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This study investigated 18 broadband teleseismic records and 451 near field GPS coseismic deformation data to determine the spatial and temporal slip distribution of the 2011 Tohoku-Oki earthquake (M 9.0). The results show a large triangular shaped slip zone with several asperities. The largest asperity centered above the hypocenter at about 5 - 30 km depth. A secondary large asperity was found in the deeper subduction zone beneath the hypocenter. The average slip on the fault is ~15 m and the maximum displacement on the biggest asperity is > 30 m. The temporal rupture process shows that the slip nucleated near the hypocenter at the beginning, and then ruptured to the shallow fault plane forming the largest asperity. The slip developed in the deeper subduction zone in the second stage. Finally, the rupture propagated toward the north and south of the fault along the Japan Trench. The source time function shows three segments of energy releases with two large peaks related to the development of the asperities. The overall rupture process is ~180 seconds. This source model coincides well with the aftershock distribution and provides a first-order information on the source complexity of the earthquake which is crucial for further studies. [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 26 December 2006 Pingtung earthquake doublet (both about Mw 7.0) were the two largest events with magnitudes close to 7.0, following the 1999 Chi-Chi earthquake. The locations and focal mechanisms of the Pingtung earthquake sequence provide good opportunities for scientists to investigate plate movement and deep tectonic structure in southern offshore of Taiwan. However, precise hypocenters and source mechanisms of this sequence are not definite due to poor station azimuthal coverage and a lack of information from historical seismicity in this area. Here we use regional ground motion full waveform records to investigate source rupture processes and determine fault planes of the Pingtung earthquake doublet. The finite fault inversion result of the first event shows two large asperities occurred near the epicenter. The rupture mainly originated from the south then propagated northward on an eastern dipping fault plane. The second earthquake is a bilateral rupture along NW-SE direction having a large asperity close to the epicenter which released most of its seismic energy within the first 15 seconds. The slips of the two earthquakes show compensative patterns when mapped on the earth's surface. We show that the Pingtung earthquake doublet are large normal and strike-slip faulting which dips toward the east and west, respectively. The mechanisms determined from inversion results infer that the first event might relate to the bending of the South China Sea Plate during its subduction to the Philippine Sea Plate. The second event could happen at a pre-existing weak zone that was triggered by transferred stress after the first event. [ABSTRACT FROM AUTHOR]

The 1909 Taipei earthquake (M 7.3) occurred beneath the Taipei metropolitan area (TMA) causing substantial damage according to the historical literature. According to the hypocenter relocation and tectonic implications provided in a previous study, we simulated ground motions within the TMA using a hybrid simulation method involving the spectral-element method (SEM) and the empirical Green's function method (EGFM). We used the SEM for simulating low-frequency components and the EGFM for simulating high-frequency components. These high and low frequency components were subsequently combined. For the EGFM we used the records from a recent ML 4.9 earthquake (11 October 2013, depth = 143.8 km) in the Taipei area as the empirical Green's function. According to the historical literature, the observed PGA (peak ground acceleration) values are 59.2 and 67.0 gal at ancient stations TAP and KEE, with periods of 1.21 and 1.34 s, respectively. By comparing the simulated PGA values at modern stations TAPB and WFSB to the historical documented ones for 12 different models, our result suggests that the 1909 Taipei earthquake was an event with a magnitude of about Mw 7.3 and stress drop of approximately 30 bars, or a smaller equivalent magnitude between Mw 6.8 - 7.3 but with much higher average stress drop of more than 100 bars. For a deep event beneath TMA a larger vertical P-wave motion and longer period shaking wave, as addressed in the historical literature, might be expected with prolonged shaking as found in the simulation. A seismic hazard assessment is necessary for metropolitan Taipei to better understand the long period shaking from deep subduction zone intra plate events. [ABSTRACT FROM AUTHOR]

The M ≥ 3 earthquakes which occurred in the Taipei Metropolitan Area from 1973 through 2010 are used to study the memory effect of earthquake sequences in the area by applying a fluctuation analysis technique in the natural time domain. The earthquakes can be divided into two groups: the first for shallow events with focal depths ranging 0 - 40 km and the second with focal depths deeper than 60 km. For both shallow and deep earthquakes, three magnitude ranges, i.e., M ≥ 3, M ≥ 3.5, and M ≥ 4, are taken into account. The calculations are also made for the events in a smaller area. Calculated results show that the exponents of the scaling law of fluctuation versus window length for all earthquakes sequences in consideration are not larger than 0.5, thus suggesting that the M ≥ 3 and M ≥ 3.5 earthquakes in the TMA are short-term corrected. On the other hand, the M ≥ 4 earthquakes are weakly corrected. [ABSTRACT FROM AUTHOR]

