Feb 10, 1994 - We assumed that a complex pressure time history could cause ...... network on Mount St. Helena Volcano and observations of long-period ...
JOURNAL OF GEOPHYSICAL
RESEARCH,
VOL. 99, NO. B2, PAGES 2637-2652, FEBRUARY
10, 1994
Array measurements and modelingof sourcesof shallow volcanic tremor at Kilauea Volcano, Hawaii Peter Goldstein
EarthSciences Division,LawrenceLivermoreNationalLaboratory, Livermore,California Bernard
Chouet
U.S. Geological Survey,MenloPark, California
We showthat resonances in near-surfacelayers (path effects),or a combinationof near-surfaceresonances and sourceresonances(sourceeffects),could be responsiblefor the sharp spectral peaks found in the records of gas-piston activity and volcanic tremor recorded near Puu Oo crater on Kilauea Volcano, Hawaii. Two types of sources are found to be compatible with observations and can not be distinguished using our data. In one model, impulsive, explosion point sources are embedded in the Puu Oo structure. In the other model, impulsive, point sources of pressure excite the resonances of a fluid-filled
crackembeddedin that structure. Both modelsrequireshallowsources(z < 100 m) and display a strong dependence of the radiated spectrum on source depth over a depth range comparable to the overall thickness of the surficial layers. Based on these results, it may be possible to track changes in source depth through temporal changes in observed spectra, a potentially useful tool for monitoring volcanic activity at this site. We estimate the depth and spatial extent of volcanic tremor and gas-piston activity using two dense arrays with respective apertures of 800 and 120 rn located near Puu Oo. Measurements of slowness
(ray parameter)and azimuth as a function of time clearly indicate that the sourcesof volcanic tremor and gas-piston activity are located beneath or in close proximity to the Puu Oo crater at depths shallower than approximately 1 km. Based on slowness and particle motion analyses we find that the records of volcanic tremor and gas-piston events at Puu Oo consist of a complex combination of body and surface waves.
INTRODUCTION
Over the last 10 years observations of seismic activity in the vicinity of and prior to volcanic eruptionshave provided an extremely useful tool for the mitigationof volcanichazards[e.g.,Malone et aI., 1981; Koyanagi et al., 1988; Watanabe,1988; Okada et aI., 1990; Chouet et aI., 1993]. Unfortunately, existingpredictivecapabilitiesare primarily empirical and may be misleadingin new environments.Therefore an important goalshouldbe an improvedunderstanding of the physics of volcanic sourcesand the signals they produce. The primary aim of this study is to improve our understanding of volcanic sources in the vicinity of Puu Oo crater, Kilauea Volcano, Hawaii, through detailed measurementsof spatial and temporal variations in near-sourceseismicwavefields. An additional objectiveis to determine how well we can constrain the spatial extent of a volcanic sourceusing
magma movements, they would provide a useful tool for hazard mitigation. Models of Volcanic Tremor
At present, there are two most commonly cited models for volcanic tremor. Those whose spectral features are controlled by resonating sources and those whose spectral features are controlled by path and site effects. Resonating source models have been the
more popularof these [e.g., Aki et al., 1977; Aki and Koyanagi, 1981; Crosson and Bame, 1985; Chouet, 1985, 1988, 1992; Feblet and Chouet, 1982; Feblet,
1983; Ferrazziniand Aki, 1992; Chouetet al., 1993] because they can explain the common spectral peaks of tremor observed at widely separated seismicstations
[e.g., Aki and Koyanagi, 1981; Chouetet aI., 1993] and can produce temporal variations in tremor spectra
[e.g., Shimozuruet aI., 1966; Kubotera,1974; Kamo
near-source dense-array recordings of seismic signals. et aI., 1977; Aki and Koyanagi, 1981; Schick et al., If accurate images could be obtained and related to 1982;Feblet,1983]. However,a numberof studieshave
presentedevidencesuggestingthat path and/or site effects could also be.responsiblefor some of the observed
Copyright 1994 by the American Geophysical Union.
spectralfeaturesof shallowvolcanictremor[e.g.,Omer,
Paper number 93JB02639.
1950; $v[inakami, 1960; Malone et aI., 1981; Gordeev
0148-0227/94/93JB-02639 $05.00
et al., 1990; Gordeev,1992]. Evidenceof path or 2637
2638
GOLDSTEIN AND CHOUET: SOURCESOF VOLCANIC TREMOR AT KILAUEA
site effects include
observations
of similarities
between
We use three-component array data to show that
shallowtremor and shallowexplosionwaveforms[e.g., the tremor waveforms consistof a complex combination of body and surface waves, but note that the surface McNutt, 1986, Gotdeer,1992]. We present new evidencethat suggeststhat many of the spectral peaks found in tremor at Puu Oo could be
waves have the largest amplitudes and durations. By itself, this observation restricts a significant portion due to resonances in near-surface layers(path effects). of the tremor source to shallow depths. Previous In particular we show that shallow impulsive sources studiesat Puu Oo [e.g.,Ferrazziniet al., 1991]did not can generate time seriesand spectra that are consistent find significant evidence of body waves because they with those that are observed. However, we also show averaged data fr.om long time windows. We also note that spectral peaks due to resonancesof shallow fluid- that the timing of gas-pistonactivity (the fountaining filled cracks could be hidden by such path effects. and drainback of lava associatedwith the bursting of Furthermore, we show that changesin sourcedepth can large gas slugs that have risen through the magma explain observed temporal variations in the time series conduit and break through ponded lava at the surface and spectra. [Swanson et al., 1979]is wellcorrelated with the largest We present a variety of additional evidence that amplitudetremor signals[Dietel et al., 1989; Chouet suggests that path effects contribute significantly to and Shaw, 1991]. This also suggeststhat shallow the waveforms and spectra of tremor at Puu Oo. sources are an important contributor to the tremor For example, we demonstrate that sources of tremor wavefield. at Puu Oo are shallow and confined to the vicinity Puu 0o Crater of Puu Oo by locating them using high-resolution The Puu Oo crater was formed shortly after the frequency-wavenumbertechniques. Evidence of shallow sources of volcanic tremor has also been found in a initial phase of the January 1983 Kilauea eruption number of previous studies. For example, McNutt and became the focus of degassing and fountaining
[1986]notedsimilaritiesbetweenB typeevents,volcanic activitiesuntil July 1986 [Wolfe et al., 1988]. The explosions, and tremor at Pavlov volcano, Alaska that activity then evolved into a more sustained, low-level suggest a shallow source. Waveform cross-correlation outpouring of lava from a new vent at Kupaianaha, studiesby Furumotoet al. [1990]and Yamaokaet al. 3 km downrift from Puu Oo. The activity at Puu Oo [1991]alsoindicatea shallowsourceof volcanictremor simultaneously changed to low-level degassingmarked by intermittent bursts associated with stronger lava beneath Izu-Oshima volcano, Japan.
