Download Plate-wide deformation before the Sumatra

SAEEAPb – 08/05.11
Plate-wide deformation before the Sumatra-Andaman
Earthquake
Stuart Crampin1,2 and Yuan Gao3
1British
Geological Survey, Edinburgh EH9 3LA, UK. E-mail: [email protected]
at School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK.
3Institute of Earthquake Science, CEA, 100036 Beijing, China. E-mail: [email protected]
2also
ABSTRACT
Rock is weak to shear-stress and the energy released by the 26th December, 2004, M ≈ 9 SumatraAndaman Earthquake, the largest earthquake for four decades, must have accumulated over
enormous volumes of crust and mantle, certainly plate-wide, possibly world-wide. Here we
report evidence for plate-wide stress accumulation. Changes in seismic shear-wave splitting
monitor stress-induced changes in the geometry of the microcrack distributions in almost all
rocks in the Earth’s crust. Such changes observed in Iceland show stress-accumulation beginning
at least four years before the Sumatra-Andaman Earthquake. These changes were recognised as
monitoring stress-accumulation before an impending large earthquake and 10 ‘stress-forecasts’
were emailed to Iceland Meteorological Office for some 27 months forecasting an impending
large earthquake. The remarkable sensitivity of critical-systems of microcrack geometry to
miniscule changes of stress had not been recognised at that time and the stress-accumulation was
expected to lead to a M ≥ 7 earthquake somewhere in Iceland. Only now is it recognised that the
changes in shear-wave splitting were monitoring stress-accumulation which would eventually
lead to the Sumatra-Andaman Earthquake at a distance of some 10,500km on the opposite side of
the Eurasian Plate. This extreme sensitivity confirms the critical nature of fluid-saturated stressaligned microcracks in the Earth’s crust.
Key words: Crack-coalescence, critical-systems, fluid-saturated stress-aligned microcracks,
plate-wide deformation, shear-wave splitting, stress-accumulation, stress-relaxation.
1. INTRODUCTION
Stress-aligned shear-wave splitting (SWS) above swarms of small earthquakes is widely observed
in the Earth’s crust (CRAMPIN and PEACOCK 2008). Caused by propagation through the
distributions of fluid-saturated stress-aligned (typically-vertical) microcracks pervading almost all
in situ rocks, changes in SWS time-delays monitor stress-induced changes to microcrack
geometry. We have found that, whenever there is appropriate source-to-seismometer recordinggeometry, stress-accumulation eventually leading to a large or larger earthquake has always been
observed by analysing characteristic changes in SWS. Such characteristic changes are often
observed at substantial distances from the impending source zone (CRAMPIN & PEACOCK
2005, 2008). Increases in tectonic stress are typically the result of subduction, magma generation,
and other interactions at plate margins. Initially such increasing stress fields carry no information
about where the stress will eventually be released, thus the observed stress-accumulations are a
widespread volumetric effect and not precursory to any particular earthquake. Additionally,
whenever there are sufficient shear-wave source data to show the behaviour immediately before
an impending earthquake, the stress-accumulation is observed to abruptly stop and start to
decrease (GAO & CRAMPIN 2004). This decrease is interpreted as stress relaxation as
microcracks begin to coalesce onto the eventual fault-break. Consequently, stress-relaxation
decreases are precursory to specific earthquakes, and measurements of stress-accumulation
increases and crack-coalescence decreases allow the fault breaks of impending earthquakes to be
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stress-forecast (CRAMPIN et al. 1999; CRAMPIN & PEACOCK 2005, 2008; CRAMPIN et al.
2008).
We refer to such estimates of earthquake occurrence as stress-forecasts rather than
earthquake forecasts or earthquake predictions to emphasise the different formalisms. Note that
stress-forecasting is an extraordinary simplifying assumption. Stress-accumulation occurs in a
heterogeneous rock mass and varies irregularly. If stress accumulates over a small rock volume,
the accumulation is rapid, but the resulting earthquake is small. If stress accumulates over a large
rock volume, the accumulation is slower, but the resulting earthquake is larger. This link with
earthquake magnitude is confirmed as logarithms of the durations of both increases and decreases
are each proportional (self-similar) to the earthquake magnitude (see Figure 2, below)
(CRAMPIN et al. 2008; CRAMPIN & PEACOCK 2008). It is our experience that stressaccumulation increases have always been followed by large or large earthquakes, often at
substantial distances from impending source zone.
Here we present evidence that stress-accumulation before the 26th December, 2004 M ≈ 9
Sumatra-Andaman Earthquake (SAE) was monitored by changes in SWS in Iceland starting at
least four years earlier. Iceland is on the opposite side of the Eurasian Plate from Sumatra at a
distance of ~10.500km where this extreme sensitivity is the result of the critical nature of
microcracks in the Earth’s crust.
