(1) Characteristics of Strong Ground Motions
Strong-motion seismograms from the 11 February, 2014 Tatunshan earthquake were recorded at stations around the source area. These recordings were used to analyze the strong-motion characteristics in the area. The largest peak ground acceleration (PGA) values of 100.7, 93.4, and 66.6 cm sec-2 in the vertical, EW, and NS directions, respectively, were recorded at station TAP056, about 4.9 km to the northwest of the epicenter. The PGA decays fast with distance, indicating high attenuation in the Tatun volcanic area. The PGA ratio of vertical to horizontal ground motions decreases with increasing epicenter distance. The PGA values in the EW component are larger than those in the NS component. This might be associated with the focal mechanism of the earthquake. The spectral accelerations decrease rapidly with increasing period. (Chen et al., 2014)
ML≥3 earthquakes (ML=local magnitude) that occurred in the Taipei Metropolitan Area from 1973–2013 are selected to study the dominant seismicity period of this area. The epicentral distribution and temporal sequences of earthquake magnitudes are simply described. These earthquakes can be divided into two groups: one for events shallower than 40 km and one for events deeper than 60 km. Shallow earthquakes are located mainly in the 0–10 km depth range north of 25.1oN, and down to 35 km for those south of 25.1oN. Deep events are located in the subduction zone, with a dip angle of about 70o. The Morlet wavelet technique is applied to analyze the dominant periods of temporal variations in numbers of monthly earthquakes in the shallow and deep ranges for three magnitude ranges, i.e., ML≥3, ML≥4, and ML≥5. The results show that for shallow earthquakes the dominant periods are 15.4, 30.8, 66.1, and 132.2 months when ML≥3 and 30.8 months when ML≥4; while for deep earthquakes, the dominant periods are 16.5 and 141.7 months when ML≥3 and 141.7 months when ML≥4. The dominant period cannot be obtained for both shallow and deep ML≥5 earthquakes. (Chen et al., 2015)
(3) Changes in Vibration Periods of High-rise Building during Typhoon
The TAIPEI 101 skyscraper (508-m) is comprised of 101 floors and five floors above and below ground, respectively. It is located in the Hsinyi District of Taipei, Taiwan. The skyscraper is equipped with a 660-metric-ton tuned mass damper – the largest of its type in the world. The fundamental-mode frequency of the skyscraper is about 0.15 Hz. Both the skyscraper and the tuned mass damper swayed during Typhoon Soudelor on 8 August 2015. The maximum vertical, E-W, and N-S displacements measured on the 90th floor are approximately 1.5, 32.2, and 44.5 cm, respectively. The Morlet wavelet technique is applied to analyze the dominant periods of three-component seismograms recorded at the 90th floor. The fundamental period of the skyscraper increases by up to 7% during the strong shaking caused by heavy winds. This change in the fundamental period may mainly correspond to a decrease in the system stiffness during the typhoon because the total mass did not change significantly.