Location of zones of anomalously high s-wave attenuation in the upper crust near ruapehu and ngauruhoe volcanoes, New Zealand

Journal of Volcanology and Geothermal Research, 10 (1981) 1 2 5 - 1 5 6

125

Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m - - Printed in Belgium

LOCATION OF ZONES OF ANOMALOUSLY HIGH S-WAVE ATTENUATION IN THE UPPER CRUST NEAR RUAPEHU AND NGAURUHOE VOLCANOES, NEW ZEALAND

J.H. L A T T E R

Geophysics Division, Department of Scientificand Industrial Research, P.O. Box 1320,

Wellington (New Zealand) (Received S e p t e m b e r 15, 1980; revised and a c c e p t e d D e c e m b e r 31, 1980)

ABSTRACT Latter, J.H., 1981. L o c a t i o n of zones of a n o m a l o u s l y high S-wave a t t e n u a t i o n in the upper crust near R u a p e h u and Ngauruhoe volcanoes, New Zealand. J. Volcanol. Geotherm. Res., 10: 125--156. Many earthquakes within the crust near R u a p e h u and Ngauruhoe volcanoes, recorded at epicentral distances less than 20 km on vertical seismometers, s h o w S-waves of lower d o m i n a n t f r e q u e n c y than the P-waves. A large n u m b e r also have amplitudes in the S-group less than those o f the P-waves. Whereas the reduced amplitude of S-waves relative to that of P-waves can be a source m e c h a n i s m effect, the corresponding r e d u c t i o n in d o m i n a n t frequency should be i n d e p e n d e n t of the source radiation pattern. The m o s t plausible reason for such a r e d u c t i o n in d o m i n a n t S-wave f r e q u e n c y is t h a t the waves have passed through a zone o f partially m o l t e n rock. The data are therefore interpreted in terms of the presence o f m a g m a in restricted zones near the volcanoes. Using ray paths f r o m 232 h y p o c e n t r e s to three p e r m a n e n t seismograph stations, together with paths f r o m three additional earthquakes to one p e r m a n e n t and t w o t e m p o r a r y stations, an i n t e r p r e t a t i o n in three dimensions has been made of the source o f the anomalous a t t e n u a t i o n at depths b e t w e e n 2 and 10 k m b e l o w d a t u m ( R u a p e h u Crater Lake). Wave paths which lie largely at depths shallower t h a n 2 km c a n n o t be used, as almost all such paths s h o w evidence o f e n h a n c e d S-wave attenuation, and this is attributed to the presence o f superficial pyroclastic and u n c o n s o l i d a t e d laharic material within 2 km of the surface. At R u a p e h u , the data suggest the presence of three principal intrusions, one underlying m u c h o f the s o u t h w e s t slopes and reaching as far east as Crater Lake, one beneath the eastern side of the S u m m i t Plateau, and one beneath part o f the northeast slopes of the volcano. All three are essentially vertical or steeply dipping structures, detectable to a depth o f b e t w e e n 7 and 9 kin. The first appears to e x t e n d to within a b o u t 5 k m of the surface, whereas the o t h e r t w o have i n t r u d e d to within 2 or 3 km. Other, less well-defined, and c o m p a r a t i v e l y small bodies exist beneath b o t h the western and eastern slopes of Ruapehu. In the Ngauruhoe area, few earthquakes have o c c u r r e d and all have been at depths less than 6 kin. Therefore, o n l y shallow attenuating areas can be defined. A small area of a n o m a l o u s S-wave a t t e n u a t i o n occurs beneath the n o r t h w e s t slopes of Ngauruhoe, and another, elongated, b o d y appears t o c o i n c i d e with a fault zone west o f the volcano. Both of these lie at depths of a b o u t 3 k m below d a t u m (less than 2 k m b e l o w surface in one locality). 0377-0273/81/0000--0000/$02.50

© 1981 Elsevier Scientific Publishing C o m p a n y

126

Finally, areas of high attenuation, at depths of 4--5 km below datum, appear to define a narrow east-west zone about 6 km long in the immediate area of Whakapapa village. Other zones exist east of the volcanic axis, defining a line which cuts the axis on the north east slopes of Ruapehu, at a point where a parasite crater formed a few thousand years ago.

INTRODUCTION Gorshkov (1958) pioneered the use of the screening effect of transverse seismic (S) waves for detecting magma in the r o o t zones of volcanoes, when he a t t r i b u t e d the r e duc t i on of S-wave amplitudes in some distant earthquakes recorded in Kamchat ka t o the presence o f magma at depths of 50--70 km beneath the Kliuchevskaya group of volcanoes. This work was followed by a n u m b e r o f o t h e r studies in K am c ha t ka (Gorshkov, 1971, 1972): see especially F e d o t o v (1965), Farberov and Gorelchik (1971), Firstov and Shirokov (1971), Gorshkov and Farberov (1974), and F e d o t o v and Tokarev (1974}: in all these cases, a t t e n u a t i o n was attributed to magma bodies in the mantle. Balesta and Farberov (1968}, however, n o t e d a t t e nua t i on of the S-phase at superficial depths beneath an erupting fissure. Bo th mantle and crustal magma bodies were inferred by K u b o t a and Berg (1967) in o r d er to a c c o u n t f or weak transmission o f S-waves in the Alaska Peninsula. T h e y used intersecting and overlapping ray paths to delineate magma bodies, b u t did n o t a t t e m p t to locate t h e m precisely, assigning t hem merely to the u p per or lower crust, or to the mantle. In this study, which followed on f r o m preliminary work by Berg et al. (1967), t hey sought to take a c c o u n t n o t only o f the reduced amplitudes of S, but o f polarization effects. By so doing, t h e y were able t o provide a theoretical explanation for their observation th at it is primarily the vertical c o m p o n e n t of the shear wave which is screened by intervening magma. M a t u m o t o and Molnar (1967) and Matum o t o and Ward (1967) also inferred the presence of magma in the same region of Alaska, and M a t u m o t o (1971) made a detailed study of it. He interpreted the data in terms of three magma chambers in the upper crust, and one in the lower crust, and showed t ha t at t enuat i on of high-frequency P-waves accompanied a t t e n u a t i o n o f S and could be used to give an estimate of magma viscosity. In Iceland, Einarsson {1978), has reported S-wave at t enuat i on beneath the Krafla caldera: and rather similar results have been obtained by Aspinall et ah (1976) in St. Lucia. Sanford et al. (1977) have used reflected P- and S-waves to map the r o o f of a magma b o d y in New Mexico. In New Zealand, M o o n e y (1970a), following similar work by Oliver and Isacks (1967) in Tonga, studied differences in the frequency c o n t e n t of seismograms, distinguishing a zone in the west and nort hw est of the North Island in which high frequencies of bot h P- and S-waves were strongly attenuated, f r o m a broad region to the east and southeast in which t h e y were not. He located the zone of anomalous a t t e n u a t i o n of high frequencies in the mantle, at a d e p t h o f a bout 75--100 km, and in a second paper (Mooney,

127

1970b) showed that travel-time residuals were consistent with this hypothesis. Hatherton (1970) confirmed that a number of significant surface phenomena, such as volcanism, heat flow and gravity, accurately mirrored the boundary between Mooney's high- and low-attenuating zones. Relative amplitudes and frequencies of P-and S-waves have not, however, hitherto been used to define zones of attenuation in detail within the crust in New Zealand. S T R U C T U R A L MODEL AND DATA

