Long-term temperature records following the Mw 7.9 Wenchuan
(China) earthquake are consistent with low friction
Haibing Li1*, Lian Xue2, Emily E. Brodsky2, James J. Mori3, Patrick M. Fulton2, Huan Wang1, Yasuyuki Kano3, Kun Yun1,
Robert N. Harris4, Zheng Gong1, Chenglong Li1, Jialiang Si1, Zhiming Sun5, Junling Pei5, Yong Zheng1, and Zhiqin Xu1
1
State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences,
Beijing, China
2
Department of Earth & Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95064, USA
3
Disaster Prevention Research Institution, Kyoto University, Gokasho, Uji, Kyoto, Japan
4
Oregon State University, Corvallis, Oregon 97331, USA
5
Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, China
METHODS AND STRATEGY
The Wenchuan Fault Earthquake Zone Scientific Drilling Project
(WFSD) has successfully collected a data set over a 4 yr period with 23
temperature profiles in a single borehole. The measurement hole (WFSD-1;
Fig. 1A) was drilled from November 2008 to July 2009 to a maximum
depth of 1201 m (Fig. 1B) with a cased interval to a depth of ~810 m.
The borehole intersects a major fault at 589 m borehole (core) depth,
i.e., as measured along the hole, which is equivalent to 578 m vertical depth
(all depths reported hereare borehole depths, unless otherwise specified).
The 589 m fault is near the boundary between the Neoproterozoic Pengguan complex, which here consists of diorite, porphyrite, pyroclastics and
other volcanics, and the Late Triassic Xujiahe Formation, which here consists of interbedded sandstone and shale (Li et al., 2013). The cataclasite,
fault breccia, and fault gouge layers in the core continue to 759 m core
depth (746 m vertical depth). Multiple faults exist throughout the 589–759
m zone, and the identification of a single principal slip surface corresponding to the 2008 Wenchuan earthquake is difficult. Togo et al. (2011b)
reported that at a nearby surface outcrop, the surface break is localized on
the Sichuan basin side of the fault zone packet, which corresponds to the
759 m depth in the core. However, internal evidence from the core suggests
that the 589 m fault is the strongest candidate for the principal slip zone of
the Wenchuan earthquake because of the fresh gouge appearance, microstructures relating to coseismic slip, high magnetic susceptibility values (Li
et al., 2013), clay mineral composition in cores (Si et al., 2014), borehole
logging data (Li et al., 2014), and drilling mud gas concentrations (Tang
et al., 2013). Breaking both the surface rupture on the basin side and the
589 m fault at depth during the Wenchuan earthquake either requires a 65°
dipping fault crossing the gouge packet (Li et al., 2013) or, more likely, slip
transfer across the packet on to the subparallel en echelon surfaces.
Repeated temperature measurements were made in the well from
October 2009 through September 2013 (Fig. 1C). For each of the 23
profiles, temperature was measured by lowering a string of two or three
temperature sensors (RBR Ltd. model 1050/2050) at a rate of ~1 m/min
downward through the well to the bottom of the casing (800 m) and then
raising the string at the same rate. Measurements were recorded every
second and a built-in pressure sensor was intended to record depth. Only
the downgoing data from the bottom sensor are interpreted because both
the trailing sensors and upgoing runs are affected by the disturbance of the
fluid in the well during temperature logging. For three profiles from June
and December 2012, a stop-go logging technique was used with the sensor held stationary for at least 90 s to allow equilibration at fixed intervals
that varied between 0.2 and 1 m. The data were then fit through the hold
time to find the asymptotic steady-state temperature for comparison with
data obtained with the sensors in motion (Harris and Chapman, 2007).
The profile from 20 September 2011 was unusable due to a faulty pressure sensor. Profiles on 23 October 2009 and 23 June 2012 do not extend
throughout the study zone and the 18 October 2010 profile was lowered
too quickly for accurate temperature gradient estimates. The remaining 19
profiles were used for the following analysis.
