國 立 臺 灣 師 範 大 學 校 務 基 金 出 國 報 告 受邀至日本與東京大學 Takashi Furumura 教授合作研究「利用高解析度 計 畫 名 稱 有限差分模型模擬台灣隱沒帶下方地震的高頻導波特徵與強地動異常的 關係」計畫 姓名 陳卉瑄 服務單位 師大地球科學系 職稱 助理教授 出 國 人 員 出 國 地 點 日本東京大學地震研究所 出 國 期 間 自 100 年 10 月 31 日起至 100 年 11 月 08 日止 報 告 內 容 一、目的： Study on slab guided wave and scattering of seismic waves. 二、過程： During the eight days visit, I worked with Prof. Furumura on the better understanding of guided wave characteristics in the Taiwan-Ryukyu subduction zone and the comparison with Japan subduction zone observations. Also, we also explored the change of scattering properties that cause spatial and temporal changes in seismic wave character associated with Chi-Chi earthquake by waveform modeling. Our results are later presented in the American Geophysical Union. 美國地球物理 年會秋季大會 (AGU) T12C 美國地球物理 年會秋季大會 (AGU) T52C 1 0 0 /1 2 /0 5 1 0 0 /1 2 /0 9 The healing of near-surface and fault zone damage induced by the 1999 M7.6 Chi-Chi earthquake High frequency waves guided by the subducted plates underneath Taiwan and their association with seismic intensity anomalies Other than working with my host in ERI, I was also able to discuss with the experts in this department during my visit: 1. Obara Kazushige: Ambient tremors observation in Taiwan 2. Naoyuki Kato: Interaction of asperities by earthquake model 3. Igarashi Toshihiro: Repeating earthquakes in Japan 4. Miyazawa Masatoshi: Mechanism of triggered tremors 5. Aitaro Kato: seismic evidence of fluids 6. Takuto Maeda: Relocation of ambient tremors 三、心得： This short-term visit went efficiently, which the result was published in next month’s American Geophysical Union annual meeting in San Francisco, US (December, 2011) as listed above. 四、建議： The financial support from school was very helpful and greatly appreciated. 五、其它（自行決定是否附論文或相片…等資料） HIGH FREQUENCY WAVES GUIDED BY THE SUBDUCTED PLATES UNDERNEATH TAIWAN AND THEIR ASSOCIATION WITH SEISMIC INTENSITY ANOMALIES 1. Introduction Seismic events traveling through subduction zone exhibit complicated wave patterns. In particular, the low-frequency arrival followed by a large-amplitude and long-duration high-frequency signal with sustained long coda are often observed at the forearc stations, where the subducting plate acts as efficient waveguide for the high frequency signals (Fig. 1) [e.g., Snoke et al., 1974; Ouchi, 1981; Chiu et al., 1985; Iidaka and Mizoue, 1991; Abers, 2000; Martin et al., 2003; Furumura and Kennett, 2005]. The guided waves are believed to travel along the plate, resulting in surprisingly large intensity in the forearc area even they are not felt near the epicenter (Fig. 2) [e.g., Furumura and Kennett, 2005; Furumura and Kennett, 2008]. The seismic events with such guided wave characteristics, therefore, are regarded as an important observation that explains the anomalous ground shaking and provides useful information for plate configuration. Taiwan, situated at the plate boundary zone between the Eurasian plate (EP) and the Philippine Sea plates (PSP), exhibits a unique interaction between the EP and PSP. In northeast Taiwan, the PSP subducts beneath the rifted Eurasian plate margin along the Ryukyu Trench, whereas in southwest Taiwan the Eurasian plate subducts underneath PSP. The anomalous seismic intensity can happen in Taiwan, if the subducted PSP/EP acts as an efficient waveguide for high-frequency seismic waves. What are the characteristics of guided waves in Taiwan subduction zones, how do they reflect the plate configurations, and under which conditions the guiding effect would cause anomalous seismic intensity are important questions towards seismic hazard assessment, however, remain unknown. The frequent occurrence of M5+ (magnitude greater than 5) earthquakes in the Taiwan subduction zones afford the opportunity to explore the association between the PGA patterns and the trapping effect of the high frequency signal in the PSP/EP. A demonstration of finite difference modeling of the scattering waveguide effects will furthermore, help us understand the physical nature of subducted slabs. 