In May and June 1976, two permanent seismographs (Fig. 1) were installed near Ruapehu and Ngauruhoe volcanoes, in the Tongariro National Park, central North Island, New Zealand. These (GSZ: Glacier Shelter, ca. 1.2 km north of Ruapehu Crater Lake; and NGZ: Ngauruhoe, ca. 3.2 km southwest of Ngauruhoe volcano), together with the seismograph at Chateau (CNZ), about 9 km distant from both volcanoes, which had been in operation since 1960, have enabled subsequent earthquakes near the volcanoes to be more precisely located than had hitherto proved possible. All three stations are equipped only with single-component vertical seismometers, and record with gains at 2 Hz of about 22,000. Frequency response is adequate over the range 1--10 Hz, but is not readable above this. Details of system response and instrumentation are given in Latter (1979a); for response curves, see Fig. 3d. The apparent foci of earthquakes depend heavily on the structural model used. This is derived from the interpretation of two timed explosions (in Ruapehu Crater Lake, and Lower Tama Lake), and of arrivals from earthquakes which accompanied eruptions at Ruapehu (hence assumed directly beneath Crater Lake): see Latter, 1979a, 1981. Deeper structure is derived from the results of long-range crustal refraction shooting (Garrick, 1968). The model is essentially provisional, pending further, more detailed refraction surveys. In agreement with a geological interpretation (Gregg, 1960) and resistivity measurements at Chateau (G.B. Dawson, personal communication, 1977), sub-horizontal Tertiary sediments are considered to underlie the area at about sea level: this layer, with a P-wave velocity (Vp) of about 2.35 km/s, is shown in Figs. 9--11. Above the Tertiary sediments, according to the model, lies low-velocity material, thought to be mainly laharic and pyroclastic in origin, with an average Vp ~ 1.4 km/s, capped by a carapace of andesite lava flows (Vp ~ 4.7 km/s) which is approximately conformable to the surface. Below the sediments a horizontal layer, about 0.65 km in thickness, with Vp ~ 3.8 km/s (weathered greywacke?) is inferred, beneath which is material interpreted as schistose greywacke (Vp ~ 5.1 km/s). This probably grades down into Garrick's average velocity (for his layer 2) of about 5.4 km/s. In the absence of good data on S-wave velocities in the area, a V p / V s ratio of 1.73 has been assumed throughout. Since May 27, 1976 (the beginning of continuous recording at GSZ), and up to October 8, 1979, 232 earthquakes have been located, at epicentral distances < 20 km from either volcano, by means of the network and the

128

PUK O

O°

•

MAN

o

o

/

°

. "~'//

•

}

cl

L.

°

v

•

o

r~4

o

~.o.-

"Jr o

",~

\''

I

o

GSZ ~e~ ~ •

• •

/

ev e

v .,. -- * /,r <, o ; .

/

.

*

o

•

•// ~=/"

•

,"

~t,

"

,/

~

^ /

ju /

•

"

~

v J

"~"

/

~

..true

0

o~

~

North

~-

~

I

I

1

l

KM

HYPOCENTRES Triangle of error(max.side)

"

'

Focal depth

5 I

O =p e r m a n e n t s e i s m o m e t e r O=temporarys eismOmeter, ,ak=Volcano ( Pukeonake, R =Ruapehu Mangatepopo ) N=Ngauruhoe

/

/e

J

I

o •

.

T U~///X

---~

MNG

I

10 I

I

I

J

l

,
4-6

•

7-9

"

>l'5km

[]

10-12

"

v

)12

"k

"~"

Fig. 1. Locality map of Tongariro National Park, showing epieentres of earthquakes used in this study, classified into five depth intervals. The zone, marked T U to W, 4 km wide, crossing Ruapehu from southwest to northeast, includes most of the earthquakes. The positions of the presently active vents of Ruapehu and Ngauruhoe volcanoes are marked, together with those of the permanent and temporary seismometers. Prominent faults (solid lines) and their inferred extensions (dashed lines), after Grindley (1960), are shown, with short dashes marking the downthrown sides. Two earthquake swarms are indicated in the figure: 1, see caption Fig. 2b; 2, see caption Fig. 2c. Inset shows regional stations used in mechanism studies.

129 inferred structural model. These, together with three earthquakes recorded during January 1975 at CNZ and two portable seismograph stations, provide the data for the present study. Epicentres of these shocks, which range in magnitude from M L -- 1.4 to 3.7, are shown, with an indication of their depths of focus, in Fig. 1 (in which earthquakes directly underlying Ruapehu Crater Lake are omitted), together with the volcanoes, and the seismometdr sites of the permanent and temporary stations. Positions and depths of focus of the earthquakes used in this study are given in annual summaries in the New Zealand Volcanological Record (see Appendix 1).

Distribution and mechanism of earthquakes For a given station of the network, within the magnitude range studied (M L -- 1.4--3.7), coverage can only be considered complete for earthquakes lying within epicentral distances of 7 or 8 km. At greater distances, many of the smaller events are obscured by noise. However, even within this restricted range of the stations, it is evident from Fig. 1 that earthquakes are not rand o m l y distributed over the area. There is a broad region north of GSZ and south of CNZ with few earthquakes. Similarly there are few earthquakes east of NGZ. Events, on the other hand, are strongly concentrated beneath the east and northeast slopes of Ruapehu.: there is a t e n d e n c y for earthquakes to occur at greater depths in the south and southwest of the area than in the north and east. The northeasterly prolongation of the main earthquake belt at Ruapehu does n o t coincide with the main belt of faulting (Grindley, 1960), which strikes between 15 and 25 ° east of north and extends from about 4 km north of Ruapehu north-northeast for more than 30 km (see Fig. 1). This is a largely aseismic region. On the other hand, the main earthquake belt, striking between 45 and 50 ° east of north from Ruapehu, coincides rather closely in direction with faults shown by Grindley which extend to the northeast from a point about 13 km northeast of Ruapehu. This is exactly the area in which the concentration of earthquakes terminates. A few earthquakes within a distance of 5 km southwest of Ruapehu seem to mark a continuation of this strike direction: t h e y also terminate close to a point where Grindley shows the beginning of a fault, which extends some 11 km to the southwest. Thus it appears that the bulk of earthquakes in the area take place in a region some 18 km in length which continue to the northeast and southwest as zones of observed faulting which are presently aseismic. It may well be, therefore, that the current earthquake activity represents a process of extension of faulting through what is at present an unfaulted block. The focal mechanism of earthquakes in this region was examined, using the polarity of first onsets (only the clearest were used) at the three local stations, and at four stations 100--150 km away (MNG, KRP, TNZ and TRZ: see inset Fig. 1). An a t t e m p t to obtain a composite focal mechanism solution for all earthquakes in the broad zone was a failure (Fig. 2a}: the wide scatter of

130

points shows clearly that more than one focal mechanism was involved. However, consistent results were obtained for two swarms of earthquakes near the northeast end of the zone (Fig. 1). The first swarm, which took place between May 29 and June 1, 1978, shows compressions near the vertical direction in all azimuths and suggests a thrust mechanism of some kind, although there are insufficient points to define the nodal planes accurately (Fig. 2b). The second swarm, which began three days after the first ended and continued intermittently till June 20, showed a different mechanism (Fig. 2c). There are insufficient points to define nodal planes, but it would be possible to interpret this in terms of a near-vertical nodal plane with a NNE or NE strike, with a second almost horizontal plane. This would accord well with the N43°E strike of Grindley's fault zone, but if this was indeed the direction along which movement t o o k place, it would indicate uplift towards the northwest, which is in the opposite sense to the secular movement on the fault in the northeasterly extension of the zone. Results are therefore inconclusive, although some at

•

O dU'o o

•

•

••

/o.

/0

°

/,e•o

•

o

, o

o.

•

B

;,

Fig. 2. D i r e c t i o n s o f first m o t i o n o f t h e initial P-wave ( o n l y clear arrivals p l o t t e d : filled circles = g r o u n d u p ; o p e n circles = g r o u n d d o w n ; (a) = u p p e r h e m i s p h e r e ; (b), (c) = l o w e r h e m i s p h e r e ) : (a) in all e a r t h q u a k e s in t h e p r i n c i p a l e a r t h q u a k e z o n e crossing R u a p e h u f r o m s o u t h w e s t t o n o r t h e a s t (TU t o W, Fig. 1); (b) in t h e s w a r m of s h a l l o w e a r t h q u a k e s t h a t t o o k place b e t w e e n May 29 a n d J u n e 1, 1978, in t h e area m a r k e d 1 in Fig. 1 ; (c) in t h e s w a r m o f s h a l l o w e a r t h q u a k e s t h a t t o o k place b e t w e e n J u n e 4 a n d 20, 1978, in t h e area m a r k e d 2 in Fig. 1. N' = t r u e n o r t h .

131

~

f

•

X

•

INI f

0 0

X

Fig. 2 (continued).

132 C,S Z -

,-'

~

-r-]O0000

,~---~

=I

::~- ~

~1

~----~1

t,, ~

./// /,"

,f

~|

F-a

i

~1

~

/

/ 2

I

c7-.,~- 8-4-Doo/ - - N .... - - ~ 1 ,' ---o .ol// c

'

I/

C -

1

J,,,,I

10

= I

=

*hH~l

i

f requency(hz) d --u

~

"--'l./ ~

-P s u

.

__----'~.~: -

.

.

'..

.

.

.

' " !