ABSTRACT
Knowledge of the shear stress on a fault during slip is necessary for a physically-based understanding of earthquakes. Borehole
temperature measurements inside the fault zone immediately after
an earthquake can record the energy dissipated by this stress. In the
first Wenchuan Earthquake Fault Zone Scientific Drilling Project
hole (Sichuan province, China) we repeatedly measured temperature
profiles from 1.3 to 5.3 yr after the 12 May 2008, Mw 7.9 Wenchuan
earthquake. The previously identified candidate for the principal slip
surface had only a small local temperature increase of at most 0.02
°C with no obvious decay. The small amplitude of the temperature
increase provides an upper bound for the frictional heat–generated
coseismic slip, but is unlikely to be a frictionally generated signal. Two
larger temperature anomalies are located above and within the fault
zone. However, neither anomaly evolves as expected from a frictional
transient. We conclude that the frictional heat from the Wenchuan
earthquake remains elusive and the total heat generated at this location is much less than 29 MJ/m2. Low friction during slip is consistent
with the temperature data.
INTRODUCTION
The magnitude of the shear stress resisting slip along a fault during
an earthquake has long been unknown. Recent measurements have shown
that a straightforward application of Byerlee’s law with a coefficient of
friction of 0.6 for most rocks and 0.2 for clays may not yield the correct shear stress at high speeds (Byerlee, 1978). At typical earthquake slip
velocities of 1 m/s, laboratory values of friction plummet and span a range
of values from 0.05 to 0.4, depending on lithology and experimental conditions (Di Toro et al., 2011). The theory controlling high-velocity friction
is vigorously debated, and nonfrictional processes can alter the local shear
stress in natural systems. Field measurements are needed to constrain the
magnitude of shear resistance during earthquakes on actual faults.
To address this observational gap, rapid response drilling projects
have measured the temperature in fault zones directly after major earthquakes. Single profiles in both shallow and deep boreholes after the
1999 Mw 7.7 Chi-Chi earthquake (Taiwan) recorded temperature anomalies interpreted to be equivalent to a dynamic coefficient of friction of
~0.1 (Tanaka et al., 2006; Kano et al., 2006) and a similar value was
inferred for the 2011 Mw 9.0 Tohoku earthquake (Japan) from the Japan
Trench Fast Drilling Project (Fulton et al., 2013). All of these experiments tracked the temperature over <1 yr.
In this paper, we report on borehole temperature measurements
made across the fault zone that ruptured during the 12 May 2008 Mw
7.9 Wenchuan (Sichuan province, China) earthquake, continuing from
1.3 to 5.3 yr after the event. We use the temperature data and thermal
conductivity measurements to place an upper bound on the coseismic
friction on the fault.
*E-mail: lihaibing06@163.com
GEOLOGY, February 2015; v. 43; no. 2; p. 163–166; Data Repository item 2015060
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doi:10.1130/G35515.1
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Published online 5 January 2015
©
2015 Geological
Society
America.
Open Access: This paper is published under the terms of the CC-BY license.
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163
sampling near 589 m under dry conditions. Saturated thermal conductivities are estimated from a composite thermal model, as described in Data
Repository equations DR1 and DR2.
Qingchuan
an
Sh
Eastern Tibet
0
135°
Beichuan
en
m
ng
Lo
WFSD-1
drilling site
A
Depth (m)
Sichuan
Basin
.
31°N
Surface
rupture
1000
200
Chengdu
103°E
50 km
0
.
Yingxiu
WFSD-1
100
Maoxian
Wenchuan
25
Neoproterozoic
Igneous rocks
and metamorphic
rocks
800
F400
uJ
ia
600
Late Triassic
800
1000
600
ng
Sedimentary
rocks
Wenchuan
earthquake
fault zone
400
200
200 m
1200
104°E
Elevation (m)
32°N
Pingwu
105°E
0
B
106°E
THERMAL OBSERVATIONS
Just below the previously identified candidate principal slip zone at
589 m core depth, a small 15-m-wide temperature deviation of 0.02 °C
persisted throughout the observation period. This perturbation to the geotherm is most clearly resolved by the highest precision experiments that
used stop-go logging (Fig. 2B). These high-precision logs also have the
best depth control of any data collected. Similar amplitude temperature
perturbations exist elsewhere in all profiles taken after the end of drilling.
