stress buildup and drop in inland shallow crust caused by
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© JSRM All rights reserved.
Volume 15, Number 1, June 2019, pp.1-4
[Summary]
Stress Buildup and Drop in Inland Shallow Crust Caused by the 2011
Tohoku-oki Earthquake Events
Kiyotoshi SAKAGUCHI*, Tatsuya YOKOYAMA**, Weiren LIN*** & Noriaki WATANABE*
* Graduate School of Environmental Studies, Tohoku University, Aobaku, Sendai 980-8579 Japan
** OYO Corporation, Minamiku, Saitama 336-0015 Japan
*** Graduate School of Engineering, Kyoto University, Saikyoku, Kyoto 615-8540 Japan
Received 15 05 2019; accepted 10 06 2019
ABSTRACT
This article is the summary of the published paper (Sakaguchi et al. 2017). In this study, to examine the change in in-situ stress
between before and after the 2011 Tohoku-oki earthquake, we performed stress measurements after the earthquake in the Kamaishi
mine. The in-situ stress measurement period was from 1991 to 2016. The results showed that the magnitudes of the
three-dimensional principal stresses and the vertical stress drastically increased during the mainshock and, at one year after the
earthquake, were more than double those before the earthquake. The principal stress magnitudes then decreased with time, and
returned almost to the pre-earthquake levels at about three years after the earthquake. The increasing and decreasing trends in stress
in the Kamaishi mine can be interpreted in terms of the effects of coseismic rupture behavior of the Tohoku-oki earthquake
mainshock and the occurrence of aftershocks in the Sanriku-oki low-seismicity region (SLSR), where the Kamaishi mine is located.
The drastic increase in stress suggests that the SLSR may act as a barrier to further rupture propagation. In addition, the consistency
between the change in measured stress and the change in seismicity in the Kamaishi regions suggests that the results of stress
measurements, even those at a much shallower depth than the earthquake source fault, can be useful for understanding rupture
propagation behavior.
Keywords: Stress change, Rock stress measurement, the 2011 Tohoku-oki earthquake, CCBO technique
1. INTRODUCTION
The Tohoku-oki earthquake that occurred on March 11,
2011 was in the largest class of earthquakes. The Tohoku
region experienced crustal movement of up to 5.3 m in the
horizontal direction and up to 1.2 m in the vertical direction
(subsidence) within a short period. This crustal disturbance
with a large displacement rate is likely to have had a major
impact on the crustal stress field at a relatively shallow depth.
Therefore, it is important that we clarify the change in in-situ
stress between before and after the 2011 Tohoku-oki
earthquake.
2. OVERVIEW OF THE IN-SITU STRESS
MEASUREMENT
The measurement location is the Kamaishi mine in
northeast Japan. The Kamaishi mine is located about 170 km
northwest of the earthquake epicenter. In this study, we
selected a measurement station that is about 5 km from the
mine opening of the 550 mL site at the Kamaishi mine. The
overburden at this measurement station is about 290 m. This
measurement station is not affected by galleries or the goaf
cavern in this mine. In addition, several stress measurements
have been performed at the 550 mL site before the
Tohoku-oki earthquake (Sakaguchi et al., 1995a; Sakaguchi et
al., 1995b; Sugawara and Obara, 1999; JNC, 1999). In-situ
stress measurements were performed using the Compact
Conical-ended Borehole Overcoring (CCBO) technique
(Sakaguchi et al., 1994). The CCBO technique is the one of
the suggested methods of the ISRM (Sugawara and Obara,
1999).
Figure 1 shows a plan view of the drift of the 550 mL site
at the Kamaishi mine. This figure also shows the
measurement stations used in this study after the mainshock
and those in other studies before the mainshock.
Figure 2 shows a plan view of the measurement station in
this study. In-situ stress measurements were performed four
times after the mainshock. The first measurement (SKO-1)
was performed from February 27 to March 1, 2012, the
second measurement (SKO-2) was performed from
December 17 to December 19, 2012, the third measurement
(SKO-3) was performed from March 10 to March 12, 2014,
and the fourth measurement (SKO-4) was performed from
March 14 to March 18, 2016, which represent, approximately,
one, two, three and five years after the mainshock,
respectively.
2 K. SAKAGUCHI et al. / International Journal of the JSRM vol.15 (2019) pp.1-4
Figure 1. A plan view of the drift the 550 mL site at the
Kamaishi mine. The measurement station used in this study
and those in other studies with the CCBO technique are
indicated by stars.