-.--180 rn-I Oldlavaflow
-•:.,• ...... ..... .-.:...:.:.....:.:...:.:.:.:.:.:. ..:.:.: ..:...:.....:.:.... ... .....:......... • .
Dense Arrays
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. . . *•,
....•
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30rn
lOOm
Fig. 1. Schematicillustrationof topography, magmatransportsystem,andearthstructurein the vicinity of Puu Oo crater.Potentialsourcemechanisms includeexplodingbubbles(grayovals)and/or a resonating fluid-filledcrack(rectangle)excitedby explodingbubbles.Tke upperboundaries of the magmatransport
system(thickdashed lines)areconstrained to beconsistent withsurface ventsandreliefinsideandoutside the crater.Interfacedepthsand layervelocitiesin the one-dimensional modelof Ferrazziniet al. [1991] are given at the left and right of the model, respectively.
GOLDSTEIN
AND CHOUET:
SOURCES OF VOLCANIC
TREMOR
AT KILAUEA
2639
movements confined within the vent. Continuous aperture array because of an apparent lack of signal weak tremor has characterized the persistent pressure coherence observed in the records at this spacing. The second array was deployed in a semicircular fluctuations in the magmatic system under Puu Oo since that time. configuration with a station spacing of 10 m and an Successive episodes of fountaining at Puu Oo apertureof 120m (Figure3). In additionto improving gradually built a spatter cone, which at the time of broadband coherence between adjacent elements, the the array experiments stood some 230 m above the size and geometry of this array was chosen so that surrounding lava fields and contained a cylindrical the statisticalanalysismethodof Aki [1957]couldbe crater with a diameter of 180 m and estimated applied and so that the array would sample roughly depth of 150 m. Two active vents were present at half a wavelength of Love wave at 5 Hz. The resulting the northeastern and southwestern perimeters of the small aperture array was very useful for measurements craterfloor (Figure 1). Visualobservations conducted of surface wave dispersion and modeling of the structure from
the
crater
rim
indicated
that
both
vents
beneathPuu Oo [Ferrazziniet al., 1991]. However,
acted
interchangeably as inlet and outlet to lava movement
the reduction in the array aperture may have been
during episodesof gas-pistonactivity [Dietel et al., unwarranted from the perspective of frequency-slowness 1989;ChouetandShaw,1991;FerrazziniandAki, 1992]. analysesfor two reasons. First, as indicated by bandDATA, INSTRUMENTATIONAND ARRAY DESIGN
pass-filtered records and narrow-band cross-correlation function from two stations separated by approximately
950 m (Figure4), the lackof coherence in the broadband During January-February1988 two densearrays of records was misleading; there were clearly coherent seismometers were deployedin the vicinity of Puu Oo arrivals propagating across the first array. We found crateron the eastrift of KilaueaVolcano[Dietelet al., similax correlation coefficients in a number of frequency
1989](Figure2). The primarygoalof thisdeploymentbands, especially in the vicinity of spectral peaks. was to obtain near-source dense-array recordings of Second, reducing the aperture of the array reduced seismicsignalsgeneratedby ongoingeruptive activity at its spatial resolution capabilities which are inversely Puu Oo. These experiments consisted of two separate proportional to the aperture. In other words, the main deploymentslocatedon a fiat expenseof denselycracked peak in the beam pattern of the second array is much pondedpahoehoelava flowscoveringan areaof 0.5km2 wider than that of the first array. Based on these I km west of Puu Oo. In the first deployment an observations, we suggest that naxrow-band or low-passarray was configuredas a mix of linear segmentsand filtered estimates of coherence should be used when grid-shaped patterns with an aperture of 800 m and determining array design. The array instrumentation consisted of 12 Mark approximatestationspacingof 100 m (Figure 3). After 10 daysthe arraywasreconfigured into a muchsmaller- Productsthree-component L-22-JD seismometers with 155ø20 ' 19ø30 '
155o10 '
I KILAUEA
-o
I CALDERA
PuuOø--Array (;yCrater
,x,,,
19o20 '
I
i
I
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Fig. 2 Map of the southeastflank of Kilauea volcanoshowinglocationsof Puu Oo crater, densearrays, and earthquakesusedin this study. The earthquakesare indicatedby solid diamondsand are labeled with their correspondingJulian day. The island of Hawaii is shown as an inset.
2640
GOLDSTEIN
AND CHOUET'
SOURCES OF VOLCANIC
TREMOR
AT KILAUEA
ß ,K6 ß
ARRAY
,
ARRAY 2 /
I
ß •-'.• ;• K2,L4 ß
Puu 0o Crater
amo
lOO METERS I
Fig. 3. Map of Puu Oo craterwith densearrays1 (solidcircles)and 2 (dashedlines).StationsK2 of array 1 and L4 of array 2 were approximatelycolocatedand were used to comparespectra from both arrays.
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Yig.4. (Top)Band-pass-filtered and(bo[•om)cross-correlated records ofvolcanic •remoronJanuary22, 1988. The band passis from 2.7 [o 3.3 Hs and con•Mnsoneof •he prominen•peaksin mos• of [he Iremot
andgas-piston spectra.Theserecords arefroms•a•ions in array1 wi•h•hegrea•es• separation (s•a[ions
A6 andK6 wi•h m950m separation; seeYigure3). They show•ha[ •hereis significan[ correla[ionbetween •he signal, within the band passof in[eres•,across•he wholearray.