.

c,sZ-'~--

L~ 'r

.....~~

=,.---C

.'~-:

P

, :~=~

,,,

~

"

,

:~ ' .

..'--

. . ~

'_'-'r=,~=

_,,C-'--"

S

~

-

-

CNZ__

j

~.-- C N Z Z

p---p:

~ . ~ -

-

- - , j t . . r - ~.r

-

-

Fig. 3. E x a m p l e s o f seismograms, s h o w i n g b o t h apparent and real a t t e n u a t i o n o f the S-wave together with response curves o f the seismographs. (a) O c t o b e r 31, 1 9 7 8 ( 0 0 : 5 1 ( m a r k e d • on record) U.T. ); earthquake o f m a g n i t u d e M L = 3.6 b e n e a t h s o u t h e r n K a i m a n a w a Mountains, ca. 4 0 km east o f C N Z ( d e p t h slightly more than 12 km). The m a x i m u m a m p l i t u d e o f the S-wave is less than that o f the P-wave at C N Z (P is faint). However, there is n o corr e s p o n d i n g r e d u c t i o n in the d o m i n a n t f r e q u e n c y o f the S-wave, which, at b o t h GSZ and

133

least of the earthquakes are consistent with nearly vertical faulting with a northeasterly strike. METHOD

OF ANALYSIS

If magma bodies exist near Ruapehu and Ngauruhoe volcanoes, wave paths from earthquakes traversing them should show anomalous features on seismograms in the area. Transverse (S) waves do not propagate through liquid, and therefore could not pass through completely molten rock. Actual magma, however, is generally partially molten. Ruapehu and Ngauruhoe erupt andesites, which, according to Cole (1978), are generally porphyritic (except for a few lavas from Ruapehu), and in which, according to Clark (1960), "porphyritic crystals are present almost ubiquitously", the percentage of phenocrysts being typically in the range 20--40%. It is likely that magma which has intruded within 10 km of the surface will contain a similar proportion of crystals, the remaining 60--80%, which at the surface solidifies to form the groundmass, being molten at depth. The fact that the feldspar phenocrysts are commonly zoned, often in an oscillatory manner, lends support to the idea of their existence for long periods before the extrusion of the magma at the surface. It is therefore reasonable to assume that subsurface magma in the area will be about 70% molten, the solid portion (the phenocryst content) giving it a finite, although low, rigidity. It will therefore transmit S-waves, but with reduced amplitudes, this effect being more marked the higher the frequency (and the shorter the wavelength) becomes. Apart from S-waves directly transmitted through the magma, there will be P- to S-wave conversion across the body, and, depending on the geometry, diffraction of S-waves around it. However, both these processes will lead to reduction in S-wave energy, compared

CNZ, is comparable to that of the P-wave (both are high-frequency, 7--8 Hz): this therefore represents apparent attenuation of the S-wave. (b) September 30, 1978 (00:13 (o) U.T. ); earthquake of magnitude M L = 3.2 directly beneath Crater Lake, Ruapehu, at a depth of 14--15 km. The m a x i m u m amplitude of the P-wave at C N Z is m u c h larger than that of the S-wave. However, the dominant frequencies of the P- and S-waves at both G S Z and C N Z are comparable, as in (a), and this is a case of apparent attenuation of the S-wave. The direction of propagation to C N Z must lie very close to a nodal plane for the S-wave. (c) July 24, 1977 (10:49 (o) U.T. ) and July 25 (20:32 (×) U.T. );shallow earthquakes of magnitude M L = 2.8, 3.5, and 2.8 beneath the northeast slopes of Ruapehu, at a distance of about 91/2 k m from CNZ. Marked lowering of the dominant frequency of the S-wave (1.8--1.9 Hz) relative to that of the P-wave (41h Hz). Note that the m a x i m u m amplitude of the S-wave is greater than that of the P-wave, and must therefore have been very considerably greater at the focus. The vibrations following the S-wave, which carry the m a x i m u m trace amplitude in these earthquakes, are surface waves. They are large because of the very shallow depth of the earthquakes (about 1 km). Real S-wave attenuation. (d) Inset showing response curves of the seismographs. T w o curves are shown for GSZ: G1 (normal) for the period before January 25, 1978, and after April 3, 1979;and G 2 for the intervening period, after development of an instrumental fault which m a d e gain approximately proportional to frequency. C = C N Z ; N = NGZ.

134

to the case in which there is no intervening magma body, so that the net effect will be a reduction in S-wave amplitude, and a lowering of d o m i n a n t frequency relative to t h a t of the less-affected P-waves. Without knowing the geometry of a magma body, and faced with uncertainties about its temperature, viscosity and elastic constants, it would be difficult to calculate the expected attenuation precisely. The earthquakes used in this study were all typical discrete shocks of normal tectonic appearance (in contrast to multiple, low-frequency earthquakes of volcanic origin). Most showed well-defined P- and S-waves, with dominant frequencies in the range 2--8 Hz (see Fig. 3). Weak S-waves, due probably to proximity of the wave path to a nodal plane in the source radiation pattern, are shown in the CNZ records of the shocks in Fig. 3a, b. These constitute examples of apparent path attenuation, as described below. A clear example of lowering of the d o m i n a n t frequency of the S-waves, relative to that of the P-waves. is shown in Fig. 3c. Note that the three earthquakes shown in Fig. 3c were very shallow (as suggested by the high-amplitude surface waves following the S-waves), and that the path attenuation, indicated by their high P/S d o m i n a n t frequency ratios, is attributed to passage of the waves through superficial pyroclastic and laharic rocks, rather than partially molten material: apart from the prominent surface waves, however, the effect is identical. As mentioned above, there are insufficient data for focal mechanism of the earthquakes to be determined, even in the case of the two swarms {Fig. 2b,c). This renders unreliable the use of reduced S-wave trace amplitudes, relative to those of the P-waves, as an index of abnormal attenuation, since this effect can be solely due to the source radiation pattern, with wave transmission lying close to a nodal direction for S-waves. Furthermore, even if source mechanisms could be established for the earthquakes, there would still be a basic difficulty in the use of S-wave amplitudes to define attenuation, since the relative generation of P- and S-wave energy at the focus remains unknown. Thus, a disturbance rich in S-waves at the source might be recorded with peak S amplitude greater than that of P, and yet have suffered marked attenuation along the path, whereas another, generating less S-wave energy at source, might be recorded with smaller amplitude and yet have undergone no such attenuation. By contrast, cases in which the d o m i n a n t frequency of the S-waves is much lower than that of the P-waves probably always represent genuine path attenuation. Normal attenuation with distance is greater in the case of S-waves than P-waves, because of differences in their mode of propagation, and becomes more marked the higher the frequency of the signal. However, it may safely be assumed that the frequency spectra for P- and S-waves are similar at the focus, and therefore sufficient allowance can be made for the distance attenuation effect to enable highly attenuating cases to be detected. The question remains at what value of the P/S d o m i n a n t frequency the threshold of normal to abnormal attenuation is to be placed. In order to establish this, it was initially assumed t h a t the majority of paths

135 along which abnormal attenuation takes place are included in the set, numbering 104, for which S amplitude is less than that of P (i.e. P/S amplitude > 1), although this set is contaminated b y an u n k n o w n n u m b e r of cases in which reduced amplitude is due to source mechanism effects, and although earthquakes with enhanced S production at source, subsequently attenuated along the paths, are n o t included in it. The null hypothesis was tested that there is no relationship between the earthquakes with reduced S amplitudes (P/S amplitude > 1) and various values of P/S dominant frequency, and was found to hold only below a value for the latter of 1.3. For higher values of the ratio there is a positive correlation with events showing S amplitude attenuation, as defined, which amounts to 7 times the standard error for a ratio of 1.4, and increases to 37 times the standard error at 2.0. The threshold value of 1.3 was accepted for P/S dominant frequency, for and above which all paths were considered abnormally attenuated. These paths, hereafter called " a t t e n u a t e d " , total 139 out of 538 (i.e. 26%) for which P/S dominant frequencies were readable (see Table 1). Of these, only 19% could have been identified b y the criterion of maximum trace amplitude of S less than that of P. Therefore, since a reduction in S amplitude must necessarily have accompanied the observed lowering of dominant frequency, it must be concluded that 81% of the attenuated paths must have yielded S amplitudes so much greater than P at source that significant attenuation could occur and still leave them as maxima on the seismograms: see Fig. 3c Furthermore, it is clear from Table 1 that, had the "apparently a t t e n u a t e d " paths, defined b y those in which the maximum trace amplitude P/S > 1, been selected as the basis for this study, this would have given drastically incorrect results. Only 28 o u t of a total of 104 such paths (27%) can be considered as having shown real S-wave attenuation: in the remaining 73%, the observed reduction of S amplitude must be ascribed to proximity of the wave paths to nodal planes for S in the source radiation pattern of the earthquakes (see Fig. 3a,b). It must be admitted that there is considerable d o u b t about the precise level at which the threshold of normal to abnormal attenuation, as defined by P/S TABLE 1 P/S dominant

