Seismic tomography of the Pacific slab edge under Kamchatka
Introduction
The Kamchatka peninsula is located at the northwestern edge of the Pacific plate (Fig. 1). The Pacific plate of Cretaceous age subducts beneath the Kamchatka arc and moves along the Bering strike-slip fault at about 8 cm/yr, increasing from 7.7 cm/yr at 55°N to 8.3 cm/yr at 47°N (DeMets et al., 1990, Steblov et al., 2003). Geological studies showed that the volcanism and convergence in Kamchatka ceased at about 55 Ma ago but resumed about 30 Ma ago (Watson and Fujita, 1985). About 10 Ma ago, island-arc magmatism extended to the north of the Aleutian–Kamchatka junction along the mid-Kamchatka volcanic belt, but those are extinct now (Honthaas et al., 1995). A chain of active volcanoes, Holocene in age (Braitseva et al., 1995), along the eastern coast of Kamchatka are underlain by about 100-km depth-contour of the subducting Pacific slab (Gorbatov et al., 1997). The Sheveluch and Klyuchevskoy volcanoes have shifted northwestward from the volcanic front. Between 54°N and 55°N, the Meiji seamounts, the northernmost segment of the Hawaii-Emperor seamount chain, enter the Kamchatka trench (Fig. 1).
The configuration of the Pacific slab under the Kamchatka region was studied by using the distributions of regional earthquakes occurring in the slab, which shows that the dip angle of the slab decreases northward from about 55° to 35° (Gorbatov et al., 1997). The maximum depth of earthquakes becomes shallower along the subduction zone from ∼ 600 km beneath southern Kamchatka to ∼ 100–200 km near the junction (Davaille and Lees, 2004).
Seismic tomography is a powerful tool for determining 3-D velocity structure and dynamic processes in the Earth. Until now several tomographic studies have been performed for the Kamchatka peninsula. Gorbatov et al. (1999) applied the tomographic method of Zhao et al. (1992) to study a 3-D P-wave velocity structure down to a depth of 200 km, and their results showed a prominent low-velocity (low-V) anomaly beneath the volcanic front and a high-velocity (high-V) zone associated with the subducted Pacific slab. But their study region was in the southeastern Kamchatka arc because of the distribution of seismic stations available for them. In order to obtain tomographic images in Northern Pacific, Gorbatov et al. (2000) conducted a regional tomographic inversion and they revealed a slab-like fast anomaly from the Earth's surface down to 900 km depth beneath southern Kamchatka, while a high-V anomaly associated with the Wadati–Benioff zone was not imaged near the junction. A surface-wave tomography shows that the subducting Pacific lithosphere terminates at the Aleutian–Kamchatka junction and no relict slab underlies the extinct northern Kamchatka volcanic arc (Levin et al., 2002a). Although Levin et al. (2002a) suggested two episodes of slab loss under northern Kamchatka, their tomographic images could not reveal the detached slab because their model is limited to 200 km depth. Recently, Lees et al. (2007) determined P-wave teleseismic tomography which showed evidence for slab shoaling toward the northern edge of the subducted Pacific slab, and they considered the thermal ablating related to asthenosphere as a possible cause for the feature.
Shear-wave splitting studies suggested that trench-parallel strain follows the seismogenic Wadati–Benioff zone, but rotates to trench-normal beyond the slab edge (Peyton et al., 2001, Portnyagin et al., 2005), indicating that the asthenospheric flow passes through a slab window beneath the junction, similar to that observed in Apenines (Wortel and Spakman, 2000). In addition, thermal modeling of the reheating of a torn slab shows that the Pacific lithosphere was already thinner well before entering the trench due to delayed thickening of the lithosphere below the Meiji–Hawaiian hotspot (Davaille and Lees, 2004).
In the present study we use teleseismic tomography to determine a 3-D P-wave velocity structure down to 700 km depth under Kamchatka. Our results provide new evidence for the loss of the Pacific slab at the slab edge, which may improve our understanding of the dynamic processes under this region.
Section snippets
Data and method
We used teleseismic data recoded by 15 portable broad-band seismic stations in Kamchatka from the SEKS experiment conducted in 1998–1999 (Lees et al., 2000), and by one permanent station PET. Fig. 1 shows the distribution of the 16 seismic stations used. All the waveform data were downloaded from the web site of IRIS (Incorporated Research Institutions for Seismology). We selected earthquakes with epicentral distances between 25° and 95° (Fig. 2), which have magnitudes larger than M 5.0 and
Analysis and results
Teleseismic tomography cannot determine the 3-D crustal structure well because the teleseismic rays arrival at stations nearly vertically and so they do not crisscross well near the surface (Fig. 4). Hence, it is necessary to correct the teleseismic relative residuals for the heterogeneous crustal structure (e.g., Lei and Zhao, 2005, Montelli et al., 2006, Zhao et al., 2006). In general, crustal structure can be obtained either from local seismic studies in the study region (using seismic
Resolution tests
To confirm the main features of the tomographic result, we conducted detailed resolution analyses. A direct way to evaluate the resolution of a tomographic result is to calculate a set of travel-time delays that result from tracing the corresponding rays through a synthetic structure as though they are data, and then to compare the inversion result with the initial synthetic structure. In the resolution tests, the numbers of stations, events and ray paths are the same as those in the real data
Discussion
According to our tomographic results, we proposed a “gap” model to describe the feature of the high-V zone, which extends below the mantle transition zone under southern Kamchatka and shortens gradually northward and ends near the Aleutian–Kamchatka junction (Fig. 13). In this new model, there exists a gap associated with the loss of the subducted lithosphere beneath Sheveluch and Klyuchevskoy volcanoes near the Pacific slab edge. The asthenospheric flow can pass through the gap around the slab
Conclusions
To understand the deep structure and dynamics beneath the Kamchatka peninsula, we applied a 3-D teleseismic tomography method to P-wave arrival time data collected from digital seismograms of 75 teleseismic events released by the IRIS data center. Our results show that a high-V zone associated with the subducted Pacific slab extends below the transition zone and beyond the leading edge of the Wadati–Benioff zone under southern Kamchatka and it shoals northward and terminates near the
Acknowledgments
We thank the IRIS data center for providing the waveform data used in this study. We are grateful to Prof. Tom Parsons and two anonymous reviewers for providing constructive comments and suggestions that improved the manuscript. This work was partially supported by a research grant (Kiban-A 17204037) to D. Zhao from the Japanese Ministry of Education and Science. Most figures are made by using GMT (Wessel and Smith, 1998).
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