Download Evidence for temporal variation of seismic velocity within the upper

JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 105, NO. B10, PAGES 23,879-23,894, OCTOBER 10, 2000
Evidence for temporal variation of seismic velocity
within the upper continental crust
GStz H. R. Bokelmann •
Department of Geophysics,Stanford University, Stanford, California
Hans-Peter Harjes
Institut fiir Geophysik, Universit/it Bochum, Bochum, Germany
Abstract. Observationsof systematictemporal variations of seismicanisotropy are
presented for an induced-seismicityexperiment at 9 km depth. These observations
were made under particularly well-controlled conditionsin the German Continental
Deep Drilling Program (KTB) borehole,usingshearwave splitting from similar
events recorded at a three-component instrument located at 4 km depth from a
hydraulic fracturing experiment at 9 km depth. In a large set of seismicevents
recorded during the experiment, many can be associatedwith multiplets exhibiting
essentially identical waveforms. Since they must have approximately the same
source location and source radiation pattern, these events are particularly useful
for testing the hypothesisof time-dependentanisotropy.Anisotropy itself is clearly
a very prominent feature in the data. A simple approachfor waveform matching
of split shear waves allows unprecedented resolution of variations in shear wave
splitting. Importantly, the variation of shear wave splitting with time is a relative
measurement, which can be performed with higher accuracy than the associated
absolute measurement. In particular, the relative measurementis not affected by
timing errors nor by event distancevariations. During the experiment the difference
betweenshear wave velocitiesdecreasesby -• 2% within -• 12 hours. After that,
the medium apparently approachesa state which is stable for at least 5 hours. We
suggestthat the temporal variation is due to the tectonic stressreleasefrom seismic
events causedby the fluid injection. This model requiresthe presenceof fluid-filled
cracks at depths larger than 4 kin.
1.
Introduction
the saxneorder of magnitude and often similar angle
dependence,which makes a distinction usually rather
There is little doubt that anisotropy is an important
difficult, unless some of the effects can be ruled out
feature of elastic wave propagation throughout many
on other grounds. Generally, however, all of these efregionsof the Earth: It affectsapparent velocitiesof P
fects are present, and the questionis only to which dewaves[e.g.,Hess,1964],polarizationof P waves[e.g., gree they contribute to the observations.An important
Bokelmann,1995a],and particularlyshearwaves[e.g., questionis whether or not effectiveanisotropy changes
Ando et al., 1983]. Sucheffectiveanisotropyat seismic
with time. Suchtemporal variations(at least thoseon
wavelengths
of l0 -• to 102km canbe causedby a va- timescalesof yearsor less),if they canbe found,narrow
riety of effects, all of which are associated with some
down the choiceto just few effects. More importantly,
type of ordering in the medium. These effectsrange
however,temporalvariationswouldshowthat (and perfrom preferentialorientationof minerals(eachwith its
haps how) anisotropyreacts sensitivelyto changesin
ownintrinsicanisotropy),layering,to preferentiallyorithe stressfield and it might perhaps serve as a simple
ented cracks or faults. These effects may have about
means for measuring crustal stresschangesat reasonable cost[Crampinand Zatsepin,1997].
Several observationshave been suggested,which ap•Previously at Institut fiir Geophysik,Ruhr-Universit/it
Bochum, Germany.
pear to show a time dependenceof anisotropy. These
observations
are in all cases associated
with
variations
Copyright 2000 by the American GeophysicalUnion.
in shear wave splitting. Gupta [1973]presentedap-
Paper number 2000JB900207.
parent observationsof very large temporal variations,
which were soon disputed on grounds that they might
also be explained by scattering and phase conversions
0148-0227/ 00/ 2000JB900207509.00
23,879
23,880
BOKELMANN
AND HARJES:
TEMPORAL
VELOCITY
within the crust [Ryall and Savage,1974]. Peacocket
a)
al. [1988]and Crampin et al. [1990]presentobservations of shear wave splitting from local events near
the Anza Gap in California, where one station apparently showsvery large variations of shearwave splitting
over just weeksbefore and after a nearby event. These
splitting delay time data, however, were disputed by
VARIATION
KTB FracNetwork (seen from above)
4OOO
OThree Components
ßVertical only
,,
2OOO
ß
Aster et al. [1990],who useda differentapproachof
•' ,•VB
o
estimating shear wave splitting: while Peacocket al.
[1988]and Crampinet al. [1990]extractedthe time
separationbetween the two shear wave phases(splitting delay) by visually inspectingwaveforms,Aster et
al. (1990) estimatedthe splitting delayas the length of
the time interval of linearly polarized shear wave motion. Most of the data both groups used were from
indiscriminate regional seismicity. We believe, however, that the argument can be sharpened considerably by specialtechniquesfor similarevents(doublets),
or at least
take
the
focal
mechanism
•,•hot
3
:
o
,
0
•,
.9.
4OOO
I1km
o
6000
4000
2000
0
2000
4000
6000
East (meters)
KTB FracNetwork (seen from South)
b)
which are nearly colocated and similar in source radiation pattern. In contrast, regional seismicity contains
eventsof strongly varying character. However, depending on the focal mechanisms,which may differ between
events, relative amplitudes of fast and slow shear wave
phasesmay differ substantially. This may affect either
method. An ideal approach would either use identical events
o
2000
2000
6E
10E
14E
II
4000
dif-
6000
ferencesinto account explicitly. In our study, similar
events are found by a cluster analysis procedure: Since
8OOO
46N j"Main
events are often remarkably similar in sourcelocation
6E 10E 14E
HB
and focal mechanism,a waveform matching technique
10000
allows determination of variations in shear wave split6000
4000
2000
0
2000
4000
6000
ting with unprecedentedresolution. Using severalclusEast (meters)
ters of events, we show that the delay between split Figure 1.
The "Kontinentale Tiefbohrung" KTB
shear waves indeed varies with time. Distance variation
in the Oberpfalz region of southeasternGermany; inof the events within a cluster are estimated with similar
strument deploymentduring the Frac Experiment 1994
resolution. Anisotropy,as recordedby the shearwave shown(a) top and (b) sideview. Three-component
receiversare shownby opencircles,single(vertical)comsplitting variesby •- 2.5% overjust a few hours.
ponent instrumentsby dots. Note the location of the
boreholeinstrument "Vorbohrung"VB in Figure lb),
which suppliedthe data in this paper, and the bottom
of the KTB drillhole("Hauptbohrung"HB), wherefluid
The "KontinentaleTiefbohrung"(German Continen- wasinjectedduringthe experiment. The locationof one
tal Deep Drilling Program) KTB is a scientificdeep particularlylarge event ("main event") and a surface
drilling project in the Oberpfalz region of southeastern shot (shot 3) are alsoshown.
