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B3-001
16:10
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16:30
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A SYSTEMATIC PREFERENCE OF LARGE (M6+) EARTHQUAKES ALONG TOMOGRAPHIC EDGE ZONES IN TAIWAN
The Taiwan orogenic belt is characterized by complicated tectonic processes involving subduction of oceanic lithosphere along the Ryukyu trench and Manila trench to the north and south of the island, respectively, and collision in between. In this study, we conducted a joint three-dimensional (3D) tomography inversion of P-wave velocity (V p ), S-wave velocity (V s ) and Poisson’s ratio (s) using a large number of high-quality P- and S-wave receiver-source pairs in the region. We also determined the 3D distributions of crack density (e) and saturation rate (z) based on the tomography results. We found that more than 70% of large (M6 and above) earthquakes during the past century occurred along tomographic edge zones (TEZ) within the crust where the spatial gradient exceeds 0.1. Specifically, the three large disastrous events in the past two decades (the 1999 M7.6 Chi-Chi, the 2016 M6.5 Meinung, and the recent 2018 M6.4 Hualien earthquakes) all appear to be associated with TEZ. We attribute the preference of large crustal earthquakes along TEZ as the consequence of fluid migration from the more saturated side to the opposite, leading to the build-up of high pore pressure along TEZ. Locations of historical M6+ earthquakes are consistent with the inference of excessive fluids from either the extensive dehydration of hydrous minerals within the subducting PSP or the sedimentary compaction of alluvium and clastic deposits in the foreland basin west of the orogenic belt. Positive correlation between M6+ earthquakes and TEZ is not unique to Taiwan. Several famous earthquakes in regions with different tectonic settings, including the 2010 M8.8 Maule, Chile, the 2010 M7.1 Darfield, New Zealand, the 2016 M6.6 central Tottori, Japan, and the 2015 M7.3 and 1934 M8.2 Nepal earthquakes all appear to be associated with local TEZ. We infer that the positive correlation between TEZ and the seismogenesis of large earthquakes can provide important insights into the likely locations of future large earthquakes in Taiwan and probably elsewhere. It can also contribute to the long-term mitigation of regional seismic hazards.
Honn Kao, Zhi Wang
long-term mitigation of seismic hazards, Seismic tomography, seismogenesis of large earthquakes
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B3-013
16:30
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16:45
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INTRAPLATE SEQUENCES AND SWARMS: STATISTICAL ANALYSIS OF TRIACASTELA REGION (GALICIAN, NW IBERIAN PENINSULA)
Despite the long-time recurrence periods typical of intraplate regions, in Triacastela region (NW Iberian Peninsula) moderate instrumental events have been registered. The seismic sequences of Sarria-Becerreá-Triacastela in 1995 and 1997, when 4.6 and 5.1 Mw earthquakes happened, it supposed the update of Spanish Building Code in the new NCSE-02. Since then, the research started to keep this intraplate region in mind from the point of view of seismic hazard. Not only instrumental registered, but also historical earthquakes have happened in the region which show a continued seismicity over the years. Some studies tried to explain the complex system of this intraplate region, but nowadays the answer remains unclear. The statistical analysis is a helpful tool to the characterization, in particular we study the widely develop Gutenberg Richter b-value and the epidemic type aftershock sequence (ETAS) model. Besides, the Omori-Utsu Law provides the solution of aftershock decay that dominate in the regional sequences. Before to analyse the spatial migration, it is necessary to make an earthquake relocation with double-difference hypocentre locations. Our study shows a scattered and clustered seismicity in that region. In the surrounding area, other locations have the same clustered seismicity of swarms or sequences, while the remarkable of this location is the combination of both. According to Mogi Classification, we could split up in eight single sequences and eleven swarms. The outcomes display how the sequences are characterising by a close to the unit b values with the highest Mc (Magnitude of Completeness), while swarms show a high b-value (b>1) and fewer Mc. Some authors related close to the unit b-values to more active seismic region, which may be the case due to in single sequences happened the highest earthquakes written on the Mc. The results of swarms indicate that they could be located in a fractured region with fluid migration as the possible origin of swarm activity. This research will help to shed light on the generation process of sequences and swarms in an intraplate region.