GOLDSTEIN
0
5
AND CHOUET'
lb
SOURCES OF VOLCANIC
115
20
TREMOR
25
AT KILAUEA
50
2641
55
40
Time(s) Fig. 5. Typical vertical component seismograms of tremor, gas-piston activity and earthquakes recorded on both arrays. Except for the earthquake on January 31, all the records are from stations K2 and L4
which are nearly colocaied(seeFigure 3). The Julian day/event type, local magnitude,and depth of each event is listed at the right of each trace (VT -- volcanictremor, GP -- gas-pistonactivity, EQ earthquake)(seeTable 1).
sensitivity of 0.5 V/cm/s and 36 Mark Products gas-pistonactivity, and a shallow(depth 1.1 km) and vertical component L-4A seismometerswith sensitivity relatively deep (depth 9.3 km) earthquake(Table 1).
of I V/cm/s.
All the instrumentshad a natural The locations of these earthquakes are indicated in
frequency of 2 Hz. Recording was performed using 12 Figure 2. GEOS digital seismographs,each consisting of a six-
channel,16-bit recorder[Botchertitet al., 1985]. All GEOS recordeddata at 200 samples/s/channel usinga commontime basewith an accuracyof I ms among all
SHALLOW
POINT
FLUID-FILLED
SOURCES
OR
CRACKS?
the channels.
In this section we describe our modeling of the Gas-piston activity and volcanic tremor were two ground motions and spectra of volcanic tremor using persistentsourcesof seismicactivity at Puu Oo during point sources and resonating fluid-filled cracks. Since the array experiments. Small earthquakes to the much of our modeling was motivated and constrained by south of Puu Oo were recorded as well. Typical our array measurements and spectra] analysis, we begin vertical componentseismograms for tremor, gas-piston by summarizing the most important results of those activity and earthquakes recordedat two approximately measurements and their implications for our modeling. colocated sensors are shown in Figure 5. Events Details of these measurementswi]] be presentedin later occurring prior to January 30 were recorded on the sections. larger aperture array. These include an example Our spectral analyses can be summarize(] by the
of volcanictremor and three earthquakes(Table 1). followingobservations:(1) individualstationsiteeffects Unfortunately,there was no gas-pistonactivity during can not explain the sharp spectra] peaks in volcanic the period of deployment of the first array. Data tremorspectraat Puu Oo, and (2) there are significant from the smaller array include an exampleof tremor, variations in tremor spectra as a function of time.
2642
GOLDSTEIN
TABLE
AND
CHOUET:
SOURCES
OF VOLCANIC
TREMOR
AT KILAUEA
1. Source Parameters of Earthquake, Tremor, and Gas-Piston Events Discussed in the Text.
Date*
Origin Time, UT
Latitude,
Longitude, M L
deg
Depth, km
deg
Earthquakes
Jan. 23, 1988 (023) Jan. 26, 1988 (026) Jan. 27, 1988 (027) Jan. 31, 1988 (031) Feb. 10, 1988 (041)
0936:37 0728:56 0124:36 0028:54 1326:58 '•
19.3395 19.3453 19.3248 19.3457 19.3517 VolcanicEvents
-155.1190 -155.1283 -155.0572 -155.0978 -155.0415
Jan. 22, 1988 (022/VT) Jan. 30, 1988 (030/VT) Feb. 2, 1988 (033/GP)
0359 0259 0449
19.3915 19.3915 19.3915
-155.0992 -155.0992 -155.0992
1.8 2.2 3.9 1.8 3.4
1.72 6.19 5.22 1.08 9.27
*Each date is followedby its correspondingJulian day.
Results from our array measurementsindicate that resonances (path effects)couldbe responsible for many (1) mostof the coherentenergyin the tremorwavefields of the sharp peaks in tremor spectra. In this section we is comingfrom shallow(z < I km) sourcesin close compare results from this model with those predicted proximity to Puu Co crater, and (2) the observed by the resonating crackmodel[e.g.,Chouet,1992].We wavefieldscontainsignificantamountsof both body and focusour modelingon shallow(z < I km), nearby(r • surface waves. I km), sourcesbut haveconsidered a few examplesof The aboveobservationsand our experiencemodeling deeper more distant sources. Our simulations include
explosionwaveforms[e.g., Goldsteinet al., 1992] both body and surface waves. Based on the above motivated us to test the hypothesis that near-surface observationsand the followingsimulations,we conclude
- Surfoce --
--
- Z=100
iSurfoc'e ' Z=100 m
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-_DATA - o•/•P DATA
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Time(s)
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Frequency (Hz)
Fig. 6. Comparisonof groundvelocitiesand powerdensityspectraof shallow,impulsive,explosionpoint sourceswith those of observedtremor and gas-pistonevents.Each syntheticis a vertical component calculatedat a distanceof I km usingthe structureof Ferrazziniet al. [1991].The top two traces correspondto a singlesourceat the surfaceand 100 rn depth, respectively.The middle three traceswere obtainedby delayingand superposing multiplecopiesof a singlewaveformfrom a sourceat the surface, 100m, and 300 m depth,respectively. We usedrandomtime delaysbetween0 an.d2 s. The bottomtraces are examplesof tremor on the first array and gas-pistonactivity on the secondarray.