2. BRIEF SUMMARY OF SHEAR-WAVE SPLITTING ANALYSIS
Only shear-waves recorded within the effective shear-wave window above swarms of small
earthquakes can be successfully recorded at the free-surface without severe waveform distortion
due to S-to-P conversions (BOOTH & CRAMPIN 1985). [The shear-wave window is defined by
incidence angles less than sin-1(VS/VP) at a horizontal free-surface, which is ~35º for a Poisson’s
ratio of 0.25.] Stress-induced changes to microcrack geometry modify crack aspect-ratios, and
SWS time-delays in Band-1 directions within the shear-wave window are sensitive to aspectratios (CRAMPIN 1999). Band-1 directions are the solid angle subtending 15º-to-45º to the
vertical crack plane. Time-delays in Band-2 directions, ±(0º-to-15º) to the crack plane, are
sensitive to crack density but crack density is not directly related to increases of stress
(CRAMPIN 1999). The effects of the shear-wave window and Band directions are specified
more fully in Appendix A.
Swarms of small earthquakes in the shear-wave windows of seismic recording networks can
be used as ‘stress-monitoring-stations’ to measure changes in SWS caused by stress-induced
changes to the geometry of the microcracks pervading the rock mass (CRAMPIN & PEACOCK
2005, 2008). Using seismic swarms as stress-monitoring-stations, characteristic stressaccumulation changes in time-delays between split shear waves have been observed
retrospectively before 14 earthquakes worldwide (CRAMPIN & PEACOCK 2008). On one
occasion when the seismic network in Iceland was monitored routinely, the time, magnitude, and
fault-break of a M = 5 earthquake in SW Iceland were successfully ‘stress-forecast’ in a
comparatively narrow time/magnitude/location window (CRAMPIN et al. 1999, 2008). Stressforecasts are usually posed in Smaller-Earlier to Larger-Later (SELL) windows (as in Table 1).
At various times it has been suggested that variations in SWS time-delays, on which stressforecasting is based, could be caused by variations in: rainfall and glacier runoff; source
earthquake migration; systematic variations in source earthquake magnitude or frequency; and
by magmatic flux variations, and other local disturbances. Initially such possible variations were
considered by VOLTI & CRAMPIN (2003a, 2003b). Many of the effects are eliminated or
weakened by only using source events deeper than 5km-depth. However, the principle
demonstration that such effects are negligible is that characteristic stress-accumulation increases
and crack-coalescent decreases have now been observed with consistent parameters before 15
earthquakes world-wide (including the SAE in this paper) in a wide variety of different
environmental, geological, and tectonic regimes. There are no exceptions. Had these local
variations been significant, the characteristic stress-accumulation increases and crack-coalescent
decreases would have been seriously disturbed. The migration issue is discussed in CRAMPIN &
GAO (2005).
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TABLE 1 HERE
3. MEASURING SHEAR-WAVE SPLITTING IN ICELAND
Iceland is on an offset of the Mid-Atlantic Ridge where several transform faults uniquely run
onshore. Onshore transform faults provide the persistent swarm seismicity recorded by the
comprehensive SIL seismic network (STEFÁNSSON et al. 1993) necessary for measuring
temporal variations in SWS in stress-monitoring-stations. Such persistent onshore seismicity is
rare, and in this optimum environment, many properties of SWS were first identified in data from
Iceland (VOLTI & CRAMPIN 2003a, 2003b; CRAMPIN & PEACOCK 2005, 2008).
The data in this paper are records of shear waves from earthquakes within the shear-wave
window beneath the specified seismic stations in Figure 1 which shows the measured SWS timedelays. The data are from all earthquakes below 5km-depth, to avoid near-surface and other
anomalies, and ~10% are omitted when seismograms are too complicated to yield good SWS
measurements. The complications are caused by: irregular surface and sub-surface topography
distorting the shear-wave window (Appendix A); P-wave coda overrunning shear-wave arrivals
so that waveforms are distorted; and where earthquakes are not well separated in time so that
again waveforms are distorted.
FIGURE 1 HERE
We use earthquake locations from the Iceland Meteorological Office (IMO) Seismic Bulletin.
It is difficult to provide statistical errors for SWS measurements that are meaningful in
geophysics. The vertical lines in the plots of normalised time-delays in Figure 1 are the observed
time-delays (in ms) divided by the range of standard errors in hypocentral distance (in km) from
the IMO locations (VOLTI & CRAMPIN 2003a, 2003b). The nearly negligible error bars in the
majority of the time-delay plots in Figure 1 suggest that most source events are sufficiently well
located to provide meaningful measures of time-delays.