Figure 2. A plan view of the measurement station. The
measurement station is located where two galleries (width of
about 5.5 m and height of about 7 m) are adjacent. In-situ
stress measurements were performed in four boreholes
(SKO-1, SKO-2, SKO-3 and SKO-4) that are denoted by
thick black solid lines and red broken line. Strains were
recorded at every 3 mm of overcoring advance using
data-logging equipment and a PC.
3. RESULTS OF IN-SITU STRESS MEASUREMENT
Figure 3 shows the orientation of the principal stresses
(1, 2, 3) by a lower hemisphere stereographic projection.
The maximum principal stress 1 is oriented in a North-South
direction both before and after the mainshock. The
intermediate principal stress 2 was in an East-West
horizontal direction, and the minimum principal stress 3 was
in a vertical direction. This is consistent with reverse fault
stress and was maintained for up to five years after the
mainshock.
Figure 4 shows (a) the magnitude of the principal stresses
and (b) the ratio of the vertical stress v to the overburden
pressure pv as estimated from the rock density and the depth
from the ground surface just above the measurement station.
The magnitudes of the principal stresses at one year after
were two to three times greater than those before the
mainshock. However, at two years after the mainshock, the
magnitudes of the intermediate principal stress and the
minimum principal stress were almost the same as those
before the mainshock, while the magnitude of the maximum
principal stress was still large compared to that before the
mainshock. The magnitudes of the principal stresses at three
years and five years after the mainshock were almost the
same as those before the mainshock.
Figure 3. The orientation of the principal stresses (1, 2, 3)
by a lower hemisphere stereographic projection. 1 (circle) is
the maximum principal stress, 2 (square) is the intermediate
principal stress and 3 (star) is the minimum principal stress.
(a) The result that were obtained before the earthquake (K-1 -
K-5). (b) The result from at one year after to five years after
the 2011 mainshock.
The magnitude of the vertical stress σv was almost the same
as the overburden pressure except at 1 year after the
earthquake: the magnitude of σv at 1 year post-earthquake
was approximately 2.4 times greater than the overburden
pressure (Fig. 4(b)). Theoretically, the static vertical stress
has to be in a state of mechanical equilibrium with the
overburden pressure. In the study area, the topography is
steep: three tall mountains with altitudes of approximate
1,300 m are located within 3 km of the stress measurement
station. These three mountains are approximately 300–400 m
higher than the ground surface immediately above the
measurement station. As the height difference is larger than
the overburden ~290 m above the measurement station, a
higher vertical stress magnitude of more than two times of the
overburden pressure can be considered to be possible for a
short period of time (1 or 2 years).
(a)
(b)
K. SAKAGUCHI et al. / International Journal of the JSRM vol.1 (2019) pp.1-4 3
Figure 4. Annual trends in the stress state. The results after
the Tohoku-oki earthquake are mean values. (a) The three
principal stress magnitudes (σ1, σ2, σ3). These mean values of
principal stresses were calculated by averaging the principal
stress at every measuring point. (b) The ratio of the vertical
stress v to the overburden pressure pv calculated as
(gravitational acceleration) × (depth) × (average density
determined from rock samples to be 2.7 ton/m3). The error of
overburden pressure pv was evaluated assuming that the
measured value of overburden (depth) has an error ±10 m.
4. DISCUSSION
Figure 5 shows the the total slip distribution of larger than
5 m of the 2011 Tohoku-oki earthquake (Yagi and Fukuhata,
2011; Ye et al., 2012). Figure 6 shows the magnitude-time
plot for the off Kamaishi region (Ariyoshi et al., 2014). In
Figure 5, the pink rectangular frame shows the Sanriku-oki
low-seismicity region (SLSR) where located near the
northern termination of the Tohoku-oki mainshock rupture by
Ye et al. (2012). A rectangular region with small slipping is
distributed off Kamaishi (Figure 5). The Kamaishi region is
believed to correspond to the outside edge of the slipping
region of the 2011 Tohoku-oki earthquake, and this
rectangular shape area may have made the slipping stop.
Earthquakes had occurred off Kamaishi every approximately
5.5 years before the 2011 Tohoku-oki earthquake (Figure 6).
