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B6-002
14:20
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14:40
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CHARACTERISTICS AND INTERPRETATION OF BROADBAND SLOW EARTHQUAKES
Almost two decades have passed since the discovery of slow slip events (SSEs) and tectonic tremors, which have become popular geophysical phenomena observed in many places worldwide including Taiwan. Often but not always, we simultaneously observe SSE geodetically and tremors seismologically. Moreover, various intermediate scale phenomena such as very low frequency (VLF) earthquakes and rapid tremor reversals occur with tremors and SSE. All these phenomena are related, or rather should be considered as different manifestations of a very broadband and outspread shear slip sequence within the same deformation structure such as plate boundaries. We may call this phenomena broadband slow earthquakes. Tremors and low frequency earthquakes are the short timescale endmembers and utilized for monitoring spatial and temporal development of slow earthquakes and to recover longer-period signals usually hidden in environmental noise.There are various theoretical models of SSEs and some models can also explain broadband nature including high-frequency tremor radiation. The very broadband characteristics of slow earthquakes are well explained by Brownian slow earthquake model (Ide, 2008; Ide and Yabe, 2019), which assume that random change of slow slip region makes very broadband signals from tremors to SSEs. This model provides scaling relations among seismic moment, seismic energy, duration, and area and statistics for size and frequency, and thus are useful to characterize regional difference of slow earthquake phenomena.
Satoshi Ide
numerical models, scaling relation, slow earthquake, Tectonic tremor
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B6-001
14:40
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15:00
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RELATIONSHIPS BETWEEN SLOW SLIP, TREMOR AND LFES: EFFECT OF ALONG-DIP POSITION
Slow slip deep in subduction zones manifests in a variety of ways, including as slow slip events (SSEs), tremor, and low frequency earthquakes (LFEs). Recently, co-workers and I have reported various behaviors of these phenomena particularly from the deep Cascadia subduction zone, which hosts quasi-repetitive SSEs of magnitude 6.6-6.8 with recurrence intervals of 12-15 months. Tremor propagation patterns illuminate steady along-strike propagation of slow slip in large SSEs. Secondary propagation patterns including streaks and rapid tremor reversals are also observed. Tidal stresses provide a well-calibrated probe of the strength of the tremorgenic zone. Tremor response to tidal stress during large SSEs is exponential, evolves toward greater sensitivity to tides over several days of slip at a spot, and constrains the intrinsic frictional coefficient to less than ~0.1, very low compared to typical experimental friction. Tidal response also evolves between large SSEs, becoming weakest midway through the cycle. We may understand this evolution in terms of the competition between a logarithmic rate of growth of strength (yield stress) and a quasi-linear rate of growth of fault stress due to continuing tectonic loading. LFE and tremor recurrence patterns delineate strong dependence on downdip position through the tremorgenic zone. At the updip edge of the tremorgenic zone, tremor and LLFEs and slow slip appear to occur only during large SSEs. In contrast, at the downdip edge of detected tremor and LFEs, LFEs are generated far more frequently (8-day recurrence) in brief swarms, whose timing reveals a strong sensitivity to the weak tidal stresses. In this region, stress and strength appear to remain relatively close together over the brief recurrence cycle. Comparisons of slow slip inverted from GPS to the spatio-temporal evolution of tremor shows that slow slip in large SSEs extends updip of the updip edge of detected tremor, raising questions about stress, strength, and capability of that far-updip region to generate tremor. Our observed relationships between tremor and slip also raise the possibility that some tremor occurs in advance of slow slip due to triggering by distant, but approaching, stresses generated by the large propagating slip pulse. We seek to integrate these observations to constrain the mechanics of the deep plate interface in warm subduction zones where slow slip and tremor are often observed. These regions are situated downdip from locked megathrusts that generate destructive great earthquakes, so their behavior may eventually help constrain seismic hazard from great subduction zone earthquakes.
Heidi Houston
Seismology, slow slip earthquakes, subduction zone
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B6-012
15:00
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15:15
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TIDAL MODULATION AND TECTONIC IMPLICATIONS OF TREMORS IN TAIWAN
We present results from 6 years of tremor observations in a continental subduction zone where continental crust of Eurasia is colliding with the Philippine Sea Plate, forming a collisional orogen. During 2007-2012, 1893 tremor events with durations ranging from 60 s to 2216 s were identified and located. Spectral analysis of tremor times reveals several sharp peaks that correlate with the annual, semidiurnal, and diurnal constituents, indicating a strong tidal and seasonal modulation. When the tidal stresses and stressing rates are compared with tremor activities, we found excess events reach the maximum of 7 % and 3 % at 1.5 kPa and -0.3 kPa/yr, respectively, showing better correlation between tremors and tidal stress amplitude. Spatially, the ambient tremors appear to form a steep, southeast-dipping ellipsoidal structure 15 to 45 km deep in the southern part of the collisional orogen where continental crust of Eurasia has been subducted. The steep dip suggests a crustal-scale ramp that imbricates the crust in this area. This section of the orogen is also characterized by high heat flow, low Vp and Vs values, high Vp/Vs ratios and relatively fast surface and rock uplift rates. We propose that the zone of tremors represents the initiation or reactivation of a brittle-ductile shear zone deep in the crust, possibly near the crust-mantle boundary.
