUMO. Superimposed on the seismicity are depth contours of the upper surface of a Wadati—Benioff (WB) zone model. This model is obtained by fitting the seismicity in the following way. First, an ocean trench model from fitting the bathymetric map of Park et al. (1990), consisting of four straight-line segments near Guam, was taken to be the initial strike of the WB zone. Seismic events were

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212

spline-fitted to obtain an initial model of the WB zone. This initial model was then perturbed by allowing the cross-sectional shape of the WB zone model, as well as both the strike and length of the four segments of this model, to vary such that the misfit between the WB zone model and seismic events, measured by events outside the model, is reduced. This is repeated, using trial and error, until the misfit is minimized. Throughout the procedure, the thickness of the WB zone was fixed at 60 km. Fig. 3 shows the profile of the WB model, together with the projected seismicity from all four segments of this model. The majority of the deeper (depth greater than 60 km) events, which are less affected by mislocation errors (H.W. Zhou, personal communication, 1991), fall inside the WB zone model. It is possible that other geometric models of the WB zone may also fit the seismicity. This uncertainty will be borne in mind in this paper, and discussed when necessary

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3. Data and observed shear-wave splitting Between 1985 and 1988, the Seismic Research Observatories (SRO) Station GUMO in northern Guam included a three-component, short-period recording system, which operated discontinuously in a bore-hole in a limestone layer. After a cornplete search through the US Geological Survey world data tapes, 14 local events with steep S-wave incident angles (less than 35°)were found to be recorded by this recording system. Steep incident angles are necessary for observing clear shearwave polarizations (e.g. Nuttli, 1961; Booth and Crampin, 1985). These events can be sorted into two groups. The first group consists of ten events which occurred beneath or on segment CDD ‘C’ of the WB. zone model plotted in Fig. 2. The second group consists of four events which occurred beneath segment BCC ‘B’. All of the events are listed in Table 1. Records from events in group 1 consistently show split shear wave arrivals with fast-arriving E—W components. Fig. 4 shows an example of the seismograms from event in group 1, together with the polarization diagrams for two windows

around the S-wave arrival. The onset of the S wave is earlier on the E—W component of this seismogram, as compared with on the N—S cornponent. Correspondingly, the horizontal particle motion starts with a linear polarization in roughly an E—W direction, and changes to a NNW—SSE direction after about 0.35 s. On two of the ten seismograms from event group 1, the S-wave signals are seriously clipped. The onset time of the S waves on these records are, however, not affected by the clipping and are earlier on the E—W components than on the N—S component. Fig. 5 shows one of these two clipped seismograms as an example. In contrast to the events in group 1, the four seismograms from event group 2 tend to be polarized in N—S or NNE—SSW direction at the onset of the S waves. Fig. 6 shows an example of these four seismograms, with S-wave polarization diagrams. In horizontal polarization diagrams, the S wave starts with a linear polarization in a NNE— SSW direction and changes to a NW—SE direction.

213

SHEAR WAVE SPLITTING NEAR GUAM

For those records from event group 1 that are not significantly contaminated by clipping, the amount of splitting, defined as the time difference between the arrivals of the faster and slower S components, was estimated by visual inspection of the horizontal polarization diagrams. For the two seriously clipped seismograms, the amounts of splitting were estimated by measuring the time differences between the onsets of the S wave on E—W and N—S component. The resulting measurements range between 0.1 and 0.4 s (Table 1; Fig. 7).

radiation pattern, off-incident plane propagation and conversion between P and S waves in the 3-D Earth structure. The exact effects of these factors, if combined, are virtually unpredictable for the paths in this study at wavelengths of a few hundred meters. We can, however, test the degree to which S-wave polarizations are affected by source radiation pattern and by heterogeneous but isotropic velocity structures. This can be done by synthetic calculations of S polarization for a velocity structure based on regional tectonic environment and seismicity. This synthetic calculation can then be repeated for a perturbed velocity structure to determine the robustness of the resuits of the synthetic calculation to the varying velocity model. To do this, we first constructed a 1-D velocity model for the Guam region, which is

