We showed that the long-period seismic radiation from the

November 29, 1975, Kalapana Hawaii earthquake, which involved

seaward displacement of the south flank of Kilauea

volcano, was best explained by invoking a near-horizontal

single force as the kinematic source of the earthquake [Eissler

and Kanamori, 1987]. In particular, the azimuthal dependence

of 100-s Love surface waves is difficult to explain by a conventional

double-couple source. The unusual Love wave pattern

was noted by Ando [1979] but not explained until our suggestion

of the single-force source model

Eissler, Holly K.

Kanamori, Hiroo

Caltech Authors - Main

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B7, PAGES 8083-8084, JULY 10, 1988 
Reply 
HOLLY K. EISSLER 
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography 
University of California, San Diego, La Jolla 
HIROO KANAMORI 
Seismological Laboratory, California Institute of Technology, Pasadena 
We showed that the long-period seismic radiation from the 
November 29, 1975, Kalapana Hawaii earthquake, which in- 
volved seaward displacement of the south flank of Kilauea 
volcano, was best explained by invoking a near-horizontal 
single force as the kinematic source of the earthquake [Eissler 
and Kanamori, 1987]. In particular, the azimuthal dependence 
of 100-s Love surface waves is difficult to explain by a conven- 
tional double-couple source. The unusual Love wave pattern 
was noted by Ando [1979] but not explained until our sugges- 
tion of the single-force source model. As explained in our 
paper, a two-lobed Love wave radiation pattern could be pro- 
duced by a double-couple source only if the fault dip angle • is 
very nearly zero. At • = 2.5 ø, the sin • (two-lobed) and sin 2• 
(four-lobed) terms already give equal contributions to the 
radiation pattern, and when • = 4.5 ø, the azimuthal pattern is 
essentially four-lobed. Thus the only geometry that produces a 
two-lobed radiation pattern with the double-couple source is 
that of a very flat fault plane, and the fault plane must remain 
flat throughout the rupture process. The single-force model 
does not have this restriction, and the force dip can be in- 
clined to horizontal and still produce the observed two-lobed 
pattern. There is also another problem. If the fault plane is 
purely horizontal at zero depth, no Love wave energy is pro- 
duced by the double-couple source. Thus, as the fault dip 
approaches zero for shallow earthquakes, the seismic moment 
must become very large to explain any appreciable Love wave 
energy. For example, the seismic moment required to generate 
the Love wave amplitudes observed in the Kalapana earth- 
quake with the .double-couple source increases by a factor of 4 
as the fault angle decreases from 10 ø to 0 ø for a source depth 
of 10 km. Ando [1979] rejected models with •i = 0 because the 
seismic moment required to match the observed surface wave 
amplitudes would produce crustal deformation much larger 
than that observed. He found that a dip angle of 20 ø best fits 
the observed subsidence and tsunami patterns. 
Wyss and Kovach [this issue] refer to Figure 4 of our paper, 
which is simply a reproduction of Ando's [1979] results, and 
argue that the radiation pattern can be interpreted as either 
two-lobed or four-lobed. Ando [1979] measured gross time 
domain Love wave amplitudes from as many World-Wide 
Standard Seismograph Network (WWSSN) stations as possi- 
ble of Gx, G 2, and G 3 passages. In Eissler and Kanamori 
[1987] we improved the Love wave pattern in several ways. 
First, we looked only at 100-s spectral data, eliminating short- 
er periods perhaps affected by heterogeneities and source ef- 
Copyright 1988 by the American Geophysical Union. 
Paper number 8B6142. 
0148-0227/88/008 B-6142502.00 
fects. We restricted ourselves to the G 2 passage, as G x can be 
contaminated by multiple S phases and G 3 is approaching 
noise level at most stations. We used several analog records 
from HGLP (high gain long period) instrumentation, not used 
by Ando [1979], which had improved long-period response at 
100 s. To assure the best possible data from WWSSN stations, 
we used only records with higher gains. The appropriate refer- 
ence to our results is Figure 7 of Eissler and Kanarnori [1987], 
which shows a clear and unmistakable radiation maximum at 
the azimuth where the four-lobed radiation pattern of the 
double-couple source predicts a node. Recent work with more 
HGLP data shows that the two-lobed pattern persists out to 
periods of at least 225 s. 
The two-lobed Love wave pattern is an important feature of 
the Kalapana earthquake and cannot be dismissd as unre- 
liable because of fault complexity, as Wyss and Kovach at- 
tempt to do. Although we expect seismic radiation at short 
periods to show the effects of rupture complexity, long-period 
(T > 50 s) waves have been successfully used to correctly de- 
termine the overall source geometry of earthquakes for many 
years. 
Harvey and Wyss [1986] appealed to variations in fault 
orientation to explain the complex nature of accelerograms 
recorded at two stations from the Kalapana earthquake. Their 
results are reproduced in Figure 2 of the comment by Wyss 
and Kovach. Note that several of the subevents have dip 
angles of about 10 ø or greater, which would certainly domi- 
nate the radiation pattern of surface waves, causing the Love 
wave radiation pattern to be four-lobed, if this were repre- 
sentative of the overall source process of the earthquake. Yet 
the observed pattern is clearly not four-lobed. In proposing 
the single-force model, we acknowledged that the first motion 
data, or short-period radiation such as the accelerograms used 
to infer these subevents, may very well be best explained by a 
conventional double-couple source. Our paper addressed the 
long-period radiation only. 
Wyss and Kovach misunderstood the point in our state- 
