奈良教育大学学術リポジトリNEAR
Stress Effect on Thermoluminescence
Intensities of Quartz Grains ―For the Establishment of a Fault Dating Method―
著者 HIRAGA Shozo, MORIMOTO Akira, SHIMAMOTO Toshihiko
journal or
publication title
奈良教育大学紀要. 自然科学
volume 51
number 2
page range 17‑24
year 2002‑10
URL http://hdl.handle.net/10105/388
1．Introduction
In order to reduce earthquake disasters due to
fault movement, it is essential to assess the potential of earthquake occurrence and to obtain the exact information on the recurrence period and the age of the
Stress Effect on Thermoluminescence Intensities of Quartz Grains
― For the Establishment of a Fault Dating Method ―
HIRAGA Shozo, MORIMOTO Akira* and SHIMAMOTO Toshihiko**
(Department of Earth Sciences, Nara University of Education, Nara 630-8528, Japan) (Received April 30, 2002)
Abstract
We have investigated the process of recovery of radiation damages at the time of fault movement, an elementary process attributed to stress, and examined the change of thermoluminescence (TL) intensities of quartz grains with diameters of 1-45 or 45-125 μ m, sheared under a slip rate of 100 μ m/s.
The following results were obtained as a result of the experiments. (1) TL intensities increase once with increasing stress, at larger rates for smaller grains. (2) With further increase in stress, the TL intensities peak then revert back to decrease, also at a larger rate for smaller grains. However, this reversion to original TL intensities occurs at higher stress for fine-grained samples. (3) Complete zeroing of TL intensities is not confirmed even at normal stress at 〜60 MPa, regardless of grain size.
These observations have been interpreted as follows. (1) The increase of TL intensities with increasing stress is due to the so-called radiation damage increment induced by generation of detached electrons at the time of new surface formation caused by shear fracture. (2) The decrease of TL intensities is due to the recovery of damages at the stress where no substantial shear fracture occurs. (3) If a grain-surface phenomenon is responsible for both the increase and the decrease of TL intensities, then the effect is expected to be greater for finer grains, because the surface areas of samples with equal mass are inversely proportional to the radii.
The stress effect may not be the only factor contributing to the complete recovery of radiation damages at the time of fault movement. The possibility remains that a complete recovery may be brought about by a rise in temperature due to frictional heating. If the temperature rise were uniform throughout a crush zone, the effect should be independent of grain size, and the complete zeroing of TL intensities should depend only on the degree of temperature rise during fault movement. This is a subject for future studies, to establish a detection method of complete zeroing using the most appropriate TL method for evaluating the thermal hysteresis of samples.
Key Words : stress effect, shear fracture, TL fault dating
* Completed the postgraduate course of Nara University of Education.
** Department of Geology & Mineralogy, Division of Earth & Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan.
Bull. Nara Univ. Educ., Vol. 51, No.２（Nat.），2002
last activity, and for this reason, many trench surveys have been conducted extensively in recent years. The age of the last fault movement is the most important for evaluating the danger level of an active fault. However, it is only estimated indirectly based on the displacement or relation of strata and rock bodies. Direct dating of intrafault materials resulting from fault activity, if possible, would be an invaluable tool for precise determination of the age of the last fault activity.
Ikeya et al . (1982) attempted an electron spin resonance (ESR) dating on quartz grains from intrafault materials of the Atotsugawa Fault. It was suggested that ESR signal intensities were zeroed by stress or heat at the time of fault movement, and it was concluded that dating of the last fault activity is possible using ESR method. Many studies of ESR dating have been vigorously attempted on various faults (e.g., Ohmura et al ., 1981; Tanaka et al ., 1981, 1984; Moriyama et al ., 1984; Fukuchi et al .,1985, 1986;
Buhay et al ., 1988; Fukuchi, 1988, 1989; Lee and Schwarcz, 1994, 1996). In later studies, the thermolumi- nescence (TL) method has also been applied to some faults (e.g., Nishimura and Horinouchi, 1989; Lin, 1989;
Hiraga and Nagatomo, 1995; Morimoto and Hiraga, 1997), based on the same principle that detects the extent of radiation damage.
However, there has been no age obtained as a result of dating by the ESR and TL methods guaranteed to be the last age of fault activity. The reason for this is that a methodology has yet to be established for confirming the complete zeroing of signal intensities at the time of the last faulting.
On the other hand, studies using the ESR method have attempted to clarify the conditions of complete zeroing experimentally (e.g., Miki and Ikeya, 1981, 1982;
Tanaka and Shidahara, 1985; Ariyama, 1985; Lee and Schwarcz, 1993) with a different approach compared to measuring signal intensities of natural intrafault materials. In contrast, the conditions of complete zeroing have not been experimentally investigated for the TL method. The details of the zeroing mechanism of TL signals at the time of fault activity are not clear, and so it is necessary to investigate each elementary process conceivable. In the present study based on shear experiments concerning the elementary pro- cesses attributable to stress, we have investigated the change of TL intensities for quartz with various degree of shear.