A temperature log taken during drilling reported a 0.15 °C anomaly at this
same location (Li et al., 2014; Fig. 2); however, as the drilling was stuck
at ~590 m for over 30 days due to the difficulties of penetrating the fault
zone, the early time data are strongly influenced by the drilling fluid temperature (see the Data Repository). For the data considered here, only the
association with the strongest geological candidate for the principal slip
zone distinguishes the 0.02 °C feature as worthy of further examination.
350
400
2013
Core Depth (m)
450
500
550
2012
600
650
700
2011
750
We used the temperature gradient change associated with a change
of lithology documented in the core and gamma ray logs at core depth of
394 m to align the logs (Figs. DR1 and DR2 in the GSA Data Repository1) because instrumental problems and variable densities in the muddy
water in the borehole made the depth inferred from the pressure transducer
inaccurate. The shifted 19 profiles overlay almost exactly, indicating that
the data quality is good and the long-term geothermal gradient of ~0.02
°C/m is stable (Fig. 1C). This inference of stability is independent of the
corrections applied here.
The temperature measurements are interpreted in conjunction with
measurements of thermal conductivity of the recovered core (see the Data
Repository). The thermal conductivity was measured by an optical scanner at 5 m intervals over the core from 350 to 800 m depth with denser
1
GSA Data Repository item 2015060, supplemental documentation of profile alignment, thermal conductivity measurement and interpretation, frictional
heat model and drilling disturbance model, including Table DR1 and Figures
DR1-DR7, as well as the original temperature data, is available online at www
.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or
Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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800
850
900
−2
A
−1.5
−1
−0.5
0
0.5
2010
1
Temperature (°C)
23-Jun-2012
29-Jun-2012
30-Dec-2012
0.03
Residual Temperature (°C)
Figure 1. Location of Wenchuan Earthquake Fault Zone Scientific
Drilling hole (WFSD-1) Sichuan province, China, and observed temperature profiles. A: Map of major active faults of the Longmen Shan
area and WFSD-1 location. Red lines show coseismic surface rupture. Red star denotes epicenter of the 2008 earthquake main shock.
B: Simple geologic cross section at the WFSD-1 site, including the
1201-m-deep hole (gray line) and candidate Wenchuan earthquake
fault. C: Temperature profiles in the WFSD-1 hole (short profile on
23 October 2009 and profile with malfunctioning sensor on 20 September 2011 are omitted). The profiles are vertically aligned using
small-scale features (see text) (Figs. DR1 and DR2 [see footnote 1]).
2013
0.02
2012
0.01
2011
0
−0.01
585
2010
B
590
595
600
605
610
Core Depth (m)
Figure 2. Temperature profiles with a constant gradient of 0.02 °C/m
removed. For clarity, profiles are shifted on the x axis in proportion
to the time since the earthquake. A: Complete data set. Data are interpretable at depths below the seasonal and hydrologically driven
changes in temperature within the casing. We focus on 350–800 m
depths. B: Close-up of the 589 m zone from the high-precision stopgo logs with arbitrary zeros. Apparent smoothing of the logs is due
to detrending. Stop-go logs are not available for other periods (Fig.
DR3 [see footnote 1]).
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Data
Synthetic
350
A
B
400
2013
450
Depth (m)
500
2012
550
600
650
2011
700
750
2010
0
0.1
0.2
0.3
0.4
−0.1
Residual Temperature (°C)
0
0.1
ANALYSIS OF THERMAL ANOMALIES
In our search for the frictionally generated heat, we first consider the
largest thermal anomalies: the 450 and 690 m depth features. Although
there is no well-developed fault plane documented at either of 450 or
690 m depth, there are fault breccias within each of the anomalies (Fig. 3
of Li et al., 2013).
If these thermal anomalies are frictionally generated, the residual
temperature provides a direct constraint on the thermal energy S generated
on the faults, i.e.,
S = ∫ ρcp ∆Tdz,
(1)
where ρ is the density and cp is the heat capacity. Estimating cp = 800 J/m3
(Beardsmore and Cull, 2001) and ρ = 2500 kg/m3 (Li et al., 2014) results
in S of 5 ± 2 MJ/m2 at 450 m and 24 ± 6 MJ/m2 at 690 m; error ranges
indicate 1 standard deviation on the estimates over the suite of profiles.
