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The distributions and characteristics of coseismic surface ruptures are the key to understand fault behaviors. However, traditional field investigations could be time consuming while toppled buildings and infrastructures may be subject to repairs due to rescue missions and transportation recovery within a few days after the earthquake, leading to urgent needs for rapid, extensive surveys before surface breakages are modified. Therefore, we take advantage of UAS photogrammetry to collect aerial images and to map surface fractures for the Mw 6.4 Hualien, eastern Taiwan, earthquake on 6 February 2018. The earthquake struck Hualien City and generated dense surface breakages over a distance of ~8 km. Together with control surveys for ground control points by using RTK-GPS, UAS photogrammetry uses the methods of the Structure from Motion and Multi-View Stereo to produce high-resolution orthoimages and digital surface models. Our mapping results show that surface ruptures follow the trace of the Milun fault and northern Linding fault. Mapped surface ruptures are usually in en échelon arrays or distributed fractures rather than a through-going fault. Estimated sinistral offset based on orthoimages is more than 60 cm in Chihsingtan and decreases toward south. The fault behavior of the 2018 Hualien earthquake shows along-strike variations and is somewhat different from the October 1951 earthquake. [ABSTRACT FROM AUTHOR]

The 2018 Mw 6.4 Hualien earthquake struck the eastern Taiwan and caused serious damage. We investigate the rupture properties of the 2018 Hualien earthquake by inverting teleseismic body wave and forward modeling GPS coseismic deformation. The rupture process and slip pattern of preferred model explain both the far-field (teleseismic data) and near-field (GPS) observations. The results show that the 2018 Hualien mainshock ruptured southward on two fault segments, with a weak but fast initiation (3.0 km s-1) in the main west-dipping segment F1 and slow (2.0 km s-1) yet significant slip on shallow east-dipping segment F2. In the past few years, several moderate-sized events, which struck eastern Taiwan and caused strong ground shaking and some seismic damage, are considered occurring on the west-dipping fault. Additional investigations are required to building up the knowledge of this not wellknown region. [ABSTRACT FROM AUTHOR]

A deadly Mw 6.4 earthquake occurred in the Hualien area of eastern Taiwan on 6 February 2018. It caused severe damage to infrastructure and creating surface ruptures in several areas mostly near the Milun Fault in Hualien City. In this study, we investigated the distribution of co-seismic surface ruptures by measuring the orientations of the ruptures, classifying the fracture patterns, and measuring the fracture geometries to calculate the principal displacement zone (PDZ) and the regional stress directions. As a result, local PDZ is observed to rotate anti-clockwise along the Milun Fault from north to south. Considering the deformation behaviors of the fractures and their relative positions along the Milun Fault, the shear zone in Qixingtan area is a horsetail structure derived from the right side of the end of the left-lateral strike-slip fault. In addition, the 170° trending fault splay in the middle segment and the fault branch corresponding to the Beipu Fault are elements of the Riedel shear structure associated with the left-lateral moving Milun Fault. Our results show that Riedel shear structures are common within co-seismic surface rupture zones in this area, and the variations in the orientations of Riedel structures reflect the influence of the pre-existing Milun Fault. We can also determine the most recent geometry, kinematics, and displacement characteristics of the Milun Fault through co-seismic surface ruptures and macro-scale Riedel shear structure analysis. This study provides a good example of understanding the relationship between the outcrop scale and the macroscale of the Riedel shear model. [ABSTRACT FROM AUTHOR]

The 2016 ML 6.6 Meinong earthquake struck southern Taiwan and caused serious damage due to the strong ground shaking. Anomalous seismicity spatiotemporal signature changes were identified prior to the 2016 Meinong event. We investigate the seismicity rate changes associated with the 2016 Meinong mainshock by applying the region-time-length (RTL) algorithm. The temporal result reveals a long obvious seismic quiescence stage soon after the 2012 Wutai event lasting until the 2016 Meinong mainshock. The spatial pattern of different background period lengths all exhibit that the 2016 Meinong event occurred near the abnormal seismic quiescence patch. Our study shows that the spatiotemporal information of the seismicity change could be a useful indication for potential seismic-hazard assessment. [ABSTRACT FROM AUTHOR]

