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We constructed a 3-D coherent velocity model by extracting the common features from recent tomographic models and utilized the forward gravity modeling and wavelength filtering method to explore the depth origin of the gravity anomalies, such as the long-puzzling Puli Bouguer gravity low in west central Taiwan. The similarity analysis among the tomographic models shows the velocity structure most reliable in the depth range from 5 to 35 km. Using empirical velocity-to-density relationships, we show that the calculated Bouguer gravity anomalies from the 3-D coherent velocity model could well reproduce the gravity observations in general. Through filtering process, we separated the Bouguer gravity anomalies into shallow (0 - 17 km) and deep gravity effects (17 - 60 km) for structural exploration. The results show that the shallow gravity effect mainly reflects the surface geological features with low gravity values for unconsolidated sediments in plain and foothill areas to the west and high values for exhumed metamorphic rocks in the mountainous areas to the east. The deep gravity effect reveals the lowest gravity anomalies located in central Taiwan beneath the western Central Range. The Puli Bouguer gravity low is likely a coincident feature arising from the mismatch of the shallow and deep gravity effects due to the N-S trending undulation of crustal thickness beneath the western plain/foothill. This crustal thickness undulation may be related to the rifting activity of Taihsi and Tainan basins in the past. Better discriminating the gravity effect from shallow and deep crust could help better understand the architecture and tectonic process of the Taiwan orogenic belt. [ABSTRACT FROM AUTHOR]

Rapid estimates of earthquake rupture properties are useful for both scientific characterization of earthquakes and emergency response to earthquake hazards. Rupture directivity is a particularly important property to constrain since seismic waves radiated in the direction of rupture can be greatly amplified, and even moderate magnitude earthquakes can sometimes cause serious damage. Knowing the directivity of earthquakes is important for ground shaking prediction and hazard mitigation, and is also useful for discriminating which nodal plane corresponds to the actual fault plane particularly when the event lacks aftershocks or outcropped fault traces. Here, we propose a 3-D multiple-time-window directivity inversion method through direct waveform fitting, with source time functions stretched for each station according to a given directivity. By grid searching for the directivity vector in 3-D space, this method determines not only horizontal but vertical directivity components, provides uncertainty estimates, and has the potential to be automated in real time. Synthetic tests show that the method is stable with respect to noise, picking errors, and site amplification, and is less sensitive to station coverage than other methods. Horizontal directivity can be properly recovered with a minimum azimuthal station coverage of 180°, whereas vertical directivity requires better coverage to resolve. We apply the new method to the Mw 6.0 Nantou, Taiwan earthquake, Mw 7.0 Kumamoto, Japan earthquake, and Mw 4.7 San Jacinto fault trifurcation (SJFT) earthquake in southern California. For the Nantou earthquake, we corroborate previous findings that the earthquake occurred on a shallow east-dipping fault plane rather than a west-dipping one. For the Kumamoto and SJFT earthquakes, the directivity results show good agreement with previous studies and demonstrate that the method captures the general rupture characteristics of large earthquakes involving multiple fault ruptures and applies to earthquakes with magnitudes as small as Mw 4.7. [ABSTRACT FROM AUTHOR]

The article presents research on the magmatic system of the Yellowstone supervolcano which is located the Yellowstone National Park region of the U.S., and it mentions the supervolcano's mantle plume and magma reservoir in relation to the Earth's crust levels. Bimodal basaltic-rhyolitic volcanism is mentioned, along with volcanic hazard assessments and the discharge of carbon dioxide gas. The formation of America's Snake River Plain is also examined.

On 26 February 2012, an inland ML 6.4 earthquake occurred near Wutai Township in Pingtung County, Taiwan and caused massive shaking in southern Taiwan. After the 2010 ML 6.4 Jiasian earthquake caused extensive damage in southern Taiwan, deep, complicated structures were revealed. This earthquake provided us an opportunity to further investigate related tectonic issues and hazard mitigation. Although the focal mechanisms of the Wutai earthquake from various institutes's readings were quite consistent, a large uncertainty was present with regard to focal depth. The relocated source parameters suggest a deeper depth of 32.3 km and a focal mechanism of a 326° strike, a 35° dip, and a 57° rake. Based on the tectonic seismic profile in southern Taiwan, the hypocenter of the Wutai earthquake is therefore likely located in the lower crust and implied that the deep blind fault system under the southwestern Central Range could even go deeper southward, revealing complicated thick-skinned tectonics. In addition, due to the abundance records of the P-wave first-motion polarities collected, we also tested the feasibility of searching the source depth by a polarity fitting for which we demonstrated several synthetic tests and showed its strength. In applying the polarity fitting method to the real case of Wutai earthquake, the results indicated that this approach may be affected by the uncertainty of the epicenter sensitively. However, the application showed the potential that a large number of polarity data existing in earthquake catalog does not only dedicate to determine the focal mechanisms conventionally but may also contribute to the source location in the future. [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]