P / S m a x i m u m trace a m p l i t u d e

frequency

unreadable

Unreada~e

Totals

< 1

> 1

130

35

2

167

< 1.3

2

323

74

399

1.3

2

109

28

139

134

467

104

705

Tota~

136 dominant frequency, should be placed, arising from the fact that P/S maxim u m amplitude > 1 does not adequately express such attenuation, and this invalidates the original assumption. The precise value of 1.3 for the ratio of P/S dominant frequency must be regarded as no more than an arbitrary estimate, yielding many events of a shadowy character in which it is impossible to assess whether significant attenuation has taken place or not. However, it is abundantly clear that these results shed d o u b t on the detailed interpretation of attenuation, based on amplitude rather than frequency, in many published studies. Attenuated and normal paths to the stations are plotted in Fig. 4a--h, at 1 + 0.5 km depth intervals, assuming straight-line propagation. This is an approximately valid assumption for earthquakes of depth ~> 3 km; shallower events, many of which gave refracted arrivals at the stations, have been neglec ted, partly because of the difficulty in defining their wave paths, but more especially because much of their energy propagated to the stations as direct waves through the shallow, heterogeneous material (assumed laharic and pyroclastic) of the upper layer (Figs. 9--11). Within this diverse and largely unconsolidated material, the majority of wave paths of more than a few kilometres in length show P/S dominant frequencies > 1.3 (Fig. 3c). This is attributed to the greater effect that lack of cohesion, and heterogeneity have on the propagation of S-waves relative to P-waves through such a medium.

h10

I

®N J

/

/

_

\

\/

" -

o

/

\

\? Fig. 4. See p. 140 for legend.

1"3

g~ O

I'D

/

~,

/ ~ ~

\

.

~, \

0

z J

-/

~

o ~

~

J

J

\

0

)"-Z

138

hT,. 0

!

2km

N"

~N

I

/

." ~ & / ////

J

\

\ ,,.

\

d

/

h 6~ 0

1

......... ~

2km

I

N"

.~

~c

®N

/

/

X

/

Fig. 4. See p. 1 4 0 f o r legend.

0

\

L_~

.

J

\

\

140

/ /

%j

JJ J 1

hi"

Fig. 4. Wave paths to the seismometers, C = CNZ, G = GSZ, N = NGZ, plotted at 1 -+ 0.5 km depth intervals below Crater Lake, Ruapehu. Straight lightweight lines represent segments of normally attenuating paths (i.e. those along which P/S dominant frequency < 1.3). Heavy unfilled rectangles indicate possible segments of abnormal attenuation, given by P/S dominant frequency ~ 1.3. Filled rectangles denote those parts of the latter to which the abnormal attenuation is attributed (the length being given by the relationship in Fig. 5). The attenuating areas are outlined by contours showing their cross-sections at each depth interval (_+ 0.5 kin), continuous lines indicating well-established boundaries, dashed lines moderately well-established boundaries, and dotted lines poorly-defined parts of the boundary: plotted at the following depths below datum: (a) 10 km, (b) 9 kin, (c) 8 km, (d) 7 km, (e) 6 kin, (f) 5 km, (g) 4 kin, (h) 3 kin. N' = true north.

141 INTERPRETATION In seeking to interpret the complex pattern of converging ray paths, shown in Fig. 4a--h, it is necessary to take account of the a m o u n t of attenuation indicated b y the P/S frequency ratios, which range from the thresh'old value of 1.3 to 3.5. In the absence of detailed frequency spectra for each earthquake, it is n o t possible to infer with any certainty a linear relationship between the P/S ratio and the path length of attenuating material, because the ratio refers to dominant frequency and there is no sure way of relating this to the spectrum as a whole. However, the assumption of linearity is a good first approximation, and allows a qualitative picture of the amount of relative attenuation along the paths to emerge. The question remains as to what values of path length of attenuating material are appropriate. Some idea of this can be obtained b y an examination of the maximum distances along which attenuation could have occurred, as shown b y the paths in Fig.4a--h. Although most attenuating paths in these figures have ample length in which to a c c o m m o d a t e considerable distances of attenuating material, some are very short, either because they originate at very shallow depths and are inclined steeply to the horizontal, or because parts of the paths are overlapped b y rays which show no abnormal attenuation. Fig. 5 is a plot of the maximum possible path lengths of attenuating material versus P/S dominant frequencies, normalised to take account of variable path lengths above a depth of 3 km. This normalisation has been carried o u t using an observed reduction in P/S dominant frequency of 0.16 between two explosions, in Ruapehu Crater Lake and Lower Tama Lake, with waves propagating entirely in the superficial low-velocity material, over a distance of 9.8 km: i.e. the P/S dominant frequency ratio reduces by 0.1 for every 6.15 km of path length in the superficial layers (9.8 km = the difference in distances, shot-station). For earthquakes deeper than 4 km, a reasonably convincing limit can be drawn in Fig. 5, satisfying all but one earthquake (an event located at 9 km depth). All earthquakes deeper than 10 km are omitted from this figure, because no a t t e m p t has been made to define path lengths of attenuating material below this depth, and it is possible that the single earthquake which does not fit the trend has therefore been mislocated and was in fact deeper than 10 km. It is likely that the reason w h y earthquakes at depths of 3 and 4 km show no relationship between maximum path length of attenuation and the P/S dominant frequency ratio is because paths from earthquakes at these depths are usually rather gently inclined and therefore pass through a greater distance of the superficial layers than do the steeper paths. It is not at all unlikely that low-velocity material may extend to greater depths in places than is shown on the model, which makes the almost certainly unwarrantable assumption of plane layering throughout the area. For the present, pending further information, it is assumed that the trend shown in Fig. 5 is approximately correct. This indicates an increase of 0.1 in the P/S dominant frequency ratio for every 0.24 km of anomalously attenuat-

142 lyl

!

-1 "9

•

o

&

1.7

•

•

'

o

o I-5

°

e

o ~

/ /

• ~

o • oe

•

•

oo oo

o

o •

£ o o

E o a

o

o

-1"1

0

o

1

2

3

4

5

Km

b

Moximum Possible Attenu~ion DistQnce Below 3Kin

Fig. 5. Maximum possible path lengths of abnormal attenuation (from Fig. 4 after eliminat. ing segments intersected or overlapped by normally attenuating paths) plotted against P/S dominant frequency, normalised for variable path lengths above 3 km (see text). Unfilled circles are wave paths from earthquakes originating at 3 and 4 km depths, and filled circles those from earthquakes at depths of 5--10 km. Data for earthquakes shallower than 3 km and deeper than 10 km are omitted. The straight line marks the relationship accepted, between the path length of abnormal attenuation and the P/S dominant frequency.

ing material, a rate o f a t t e n u a t i o n a b o u t 26 times greater t h a n t h a t inferred f o r the superficial layer, f r o m the records o f the t w o explosions m e n t i o n e d above. At t h e c h o s e n t h r e s h o l d o f 1.3 f o r the P/S f r e q u e n c y ratio, this gives an a n o m a l o u s l y a t t e n u a t i n g p a t h length o f a b o u t 300 m, while t h e largest P/S f r e q u e n c y ratio observed, 3.5, is equivalent t o a b o u t 5.6 k m (this, being observed in t h e case o f an e a r t h q u a k e at a b o u t 30 k m d e p t h , is a t t r i b u t e d to a t t e n u a t i o n d e e p e r t h a n 10 k m : the largest ratio f o r an event shallower t h a n 10 k m was 1.95, equivalent t o a p a t h length o f a t t e n u a t i o n o f a b o u t 1.9 km). An i n d i c a t i o n t h a t this relationship b e t w e e n a t t e n u a t i o n p a t h length and P/S d o m i n a n t f r e q u e n c y is o f the c o r r e c t o r d e r is given b y Fig. 6, in which the ratio is p l o t t e d against the a t t e n u a t i o n distance expressed in t e r m s o f the n u m b e r o f wavelengths. T h e wavelength ~ = V s / f s , where Vs is the v e l o c i t y o f the S-wave in t h e a t t e n u a t i n g m e d i u m , and fs is the observed d o m i n a n t freq u e n c y o f the S-wave. Vs is t a k e n as 2.57 km/s, f r o m Murase and M c B i r n e y ( 1 9 7 3 , fig. 13c), which shows P- and S-wave velocities f o r an andesite f r o m M o u n t H o o d , Washington. This is the value a p p r o p r i a t e t o a melt o f a b o u t

