The reactivation examination of pre-existing surface fractures in NE Taiwan is based on calculating and analyzing slip tendencies of the 120 fracture planes under the opening of Okinawa Trough. The potential fractures slipped under the stress field are highlighted by red lines (Fig 5.10). In the counting, only 18, 9& 14 nucleated fracture planes of III-D, III-C& II-D cells (Fig 5.10D, Fig 5.10E, and Fig 5.10F), respectively, are going to reactivate under the operation of reverse faulting regime and strike-slip faulting regime. And, in the comparison, the reactivated fractures statistics in Domain I (Fig 5.10A and Fig 5.10B) and lower crust of Domain II (Fig 5.10C) show that the number of fracture planes is prominently reactivation. 65, 48, and 40 surface fracture planes are supposedly reactive due to the stress regime evolution and exhumation.
In Domain III, 18 fractures distributed in domains I and II can be reactivated under the III-D reverse faulting stress. Only one fracture plane could be reactive in Domain III (Fig 5.10F). The mean strike of the fracture plane reactivated by II-D stress shows the N-S orientation in the lower hemisphere. At 7.5km for III-C, II-C, and I-C,
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when 2 became the horizontal components instead 3 (Fig 4.17), the number of reactivated fracture planes in the study area are different 6/9 fracture planes are located in Domain III, and the mean fracture plane displayed a NE-SW striking (Fig 5.10E).
In domain II, the number of reactivated fractures under the strike-slip faulting regime (II-D) was less than reactivated fractures under the reverse faulting regime (III-D). 14 fractures planes were recognized in II-D cell, only 2/14 fracture planes in the domain III could be reactivated. The 14 reactivated fractures present an N-S strike in the lower hemisphere (Fig 5.10D). The result of slip tendency analysis under II-C stress represented that the reactivation of surface fracture is more predominant. 40 surface fractures can be reactivated, and most of the reactivated fractures found in Domain I and III (Fig 5.10C). In the lower hemisphere, the mean fracture of 40 reactivated fracture at II-C marked NE-SW orientation.
In the Domain I, surface fractures along the coastline of NE Taiwan strongly respond with the strike-slip faulting regime. At 10km, 65/120 (54%) surface fractures are able to reactivate (Fig 5.10B). These reactivated fractures were located in three domains, and the ENE-WSW striking of mean fracture was represented in the lower hemisphere. At 7.5km, 40/120 (30%) of surface fractures could be reactivated under the I-C stress (Fig 5.10A). And, in the lower hemisphere, the mean strike of the fracture plane at I-C presented the NE-SW orientation. It is different from the mean strike of fracture at III-D in the comparison; the mean fracture plane of I-D cell presented ENE-WSW striking. Therefore, the mean strike of fracture at the study area has clockwise rotation, from N-S orientation in III-D and II-D via NE-SW orientation in III-C, II-C, and I-C to ENE-WSW orientation in I-D.
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Figure 5.10: Reactivation of surface fractures in the scenario of tectonic evolution in NE Taiwan. Red and green lines described the reactivated and inactivated fracture plane, respectively. n is the number of the reactivated fracture planes in given stress state andmean reactivated fractures (red great circle) are showing in the lower hemisphere projection.
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As a result, it supported that a new stage of normal faulting is influenced on NE Taiwan, which was provided by Huang et al. (2012). The scenery declared that the opening of the Okinawa Trough became a dominant driving force in the regional stress regime. In our study, the result presented that earthquake events near southern Ilan Plain (Domain I) is specified as strike-slip faulting, and the focal mechanism cluster located in Domain II also is characterized by strike-slip faulting. These seismic events of tectonic motion between the PSP-EP subduction system and back-arc opening of Okinawa Trough along the Ryukyu trench delivered high motivation in reactivation of pre-existing surface fractures in NE Taiwan.
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CONCLUSIONS
This study aims to evaluate whether fracture development can correspond to the stress evolution from compression to extension along NE Taiwan. To accomplish this study, the work contains three parts: (1) To measure and calculate attitudes of surface fracture plane and foliation based 5-meter DEM data. (2) To investigate the regional stress state by analyzing focal mechanism data in period 1991-2015, from the southern part (Domain III) to central (Domain II) and northern NE Taiwan (Domain I) and (3) To correlate the surface fracture pattern with the evolution of stress state based on fracture instability calculation. In summary, the following points can be concluded:
1、 After examining the DEM data, the fracture system at NE Taiwan can be recognized and constructed in 3D view based on the interpretation of the cross-cutting relationship between foliation and fracture. 187 fracture planes have recognized and calculated their attitudes. confirmed to play a crucial role of manipulating surface fractures pattern from N-S
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strike in the south to ENE-WSW strike in the north along the NE coastal area of Taiwan.