Due to the complexity and temporal and spatial variations of SWS waveforms (CRAMPIN et
al. 2004), wholly automatic measuring techniques cannot successfully identify and measure SWS
(CRAMPIN & GAO 2006). We measure orientations and time-delays of SWS on the Iceland
seismograms with the semi-automatic Shear-Wave Analysis System (SWAS) of HAO et al.
(2008). SWAS automatically selects and plots polarisation diagrams (hodograms) of threecomponent SWS waveforms in screen images, which allow visual assessments and manual
adjustments of picks for optimising time-delay arrivals. Specifically designed for Iceland data,
SWAS gives rapid reliable measures of polarisations and time-delays for about 80% of all
earthquakes below 5km-depth in the shear-wave windows of stress-monitoring-stations (HAO et
al. 2008).
During routine analysis of IMO data from Iceland, we noticed what we thought was longterm stress-accumulation before an impending large earthquake at seven seismic stations above
swarms of small earthquakes. Figure 1 shows variations in measured time-delays for six and a
half years from January 2000 at the seven seismic stations in Iceland where there were sufficient
small earthquakes within the shear-wave window to monitor temporal variations in SWS timedelays. The lower half diagrams are arrivals in Band-1 directions (sensitive to aspect-ratio and
hence to changes of stress, CRAMPIN 1999). Band-1 directions are specified in Appendix A.
The upper half diagrams are arrivals in Band-2 directions specified in Appendix A that are
sensitive to crack density, but crack density does not appear to have a direct relationship with
changes of stress, and Band-2 variations are not used (CRAMPIN 1999). The least-squares lines
in Band-1 show increases and decreases before the 2004 Sumatra-Andaman Earthquake (marked
by a bar in the time axis). Diagrams to the left (BJA, KRI, SAU) of Figure 1 are stations from
southern Iceland above seismicity associated with the SW Iceland Transform Fault. Diagrams to
the right (BRE, FLA, GRI, HED) are stations over 300km to the north above seismicity associate
with the Húsavík-Flatey Transform Fault.
The relative north-south locations of the seismic stations were used in the initial interpretation
of emails in Table 1, when we thought the stress-accumulation referred to an impending large
earthquake in Iceland, and are recorded for completeness in Table 1. This interpretation was
inappropriate when it was realised in retrospect that the stress-accumulation referred to the
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Sumatra-Andaman Earthquake. The network of seismic stations in Iceland subtends less than 2º
at Sumatra and is essentially a single direction. Consequently, the variations in Figure 1 are
believed to be caused by local variations in the shear-wave source seismicity in Iceland, local
geology and tectonics, and interactions with irregular surface topography.
The diagrams in Figure 1 show characteristic increases and decreases in time-delays in a time
window spanning the Sumatra-Andaman Earthquake that are typical of temporal variations in
SWS time-delays observed before impending earthquakes elsewhere (CRAMPIN et al. 1999,
2008; VOLTI & CRAMPIN 2003a, 2003b; CRAMPIN & PEACOCK 2008), but with much
longer time scales. The large scatter about the mean (sometimes referred to as a ‘±80%’ scatter)
is a real geophysical effect observed in all measurements of SWS time-delays that is understood
and can be modelled but the scatter cannot be eliminated from the measurements (discussed in
Appendix B) (CRAMPIN et al. 2002, 2004).
4. INTERPRETING STRESS-FORECASTS EMAILED TO ICELAND
The long term increases in time-delays in Figure 1 were first recognised in September, 2002, at
Stations BJA, SAU, and BRE, and the first of 10 stress-forecasts was emailed to the IMO
warning of an impending large earthquake. Table 1 summarises these emails.
TABLE 1 HERE
Note that the least-squares fits to increases and decreases in time-delays in Figure 1 were
cited retrospectively to the impending earthquake. Increased understanding of the phenomena
was ongoing during this period, and some of the earlier stress-forecasts in Table 1 are not wholly
consistent with the up-dated least-squares lines in Figure 1.
The initial stress-forecast, SF1 (13/09.2002), Table 1, was based principally on stressaccumulation increases at Stations BJA, SAU, and BRE. Table 1 distinguishes reports from
stations in SW Iceland by bold-face font from those in northern Iceland in regular-face font.
These geographical locations guided the interpretation of stress-forecasts summarised in Table 1.