143

70%, at a temperature of ca. 1040°C. This rock varies somewhat in chemical analysis from Cole's (1978) averages for the Tongariro Volcanic Centre andesites, being more siliceous and aluminous, and with lower lime and magnesia, but the variation is within 5%, and the velocity is probably therefore comparable. Fig. 6 shows that the limiting threshold of 1.3 for the P/S dominant frequency corresponds to a b o u t 0.6X. For attenuation to become detectable only at distances greater than a b o u t half a wavelength is very much what one would expect. Path lengths of anomalous attenuation, derived from the relationship shown in Fig. 5, and corrected for variable distances in the superficial material above 3 km, are marked in Fig. 4 a--h, in such a w a y as to group intersecting and neighbouring attenuated paths as closely as possible together in three dimensions, and to maximise their distance from paths which show no such attenuation. This is essentially the m e t h o d of overlapping and intersecting paths used by K u b o t a and Berg (1967) in Alaska, but because of the small area of the 5-

3

0~

~2

E z 1

o

I ¢ )'0

1"1

I

)

I

I

)

)

)

I

I

1"2

1'3

1-4

1"5

1"6

1'7

1"8

1'9

2"0

Norrnaii~td P/S Dominant Frequency (normaiimd for path ler~th alx~e 3Kin)

Fig. 6. Path lengths o f a b n o r m a l attenuation, expressed as number of wavelengths, plotted against P/S dominant f r e q u e n c y .

144 Tongariro National Park and the larger number of wave paths which traverse it, it is here possible to apply the m e t h o d in far greater detail. It is clear t h a t the resulting areas of anomalous attenuation marked in Fig. 4a--h, are not the only ones that could have been inferred. They do, however, provide a mutually better fit than other possibilities, although there is no doubt that this interpretation will need to be modified as subsequent earthquakes fill in the picture. Note that some paths are of much greater value than others in establishing where attenuation takes place. Earthquakes at very shallow depths, 3 and 4 km, generally have gently inclined paths which result in long segments for each depth interval: these may be hard to interpret, as there is often a wide choice as to where to allot the anomalous path length. On the other hand, such shallow earthquakes are valuable, because ambiguity about where attenuation takes place is restricted to only one or two possible depths, providing it is accepted that the structure above 3 km has been accurately modelled and the attenuation in this area satisfactorily accounted for. Earthquakes deeper than 10 km are of little value in the present study, and their paths have been left uninterpreted unless they show a good mutual fit with others, since attenuation in these can be attributed to levels deeper than 10 km, for which there is presently insufficient information. The most useful earthquakes fall in the depth range 5--7 km, for which there is neither too great an ambiguity about the depth to which attenuation should be assigned, nor individual segments of the path which are too long to be easily interpreted. Fig. 7 summarises all attenuated paths, segments overlapped by normal paths being omitted; depths of the earthquakes and of inferred areas of attenuation are marked. Zones of abnormal attenuation, shown in Figs. 4a--h and 7, are shown contoured in Fig. 8, in relation to the volcanoes, surface faults, and other points of reference in the area. Relative degrees of uncertainty in the definition of the zones are indicated on the figure. The shape of the bodies, and their general disposition, is in good agreement with the hypothesis that they represent intrusions of magma. Although none precisely underlie active or recently active vents in the area (Ruapehu Crater Lake, the parasite crater on the northeast slopes, Lower and Upper Tama Lakes, Ngauruhoe, Pukekaikiore and Red Crater), several, especially the body west of Crater Lake, and that beneath the western slopes of Ngauruhoe, very nearly do so. In view of uncertainties in the structural model, and in location of the earthquakes, as well as those in assigning attenuation to particular segments of the paths, the close approximation to the present active volcanoes is remarkably good. So too is the fit to several of the surface faults in the area, notably the inferred extensions of the faults on the southwest slopes of Ruapehu, the fault running north from near Te Heu Heu (T in Fig. 8), and the most westerly of the faults near Pukekaikiore (PK, Fig. 8). There appear to be three large bodies beneath Ruapehu. One underlies part of the western and southwestern slopes between Crater Lake and Turoa (TU); a second lies beneath the eastern side of the Summit Plateau {roughly

145

........

]

?km

i" 12" e

30"

~18"

9"

5-clepth of attenuation 5"-d~th of earthquake 5"-depth of earthquake and attenuation eS-earthcluakein magma (all depths are km Ruapehu Crater Lake, +-0'5km )

(/'~_~

L'7, 7"7 +11,12,16,22

16"

6"

Fig. 7. Summary of all abnormally attenuating wave paths, between depths of 10 -+ 0.5 km and 3 -+ 0.5 km below datum. Segments overlapped or intersected by normally attenuating paths have been omitted. The parts of the paths to which attenuation is attributed are shown in black, and their depths noted. The depth of origin of each earthquake is marked (figure followed by a long dash), except for events below Crater Lake. Shaded areas on paths denote lengths of attenuation for which there is insufficient evidence to show where along the path they should be placed. C, G, N, see caption Fig. 4. N' = true north.

the triangle defined by the peaks TA, P and T): while the third extends under much of the northeast slope of the mountain, although it does n o t extend as far north as the parasite crater. The first two of these bodies are detectable to depths of about 9 km below Crater Lake. Assuming t h e m to be magma, t h e y have intruded, perhaps along fault zones, from the southwest and north respectively. The latter underlies part of the North Crater of Ruapehu, which was extremely active between 5000 and 10,000 years ago, and towards its southeast side reaches within 3 km of the surface. Between the closed contour

146 II

1

RC

MP rf

I

I w

11

~,i t i}~,

I

_•

t

F"

B" /

UT

. ,-:.~.r." / '¢"~i

TF

C

/x

/i I

'-i

"

I

iv

/

,_ll

"

t,",# I

~

.

I

,

/.,

G

..... "......

P. *

/

,, ....

.4"~~-z

,.,-i')f," ~.. L-~," TU i~/~ f ,-~

",..~,'Tr

/~"~

#~"

..

/, /

_

")

.-, : 'fT"

, J

....

OI

ll~

,'T3 'J

t,t~,,

ZTA •

3

I

"

;,,-_6,-., ,~

,,"~ , / ". 5 ~ t//

?~ t/ )

/9

~o-.
/ l

~_._~4

1 " km

2l

• Peak ® S~ismameter ,n, Hut i Lake I~ Crater Lake Q Crater • 'Magmatic' earthquake;seet e x t D'

Fig. 8. Map showing inferred attenuating bodies, interpreted as magma bodies. The figures indicate the d e p t h in kin below Crater Lake, Ruapehu, to which each c o n t o u r refers. Solid c o n t o u r s d e n o t e well-established boundaries, short dashed c o n t o u r s m o d e r a t e l y well-established boundaries, and d o t t e d c o n t o u r s poorly-defined parts of the boundary. The seism o m e t e r sites, volcanoes, and p r o m i n e n t localities are marked. Double lines represent a main highway, c o n t i n u o u s lines o t h e r roads, and short dashed lines selected tracks in the area. Long heavy dashes are p r o m i n e n t faults (with mark on d o w n t h r o w n side), and very short dashes their inferred extensions (after Grindley, 1960). Sections A ' - B ' , C ' - D ' , and E ' - F ' (see Figs. 9--11) and their widths, are marked. Abbreviations are as follows: A C = New Zealand Alpine Club Huts, near Iwikau and T u k i n o ; C -- CNZ (Chateau) seismometer; C L = Crater Lake, R u a p e h u ; G = GSZ (Glacier Shelter) seismometer; I W = Iwikau village;

