| PaperNO | Paper / Abstract |
|
A0-013
14:20
|
14:35
|
ROLE OF SURFACE PROCESSES ON THE LOCATION OF LARGE SEISMOGENIC FAULTS IN TAIWAN
In Taiwan, due to the high convergence rate (8 ~ 9 cm/yr) between Eurasia and Philippine Sea Plate (PSP), deformation and erosion rates are extreme. Big earthquakes (M>7), occur mainly in the frontal part of the orogen and along the arc-continent plate boundary. Most of the middle term shortening is accounted for by just a few faults on the western foreland side of the wedge and along its backside hinterland against the Philippine Sea upper-plate; about 4 cm/yr are absorbed across the frontal faults, whereas on the backside ~3 cm/yr are absorbed on the onshore Longitudinal Valley faults and ~2 cm/yr offshore within the PSP along the submerged flank of the Coastal Range. On the opposite, little horizontal shortening occurs within the main body of the wedge due to a strong partitioning of deformation. Such a kinematic pattern closely matches the behavior of experimental erosional wedges with décollements. Experiments are analyzed to investigate the effects of erosion on the active bivergent orogenic wedge of Taiwan. Models, are submitted to erosion under flux steady state conditions. A weak layer mimics a décollement in the entering sequence favouring strain partitioning. A rapid uplift of underplated material occurs in the rear part of the wedge controlled by backthrusting, whereas frontal accretion characterizes the front of the growing prism. The whole material resting above the abandoned décollement is passively uplifted suffering very little horizontal shortening. Growth mechanisms can be described by frontal accretion in the foreland Foothills and basal accretion of tectonic units at depth under the hinterland. Intra-crustal décollements involving flats and ramps and inherited structures localized within the subducting continental margin of Eurasia favor such a style of deformation partitioning and wedge growth. Basal accretion is at the origin of rapid uplift and exhumation of wedge material accounting for most of the vertical component of deformation. Thus, a direct relationship exists between tectonics (shortening inducing the partitioning between horizontal and vertical displacements on faults) and surface processes contributing to huge material transfer. It has major consequences: The main seismogenic faults are located at the boundaries of the wedge both in front (foothills) and along its backpart (in the Longitudinal valley and offshore) where horizontal shortening rates are high. Most of the deformation that is responsible for the fast uplift of the hinterland is taken into account by aseismic ductile strain at depth.
Jacques Malavieille
Active faults
|
|
A0-012
14:35
|
14:50
|
APPLICATION OF THE NEW MORPHOLOGICAL ACTIVE FAULT DATABASE ON SEISMIC SCENARIO
Seismic source parameters are the key points in seismic scenario and its consequent disasters. Definitely, an important work in seismic study is how to choose reasonable seismic source parameters. According to this demand, the three-dimensional planes of active faults are investigated and plotted based on lots of up-to-date data, i.e., the tomography, historical earthquake events, seismic cross-section distributions, and geological surveys in recent years. Therefore, the seismic source parameters for each fault could be redefined. Until now, 39 active faults, which are released by the Central Geology Survey or academic results of geologists, have updated their seismic source parameters. We compare two different types of seismic source parameters in this study. Taking the Hsin-Cheng Fault as an example, the maximum earthquake magnitude can reach a value of 6.8 in local magnitude (ML) based on the former version of fault-plane geometry and historical earthquake events. The newly maximum earthquake magnitude is ML6.3 according to the three-dimensional fault plane geometry replotted in this study. And the source location and the fault sliding range are limited from the actual fault plane shape. The simulated distribution of the peak ground acceleration by the three-dimensional plane would be closer to the actual situation than before.
Ming Chun Ke, Ming Wey Huang, Yi Kai Lin
active fault, seismic source parameter, three-dimensional fault plant
|
|
A0-015
14:50
|
15:05
|
QUASI-REGULAR BEHAVIER OF AN INTRAPLATE REVERSE FAULT INFERRED FROM THE FLIGHT OF DISPLACED TERRACES: AN EXAMPLE FROM THE KAMISHIRO FAULT, CENTRAL JAPAN
Revealing spatiotemporal variability of slip through seismic cycles is crucial because it is directly related to understanding of fault behavior and fault displacement hazard assessment. High-resolution topographic data and quantitative dating techniques have played significant roles in reconstructing slip of paleo-earthquakes, especially on a plate-bounding strike slip fault. On the other hand, due to relatively long recurrence interval and slow slip rate, resolving a series of paleo-slips on an intraplate fault is difficult and often accompanies larger uncertainties compared to examples on plate-bounding faults.Here we explore variability in slip through seismic cycles using a flight of offset terraces along the Kamishiro fault. The Kamishiro fault is a reverse fault located at the northernmost part of the Itoigawa-Shizuoka Tectonic Line active fault system, one of the largest intraplate active fault systems in Japan, and vertical slip rate of the Kamishiro fault is ~1.5-2.5 mm/yr. Advantages of studying the Kamishiro fault are that the co-seismic slip distribution of the most recent event, the 2014 Nagano-ken-hokubu earthquake (Mw 6.2), is precisely measured by field observation and differential LiDAR analysis, and that there is a flight of displaced terraces that record slips of recent earthquakes. Several paleoseismic trenches and historical accounts indicate the penultimate and the antepenultimate earthquake occurred in AD 1714 and ~1.2 ka (AD 841 or AD 762) respectively. We first measured cumulative net slip for each displaced terrace using a tool developed by Wolfe et al. (2018) which enables objective measurement by incorporating topographic and fault properties with uncertainties. We also determined their ages using Oxcal v4.3.2. Based on terrace ages and paleoseismic records, we estimated slips of the penultimate and the antepenultimate earthquake to be 1.5 ±0.2 m and 2.7 ±0.4 m respectively. Next, we performed a Monte Carlo simulation to estimate slips of earthquakes prior to the antepenultimate event, and calculated CV (a ratio of a data set’s standard deviation to mean) for slip. Because available paleoseismic studies suggest one to three earthquakes occurred between the antepenultimate event (1.2 ka) and the abandonment of the oldest terrace (2.7-5 ka), we iteratively computed one to three co-seismic slips so that the sum of slips of the most recent, penultimate, antepenultimate and simulated events can reproduce the observed cumulative terrace offsets. Calculated CV ranges 0.25-0.48 (1σ), which indicates the Kamishiro fault has behaved in a quasi-regular manner in the last ~3-5ka.