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4. Probable causes of shear-wave splitting

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TABLE 1 Events studied and the estimated amount of shear-wave splittings Event date

Origin time

Focal depth (km)

Distance to GUMO (km)

Back azimuth (deg)

Event group

Amount of splitting (s)

12-04-85 29-05-85 23-11-85 25-11-86 16-02-86 11-03-86 15-05-86 25-06-86 29-07-86 18-12-87 18-03-88 24-07-88 16-08-88 26-09-88

07:50:37.0 18:20:47.8 16:25:30.5 00:50:56.3 23:26:01.4 22:11:18.1 15:47:11.2 11:33:04.0 10:22:59.0 10:17:51.3 00:54:45.6 15:47:56.5 21:55:58.5 04:58:09.1

78.0 99.0 141.0 142.0 107.0 126.0 218.0 129.0 148.0 114.0 141.0 182.0 57.0 144.0

4.28 57.8 25.7 31.3 114.1 13.2 177.5 43.9 46.0 79.7 27.7 180.0 34.6 48.4

36.2 193.8 278.0 293.4 235.4 20.0 252.9 295.7 300.7 244.8 313.3 246.0 211.3 323.6

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adapted from that of Nishimura and Forsyth (1989) for areas in the Pacific aged 0—4 Myr. The adaptation made was that the 2.9 km water layer in that model (Table 3 of Nishimura and Forsyth (1989)) was replaced by two layers, with thicknesses of 3.0 kin and 0.25 km, and with S-wave velocities of 2.1 km s~ and 2.8 km s~,respectively. This adaptation was made to account for a limestone layer of roughly 250 m thickness at the station site (Ganse and Hutt, 1982), and for a volcanic layer, which is roughly 3.0 km thick in the middle segment of Mariana arc (Hussong et al., 1982). A 3-D model was then constructed by

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embedding the WB zone model described in the earlier section into the 1-D model, and assuming that the velocity inside the WB zone is 5% higher than that outside. Of the events in group 1, event 112385 has a published Harvard moment tensor solution. Dynamic ray tracing, using a computer code by Cerveny et al. (1988), was performed in both the 3-D and 1-D velocity models for this event, which is located inside the WB zone model. The direction of S-wave polarization predicted by these calculations at GUMO are N29.1°Wand N28.9°W for the 3-D and 1-D models, respectively. These

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5.1. The location and amount of anisotropy The number of seismograms from event group

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whether a NNE—SSW (or N—S) polarization direction of faster-arriving S waves persists for the corresponding paths. We therefore focus on the probable cause of anisotropy responsible for the shear-wave splitting observed from event group 1. This group includes raypaths which are within the

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two directions are nearly identical, and both differ by about 60°from the observed, roughly E—W direction of the polarization of the onset of the S waves. Even if we assume that there are some additional isotropic 3-D velocity heterogeneities at various depths not taken into account in our dynamic ray tracings, it is still very unlikely that the corresponding additional changes in S-wave polarities could be large enough to explain the large (about 60°) discrepancy. The same argument also applied to changes in S-wave polarity as a result of changes in the geometry of the WB zone. At small incident angles S—P conversions should produce vertical amplitudes which are larger than horizontal. Observations in this study, however, show the opposite (Figs. 4 and 5). Near-source P—S conversions should not persist for events occurring in a depth range as large as that of event group 1 (57—148 km). Thus it is unlikely that the E—W polarizations of fast-arriving S waves from events in group 1 are due to some similar source radiations into, or P—S and S—P conversions within, a 3-D isotropic structure. Anisotropy is likely to be responsible for the observed polarization/splitting,