Earthquake early warning (EEW) is an effective approach to mitigating earthquake damage. This is the first study evaluating borehole seismograph application to EEW in Taiwan. We selected inland and offshore earthquakes with ML larger than 4.0 occurring between 2012 and 2014 for this study. We investigated the Pd attenuation relationship as a function of the hypocentral distance (R) and magnitude (M). The new Pd attenuation relationship specific for the borehole records is expressed as: log(Pd) = 0.689ML - 0.741log(R) - 4.608 ± 0.248. Once the earthquake location is determined, this regression equation is used to quickly estimate Pd magnitude (MPd). According to the new regression equation formulated specifically for borehole observations in Taiwan, our result shows that the standard MPd deviation is about 0.21 relative to ML. This smaller standard deviation of 0.21 compared to that of the free-surface records might be attributed to the reduced influence of near-surface effects in the borehole records. We propose a new robust Pd regression equation for the Taiwan borehole seismic network. [ABSTRACT FROM AUTHOR]

The Taiwan Earthquake Model (TEM) was established to assess the seismic hazard and risk for Taiwan by considering the social and economic impacts of various components from geology, seismology, and engineering. This paper gives the first version of TEM probabilistic seismic hazard analysis for Taiwan in these aspects. We named it TEM PSHA2015. The model adopts the source parameters of 38 seismogenic structures identified by TEM geologists. In addition to specific fault source-based categorization, seismic activities are categorized as shallow, subduction intraplate, and subduction interplate events. To evaluate the potential ground-shaking resulting from each seismic source, the corresponding ground-motion prediction equations for crustal and subduction earthquakes are adopted. The highest hazard probability is evaluated to be in Southwestern Taiwan and the Longitudinal Valley of Eastern Taiwan. Among the special municipalities in the highly populated Western Taiwan region, Taichung, Tainan, and New Taipei City are evaluated to have the highest hazard. Tainan has the highest seismic hazard for peak ground acceleration in the model based on TEM fault parameters. In terms of pseudo-spectral acceleration, Tainan has higher hazard over short spectral periods, whereas Taichung has higher hazard over long spectral periods. The analysis indicates the importance of earthquake-resistant designs for low-rise buildings in Tainan and high-rise buildings in Taichung. [ABSTRACT FROM AUTHOR]

The northeastern convergence of the Philippine Sea plate toward the Eurasian plate causes the major western Philippine Sea plate boundary to subduct toward the northwest or west directions. However, this phenomenon is not clearly observed along the plate boundary between Luzon and Taiwan. Careful examination of deep seismicity in the southern Taiwan area from the earthquake catalog reported by the Central Weather Bureau shows two seismic zones dipping toward the opposing directions. The first dips toward the east from the surface down to 150 km in depth, while the second dips westward at depths between 150 and 200 km. These two seismic zones are confirmed further by seismogram observation and modeling results generated by two deep faults in the southern Taiwan area. The eastward seismic zone clearly results from the Eurasia plate subduction along the Manila trench, while a small section of the westward seismic zone might likely be a residual slab from the ancient subducted Philippine Sea plate. Based on the subduction speed obtained from GPS observations and the subducted Eurasian plate geometry, we can further estimate the eastward Eurasian plate subduction started at least 3.35 million years ago. This result is roughly consistent with the volcanic ages (3 - 4 Ma) observed in the arc between Luzon and Taiwan. [ABSTRACT FROM AUTHOR]

One decade after the 1999 Chi-Chi earthquake, central Taiwan experienced more strong ground shaking [Central Weather Bureau (CWB), intensity VII] induced by a ML 6.2 earthquake on 5th November 2009. This earthquake occurred in the Mingjian Township of Nantou County, only 12 km southwest of the Chi-Chi earthquake epicenter. The broadband micro-earthquake monitoring network operated by the National Center for Research on Earthquake Engineering (NCREE) observed numerous aftershocks in the five days following the mainshock. The relocated aftershocks and the mainshock focal mechanism indicated a NE-SW striking fault dipping 60° toward the northwest. This fault plane is inside the pre-Miocene basement and the rupture extends from the lower crust to 10 km depth just beneath the basal décollement of the thin-skinned model that is generally used to explain the regional tectonics in Taiwan. The fault plane is vertically symmetrical with the Chelungpu fault by the basal décollement. The NW-SE compressive stress of plate collision in Taiwan, as well as the deep tectonic background, resulted in the seismogenic structure of the Mingjian earthquake at this location. [ABSTRACT FROM AUTHOR]