Germany(Figure 1). Since1990,drillinghasprogressed
2. Experiment
to the final depth of 9101 m below the surface. One of
the last experiments performed at that depth was the
KTB Frac Experiment 1994, which is discussedin detail
large number of seismicevents were generatedduring
by Zobackand Harjes [1997].The goalsof that experi- eachof thesephases.There were 376 eventswith magment were related to studying the behavior of the crust
at that deeplevel in the crust during hydrofracturing;in
particular, onegoal wasto determinethe stressfield acting at that depth, which has never beforebeenobtained
by direct observation.Pumping started at 2202:15UTC
on December 17, 1994, and lasted for about a day. During and after the experiment, flow rate, pressure,and
nitudes of at least -2.1, the b value is 1.1. More details
of the event location distribution
and focal mechanisms
are givenin Zobackand Harjes [1997].
2.1.
Main Event and Anisotropy
During the 2-day interval a magnitude 1.2 event occurred, which was more than a magnitudelarger than
seismicitywere closelyrecorded(Figure 2). The exper- the secondlargest event. That event, which we refer to
iment was conductedin two phases:first the frac-phase as the "main event", a recordedwith excellentsignal-tolasting for -• 9 hours and secondthe main fluid injec- noise ratio on all stations temporarily installed for the
tion phase lasting until •- 23 hours after the start. A experimentand alsoon stationsof the German Regional
Flow
23.881
i
I
i
15
20
25
I
I
1
600 prE' 500
• 400
•-. 3oo
200
loo
I
o ,Ir•I=-'11
i
0
5
,..P
10
4O
Time [h], To=1994:351:22 02:15.0 UTC
Temporal Event Distributionand Pressure
b)•,60
I
I
5
10
I
I
I
I
I
15
20
25
30
35
•o
i4o-
• 30
,,>,
20
-•o
o
o
Time [hi, To: 1994:351:22 02:15.0 UTC, dt=10 min
Figure 2. (a) Fluid flow and (b) pressureand inducedseismicity(histogram,light shading)
during the KTB Frac Experiment. Events with magnitude-1.0 or larger are shownin the solid
histogram.
b)
a)
Component Orientations (Borehole Instrument)
Main Event on Original Components
Shear Waves
North
i
2 Az.317deg
•' 1 Az.43deg
~ horizontal
'A.•-- ' ' 6.0
62
6.4
6.6
Time in Seconds
Figure 3.
Borehole instrument orientation and waveformsof P and $ wavesfor the main event:
(a) the instrumentorientationas seenfrom above(the third componentis essentiallyverticalwith
positiveupward). (b) The "Main Event" (origintime, day 352 at 1526:30.225UTC) showing
excellentP and S waveforms,recordableat suchboreholesites. The S wave window is enlarged
to showthe typical S wavedelaybetweenthe first (horizontal)componentrelativeto the second.
That delay was consistentlyobservedfor all eventsin the boreholedata analyzed in this study.
By chance, orientations of components1 and 2 correspondmore or less to the slow and fast
directions(Az.-azimuth).
40
23,882
BOKELMANN AND HARJES:TEMPORAL VELOCITY VARIATION
a)
Shot
3
Raw
Data
Shear
Waves
•10• "110 11• 11, 116
' '•
Time in Seconds
•)
Lows-pass-Filtered
Data
Shear
•aves
Time in Seconds
Figure4. P andSwaveforms
fora surhce
sho[(sho[
3 onday350a[ 14:34:50.000
UTC)on
[heoriginal
componen[s.
Bo[hrawda[ainFigure
4aandlow-pass-fil[ered
da[a(8Hz,causal)
in
Figure4b showS phases
arrivingon [hesecond
componen•
earlier[hanon•hefirst. The
polariza[ion
agrees
qui[ewellwi[h[heorien[a[ion
of[hesecond
componen[.
Manualde[erminalionofspli[[ing
delayisdifficul[
[hough,
asdocumen[ed
by[hediffering
es[ima[es
of39msand
23msfor[hespli[[ing
delayts2- tsz,depending
onwhe[her
[heband-pass
is usedor no[. If
goodaccuracy
is required,
moresophis[ica[ed
[echniques
mus[be used.
Seismic
Network(GRSN)in southern
Germany
andthe pendiculardirection(Figure 1). Still, the S arrival is
GermanExperimental
Seismic
System(GERESS)ar- fasteron the secondcomponentfrom the surface.This
ray,whichislocated-• 160kmto thesoutheast.
Figure feature,an indication
of anisotropy,
is in generalagree3 showsa recordingof that event on the three channels mentwith previous
studiesin the area[Liischenet al.,
of theborehole
instrument
in the"Vorbohrung"
(VB). 1991],and it also agreeswith valuesfor the crust in
Theinstrument
orientation
wasdetermined
byusingP CentralEurope[Babu•kaand Cara,1991].FromFig-
wave polarization data recordedfrom four surfaceshots.
The instrument is tilted to the south in accordancewith
the local deviation of the boreholeout of the vertical to
the north. The early secondcomponent
is seenfor all
eventsin this study,whichis illustratedin Figure4
usinga calibrationshotarrivingfrom an almostper-
ure 3 we estimatethe time separationbetweenthe shear
waveson the two horizontalcomponentsas -• 14 ms for
the mainevent.We obtainthe relativedifference
6/•
betweenthe two shearwavevelocities/•1 and/•2, aver-
agedoverthe path length(fromHB to VB, Figure1),
if we can estimate the source-receiver distance D:
BOKELMANN
AND HARJES' TEMPORAL
VELOCITY
VARIATION
23,883
izontal interface. The latter would predict a radially
approximatelynorth-southpolarizedfast arrival. Similarly, the effectof conversionoff a dippinginterfacecan
with the velocityratio ? = a/• m x/•. This shows be ruled out, sincethe structure dips to the northeast.
thatwemayobtain6• frommeasurement
of threearrival times only with knowledgeof ?. We do not need
to know the source-receiver
2.2. Spatial Distribution
of Sources
distance D. The calibration
All of the eventsoccurin a spatially confinedregion:
Figure 5 showsdirectionsfrom the VB boreholeinstru(1.9%or 3.2%) than the mainevent,nomatter whiches- ment to the locationsof the 35 strongestevents(lower
timate of ts2-ts• we use. The most likely explanationis hemisphereplot). Incidenceis alwaysrather steepand
that effectiveanisotropygenerallyincreasestoward the all azimuths are in the third quadrant near 120ø. Time
surface, for example, due to increasingcrack-induced separations
t$• - tp (approximatelydistance)are also
anisotropy. This increase in anisotropy near the sur- given. The scatter indicatesthat distancesof the events
face is consistentwith severalvertical profiling studies from the VB instrumentvary by lessthan +8%, which
[e.g., Aster and Shearer,1991]. There are indications includes the statistical scatter of observations. In fact,
that seismic anisotropy near the KTB borehole may we will see in section 2.3 that most of these events can
alsobe affectedby foliationdirections[Rabbel,1994], be associatedwith severalclustersof very tight spatial
shot(Figure4) givesriseto muchlargervalues
of •
as was found to be the dominant
effect under the G ER-
ESSarray160km to the southeast[Bokelmann,
1995b].
