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B6-004
14:20
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14:40
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MILLIHERTZ GROUND MOTION ON OCEAN BOTTOM PRESSURE DATA EXCITED BY LARGE REGIONAL EARTHQUAKES
The effects of sedimentary basins on strong ground motion have been recognized through the amplification and excitation of long-duration coda. Most studies evaluating these effects using seismograms have been land-based. Ocean bottom seismometers have recently been deployed in several subduction zones, that commonly use high-sensitivity and/or broadband sensors in the ocean bottom networks, which makes it difficult to measure large or strong ground motion without saturating the recorded seismograms. Herein, we analyze the ground motion retrieved from ocean bottom pressure gauges (OBPGs) as strong-motion seismometers, and discuss the effects of shallow submarine structures on the amplification and excitation of coda waves, particularly the Hikurangi Trough accretionary wedges. The dynamic pressure changes (caused by strong motion) on the OBPGs can be converted to ground motion acceleration, based on the ground motion frequency and the water depth of a given OBPG. The water depth can be expressed in terms of the fundamental acoustic resonant frequency, which depends on the velocity of the ocean acoustic wave and the water depth above the OBPG. When the ground motion frequency is sufficiently low compared to the acoustic resonant frequency, the vertical acceleration of the ground motion at the seafloor can be estimated from the dynamic pressure change observed on the OBPG. We investigated the ocean bottom seismograms of both M7.8 Kaikoura and M7.1 Te Araroa earthquakes from ocean bottom pressure recorders. Our results show that anomalous long-period waves, dominating in the range of 0.005–0.05 Hz, occurred near the coastline. However, the long-period waves have not been observed at onshore stations. This suggests that the millihertz ground motion on OBP might be caused by interaction between the seafloor and ocean at shallow water depth near the coastline.
Ryota Hino, Yoshihiro Kaneko, Yoshihiro Ito, Spahr C. Webb, Laura M. Wallace
ocean bottom pressure, sedimentary basin, strong ground motion
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B2-012
14:40
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14:55
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THE 2018 M W 6.4 HUALIEN EARTHQUAKE: DYNAMIC SLIP PARTITIONING REVEALS THE SPATIAL TRANSITION FROM MOUNTAIN BUILDING TO SUBDUCTION
The Hualien earthquake of 6 February 2018 occurred on the east coast of Taiwan in the collision zone between the Eurasia and Philippine Sea plates. Even though an event with a moderate magnitude of M W 6.4, it caused widespread surface rupture in and around the city of Hualien, leading to significant loss of life and property damages and signaling possible involvement of multiple faults. In this study, we investigate the rupture process of this earthquake using teleseismic, strong motion, GPS and SAR observations. We first employ the geodetic data to search for the geometry of multiple fault planes, followed by a multi-point source inversion using teleseismic and strong motion waveforms to determine the possible change in mechanism during the rupture process. We parameterize the fault model with three curved segments to represent the complicated fault system involved in the rupture process, including an offshore fault, which appears to be the plate interface between the subducted Costal Range and Meilun Tableland. This fault plane presents a curved geometry, changing from shallow dipping (~45°) in the north to sub-vertical (~80°) in the south. Finally, a joint inversion of all seismic and geodetic data is carried out to constrain the complete rupture process of the earthquake. The inversion result reveals that the rupture initiated on the plate interface and propagated unilaterally towards south-southwest. The rupture then jumped on to the sub-vertical Meilun Fault and reached the ground surface. Significant dynamic slip partitioning occurred during the rupture process near the Meilun Fault, with the majority of thrust and strike-slip motions occurring on the plate interface and the Meilun Fault, respectively. Our result indicates that the 2018 Hualien earthquake does not follow a regular slip pattern: thrust motion may occur on the two side walls of the Meilun Tableland with a “teeterboard” uplift pattern in different earthquake cycles. The change in geometry and slip on the inter-plate fault appears to be related to the transition of Coastal Range from mountain building to subduction.
Yi-Ching Lo, Han Yue, Jianbao Sun, Li Zhao
2018 M W 6.4 Hualien earthquake
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B7-015
14:55
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15:10
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Coulomb stress changes triggering surface uplift in 2016 Mw 6.4 Meinong Earthquake and their implications for Earthquake-induced mud diapiring
We investigate the 2016 Meinong earthquake (Mw 6.4) in southwestern Taiwan, which caused surface uplift in an area of about 10x15 km2 with a maximum of 10 cm. In this study, we calculate 3D strain tensor in the epicentral area due to the Coulomb stress change derived from the coseismic slip on the fault plane and based on a dislocation (Okada) model. We adopted a published fault model by Lee et al. (2016) with fault geometry and coseismic slip distribution on the fault. We found substantial shortening strain (10-5-10-6) occurred in a lobe of several km above the coseismic fault plane at the depth of 14-18 km, which is situated right below the coseismic uplift area on the surface. Our results also show the “squeezing” characteristics of the principal strain axes of the shortening based on the distribution of 3-D principal strain projected axes on to the horizontal and vertical planes. On the other hand, at the shallow part near the surface level (0-3 km), the “explosion” characteristics are revealed by the maximum lengthening principal strain axes in and around the surface coseismic uplift area. Combining our results with the local geological rocks, which are mainly composed of Miocene-Pliocene thick mudstone in the fold-thrust belt, we tend to interpret that the 2016 Meinong earthquake coseismic surface uplift was closely related to the mud diapirs and mud volcanoes, which were likely reactivated by sudden increase of compression in the deeper part of the mud diapirs and extension in the shallow level.