The cause of the large scatter in time-delays was not initially recognised (see discussion in
Appendix B), nor were the implications of increasing time-delays abruptly decreasing in stressrelaxation as microcracks coalesce onto the fault-plane (GAO & CRAMPIN 2004). In the
analyses, crack-coalescent decreases were first identified in SF4 (07/03.2003) at northern Stations
BRE and FLA, which suggested that the impending earthquake would be in northern Iceland, SF5
(04/04.2003). Later, in SF9 (29/09.2004), crack-coalescence decreases were identified on all
seven available stations, and SF10 (22/12.2004) suggested that the impending M = 7+ earthquake
could be anywhere in Iceland. The situation did not change in the final stress-forecast SF11
(18/02.2005), nor subsequently to the time of writing in May, 2011. The anticipated M ≥ 7
earthquake in Iceland has not occurred, and by now would be far too late to be associated with the
stress-forecasts in Table 1. Consequently, the variations in Figure 1 are now associated with the
only candidate earthquake, the Sumatra-Andaman Earthquake of December, 2004 The stressaccumulation increases and crack-coalescence decreases in Figure 1 are characteristic of the
changes in SWS observed before 15 earthquakes worldwide (CRAMPIN & PEACOCK 2008),
but with significantly larger time durations appropriate to the larger magnitude earthquake.
5. PLOTS OF DURATIONS OF STRESS INCREASES AND STRESS DECREASES
The association between changes in shear-wave-splitting time-delays and earthquakes is
confirmed by the linearity (self-similarity) of logarithms of the durations of both increases and
decreases with earthquake magnitudes, analogous to the linearity of the Gutenberg-Richter
relationship (GUTENBERG & RICHTER 1954). Figure 2 shows plots against magnitude of
logarithms of the durations of stress-accumulation increases (Figure 2a) and crack-coalescence
decreases (Figure 2b) of all available data, where the solid data points are identified in
CRAMPIN & PEACOCK (2008). The data for the open circles for the Sumatra-Andaman
Earthquake in Figure 2 are taken from Figure 1. The initial points of these Sumatra-Andaman
increases in Figure 1 are hidden in noisy data, and are likely to have begun possibly tens of years
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before the start of the time interval shown in Figure 1. Thus the arrow head to the right of the
open circles in Figure 2a indicates that the data points should be located at (unspecified) greater
durations. Extrapolation of the least-squares line reaches M ≈ 9 at ~10000days (~27 years) which
may be associated with the world-wide return period of M ≈ 9 earthquakes.
FIGURE 2 HERE
The least-square line through the solid data points in Figure 2a is thought to represent the rate
of increasing tectonic stress generated at plate boundaries. The solid-circle outliers at M = 6
(1986, North Palm Springs Earthquake, California) and M = 7.7 (1999, Chi-Chi Earthquake,
Taiwan), respectively, refer to earthquakes at different plate boundary faults, where the rates of
stress-accumulation are likely to be different from those elsewhere (CRAMPIN & PEACOCK
2008).
In Figure 2b, the open circles at M ≈ 9 for the Sumatra-Andaman Earthquake from Figure 1
are believed to be correctly located. The stress-relaxation least-squares line in Figure 2b is
thought to be related to the rate at which networks of cracks coalesce onto a principal fault break.
This suggests that the linear relationship for the smaller magnitude data ends at about M = 6 and
curves upwards for larger magnitude events. Such effects might be expected. The linear data
below M = ~6 are expected to refer to data from coalescing crack networks in the Earth’s crust,
whereas data for larger earthquakes include crack-coalescence in crack networks which are at
least partially located in the upper-mantle. Thus the upward trending curve in Figure 2b may
indicate that crack distributions in the mantle coalesce more rapidly than those in crust. Since the
fluid in the cracks in the mantle is likely to be films of high-temperature liquid melt (CRAMPIN
2003) rather than the lower-temperature water or water-based salt solutions common in the crust,
faster crack coalescence in the mantle may be expected.
Estimates based on duration/magnitude plots, both for stress-accumulation and crackcoalescence as in Figure 2, were also included in the emailed stress-forecasts from SF9
(29/09.2004) onwards listed in Table 1. Current studies of the Sumatra-Andaman Earthquake
reported elsewhere generally focus on co-seismic and post-seismic behaviour are discussed in the
Conclusions. To our knowledge, this is the first report of plate-wide pre-earthquake stressinduced deformation.