However, even at 9 km depth, there may still be a component of crack-inducedanisotropy if high fluid pressuresare present. Figures 3 and 4 together give clear
evidence that the time separation t s2 -ts• is caused
by anisotropyand not by S-to-P conversionfrom a hor-
confinement.
An ideal experiment for testing the time-dependent
anisotropy(TDA) hypothesiswouldconsistof repeated
identicalsources(samefocalmechanismand samesource
time function)at the samelocation.High-qualitythreecomponentreceiverswould be distributed such that
identical ray paths through a crustal region are ob-
Histogram of S-P Differential Times
Direction to the Sources (Lower Hemisphere)
i
i
i
i
2O
i 9o
18
16
14
12
10
4 -
0
I
0
0.2
0.4
0.6
0.8
Time Delay in Seconds
Figure 5. Receiver(borehole
instrument)to sourcedirections
(lowerhemisphere)
and ts• - t•
differentialtime delaysfor the strongest35 events. The eventsare within a spatiMlyconfined
regionasdemonstrated
by the smallscatterin distance(• ts•- t•) andanglesfromthe borehole
instrument. Closerstudy showsthat many of them form groups(clusters)of eventshaving
remarkablewaveformsimilarity.This suggests
specialtechniques
of waveformmatchingto find
subtletemporalchangesof material parametersalongthe sharedpath.
23,884
BOKELMANN
AND HARJES: TEMPORAL
VELOCITY
VARIATION
i
12
--Frac
Phase
--
--Injection Phase-o
10
o
oo
o
o
o
o
O
0
o
co
O0
5
G•D o
o
o
I
o
o
of Main Event
oo
I
1•0
o
o
-Time
I
0
15
2•0
2•5
Time in hours
Figure 6. Temporaldistribution(in hours)of five clustersof similarearthquakesat a required
similarity level of at least 0.8 . The diameter of the symbolsshowsthe magnitude of the events
(maximumof-0.3, minimumof-1.2). A few otherclusters(2,3,4,6,7)are too confinedin time to
be of interest in this study.
served,which may be describedby a simple model. The
receiverswould preferably be located in deep boreholes
to circumvent complex interferencesfrom near-surface
structure and to produce a high signal-to-noiseratio.
Essentially,all of thesecriteria are met by the KTB Frac
Experiment: Figure 3 showsthat the three-component
instruments record remarkable P and S waveforms,even
at frequenciesof 150 Hz. As sourceswe use data from
well-defined clusters of events. We also have some prior
effectsthat must be soughtby highly accuratewaveform
comparisonof the events within a cluster.
As will be seenin section2.4, the degreeof similarity
between events of the same cluster is in fact remarkable. The existence of such "doublets" is well-known
for seismicenergyreleaseacrossmanydifferentlength
scales,rangingfrom earthquakeseisinology
[Gellerand
Mueller, 1980; Poupinet et al., 1984; Nadeau et al.,
1995]to inducedmicroseismicity
[Moriyaet al., 1994].
knowledge
of effectiveanisotropy
in the region[Liischen The long time window in this study containsboth
et al., 1991; Rabbel,1994]. The clusteranalysisis de- the P and S phasesand the coda and secondaryphases.
scribed in section 2.3.
Highly similar events(and only these)must sharethe
2.3.
Event
Clusters
samepath, sinceit is extremely unlikely that any other
path can causethe same(kinematicand dynamic)pat-
We obtain nearly colocatedeventswith similar source tern of reverberationsat the receiver. Clearly, locations
radiationby clusteranalysis[$chulte-Theis,
1995]of the of sucheventsare displacedfrom eachother by at most
100 strongestevents. In that procedure, waveform sim- a small fraction of the wavelength. Geller and Mueller
ilarity is measured by the maximum value of the cross [1980]estimatedthe maximumdistancebetweentwo
correlation(we requireat least0.8). Successively,
event eventsin a doublet, which are similar up to a wavelength
pairs are distributed into different clusters of events. A, as A/4. This was confirmedby numerousauthors
Generally,we obtain more clustersfor longertime windows, but with a reduced maximum cross correlation
value. For our purpose, we use time windowsof 1.5 s,
containingboth P and S waveenergy.
Could this procedurebias the time dependenceanalysis? Such a cluster analysisprocedurewould bias a
possibletime dependenceto smaller valuesrather than
to larger onesbecauseevent pairs displayingtemporal
variations are then lesssimilar and might therefore be
rejected from the cluster. On the other hand, the existenceof highly similar doubletsover sometime interval
showsthat temporal variationscan not be very large;
instead, subtle variations are likely to be second-order
thereafter,with somestudiessuggesting
that A/4 may
be overly restrictive, perhaps due to the strong effect
of somestation site responses.By the same reasoning,
theseeventsmust also have nearly identical focal mechanisms. Thus such events are ideal sourcesfor testing
the TDA hypothesis, since they give nearly identical
signalspropagatingalong nearly the same paths with
appreciabletime separations.
Out of the 100 strongestevents,57 can be associated
with 11 clusters at the similarity level of at least 0.80
(crosscorrelationmaximum). Sevenof theseclusters
are quite confinedin time, each lasting not longerthan
5 hours. On the other hand, four clusters are more
BOKELMANN AND HARJES: TEMPORAL VELOCITY VARIATION
spreadout in time with durationsof up to 12 hours.
We may conceptuallyseparatethe time spanof the experiment into three intervals,the frac phaseand the
early and late injectionphases,whichrefer to the injection periodsbeforeand after the main event. Two
of the clusterscoveralmostall three time intervals(see
Figure6). Theseare the mostvaluableclustersfor our
study,but we alsousethree other clusterswith useful
temporal coverageas they give us the opportunity to
study the variation of shear wave splitting over time
23,885
aresufficiently
large. Shearwavesagreeverywellon all
components:
To seesubtlechanges
in the waveforms,
closerinspectionis required.