ment that the south flank of Kilauea is mobile and had under- 
gone extensions of several meters in the last century. We have 
no disagreement with the observation of short-term contrac- 
tion events on Kilauea's south flank that may reflect cyclical 
changes in the volcano's magmatic system. We would expect 
to see crustal shortening and compression prior to the earth- 
quake, regardless of whether it was gravity-driven or magma- 
driven, since there will always be a frictional force on the 
failure plane or planes that must be overcome. Our point is 
that the long-term deformational character of the south flank 
shows extension, or seaward displacement. We do not think 
we misinterpreted the facts presented in the detailed defor- 
mation study of Swanson et al. [1976, p. 13] where they state 
8083 
8084 EISSLER AND KANAMORI.' COMMENTARY 
that 
Long-term horizontal displacements, as measured over periods of 
years to decades, are of considerable magnitude at Kilauea .... 
The entire south flank moved southeastward in a direction 
almost exactly perpendicular to the east riff zone and the Koae 
fault system; this direction is more nearly perpendicular to the 
trend of the riff zone as a whole than to individual fissures within 
the rift zone. 
Swanson et al. [1976] repeatedly describe the south flank as 
mobile to some depth that may be between 5 and 8 km, or a 
gradual decrease in mobility with depth. In a study of the 
ground deformation from the Kalapana earthquake, Lipman 
et al. [1985] showed that the vertical displacements every- 
where on the south flank were negative, that is, the area sub- 
sided. Many researchers have appealed to the volcano-oceanic 
crust interface, presumably lubricated by the accumulation of 
deep sea sediments prior to the development of the volcano, as 
the fault plane of the Kalapana earthquake or the sole of the 
large-scale block slump [Nakamura, 1982; Furumoto and 
Kovach, 1979; Crosson and Endo, 1981; Lipman et al., 1985]. 
Presumably, this interface dips gently under the volcanic load, 
so that a rigid displacement of the south flank along it would 
have a reverse sense. However, if the overall event were a 
low-angle thrust, some uplift should have been observed on 
the south flank. In fact, our model is very similar to Lipman et 
al's [1985] interpretation of the deformation data: regardless 
oi • the details or location of the initial seismic event, a large- 
scale gravity slump is responsible for the overall displacement 
(and hence long-period seismic waves). We envision large-scale 
gravity slumping of the south flank on many subsidiary slide 
planes that may merge with a basal plane producing block 
rotations, such as shown by Lipman et al. [1985, Figure 20], 
Holcomb [1987, Figure 12.61-1, or Hill and Zucca [1987, 
Figure 37.10]. Whether the basal plane coincides with the 
lubricated interface is not known, but it is an appealing idea. 
As we discussed above, we believe that the two-lobed Love 
wave radiation pattern is real and can be best explained by a 
single-force model. Obviously, the actual source process is 
likely to be more complex than one that can be represented by 
a simple single force. However, more complicated kinematic 
source models were not warranted because of the limited qual- 
ity and quantity of the data available to us. The interpretation 
of the data from the Kalapana earthquake is not as clear as 
for the classic landslide associated with the eruption of Mount 
St. Helens, discussed by Kanamori and Given [1982]. Improve- 
ment of the source model may be possible in the future; for 
example, Kawakatsu [1987] has used digital HGLP data from 
the Kalapana earthquake at a few stations only recently com- 
piled by the U.S. Geological Survey. These data are probably 
better than the analog records used by Eissler and Kanamori 
[1987]. Further analysis of the improved data set may be able 
to resolve more details of the source process associated with 
the 1975 Kalapana earthquake, but we believe that our pri- 
mary conclusion that the long-period Love wave radiation 
pattern can be best explained by the single force will remain 
unchanged. We thank Wyss and Kovach for this opportunity 
to clarify our results. 
REFERENCES 
Ando, M., The Hawaii earthquake of November 29, 1975: Low dip 
angle faulting due to forceful injection of magma, J. Geophys. Res., 
84, 7616-7626, 1979. 
Crosson, R. S., and E. T. Endo, Focal mechanisms and locations of 
earthquakes related to the 29 November 1975 Kalapana, Hawaii 
earthquake: The effect of structural models, Bull. Seismol. Soc. Am., 
71, 713-729, 1981. 
Eissler, H. K., and H. Kanamori, A single-force model for the 1975 
Kalapana, Hawaii, earthquake, J. Geophys. Res. 92, 4827-4836, 
1987. 
Furumoto, A. S., and R. L. Kovach, The Kalapana earthquake of 
November 29, 1975: An intra-plate earthquake and its relation to 
geothermal processes, Phys. Earth Planet. Inter., 18, 197-208, 1979. 
Harvey, D., and M. Wyss, Comparison of a complex rupture model 
with the pre-cursor asperities of the 1975 Hawaii M s = 7.2 earth- 
quake, Pure Appl. Geophys., 124, 957-973, 1986. 
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Holcomb, R. T., Eruptive history and long-term behavior of Kilauea 
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Swanson, D. A., W. A. Duffield, and R. S. Fiske, Displacement of the 
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1976. 
Wyss, M., and R. L. Kovach, Comment on "A single-force model for 
the 1975 Kalapana, Hawaii, earthquake" by Holly K. Eissler and 
Hiroo Kanamori, J. Geophys. Res., this issue. 
H. K. Eissler, Institute of Geophysics and Planetary Physics, 
Scripps Institution of Oceanography, University of California, San 
Diego, La Jolla, CA 92093. 
H. Kanamori, Seismological Laboratory, California Institute of 
Technology, Pasadena, CA 91125. 
(Received March 29, 1988; 
accepted April 11, 1988.) 


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