6. TECTONIC IMPLICATIONS
FIGURE 3 HERE
The map in Figure 3 shows approximate plate boundaries and the great-circle path from SAE to
Iceland. Plate-wide observations of stress-accumulation at seismic stations in Iceland at
~10,500km from SAE confirm the extreme sensitivity expected of pervasive distributions of
microcracks in crustal rocks (CRAMPIN & PEACOCK 2005; CRAMPIN 2006). Note that
stress is a volumetric effect not confined to straight-line transmission. The huge stressaccumulation before the SAE was clearly plate-wide as indicated by this paper, and potentially
worldwide, as is suggested by the approximate coincidence of start of stress-accumulation (~27
years) with the ~40 years since the last M ~9 earthquake. This was the 1964, M ~9.2, Alaskan
Earthquake - although the relationship is complicated by the 2011, M ~9, Tohoku Earthquake in
offshore Japan.
The underlying reason for such extreme sensitivity is that the distributions of fluid-saturated
microcracks in situ rocks are so closely-spaced they verge on failure by fracturing and hence are
critical-system (DAVIES 1989; CRAMPIN et al. 2003; CRAMPIN & PEACOCK 2005). This
means that the dominant mode of low-level deformation in crustal rocks (at levels of deformation
below those of fracture-criticality when the rock fractures and earthquakes occur) is modifying
microcrack geometry of the stress-aligned fluid-saturated microcracks that stress-aligned SWS
indicates pervade almost all in situ rocks (CRAMPIN & PEACOCK 2008). Such critical-systems
impose fundamentally new properties on conventional sub-critical geophysics (CRAMPIN 2006)
and geology (CRAMPIN & CLARKSON 2009). Consequently, the crust of the Earth is not
conventional brittle-elastic rock. In situ rock is highly compliant (CRAMPIN 2006) and responds
to stress changes. Deformation of fluid-saturated stress-aligned microcracks in both crust and
upper mantle explains how the solid earth responds to changes: before earthquakes and volcanic
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eruptions; in earth and ocean tides; and during CO2-sequestration, and hydrocarbon production,
and potential changes in radioactive waste repositories, all of which can be monitored by SWS.
7. DISCUSSION OF SENSITIVITY
Critical-systems of fluid-saturated microcracks are expected to be sensitive to small changes of
stress so the plate-wide deformation before the Sumatra-Andaman Earthquake is not unexpected.
We can estimate the sensitivity:
CRAMPIN et al. (2003) at the prototype borehole Stress-Monitoring Site (SMS) in Iceland
observed classic seismic anomalies at 70km distance from micro-earthquake swarm activity with
the seismic-energy equivalent of a M ≈ 3.5 earthquake. Assuming similar energy density (ED)
over the whole thickness of the crust for the (energy equivalent) M = 3.5 earthquake, where
stress-accumulation is observed over a distance of at least 70km, the energy density ED is
distributed over an area greater than π 702 = ~15000km2. Since earthquake magnitude is
proportional to (2/3)log10(E), where E is the radiated seismic energy (CHOY and
BOATWRIGHT 1995), the energy for a M = 5 earthquake, say, would be 102.25 (=178) times
greater than that of the M = 3.5 earthquake. If the detectable level of ED is the similar to that of
the M = 3.5 event, ED for a M = 5 earthquake would be detectable at a SMS over an area of 178 x
15000km2 = ~2670000km2 , that is to a distance of ~930km, say, 1000km. The detectable levels
of ED for the M ≈ 9 earthquake would be visible worldwide and plate-wide deformation must be
expected as observed in this paper.
This exceptional sensitivity, well beyond that expected in a conventional sub-critical
geophysics, is merely extending the sensitivity observed for the energy equivalent of a M = 3.5
earthquake at the SMS in Iceland (CRAMPIN et al. 2003) to larger magnitudes. This estimate of
sensitivity makes three assumptions:
(1) The same observable change in energy density at the source is assumed for M = 3.5 and M =
5 earthquakes dispersed over appropriate volumes;
(2) Observations of the M = 3.5 change in stress at 70km are so distinctive (CRAMPIN et al.
2003) that the effects are expected to be visible at well over 70km;
(3) Similar SMS sensitivity for Iceland as for elsewhere in the crust - note that the sensitivity of
critical-systems of microcracks is likely to be universal with similar effects over all available
space (CRAMPIN 2006; CRAMPIN & PEACOCK 2005,2008).
There are two key questions:
(1) With this exceptional sensitivity indicated above how is it that such effects have not been
observed elsewhere before the Sumatra-Andaman Earthquake? The difficulty is that suitable
swarms of small earthquakes to use as stress-monitoring-stations are extremely scarce. There no
suitable shear-wave source events to monitor SWS elsewhere. Had there been suitable monitored
swarms, we would expect the stress-accumulation to have been observed.