geographic extent of surface segment CDD’ C’ of the WB zone model. Figure 7 shows the amount of splitting between the faster- and slower-arriving S waves vs. focal depth for events in group 1. There appears to be a tendency for records from shallower events to exhibit less splitting, but a quantitative correlation analysis is not practical for this plot because of the small number and the scatter of the points. This tendency may suggest that anisotropy is occurring, at least partly, in the depth range of these events (about 50—150 km). To explore further the location of anisotropy, kinematic ray tracing in both the 3-D and 1-D velocity models mentioned above was performed for all 14 records from the two event groups. The horizontal deviations of the rays in the 1-D structure from the corresponding rays in the 3-D structure are no greater than 1 km at a depth of 10 km. These deviations are about an order of magnitude smaller than the expected Interna. tional Seismological Center location error for intermediate focus events (about 10 km). This is mainly because of the steep incident angles, which make the ray patterns near the receiver insensitive to the velocity structure. It is therefore reasonable to assume that, at depths less than 10 km, ray patterns in the real velocity structure are close enough to those calculated in both 1-D and 3-D velocity models to allow us to use the features of the calculated ray patterns to infer the shallowness to which the anisotropy extends. The inset of Fig. 3 shows a 3-D view of the ray patterns obtained in the 3-D structure, with the upper surface of the WB zone plotted mainly as a reference geometrical structure. The ray seg-

218

ments beneath that surface are not plotted in this figure. In both the 1-D and 3-D models, the rays corresponding to event group 1 come very close to those corresponding to event group 2 (within 3 km) at a depth of 10 km. Above this depth, rays converge rapidly to the station. As only records from events in group 1 show shear-wave splitting with a fast E—W arrival, the probable anisotropy causing this shear-wave splitting is more likely to be located somewhere beneath the crust, rather than within it. Assuming a depth range between 120 and 10 km for the location of anisotropy, the amount of seismic velocity anisotropy is 1% from the ray tracings in the 1-D and 3-D velocity models, as well as in the Jeffreys—Bullen model. The anisotropy could be much greater if it is localized in a smaller depth range.

5.2. The cause of anisotropy From the above discussion, the probable location of the anisotropic region may be in the mantle wedge above the WB zone and/or within the WB zone. This is consistent with previous observations in similar tectonic environments (Ando et al., 1983; Bowman and Ando, 1987; Shih et al., 1991). If the geometry of the CDD ‘C’ segment of the WB zone model in Fig. 2 is roughly correct, then its strike, which is in the E—W direction, is parallel to the direction of polarization of the fast-arriving S waves from event group 1. This is also consistent with previous observations in Honshu (Ando et al., 1983), Fiji (Bowman and Ando, 1987) and Bucaramanga, Colombia (Shih et al., 1991), where the anisotropy was aligned to the strike of the slabs, Previously proposed causes of the anisotropy have included mantle flow related to slab subduction and lava migration through cracks in the upper mantle. In the current study, the upper-mantle flow may be associated, at small scale, with the opening of the Mariana Trough, which could be locally in an ESE—WNW direction (Carison and Morterra-Gutierrez, 1990). In addition to these causes, water vapor from dehydration of the subducting slab at depths around 100 km may mi-

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grate toward the surface through sheet-like channels (Wyllie, 1984). These channels should tend to be vertical and aligned along the direction of slab strike. This could also have caused the observed anisotropy. Unfortunately, we cannot correlate with confidence any one of these causes with observations so far.

6. Conclusions In this study I have used shear waves with steep incident angles (less than 35°,according to various velocity models) observed at Station GUMO. Shear-wave splitting, with fast-arriving E—W components, were observed for records from intermediate subduction zone events inside and above a portion of the Mariana slab near Guam. Results of 1-D and 3-D ray tracing suggest that the effects of source radiation and 3-D velocity structures on the S waves are unlikely to be the cause of the observed polarization anomalies, and that shear-wave splitting is most probably caused by seismic anisotropy in and/or above the subducting slab. Records from the same station, but for events in a different geographical portion of the slab, do not show the same type of shear-wave splitting, suggesting that anisotropy which causes faster arrival of the E—W component is likely to be occurring beneath the crust, as rays from all of the events converge rapidly in the crust. Plausible causes of the proposed seismic anisotropy near Guam include (1) mantle flow, (2) lava migration through aligned cracks or channels in the upper mantle, (3) vapor migration, associated with dehydration of the subducting slab at depths of around 100 km, through aligned sheet-like channels, and (4) preferential alignment of olivine grains. Deciding between the causes will require greater spatial coverage of the region with higher-quality S-wave data.

