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A2-011
14:20
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14:35
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ACTIVE TECTONICS AND STRUCTURAL ARCHITECTURE AT THE PIEDMONT OF SOUTHWESTERN TAIWAN IN RELATION TO RECENT SEISMICITY AND OROGENY
The foothills of southwestern Taiwan accommodate rapid shortening of 45 mm/a at the latitude of Tainan City. To document the structure and the distribution of shortening across the region, we build two east-west regional balanced cross-sections based on surface and subsurface geology, as well as geodetic data. We also examine how the piedmont structures relate to the inner mountain belt using seismic tomography. From west to east, we propose a series of three active west-dipping backthrusts, rooted on the 4.0-km-deep Tainan detachment, within the base of the Plio-Pleistocene Gutingkeng mudstone. Syntectonic sediments and rapid shortening and uplift observed from geodetic measurements attest for the activity of these structures since the Late Pleistocene. Further east, the Tainan detachment ramps down to ~7 km depth, allowing the east-dipping Lungchuan and Pingchi thrusts to bring Upper Miocene formations to the surface. The cross-section restoration indicates ~5-7 km shortening since ?450 ka on the Tainan detachment and the frontal backthrusts, while the east-dipping Lungchuan and Pingchi thrusts each consumed ~10 km shortening. Another ramp from ~7 to ~11 km depth is expected further east based on older sediments and slates exposed on the hanging wall of faults in the inner part of the mountain belt. This ~11-km-depth detachment seems to correspond with an inversion in seismic velocities at ~12 km depth beneath the slates belt, interpreted as slates over-riding lower-velocity passive margin sediments. Therefore, the detachment and thrusts system proposed from our cross-section appears to correspond to the main plate interface, where significant shortening was consumed in a thin-skinned deformation style with significant aseismic slip on the frontal structures. In contrast, moderate-magnitude earthquakes, such as the 2010 Jiashian and 2016 Meinong events, nucleate at 15-20 km depth, below the main detachment and near the interface between Cenozoic basement rocks and post-rifting sediments. This interface may have developed as a secondary detachment level with limited total shortening based on tomographic models.
Hsin-Hua Huang, Maryline Le Béon, John Suppe, Mong-Han Huang, Yu-Huan Hsieh, Chih-Tung Chen
balanced cross-section, Meinong earthquake, tomography, Western Foothills
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A1-015
14:35
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14:50
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LANDFORM DEVELOPMENT PROCESSES OF THE WESTERN HENGCHUN TABLELAND IN SOUTHERN TAIWAN BASED ON UPLIFTED COASTAL FEATURES
The Western Hengchun Tableland is a notable topographic feature on the Hengchun Peninsula in the southern tip of Taiwan. The formation of this feature is debated, since there are active faults on the peninsula, but numerous mud diapirs are also present offshore, producing deformations of offshore sediments on the continental shelf. In this study, we used uplifted coastal features surrounding the tableland to understand the formation and uplift processes of the tableland. If the uplift pattern appears to be continuous and is spatially related to existing mud volcanoes, it may be produced by mud diapirism. Instead, if the pattern shows episodic characters, it would be more likely produced by co-seismic movement of active faults. In our field survey, we identified uplifted Porites coral colonies along the coast, and found that they can be separated into several groups based on their elevations and ages. This suggests that the uplifted corals may be the products of paleo-earthquake events during recent several thousand years. Moreover, we surveyed Holocene marine terraces near a major mud volcano, Gueishan, and found there is no apparent deformation of the terraces by the mud volcano. Based on our results, we believe the Western Hengchun Tableland was uplifted by the Hengchun offshore structure, a proposed active structure of the area. This structure is a potential seismogenic fault, and the earthquakes produced by this fault may have a clustered pattern. Our results would have important implications for earthquake hazards of the area in the future.
J. Bruce H. Shyu, Sze-Chieh Liu, Yuan-Lu Tsai, Chuan-Chou Shen
Hengchun offshore structure, uplifted coastal features, western Hengchun Tableland
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A1-011
14:50
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15:05
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INTERACTION BETWEEN SLIP EVENTS, EROSION AND SEDIMENTATION ALONG ACTIVE STRIKE-SLIP FAULTS: INSIGHTS FROM GEOMORPHIC EXPERIMENTS
Recovering information on past (i.e., last 102-104 yrs) large earthquakes on faults is a challenge and a critical issue for this information might help determining the magnitude and the recurrence mode of the largest events on a given fault. Morphologic features provide useful markers to investigate this issue. They are shaped by the large earthquake slips that repeated on this fault over its seismogenic history. Therefore, offset markers preserve information on past large earthquakes, especially on the slip amplitudes they produced at surface. Yet the relevance of this information depend on whether the slip-recording markers that are used for slip analyses maintained well preserved in the morphology while the fault and the landscape were evolving. This important question is difficult to tackle with natural data, for fault and landscape evolution cannot be monitored over the relevant long time spans. To address it, we have developed an original experimental set-up made to simulate repeated slip events on a strike-slip fault placed under arainfall system, sustaining significant sedimentation and erosion. The analog material is a mix of granular materials, whose mechanical properties lead to a geometric scaling of about 1:10 000 and to a temporal scaling on the order of one second equivalent to a few dozens of years. Under the combined effects of accumulating slip, erosion and sedimentation, the model surface exhibits tectonic and morphological structures similar to natural features (Riedel's shears, pressure and shutter ridges, pull-apart basins, alluvial fans, terrace risers, braided rivers, etc), whose space and time evolution is analyzed. Deformation partitioning, sequential formation of alluvial terraces, stream captures, development of ‘traps’ filling with sediments, etc, are observed. Impact of the rainfalls on the fault morphology and on the evolution of the associated morphological markers is analyzed. Finally, we can compare the imposed slip events (number, amplitudes, repeat times) with the cumulative offsets eventually preserved and measurable at the model surface. Significant discrepancies between originally imposed and finally preserved offsets shed light on the large uncertainties that may affect the paleoseismological analyses performed on natural faults.