2．Experiments
２．１．Sample preparation
The quartz samples used for shear experiment were separated from St. Peter Sand, also known as Ottawa Sand, derived from the Cambrian St. Peter Sandstone. The sand consists of rounded quartz grains with minimal impurities.
Two factors, procurement of samples and comminution due to shear fracture, were considered, and we collected grains of 150-250 μ m by dry sieving.
Impurities were removed by isodynamic separation and handpicking. The samples were cleaned in an ultrasonic bath with acetone and distilled water for 10 min each, and then dried at 40℃, avoiding exposure.
２．２．Shear experiments
The shear experiments were performed using the high-temperature biaxial testing machine (MARUI Co., Ltd.) installed at Kyoto University. Fig.1 is a photograph of the whole equipment and the scheme of the sample assembly.
In the experiment, 1.6 g of quartz grains, cleaned by the above method, were placed between 3 blocks of gabbro as a 0.3 mm-thick shear in the biaxial machine (Fig. 1). Loads of 2, 4, 6, 8, and 10 tons were applied to the three-block sample assembly for 30 min to create compact quartz layers, before setting the specimens in the biaxial machine. Experiment runs were performed on the specimens at normal forces equivalent to the above loads, and under a slip rate of 100 μ m/s, until the total displacement reached the limit of 20 mm. The slip rate was selected with consideration to avoid the effect of frictional heating so that the factors to be considered in this experiment should only be elementary processes attributable to stress in paleodose zeroing.
Normal stress of about 20-60 MPa was calculated from the normal force divided by the sample area measured after each run.
Furthermore, the equipment was blacked out throughout the runs, to avoid exposure as much as possible. The exposure was less than 15 lx of red light, and this condition was satisfied throughout the whole procedures from the shear experiments to pretreat- ment for TL measurements.
Hereafter, the recovered samples are called HIRAGA Shozo, MORIMOTO Akira and SHIMAMOTO Toshihiko
18
samples A, B, C, D and E, in ascending order of normal forces.
２．３．TL measurements
The recovered samples were sieved into fractions of >125 μ m, 45-125 μ m, and <45 μ m. The second and third fractions, which are definitely affected by shear fracture, were named the coarse and the fine samples, respectively. The ratios (in percent) of recovered masses of coarse and fine samples were 40:50 for sample A, 35:55% for sample B and 20:75% for samples C, D and E.
２．３．１． Coarse samples
The coarse samples were cleaned in an ultrasonic bath with acetone and distilled water for 10 min each, dried at 40℃, and used for TL measurements with the quartz inclusion method (Ichikawa, 1965,1967; Fleming, 1970).
The measurements were performed in an atmosphere of nitrogen using a TL 2000A & 2080 analyzer (Harshaw Co., Ltd.). An optical filter of transmitting 350-570 nm lights was used, and the aliquots of 18 mg sample were heated from 50℃ to
470℃ at heating rate of 10℃/s.
２．３．２．Fine samples
The fine grains of 1-45 μ m were used for the TL measurements, because there were only a small amount of grains of 1-8 μ m, which are normally used for in the fine-grain technique (Zimmerman, 1971). The sample preparation procedure for TL measurements was as follows:
(1) Samples were cleaned in an ultrasonic bath with distilled water for 10 min. The supernatant was discarded after letting the sample stand for 24 hours.
The samples were dried at 40℃ until slightly damp.
(2) The above samples were suspended in acetone for 10 min in an ultrasonic bath, and left to stand in acetone 6-cm deep for 20 min. The supernatant was discarded and the sample dried at 40℃.
(3) The samples obtained in step (2) were suspended in a small amount of acetone, and the suspension was dropped into acetone 2.5-cm deep to achieve a sample mass of 0.8 mg per aluminum disc of 5 × 5 × 0.5 mm, and the sample was dried at 40℃.
The conditions of measurements were the same as for the coarse samples except for the sample mass per Fig. 1 Ａ：Photograph of the high-temperature biaxial testing
machine.
Fig. 1 Ｂ：Schematic diagram showing the sample assembly (modified from Kawamoto, 1996).
TL glow curve.
3．Results
Fig. 2 shows the TL glow curves for each sample, on the average of 4-6 measurements with background subtracted. For the coarse samples, TL intensities decrease monotonously with increasing normal force (sample A to E). But the most noticeable feature of this figure is that the TL intensity of the natural sample falls within the range of intensities for samples A-E.