147 at 3 km depth (south of T and northwest of AC) and the seismograph at GSZ (G), is a source of shallower attenuation. This has not yet been mapped. The third large b o d y under Ruapehu is detectable to a depth of about 7 km and rises to within at most 3 km of the surface. It does not appear to be faultcontrolled, nor is it related to any surface feature: however, together with the other two bodies, it roughly parallels the northeasterly strike of the zone containing the active volcanoes, and, at its northern end, it underlies part of the northeast slopes of Ruapehu from which recent lava flows have been extruded, possibly on a fissure upslope of, and linked to the parasite crater (G.W. Grindley, personal communication, 1980). There are other, smaller bodies beneath the eastern slopes of Ruapehu, north and west of Tukino (TK, Fig. 8). Others lie to the west, one of which, at about 3 km depth, is well-defined. The other, plotted at a depth of 8--10 km, is no more than a guess, since it depends on two wave paths which originated at about 15 km depth. None of these bodies has any obvious connection with surface volcanism or observed faults. Several of t h e m show a conspicuous east-west elongation, which is also apparent in the eastern arm of the wedge-shaped body that underlies the northeast slopes of Kuapehu. In the Ngauruhoe area, few earthquakes have taken place, none deeper than 6 km. Hence there is insufficient evidence to do more than point to the existence of two small bodies, both at a depth of about 3 km. One underlies the western flank of the small cone of Ngauruhoe, and the other follows closely the strike of a fault to the southwest of Pukekaikiore (PK, Fig. 8). It is notable t h a t both are in areas in which small low-frequency, very shallow earthquakes of unusual appearance take place; these are interpreted as probably of volcanic origin (i.e. as having taken place in, or very close to magma). East and southeast of the Tama Lakes are two bodies; one is well-defined at a depth of about 3 km near Waihohonu Hut, close to the southwest end of Grindley's (1960) fault zone; the other lies to the west, at a depth of 5--6 km. A prolongation of the line joining these two intersects the line passing through Ruapehu Crater Lake, both Tama Lakes, and Ngauruhoe, along which almost all recent volcanic activity has taken place, at about the location of the parasite crater on the northeast slopes of Ruapehu, which was the source of lava flows erupted between 5000 and 10,000 years ago. West of the Tama Lakes, there are two, and possibly three small bodies, at depths of 4--5 km. The most easterly of these is based on a wave path originating at 15 km depth, and is therefore only a valid estimate if the attenuation

LT = Lower Tama Lake; M = Mangaturuturu Hut; MP = Mangatepopo Hut; N = NGZ (Ngauruhoe) seismorneter; NG = Ngauruhoe volcano (crater); P = Paretetaitonga; PC = P a r a s i t e Crater o n n o r t h e a s t s l o p e s o f R u a p e h u ; P K = P u k e k a i k i o r e ; P N = P u k e o n a k e ; R C = R e d Crater ( T o n g a r i r o ) ; T = T e H e u H e u ; T A = T a h u r a n g i ; T F = T a r a n a k i Falls; T K = T u k i n o ; T U = T u r o a , U T ffi U p p e r T a m a L a k e , W = W a i h o h o n u H u t , WH = W h a k a p a p a village, x ffi e a r t h q u a k e , J u l y 9, 1 9 7 9 ( 2 1 : 4 7 U . T . ); M L ffi 3 . 1 ; d e p t h 5 k m ; i n f e r r e d t o h a v e o r i g i n a t e d in m a g m a (as o t h e r s b e n e a t h Crater L a k e , R u a p e h u ) . N' = t r u e n o r t h .

Pipe

7"7

4.7~

Lake.depth h lain

~uapehuCrater

i

i

i

i'--~ >

..

--'

~,

GST.

i

'/

•

l

I/

\

/

I ~

liT/

,

"I

-'-~

l

.i

I-

,~ ./.

,'/-

.'/

.

"~'-il / • .It'# ".g2

./

i

-

///

//

~d

----'-

i

,

~

/

/

I

y/.///,"

i"l

l

\

,,

1.

Ngauruhoe

,

if

NE 20kin l i

~ ~ ~"~

i

• = earthquake in magma

\\

/ i ' l l

i

,..- ~, , i . . l / ' t -

_ S//,- "///'/ ,¢. / / / /

/~ f / :/ // ~---- - - ' ; /

,, 1,."

_l,l.'~i,,aLl

"2/.~ ~ /

I I ,; ~

~ _..d~x\/ /

/

10 I

. " 4 ~ ~

~ N

i

/

'V /

~--

i

~,x_

,

f # . ~ ' # # ' / . . / / #iV

I

.e.#;-y

,

~

~

/ ../.l i \ \

iluop~hu

I

, i ~ _ . g f/I,m' ~ II I //! f~ll

ti~l

i

,Z'

i

~22

i-

s-1

3.8

,

2km

11

t

l

-6

-5

-'

-3

K

0 I

SW

Oo

149

is arbitrarily assumed to have taken place above 10 km. The b o d y lies close to the intersection of t w o faults, one of which is inferred, from the preceding discussion, to be associated with magma intrusion at b o t h its extremities. The central of the three bodies lies very close to the seismograph at CNZ, and may well be the source of weak volcanic tremor, and of very small low-frequency volcanic earthquakes, which are occasionally recorded solely at tl~is station. Apart from earthquakes giving rise to waves which are abnormally attenuated along paths to the stations, there are other events which yield P/S dominant frequencies/> 1.3, for which the entire path length to some or all of the seismographs is overlapped by normally attenuating paths (i.e. P/S dominant frequency < 1.3). With the single exception of a well-located earthquake at 5 km depth, of magnitude M L = 3.1, beneath an inferred fault, and very close to the northern end of the presumed magma b o d y underlying the Summit Plateau at Ruapehu (X in Fig. 8), all such earthquakes have been directly beneath Ruapehu Crater Lake, at depths ranging between 31/~ and 22 km (see Figs. 9 and 10). It is inferred that these earthquakes t o o k place actually in magma, or in a volume of rock extensively permeated by magma, perhaps after the manner of a stockwork intrusion (Challinor, 1967). Many other earthquakes, with normal attenuation, take place directly beneath Crater Lake, giving rise to the long wave paths, composed of waves from many earthquakes at various depths, which are conspicuous in Fig.4a--h (see especially Fig. 4a). Because some of these occurred at the same, or almost at the same depth as those which showed the abnormal attenuation, it is likely that the latter originated in a narrow zone with horizontal dimensions smaller than the standard error of epicentre location, which for these events has varied from a b o u t 0.3 km to more than 1 km. Because of the pronounced vertical distribution of the earthquakes, this almost certainly should be construed as a vertical pipe. The earthquakes that show normal attenuation probably take place in the surrounding wall rock, and those abnormally attenuated within the pipe itself, the accuracy of epicentre location being insufficient to resolve the two classes. Earthquakes interpreted as within the pipe have, up to the present, totalled ten, ranging in magnitude from M L 1.4 (at 31/2 km depth) to 3.5 (at 22 km depth), with an overall tendency for magnitude to increase with depth (but no

Fig. 9. Vertical section along line A'-B' (Fig. 8), R u a p e h u to Ngauruhoe, showing abnormally attenuating segments of wave paths (filled rectangles) and boundaries of the attenuating areas (identified as m a g m a bodies): see c a p t i o n to Fig. 8 for degrees of reliability of solid, dashed and d o t t e d boundaries. N o r m a l l y attenuating paths (dashed lines, with crosses where t h e y pass behind, or in front o f inferred m a g m a bodies) are also shown, together with assumed structure (P-wave velocities in kin/s), and earthquakes inferred to have taken place within m a g m a (for X, see c a p t i o n to Fig. 8). Relative S-wave a t t e n u a t i o n in the latter is shown, in the directions of the seismometers (C = CNZ, G = GSZ, N = NGZ), in vertical sections viewed horizontally f r o m the east, with wave paths p r o j e c t e d o n t o a north-south plane. The heavy arrow indicates a b n o r m a l a t t e n u a t i o n at shallower levels, in the direction shown.

I

I

1

I

I

1

I

I

Gs2

I

.

R+

10 I

Y

I

\

I

I

I

Fig. 10. Vertical section along line C’-D’ (Fig. 8), at right angles to that in Fig. 9, showing (magma), together with normal wave paths, and assumed structure (see caption, Fig. 9).

t 12L km

11

10-

9-

8-

7-

6-

5-

4-

3-

O-

C'

0

NW

inferred

I

1

I

abnormally

in

w

I

zones

magma

SE

attenuating

l=earihqwke

2.35

1

km

D

_

/

/

\

\

,~

"l-

,~

,

~',,

'

~

~

l

I

i

I

/

:.