GOLDSTEIN
AND CHOUET:
SOURCES OF VOLCANIC the
TREMOR
data
for
AT KILAUEA
sources
between
2643
that resonancesin the surficial layers are a plausible
with
the
surface
and
explanationfor the observedspectralfeaturesbut that
100 m depth (Figure 6), and suggestthat a complex
time-dependent excitation of shallow point sourcesin a a resonating source couldalsobe present. We tried to determineif the spectralpegksin layered medium might explain the observations.
volcanic tremor and gas-pistonrecords from Puu Oo Shallow,resonating,fluid-filledcrackmodels[Chou•t, were sourceor path related using forward modeling. 1988, 1992]can alsoexcitenear surfaceresonances and We simulatedpath effectswith impulsive,point-source generate spectra that are consistent with the observed
explosionreflectivity syntheticseismograms [•f•'ller, spectral peaks. For example, Figure 7 shows synthetic 1985]in thelayeredvelocitystructureof Ferrazzini•t al. seismogramsand spectra of the vertical component of [1991]IFigure 1). We computedverticalcomponent ground velocity calculated in the near field of a vertisyntheticsfor a variety of sourcedepthsbetween0 and cal fluid-filled crack of rectangular shape embedded in 1 km and for epicentraldistancesbetween800 and 1200 a homogeneousmedium and the layered structure dem. We found that both time series and spectra were
rived by Ferrazziniet al. [1991](Figure 1). This cal-
quite sensitiveto sourcedepthwith muchlesssensitivity to distance. Spectra of sourcescloseto and within the uppermostlow-velociWlayersvariedrapidly with depth and have spectral features that are similar to those of
culation was performed using the discrete wavenumber
velocities in the homogeneous half space are the same
the data (Figure6).
as those found
We modeled the nearly continuoustremor and gaspiston data by assuming that a number of point sourceswere occurring in a random time sequence.We superposedidentical syntheticwaveformswith random time delaysbetween0 and 2 s. Waveformsand spectra of the resulting time seriesare in qualitative agreement
(Figure 1). For the fluid in the crackwe assumedan
0
2
4
6
8
1•)
method [Bouchon,1979]and propagator-based formalism of Chouet[1987]. Compressional and shearwave at the bottom
of Ferrazzini
et al's model
acoustic wave speed a- 1.25 km/s, a value compatible with a basaltic magma containing void fractions of gas
rangingup to a few percent[Chouet,1992]. A sketch of the source-receivergeometry used in our model is given in Figure 7. Crack excitation is provided
4
Fig. 7. Comparisonof ground velocities and power density spectra of a vertical fluid-filled crack with those of observedvolcanic tremor and gas-pistonevents.Each synthetic is a vertical component calculated at a distance of 1 km and an azimuth of 25 ø. The top two traces correspondto a single step in pressure at the top of a crack that intersectsthe free surfacein a homogeneoushalf-spaceand the layered structure
of Ferrazzir•iet al. [1991],respectively. The next three traceswereobtainedby delayingand superposing
multiple copiesof crack syntheticsin a layered medium. The first two correspondto sourcesat the top
(z = 0 m) and middle(z -- 100 m), respectively, of a crackthat intersects the free surface(h -- 0 m). The third tracecorresponds to a crackwith a sourceat its midpoint(z -- 400 m) whosetop is at
a depth of 300 m. Medium type, number of sources,and source and crack depth are identified at the left of the spectra. Time delays were the same as those used to superposepoint sources. The bottom traces are examplesof volcanictremor on the first array and gas-pistonactivity on the secondarray. The source-receivergeometry is shown in Cartesian coordinates as an inset. The dark patches on the crack axis represent the locations and extent of the pressuretransients triggering crack resonance.
2644
GOLDSTEIN AND CHOUET' SOURCES OF VOLCANIC TREMOR AT KILAUEA
by a suddenstep in pressureappliedover a small area surficiallayersagreequalitatively with our observations of the crack wall located on the main crack axis near (Figure7}. Again,we find that the spectrafor sources the top, center,or bottomof the crack.The top of the within the uppermost low-velocity layers depend on crack is buried at a depth/• and the receiveris located source depth and structure. Differencesbetween the at a distancer from the epicenterand azimuth • from shallow sourcesynthetics and observedspectra could be accountedfor by changing the crack geometry or the vertical crack plane. We assumed a crack with horizontal and vertical
dimensionsof 100 and 200 m, respectively,on a plane
acoustic velocity in the fluid. In contrast, spectra correspondingto resonatingcracksbelow the surficial
alignedwith the rift axis as definedby the eruption layers(a=105, 300, and 500 m} are fairly insensitive
fissuresin Figure 1, and choser = 1000m and • = 25ø to source depth and have much less bandwidth and for the receiver. This receiver position coincideswith complexity than those of the data. These results that shallow(a < 105m} resonating cracksmay the location K2 of the large array or location L4 of suggest the small array (seeFigure 2). We considered four be a component of a complex sourceand structure at crack depths, /• = 0 m, /• = 105m, /• = 300m, and Puu Oo. In combination with results from the following array h = 500 m, and three sourcedepths along the main and spectral analysis, the above results suggestthat crackaxis,z = h, z = h+ 100m, and z = h+ 200 m. The volcanic tremor and gas-piston activity at Puu 0o syntheticswereobtainedovera bandwidthof 10 Hz or crater are generatedby a near-surfacesourceand that greaterusinga windowof 6 s that includesthe entire path effects or a combinationof sourceand path effects historyof groundmotionat thereceiver(e.g.,Figure7). could explain the observedspectral complexity. We assumedthat a complex pressuretime history could cause sequentialexcitation of the crack. We SPECTRAL ANALYSIS modeledthis complexsourceexcitationby superposing identical waveforms with the same random time delays In this section we compare array-averagedspecused to superposepoint sourcesynthetics.Waveforms tra from a variety of seismicevents and show that and spectra of resonating cracks that penetrate the individual-station site effects are an unlikely explana-
Hdrmoni'c Tre•or
LargeArray I
Hdrmoni'c Tre•nor SmallArray
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Frequency (Hz) :Fi•. 8. A•a,y-avera,•ed power density spectra of ve•tica,] components of events sbow• i• :Fi•u.re 5. "['be spectrum of each station is obtained using It, overlapping, 30-s long windows and 40 s of data.