2. Anomalous seismic intensity pattern from intermediate events Distorted seismic intensity patterns have been recognized in our present work, as shown by the examples in Fig. 3. The peak ground acceleration (PGA) distribution for two M>5 earthquakes with different focal depth show that the events at shallow depth of 7 km have the PGA concentrated near the epicenter and Taipei basin. In contrast the intermediate-depth event (175 km) does not seem to be confined to the nearest point of land or Taipei basin, instead, it shows a shift towards TWT and WHF stations. The PGA contours from the intermediate-depth event exhibits a southeast elongation with the largest PGA value at the WHF station ~100 km apart from the epicenter. In order to understand the possible causes of such PGA anomaly, Fig. 4 displays vertical component broadband seismograms along the A-A' profile in Fig 3. A clear difference in the appearance of waveforms between the shallow and intermediate events is shown. At station TDC, there is strong high-frequency energy with long coda for the intermediate event but not for the shallow event. Such elevated high-frequency energy of TDC station does not appear at ANP station located to other side of subducted slab, suggesting the excitation of high frequency signals while traveling along the subducting plate with high wave velocity and high Q plate (Fig. 5). Fig. 6 shows the spectral ratio of the 10-s P-wave the guiding station (TDC) and non-guiding (ANP) station for shallow and intermediate events. Note that the seismograms for the intermediate event at TDC station are characterized by stronger > 3Hz frequency energy compared to the ANP station. The spectral ratio of the seismograms at TDC relative to ANP (green line in Fig. 6) also shows a significant increase at higher frequency. The elevated high frequency energy at some other stations (e.g., SSL, NAC) is also found, which confirms the association with wave guide effect of the subducting slab instead of the localized site amplification effect. 3. Guided waves observation Change in the character of guided waves tie closely to the nature of the subduction zone. Here we plan to carefully explore different classes of guided waves characters for the Taiwan subduction-zones earthquakes. We will analyze the seismic characteristics of M5+ earthquakes recorded at CWBSN (Central Weather Bureau Seismic Network) and BATS (Broadband Array in Taiwan for Seismology) in Fig. 7. The M5+ events in the box are first examined in our present work. In Fig. 8 we show the depth dependent waveguide behavior for these M5+ earthquakes recorded at TDC station. A clear drop in high frequency energy is evident for the shallow events. Among this selected subset of seismograms, most of the deeper events (> 60 km) have dominant frequency at 6-7 Hz, whereas the shallow events (< 10 km) show the peak frequency below 1 Hz. The faster arrival of low frequency signals is visible for all > 60 km events but not for shallow events. This indicates that slab structure is responsible for the different frequency content between deeper and shallower event groups. Among the > 60 km events, we also found the time separation between low- and high-frequency signals is not consistent with focal depth. The faster arrival of low-frequency P-wave energy for has a ~1 s time separation with the high-frequency energy for the deeper than 90 km earthquakes. The sharp low- and high-frequency onsets seem to diminish with depth, which makes it difficult to identify the time separation. It is worth noting that uncommonly, the high-frequency guiding is still visible at least at 60 km events comparing to > 100 km guided waves observations in other subduction zones. This is probably associated with different roles of discrete low-velocity zone and stochastic heterogeneity on trapping high frequency energy. To better understand the relationship between waveguide behavior and slab property, there is a need to calibrate the expected behavior in Taiwan subduction zones. 