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SS3-001
10:50
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11:20
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LEARNING ABOUT LARGE EARTHQUAKE RUPTURES USING FAULT ZONE DRILLING FROM TCDP TO JFAST
Drilling directly into the areas of significant fault slip soon after large earthquakes, has yielded exciting new information about the physical properties of the faults that are responsible for causing these important seismic events. The Taiwan Chelungpu Drilling Project (TCDP) after the 1999 Chichi, Taiwan eathquake (M w 7.6) and the Japan Trench Fast Drilling Project (JFAST) after the 2011 Tohoku-oki earthquake (M w 9.1), were both able to collect cores of the fault zone and carry out related geophysical monitoring. One of the main goals for both of these efforts was to measure the frictional heat generated by the earthquake, which is used to estimate the level of dynamic friction on the fault surface during the time of the large slip. The temperature anomalies measured across the fault zones following both earthquakes were small, 0.06oC for the Chelungpu fault (5 years after the earthquake) and 0.3oC for the Japan Trench (1.5 years after the earthquake). These observations indicate that the shear resistance to slip was quite low with a coefficient of friction of about 0.1. These estimates are small compared to typical values of static friction for rocks, which is usually about 0.5 to 0.7. The low values of dynamic friction indicate that there is strong slip weakening (reduction of friction) from the static friction to the sliding friction. This conclusion is also consistent with high-speed laboratory experiments on the fault zone samples, which show similar low values for the sliding friction. The reduction of the shear resistance on the fault at the time of slip initiation is an important process that facilitates the fault slip. This process helps explain how large displacements can occur on rock surfaces without causing the large amount of heat that would melt the fault zone. The physical mechanisms that cause the reduction in friction are still unclear, however, fluids in the fault zone likely play an important role. Mechanisms such as thermal pressurization can significantly reduce the fault friction and allow large fault slips, such as the 50 to 60 meter displacement for the case of the Tohoku earthquake.
James Mori
1999 Chi-Chi Earthquake, 2011 Tohoku-oki Earthquake, fault friction, fault zone drilling, Seismology
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SS3-002
11:20
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11:50
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VARIABILITY AND TIME-DEPENDENCIES OF EARTHQUAKES PROPERTIES: LESSONS LEARNED FROM LARGE GROUND-MOTION DATASETS ANALYSIS
One of the key challenges of seismology is to be able to evaluate the physical factors that control the variability of earthquakes source properties (e.g stress-drops) for a given earthquake size (magnitude). The calibration of these source properties variabilities is essential to develop future physics based probabilistic seismic hazard assessments (Cotton et al., SRL, 2013). The exponential growth of seismological near-?eld records and modern ground-motion analysis technics allow to better calibrate source properties variabilities. Between-event residuals (residuals between observed and predicted ground motion for individual earthquakes averaged over all stations) can then be computed in a robust way on large numbers of earthquakes (Kotha et al., Earthquake Spectra, 2017) and provide a new way to analyse the physical factors that control earthquakes properties variabilities. We evaluated the between-events variabilities using recent high-quality global accelerometric datasets (NGA-west2, Kiknet and European Strong Motion databases) and datasets from recent earthquakes sequences (Aquila, Iquique, Kunamoto). Surprisingly, the between-events variabilities of earthquake of similar magnitude occurring on the same system of fault (e.g. Central Italy) are shown to be still significant which confirms the high variability of earthquakes rupture properties even for earthquakes occurring on the same fault system. Our analysis also confirms the frequency-dependent correlation between the Brune stress drop Δσ and the between-events but also show that the stress-drop only cannot explain high-frequency variabilities. These new data and analysis also capture the temporal variability of the ground shaking at high frequency We will show that between-events and the stress-drops show large changes after the occurrence of mainshocks such as the 2009 Mw 6.3 L'Aquila, the 2016 Mw 6.2 Amatrice, and Mw 6.5 Norcia earthquakes. The temporal variability of Δσ mirrors the relative seismic-velocity variations observed in previous studies for the same area and period, suggesting that both crack healing along the main fault system and healing of microcracks distributed at shallow depths throughout the surrounding region might be necessary to explain the wider observations of postearthquake recovery (Bindi et al., BSSA, 2018). We will finally discuss the hability to detect stress-drops changes potentially associated to the preparation phase of large earthquakes (Pina-Valdes et al., BSSA, 2018, Specht and Cotton, 2019)
Fabrice Cotton, Sreeram Reddy Kotha, Sebastian Specht, Dino Bindi
fault healing, Ground-motion variability, Physics of earthquakes, stress-drops
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SS3-003
11:50
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12:20
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SLIP BEHAVIOR OF THE SHALLOW SUBDUCTION INTERFACE ALONG THE JAPAN TRENCH
The 2011 Tohoku-Oki Earthquake was associated with massive trench breaching seismic slip on the shallow subduction interface. This presentation reviews recent results of deep-sea researches composed of seismic surveys, marine paleoseismological researches, and seafloor geodetic/seismological observations to show the along-trench segmentations of the slip behavior and of the sub-seafloor structure. Along-trench extent of the 2011 coseismic slip was constrained within the central part of the margin based on the coseismic and postseismic deformation data. The spatial extent of the thick turbidite units related to the past massive earthquakes in 869 and 1454, as well as that of the 2011 earthquake, correspond well to the coseismic slip area of the 2011 earthquake. This suggests that the M~9 class earthquakes can be regarded as recurrent ruptures of almost the identical segment with ~ 600 year interval. Along trench variation of the seismic image near the trench provides us a hint to understand the segmentation persistent through the supercycle of the massive events. The northern end of the 2011 rupture area seems to be terminated by a spot of remarkably thin sediments on the incoming plate located ~ 39°N. To the south of ~ 37.5°N, where extensive afterslip is taking place on the shallow megathrust, development of a low-V channel layer along the plate boundary is identified. These observations suggest that the propagations of recurring slips have been prohibited by the along-trench changes of frictional properties on the shallow megathrust controlled by the sedimentary structure. Outside of the segment causing M~9 earthquakes, various types of slow-slip phenomena were identified along the shallow megathrust. To the north of the area, small rate of post-2011 deformation near the trench is observed and suggests that the periodic slow slip may propagate to the trench making the interplate coupling low. To the south of the mainshock rupture, rapid afterslip is taking places along the subduction interface near the trench axis. Interestingly, tectonic tremor activities are observed in the afterslip zone, suggesting coexistence of long-term postseismic slip and intermittent short term slow slip on the fault. The observations of various kinds of slow slip phenomena support our hypothesis that the segmentation controlled by structural heterogeneities on the shallow subduction interface allows M~9 earthquakes to occur only in the central part of the Japan Trench.
Ryota Hino
2011 Tohoku-Oki Earthquake, ocean bottom observations, seismic surveys, turbidites paleoseismology
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