Hue Anh Mai, Jian-Cheng Lee, Kuo-Liang Wen
Coulomb stress changes, Crustal deformation, Earthquake triggering, The 2016 Meinong Earthquake
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B5-013
15:10
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15:25
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LESSONS LEARNED FROM THE 2018 HUALIEN EARTHQUAKE: I. CHARACTERISTICS OF STRONG GROUND MOTIONS AND ITS CORRESPONDENCE TO DAMAGES FROM QUESTIONNAIRE SURVEY FOR HIGH-RISE RESIDENTIAL BUILDINGS
During the 2018 Hualian earthquake, high amplitude and pulse-like motions with period of 2 sec in the EW direction (almost fault-normalized direction) and 1sec in the NS direction (almost fault-parallel direction) were recorded near the Milun fault, which are responsible for damage of buildings. To examine the effect of the long-period ground motions to high-rise buildings, in this study, we selected 9 high-rise RC residential buildings locating at the southwest area, the northeast area, and the central area of the east side of the Milun fault to performe the questionnaire survey on feeling of shaking, structural/nonstructural damages, indoor damages, and so on. After the characteristics of ground motions near the Milun fault were studied, the results of the answers were analyzed regarding to the height of buildings, i.e. the upper level, the middle level, and the lower level. Furthermore, considering the different sites and directions to the fault, answers are analyzed regarding to different locations, i.e. the southwest area, the northeast area, and the central area. The natural periods extracted from microtremor data and estimated peak floor accelerations of target buildings are presented in companion presentation, the part II.
Xin Wang, Wen-Yen Chang, Masayuki Nagano, Yixuan Sun, Hongjun Si
2018 Hualien earthquake, feeling of shaking, indoor damage, microtremor measurement, questionnaire survey, structural/nonstructural damage
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B5-015
15:25
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15:40
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Lessons learned from the 2018 Hualien earthquake: II. Nonlinear response analysis of high-rise reinforced-concrete buildings to pulse-like ground motions
Four medium-to-high-rise buildings collapsed near the Milun Fault during the mainshock of the 2018 Hualien Earthquake. The long-period pulse-like ground motions that occurred adjacent to the Milun Fault, could be one of the reasons for the building damage. In this paper, we focus on the earthquake response characteristics of high-rise reinforced-concrete buildings in Hualien City via nonlinear response analyses. We first conducted microtremor measurements at 10 target buildings to extract their fundamental natural periods, and the relationship between the natural periods and number of stories was compared to the calculation formulas outlined in the building design codes of Taiwan and Japan. We then performed a nonlinear response analysis on three buildings using the generalized multi-degree-of-freedom building system models, whose parameters were determined considering the measured natural period and shear strength from the design codes of Taiwan and Japan, respectively. The fundamental natural periods that were detected from the microtremor measurements were in good agreement with the previous regression equation. However, they were much shorter than the values from the design codes of Taiwan and Japan, which is likely due to the rigidity of nonstructural members. The maximum responses of the Japan models are generally larger than those of the Taiwan models. The trend in inter-story drift angle corresponds to the questionnaire survey results for “Cracks in structural material,” and the peak floor accelerations estimated from the questionnaire survey results using the regression equation from past damage earthquakes are almost consistent with the results from the nonlinear response analyses.
Tetsushi Watanabe, Xin Wang, Hong-jun Si, Wen-Yen Chang, Masayuki Nagano
2018 Hualien earthquake, high-rise RC buildings, microtremor measurement, nonlinear multi-degree-of-freedom system model, questionnaire survey, response analysis
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B5-014
15:40
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15:55
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LESSONS LEARNED FROM THE 2018 HUALIEN EARTHQUAKE: III. GENERATION OF LONG-PERIOD PULSE-LIKE GROUND MOTIONS
During the main shock of seismic sequence of 2018 Hualien earthquake, long-period pulses with period of 2 to 4 sec and significant crustal deformation were observed near the Milun fault traveling under the Hualien City in almost north-south direction. The long-period pulses are assumed to be generated by the fling-effect in the fault-parallel direction and the directivity-effect in the fault-normal direction. These long-period pulses are considered to relate to the damage of buildings. In this study, in order to investigate the generation mechanism of long-period pulses near the Milun fault during the main shock, a characterized seismic source model was constructed which is composed of strong motion generation areas (SMGAs) and long-period motion generation areas (LMGAs) located at the shallow part of the Milun fault with different slip patterns. One thrust SMGA is located at the Milun fault. Two thrust SMGAs are at the Linding fault. Based on the reproduced results of the observed records, we found that (1) the SMGA1 with thrust components located in the focal fault contributes to the first motions in three directions; (2)the LMGA1 with left-lateral strike-slip components, which includes the surface slip at the northern part of the Milun fault, contributes to the fling-pulse in the NS direction; (3) the SMGA2 with thrust components located in the southern deep part of the Milun fault generated the directivity pulses in the EW direction. Furthermore, based on the proposed source model theoretical simulation of seismic motions was performed using the Thin Layer Method (TLM), which has been employed as an efficient numerical tool to evaluate Green’s function in layered soil structure. Base on the comparison between the crustal deformation distribution produced by InSAR and that from the simulated results, it was found that because of the change of strike and slip angle of the Milun fault there was crustal deformation including uplift in the northeastern part of the Milun fault. It is highly probable that left-lateral fault contributes to crustal deformation in the northeast direction.
Xin Wang, Wen-Yen Chang, Masayuki Nagano, Hongjun Si, Kazunari Suzuki, Che-Min Lin, Chun-Hsiang Kuo
2018 Hualien earthquake, Characteristic source model, near-fault long-period ground motions
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