(2) If the exceptional sensitivity had been recognised, could we have stress-forecast the time,
magnitude, and location of the Sumatra-Andaman Earthquake before the event? Stress-forecasts
SF8 (25/06.04), SF9 (29/09.04), and SF10 (22/12.04) in Table 1 did stress-forecast the occurrence
of an imminent M = 7+ earthquake, although the earthquake was expected to be in Iceland. If the
exceptional sensitivity had been recognised, the impending earthquake could have been anywhere
worldwide. If precursory anomalies could have been recognised, the impending fault line of the
Andaman-Indonesian subduction arc might have been identified as the potential site of a large
earthquake. In our current understanding of the phenomenon, a single SMS gives no indication
of the location of impending earthquakes. It is possible that networks (arrays) of SMS, as
suggested inter alia by CRAMPIN et al. (2010), might provide some indication of location or
proximity of impending large earthquakes but there is no information currently available.
8. OTHER OBSERVATIONS
There have been extensive studies of the behaviour of various geophysical parameters at the time
of and following the 2004 MW = 9.1 Sumatra-Andaman Earthquake (SAE) the largest
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(megathrust) earthquake of the past forty years. These studies include tidal triggering (TANAKA
2010), earthquake-induced thermal anomalies (DONNE et al. 2010), and gravity anomalies
(EINARSSON 2010), and many others. SHEARER & BÜRGMANN (2010) is a comprehensive
summary of such studies. However, to our knowledge there are no observations of anomalies
before SAE, and this paper appears to be the first to report temporal variations several years
before SAE, observed in Iceland at a distance of ~10500km.
Anomalies at such distances have been previously suggested. BOWMAN et al. (1998)
estimate the radius of the volume of rock approaching criticality (defined as a region of
accelerating seismicity and Benioff strain release) as 100.5Mkm, which for M = 9 equals 31600km,
that is worldwide, as would be expected from our analysis above. DOBROVOLSKY, et al.
(1979) by a different approach (the distance to which precursors are observed derived mostly
from data from RIKITAKE 1975) obtains a ‘strain radius’ of 100.43Mkm, which for M = 9 equals
7400km. This also agrees with the pattern recognition techniques of KEILIS-BOROK and
KOSSOBOKOV (1990). Thus other observations appear to support the plate-wide deformation
we observe in this paper.
9. CONCLUSIONS
The least-squares lines for the stress-accumulation increases and crack-coalescent decreases
through the ‘±80%’ scatter in data points in Figure 1 may appear somewhat arbitrary to the casual
eye. Consequently, the list of emailed stress-forecasts in Table 1 is important as indicating that
familiarity with the phenomenon led the authors to recognise the characteristic stressaccumulation (and crack-coalescence) 27 months before the time of the SAE. Unfortunately, the
exceptional sensitivity of SWS to miniscule deformation of fluid-saturated stress-aligned
microcracks had not been recognised at that time and the changes were attributed to an impending
large earthquake in Iceland. The familiarity was based on retrospective stress-forecasts of 14
earthquakes worldwide and one successful real-time stress-forecast (CRAMPIN & PEACOCK
2008; CRAMPIN et al. 2008).
The fluid-saturated stress-aligned microcracks throughout the Earth’s crust and upper mantle
are confirmed as the most compliant elements of in situ rock, and because the microcracks are so
closely-spaced, all in situ rocks verge on failure in fracturing and earthquakes and are criticalsystems. Stress-induced changes can be monitored by SWS and the time, magnitude, and in some
cases location of impending earthquakes stress-forecast.
This calculable compliance of in situ rock, where microcracks are so closely spaced that they
are critical-systems, imposes a range of new properties on conventional sub-critical geophysics
including monitorability, calculability, predictability, universality, and extreme sensitivity. The
observations of stress-accumulation before the Sumatra-Andaman Earthquake are demonstrations
of all these properties (except perhaps calculability) and confirm that cracks in the crust are
critical-systems (CRAMPIN 2006; CRAMPIN & PEACOCK 2005, 2008).
Acknowledgements
We thank a Reviewer, Angelo De Santis, for drawing our attention to papers by BOWMAN et al.,
DOBROVOLSKY et al., and KEILIS-BOROK and KOSSOBOKOV. We thank Xiqiang Liu and
Jing Wu for assistance with the analysis, and Francesca Bianco, David Booth, Sheila Peacock,
and an anonymous Reviewer for valuable comments that helped to improve the manuscript. We
thank the Iceland Meteorological Office for providing access to their seismic data. YG was
partially supported by National Natural Science Foundation of China project 40774022.
The work of SC was published with the approval of the Executive Director of the British
Geological Survey (NERC).
