中央研究院
地球科學研究所
地震學的研究是利用地震與其他各式自然振動源(如海浪)所產生的波傳訊息來了解震源與地下傳播介質的物理特性與構造形貌，進而探究其與斷層、火山岩漿系統、造山運動、內核演化等從岩石圈到地球深部構造活動的關係，應用涵蓋的主題非常廣泛。
地球所近期主要的研究方向包括了（1）以數值模型探討大地震孕震與破裂的行為特性、（2）利用高精度波傳模擬反演斷層破裂過程、（3）以非傳統地震學方法進行火山與山崩活動的監測、（4）利用剪力波分離特性探討隱沒帶動力流場與造山帶模型、（5）使用地震陣列方法研究地震引起的地動旋轉特性、（6）對中大型地震進行強地動分析與預警、（7）利用地震層析成像解析隱沒板塊與火山岩漿庫的幾何形貌、（8）從海底震測剖面分析沉積構造歷史與甲烷水合物分布、以及（9）整合多重物理量觀測的地震前兆研究等。
地球所也致力在台灣與海外佈設與運行多個獨特的地震測站網如臺灣地區寬頻地震觀測網（BATS）、臺北盆地井下強震儀陣列、臺灣山區強震儀陣列、大屯火山地震觀測網，以及在東南亞與中亞的流動式地震陣列；通過合作與衛星通訊接收全球上千個地震站的即時波形數據。技術開發方面，地球所更與中山大學海下科技研究所以及國研院海洋中心自主開發台灣的寬頻海底地震儀；與德國GFZ研究中心和新加坡南洋理工持續合作與跟進旋轉地震儀與次聲波儀器的開發與最新進展。結合多元的研究與觀測網，地球所的地震學研究旨在深入了解地球內部的各種構造系統；並對台灣以及東南亞地區的地震與地質災害做出實質的貢獻。
The shear-wave velocity structures of the crust and uppermost mantle of northern Vietnam were analyzed using the receiver function (RF) method at 25 broadband stations to investigate the regional crustal structure and its tectonic evolution. In this study, we presented a new crustal shear-wave velocity structure of northern Vietnam determined through RF analysis. Our results revealed significant variations in crustal thickness and deep crustal velocities across the study area. Along the Red River shear zone (RRSZ), the patterns of the crustal structure were distinct on both sides; they were simple and complex, respectively, in the blocks on northeast and southwest. A low-velocity zone (LVZ) was widely observed in the northwestern corner of the study area, and significant lateral variations in the thickness and strength of the crustal structure were observed from north to south. This LVZ was distributed as a thick and deep zone in the north and became thinner and shallower in the central region; the LVZ finally disappeared in the south. Two end members of the origin of the LVZ were proposed. The LVZ can be considered a weak crustal layer that escaped from the southeastern margin of the Tibetan Plateau, or it may have been formed from a paleo-subducted slab beneath it because of an onsite mantle heat source. The existence of this LVZ suggests that the movement of the RRSZ is possibly concentrated above the LVZ and that extension to the upper mantle is not necessary in the present stage. The above tectonic regime supports the possibility that the RRSZ is a strikeslip fault with a feature restricted in the crust.Full Article: https://doi.org/10.3319/Tao.2020.03.05.01
We initiated an earthquake reporting project in 2016 to collect field observations of ground damages caused by large earthquakes from trained volunteers and interested citizens. After a potentially damaging earthquake occurs in the Taiwan area, our system, the Taiwan scientific earthquake reporting system (TSER), would send a notice to the participants, who are encouraged to visit the epicentral area to survey and describe in as much detail as possible the variations of the ground damages using a Usahidi-based mapping platform. They may also upload relevant images in the field when the condition permitted (i.e., good mobile signal). This collective information will be shared with the public after a quick check by the on-duty scientists. Statistically, in Taiwan damaging inland earthquakes, e.g., magnitude greater than 6, occurred every 2–3 years. During the intermittent time, the platform serves to share educational materials such as pictures of geological structures and landscapes, which are beneficial to many of the volunteers, who are high school science teachers. This experimental, science-oriented crowdsourcing system was first tested during the February 6, 2018 Mw 6.4 offshore Hualien, Taiwan earthquake. We received 19 field reports in the first 3 days after the earthquake. Most of these reports provided surface damage details along the Milun fault, which also ruptured during the 1951 ML 7.1 Longitudinal Valley earthquake sequence. The crowdsourcing approach of TSER has proven to be effective in enhancing public awareness and the potential for scientific advancement in hazard mitigation.