To test the consistency of the data with a frictional model, we calculate the evolution of the temperature field over time assuming that the
thermal energy is generated on fault planes at 450 and 690 m depth and
diffuses into the surrounding rock (Fig. 3B). The diffusive model predicts
that the anomalies should widen and decay resolvably in amplitude over
time. Although some reduction is seen in the 450 m depth anomaly, neither observed anomaly widens over time and both are narrower than predicted at the end of the study period.
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0.3
0.4
Residual Temperature (°C)
A larger feature in the temperature profile is centered at ~690 m
depth with a width of ~130 m, which nearly spans the fault zone (Fig.
2A). The amplitude decays from 0.25 °C to 0.20 °C over the observation
interval, but the width does not change. Above the primary fault zone, a
feature with a similar width but smaller amplitude is centered at ~450 m.
All of these features exist in both the continuous and the stop-go logs,
indicating that sensor reequilibration is not a factor in their identification.
We follow standard interpretive procedure and examine temperature
as a function of thermal resistance to form a Bullard plot (Beardsmore and
Cull, 2001). Thermal resistance incorporates variations in thermal conductivity and the procedure allows us to estimate the steady-state conductive heat flow. The mean resultant heat flow is 69 mW/m2 with a range of
68–70 mW/m2 over all the profiles (see the Data Repository). We remove
the steady-state conductive temperature profile at each depth and define
the residual temperature as the anomaly (Fig. 3A). The resultant anomalous temperature rise over the 589 m fault is still small and no anomaly
>0.01 °C exists over the second candidate fault zone at 759 m. The wider
features noted in the raw data are also anomalies in the residual temperature centered at 450 m and 690 m depth.
GEOLOGY
0.2
We conclude that the lack of observed time evolution implies that the
450 m and 690 m anomalies are unlikely to be due to diffusion of frictional
heat away from a fault. These anomalies could be the result of frictional heating combined with another source of heat transport. For example, advective
flow could generate a narrow thermal pulse limited by the high hydraulic
diffusivity structure in the damage abutting the main fault zone (Fulton et
al., 2010; Xue et al., 2013). A fluid flow explanation is qualitatively the most
attractive possibility, and detailed modeling of such a fluid flow is the focus
of future work. We confine ourselves here to concluding that the temperature
data provide an upper bound of 29 MJ/m2 of frictionally dissipated energy.
The heat energy could have been dissipated on any number of planes within
the region; there is no direct constraint on the localization of the energy.
The geologically significant 589 m depth fault has a subtle temperature increase. However, the width of a frictionally generated anomaly is
predicted to be much greater than observed. For a homogeneous medium
with a thermal diffusivity of 1.5 × 10−6 m2/s and heat generation at the time
of the earthquake, the anomaly is expected to be ~100 m wide at the time
of measurement, which is not observed (Fig. 2B). Furthermore, the signal
does not decay or widen (Figs. DR2 and DR3).
The temperature increase at 589 m could be attributed to a gradient
change associated with local thermal conductivity structure. We evaluate
this possibility in Figure 4 by calculating the steady-state thermal profile
Depth (m)
800
−0.1
Figure 3. Residual (anomalous) temperature after the
conductive geotherm is
removed. A: Observed residual temperature for each
profile color coded by date.
Thermal conductivity structure is in Table DR1 (see
footnote 1). B: Modeled residual temperature for frictional dissipation on 450
m and 690 m surfaces with
diffusive heat transport.
Total dissipated energy
is constrained by the observed residual thermal energy (see the Data Repository). The time evolution in
the model in B is absent in
the observations in A, suggesting that the anomalies
are not due to the diffusion
of a frictional heat.
585
585
590
590
595
595
600
600
605
605
610
−0.1 −0.05
0
0.05 0.1
Residual Temperature (°C)
610
0
1
2
3
4
Thermal Conductivity (Wm-1K-1)
Figure 4. Steady-state temperature profile with variable thermal conductivity. At each depth, we divide the heat flow of 69
mW/m2 as constrained by Figure 3 by the local thermal conductivity measured on the core to infer the local thermal gradient.
