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.