The Tatun Volcanic Group (TVG) is proximal to the metropolis of Taipei City (population of ca. 7 million) and has long been a major concern due to the potential risks from volcanic activity to the population and critical infrastructure. While the TVG has been previously considered a dormant or extinct volcano, recent evidence suggests a much younger age of the last eruption event (~ 6000 years) and possible existence of a magma reservoir beneath the TVG. However, the location, dimension, and detailed geometry of the magma reservoir and plumbing system remains largely unknown. To examine the TVG volcanic plumbing structure in detail, the local P-wave travel time data and the teleseismic waveform data from a new island-wide Formosa Array Project are combined for a 3D tomographic joint inversion. The new model reveals a magma reservoir with a notable P-wave velocity reduction of 19% (ca. ~ 19% melt fraction) at 8–20 km beneath eastern TVG and with possible northward extension to a shallower depth near where active submarine volcanoes that have been detected. Enhanced tomographic images also reveal sporadic magmatic intrusion/underplating in the lower crust of Husehshan Range and northern Taiwan. These findings suggest an active volcanic plumbing system induced by post-collisional extension associated with the collapse of the orogen.
It is conventionally believed that magma generation beneath the volcanic arc is triggered by the infiltration of fluids or melts derived from the subducted slab. However, recently geochemical analyses argue the arc magma may be formed by mélange diapirs that are physically mixed by sediment, altered oceanic crust, fluids, and mantle above the subducted slab. Further numerical modeling predicts that the mantle wedge diapirs have significant seismic velocity anomalies, even though these have not been observed yet. Here we show that unambiguously later P-waves scattered from some obstacles in the mantle wedge are well recorded at a dense seismic array (Formosa Array) in northern Taiwan. It is the first detection of seismic scattering obstacles in the mantle wedge. Although the exact shape and size of the scattered obstacles are not well constrained by the arrival-times of the later P-waves, the first order approximation of several spheres with radius of ~ 1 km provides a plausible interpretation. Since these obstacles were located just beneath the magma reservoirs around depths between 60 and 95 km, we conclude they may be mantle wedge diapirs that are likely associated with magma generation beneath active volcanoes.
An MwMw 7.5 earthquake struck Palu in the northern coast of Sulawesi island, Indonesia, on 28 September 2018. Its focal mechanism was determined to be a left‐lateral strike‐slip fault, which is generally expected to not produce a tsunami. However, a large tsunami with runup heights of more than 6 m was observed along the coast of Palu city. Here, we show a complex triggering supershear source model as determined by teleseismic waveform inversion. Three asperities with different slip characteristics were found on the 120‐kilometer‐long rupture zone. Significant triggering rupture with a supershear speed was observed south of the epicenter, which was just beneath Palu city. This special rupture process can cause a strong directivity effect that produced anomalously large ground shaking with nonlinear effects in Palu area. The coseismic deformation determined from the inverted source model showed large horizontal displacements. These horizontal movements combined with complex bathymetry and topography could have pushed seawater to generate a tsunami even though the Palu earthquake was a strike‐slip event.