Jacques Malavieille
geomorphic experiments
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A1-013
15:05
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15:20
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PALEOSEISMIC STUDY OF THE MILUN FAULT ACTIVATED DURING THE 2018 MW 6.4 HUALIEN EARTHQUAKE RUPTURE IN EASTERN TAIWAN
The Milun Fault is the northward extension of the Longitudinal Valley Fault which has been recognized as the suture of the Eurasian and the Philippine Sea Plates since early 1970s. Historically, it was linked to two destructive earthquakes on October 22nd, 1951, and February 6th, 2018, respectively. The 1951 ML 7.3 earthquake and the 2018 MW 6.4 earthquake both resulted in surface ruptures along the Milun Fault trace, causing casualties and damage on buildings in Hualien city, which is the most populated area in the eastern Taiwan. Although the Milun Fault is surely intimidating, its configuration and history remain little exposed. Two sites on the 2018 earthquake ground ruptures located about 2.6 km north of downtown Hualien were excavated 6 months after the earthquake along with 4 boreholes drilled. The walls of one trench revealed a high dip angle of 80° to the east for the Milun main fault and its branch faults dip 36°, 56°, and 70° to the east, respectively. The growth strata in the footwall of the branch faults were folded. Distinctive sedimentary features such as the colluvial-wedge deposits and soil liquefactions were also observed on the trench walls. By compiling the results of restoration and stripping of strata in conjunction with the age constraints, five paleo-earthquake events associated with the Milun Fault are resolved: (1) the 2018 event, (2) the 1951 event, and the earlier ones occurred at (3) 686-380 yr BP (4) 2184-1396 yr BP (5) 3232-2630 yr BP. Although the recurrence interval was estimated as ca. 70 yrs earlier, our data suggest that the recurrence interval could be longer than previously expected. However, due to the limited records of stratigraphy and possible absence of paleo-earthquakes, further investigation will be needed to generate a more sophisticated estimate of the recurrence interval. In addition, based on the borehole data, the uplift rate of the Milun fault is about 6.0 to 6.5 mm/yr within the past 5000 yrs. This uplift rate corresponds to all the previous studies, which suggest the vertical slip rate of the Milun fault shall be larger than 4.5 mm/yr. These new findings shall benefit the earthquake hazard assessment for the region.
Wen-Jeng Huang, Ya-Chu Tseng, Wen-Shan Chen, I-Chin Yen, Shao-Yi Huang, Jiun-Yee Yen
2018 Hualien earthquake, active fault, disaster, Milun fault,, paleoearthquake
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A1-012
15:20
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15:35
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A PROPOSED METHOLOGY FOR STUDING EARTHQUAKE DAMAGE ORIENTATION
The information gained from past earthquakes is essential to seismic hazard studies and to mitigate losses from future earthquakes. A multidisciplinary combination of paleoseismic, historical and archeological data can provide valuable information about the recurrence, location, ground motion, magnitude, seismogenic fault or socioeconomic effects in early seismic events. All this information is crucial to complete seismic catalogs to assess seismic hazard and building requirements. The orientations of the earthquake damage (ED) that affected architectonic elements during earthquakes (e.g. Fallen columns, conjugated fracture sets in walls, sliding dropped keystones in arches) has been used to infer seismic parameters of historical and ancient earthquakes such as seismogenic source, ground motion or epicenter. This is of special interest in the non-instrumental period and where no surface rupture is preserved or identified. However, it is not applied a common methodology to measure this orientation damage, which leads to controversial results and interpretations in the literature. For example, the tilting and toppled walls are one of the most used elements to measure the horizontal ground movement in non-instrumental earthquakes. However, according to the shape of the architectonic element (a wall in this case) it have only certain degrees of freedom to fall (two in this case) and it does not have necessarily correspond with the ground motion pulse. Even the elements that are likely to be used are not accepted by all the scientist. Therefore, the fallen direction is conditioned not only by the ground pulse but by the shape of the element. This way, only the measure of the azimuth of the element damage is not a reliable indicator of the ground motion direction. In this work a methodology is proposed, taking into account no only the frequency of the damage orientations but considering the degree of freedom of each element to be oriented by the ground motion pulse. For example, elements with quadrangular or rectangular bases have 90º of uncertainty to record the orientation of an incoming pulse. Others elements such as walls or headstones have only two possible direction of falling in the orientation of the ground motion pulse that caused the fall. In the same way, a keystone can drop only if the arch that contains it is oriented favorably to the pulse orientation (90º). Moreover, this methodology has been tested modelling different scenarios with different seismogenic source locations demonstrating that it is a reliable method to infer fault seismic parameters
Fidel Martin-Gonzalez
earthquake damage orientation(EDO), historical earthquakes, non-instrumental earthquakes, quantification of earthquake effects, seismic parameters
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