We integrate the gradient and detrend the resultant 590–610 m
depth temperature profile for comparison with Figure 2B.
165
consistent with the full suite of thermal conductivity measurements in the
589 m zone and a constant heat flow of 69 mW/m2 constrained by the analysis in Figure 3. The procedure is similar to that of Tanaka et al. (2007).
Although the symmetry of the observed temperature increase is not reproduced by a realistic thermal conductivity structure, the measured variation
in thermal conductivity suggests an even larger range of temperatures than
observed. Here we use only the dry laboratory values and therefore Fig. 4
may over-estimate variations of thermal conductivity. In the model of Fig.
DR4, we took the opposite approach and designed an inversion in which
porosity completely compensates for thermal conductivity variations in
the laboratory samples. Figure 4 is primarily useful as a warning of the
potential effects of heterogeneous structure, but is unlikely to provide an
accurate estimate of the residual temperature as saturation effects are not
included. We have already concluded based on the width and lack of time
dependence that the temperature increase below 589 m is unlikely to be
frictionally generated, and now modeling shown in Figure 4 permits the
feature to be the result of heterogeneous thermal conductivity structure.
The lack of a resolvable frictional heat anomaly yields an upper
bound for frictional heat dissipated here during the earthquake. For frictional heating during slip, the decay of the peak temperature, T, on a planar
fault due to one-dimensional heat conduction away from the fault is
T = (µ σn d/cp ρ)/[2 (π κ t)1/2],
(2)
where µ is the average coefficient of friction during slip, σn is the effective
normal stress, d is the slip, cp is the specific heat capacity, ρ is the density,
κ is the thermal diffusivity, and t is the time elapsed since the event (Carslaw and Jaeger, 1959). The model of Equation 2 is for an infinitesimally
thin fault. The times of the data recorded here are sufficiently long after
the earthquake that a thermal anomaly would have spread over a region
larger than the shear width and the planar solution is indistinguishable
from the finite shear zone solution. Based on Equation 2 and a normal
stress on the fault equal to the lithostatic overburden less the hydrostatic
pore pressure, the 0.02° C upper bound in the 589 m zone implies that
the effective coefficient of friction during the earthquake is <0.02 if 7 m
of slip happened on this surface (see Fig. DR7 caption for parameters).
Although the bound in terms of the coefficient of friction enables easy
comparison with previous work, dynamic reduction of normal stress or
inaccurate estimate of slip are also acceptable explanations of the data.
The data most directly constrain the dissipated heat energy, which over the
depth of Figure 2B is <1.2 MJ/m2.
In summary, the total energy dissipated frictionally at the locale penetrated by the WFSD-1 borehole is <29 MJ/m2 and the dissipated energy
on the previously identified fault surface is <1.2 MJ/m2. The upper bound
for the entire zone (29 MJ/m2) is less than would be anticipated with a
coefficient of friction of 0.6, but could be consistent with the range of
coefficients of friction seen in dynamic weakening experiments (Di Toro
et al., 2011). Other studies have inferred dramatic weakening for the Wenchuan fault material based on laboratory studies of fault zone rocks from
surface outcrops subject to high-velocity shear (Togo et al., 2011a; Chen
et al., 2013; Yao et al., 2013; Zhang and He, 2013). The high organic and
clay content or presence of graphite may be critical factors (Zhang and He,
2013; Kuo et al., 2014). All of the cited studies concluded that the coefficient of friction during slip was <0.2, which would be consistent with
the upper bound on the total dissipated energy. The Wenchuan earthquake
fault appears to have been weak during slip.
ACKNOWLEDGMENTS
This research was supported by the Chinese National Science and Technology Planning Project (Wenchuan Earthquake Fault Zone Scientific Drilling Project,
WFSD), National Science Foundation of China grant 41330211 (to Li) and U.S.
National Science Foundation grant EAR-1220642 (Brodsky). We thank W. Zhang,
G. Yang, R. Guo, and Y. Huang for data collection assistance. Detailed reviews from
T. Shimamoto, S. Nielsen and an anonymous reviewer are enormously appreciated.
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Manuscript received 29 January 2014
Revised manuscript received 4 December 2014
Manuscript accepted 5 December 2014
Printed in USA
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