Crespo-Martin Cristina, Martin-Gonzalez Fidel
b-value, ETAS, NW Iberian Peninsula, p-value, SCR, swarm
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B3-012
16:45
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17:00
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GPS HORIZONTAL DISPLACEMENTS AND SURFACE DEFORMATION DURING EARTHQUAKE CYCLE IN THE NORTHERNMOST LONGITUDINAL VALLEY, EASTERN TAIWAN
We conducted a dense spacing GPS network in the northernmost Longitudinal Valley, eastern Taiwan from 2007 to 2019, in order to characterize the surface deformation in this tectonic junction where the Luzon arc (the Coastal Range) and the underlying Philippine Sea plate is northward subducting and against the Eurasian continental crust. We installed 17 low-cost single frequency GPS stations and also carried out repeated campaigned dual-frequency GPS measurements in the study area in 2007-20012. While the 2018 Mw6.4 Hualien earthquake produced significant co-seismic surface deformation with maximum GPS horizontal displacement up to 1 meter, we conducted two campaigned GPS measurements following the earthquake a few weeks later and one year after. Combining the pre-, co- and post-earthquake GPS results, we interpret that 1) during the pre-earthquake (interseismic period?), the northernmost Longitudinal Valley underwent an steady, rapid eastward extrusion, as being initiated by the northwest indentation of the Coastal Range, which pushed the northeastern Hualien not only to move upward but also eastward, whereas the Milun fault, the major active fault in the area, remained mostly locked ; 2) the 2018 Hualien earthquake indeed ruptured the Milun fault in mostly left-lateral movement and produced significant surface ruptures along the fault; 3) following the earthquake, very quickly the surface deformation and GPS horizontal displacements returned to its pre-earthquake (and interseismic) pattern of eastward extrusion, however much larger amounts of deformation and displacement occurred in one year following the earthquake.
Jian-Cheng Lee, Horng-Yue Chen, Chia-Yu Chen
2018 Hualien earthquake, earthquake cycle, GPS, surface deformation
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B3-017
17:00
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17:15
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THE SEISMOGENIC STRUCTURES BENEATH THE NORTHERN LONGITUDINAL VALLEY, TAIWAN: APPLICATION IN 2018 HUALIEN EARTHQUAKE
The 2018 Hualien earthquake occurred at the junction of the deformed continental crust of Eurasian plate and the Heping sea basin, where a northeast trending seismic belt exists. In this study, a 3D seismic tomographic inversion is used to investigate the seismogenic structures of the 2018 Hualien earthquake sequence. An earthquake relocation procedure is performed and the focal mechanisms are analyzed to study the faulting behavior. Our results indicate that the source region of the 2018 Hualien earthquake consists of complex high-angle eastward and westward dipping reverse faulting. We also observe that most earthquakes that have occurred in the area exhibit considerable variation in Vp/Vs ratios. Fluid migration may have played an essential role in causing the Vp/Vs ratio variation. Meanwhile, we also analyze the focal mechanisms and conduct stress inversion for the seismicity. The results show that some seismic clusters are energetic and short in duration and were dominated by right and left lateral strike-slip events along a prominent lineation in EW and NW directions, respectively. Due to the collision process is still active and this compression produces micro-earthquakes likely linked to E-W or NW-SE pre-exiting faults which are triggered with oblique-slip faulting. Therefore, the results are not only giving better understanding the seismogenic structure beneath the Longitudinal Valley in Taiwan, but also can provide key information to properly assess seismic hazard analysis for the urban area. We suggest that the mainshock of the 2018 Hualien earthquake may be associated with a west-dipping fault, which is a blind fault tending toward the Heping sea basin and possibly belonging to the Central Range fault system.