For the five clusters studied in this paper, Table 1
givesinformation
for the associated
events.The relative time delaysA(tse -tsx) and A(tsx -tp) with respectto a reference
eventcanbe determined
with high
accuracy.Givenestimatesfor the reference
event,the
time separations
tse - ts• andts• - tp are determined
toestimate
thedifference
ofshear
wavevelocities
SlY(t).
and also between these intervals.
2.4.
Similarity
2.5. Waveform Matching Technique
of Waveforms
In Figure 7 we have seen how remarkablysimilar
Figure7 showsP and S waveforms
for the four events waveforms are within the same cluster. Now we turn to
in cluster 9. It is evident that both P and S waveforms
are indeedvery similar,at leastwheresignalamplitudes
the determinationof relative delaysbetweenthe phases
for two events,particularlythe relative splitting delay,
the observable
of prime interestin this paper. Are the
small variations between the waveforms visible in Fig-
a)4[Component
2 pWaveforms
for
Four
Events
._-_:
ure 7 systematicor not? To determinethesevariations,
we displaypairsof seismograms
u• (t) and u2(t) (each
component)usingan initial alignmentby waveformcorrelationon a large time windowfor eachcomponent,to
roughlyalign the waveforms.For eachcomponentwe
E
comparethe two seismograms
in detail(seeFigure8).
Figure8 showsthat waveforms
are virtually identical
(•4[Component
3
aside from the small time shifts. Thus the waveform
,...
comparison
is donemostconvenientlyby a slidingcross
correlation window, interpolating and monitoring the
<• 0
10
20
S Waveforms
30
Time in msec
delayof the maximum[seealsoBokelmann,
1992],dependingon time. We obtainmeasurements
of At• and
Ate with very high accuracy,far below the sampling
interval of 1 ms. The capabilityof measuringsuchsubsamplingtime delaysis in agreement
with otherstudies
for Four Events
2
[seee.g.,Poupinetet al.,1984]:Thelimitingaccuracy
is
controlledby waveformsimilarity,not by the sampling
interval. We especiallywant to determine variations
4
A(t$2 - t$•) of the delayt$2 - t$• betweenthe two
shearwave phases,which we obtain simply from the
two quantitiesAtse and At$•, whichwe can determine
Component 2
very well
I
2
i
i
i
i
i
i
i
Component 3
10
A(t$2-- rS1)-- (t$2-- rS1)-- (t$2-- t$1)ref
=
(rS2-- +ref- (ts
20
3•0 Time
• in'msec
Figure 7. P and $ waveformsfor eventsin cluster9,
+ref
)-
-- •S1
-
'
(2)
We usethe delayestimatesof the fast (S1) waveon
the secondcomponentand the slow(S2) waveon the
first component,sincetheir amplitudesare largest on
thesecomponents.Our delay estimatesare composed
of the crosscorrelationmaximumlagsaveragedwithin
(a) P ontheradial(2) andvertical(3) components,
and the chosen time windows for the fast and slow shear
(b) shows$ on all components.
Notethe highdegreeof wave phases.
similarityof eventswithin clusters.This is remarkable, The standarderror of the delay betweenthe two shear
considering
that dominantfrequencies
are•,, 150Hz. As waves
is 5 = V/(St$•)2 +(Stse)2, withthescatter
a consequence,
wemayusedetailedcomparisons
of such and 5tse of the delay relative to the referenceevent.
eventsto study time-dependentvariationsof medium
properties. For this figure, raw waveformdata have This is a very simplebut effectivemethodfor determinbeen interpolated,time-shifted,and amplitude-scaled ing the relativesplittingdelay(or moregenerallybeonly.
tweenany two phases).The approachdoesnot require
23,886
BOKELMANN
AND
HARJES:
TEMPORAL
Table 1: Clustersof Similar Events Used in This Studya
ml
ms
ms
ms
Cluster
ms
•2•
1
0.00
579.00
0.00-t-0.05
14.50
1.058
25
352-0421:19.808
-0.8
-0.86
578.14
-0.18+0.07
14.32
1.047
16
352-0519:32.028
-0.7
-O.O6
578.94
-0.10+0.03
14.40
1.051
19
352-0521:59.744
-0.7
-0.44
578.56
-0.10+0.06
14.40
1.052
76
352-0533.54.897
-1.2
-0.36
578.64
-0.15-t-0.05
14.35
1.048
34
352-0538:03.481
-0.9
-0.08
578.92
-0.15-t-0.05
14.35
1.048
29
352-0559:09.019
-0.8
-0.38
578.62
-0.10-t-0.06
14.40
1.052
67
352-0654:51.035
-1.1
-0.56
578.44
-0.13-t-0.05
14.37
1.050
43
352-1317:15.312
-0.9
-0.02
578.98
-0.26-t-0.06
14.24
1.040
49
352-1321:39.923
-1.0
-0.30
578.70
-0.23-t-0.08
14.27
1.042
(24) 352-0351:30.880-0.8
39
352-1432:19.950
-0.9
0.16
579.16
-0.23-t-0.08
14.27
1.044
48
352-1527:21.615
-1.0
0.16
579.16
-0.34+0.07
14.15
1.036
47
352-1531:22.067
-1.0
-O.36
578.64
-0.37+0.12
14.13
1.032
1.063
Cluster
26
352-0121:10.691
-0.8
(17) 352-0252:37.508-0.7
554.40
-0.06+0.21
13.94
0.00
554.50
0.00-t-0.05
14.00
1.067
32
352-0353:53.902
-0.8
0.18
554.68
-0.08+0.07
13.92
1.060
27
352-0530:39.227
-0.8
-0.34
554.16
-0.09-t-0.18
13.91
1.061
86
352-1338:34.507
-1.2
-0.08
554.42
-0.14+0.25
13.86
1.057
75
352-1514:51.335
-1.2
-0.02
554.48
-0.13-t-0.20
13.87
1.057
97
352-1739:11.455
-1.2
-0.02
554.48
-0.39-t-0.13
13.61
1.037
Cluster
0.00-t-0.05
13.00
1.027
352-2129:03.308
-0.3
-0.02
534.98
-0.04+0.04
12.96
1.024
42
352-2308:11.075
-0.9
-0.04
534.96
0.06ñ0.08
13.06
1.032
18
353-0040:43.140
-0.7
0.06
535.06
0.02+0.04
13.02
1.029
0.00
576.40
0.00:h0.10
14.50
1.063
0.04
576.44
-0.19ñ0.22
14.31
1.049
0.00
535.00
Cluster
(35) 352-0805:07.707-0.9
90
352-1510:50.274-1.2
Cluster
583.20
0.00+0.10
14.50
1.051
-0.9
-0.32
582.88
-0.21+0.09
14.29
1.036
62
352-1445:21.357
-1.0
0.38
583.58
-0.01+0.14
14.49
1.049
0.00
581.07
0.00+0.10
14.50
1.055
0.34
581.41
-0.05+0.22
14.45
1.050
Cluster
(20) 352-1153:46.358-0.7
69
352-1625:46.000
-1.1
The only interactive part in the approachis in choosing time windows for the procedure to operate on. At
that time the user is shown only the waveforms,and
no indication of the time of the event, which prohibits
subjective biases.