GOLDSTEIN
AND
CHOUET:
SOURCES
OF VOLCANIC
TREMOR
AT KILAUEA
2645
tion for mostof the largespectralpeaksin the volcanic array (event 022). If a site effect were responsible tremorandgas-piston eventsobserved at Puu 0o. We for the sharp peaks in the tremor spectra, we would alsoshowthat there are significanttemporalvariations expect to see it in the spectra of earthquakesas well. in tremorspectraandsuggest that thesevariationsmay This argument is lesspersuasivefor tremor recordedon the secondarray (events030 and 033) becausethere be dueto temporalvariationsin sourcedepth. With the exception of an earthquake on January 23, are some notable peaks in the spectra of earthquakes spectralestimateswere derivedby computinga power recordedon that array (events031 and041). However, densityspectrum[Jenkinsand Watts,1968]for each the locations of spectral peaks of tremor recorded on vertical component of motion using 11 successive30- the first and second arrays are well correlated, while s windows with 1-s overlap, and then averaging the the peaksin the spectraof earthquakesrecordedon the resultingspectraacrossthe array. The sameprocedure secondarray appear to be more randomly distributed. Evidence of the temporal variability of tremor was applied to the January 23 earthquakeexceptthat spectra at Puu Oo is given'in Figure 9. In this figure we the overlappingwindowswerelimited to a length of 10 s see the tremor spectrum evolve from one with a single because of the smaller signal duration of that event. fairly sharp dominant peak to a complex spectrum We looked at temporal variations in tremor spectraby containing multiple peaks in the 2.5 to 4.0 Hz band. comparingresultsfrom consecutive40-s windowswith Variations similar to these could easily be replicated
20 s of overlap.
Our primary evidence that sharp spectral peaks by superposingspectra from successiveexplosionpoint in tremor spectra are not site effects is indicated sourceswith varying depth (e.g., Figure 6), or from in Figure 8. In this figure we see that the array- successiveimpulseresponsestriggered at variousdepths averagedspectra of earthquakesrecorded on the first in a shallow fluid-filled crack embedded in the Puu Oo
array (events023, 026, and 027) are fairly smoothin structure(e.g.,Figure7). comparisonto those of tremor recorded on the first
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02
0.40,2o. o.
o. o 0.t
O•0.2L .
0.• 0.1• 0.4-
0,2
0,4'
0 0
Fig. 9. Array-averaged power density spectra of consecutive, 40-s long, overlapping windows of volcanic tremor recorded on January 22, 1988. Each spectrum is obtained using 11, overlapping, 30-s long windows. There is 20 s of overlap between successivespectra.
2646
GOLDSTEIN
AND CHOUET'
SOURCES OF VOLCANIC
TREMOR
AT KILAUEA
Multiple SignalClassification technique[Schmidt,1986; Goldstein and Archuleta, 1987], and seismogram alignment[Goldstein,1988; Goldsteinand Archuleta, In this section we use frequency-slownessanalyses, 1991a]. The advantagesof these techniquesover SPATIAL
EXTENT
OF TREMOR
AND DEPTHS
SOURCES
more conventional array signal processing techniques
particle motions, and ray tracing to estimate the spatial extent and depths of the sources of volcanic tremor and gas-piston activity beneath Puu Oo. We perform similar calculations with data from small earthquakes to identify any trends in ray parameter and azimuth that might be due to path or site effects.
has been demonstratedin detail by Goldstein[1988] and Goldsteinand Archuleta[1991ab]. We beganby
aligning the peak of a prominent arrival in the vertical component waveforms of each event. For example, the first peak in each waveform was used for most of the We measuredtotal slowness(ray parameter)and earthquakes. Volcanic tremor and gas-piston records azimuth as a function of time using MUSIC, the were scanned in record section format to identify
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_
,
-1.0 ' f=Z7 •-'-...'d•b= 1_2 • '
1.0 , • ,
, • ,
.
I , , I , f= 3.1 db•b= 1,5
sx=-0320 sy=-OJ'20
sx=0.000sy=-0240
f-- 3g dbd>=
f= 3.5 db•b= 1
sy= sx=-0280 sy=-O.•20 sx=-0',360
.
-
0.0
'
-
0.••••.•
0.5
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-
--
(
-0.5 -
,
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--
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ß
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_
-0.5 .
-
,
-1.0
4--.,
,
b= 4.7 dbob=2.2
sx=-0.360sy=-•1-60 -
0.5
,
,
0,0
I
0,5
,
1.0-1.0
-0,5
0,0
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...
-
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I
-1,0 -0.5
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•
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-1:-01,0 -0,5 0.0 0,5
-
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1,0-1,0
-0,5
0,0
0,5
1,0
x componen[ of slowness Fig. 10. Narrow-band frequency-slownessspectra and the correspondingstacked spectrum from a single
windowof volcanictremor on January22, 1988.The centerfrequency(/), peak spectralamplitudein dB abovethe baselinespectrallevel (dbab;seetext), and corresponding components of slowness at the peak (sx, sy) are listedat the top of eachnarrow-bandspectrum.The stackedspectrumis at the bottomof the secondcolumn.It is labeledby bandwidth(b) insteadof centerfrequency.Sourcedepthsand azimuths were estimated from slownessescorresponding to peaks in the stacked spectra.
1,0
GOLDSTEIN
AND
CHOUET:
SOURCES
OF VOLCANIC
TREMOR
AT KILAUEA
2647
prominentarrivals. The slowness/rayparameterof a
slownesswas converted to ray parameter and azimuth
reference arrival was determined by fitting a plane wave to its time delays across the array. After alignment, we computed slowness spectra during successive,2.5s long time windows with 2.0 s of overlap between adjacent windows. Slowness spectra were estimated over a bandwidth from 2 to 6 Hz using slownessstacking
using
q•- 9/2- arctan(Sy/Sx)
whereP is theray parameter,$x and$y arethe z (east) of slowness, respectively, and [Spudichand Oppenheimer, 1986]. A typical set of andy (north)components • is the apparent direction of propagation or signal narrow-band(0.4 Hz bandwidth)slowness spectraand azimuth
their corresponding stacked spectrum, for one time window, are shown in Figure 10. Vector slownesswas estimated from the locations of peaks in the stacked spectra provided their amplitude was at least i dB
The baseline spectral level is defined as two standard deviations above the mean spectral level. Based on both simulations and experience with data, this choice filters our peaks due to noise and signals with poorly constrained slowness. Both components of slowness
were initially allowedto range over a 4 s/km window on
the
reference
arrivals
slowness.
clockwise from north.