Through a proposed multiple-event analysis of teleseismic P-waves, this study investigated the radiated seismic energy and rupture process of the 2010 JiaSian earthquake. Results showed that the earthquake comprised at least two sub-events. The first sub-event was followed by the second sub-event, ˜1.7 s later. The entire source duration was 5.4 s. Let the two sub-events occur on the same fault plane with a strike of 304° and a dip of 28°; the first sub-event had a relatively smaller seismic moment (M0) and larger radiated seismic energy (Es) than the second sub-event, and this leads to the Es/M0 of the first sub-event larger than that of the second sub-event thus. This feature implies that the first sub-event probably had a higher static stress drop during faulting. The total M0 was estimated to be 2.17 x 1018 Nm, corresponding to Mw = 6.15, and the total Es was ˜2.91 x 1013 Nm, larger than that estimated only from a single source. Subsequently, the Es/M0 was approximately 1.3 x 10-5, lower than ordinary earthquakes. The low static stress drop was probably responsible for the low Es/M0. Overall, the 2010 JiaSian earthquake was characterized by a relatively low ES/M0 and low static stress drop, and then the partial stress drop model would be relatively appropriate to interpret its rupture process. [ABSTRACT FROM AUTHOR]

Moment tensor inversion is a routine procedure to obtain information on an earthquake source for moment magnitude and focal mechanism. However, the inversion quality is usually controlled by factors such as knowledge of an earthquake location and the suitability of a 1-D velocity model used. Here we present an improved method to invert the moment tensor solution for local earthquakes. The proposed method differs from routine centroid-moment-tensor inversion of the Broadband Array in Taiwan for Seismology in three aspects. First, the inversion is repeated in the neighborhood of an earthquake's hypocenter on a grid basis. Second, it utilizes Green's functions based on a true three-dimensional velocity model. And third, it incorporates most of the input waveforms from strong-motion records. The proposed grid-based moment tensor inversion is applied to a local earthquake that occurred near the Taipei basin on 23 October 2004 to demonstrate its effectiveness and superiority over methods used in previous studies. By using the grid-based moment tensor inversion technique and 3-D Green's functions, the earthquake source parameters, including earthquake location, moment magnitude and focal mechanism, are accurately found that are sufficiently consistent with regional ground motion observations up to a frequency of 1.0 Hz. This approach can obtain more precise source parameters for other earthquakes in or near a well-modeled basin and crustal structure. [ABSTRACT FROM AUTHOR]

The response of Taipei basin upon earthquake excitation was studied using records of recent earthquakes. The strong-motion database includes records obtained at 32 stations of the Taipei TSMIP network from 83 deep and 142 shallow earthquakes (M > 4.0) that occurred in 1992 - 2004. The characteristics of frequency-dependent site response were obtained as spectral ratios between the actual earthquake records (horizontal components) and those modelled for a hypothetical Very Hard Rock (VHR) condition. The models for VHR spectra of Taiwan earthquakes had been recently proposed by Sokolov et al. (2005b, 2006). Analysis of site response characteristics and comparison with simple 1D models of the soil column resulted in the following conclusions: (1) The special ratios throughout the basin obtained from deep earthquakes (depth > 35 km) exhibit good agreement with the theoretical ratios calculated using the 1D models constructed using available geological and geotechnical data. (2) The spectral ratios obtained from shallow earthquakes show influence of: (a) surface waves generated when travelling from distant sources to the basin and (b) relatively low-frequency (< 1 - 2 Hz) waves generated within the basin. (3) Some shallow earthquakes produce extremely high amplification at frequencies 0.3 - 1 Hz within the basin that may be dangerous for high-rise buildings and highway bridges. (4) The obtained results may be used in probabilistic seismic microzonation of the basin when many possible earthquakes located at various distances are considered. 2D and 3D simulation is necessary to model the seismic influence from particularly large earthquakes. [ABSTRACT FROM AUTHOR]

To monitor the fault activity in the Taipei area, a new broadband downhole seismic network comprised of three stations was established in the Taipei Basin over a period of three years, 2005 - 2007. The network geometry is a triangle with a station spacing of about 12 km covering the entire Taipei Basin. Each station has two holes of different depths containing modern instruments, including a low-gain broadband seismometer. The largest depth is 150 m. We report our first experience on the installation and operation of the broadband downhole seismic network in the Taipei Basin. Some representative records from the Pingtung earthquake doublet in December 2006 are shown here. Ground displacement during the Pingtung earthquake doublet can be recovered from the velocity records without the baseline corrections that are required for the acceleration records. Our network offers excellent data for accurate and effective characterization of seismic motion in the study area. Seismic data from this network will be shared with other research institutions in Taiwan and abroad for further research. [ABSTRACT FROM AUTHOR]