The first stage of field experiments involving the design and construction of a low-power consumption ocean bottom electro-magnetometer (OBEM) has been completed, which can be deployed for more than 180 d on the seafloor with a time drift of less than 0.95 ppm. To improve the performance of the OBEM, we rigorously evaluated each of its units, e.g., the data loggers, acoustic parts, internal wirings, and magnetic and electric sensors, to eliminate unwanted events such as unrecovered or incomplete data. The first offshore deployment of the OBEM together with ocean bottom seismographs (OBSs) was performed in NE Taiwan, where the water depth is approximately 1400 m. The total intensity of the magnetic field (TMF) measured by the OBEM varied in the range of 44 100–44 150 nT, which corresponded to the proton magnetometer measurements. The daily variations in the magnetic field were recorded using the two horizontal components of the OBEM magnetic sensor. We found that the inclinations and magnetic data of the OBEM varied with two observed earthquakes when compared to the OBS data. The potential fields of the OBEM were slightly, but not obviously, affected by the earthquakes.
Earthquake slip leads to stress relaxation in the crust, whereas healing of the damage induced by strong ground motion predominantly occurs in the near surface. Temporal changes in the seismic velocity structure after large earthquakes can be driven by diverse mechanisms, such as aseismic slip or fault zone healing, but the timescales governing these processes are very similar, making them difficult to distinguish. We detect temporal velocity changes in the crust since the great 2004 Sumatra and 2005 Nias earthquakes using the high‐frequency late‐arriving scattered waves after the S phase and long‐period Rayleigh waves of repeating earthquakes. We find that the temporal velocity changes in the scattered waves exhibit steady logarithmic recovery from 2005 to 2015, whereas the Rayleigh wave velocity recovery was interrupted by several large earthquakes after late 2007. The difference between these two temporal trends in velocity change is the key to distinguishing between a damage/healing/redamage cycle near the surface and slow deformation (e.g., afterslip and postseismic relaxation) at depth. Rayleigh waves are highly sensitive to the near‐surface damage and healing after the 2004/2005 events and also the repeated damage induced by the 2007 and 2008 earthquakes. Steady velocity recovery of the scattered waves primarily corresponds to slow deformation at depth.
Turbulent mixing in the deep ocean is not well understood. The breaking of internal waves on sloped seafloor topography can generate deep-sea turbulence. However, it is difficult to measure turbulence comprehensively due to its multi-scale processes, in addition to flow–flow and flow–topography interactions. Dense, high-resolution spatiotemporal coverage of observations may help shed light on turbulence evolution. Here, we present turbulence observations from four broadband ocean bottom seismometers (OBSs) and a 200-m vertical thermistor string (T-string) in a footprint of 1 × 1 km to characterize turbulence induced by internal waves at a depth of 3000 m on a Pacific continental slope. Correlating the OBS-calculated time derivative of kinetic energy and the T-string-calculated turbulent kinetic energy dissipation rate, we propose that the OBS-detected signals were induced by near-seafloor turbulence. Strong disturbances were detected during a typhoon period, suggesting large-scale inertial waves breaking with upslope transport speeds of 0.2–0.5 m s−1. Disturbances were mostly excited on the downslope side of the array where the internal waves from the Pacific Ocean broke initially and the turbulence oscillated between < 1 km small-scale ridges. Such small-scale topography caused varying turbulence-induced signals due to localized waves breaking. Arrayed OBSs can provide complementary observations to characterize deep-sea turbulence.
The average anisotropy beneath Anatolia is very strong and is well constrained by shear-wave splitting measurements. However, the vertical layering of anisotropy and the contribution of each layer to the overall pattern is still an open question. Here we construct anisotropic phase-velocity maps of fundamental-mode Rayleigh waves for the Anatolia region using ambient noise seismology and records from several regional seismic stations. We find that the anisotropy patterns in the crust, lithosphere and asthenosphere beneath Anatolia have limited amplitudes and are generally consistent with regional tectonics and mantle processes dominated by the collision between Eurasia and Arabia and the Aegean/Anatolian subduction system. The anisotropy of these layers in the crust and upper mantle are, however, not consistent with the strong average anisotropy measured in this area. We therefore suggest that the main contribution to overall anisotropy likely originates from a deep and highly anisotropic region round the mantle transition zone.