SourceAzimuths and Spatial Extent
higher than our baselinespectra level (dbab _• 1).
centered
as measured
These
The azimuthal distribution of earthquake,volcanic tremor, and gas-pistonsignals,measuredduring a 40s time window, are indicated in Figure 11. This plot shows that volcanic tremor and gas-piston signal azimuths are concentrated in a cone approximately 25ø wide pointing in the direction of Puu Oo with a slight trend in the down rift direction towards Kupaianaha. In contrast, earthquake azimuths axe more evenly distributed and typically span a much larger range than
windowswere narrowedto 2 s/km windowswhen we those of gas-piston and tremor activities. The cluster found them sufficient to capture all the significant in azimuths near 270ø for the January 31 earthquake arrivals. The initial range was chosen to be large is due to the superposition of this small event on enough to include both body and surfacewaves. Vector a background of volcanic tremor. The number of 2,0
'
'
'
'
I
'
'
'
'
o•- La, rqe, Ar?y I ....
2,0,
I
'
I
....
'
'
'
I
'
I
....
'
'
'
I
i
.
-
....
1-'Z_
•m-Harmohio Tremor 0,5S,maJJ Ar[ay
•
••••,•••)k • •/HT-_
(;as-Piston Even'[
-
•
all Ar,r ay Sin,
-
-,-'
Earthquake
-
!,_
"
'
'""
I
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{
'
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r'L - Earthquake •c•
- ' •k,a, rqe, Ar[ay I '
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J '
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lm- E•hquake
2,0 -
,Ar[ay• ....
i ....
• ....
•
-
om-,Smali, Array•, , , , • , , , , • , , , .•
1•
1.•
!
•
i
20o
Azimuth (deg) Fig. 11. Plots of ray parameteras a functionof azimuthfor mostof the eventsshownin Figure5. The first 40 s of eacheventweredividedinto consecutive 2.5-slongwindowswith 2 s of overlap.Azimuth is measuredclockwise from north. The azimuthfrom eacheventis indicatedby the vertical]Line through
each corresponding data set.
2648
GOLDSTEIN AND CHOUET: SOURCES OF VOLCANIC TREMOR AT KILAUEA
Ray Parameter(s/km) 0.5 1.0 1.5
coherent arrivals in the volcanic tremor and gas-piston records and their clustering in azimuth around the direction of Puu 0o indicate that sources localized in
closeproximity to Puu 0o crater are responsiblefor
I
the observed volcanic tremor and gas-piston activity.
0.2'
Assumingthe tremor sourcesare at a distanceof 1 km, the approximatedistancebetweenthe arrays and Puu 0o crater, the horizontal extent of the source
0.4-
region(œ----re) is approximately 400m. This is roughly twicethe sizeof Puu Oo crater (Figure1). Basedon (ee of GoldsteinandArchuleta[1991a]),the uncertaintyin
0.8'
this estimate of spatial extent is approximately 100 m for the larger array and 300 m for the secondsmaller array. Differencesin precisionof the arrays are due to differencesin aperture, averagestation spacing,and uncertainties in delay times used to align the data.
I
I
I'
I ,
0.6F
1.0"
Fig. 12. UpgoingP and S waveray parametersas a function
of sourcedepth, in the model of Ferrazziniet al. [1991]. S wave ray parameters are indicated by the thin curves. These curvesprovide a bound on the maximum sourcedepth because reflections from sources within the top two layers could have a smaller ray parameter than a direct wave.
Ray Parameters and SourceDepths
Most of the volcanictremor ray parameters(Fig- or in the vicinity of the shallowsurficial layers. We base ure 11), especiallythosewith azimuthscorrespondingthis conclusion on our observation that only shallow to the direction of Puu Oo, range betweenslownesses of sources are expected to generate significant surface about0.6 and 2.0 s/km. The low endof thisrange,less waves and that surfacewaves are a large componentof
than approximately1.4 s/km, is incompatible with sur- the observed tremor wavefield. However, we also show face waves in low-velocity layers such as the onesfound that a significant amount of body waves are present by Ferrazziniet al. [1991].Basedon this observation,and note that their correlation with the smallest ray we assumethat the low slownessarrivals are body waves parameterswe observesuggeststhat there may be some and use this range of ray parameters to constrain the contributions to the tremor wavefields from sources as maximum sourcedepth as follows. First, we use Snell's deep as I km. We began by looking for body or surface wave law to relate the observed ray parameter $ to a corpolarizations in individual three-componentrecordings respondingdepth (z) for an upgoingbodywavesource but found significantvariability from station to station; as not suprising in light of the complex signal properties we have identified thus far. In order to identify those features that were common to most or all of the stations, we compared record section plots of the product of band-pass-filtered vertical and radial componentswith where ci and hi are the layer velocitiesand thicknesses the product of band-pass-filtered radial and transverse (Figure1), and r is the epicentraldistancebetweenthe components. In these plots, P and S V waves should source and receiver, which is fixed at I km. We only appear primarily on the vertical[radialrecordsas a considerdirect upgoingwaves becausethey correspond pulse or series of pulses that are consistently positive to the greatest sourcedepth for a given ray parameter. or consistently negative. Rayleigh waves should also Sourcedepth is plotted as a function of P and $ wave appear primarily on the vertical[radial recordsbut ray parameters in Figure 12. Given the lower cutoff should oscillate about zero. SH and Love waves should
in observedray parametersof about 0.6 s[km, this be observedon the radial[transverserecordswith
restricts a P wave source to a depth shallower than about 250 m. The maximum depth for an S wave source is about I km. Uncertainties in maximum sourcedepth, based on uncertainties in slownessat the large array, are approximately 100 m for P waves and 300 m for S waves. These results clearly show that the source of volcanic
tremor
at Puu
Oo is close to or within
the
upper kilometer of the crust.