The time history and spatial dependence of seismic-wave propagation on the ground surface and through the ionosphere following the 2011 off the Pacific coast of Tohoku Earthquake were reconstructed from dense seismic networks and from Global Positioning System (GPS) array observations, respectively. Using total electron content (TEC) data recorded by a dense GPS receiver network, the near-source ionosphere perturbations induced by this giant earthquake were analyzed and high-resolution images of seismic-wave propagation in the ionosphere are presented. Similar spatial images of ground motions were reconstructed from observations by a dense seismic array. Observations of this event provide, for the first time, the opportunity to compare near-source ground motions with the near-field seismo-traveling ionosphere disturbance (STID) excited by the ground motions. Based on the results, the nature of the source rupture and seismic-wave propagation are discussed. Both seismic and ionosphere observations indicate that seismic energy propagated radially outward initially from the hypocenter, but that the circular shape of the propagation front became gradually distorted as the source rupture became extended. Coherent wavefronts from the two analyses show contrasting patterns during the later stage of propagation, possibly due to different patterns of spatial variations in the physical properties of the solid earth and of the ionosphere. [ABSTRACT FROM AUTHOR]

This paper tracks the time history and spatial variation of the long-period (1 - 10 sec) strong ground motions recorded in the Taipei basin during the magnitude 7.1 earthquake of 31 March 2002, offshore of eastern Taiwan. The two-dimensional ground motions of this event were reconstructed from 89 free-field strong motion records over an area of 40 square km in northern Taiwan. The observed basin ground motions show complex waveforms, extended durations and multiple propagation directions in later phases. The dominant basin ground motions are identified to shake in its radial directions after the S-wave arrivals. Within the analyzed period band, results show that seismic waves amplifications were observed inside the sedimentary basin and its major amplifications were located on the eastern edge, the thick sediment portion of the basin. Across the Taipei basin, large ground motions were still maintained at the Linkou Tableland. Employing moving-window and waveform stacking techniques to analyze array seismograms revealed that high amplitude later phases have lower apparent velocities than the incident S-waves and cross the basin in multiple directions. We interpret these long period later arrivals to be surface waves, which are generated by a body wave interacting with the thick soft sediment of basin. [ABSTRACT FROM AUTHOR]

The magnitude 7.1 Pingtung offshore earthquake occurred on 26 December 2006 at a depth of 40 - 60 km within the subduction zone of the Eurasian plate. This event exhibits an emergent onset. The first large amplitude arrivals were delayed by several seconds with respect to the origin time and were preceded by small-scale slip. This event is one of a few large intermediate depth earthquakes well recorded in a near source region. Based on beamforming analysis, some near source recordings were used in this study to investigate the initial rupture process of this event. Analyzed results indicate that initial fault rupture of this event at the hypocenter was of short duration with initial resultant fault slip extending southward toward the shallower portion of the fault plane. Thereafter, a detected rupture front propagated both northward and downward. Following small-scale initial slip, a major rupture arose. A detailed rupture process has been reconstructed using backward projection of array seismograms from the fault plane to image fault slip. The spatial and temporal evolution of initial onset presents the same rupture behavior determined by beamforming analysis. Our analysis appears to indicate the distinct possibility of an initial seismic nucleation phase; however, uncertainties remain and a cascade process cannot be ruled out completely. [ABSTRACT FROM AUTHOR]

Despite a moderate magnitude, Mw = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (Mw = 6.9) and the 1994 Northridge, California, earthquake (Mw = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of Mw = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical "soft" and "hard" crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes. [ABSTRACT FROM AUTHOR]

The 2013 ML 6.4 Ruisui earthquake struck a seismic gap around the northern part of the Longitudinal Valley where rare earthquakes have occurred for at least two decades. Seismic data with different frequency bands connote a diverse scale of source characteristics. The slip model derived by a previous study using long-period seismic data provided us with the preliminary earthquake source properties in low-frequency energy radiation. However, the high-frequency part of the seismic energy needs to be considered for the strong motions in the comprehensive frequency band. As a case study, we carried out broadband strong motion simulations through the hybrid scheme, which simulated waveforms combining the low-frequency motions (frequency-wavenumber integration method) and high-frequency motions (empirical Green's function method) to reveal the strong ground motion and source characteristics of the 2013 Ruisui earthquake. The results show that even though the local structure is complicated this hybrid simulation can effectively rebuild the overall broadband strong ground motion characteristics for the 2013 Ruisui earthquake. Additional study issues are required to better understand the nature of localized high peak ground acceleration and the related seismic hazards for future earthquakes. [ABSTRACT FROM AUTHOR]

The Taipei basin, historically low in seismicity, is located in northern Taiwan. A dense broadband seismic array was deployed in the basin in June 2004 to monitor seismic activity. During the period of operation, three felt earthquakes occurred near the eastern part of the Taipei basin, about 3 km to the south of Taipei 101 then the tallest building in the world. Relocated earthquakes show a southeast-dipping distribution of hypocenters beneath the Taipei basin. The seismicity pattern and focal mechanisms of the three felt events suggest the existence of a blind normal fault whose surface projection is along the river channel in the middle of the basin. [ABSTRACT FROM AUTHOR]