5、 The number of nucleated surface fracture planes of Domain I is highest (64%), and the surface fractures from DEM-derivation of NE Taiwan can be correlated with the evolution of the stress state.
6、 54% fracture planes in Domain I can reactivate under the activity of the back-arc opening of Okinawa Trough in the north of NE Taiwan.
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Appendix A Surface fractures orientation of domain I
Plane No. Strike Dip Long Lat R2
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Appendix B Surface fractures orientation of domain II
Plane No. Strike Dip Long Lat R2
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Appendix C Surface fractures orientation of domain III
Plane No. Strike Dip Long Lat R2
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Summary of questions/answers and suggestions
Prof. Chang, Kuo-Jen
1. How do you identify the foliation in the 5m DTM data?
Ans: In the 3D environment, the small parallel, planar, triangle facets are presented for foliation planes (Figure 6.1A).
2. How to define the foliation in your thesis (planar structure defined metamorphic rock)?
Ans: updated the text in Section 3.1.1, p.22.
3. How to trace foliation on 5m DTM? Is the resolution of DTM enough to identify the geological structure?
Ans: updated the text in Section 4.1.1, p.42.
Suggestion: change the foliation term to the planar structure.
Ans: We already discussed the definitions and characteristics of foliation in 3D environment of DEM data (text in Section 3.1.1, p.22 and Section 4.1.1, p.42). We supposed that those characteristics are presented the foliation planes in schist rock area.
Beside that, to verify our methodology, we want to compare the foliation attitude from DEM-derivation with the foliation attitute in geological map (Central Geological Survey, 1995). Therefore, we still keep the foliation term in our study.
4. Are they fractures or faults?
Suggestion: faults, because the length of planes is a few kilometers.
Ans: Because, no further information of fault characteristics are recognizable in 3D enviroments of 5 meter DEM data. Therefore, our traces results in Fig 4.17 could be represented for faults or joins. And, we assumed that the fractures term in our study
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composed of faults (shearing-models fractures) and joins (extension-models fractures) (text in Section 3.1.2, p.22). The fracture definitions are resonable for our study hypothesis.
5. Are fractures generated by single or multi of earthquake events?
Ans: In our research, we assumed that the fractures are only generated by single earthquake events.
Prof. Wang, Tai-Tien
1. Did you compare your fracture results with the other research?
Ans: Yes, we did compare fracture results with Liu et al. (1982) results in figure 2.4, p.17, section 2.2.
2. Did you compare your foliation system with the other research?
Ans: We did compare the results of the foliation attitude with the public database from the Geological map (Central Geological Survey, 1995).
Suggestion: Compare with the public reported, because of the fracture plane located in the lithology boundary of different rock types.
3. What is the component of the fracture plane on the map view?
Ans: In the map view (Fig 4.7), we displayed the fracture traces.
4. How can you make sure that trace is right?
Ans: To verify the accuracy of fracture planes, we guaranteed three main factors:
• Fracture traces were mapped along the lineaments.
• To distinguish the fractures and foliation, we used the cross-cutting relationship.
• Traces are cut across the ridges or rivers, and traces are confirmed as a certain plane in a certain view direction.
5. What do we have with the fracture locate in the same rock type?
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Ans: We suppose that it could be fracture or shear zones.
6. Why do you want to correlate the earthquake events in the 10km depth with the surface fractures?
Ans: Due to the exhumation (updated the text in Section 4.2.2, p.68) and tectonic setting (Section 2.1, p.9) of the NE Taiwan region, our assumptions supposed that the events of the earthquake can manipulate fractures at 10 km in maximum and these fractures can be identified on the landscape at present.
7. Why we have the average values of friction angle and cohesion from rock mechanics?
Ans: From the geological map, we calculated the area of 3 representative rocks of NE Taiwan in Arc Map software, and we obtain the area ratio. We also summarized the average values of rock mechanics data for three rocks. Then we have calculated the average rock mechanics with the weighting of area ratio. Finally, we had the average cohesion and friction angle (updated the text in Section 3.2.5.3, p.39 and the table 3.1 ) 8. What are the unknown parameters to calculate fault instability?
Ans: The stress inversion result only presents the principal stress axes and relative size od principal stress values in Mohr’s circle diagram. Therefore the values of principal stresses are unknown.
9. How can you estimate the normal and shear traction of the fracture plane?
Ans: From focal mechanism data, we measured the Mohr’s circle by calculated the
Ans: From focal mechanism data, we measured the Mohr’s circle by calculated the