| PaperNO | Paper / Abstract |
|
A3-002
10:50
|
11:10
|
A PRELIMINARY RESULT OF TEMPERATURE DEPTH PROFILE IN A DRILLING BOREHOLE PENETRATED THE FUTAGAWA FAULT RUPTURED DURING THE 2016 KUMAMOTO W M 7.1 EARTHQUAKE
Measurements of temperature depth profile in scientific-drilling boreholes and determination of heat flow from the measured temperature profiles are important approaches to understand the temperature distribution and thermal structure around volcanic regions and seismogenic fault zones. A scientific-drilling borehole down to ~700 m deep was penetrated through the Futagawa Fault ruptured during the 2016 Mw 7.1 Kumamoto, Japan earthquake in Aso volcanic region. To obtain the fundamental features of the temperature profile in the borehole, we have repeated measurements of temperature depth profile totally four times, i.e., once per two – three months from the borehole penetration was completed in March 2018. As a preliminary result, the temperature depth profile showed a popular trend between depth ranges of 200 – 310 m and 430 – 650 m where the formation temperature may not be influenced by weather and seasonal temperature change on surface. The temperature in the two depth ranges linearly increased with depth increasing, and showed a geothermal gradient of ~55 °C/km. This geothermal gradient is much higher than the global average geothermal gradient 20 – 30 °C/km, being considered reasonable in a volcanic region. However, a very unexpected temperature distribution feature was observed in the depth range of 310 – 430 m. The temperature was almost the same in the ~120 m depth interval; and the geothermal gradient was only ~1 °C/km. To make sure if this special temperature distribution is a natural phenomenon, we will continuous our temperature measurements and monitoring in the borehole. I am grateful to Koichiro Sado, Susumu Shibutani, Kazuya Ishitsuka, Nana Kamiya, Tatsuhiro Sugimoto, Terasu Sano, Xiaoqiu Yang, Aiming Lin for their great helps to the temperature measurements.
Weiren Lin
2016 Kumamoto Earthquake, Aso Volcano, Drilling borehole, Temperature Anomaly
|
|
A3-011
11:10
|
11:25
|
STRESS STATE HETEROGENEITY OBSERVED ALONG THE TCDP WELLS AND ITS RELATION TO LITHOLOGICAL VARIATIONS
In-situ stress is an important parameter to characterize when studying the driving forces of earthquakes. Previous geomechanical studies have characterized the in-situ state of stress in the Chelungpu fault system based on image logs, extended leak off tests, and rock strength measurements from the Taiwan Chelungpu fault Drilling Project (TCDP). However, results are not necessarily consistent with each other, suggesting faulting environments ranging from normal to reverse faulting environments depending on the study. We revisit previous literature, reanalyze image logs and conduct experiments on cores collected from TCDP to deduce that the stress state in the region fluctuates due to the variation in lithology encountered along the well. Observation of wellbore failures show that their occurrence varies with lithology. Drilling-induced tensile fractures (DITF) are only observed in sandstone layers. We also observe that breakouts are ubiquitous features along the well, although in sandstone layers, their average width is smaller at 27 degrees compared to 31 degrees in siltstone layers. The DITF observations can be explained by the lower tensile strength of sandstone cores (0.5 MPa) compared to the siltstone cores (8 MPa) found from laboratory measurements. However, the narrower breakouts in the sandstone layer cannot be explained by the lower UCS (uniaxial compressive strength) of the sandstones, therefore it suggests that the maximum horizontal stress (S Hmax ) magnitude is lower in the sandstone layers. Such lowered S Hmax magnitude may potentially be explained by the lower elastic moduli of the higher-porosity sandstones. If formations experience uniform horizontal compression due to tectonic loading, stress accumulation in compliant layers would be less than in stiff layers. Hydraulic fracturing stress measurements carried out in Hole-B also suggests that the minimum horizontal stress (S hmin ) gradient varies with lithology. In test carried out in siltstone/ shale layers, S hmin values (24 MPa) were observed to be only slightly less than the vertical stress (S v =27 MPa); whereas in a sandstone layer, S hmin (16 MPa) was much smaller than the S v at that depth (25 MPa). Higher S hmin in the siltstone layers can be the result of stress relaxation that occurs in clay-rich siltstone layers. Rocks with higher clay content are known to exhibit ductile properties which helps to relax stress anisotropy and increase the S hmin magnitude over time. It is essential to carefully acknowledge these stress heterogeneities when discussing the relation between in-situ stress and earthquake faulting.
Li-Wei Kuo, Hiroki Sone, Mayukh Talukdar
hydraulic fracturing, in-situ stress, lithological variation, wellbore failures
|
|
A3-003
11:25
|
11:45
|
CO-SEISMIC FOCAL MECHANISM OF CHICHI EARTHQUAKE (1999, M W 7.6) DEDUCED FROM GOUGE MAGNETIC FABRIC
It is known that the gouge fabric can provide the elements of the focal mechanism of an ancient earthquake. However, the comparison between the gouge fabric and the focal mechanism of a modern earthquake remains to be established. Here, we present magnetic fabric investigation of gouges and wall rocks of the Chelungpu fault, where Chi-Chi earthquake took place (1999, Mw 7.6). Rocks are obtained from fresh surface outcrop and 1 km deep borehole (Hole B, Taiwan Chelungpu-fault Drilling Project). Deep gouge hosts the 3-mm thick principal slip zone of Chi-Chi earthquake, which is also analyzed. Wall rocks display contrasting magnetic fabric. Deep siltstones show magnetic foliation parallel to bedding, while for surface siltstones, it is oblique. In both gouges, the magnetic foliation is oblique with an angle near 40° with respect to the gouge plane, compatible with thrust movement. The reconstructed focal mechanism match closely with CWB and CMT solutions of Chi-Chi earthquake. It turns out that the pole of magnetic foliation parallels reconstructed extension axis. This are also consistent with the co-seismic displacement.
