Takahashi Furumura

Subduction zone guided waves and the heterogeneity structure of the subducted plate: Intensity anomalies in northern Japan

[1] The subducting Pacific plate acts an efficient waveguide for high-frequency signals and often produces anomalously large intensity on the eastern seaboard of northern Japan during deep earthquakes. The waveform records in the region of high intensity show a low-frequency (f 2 Hz) later arrivals with a long coda. This behavior is not explained by a simple subduction zone model comprising a high-velocity plate with low attenuation. From the analysis of observed broadband waveforms and numerical simulation of seismic wave propagation in the Pacific subduction zone we demonstrate that the high-frequency guided waves traveling in the subducting plate arise from the scattering of seismic waves by heterogeneity in plate structure. Our preferred model of the heterogeneity has elongated scatterers parallel to the plate margin described by a von Karmann function with a downdip correlation length of about 10 km and much shorter correlation length of about 0.5 km in thickness. The standard deviation of wave speed fluctuations from the averaged background model is about 2%. This new heterogeneous plate model generates significant scattering of seismic waves with wavelengths shorter than correlation distance in thickness, but low-frequency waves, with long wavelengths, can easy tunnel through such lamina structure. The result is frequency-selective propagation characteristics with a faster low-frequency phase followed by large and high-frequency signals with very long coda. A low-wave speed channel effect from the former oceanic crust at the top of the subducting slab is not necessary to explain the observed dispersed signals and the very long high-frequency coda. Three-dimensional simulations, using the Earth simulator supercomputer for modeling of high-frequency seismic wave propagation in the Pacific subduction zone including plate heterogeneity, clearly demonstrate the scattering waveguide effects for high-frequency seismic waves traveling in the plate. The region of large intensity for the heterogeneous model migrates away from the hypocenter into northern Japan with an elongated zone along the Pacific coast, almost comparable to the observations from deep events in the Pacific plate.

Variations in Regional Phase Propagation in the Area around Japan

The high level of seismicity and dense network of short-period stations in Japan allows a detailed characterization of regional phase propagation. There are substantial differences depending on the location of the source and stations. Despite the structural complexity, Lg is clearly seen in many parts of Japan. Lg is particularly well developed in the western part of the main island, Honshu, and will propagates through some volcanic zones with loss of high-frequency components. There are also zones of Lg blockage in northeastern Honshu and for subduction zone events with significant oceanic paths for which the mantle phases Pn and Sn are particularly clear. The oceanic region in the Sea of Japan blocks Lg propagation paths for events in mainland Asia at many stations, but there are clear Lg propagation corridors through Korea to stations in the west (Kyushu, western Honshu) and in the north into Hokkaido. The Lg phase carries substantial energy for large events, and the differences in efficiency of propagation influences the intensity of ground shaking. Thus the 1995 Hyogo-ken Nanbu (Kobe) earthquake shows intensity contours extended to the west in the region of efficient Lg-wave propagation. The Rg phase and the fundamental mode of Love waves are very significant for the shallow 2000 Tottori-ken Seibu event, and the effect of the combination of Lg with these slightly lower frequency surface waves would help to explain the discrepancy between the Japan Meteoro- logical Agency magnitude from regional stations (Mj 7.3) and the moment magnitude from distant observations (M w 6.6).

Visualization of 3D Wave Propagation from the 2000 Tottori-ken Seibu, Japan, Earthquake: Observation and Numerical Simulation

The dense networks of strong-ground-motion instruments in Japan (K- NET and KiK-net) provide a means of direct visualization of regional wave propa- gation during large earthquakes. For the 2000 Tottori-ken Seibu earthquake (Mw 6.6) in western Japan, snapshots of ground motion, derived directly from interpolation of a large number of array observations, demonstrate clearly the nature of the source radiation and the character of the seismic wave field propagating to regional dis- tances. In western Japan the wave field from the earthquake is characterized, in most parts, by the dominance of high-frequency (0.2-5 Hz) Lg waves on three-component acceleration records and longer-period (T 10 sec) fundamental-mode Love waves in tangential displacement. The presence of strong lateral variations in the crust and upper-mantle structures, such as the low-velocity superficial layer and the high- velocity Philippine Sea plate with shallow subduction into the mantle, impose sig- nificant modifications on the regional wave field. Further insight into the nature of the seismic wave field is gained by comparison of the observations with numerical simulation for a 3D model, including sedimentary basins by an embedded submesh. A multigrid, parallel computation using a hybrid Pseudospectral/Finite Difference Method allows the inclusion of a realistic model of the source process for the 2000 Tottori-Ken Seibu earthquake. There is good agreement in the dominant features of the regional wave field propagating through the complex structure of western Japan. However, the differences between the observations and the computer simulations indicates the need for further refinement of the source and structural models. The inclusion of amplification effects for the population centers in sedimentary basins means that the modeling procedure is already suitable for estimating the main pattern of ground motion for future earthquake scenarios.

Toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity

The character of the high-frequency seismic phases Po and So, observed after propagation for long distances in the oceanic lithosphere, requires the presence of scattering from complex structure in 3-D. Current models use stochastic representations of seismic structure in the oceanic lithosphere. The observations are compatible with quasi-laminate features with horizontal correlation length around 10 km and vertical correlation length 0.5 km, with a uniform level of about 2% variation through the full thickness of the lithosphere. Such structures are difficult to explain with petrological models, which would favor stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts. Petrological evidence mostly points to smaller-scale features than suggested by seismology. The models from the different fields have been derived independently, with various levels of simplification. Fortunately, it is possible to gently modify the seismological model toward stronger basal heterogeneity, but there remains a need for some quasi-laminate structure throughout the mantle component of the oceanic lithosphere. The new models help to bridge the gulf between the different viewpoints, but ambiguities remain.

Multiscale seismic heterogeneity in the continental lithosphere

We examine the nature of seismic heterogeneity in the continental lithosphere, with particular reference to Australia. With the inclusion of deterministic large-scale structure and realistic medium-scale features, there is not a need for strong fine-scale variations. The resulting multiscale heterogeneity model gives a good representation of the character of observed seismograms and their geographic variation, and is also in good agreement with recent direct results on P wave reflectivity in the lithosphere. Fine-scale heterogeneity is pervasive, but strongest in the crust. There is a weak quasilaminar component above the lithosphere-asthenosphere transition with horizontal correlation length of 10 km and vertical correlation length of 0.5 km. Within the transition, the aspect ratio of heterogeneity changes and can be well represented with a horizontal correlation length of 5 km and vertical correlation length of 1 km. For the Australian cratons, this transition zone needs low intrinsic attenuation (high Q) to sustain the long high-frequency coda of both P and S waves. The interaction of the different aspects of the heterogeneity is complex and produces a diversity of behavior depending on the relative thickness of the different lithospheric zones. The multiscale model reconciles many of the divergent concepts of the character of heterogeneity based on interpretations of particular aspects of the seismic wavefield. The varying nature of the heterogeneity also ties well with the variations in tectonic character across the Australian continent.