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9
APPENDIX A
Ray-path geometry of the effective shear-wave window
FIGURE 1A HERE
Figure A1 shows the necessary recording geometry at Seismic Station S on a horizontal
free-surface. ABSCD is a crack plane of a distribution of parallel vertical microcracks,
through S, the position of the recorder on a horizontal free-surface. The effective shearwave window is the solid angle DEFGA-S and its reflection in the crack plane. SWS
time-delays in Band-1 directions to the horizontal free-surface, within the solid angle
EFGH-S subtending 15º to 45º to the crack plane, are sensitive to crack aspect-ratios and
hence to changes of stress (CRAMPIN 1999). Band-2 directions to the horizontal freesurface, where shear-wave time-delays are within the solid angle ADEHG-S subtending
0º to 15º to the crack plane, are dominated by crack density. Both Band-1 and Band-2
directions include equivalent solid-angle directions on both sides of the imaged crack
plane ABCD through S.
Note that the theoretical shear-wave window (BOOTH and CRAMPIN 1985) for an
isotropic half space with a Poisson’s ratio of 0.25 is for incidence less than ~35º.
However, in field conditions with low-velocity near-surface layers, ray paths are curved
upwards, and the effective shear-wave window can usually be extended to 45º or 50º. In
this paper and in Figure 1A, we conservatively assume 45º.
Note also that irregular topography of the free-surface or of sub-surface interfaces can
seriously distort the circular symmetry of the shear-wave window. The effect of irregular
topography can be recognised in field observations by scatter in the observed SWS
polarizations.
APPENDIX B
The cause of the large scatter in shear-wave time-delays
The large ‘±80%’ scatter in SWS time-delays is typical of all measurements of SWS
above small earthquakes. Microcrack orientations, and hence SWS time-delays, are
sensitive to the effects of the critically-high fluid-pressures (CRAMPIN 1999;
CRAMPIN et al. 2002, 2004) near all seismically-active fault planes. The key effect is
that the critically-high pore-fluid pressures, necessary to relieve friction on all
seismogenic fault-planes, change microcrack orientations by typically 90º near the faultplane, and hence cause ‘90º-flips’ in SWS polarizations. In normally pressurised rock,
faults and cracks are typically parallel to the direction of maximum horizontal stress
which is typically at large angles to the strike of the fault-plane. When pore-pressures on
seismogenic faults exceed the maximum horizontal stress (as can be expected on
seismically-active faults), the microcrack orientations change by, typically, 90º, causing
the polarizations of the leading split shear-waves to change by 90º (CRAMPIN et al.
2002, 2004). This causes the faster and slower split shear-waves to exchange
polarizations, in what is known as ‘90º-flips’ (ANGERER et al. 2002). Consequently, in
the immediate neighbourhood of the fault-plane where pore-pressures are critically high,
the faster split shear-waves display 90º-flips, which are reversed in the remainder of the
normally-pressurised ray paths to the free surface. The observed time-delays at the
surface immediately above small earthquakes are the sum of the ‘flipped’ negative timedelays and the positive time-delays through the unpressurised portion of the ray path.
The occurrence of each earthquake in a swarm of small earthquakes changes details
of the local stress-field and including pore-fluid pressures. This modifies microcrack
10
geometry within the focal zone of the swarm so that details of the stress-field and porepressure are modified. Consequently, small differences in the ratio of path lengths of
normally-pressurised paths to path lengths of critically high-pressurised paths, caused by
small differences in focal depths and small differences in proximity of ray paths to the
fault plane, can easily cause the ±80% scatter (CRAMPIN et al. 2004). Although the
scatter can be calculated, given accurate structural and stress and pressure parameters, the
scatter cannot be eliminated from observations because there is never sufficient
knowledge of the in situ stress, pressure, and local geology at each earthquake source to
make meaningful waveform evaluations (CRAMPIN et al. 2004). The ±80% scatter is
observed to be always present in observations of shear-wave time-delays above small
earthquakes confirming that all seismogenic fault-planes are pervaded by critically-high
pore-fluid pressures.
11
TABLE 1
Summary of emailed stress-forecasts (SF) from University of Edinburgh to Iceland Meteorological Office.
Date
Seismic stations*
with stressaccumulation
Increases.
Seismic stations* with a) Smaller-Earlier to Larger-Later (SELL)
stress-relaxation
window.
(crack-coalescent)
b) Estimate of location.
decreases.
c) Plots of duration/magnitude as in Fig.2.
SF11
13/09.2002
SF21
11/11.2002
BJA, SAU,
Not reported.
BRE
over ~15 months.
BJA, SAU,
Not reported.
BRE, FLA
SF31
05/12.2002
BJA, SAU,
BRE, FLA
SF41
07/03.2003
BJA, SAU, KRI, BRE, FLA
BRE, FLA, GRI.