Sheng-Rong Song, En-Chao Yeh, Yu-Min Chou, Charles Aubourg, Xiaodong Jiang, Yi-Kai Lin, Fabien Humbert, Teh-Quei Lee
CHICHI EARTHQUAKE
|
|
A3-012
11:45
|
12:00
|
DEFORMATION STYLES WITHIN SHALLOW CREEPING FAULT ZONE OF THE CHIHSHANG FAULT, TAIWAN
The Chihshang Fault located in eastern Taiwan is a present-day plate-suture boundary fault between the Eurasian continental margin and the Philippine Sea Plate. By the microanalysis on rock cores retrieved from WAN-2 borehole, amorphous materials found at shallow depth in creeping zone of the Chihshang Fault occurred in different deformation styles. We investigate the rock deformation by conducting extensive microstructural observations on the basis of previous study. The petrographic thin sections were made from cores at different depths across lithological boundary where may most contribute to the deformation of fault zone. However, the heterogeneity of Mélange plays important role for the deformation styles in fault zone, so the deformation need to be recognized as a fault zone with wide heterogeneous slips. The preliminary results show ultra-comminuted particles occur not only in lithological boundary but in the certain depth, implying the competent to incompetent materials in Mélange could influence the slips in fault zone. Further microanalysis will be conducted in the near future.
Li-Wei Kuo, Wen-Jeng Huang, Wen-Jie Wu, Jian-Cheng Lee, Jia-Jyun Dong
Chihshang fault, creeping, Deformation style, Mélange
|
|
A3-001
12:00
|
12:20
|
RELATING POTENTIAL SIGNATURES OF FAULT HEALING AND DISTRIBUTED DEFORMATION IN FAULT DAMAGE ZONES
We examined in detail geophysical logs collected in the Taiwan Chelungpu fault Drilling Project (TCDP) in order to study the cause of spatial stress variations observed around fractures, faults, and its damage zones. The study aims at providing insights into how fault shear stress may be dissipated by distributed deformation in the damage zone during the interseismic period. TCDP is a suitable location for this study because the vertical boreholes encounter three large reverse faults and its damage zones of the Chelungpu fault system, including the slip plane of the 1999 Chi-Chi Earthquake. Examination of fracture densities in recovered cores show that there is an asymmetric distribution of damage about all three fault planes where the fracture density is higher in the hanging wall side. This is consistent with modeling of plastic deformation around reverse faults predictions in the literature. We find evidences of healing in these damage zones which increases with depth. First, abrupt changes in sonic wave velocities at the fault plane caused by the asymmetric distribution of damage zones become much less pronounced with depth. Also, borehole enlargement in the damage zone section, caused by the weakness of damage zone rocks, diminish with depth. These observations suggest that the ages of the deeper fault slips are older allowing more time for fault healing. A study on clay mineral assemblage have also suggested that the age of the 3 major faults become older with depth. These observations suggest that fault healing is a process taking place inside fault damage zones. We also examined how the azimuths of borehole breakouts change upon intersection with a minor fault fracture plane. We find that abrupt changes in breakout azimuth tend to occur more frequently outside of the Chelungpu fault system, whereas gradual changes in breakout azimuth is more common within. Changes in breakout azimuths reflect the stress perturbation caused by slip along the intersecting faults. We suggest that the relative absence of abrupt azimuth changes and abundance of gradual azimuth changes inside the Chelungpu fault system is a result of the relaxation of stress perturbation around the individual minor fault planes. Such relaxation is pronounced inside the Chelungpu fault system because damage zone rocks can deform by slow distributed deformation. We suggest that fault healing is facilitated by post-seismic distributed deformation within damage zones.
Li-Wei Kuo, Hiroki Sone, Mayukh Talukdar
breakout azimuth, fault damage zone, geophysical logs, healing
|
|
A3-013
12:20
|
12:35
|
CURVED SLICKENLINES PRESERVE DIRECTION OF RUPTURE PROPAGATION
Slip-parallel striations on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording details of earthquake rupture dynamics has received little attention. During the 2016 M7.8 Kaikoura earthquake, more than 10 m of dextral strike-slip on the steeply dipping Kekerengu Fault exhumed fresh fault exposure where it crossed deep gullies in bedrock. Inscribed upon these surfaces, we observed individual fault striations up to 2 m long, which are typically curved. Collectively, the striations recorded a temporal rotation of the co-seismic slip vector that was common between sites. Using simulations of spontaneous dynamic rupture on a vertical strike-slip fault employing slip-weakening friction laws, we reproduce the observed, curved morphology of near-surface striations on the Kekerengu fault. Our dynamic models with a strike-slip pre-stress reveal that vertical tractions are induced co-seismically by fault slip in the so-called cohesive zone. These result in some local dip slip and temporal changes in fault slip direction. We show that the sense of striation curvature is sensitive to the direction of rupture propagation. To match the geometry of the striation observed on the Kekerengu fault, our simulations require the rupture propagating from southwest to northeast, which is in agreement with the known rupture direction of the Kaikoura earthquake. Our study highlights the potential for fault striation to record aspects of earthquake rupture dynamics, including the rupture propagation direction of paleo strike-slip earthquakes.
Yoshihiro Kaneko, Jesse Kearse, Tim Little, Russ Van Dissen
Dynamic rupture simulations, Earthquake dynamics, Fault striations, Paleoseismology, Strike-slip earthquakes
|