consistently positive or consistently negative motions. We differentiate $H and Love wavesbased on expected differencesin their apparent velocities. We found that the polarization of a few arrivals were consistent acrossmost of the array. For example, in Figures 13 and 14 we show 2-Hz low-pass-filtered
vertical[radial and radial[transverserecord section plots for the first 20 s of the gas-piston event on February 2. Based on its oscillatory waveform, and
Particle Motions and Wave Types
large slowness (• 1.5 s/km), the arrival identifiedby
In this sectionwe use array measurementsof particle motions to identify wave types and argue that most of
the subvertical
the tremor
arrival marked by the line between 12 and 14 s in
source or sources must be confined
to within
line in the vertical-radial
record
section
(Figure13) is a Rayleighwave.Similarly,the impulsive
GOLDSTEIN
AND CHOUET:
SOURCES OF VOLCANIC
TREMOR
AT KILAUEA
2649
the most vigorous bursting of gasbubbles at the surface
of Puu Oo crater [Dictelet al., 1989;Ohouetand Shaw, 1991]. This suggests that point sourcesare associated with the explosionof large gas bubbles, someof which have risen through the magma conduit to the surface. This model may have important implications for monitoring. Our observation that the spectra of point
sourcesare sensitiveto depth (Figure6) suggests that it may be possibleto determine the depth of the most prominent source of energy from a single record. It
mightevenbe possible to trackchanges in sourcedepth through temporal changesin observedspectra. If such changes in source depth are correlated with and occur prior to changes in volcanic activity they would be a useful tool for monitoring. Even if correlations between activity and source depth do not exist, observationssuch as these provide useful indications of the subsurface plumbing and magma dynamics. For example, our observation that most of the sourcesare located in the upper few hundred meters beneath or in close proximity to Puu Oo with sources outside this region trending downrift toward Kupaianaha suggeststhat most of the magma dynamics is occurring in shallow conduits or chambers beneath Puu Oo with some activity extended in the direction of Kupaianaha. This is consistentwith visual observations 0
/5
10
1/5
20
Time (s)
Fig. 13. Record sectionplot of the product of the vertical and radial components of motion. Each component was
band-pass filtered below 2 Hz before multiplication. The solid line near 6 s indicates the start of a Rayleigh wave.
the radial-transverse recordsection(Figure 14) has a slowness of approximately0.9 (muchtoo low to be a Love wave) and is an S/-/wave. Althoughvolcanic tremor records from both arrays were quite variable, they both had consistentphasesthat could be identified as body or surfacewaves;with surfacewavesbeing more common. Based on this analysis, volcanic tremor at Puu Oo appears to be dominated by sourceswithin or in the vicinity of the surficial layers, although sources as deep as 1 km may also be contributing. DISCUSSION
In this section we discuss plausible mechanisms for and implications of point source models of shallowvolcanic tremor. We compare advantagesand disadvantagesof thesemodelswith those of resonating
fluid-filledcrackmodels[e.g.,Chouet,1988,1992].We provide a few additional observations that should be
useful constraints on further modeling and conclude with some suggestionsfor future work.
I
0
/5
10
I
[
I
I
I
15
I
20
Time (s)
Mechanism and Implications of Point Sources
Fig. 14. Record section plot of the product of the radial and transverse components of motion. Each component was Visual observationsindicate that the large ampli- band-pass filtered below 2 Hz before multiplication. The
tudes in gas-pistonrecordingsare well correlated with
solid line near
12 s indicates
the start
of an $H wave.
2650
GOLDSTEIN
AND CHOUET:
SOURCES OF VOLCANIC
TREMOR
AT KILAUEA
of the Puu Oo crater and Kupaianaha lava pond made at the time of this experiment. A point sourcemodel for volcanicsignMsat Puu Oo also has implicationsfor the rheology and composition of the magma. For example, if gas bubbles are the primary source, their frequency and magnitudes suggest a significant percentage of gas in the magma composition. The ability of the magma system to transport these gas bubbles should provide some constraint on magma viscosity.
spectra. These models are also consistent with visual observationsof magmatic activity in Puu Oo crater. Based on the above observations, either a model consistingof point sourcesembeddedin shallowsurficial layers or a model consistingof point sourcesin a fluid-
Point
Additional
Sources Versus Fluid-Filled
Cracks
The primary advantage of the point sourcemodel of
shallow tremor is its simplicity and ability to explain the observeddata. A potential disadvantageis that it does not provide a formal descriptionof the coupling between processesin the gas, liquid and solid. Such a description would aid in our understanding of the volcanicsourceprocessbut may not be identifiablefrom this data. This model may also be inappropriate when well-correlated spectral peaks are observedat stations with large separations/ncethey would most likely have different path effects;but this is clearly not the casefor
filled crack embeddedin shallowsurficiallayers appear to be appropriate for observations of shallow volcanic tremor in the open vent of Puu Oo. The selectionof one of thosemodelswould require a more extendedcoverage of seismic stations
than
Observations
available
at Puu Oo.