Acknowledgments This research was partially supported by the National Science Foundation Grant EAR88-

SHEAR WAVE SPLITTING NEAR GUAM

04355. Brain Mitchell initiated my interest in the subject and provided much assistance during this work. This work benefitted from discussions with Don Anderson, Hiroshi Inoue, Toshiro Tanimoto, Peter Wyllie and Lianshe Zhao. I appreciate assistance by Ludek Klimes and Jiang Qu in using the 3-D ray tracing code. References Ando, M., Ishikawa, Y. and Yamazaki, F., 1983. Shear-wave polarization anisotropy in upper mantle beneath Honshu, Japan. J. Geophys. Res., 88: 5850—5864. Booth, C.D. and Crampin, S., 1985. Shear-wave polarizations on a curved wavefront at an isotropic free surface. Geophys. J. R. Astron. Soc., 83: 31—45. Bowman, R.J. and Ando, M., 1987. Shear-wave splitting in the upper mantle wedge above the Tonga subduction zone. Geophys. J.R. Astron. Soc., 88: 25—41. Carlson, R.L. and Morterra-Gutierrez, C.A., 1990. Subduetion hinge migration along the Izu—Bonin—Mariana arc, Tectonophysics, 181: 331—344. Cerveny, V., Klimes, L. and Psencik, I., 1988. Complete seismic ray tracing in three-dimensional structures. In: D. Doornbos (Editor), Seismological Algorithms—Computational Methods and Computer Programs. Academic Press, San Diego, 469 pp. Ganse, R. and Hutt, C.R., 1982. Directory of world digital

219 seismic stations. In: World Data Center A for Solid Geophysics, Rep. SE-32, Boulder, CO, 439 pp. Hussong D.M., Uyeda, S., Knapp, R., Ellis, H., Kling, S. and Natland, J., 1982. Deep Sea Drilling Project Leg 60: cruise objectives, principal results, and explanatory notes. In: Initial Reports of the Deep Sea Drilling Project. NSF Rep. NSFSP-60, LX. US Government Printing Office, Washington, DC, pp. 3—30.’ Nishimura, C.E. and Forsyth, D.W., 1989. The anisotropic structure of the upper mantle in the Pacific. Geophys. J. Int., 96: 203—229. Nuttli, OW., 1961. The effect of the earth’s surface on the S-wave particle motion. Bull. Seismol. Soc. AM., 51: 237— 246. Park, C.-H., Tamaki, K. and Kobayashi, K., 1990. Age—depth correlation of the Philippine Sea back-arc basins and other marginal basins in the world. Tectonophysics, 181: 351— 371. Tracy, Jr. 1.1., Stensland, C.H., Doan, D.B., May, H.G., Schlanger, S.O. and Stark, J.T., 1959. Military geology of Guam, Mariana Islands. Prepared under the Direction of the Chief of Engineers, U.S. Army, by the Int. Div., Office of the Engineer, U.S. Army Pacific, with personnel of the U.S. Geol. Surv., Pacific, 282 pp. Shih, X., Schneider, J.F. and Meyer, R.P., 1991. Polarities of P and S waves, and shear wave splitting observed from the Bucaramanga Nest, Colombia, J. Geophys. Res., 96: 12069—12082. Wyllie, P.J., 1984. Sources of granitoid magmas at convergent plate boundaries. Phys. Earth Planet. Inter., 35: 12—18.