The magnitude of these earthquakes ranged from M = 4.7 to
M = 5.1. However, the interval between these earthquakes
decreased and the magnitude of the earthquakes off Kamaishi
increased after the Tohoku-oki mainshock. Furthermore, the
interval between earthquakes off Kamaishi gradually
increased at one year after the Tohoku-oki mainshock. At
about the same time, the magnitudes of these earthquakes
returned to the same level as those before the Tohoku-oki
mainshock.
Figure 5. The positional relationship between the Kamaishi
mine and the epicentre of the Tohoku-oki earthquake (yellow
star with a red outline). The epicentre of repeaters off
Kamaishi is marked by a red star and the location of the
Kamaishi mine is indicated by a black star (modified from
Ariyoshi et al., 2014). The total slip distribution of slip
greater than 5 m for the 2011 Tohoku-oki earthquake is also
plotted, from Yagi and Fukuhata (2011) and the epicentres of
earthquakes (M > 5) in the Sanriku-oki low-seismicity region
(SLSR), from Ye et al. (2012).
Figure 6. Magnitude-time plot for the off Kamaishi region
with close-up in the sky-blue-colored time window (insite),
lower figure, from Ariyoshi et al.. The vertical red line shows
the occurrence time of the 2011 Tohoku-oki earthquake.
These results suggest the following scenario regarding the
changes in the crustal stress in the Kamaishi mine after the
(a)
(b)
4 K. SAKAGUCHI et al. / International Journal of the JSRM vol.15 (2019) pp.1-4
Tohoku-oki mainshock. Slipping behavior off Kamaishi
increased the magnitude of the crustal stress at the Kamaishi
mine at one year after the Tohoku-oki mainshock. As a result,
the number of earthquakes off Kamaishi increased. These
earthquakes led to a decrease in the magnitude of the crustal
stress at the Kamaishi mine beyond one year after the
Tohoku-oki mainshock. The magnitude of the crustal stress in
the Kamaishi region then decreased beyond two years after
the Tohoku-oki mainshock, and the earthquakes off Kamaishi
decreased.
Iinuma et al. (2016) showed that the cumulative
postseismic slip (for the period from 23 April 2011 to 10
December 2011) of the 2011 Tohoku-oki earthquake in
offshore Kamaishi was larger than that of surrounding regions.
Moreover, Toda et al. (2011), Hiratsuka and Sato (2011) and
Sato et al. (2012) showed that the Coulomb stress change
(ΔCFF) around the Kamaishi mine exhibited a positive trend
after the Tohoku-oki earthquake, and Bletery et al. (2014)
demonstrated that the stress drop altered to a negative trend
after the mainshock. Additionally, Ishibe et al. (2017) showed
that the temporal changes in median ΔCFF from 2000 to the
middle of 2015. The median values of ΔCFF rapidly
increased just after the Tohoku-oki mainshock, after which
the median ΔCFF gradually decreased to background levels
approximately 3 years after the mainshock. Uchida et al.
(2013) showed that the stress drop due to earthquakes off
Kamaishi (20 Mar. 2011–23 Sep. 2011) was 2.4 MPa to 10.4
MPa. These observations also support our interpretation of
the stress change pattern examined in this study.
In addition, the consistency between the change in
measured stress and the change in seismicity in the Kamaishi
regions suggests that the results of stress measurements, even
those at a much shallower depth than the earthquake source
fault, can be useful for understanding rupture-propagation
behavior.
5. CONCLUSION
The direction of the maximum principal stress did not
change much between before and after the Tohoku-oki
mainshock. However, the directions of the intermediate
principal stress and the minimum principal stress after the
mainshock were different than those before the mainshock;
i.e., the intermediate principal stress was in the East-West
direction horizontally, while the minimum principal stress
was in the vertical direction.
The magnitudes of the principal stresses at one year after
were two to three times greater than those before the
mainshock. However, at two years after the mainshock, the
magnitudes of the intermediate principal stress and the
minimum principal stress were almost the same as those
before the mainshock. The magnitudes of the principal
stresses at three years and five years after the mainshock were
almost the same as those before the mainshock. The
magnitude of the vertical stress at one year after the
mainshock was about 2.4 times greater than the weight of the
overburden. Moreover, the magnitude of the vertical stress at
five years was almost the same as the weight of the
overburden.
The increasing and decreasing trends in the crustal stress
in the Kamaishi mine are believed to be influenced by both
the slipping behavior off Kamaishi at the Tohoku-oki
earthquake and repeated earthquakes off Kamaishi.
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