3.
Results
Another waveform example from cluster i is displayed in Figure 9, which showsa much smaller delay
t$2 -- rS1 compared with Figure 8. Figure 10 shows
showsthe results of this analysis for all events in cluster 1. Figure 10a comparesdifferent sets of time delay
t$1 -- t?. Two sets of estimates obtained from manual
picking scatter much more than the data set obtained
from our procedure. Obviously, the uncertainty of estimates from the waveform matching is at least an order
of magnitude smaller. The standard deviation is 0.3 ms.
This confirmsthat (1) all eventlocationsin the cluster
are very near eachother and (2) relative arrival times
can be determined
with accuracies of at least a third of
the samplinginterval which is I ms. The limits of uncertainty which can be reachedis primarily determined
by waveformsimilarity,not by the samplingrate [Bokelmann,1995b].The quantityts•- tp = D f(/31,c•),with
f(/31,c•) -- [1//•1 - 1/c•], which is computedfor each
5
352-1323:04.487
0.00
between the waveforms of S1 and S2.
11
44
(61) 352-1115:20.956-1.0
the approach makes no assumption about the relation
9
3
(8) 352-1931:49.888-0.5
ple [seefor exampleBokelmann,1992]. In particular,
10
-0.10
VARIATION
strong assumptionson the type of anisotropycausing
the effects.Basically,the weak assumptionit doesmake
is that as elastic propertiesvary with time by a small
amount, eachobservedwave phasechangesprimarily in
travel time and amplitude but not in waveformshape.
This assumptioncan be justified from Fermat's princi-
Event
6OriginTime, Mag,Actsl_p,t•l_p,
Day:-UTC
VELOCITY
event, dependson the source-receiverdistance D and
on medium velocities/31 and c•. Since D and the latter
are unrelated quantities, we infer from the constancy
of t$1 -t? for all events in cluster 1 that both D •
8
constand f(/3•, c•) , const= c. The formermeansthat
distancesvary by < 2 meters. The latter imposesa con-
a A few eventsare left out due to low signal/noise:Theseare events77,
89, and 95 for cluster 1 and event 94 for cluster 9, Event 7 was discarded
dition relating the variationsof c•(t) and /31(t). If we
write c•(t) = c•(0)+ Ace(t)and/3•(t) =/3• (0)+ A/3•(t),
from cluster 5, becausethe magnitude was very different from the others
we obtain
(-0.4).
I q-C•1(0)
I - c/• (0)
• Eventnumbersweregivenbasedon decreasing
magnitude.Reference
events(24, 17, 8, etc) are indicatedby parentheses.
Theseeventsare used
(t),
(a)
tocompute
A/•. Forthese
events,
uncertainties
forA(ts2-ts•)arespecified
subjectively.
c A refersto the differenceof a quantity with respectto the value for the
referenceevent. In both cases,(1) for determiningthe relative shearwave
splittingdelayand (2) the relativedelaybetweenfasterS waveand P wave,
the high-resolution
methodis used;accuracies
for ACts2_s•are typicalfor
both.
d Herets2-s• = rs2 - ts• andts•-? = tsl - t?.
e Here 5/3 is the differencein shearwavevelocitybetweenfast and slow
whichsuggests
Ace(t) m 2.4A/3•(t). However,this can
be explainedmorenaturally,if both c•(t) and/31(t) are
constant. After all, the constancyof f(/•l, o•) for varying c• and /3• may not be coincidental. Therefore it
appearsmore likely that the temporal changeoccursin
the slower shear velocity/32.
Observedshearwavesplitting delaysshowa remarkable decrease,which is statistically significant. With
velocity
averaged
overthe ray path5/• = /32-/3•//31withfll ---6.2/•¾-• estimates of ts• - t• and ts2 - ts• we obtain estimates
km/s.
of5/•fromequation
(1) using
r/ - x/•. Thetimede-
BOKELMANN AND HARJES: TEMPORAL VELOCITY VARIATION
23,887
EventComparison
afterFitting:352152715(solid)and352035125(dashedline),D•fference:
-0.32 +/- 0.06msec
>'
ß
•
•Component
1
,....',.•_,
•
-0.5
-• •
(D
t0.15
+0.02
msec'•
•
-1
-1.51
Ii
•
•
I
I
I Component
3
0.5
I
I
I
I
-•
.
•
•
I
I
f•-'..- -.
•
I
f' \
•o
.5
1
i
I
I
10
20
30
Time in msec 40
Figure 8.
Illustrationof the waveformmatchingtechniquefor determiningrelative delays
betweentwo seismograms.Overlayingtwo three-component
seismograms
allowsdetermination
oftherelative
splitting
delayA(t$2- tsz)- (rs2-*ref
ref (thickdashedline,in ms),The
•$2)-(t$• -tsz)
crosscorrelationoperateson a sliding15-mswindowwith a Hanning-typetaper. Averagingover
the windowsshowsby the verticaldashedlinesresultsin a relativesplittingdelayA(ts2 - tsz)=
-0.32 + 0.06 ms. Clearly, this procedureallowssubsampleaccuracy.
EventComparison
afterFitting:352051925(solid)and352035125(dashed
line),Difference:
-0.10 +/- 0.03 msec
1[ ' ' ' ' ' ' ',f•,' i' ' ' '
>, 0.5 Component 1
.•
-0.5
•
0.19
Component
3
A
"
x• •x•
\.,,/'"
10
/ "•
/ ',\
20
-• •
" ••I
I
30
'•/'•
'/ ',•
;/ ',/
/•
4•
,/•,'
/ '•
'1
,
I
50
60 Timeinmsec
Figure 9. Waveforms
andrelativesplitting
delays
foranother
eventpairfromcluster1 (24-25).
The timelagis shownonlyfor the chosen
timewindows
of S1 andS2 (displayasin Figure8).
After approximatealignmentthe pair shows"earlier"time delaysfor both shearwavephases,
while the examplein Figure 8 showeda later S2. This documentsan increasingtime delay
between
the two shear waves from event to event.