The strong‐motion downhole array (SMDA) in Taipei basin is examined, and the data quality, glitches, and systemic errors in its data are discussed. This seismic network is an array of arrays: the SMDA comprises a total of 32 triggered strong‐motion acceleration seismometers spanning eight sites. Each site has one seismometer at the surface and an additional two to four seismometers each collocated at the individual boreholes. Polarity reversals, swapped components, clock desynchronization, and bad components (flat‐line signal or aseismic noise) have all been observed and are shown to be occasional issues. Signal‐to‐noise ratios are generally excellent. The lack of known orientations at depth is the primary issue regarding data quality of the SMDA. Orientations of each borehole seismometer were recorded at the time of installation, but those initial values are not reliable for subsequent events. Furthermore, redetermined azimuthal orientations are inconsistent from event to event, suggesting that the borehole seismometer orientations change over time. Orientation wander is not associated with instrument maintenance. An iterative method to reliably determine borehole seismometer orientations is introduced. Data from each borehole seismometer are rotated and compared with that from a collocated reference station on the surface with known orientation until a maximum correlation is reached. This method is reliable for most events, but may become unreliable for local events in which the incidence of incoming seismic waves is near vertical and shorter wavelengths introduce complicated wave propagation within Taipei basin. Temporal analysis of borehole seismometer orientations shows not only drifting of azimuthal orientations over time, but also erratic changes in orientation in multiples of 90°. This behavior is suggestive of frequent polarity reversals and/or swapped components on one or both of the two horizontal components of each borehole seismometer.
The Taiwan Earthquake Model (TEM) published the first version of the Taiwan probabilistic seismic hazard assessment (named TEM PSHA2015) 5 years ago. For updating to the TEM PSHA2020, we considered an updated seismogenic structure database, including the structures newly identified with 3D geometry, an earthquake catalog made current to 2016, state-of-the-art seismic models, a new set of ground motion prediction equations, and site amplification factors. In addition to earthquakes taking place on each individual seismogenic structure, the updated seismic model included the possibility of an earthquake occurring on multiple structures. To include fault memory for illustrating activity on seismogenic structure sources, we incorporated the Brownian passage time model. For the crustal seismicity that cannot be attributed to any specific structure, we implemented both area source and smoothing kernel models. A new set of ground motion prediction equations is incorporated. In addition to the calculation of hazard at engineering bedrock, our assessment included site amplification factors that competent authorities of governments and private companies could use to implement hazard prevention and reduction strategies.
The intermediate-depth seismicity below the Hindu-Kush orogen is thought to mark the Indian-plate subduction with the bottom half of the slab currently breaking off. Unique features of this continental subduction are the near-vertical slab and the roughly stationary convergence boundary. How this subduction affects the mantle flow patterns remains to be understood. In this study we measured source-side shear wave splitting on the S waves from Hindu Kush intraslab events to sample the surrounding mantle. The observed fast polarization directions exhibit a circular pattern around the slab resembling that predicted for the toroidal flow driven by slab rollback. However, the rollback scenario is not favored because it hardly sustains in dynamic models without a considerable retreat of convergence boundary. We propose that the observed pattern is produced by the sub-vertical shear flow entrained by the steep descent of the slab and the ongoing breakoff. This scenario requires the existence of A-type or AG-type olivine fabrics with strong orthorhombic anisotropy in mid- to lower upper mantle, which is consistent with the global models of azimuthal and radial anisotropy. This interpretation circumvents the debate on the cause of trench-parallel anisotropy in some oceanic subduction zones where slab entrainment and rollback may coexist, and supports the notion that orthorhombic anisotropy of olivine may play an important role in shaping mantle anisotropy.