For the fine samples, TL intensities of the samples B, C and D are higher than that of the sample A with the lowest normal force applied, though TL intensities decrease monotonously with increasing normal force (sample B to E).
The TL glow curve shown by the broken line is the reference line representing the fine natural sample, obtained by converting the data for coarse grains of 125 -150 μ m based on sample mass. This conversion was performed because fine samples unaffected by shear fracture could not be obtained. The noticeable feature of this figure is that TL intensities of all samples A-E are higher than that of the reference data.
Fig.3 shows the relationship between normal stress and TL intensities integrated over the full width at half maximum. TL intensities increase once at low normal stress then revert back to decrease in both the coarse and fine samples. It can be seen that both the
increase and decrease rates are larger for the fine samples, and that complete zeroing of TL intensities was not achieved at normal stress of 〜60 MPa regardless of grain size. Furthermore, it can be seen from the plot that the reversion to original TL intensities occurs at higher stress for fine-grained samples.
4．Discussions
４．１．Increase of TL intensity by shear fracture TL intensities increase once with increasing normal stress for both coarse and fine samples (Fig. 3).
Generally, the transparency of fine samples is significantly lower than that of coarse samples so that the TL signals for an equal sample mass is less detectable, and so the TL intensity presented as reference data for the fine natural sample may have been overestimated, as it was calculated from the data for coarse natural sample (Fig. 2, 3). Therefore, the level is considered to be adequate, being slightly lower than the level shown by the broken line in Fig. 3. If so, the results correspond better to the result obtained from coarse samples.
It has been reported in past studies that ESR intensities decrease with increasing compressive stress (Miki and Ikeya, 1981, 1982), differential stress (Tanaka and Shidahara, 1985), normal stress (Ariyama, 1985) and HIRAGA Shozo, MORIMOTO Akira and SHIMAMOTO Toshihiko
20
Fig. 2 TL glow curves of quartz from the St. Peter Sand. The left-hand figure is for coarse samples of 45-125μ m, and the right-hand figure for fine samples of 1-45μ m. Curves A, B, C, D, and E are for samples that have experienced shear fracture at normal forces of 2, 4, 6, 8, and 10 tons, respectively. The broken curve nat. is the reference curve calculated from the data for coarse grains of 125-150μ m based on sample mass. Each glow curve is the average of 4-6 measurements corrected for background.
shear strain (Lee and Schwarcz, 1993). Additionally, it was found in the present experiment that TL intensities increase once with increasing normal stress.
This implies that normal stress in shear experiments has the effect of not only decreasing but also increasing TL intensities. The following mechanism is examined which explains both of these effects.
The decrease of signal intensities may be caused by strain energy liberated at the end of shear experiments that recover radiation damages. Strain energy per unit volume has the same dimension as stress. The energy corresponding to the stress drop or stress at time of shear experiments is used to recover the radiation damages. This line of thinking can explain the well-known phenomenon of decreasing radiation damages, which exist originally, with increasing stress, which in turn decreases the signal intensities.
The increase in signal intensities once with increasing normal stress found in the present experiment can be explained if new surface creation occurs in sample grains that suffer from shear fracture.
Such surface creation will no doubt generate released electrons, which are indistinguishable from those generated by radiation exposure. Therefore, surface creation accompanying shear fracture will increase the quantity of the so-called radiation damage, and consequently produce increased signal intensities. This
increase in signal intensity should be proportional to the newly created surface area. Therefore, samples more comminuted with progressing shear fracture should display a larger increase. The observation from Fig. 3 supports this view.
Furthermore, Lee and Schwarcz (1993) have investigated the grain-size dependency of ESR intensities at various shear strains. Though they noted an increase in intensity with decreasing grain size to values exceeding even that of the unsheared sample, they did not present any detailed discussion on the matter. Their results agree well with the present observations according to the above discussion, since both the ESR and TL methods detect the quantity of the so-called radiation damage.
４．２．Relation between stress and signal intensity The liberated strain energy both decreases and increases the quantity of damage, depending on the degree of stress or comminution. Note that shear stress, which is an important parameter at the time of shear fracture, is calculated as normal stress multiplied by the friction coefficient. If the general friction coefficient of 0.85 is adopted (Byerlee, 1978), both stresses are on the same order.
At a stress lower than the threshold value, sample grains do not suffer from shear fracture. Therefore, there should be no increase in the quantity of the so Fig. 3 TL intensity vs. normal stress for quartz from the St. Peter Sand. The left-hand figure is for coarse samples of 45-125μ m, and
the right-hand figure for fine samples of 1-45μ m. Normal stress is the estimate of normal force divided by the sample area measured after shear experiment. TL intensity is the integrated value over the full width at half maximum. The broken horizontal line shows the level of the coarse nat. sample for reference.