23,888
BOKELMANN AND HARJES' TEMPORAL VELOCITY VARIATION
a)Cluster
1-Estimating
tsl-tP
b) Observed Splitting Delay
,
590
585
+++
++
.06
+
)55
+
++
c) Temporal Variation of S Velocity Difference
,
.05
)45
580
)35
575
+: Routine Processing
x: Own Manual
.03
Estimates
o: Relative Delay Analysis
10
20
Time in hours
)25
0
10
20
Time in hours
0
10
20
Time in hours
Figure 10. Resultsfrom cluster1. (a) A comparisonof differentsetsof is1 - tp valuesfor the
eventsin cluster 1. Plusesindicate data from routine manual picking, crossesindicate data from
an additional attempt using manual picking, and circlesindicate valuesobtained from waveform
matching. Clearly, the waveformmatchingproducesstandard errors,which are at least an order
of magnitude smaller than those of manual picking, and distance variations of the events in
cluster 1 are all within 10 meters (seetext). (b) Observedvaluesof relative splitting delay as
a functionof timeduringthe experiment
assuming
r/= x/• (timeis givenrelativeto 1994,day
351 at 2202:15.0UTC), (c) The shearwavevelocitydifferencein percentas a function of time.
There is a clear decreaseby -• 2% overjust 12 hours.
pendenceof that effectis clearly statistically significant
with a variation of -• 2% within
12 hours. The effect
of the time window choice has been extensively tested
insuring that it has only minor influenceon the result.
Hence the method is objective in the sensethat its resuits do not contain subjective biases.
eventerrorsenterintotheoffsetlevelof rS/}(absolute
measurement),
say(•/• + •/•2)/2,morestrongly
byan
orderof magnitude.
Theimp•ortant^feature
is that the
timedependence
of•/•, say•/•2- 6fi2isnotaffected.
In addition to the larger errors in the relative delaysof the referenceeventdifferentialarrival times,one
must check effectsof timing errors during the experi3.1. Error Analysis
ment. Generally,timing errorsmay be groupedinto two
types,
both of which might affectthe waveformdata of
Whicherrors
mayaffectestimates
of6/•andmayperour
experiment:
First, timing might sufferfrom a conhapscause
artifacts?We notethat results
of •/• are
stant
offset,
and
second,there might be a drift in the
alsoaffectedby errorsin referenceeventvalues[seealso
Bokelrnann,2000]. To understandits influence,con- time signal, which would effectivelyspread or shrink
the waveform data along the time axis. Since we exsider that equation (1) is in our procedureusedas
clusivelyuse relative arrival times in this study, errors
of the first type can not affect our analysis. On the
other hand, a time drift with constant rate over the 0.6
s windowcontainingboth P and S phases,wouldspread
or shrink waveformsand consequentlyaffect estimates
of ts• - tp and ts2 - ts• similarly by a constant facwhere the first terms in numerator
and denominator
tor. That is the caseif we use either equation(1) or
(referenceeventvalues)are of size 102 and 103 ms. (4). Equation (1) showsthat throughthe quotient,this
outandtheestimates
of 6/• areunThese valuescarry errorsof the order of 1 ms. The other factorwouldcancel
(ts2
--tp)
ts)er
+X(ts2
- ts)V-r/ ' (4)
rer+A(t$-tP)
termsare of size10-• msandcarryerrorsof 10-2 ms. affected. For the data set in this paper, in fact, we are
Weareinterested
inthetemporal
variation
of• be-
fortunate to have good knowledge of the true timing
tweentwoevents,
say•2 -6fi•. Wechoose
thesecond signal during the experiment. Variation in this timing
event as the referenceevent and develop the denomina-
signal is only of the order of microseconds.However,
also in the caseof a less-knowntiming signal, the sugis of order10-6). Thenweseethat the temporalvaria- gested type of analysis is robust in the sensethat it
tion is of size10-2 (percent)with errorsof order10-3 offersat the same time very high accuraciesand is also
arisingprimarily from the errorsin A(t$1 - tp). The insensitiveto timing errors, which may easily bias other
reference events errors enter into the order 10-4. Theretypes of studiesof time-dependenteffects,at least if the
fore,thetemporal
variation
of• isnotaffected
byref- size of the temporal variations are of the order of only
erence event errors. On the other hand, the reference a few percent.
tor of •/•2 keeping
thefirst-order
term(thenextterm
BOKELMANN
3.2.
Other
AND HARJES: TEMPORAL
3.3.
Clusters
VELOCITY
VARIATION
23,889
Total Temporal Variation
In order to study temporal variationsalong the total
duration of the experiment we need to take into account
The samewaveformanalysiswas applied to data from
the other clusters. For cluster 10, waveform similarity
different
absolute
levelof 5• fordifferent
is generallysomewhatsmallerthan for cluster 1 (Ta- theperhaps
ble 1). Nevertheless,we obtain a similar result: small clusters.
A conservative
approac•h
of doingthisis to fit
shearwavesplitting variationsfor event pairs with time a straight line to the valuesof 5fi for eacheventand to
separationsof just a few hours. For larger separation, constrain these extrapolations to coincidein the center
splitting delays vary more. Differential times t$1 - tp of overlappingtime intervals or the center of the gap
again show a near constancywith time, while splitting for nonoverlappingtime intervals. This proceduregives
delays
decrease
significantly.
The5• clearly
varies
with the temporal variation shownin Figure 12, where the
time consistent with the results of cluster 1, although adjustmentis in agreementwith the data for the three
uncertaintiesare larger for cluster 10. Clusters 1 and small clusterscoveringthe transition (Table 1), which
10 essentially have similar temporal coverageand are shows
a milddecrease
in 5•. Thetotalvariation
is of
hence easily comparable: Both consistentlyshow that the order of 2%. Interestingly, the differenceof shear
the difference between the two shear wave velocities dewave velocities decreasestill the main event and stays
creases with
time.
constant after that. Polarization
directions do not show
How does5• behaveafter the mainevent? The suchsystematicchangeswithin clusters,neither for P
largest remaining cluster, cluster 9, coversa time inter- waves nor for the faster S waves.
val of 11 hours after the main event. Figure l l shows
waveforms and relative splitting delays for an example event pair from cluster 9. Waveform similarity for 4. Discussion
events in cluster 9 is remarkable. Visually comparing
The main results from this study are (1) that the
the event pairs, we notice that time variation of shear
medium
propertiesdo in fact vary with time and (2)
wave splitting is clearly small. This is also apparent
that
we
may
indeedobservesucheffects.However,it is
in the results (Table 1), where uncertaintiesare very
also
of
great
interestto understandthe nature of this
small, reflectingthe excellent similarity. Variations of
time
dependence.