~

~

Ngauruhoe Pukekaikiore / L / / ~~ \ /

~

,l

2"35

' .,o.ga~,ro "-

!__

~....~_/y . ~ -- ~.~--,..__ :~-..'~',.._~ ---__~---_ ~

1

. ""-'~ /

i-

I

/" "~fi-.~ +-. . . . . . . . . . . . . . . . . . .

*.

,~ ",, ,,

I ~ ~ ~. I x a" ~

./..-f:

/

.

,// ,,

CNZ

I

1t0

20km F"

ENE

Fig. 11. Vertical section along line E ' - F ° (Fig. 8), passing through CNZ and Ngauruhoe, showing inferred abnormally attenuating zones (magma), together with normal wave paths, and assumed structure (see caption, Fig. 9).

10'-km

9

8

7.

6~

5-

4-

3 -

2sea le,,el--

0

1E"

wSW 0

O1

152

definite depth-magnitude relationship). It is noteworthy that in only 30% of these cases was the trace amplitude of the S-wave less than that of the P-wave. The most extreme attenuation within the pipe has taken place in an earthquake at a depth of 16 km. From the relationship between P/S dominant frequency and path length of attenuation, derived in Fig. 5, this wave path should indicate attenuation over a distance of about 3.7 km, whereas a maximum of about 0.5 km is available. The condition of similarity between the frequency spectra of P- and S-waves at the focus of these earthquakes is, however, no longer valid, and therefore the relationship in Fig. 5 no longer applies. It is more likely that the reduction in dominant frequency for earthquakes in which magma is present at the source, which implies a rate of attenuation at least 7% times more rapid than that deduced from Fig. 5, derives from the fact that only low-frequency S-waves are produced at the focus, rather than that the melt is substantially more complete than the 70% hitherto inferred. Thus only relative attenuation in the directions of the seismometers can be measured within the pipe. This is shown, for event X and earthquakes below Crater Lake, in Fig. 9 (vertical section viewed horizontally from the east, with wave paths projected onto a north-south plane). Wave paths, with both normal and abnormal attenuation, are shown in vertical section in Figs.9-11, together with an interpretation of the shape and interconnections of the attenuating areas, shown contoured in Fig. 8. Fig. 9 is a section along the strike of the volcanic axis, passing through Ruapehu Crater Lake and Ngauruhoe volcano: Fig. 10 is a section at right angles to the above, passing through Ruapehu Crater Lake: and Fig. 11 is a section along a line passing through the CNZ seismograph and Ngauruhoe volcano. These profiles are marked in Fig. 8. DISCUSSION It is questionable to what extent a narrow magma body can be detectable through ray tracing, when it is aligned parallel to the direction of the ray. The greater part of the energy of a wave-front may avoid such a body, and travel only in the wall rock, in which compressional and shear-wave velocities will generally be higher. In such a case, the seismograph will simply fail to “see” any attenuation anomaly. When therefore such an anomaly is detected, yielding on analysis a linear body like many of those shown in Fig. 8, it is likely that its true size will be substantially larger than that indicated. In the present study, this is probably particularly true of the intrusion beneath the east side of the Summit Plateau at Ruapehu (X to AC, Fig. 8). A careful examination of Fig. 4a-h, will show the directions in which enlargement is possible. All the intrusions shown should be regarded as of minimum size, especially the narrow and elongated ones. Quantitative estimates of attenuation across magma bodies have been made by Matumoto (1971) and by Aki et al. (1978), the former on rhyolitic, and the latter on basaltic magma. Matumoto estimated a viscosity value of lo8 cgs

153

units, b y means of Fourier analyses of P-waves, and discussed some implications of this estimate on the temperature of subsurface magma. His results, however, cannot be applied directly to the New Zealand andesites. Aki et al. recorded 9 Hz S-waves (horizontally polarised), which t h e y convincingly proved had been transmitted through a thin magma lens, attributing this to the presence of vesicles, causing the magma to behave as a solid below, and as a liquid above, a certain threshold stress. To apply these results in the Tongariro National Park, and calculate attenuation using visco-elastic theory, so that P/S dominant frequency might be estimated across magma bodies of various shapes and sizes, would require a careful evaluation of the petrological evidence in order to estimate critical properties of the melt. Murase and McBirney (1973) made direct high-temperature measurements of a number of important properties of a somewhat comparable andesite. However, it is not possible to use these measurements without modification, because t h e y were made at atmospheric pressure on an essentially anhydrous melt, of somewhat higher acidity than those typical of Ruapehu and Ngauruhoe. Whereas compositional differences, between Murase and McBirney's Mount H o o d andesite and our andesites, are sufficiently small to allow a reasonable estimate of the rock velocity to be made, they are still t o o great for a ready estimation of viscosity, on which the calculation of attenuation largely depends. There is petrological evidence that viscosity calculated from the wholerock chemical analyses of typical Ruapehu and Ngauruhoe andesites will be many orders of magnitude t o o low if applied to the molten fraction of subsurface magma. This is due to the fact that there are marked differences between the chemistry of the groundmass and that of the whole-rock, in magma erupted from Ruapehu during 1969 (C.P. Wood, personal communication, 1980), the former being rhyolitic, and the latter dacitic. This complex question will be examined in a future paper, in which an a t t e m p t will also be made to locate sources of abnormal attenuation in the shallow layers, above 2 km, near Ruapehu and Ngauruhoe. CONCLUSIONS

S-wave attenuation, in the form of lowering of the dominant frequency of the S-, relative to that of the P-waves, has been mapped b y a m e t h o d of threedimensional ray tracing, using overlapping and intersecting paths, in the immediate vicinity of Ruapehu and Ngauruhoe volcanoes, for depths between 2 and 10 km. Abnormally high attenuation is attributed to passage of the waves through magma, which is estimated, on petrological evidence, to be a b o u t 70% molten. Inferred magma bodies have been found by estimating the path length of anomalous attenuation, through a consideration of the maximum possible path lengths over which given P/S dominant frequency ratios are observed, correcting as well as possible for the effect of transmission of the waves through the superficial layers.

154 In the absence of reliable focal mechanism determinations for the earthquakes, the use of reduced S-wave amplitudes, relative to those of the P-waves, has been shown to be unreliable. Nearly 75% of cases in which the m a x i m u m trace amplitude of the S-wave is less than that of the P-wave are unaccompanied by any significant reduction in the dominant frequency ratio, so that in these cases the reduction in S amplitude must be attributed to source radiation and the proximity of the wave paths to nodal planes for the S-wave. Similarly, in more than 80% of the cases in which true attenuation, as defined by a reduction in P/S d o m i n a n t frequency, was observed, the maximum trace amplitude of the S exceeded that of the P-waves, thus indicating that at source the generation of S-waves must have been greatly in the ascendant. These results underline the dangers of using amplitude rather than frequency of S-waves to define attenuation. Inferred magma bodies are convincing in terms of their geometry and location. They lie close to the existing active vents and show elongation parallel to the trend of the volcanoes. Several are closely associated with observed faults, or their inferred extensions. Three principal magma bodies underlie Ruapehu; one beneath much of the southwest slopes, extending as far east as Crater Lake, is mapped between 9 and 5 km below datum; a second underlies part of the North Crater of Ruapehu, and is conspicuously aligned parallel to a fault; and a third underlies much of the northeast slope of the volcano, but does not extend as far to the northeast as the recently active parasite crater. Both the second and third extend to within 2--3 km of the surface. Ngauruhoe volcano is underlain by a small body of magma at 2--3 km depth, and another lies, apparently in a fault zone, a short distance to the west. A lack of earthquakes, however, limits the extent to which magma can be mapped in this area. Other small bodies appear to define two zones, at about 60 ° to the strike of the volcanic axis, and on both sides of it. One of these lies close to the seismometer site at Chateau, and may be the source of occasional volcanic earthquakes and volcanic tremor recorded in this area. Earthquakes, in which a reduction in dominant S-wave frequency cannot be explained by attenuation along the path, underlie Ruapehu Crater Lake at depths between 31/~and 22 km. Such events do not occur elsewhere, with the single exception of an earthquake closely associated with one of the magma bodies north of Ruapehu. These are considered to represent events generated in, or very near magma. Those under Crater Lake are interpreted as taking place within a vertical pipe which is less than about 300 m in diameter over all or most of its length. ACKNOWLEDGEMENTS I wish to t h a n k m a n y of m y colleagues at Geophysics Division, D.S.I.R., Geological Survey (Lower Hutt), and Victoria University of Wellington, for helpful criticism of this paper. Particular thanks are due to Professor R.H.