-called radiation damage due to the new surface creation, and consequently, neither should there be an increase in signal intensity. Therefore, only the decrease in signal intensity with increasing stress is evident.
When shear fractures start to occur at a stress higher than the threshold value, an increase in signal intensity is expected as mentioned in the previous paragraph. But this increase does not continue endlessly, since the shear fracturing becomes more difficult as the sample is comminuted with increasing stress. This view is supported by the fact that the percentage recovery of fine grains is the same at 75%
for samples C, D and E. Furthermore, although the signal intensity actually observable is the one with the decrease due to stress increase subtracted, it seems that most of the liberated strain energy contributes to the increase in signal intensity while shear fracture continues.
It must be considered that signal intensity will revert back to a decreasing trend with further increase in stress, when the comminution due to shear fracture has substantially reached a peak. Although the complete zeroing of signal intensity has not been achieved in the present study, it is expected to be achieved at higher stress. Furthermore, the rate of signal intensity decrease at higher stress is larger for fine samples. This may correspond to the reports of
finer sample showing lower signal intensities (e.g., Miki and Ikeya, 1981; Buhay et al ., 1988). The recovery of damages is considered to be a surface phenomenon of grains (Hiraga and Nagatomo, 1995; Toyoda and Schwarcz, 1996) since the surface area of the equal mass sample is inversely proportional to grain size.
On the basis of the above discussions, a schematic relationship between stress and signal intensity is summarized in Fig. 4. In the figure, σB or σ T represent the stress at which substantial shear fracture begins or terminates, respectively, and σ Z represents the stress at which the complete zeroing is achieved. The characters IN and IMAX represent the signal intensities of the natural sample or the expected maximum value of TL intensity during shear fracture. The subscripts C and F indicate coarse or fine samples, respectively.
The σ T for fine sample is larger than for coarse sample because of progressive comminution with increasing stress.
４．３．Zeroing of signal intensity during fault activity The actual stress during fault activity is, for example, 〜10 MPa for the 1968 Saitama earthquake (Abe, 1975) and 10-20 MPa for the 1995 Hyogo-ken Nanbu earthquake (Kikuchi, 1995). In spite of the maximum stress being around 60 MPa for normal stress, and 51 MPa for shear stress in the present experiments, there is no remarkable decrease in signal intensities. The decrease is around 15% for coarse samples and 10% at most for fine samples. If the signal intensity of fine natural sample had been overestimated as mentioned previously, the signal intensity of sheared sample should increase rather than decrease. This implies that there is contribution from the increase in signal intensity with comminution. Judging from the present results, it is concluded that complete zeroing cannot be achieved by a single exertion of stress during usual fault activities.
For a fault with repeated activities and also for which more extensive comminution cannot be expected, signal intensities decrease only during a subsequent activity, with a rate larger for fine sample than for coarse ones, as mentioned in the previous section. However, the signal intensity increased by the previous comminution is also larger for fine samples.
Therefore, it can no longer be assumed that the signal intensity expected immediately before the last activity is the same regardless of grain size (Fig. 4), and that the HIRAGA Shozo, MORIMOTO Akira and SHIMAMOTO Toshihiko
22
Fig. 4 Schematic diagram showing the relationship between stress σ and signal intensity I . σ B, σ T, and σ Z
indicate the stress at which shear fracture is initiated,
of substantial termination, and of complete zeroing, respectively. IN indicates the signal intensity of the natural sample, and IMAX for the expected maximum value of TL intensity during shear fracture. The suffixes C and F represent coarse and fine samples, respectively.
signal intensity after the last activity is smaller for fine samples than for coarse ones.
Furthermore, even if the same paleodose or age is estimated regardless of grain size, there is the possibility that it does not represent the age of the last fault activity.
If so, is it, in principle, impossible to date the last fault activity based on detection of the quantity of radiation damage? Not necessarily. It has been pointed out that the factor contributing to decreasing the signal intensity is not only the stress effect but also the thermal effect during fault activity (Ikeya et al ., 1982).
If the temperature rise due to frictional heating during fault activity is regarded as being uniform throughout the whole crush zone, the effect should be independent of grain size. Therefore, the complete zeroing of signal intensity due to fault activity is dependent only on the degree of temperature rise at the time of activity. The feasibility of fault dating still rests on establishing the detection method of complete zeroing, using TL method which is the most suitable for evaluating the thermal hysteresis of samples.
Acknowledgments
The authors would like to thank Dr. E. Kawamoto, postdoctoral researcher of the Japan Society for the promotion of science, for many helpful suggestions during the course of our shear experiments, using the high-temperature biaxial testing machine installed at Kyoto University.
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