and Future
Work
During the processof modeling shallow tremor and gas-piston activity we found that shallow attenuation
mustbe fairly high (Q, • 20) to explainthe relativesize
of spectralamplitudes above6 Hz (e.g.,Figure6). We also found that long-term averagesprovide a different perspective than our analysis because surface waves,
whichhavelargeramplitudes,are emphasized.This who I0180 s) windows,found that surfacewavesdominated
their observations. We looked at 2.5-s long windows and found a significant amount of body waves. Based on the above observations,further comparthe Puu Oo arraydata (Figure4). Strictly speaking, the fluid-driven crack model is a isons of impulsive point source and fluid-filled crack composite of a triggerelement(the energysource)and models in a variety of volcanic settings are warranted. a resonator(the fluid-filledcrack). In the modelswe Additional observationswith large aperture arrays or considered,the trigger is again a point sourcewith the multiple arrays would be most usefulfor examining the essentialdifferencefrom the earlier model being that spatial as well as temporal characteristicsof volcanic the point source is now embedded in the fluid rather sources.The developmentof a model of time-dependent than the elastic solid. Thus the fluid becomes an active gas bubble generation and transport that is consistent participantin the generation of the elasticwavefieldwith the short-term rise and fall in activity within an via the process of acoustic resonance in the crack. ifidividual gas-pistonevent and the longer-termperiodThe primary advantage of this model is the direct link icities thai were observedbetweenmultiple gas-piston
betweenthe observedwavefieldand the fluid dynamics. Another advantageis the ability of the model to explain commonpeaks in the spectra of tremor seenat widely spacedstations[e.g.,Aki and Koyanagi,1981; Chouet
events is also needed. CONCLUSIONS
We have suggestedthat point sources,within and
et al., 1993]. When the crackpenetratessurficiallow- in the vicinity of shallow surficial layers, can explain velocity layers,path resonances in thoselayerscombine observationsof shallowvolcanictremor and gas-piston with sourceresonancesto produce a broadband multi- activity at Puu Oo crater, Kilauea volcano, Hawaii.
peakedspectrum.Whenthe sourceisremoved fromthe Resonating fluid-filledcracks[e.g., Chouet1933, 1992] surficiallayers,lessenergyis pumpedinto thoselayers that penetrate the surficial layers can also excite nearand the spectrumdisplaysa narrowerband dominated surfaceresonancesand would be difficult to distinguish
by sourceresonances. Thus a shallow(h < 100m, z < from point sources. Waveforms and spectra of both 100 m) resonatingcrackcouldbe presentat Puu Oo of these models are very sensitive to source depth, an but probably could not be distinguishedfrom shallow observation that may be useful for volcano monitoring. impulsive point sourcesbecause our arrays did not These observationsalso suggestthat path effectsshould providea wideenoughcoverageto answerunequivocally be understood and accounted for before empirical the questionof whetheror not sourcepeaksare present models for predicting the size and timing of eruptions are used in new environments. in the observedspectra. Measurements of ray parameter and source azimuth The point sourceand fluid-drivencrack modelsin a layered structure can both explain the waveforms, indicate that both tremor and gas-piston events are frequencycontent,ray parameters,azimuths,and wave localized beneath or in closeproximity to Puu Oo crater types of the observedtremor and gas-pistonsignals and are due to a nearly continuoussuccessionof sources. at Puu Oo. Assuming the position of the source We show that most of the sources must be located
and/or geometryof the crackcan vary, both models within or in the vicinity of the suficial layers to explain can also explain time-dependent variations in these the composition of the observedwavefields. Body wave
GOLDSTEIN AND CHOUET: SOURCES OF VOLCANIC TREMOR AT KILAUEA
2651
Chouet, B., A seismic model for the source of long-period events and harmonic tremor, in IAVCEI Proceedings in Volcanology,vol. 3, Volcanic Seisinology, edited by Based on ray parameter and particle motion P. Gasparini, R. Scarpa, and K. Aki, pp. 133-156, Springer-Verlag, New York, 1992. analyses,tremor and gas-pistonrecordingsat Puu Oo Chouet, B., and H. R. Shaw, Fractal properties of tremor consistof a complex combinationof body and surface and gas-piston events observed at Kilauea Volcano,
ray parametersconstrainthe maximumsourcedepthto about
i km.
waves. Analysis of long-term averagesof these signals Hawaii, J. Geophys. Res., 96, 10,177-10,189, 1991. obscuresthe body wave contributionsbecausethere are Chouet, B. A., R. A. Page, C. D. Stephens, J. C. Lahr, and J. A. Power, Precursory swarms of long-period events at more surfacewaves and they have greater amplitudes. Redoubt Volcano(1989-1990), Alaska:Their origin and Based on these observations,we suggest that more use as a forecasting tool, J. Volcanol. Geotherm. Res., in press, 1993. detailed measurementsof the spatial extent of similar R. S., and D. A. Bame, A spherical source model volcanic sources are possible using larger aperture Crosson, for low-frequency volcanic earthquakes, J. Geophys.Res., arrays or pairs of arrays and that the choiceof array 90, 10,237-10,247, 1985. aperture shouldbe basedon narrow-bandor low-pass- Dietel, C., B. Chouet, K. Aki, V. Ferrazzini, P. Roberts, filtered coherence estimates.
If such measurements
can
be used to estimate the extent of volcanic sources, they
may improveestimatesof volcanichazardsby helpingto constrain the amount of magma and available pathways to the surface.
and R. Koyanagi, Data summary for dense GEOS array observations of seismic activity associated with magma transport at Kilauea Volcano, Hawaii, U.S. Geol. Surv. Open File Rep., 39-113, 1-171, 1989. Fehler, M., Observations of volcanic tremor at Mount St. Helens Volcano, J. Geophys. Res., 88, 3476-3484, 1983. Fehler, M., and B. Chouet, Operation of a digital seismic network
Acknowledgments. Paul Kasameyerdeservescredit for motivating the first author to emphasizehypothesis testing in his work and for carefully reviewing this manuscript. We are also grateful to Phil Harben, Dave Harris, Howard
Patton, Steve Jarpe, Mike Fehler, and Yoshiaki Ida for their thoughtful reviews of this manuscript. Charles Carrigan, Bill Walter, and George Zandt provided many helpful comments. Christopher Dietel was especiallyhelpful in dearchiving data and answering questions about the field experiments. We are grateful to Keiiti Aki, Valerie Ferrazzini, Christopher Dietel, Peter Roberts, and Robert Koyanagi for their participation in the field experiments. We also thank David Okita of Hilo, Hawaii, for providing superb helicopter assistanceand a constant helping hand in the field. This work was supported in part by the National Science Foundation under grant EAR-8618107, through funding from the University of Southern California, and under the auspicesof the U.S. Department of Energy by the Lawrence Livermore National
Laboratory
under contract
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SOURCES
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(Received May 17, 1993;revisedSeptember 7, 1993; acceptedSeptember13, 1993.)