A network of ground-based GPS receivers, an ionosonde, and a Doppler sounding system have been employed to monitor the total electron content (TEC), electron density, and vertical motion of the ionosphere above Taiwan. Two M 7.0 offshore earthquakes occurred near Pingtung, the most southern county of Taiwan, on 26 December 2006. Analysis has revealed that the ionospheric TEC and F2-pear electron density (NmF2) surrounding the epicenters of the events decreased abnormally in the afternoon of the fourth day before the earthquakes, and quasi 3-minute fluctuations of pronounced vertical motion in the ionosphere also appeared over the last two days prior to the earthquakes. This is for the first time the pre-earthquake fluctuation signatures in the ionosphere being reported. [ABSTRACT FROM AUTHOR]

We report the retrospective analyses of frequency-magnitude distributions (FMD) and pattern informatics (PI) for the Pingtung offshore doublet earthquakes having magnitudes 6.7 and 6.4. The frequency-magnitude distributions of earthquakes that occurred in the study area preceding the Pingtung doublets demonstrate the activation of moderate events since March 2004, about three years before the Pingtung doublets. In addition, in the calculated PI map, the hypocentral area of the doublets exhibits the signatures of anomalous activity associated with the precursory seismic activation. Our study therefore suggests that, by inspecting temporal variations in the frequency-magnitude distributions of earthquakes, together with the hotspot locations with intense seismicity changes in the PI map, it is possible to shed light on the relevant preparation processes of the forthcoming large earthquakes. [ABSTRACT FROM AUTHOR]

We characterize fault geometries for moderate and large earthquakes in and offshore southwest (SW) Taiwan. Stress orientation estimates suggest that the first shock of the Pingtung earthquake may be characterized by slab flexure. A region of predominately normal-faulting earthquakes is also defined. Using the maximum-likelihood Gutenberg-Richter relation, we find that the repeat times for magnitude, M ≧ 6.5 and M ≧ 7.0 earthquakes are approximately 30 and 80 years, respectively. By assuming a Gaussian probability distribution with a standard deviation of 0.4 the recurrence time, we estimate a 50% probability of an M ≧ 6.5 earthquake recurring in the next 30 years, while only a 20% probability of an M ≧ 7.0 in the next 50 years. To test the empirical relation that is currently being used to convert ML to seismic moment (M0), we test the well-defined energy to moment ratio (E/M0) for the region. We additionally utilize this method to identify the lack of slow-source earthquakes in several recent offshore events in Taiwan. [ABSTRACT FROM AUTHOR]

In this study, ground strains over a soft sediment-filled plain were estimated for nineteen moderate earthquakes in northeastern Taiwan. To this end, abundant accelerograms recorded by two independent seismic arrays, LLSST and SMART1, were analyzed. These two dense seismic arrays are both highly overlapping in terms of space and operation duration. Spatial analyses herein for the LLSST array suggest that good estimates of ground strains can be obtained using only three 3-component seismometers recording at stations configured in a simple triangle with maximum separation of about 100 m. In the case of the Hualien earthquake (ML = 6.5) of 20 May 1986, a peak shear strain of 210 μ occurs with direct S-waves whilst strain of 186 μ occurs with surface waves dominated by a period of about 2.5 sec. However, the peak vertical gradient accompanied by shear waves is in the range 1500 ~ 2000 μ, which is larger than that of horizontal differential motion by a factor of about 10. For further applications in mapping ground deformation fields at a given region, the single-station method is examined by comparing its estimates with those of a displacement gradient algorithm. A moving cross-correlation method is proposed to obtain phase velocities for a sequence of waves, particularly when ground motion contains obvious surface-wave energy. Consequently, estimates of peak horizontal strain at the LLSST site are in the range 1 ~ 150 μ for the nineteen moderate earthquakes and the logarithm of these appears to be proportional to earthquake magnitudes. Finally, a simple test was implemented to verify the feasibility of this method when used with large station spacing (2 ~ 3 km), simulated by the SMART1 array. In general, when taking strain estimated from LLSST data as a basis, the results from SMART1 reveal that differences would not be over 50%. [ABSTRACT FROM AUTHOR]