We will in the followingsectiondisdifferential times t$l -tp are never larger than 0.6 ms.
cuss
possible
causes
for the temporal variation which
Also the shear wave delays hardly vary: Temporal variin mechaniation of shear wave splitting in the latter part of the are (1) the injectedfluid, and (2) changes
cal stressdueto (a) the inducedseismicityor (b) solid
experiment is negligiblefor cluster 9.
EventComparison
afterFitting:352212855(solid)and352193145(dashed
line),Difference:
-0.04 +/- 0.04 msec
Componont•
•"
11
%
i
i
I
,
Component
2
(1)•0
, /'%
•
•
, •'•
i
•
,,,
i
•
•
i
•
I
-
rr
0.03
__21
I
i
0.5
Comp'onon,
:•
/
--
>•o
-1'
I
I
10
I
I
20
I
I
30
I
40
50
Timein msec
Figure l l. Waveformsand relativesplittingdelaysfor an exampleeventpair from cluster9
(events8 and 3). The displayis similarto Figure 8.
23,890
BOKELMANN
AND HARJES'
TEMPORAL
VELOCITY
VARIATION
I
Main ,Event
1.06
1.05
()
1 .o4
1.03
0
5
10
15
Time
20
25
in hours
Figure 12.
Temporal variation of shear wave splitting over the length of the experiment
(clusters1 and 9). Seethe text for discussion.
Earth tides. Can any of thesefactorsexplain a variation
of-• 2%in •/• (oralternatively
in
jes, 1997].Theseare consistent
with the regionalstress
field (NW-SE compression),
suggesting
that a substan-
Figure 13 showsthe volume of the first Fresnel zone tial fraction of the releasedseismicenergyis due to a refor the dominant frequencyof 150 Hz, which consists laxation of tectonicstress.Changesof stressdifferences
of all pointsgivingrise to time delaysAT _<T/4 rel- may have a considerableeffect on effective anisotropy
ative to the first arrival time. A good approximation [e.g., Zatsepinand Crampin, 1997], especiallywhen
to the wave field can be made by assumingthat only fluid-filled cracks are present throughout the medium.
this volumecontributesto the data [$niederand Lo- Events in this study produced stressdrops on the order
maz, 1996].The volumeof that bodyis approximately of 10s Pa (bars)andfracturesizesof fewtensof meters.
l08 m3. Prior to the main event, about 126 m3 of On the other hand, the sensitivity of relative velocity
fluid was injected at 9 km depth. We take the most changeswith respect to stresschangeshas been sugfavorableview and assumethat all fluid propagates gestedto be of order10-s or even10-7 Pa-1 nearthe
into the Fresnel volume where it can affect the obsersurface[De Fazioet al., 1973].A studyin southernGervations. Although unlikely,it givesa viable argument many usingcontinuousman-madesignals[Baischand
1999]produced
an upperboundof 10-s
about whether the injected fluid can explain the obser- Bokelmann,
vationsat all. The 126m3 offluidmaychange
themean Pa-1 With an averagestressdropof several104Pa
fluid contentin the volume(mean excessporosity)by
(assumingthat stressreleaseoccurson a "fault plane"
10-6. Can sucha smallchangein fluid contentcause throughoutthe Fresnelvolume) we may, in principle,
su•ciently large variationsin •/? Randomlyoriented explain the magnitude of the observedeffectsby teccircular cracksas computedfrom a self-consistent
ap- tonic stresschangein an anisotropicmedium containing
proachfollowingO'ConnellandBudiansky
[1974]would fluid-filled cracks. This mechanism should also explain
require a mean excessporosity of the order of a few percent to explain the temporal variation of 2%. This is
further characteristicsof the observations,namely that
4 ordersof magnitudelarger than can be explainedby
variation occurs(most likely) in the slowerS velocity
(1)5• decreases
withincreasing
stress
dropand(2)the
the injectedfluid(10-6). Thusthe injectedfluidcannot /•2, which is polarized in NW-SE direction.
Figure 14 illustratesour interpretation. The cracks
explain the observedeffects,at least if the pore spaces
are isotropic.In the followingsectionwe will be dealing are oriented in or1direction, perpendicular to or3.Figure
14b showsthat tectonic stressdrop causesthe normal
with anisotropicpore spaces.
stress on the cracks an = or3to increase. This leads to
4.1.
Tectonic
Release
The hydraulicfracturing set of a largenumberof seis-
miceventswith strike-slipmechanism
[ZobackandHat-
a flatteningof the cracks(decreaseof aspectratio c•).
Figure 14cpresentsthe resultsfrom a modelof random
alignedcracks[Hudson,1981]. Specifically,the two S
velocities are shown for a number of aspect ratios. We
BOKELMANN
Partial KTB frac-Network
I
-3000
J
-
AND HARJES' TEMPORAL
VELOCITY
VARIATION
23,891
(seen from South) see that indeed 5• = • -/• decreasesfor decreasing
aspectratios for all angleswhich produceconsiderable
I
S wavesplitting. Furthermore,this model predictsthat
the slower S wave velocity •2 varies, while • is constant. We can match the size of the observationsby a
1% changeof the aspectratio if the angleof propagation
is 45ø from the symmetry axis. This would be the case
if the cracks were roughly aligned with the foliation,
VB
whichdipswith -• 50ø to the northeast[Harjes et al.,
1997]. A moreverticalorientationof the crackswould
requirelarger changesin aspectratio in this model.
-4000
We illustrated that the nature of the velocity change
canbe explainedby a changein meanaspectratio of the
cracks. The crack density was chosenas e - 0.05. This
model is certainly simplistic. A more realistic treatment would also allow a varying orientation distribu-5000
tion of opencracks[e.g.,Zatsepinand Crampin,1997].
-
However,this simplecrack model doesqualitatively reproducethe relevantfeaturesof the observedtemporal
variation.
4.2.
-6000
Solid-Earth
Another
-
Tides
stress field which we need to address is that
arisingfrom solidEarth tides, whichgive riseto periodic
changesof stresswithin the crust. These are of the or-
>
o
derof 103Pa, smallerthan the coseismic
stresschanges.
J•
However,they act throughout the entire crust. Unfortunately, the time duration of our experiment was not
o
longenough
to ruleouta periodic
change
of 5• with
>
-7000
period of about 1/day and 2/day directly. Therefore
-
-8000-
the possibility of tidal effects cannot be ruled out entirely given the uncertaintiesinvolved. A longer-time
observationof shear wave splitting using doublet data
would be an important test to better distinguishthis
effect from tectonic causes. In any case, such a test
givesa measurefor the extent of open compliantcracks
in deeper levels of the crust. The possibilityfor such
a long-time monitoring of seismicvelocitieshas been
Main
addressed
for continuouslygenerated(machine)signals
from the surface[Bokelmannand Baisch,1999;Baisch
and Bokelmann,1999].