SF52
04/04.2003
BJA, SAU,
BRE, FLA
BRE, FLA
SF61
18/11.2003
BJA, SAU,
BRE, FLA
Not reported.
SF71
05/12.2003
Not reported.
SF81
25/06.2004
SF93
29/09.2004
BJA, SAU, KRI,
BRE, FLA, GRI,
HED
BJA, SAU, KRI,
BRE, FLA, GRI,
HED
BJA, SAU, KRI,
BRE, FLA, GRI,
HED
SF103
22/12.2004
BJA, SAU, KRI, BJA, SAU, KRI,
BRE, FLA, GRI, BRE, FLA, GRI,
HED
HED
►
26/12.2004
Sumatra-Andaman Earthquake
Not reported.
Not reported.
BJA, SAU, KRI,
BRE, FLA, GRI,
HED
a) M 5.6 soon or M 6+ in 6 months.
b) No estimate of location.
c) No plots of duration/magnitude.
a) M 6.6 soon or M 7+ in 4 months.
b) Húsavík-Flatey Fault or Grímsey
Lineament.
c) No plots of duration/magnitude.
a) M ~ 6.
b) Húsavík-Flatey Fault.
c) No plots of duration/magnitude.
a) M 7 soon or M 8 in 5-8 months.
b) No estimate of location.
c) No plots of duration/magnitude.
a) M 7 soon or M 8 within 4-7 months.
b) Northern Iceland.
c) No plots of duration/magnitude.
a) M 7+ soon or within a few months.
b) No estimate of location.
c) No plots of duration/magnitude.
a) M 7+ soon or within a few months.
b) No estimate of location.
c) No plots of duration/magnitude.
a) M 7+ soon or within a few months.
b) No estimate of location.
c) Plots of duration/magnitude.
a) M 7+ imminent.
b) Húsavík-Flatey Fault or Grímsey
Lineament.
c) Plots of duration/magnitude.
a) M ~ 7 imminent – consistent stressaccumulation and crack-coalescence.
b) Somewhere in Iceland.
c) Plots of duration/magnitude.
◄
*Bold-face font: seismic stations in the South Iceland Seismic Zone in SW Iceland. Regular-face font:
seismic stations over 300km to the north, near the Húsavík-Flatey Fault and the Grímsey Lineament.
1
Compiled by Stuart Crampin, and Yuan Gao.
2
Compiled by Stuart Crampin, and Xiqiang Liu.
3
Compiled by Stuart Crampin.
12
FIGURE CAPTIONS
Figure 1
Measured shear-wave time-delays, from January 2000 to June 2006, normalised to
ms/km, at the seven seismic stations in Iceland with sufficient data to show variations
before and after SAE (date bar): (a) SAU; (b) BJA; (c) KRI; (d) GRI; (e) FLA; (f)
BRE; and (g) HED. Lower-half diagrams are arrivals in Band-1 directions; upper-half
diagrams are in Band-2 directions (see Appendix A). The irregular lines are nine-point
moving-averages, which show the overall variations in each half diagram but are not used
in the analysis. The least-squares lines show increasing stress-accumulation and
decreasing stress-relaxation (crack-coalescence) before SAE.
Figure 2
Variations of logarithms of duration in days against earthquake magnitude of (a) stressaccumulation increases, and (b) stress-relaxation crack-coalescent decreases, of SWS
time-delays. Solid data points are specified in CRAMPIN & PEACOCK (2008). Open
circles are data for variations before SAE from Figure 1. (a) Duration in days of
increasing time-delays with impending earthquake magnitudes for 11 earthquakes
worldwide with parameters of the least-squares fit in the inset box (CRAMPIN and
PEACOCK 2008). The arrow head to the right of the open circles indicates that the true
durations are likely to be much larger. (b) Duration in days of stress-relaxation (crackcoalescent) decreases in time-delays with the least-squares fit in the inset box. The open
circles in Figure 2b are approximately correctly located. Modified from CRAMPIN and
PEACOCK (2008).
Figure 3
Map showing approximate plate boundaries (dashed lines) and great circle path (dotted
line) between Sumatra-Andaman Earthquake (circle) and Iceland seismic network
(triangle).
Figure A1
Necessary recording geometry for recording SWS within the shear-wave window at
Seismic Station S on a horizontal free-surface for propagation through a distribution of
parallel vertical stress-aligned microcracks.
13
FIGURES
Figure 1
14
a)
►
b)
M = (1.17 ± 0.17) log10(days) + 4.01 ± 0.20
Correlation coefficient = 0.93
Figure 2
15
Figure 3
16
Figure A1
17