4. Future works The objective of this research is to investigate the possible relationship between the anomalous PGA patterns and the trapping effect of the high frequency signal in the PSP/EP. By detection and quantification the subduction zone guided waves, the geometry, thickness, velocity gradient, and heterogeneities of the plates can be further inferred through the numerical 2D simulations and the comparison with other well-established subduction zones. Additionally, knowledge of waveguide characters and modeled parameters that fit the observations are critical inputs to connecting with the seismic intensity anomalies for ground motion and earthquake hazard estimation. 5. References Abers, G. A., 2000. Hydrated subducted crust at 100-250 km depth, Earth Planet. Sci. Lett., 176, 323-330. Chiu, J.-M., B. L., Isacks, and R. K. Cardwell, 1985. Propagation of high-frequency seismic waves inside the subducted lithosphere from intermediate-depth earthquakes recorded in the Vanuatu, J. Geophys. Res., 90, 12,741-12,754. Furumura, T. and Kennett, B.L.N., 2005. Subduction zone guided waves and the heterogeneity structure of the subducted plate—intensity anomalies in northern Japan, J. geophys. Res., 110, B10302, doi:DOI: 10.129/2004JB003486. Furumura, T. and Kennett, B.L.N., 2008. A scattering waveguide in the heterogeneous subducting plate, in Advances in Geophysics: Scattering of Short-Period Seismic Waves in Earth Heterogeneity, eds Sato, H. & Fehler, M., Elsevier, Amsterdam. Iidaka, T., and M. Mizoue, 1991. P-wave velocity structure inside the subducting Pacific plate beneath the Japan region, Phys. Earth Planet. Inter., 66, 203-231. Martin, S., A. Rietbrock, C. Haberland, and G. Asch, 2003. Guided waves propagating in subducted oceanic crust, J. Geophys. Res., 108(B11), 2536, doi:10.1029/2003JB002450. Ouchi, T., 1981. Spectral structure of high frequency P and S phases observed by OBS's in the Mariana basin, J. Phys. Earth, 29, 305-326. Snoke, J. A., I. S. Sacks, and H. Okada, 1974. A model not requiring continuous lithosphere for anomalous high-frequency arrivals from deep-focus South American earthquakes, Phys. Earth Planet. Inter., 9, 199-206. Fig. 1. Radial component broadband records of ground velocity from an earthquake occurred on 12 June 2006 (M6.2, depth 146 km). (Left) seismic waveforms at stations in the backarc region. (Right) seismic waveforms at stations in the forearc region, which shows the characteristics of typical subduction zone guided wave (i.e., large amplitude of high-frequency S-wave signal with very long coda. The contrast in the appearance of the waveforms across the volcanic front is striking [Furumura and Kennett, 2005]. Fig. 2. Anomalous intensity pattern for deep earthquake on 12 Nov., 2003 (depth=360 km). The triangle indicates the strong motion station. The dashed line indicates the volcanic front. Number in the small box indicates the seismic intensity. The area of felt ground shaking from such a deep and distant event covers almost the whole extent of the Pacific coast of northern Japan (forearc region), but no anomalous intensities are found in the area above the hypocenter [Furumura and Kennett, 2005]. Fig. 3. Example of anomalous PGA pattern due to deep plate event. The PGA patterns for M>5 earthquakes with varying depth. (a) 2005/3/5 M5.96 earthquake at 7 km (b) 2002/9/16 M6.5 at 175 km. Epicenter is denoted by red star with fault plane solution. Strong motion stations are denoted by grey triangles. Profile and broadband stations shown in Figs. 4 and 6 are denoted by blue triangles. Fig. 4. Waveform comparison of events with guiding and non-guiding effect. (a) shallow earthquake at a depth of 7 km. (b) intermediate-depth earthquake at a depth of 120 km. Fig. 5. Schematic illustration of plate guiding. Fig. 6. Fourier spectrum of the broadband waveforms at guiding station (TDC) and non-guiding station (ANP) for (a) shallow earthquake and (b) intermediate-depth event as indicated in Fig.4. Spectral ratio of TDC relative to ANP is shown by green line. Fig. 7. The distribution of BATS stations and the M>5 earthquakes from 1991 to 2009 July. Earthquakes in the box are the selected events for studying depth-dependent waveguide behavior in Fig. 8. Fig. 8. Aligned P onsets of selected earthquakes with varying focal depth (60-s unfiltered vertical-component broadband seismograms) recorded at TDC station (station location shown in Fig. 7) and corresponding spectra.