Static stress changes after major earthquakes are generally believed to influence the spatial and/or temporal distribution of aftershocks and subsequent earthquake events. Aftershock sequences, for example, tend to occur in areas with an increase in Coulomb stress with similar mechanisms to the main shock. However, in regions with pre-existing crustal structures with different mechanisms, the corresponding Coulomb stress change may result in different seismic characteristics. In this study, we demonstrate that a transtensional aftershock sequence was induced by the 1999 Chi-Chi earthquake on a pre-existing normal fault, in the compression- dominated western Taiwan fold-and-thrust belt. We used relocated seismicity and earthquake focal mechanisms to show that this sequence is likely occurred along a subsurface fault that strikes N10°W and dips 80° to the east, at depths greater than 5 km. A Coulomb stress change analysis also shows that this aftershock sequence were located in the area of stress increase due to the Chi-Chi earthquake. Such pre-existing normal faults are found throughout the western coastal plains of Taiwan, thus may pose important earthquake hazards for the populous cities located in the area. [ABSTRACT FROM AUTHOR]

On 27 March 2013, a 6.2 ML earthquake occurred at 19 km depth in eastern Nantou, central Taiwan. Over a 2 week period it was followed by more than 680 aftershocks that ranged to 5 ML. Most events occurred below the ~10-km-deep detachment fault predicted for this part of the mountain belt, coinciding with other precisely located hypocenters that indicate that much of the crust in this area is seismically active. We combine geological data with a three-dimensional (3-D) P-wave velocity model derived from local tomography and earthquake hypocenters to determine a model for the structure of central Taiwan. Much of the surface geology of the area comprises the uplifted Eocene rocks of the Hsuehshan Basin. The 3-D P-wave velocity model shows a shallowing of higher velocities across the Hsuehshan Basin and hypocenter data indicate that its western bounding fault is clearly defined by an eastward-dipping band of events that extends to >20 km depth. The eastern bounding fault is interpreted to coincide at depth with a cluster of events between 20 and 30 km depth. These data suggest that the preexisting, rift-related extensional faults of the Hsuehshan Basin are currently being reactivated and the basin is being inverted. We present hypocenter data from the Nantou sequence that corroborate this interpretation and show the importance of choosing the correct structural model when assessing seismic risk. [ABSTRACT FROM AUTHOR]

Signals from ten explosions were used to examine earthquake location uncertainty in Taiwan. Location errors for explosion sites determined using a relocation process were expressed in terms of statistical measurements for standard errors in the depth (ERZ), the epicenter (ERH), the root-mean-square of the travel time residuals (RMS), and the station coverage gap (GAP). In general, for this study, major factors in location errors resulted from the poor coverage of seismic stations (e.g., a large value of GAP). Using the relationship between location errors and parameters from the explosion location assessment, uncertainties for earthquake locations in the Taiwan region for a total of 384064 events from 1991 to 2011 were evaluated. Offshore regions in southwestern and northeastern Taiwan had larger location errors. For the inland locales, location errors in longitude, latitude, and depth were approximately 3.1 ± 2.7, 1.3 ± 1.6, and 4.6 ± 3.9 km, respectively. The uncertainties estimated from this study could offer a good reference for other related studies. [ABSTRACT FROM AUTHOR]

The first triple-stranded polymeric ladderphanes 6with multilayer planar oligoaryl linkers having C3symmetry are synthesized by Grubbs I catalyst-mediated ring-opening metathesis of tris-norbornene derivatives 5. All linkers are coherently aligned perpendicular to the longitudinal axis of the polymer. Interactions between these linkers lead to fluorescence quenching or perturbation of frontier orbitals, resulting in shift of the emission maximum. These triple-stranded polymeric ladderphanes tend to self-assemble to form an ordered pattern as revealed by TMAFM. [ABSTRACT FROM AUTHOR]