-9000
4.3.
-
Comparison With
Other Studies
Poupinetet al. [1984]found temporalvariationsof
i
-1000
-500
0
i
500
1000
East (meters)
Figure 13. Closerview of the geometryof the experiment: A circle of 50 m radius is drawn around the point
of fluid injection(HauptbohrungHB); The rayspropagate from the sourcelocation,for example,"main"to
the VorbohrungVB. The first Fresnelvolume(zone)
is shownfor 150 Hz (100 timesexaggerated
in width).
S velocitiesby • 0.2% before and after a magnitude
5.9 event in California using a coherence-based
method
operating on full seismogramsof events with similar
waveform. In contrast to our paper, the split shear
waveswere not specificallyaddressed,but that method
is also capable of resolvingvery small time shifts. In
fact, the size of temporal variation that it suggestsis
similar to the effectin our paper. The ideal approachto
studyingtime-dependentvariationsis certainly the approachusingdoubletsto eliminate alternative apparent
time dependences(distancevariation of events;variation of focalmechanism),unlessthere are sourceswhich
The observedshear wave effect can be interpreted as
are indeedfixed in space[Bokelmann,1997]. A num-
beingdue to contributions
from this Fresnelvolume.
ber of studies have claimed to observe temporal varia-
23,892
BOKELMANN
AND HARJES:
TEMPORAL
a)
VELOCITY
VARIATION
b)
Mohr circle
before stress drop
er•.•
foliation
cracks
c)
Shear Wave Velocitiesfor a Numberof Crack Aspect Ratios
i
o 3.55
i
i
I
i
i
I
_
i
S1
_
._
• 3.5
•3.45
o
03
3.4
3.350
......
..j:
;o
;o
.,
S2
;o
s'o
6'o
90
AnglefromSymmetry
Axis(•,•) indegrees
Figure 14. (a) Orientationof maximumand xninimumhorizontalcompressive
stress(O'H,O'h).
Cracksopeningnormal to c3 (= c• if c2 vertical) havesimilar strike as the foliation direction.
(b) Increasein cracknormalstressc, - c3 duringa quake,leadingto a flatteningof the cracks
(decreasingaspectratio a). (c) Dependenceof S velocitieson crack aspectratio. Decreasing
aspect ratio a producesa decreasein 6fl for angleslarger than 30ø. Note that the slower$
velocity fi2, which is polarized in NE-SW direction, is predicted to vary, while the faster fi• is
constant.
tions,mostlyin the ratio a/fi [seeLukk and Nersesov, upper bound does not preclude temporal variation of
1978]. However,observation
of smallvariationsposes the size observedin our study or in that of Poupinet
very strongrequirementson eliminating errorsfrom dis- et al. [1984]. In our experience,it is often difficult
tance variation and timing, which are very difficult to to unambiguouslydetermine time delaysbetweenfast
satisfy with other methods than ones based on wave- and slow shear wave phasesby visual inspectionof a
form correlationof doublets. Also for the Anza Gap singlethree-componentseismogram.In fact, initial estidata set, seismic doublets have been used to show that matesof shearwave splitting from our data, which were
temporalvariationsare at most5 to 10% [Asteret al., basedon visual analysisalso showedlarge excursions.
1990],which disagreeswith large temporalvariations After allowing only clustersof doubletsand performwhichwereclaimedby otherresearchers
[Peacock
et al., ing the objective procedure,temporal variationsalmost
1988; Crampinet al., 1990];on the other hand, that vanishedand the stable smaller-scalepattern emerged.
BOKELMANN
5.
AND
HARJES:
TEMPORAL
VELOCITY
VARIATION
23,893
Bokelmann, G.H.R., Azimuth and slownessdeviations from
Conclusions
the GERESS regional array, Bull. Seismol. Soc. Am.,
85, 5, 1456-1463, 1995b.
The main result of this study is that shearwave splitting hasbeen observedto vary with time at a depth level Bokelmann, G.H.R., Messung zeitlicher Variationen von
Wellengeschwindigkeiten
in der Erdkruste, Mitt. Dtsch.
of 4 to 9 kin. The analysis procedure is objective in
Geophys. Ges., 3, 6-12, 1997.
the sensethat subjective influencesare eliminated; im- Bokelmann, G.H.R., A method for resolving small tempoportantly, the procedure is able to determine temporal
ral variations of effective elastic properties, SEG Special
Volume, in press, 2000.
variations of the relative shear wave velocity difference
with veryhighaccuracies
(• 10-4). The anisotropy
itself, as documentedby shear wave splitting, has a size
of ~1% and is generally consistentwith previousobservations. Intriguingly, shear wave splitting decreases
for ~ 12 hours and is essentially constant afterward.
No assumptionis made that the events are colocated.
Nevertheless,the analysis showedthat the events actually do occur very closeto each other, confirmingthe
hypothesisof similar events. No progressivechangeof
sourcelocation is apparent from either the time delays
nor the P wave polarizations.
It is interesting that the changein medium configuration occurs apparently almost instantaneously, with
a lag of at most a few hours. The injected fluid volume
cannot explain the temporal changeby itself. On the
other hand, the effect of the stressdrop due to induced
seismicitymay possibly explain the data if fluid-filled
cracksare presentat depth larger than 4 kin. A longertime experiment in a deep borehole as KTB would help
to further understand the mechanismsof temporal variation in the Earth's
crust.
Bokelmann, G.H.R., and S. Baisch, Nature of narrow-band
signalsat 2.083Hz, Bull. Seismol.Soc. Am., 89, 156-164,
1999.
Crampin, S., and S.V. Zatsepin, Changesof strain before
earthquakes;The possibilityof routine monitoringof both
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Acknowledgments.
We wish to thank the KTB team
which assistedin performing the injection experiment. Markus
Janik, Hartwig Schulte-Theis,Matthias Bahlo, and Thorsten Lukk, A.A., and I.L. Nersesov,Character of temporal changes
in the velocities of elastic waves in the Earth's crust of the
Bfisselbergprovided valuable help in analyzing the waveGarm region, Izv. Phys. Sol. Earth, 1•, 387-396, 1978.
form data. We profired from discussionswith Fritz Rumreel. Reviews by George Helffrich, Peter Shearer, and an L/ischen, E., W. SSllner, A. Hohrath, and W. Rabbel, Inteanonymousreviewer greatly improved the manuscript.
grated P- and S-waveboreholeexperimentsat the KTB
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