Strong Wen, Yi-Ying Wen, Kuo-En Ching, Yu-Lien Yeh, Yuan-Hsi Lee
2018 Hualien earthquake, 3D velocity structure, focal mechanism, source rupture
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B3-015
17:15
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17:30
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RUPTURE CHARACTERISTIC AND SEISMOGENIC STRUCTURE OF 2018 M L HUALIEN EARTHQUAKE SEQUENCE
The northernmost Longitudinal Valley where experience the transition from the oblique collision to subduction is a tectonically complex area with a potential of producing large earthquakes such as 1986 M L 6.8 Hualien offshore earthquake. About three decades after, a M W 6.4 earthquake stroke Hualien city on February 6 2018 at the similar offshore location of the 1986 event and caused building collapses and extensive surface ruptures particularly along the Milun fault. Aftershock sequence of this 2018 Hualien earthquake migrated landward from the offshore to the northern end of Linding fault, provoking slips on multiple fault structures. Complex coseismic deformation revealed by InSAR and leveling data suggests complicated faulting senses involved from north to south. To untangle the seismogenic structure of the northernmost Longitudinal Valley, we first analyze the rupture characteristic of the 2018 Hualien earthquake using directivity moment tensor inversion, and then relocate and determine the focal mechanisms of the aftershock sequence to investigate the structure geometry and spatial stress variations. The rupture directivity analysis suggests an additional E-W trending fault that is close to the offshore hypocenter and is possibly responsible for the rupture initiation, which then trigger the main N-S trending west-dipping fault rupturing landward into the Hualien city. The spatial stress variations further reveal a complicated faulting sense between the southern Milun Tableland and the northern end of Linding fault, where clear surface ruptures were also found. From that, a seismogenic structureal model in the Hualien area is proposed and discussed.
Hsin-Hua Huang, Kuan-Fu Feng, Tzu-Hsin Lai, Yen-Lin Chen
2018 Hualien earthquake, rupture characteristic, seismogenic structure
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B3-014
17:30
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17:45
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Discussion on anomalous crustal structures along the convergent zone in eastern Taiwan : Insights from the new airborne magnetic survey and updated seismic tomographic models
The plate tectonics of Taiwan have been suggested as from ancient oceanic-oceanic plate subduction transforming to the continental-oceanic plate collision as the proceeding of the accreted volcanic arcs. The major suture of the plate boundary in eastern Taiwan contributes most of the seismic activities and absorbed about half of the plate convergence. How the shortening and the exhumation trajectory of the oceanic plate provoked multidisciplinary researches in discussing the kinematics and the related earthquake structures.Local seismic tomography is a useful technique for unraveling the subsurface structures. The Taiwan seismic tomographic studies that are advanced by large projects or by integrating multiple data sets have been proposed in recent years. Furthermore, since 2012 the seismic signal digitization of the Central Weather Bureau (CWB) seismic network was upgraded from 12 to 24 bit and equipped with onsite GPS time system. Additionally, the new airborne magnetic survey in eastern Taiwan has been carried out in eastern Taiwan by Central Geological Survey (CGS) and Industrial Technology Research Institute (ITRI) since 2017. High magnetic anomalies were detected not only in the volcanic arc but also in the eastern flank of the Central Range along the north-south trending Longitudinal Valley. These observations motivate us to revisit the subsurface structures with updated seismic tomographic models using new CWB data sets from 2012 to 2017.The iso-velocity contours of the updated model keep similar structures with the previous model at first order but further reveal more details. By combing the Vp and magnetic models, the geometry of the arc crust was clearly imaged along the Coastal Range area. Another high magnetic stripe characterized with magnetic polarity reversal, located beneath the eastern Central Range and the Longitudinal Valley, coincided with the uppermost part of the ascending channel characterized by high Vp values. The anomalous high-Vp channel has been interpreted as a high-pressure rock exhumation. Such findings of these integrated geophysical analyses may imply the incorporation of the oceanic plate in the convergent zone in the Taiwan orogency.
Chi-Hsuan Chen, Hsin-Hua Huang, Lun-Tao Tong, Wayne Lin, Po-Tsun Lee, Shou-Fang Chang, Jiin-Fa Lee
Airborne magnetic survey, Magnetic polarity reversal, Oceanic Crust, Seismic tomography
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