155 Clark, J.W. Cole, R.R. Dibble, G.W. Grindley, W.R. Hackett, R. Robinson, E . C . G . S m i t h , W . D . S m i t h , a n d C.P. W o o d . T h e r e s e a r c h w a s a s s i s t e d b y t h e provision of a grant by the Lottery Profits Scientific Research Distribution Committee of New Zealand. APPENDIX 1 : EARTHQUAKES USED IN THIS STUDY Origin-times, magnitudes, epicentres and depths of focus of earthquakes used in this work are included in annual summaries (Latter, 1977, 1978, 1979b (for 1976, 1977 and 1978 data, respectively): 1979 data are in preparation). The earthquakes used are those identified in these summaries as " A - t y p e " volcanic earthquakes at Ruapehu, Ngauruhoe or Tongariro, together with events listed as in the areas of Whakapapa village (ChateauTongariro), Taurewa, Waihohonu Hut, Tama Lakes, southeast slopes of Ruapehu, Mangawhero Lodge and Hauhungatahi Wilderness. Earthquakes with very poor locations, and those in which the seismic traces overloaded, have been omitted, and only those events within the crust have been used. The three 1975 earthquakes used are Nos. 5--7 in Latter (1976). Earthquakes during 1977, marked ** in the summary, together with event No. 64 and three additional events beneath Ruapehu Crater Lake in 1976, and the three 1975 earthquakes (all of which were determined without station corrections), have been recomputed to bring them into line with events for which station corrections have been used (those marked *** in the 1977 list). These recomputations will be published with the 1979 data, in a future number of the New Zealand Volcanological Record. Dominant frequencies of P- and S-waves, which, together with the hypocentres, comprise the basic data for this paper, are unpublished, but can be provided on request, as also can trace amplitude data. REFERENCES Aki, K., Chouet, B., Fehler, M., Zandt, G., Koyanagi, R., Colp, J. and Hay, R.G., 1978. Seismic properties of a shallow magma reservoir in Kilauea Iki by active and passive experiments. J. Geophys. Res., 83(B5): 2273--2282. Aspinall, W.P., Michael, M.O. and Tomblin, J., 1976. Evidence for fluid bodies beneath the Sulphur Spring geothermal region, St. Lucia, West Indies. Geophys. Res. Lett., 3(2): 87--90. Balesta, S.T. and Farberov, A.I., 1968. Seismic studies of Piip Crater break-through. Bull. Volcanol., 32(2): 395--399. Berg, E., Kubota, S. and Kienle, J., 1967. Preliminary determination of crustal structure in the Katmai National Monument, Alaska. Bull. Seismol. Soc. Am., 57(6): 1367--1392. Challinor, J., 1967. A Dictionary of Geology, University of Wales Press, Cardiff, 3d ed., 298 pp. Clark, R.H., 1960. Petrology of the volcanic rocks of Tongariro Subdivision. In: D.R. Gregg, The Geology of Tongariro Subdivision. N.Z., Dep. Sci. Ind. Res., N.Z. Geol. Surv., Bull., N ser., 40: 107--123. Cole, J.W., 1978. Andesites of the Tongariro Volcanic Centre, North Island, New Zealand. J. Volcanol. Geotherm. Res., 3: 121--153. Einarsson, P., 1978. S-wave shadows in t h e Krafla Caldera in NE-Iceland, evidence for a magma chamber in the crust. Bull. Volcanol., 41(3): 187--195. Farberov, A.I. and Gorelchik, V.I., 1971. Anomalous seismic effect under volcanoes and some features of deep-seated structure of volcanic areas. Bull. Volcanol., 35(1): 212--224. Fedotov, S.A., 1965. Upper mantle properties of the southern part of the Kuril Island arc according to detailed seismological investigation data. Tectonophysics, 2(2-3): 219--225.

156 Fedotov, S.A. and Tokarev, P.I., 1974. Earthquakes, characteristics of the upper mantle under Kamchatka, and their connection with volcanism {according to data collected up to 1971). Bull. Volcanol., 37(2): 245--257. Firstov, P.P. and Shirokov, V.A., 1971. Seismic investigation of the roots of the Kliuchevskaya group volcanoes, Kamchatka. Bull. Volcanol., 35{1): 164--172. Garrick. R.A., 1968. A reinterpretation of the Wellington Crustal Refraction Profile. N.Z. J. Geol. Geophys., 11(5): 1280--1294. Gorshkov, G.S., 1958. On some theoretical problems of volcanology. Bull. Volcanol., 19: 103--113. Gorshkov, G.S., 1971. Prediction of volcanic eruptions and seismic methods of location of magma chambers -- a review. Bull. Volcanol., 35( 1 ): 198--211. Gorshkov, G.S., 1972. Progress and problems in volcanology. In: A.R. Ritsema (Editor), The Upper Mantle. Tectonophysics, 13(1-4): 123--140. Gorshkov, G.S. and Farberov, A.I., 1974. Magma chambers and roots of volcanoes in island arcs. Acta Geol. Acad. Sci. Hung., 18{3-4): 235--242. Gregg, D.R., 1960. Volcanoes of Tongariro National Park. N.Z. Dep. Sci. Ind. Res., Inf. Ser., 2 8 : 8 2 pp. Grindley, G.W., 1960. Geological Map of New Zealand, 1 : 2 5 0 000. Sheet 8 - - T a u p o . New Zealand Department of Scientific and Industrial Research, New Zealand Geological Survey. Hatherton, T., 1970. Upper mantle inhomogeneity beneath New Zealand: surface manifestations. J. Geophys. Res., 75(2): 269--284. Kubota, S. and Berg, E., 1967. Evidence for magma in the Katmai Volcanic Range. Bull. Volcanol., ;51 : 175--214. Latter, J.H., 1976. Shallow seismicity of the Taupo Volcanic Zone and Mt. Egmont area. N.Z. Volcanol. Rec., 5: 13--22. Latter, J.H., 1977. Shallow seismicity of the Taupo Volcanic Zone and Mr. Egmont area. N.Z. Volcanol. Rec., 6: 11--18. Latter, J.H., 1978. Shallow seismicity of the Taupo Volcanic Zone and Mt. Egmont area. N.Z. Volcanol. Rec., 7: 12--19. Latter, J.H., 1979a. Volcanological observations at Tongariro National Park, 2. Types and classification of volcanic earthquakes, 1976--1978. N.Z. Dep. Sci. Ind. Res., Geophys. Div. Rep., 1 5 0 : 6 0 pp. Latter, J.H., 1979b. Shallow seismicity of the Taupo Volcanic zone and Mt. Egmont area. N.Z. Volcanol. Rec., 8: 6--26. Latter, J.H., 1981. Volcanic earthquakes, and their relationship to eruptions at Ruapehu and Ngauruhoe volcanoes. J. Volcanol. Geotherm. Res., 9{4): 293--309. Matumoto, T., 1971. Seismic body waves observed in the vicinity of Mount Katmai, Alaska, and evidence for the existence of molten chambers. Geol. Soc. Am. Bull., 82: 2905--2920. Matumoto, T. and Molnar, P., 1967. A shadow effect on S waves observed in the vicinity of Mount Katmai, Alaska. Trans. Am. Geophys. Union, 48: 199. Matumoto, T. and Ward, P.L., 1967. Microearthquake study of Mount Katmai and vicinity, Alaska. J. Geophys. Res., 72(10): 2557--2568. Mooney, H.M., 1970a. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75(2): 285--309. Mooney, H.M., 1970b. Theoretical and observed travel times for New Zealand deep earthquakes. N.Z.J. Geol. Geophys., 13(3): 703--717. Murase, T. and McBirney, A.R., 1973. Properties of some common igneous rocks and their melts at high temperatures. Geol. Soc. Am. Bull., 84: 3563--3592. Oliver, J. and Isacks, O., 1967. Deep earthquake zones, anomalous structures in the upper mantle, and the lithosphere. J. Geophys. Res., 72( 16): 4259--4275. Sanford, A.R., Mott, R.P., Shuleski, P.J., Rinehart, E.J., Caravella, F.J., Ward, R.M. and Wallace, T.C., 1977. Geophysical evidence for a magma body in the crust in the vicinity of Socorro, New Mexico. Am. Geophys. Union, Geophys. Monogr., 20: 385--403.