Naoya Takahashi, Shinji Toda
Monte Carlo simulation, Paleoseismology, reverse fault, slip variability
|
|
C2-012
15:05
|
15:20
|
OBSERVATIONS AND MODELING OF CO-SEISMIC STRESS CHANGES IN THE M7.6 CHI-CHI EARTHQUAKE TAIWAN – APPARENT EVIDENCE FOR COMPLETE STRESS DROP ON A SMALL FAULT PATCH
Following the 1999, M 7.6 Chi-Chi earthquake, the Taiwan Chelungpu Drilling Project (TCDP) cored through the Chelungpu Fault in a 2-km-deep scientific borehole, near Dakeng, Taiwan. Comprehensive suites of geophysical logs were collected to measure physical properties of the fault zone and borehole image logs were used to determine variations in stress orientation with depth through observations of stress-induced borehole breakouts. These data show that near the fault, the azimuth of the maximum horizontal principal stress (S HMAX ) changes by about 90º from the regional tectonic stress direction (N130ºE), which is observed in the borehole both above and below the Chelungpu Fault. Hydraulic fracturing tests were used to determine the magnitude of the minimum principal stress (S 3 ) at multiple depths. Through dislocation modeling, we used these post-earthquake observations to constrain stress orientations and magnitudes before the earthquake as well as the stress changes on and near the fault as a result of the earthquake. Our simulations replicate the abrupt stress rotation observed in the image logs at the depth of the Chelungpu Fault. In addition, the modeling suggests that the magnitudes of S Hmax and the minimum horizontal principal stress S hmin changed markedly during the earthquake. In order for the co-seismic stress changes to result in a ~90º stress rotation near the fault, the state of stress prior to the earthquake had to have been a reverse faulting stress regime (as expected), but with S HMAX ? S hmin >>S v ..
Kuo-Fong Ma, Hung-Yu Wu, Mark D. Zoback
Chi-Chi earthquake, geomechical model, horizontal principal stress, stress drop
|
|
A0-014
15:20
|
15:35
|
THE NEW REVEAL OF CHIHSHANG FAULTING AT TAPO, EASTERN TAIWAN
Surface rupture and escarpment are some of many features that could indicate the location of the fault trace. But, only depending on these two features could lead to an unsatisfying conclusion. Related to this problem, six boreholes have been constructed and one inclinometer has been installed in one borehole from May 2016 to October 2018 and then relocated to another newly drilled borehole at Tapo Elementary School. This area is located precisely on the trace of the Chihshang fault. There is an escarpment at this location, which had been expected as the trace of this fault. It is supported by one of the deformation evidences; that is an inclined school facility located on this escarpment. From the borehole cores, we observed three different lithologic units: Lichi mélange, colluvial gravels and fluvial gravel, and fault zone contacts where Lichi mélange overrides fluvial gravel. It turns out an unexpected inference that the trace of Chihshang fault is located about 60 meters to the east from the escarpment, which had been expected as the trace of the fault. Meanwhile, if we correlate the cores with the ERT result, it shows that the low-resistivity mélange near surface was distributed like the sliding material. We expected the escarpment at Tapo elementary school is more likely generated as the toe of landslide. The sense of the deformation beneath the surface at the research area also has been inferred by inclinometer data analysis. We found three active deformation zone within the inferred sliding material with eastward direction. Based on all of these evidences, we considered that two mechanisms, that is faulting and landsliding, contribute to the deformation of the escarpment at Tapo Elementary School.
Mohammad Tri Fitrianto, Wen-Jeng Huang, Yi-Wei Chiou, I-Chi Yen, Chien-Chih Chen, Ping-Yu Chang
Chihshang fault, creeping, lichi mélange
|