Kate Huihsuan Chen, Hsin-Ju Tai, Satoshi Ide, Timothy Byrne, Christopher W. Johnson
brittle-ductile shear zone, Taiwan, tidal triggering, tremors
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B6-003
15:15
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15:35
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THE EVOLUTION OF FAULT SLIP RATE BEFORE EARTHQUAKE: THE INTERPLAY OF SLOW AND FAST SLIP
The earthquake nucleation process is inherently complex, due to an involvement of several deformation mechanisms with multiple spatial and time scales. Natural fault hosts a wide spectrum of slip rate from fast to slow slip. Before large earthquakes, the number of smaller magnitude events often increases, retrospectively named foreshocks. Foreshock can be interpreted to be a physical process implying unlocking of fault by fast slip mode. Recent seismic and geodetic studies of foreshock sequences accompanying with migrations suggest that partial unlocking of fault took place episodically through interplay between fast and slow slip modes before some large earthquakes such as the 2011 Tohoku-Oki, the 2014 Iquique and the 2016 Kumamoto earthquakes. The partial unlocking causes stress loading onto the nearby critically-loaded fault segments, resulting to triggering of subsequent dynamic and unstable slip. Alternatively, the partial unlocking of fault enhances strength-weakening of the earthquake nucleation area through slip invasion or fluid migration, that ultimately initiates the subsequent dynamic rupture. However, the manner of the unlocking is “episodic”, not “smooth acceleration” which has been typically observed as nucleation phase in laboratory experiment and numerical simulation model having simple fault zone structure. This episodic manner precludes a possibility of forecasting the subsequent large earthquake with a high degree of accuracy. The triggering of a subsequent large earthquake on nearby fault segments depends on the areal extent of the critically-loaded seismic patches and how close these areas are to failure, even though the partial unlocking by both fast and slow slip processes is observed. An important research area is the development of methods for assessing the degree of criticality within fault segments adjacent to already ruptured portions. In addition, earthquake triggering probability by slow slip transient shall be incorporated into operational earthquake forecasting scheme as future challenge, especially during the latter phase of the inter-seismic periods.
Aitaro Kato
foreshock migration, slip rate evolution, Slow slip, unlocking of fault
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B6-011
15:35
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15:50
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THE NATURE OF ASEISMIC SLIP IN SOUTHERN TAIWAN: THE MUD DIAPIR/VOLCANO
How to estimate the seismic hazard of the creeping fault is an essential but not well-resolved issue because the mechanisms to result in the aseismic slip on the creeping fault are still debated. The Chihshang creeping fault in eastern Taiwan is generally located along the Lichi mélange, which is composed of rich clay minerals. Therefore, do we have creeping faults in southern Taiwan because a thick mud layer, Gutingken formation, is well-known in this region? An islandwide geodetic network of 739 campaign-mode GNSS stations, 495 continuous GNSS stations and 25 precise leveling routes was mainly constructed by the Central Geological Survey in Taiwan since 2002 to understand the characteristics of Taiwan crustal deformation. The Chishan and Chegualin fault system and the Fengshan fault has been detected as the creeping faults based on the secular velocity field during 2002-2018. The maximum uplift rates across the Chishan fault and the Chegualin fault are about 100 mm/yr and 40 mm/yr, respectively. The maximum extension rate across the Chishan fault is about 50 mm/yr while the contraction rate of about 40-50 mm/yr is detected across the Chegualin fault. This ultra-rapid deformation rate has been proposed to be caused by the active mud diapirism under the fault system. After the material pushing up by the mud diapir, the geometry of the shallow fault system controls the deformation pattern of this material, which also result in the aseismic slip on the Chishan and Chegualin faults. In addition, the slip rate of > 15 mm/yr on the Fengshan sinistral strike-slip creeping fault has been estimated based on the surface velocities. The creeping segment of the Fengshan fault is identified by two mud volcanoes. Similar geochemical compositions in these two mud volcanoes implies that the fluid provided by the mud volcanoes induces the interseismically aseismic slip on the Fengshan fault. Although the aseismic slips on the Chishan and Chegualin fault system and the Fengshan fault are all related to the growth of the mud diapir or the mud volcano in the thick mud layer of southern Taiwan, the kinematics of the Chishan and Chegualin creeping fault system is the mud diapirism-induced while the kinematic of the Fengshan fault is the tectonically-dominated.
Kuo-En Ching
Chegualin fault, Chishan fault, creeping fault, Fengshan fault, geodesy
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