Jökull - 01.12.1976, Page 18

TIME-SPACE DISTRIBUTION OF EARTHQUAKES
_ RECORDED NEAR HEKLA JUNE22-AUG.5 1970
' i i i i ’ i i i i i i i i ' i
I—L-L-L-l—1—LJ-J
JUNE
30 'august
Fig. 6. Time-space distri-
bution of earthquakes
near Hekla recorded by
the temporary seismic ar-
ray. The distance of the
epicenter from the center
of the cluster of earth-
quakes (see Fig. 5) is
plotted versus the time of
occurrence.
Mynd 6. Dreifing skjdlfta
i goslok. Myndin sýnir,
hvernig upptök skjálft-
anna farðust burt frd eld-
stöðvunum eftir að gosi
lauk 5. júlí.
tions of the P-waves were recorded. A mono-
polar source of the earthquakes is therefore un-
likely, they appear to be of tectonic origin.
The magnitude-frequency relationsliip
For most earthquake sequences the Guten-
berg-Richter formula
log n(M) = a — bM
is a reasonable approximation. In this equation
n(M)dM is the number of earthquakes with
magnitude between M and M + dM, a and b
are constants. If N(M) is the number of earth-
quakes of magnitude larger or equal to M a
plot of log N(M) versus M should form a
straight line with the slope of — b.
In Fig. 7 N(M) is plotted versus M for the
Hekla earthquake sequence as recorded at AKU
during the period May 6 to July 14, 1970. The
earthquakes during the initial phase of the
eruption on May 5 are not included in this
study since they may be of a different physical
origin and many of them were masked by the
volcanic tremors. The distribution becomes
nonlinear below M = 3.4 indicating that some
earthquakes below that magnitude were missed
in the count. The maximum likelihood esti-
mate (Utsu, 1971) of the b value for earth-
quakes with M =2 3.4 is 1.6 ± 0.3 (standard de-
16 JÖKULL26. ÁR
viation). The b value is not significantly dif-
ferent if only earthquakes in the magnitude
range 3.4 to 4.2 are used for the calculation.
The physical origin of these earthquakes is
poorly understood, and they may not even have
a common cause. Therefore, large physical
significance should not be attached to the b
value, it is reported here mainly for the reason
of documentation. It should be mentioned,
however, that the b value for the Hekla earth-
quakes is larger than typical values for tectonic
earthquakes and volcanic earthquakes of A-type
and smaller than typical values for B-type vol-
canic earthquakes (Minakami, 1960) but is
similar to the b value reported by Francis
(1968) for earthquakes of the median rift of
the mid-Atlantic ridge. A-type volcanic earth-
quakes are considered to be of tectonic origin,
caused by the stresses within and around the
volcano. B-type earthquakes originate at shal-
low depth beneath the craters of a volcano and
are in some way associated with the eruption
process.
Seismicity outside the Hekla area
The temporary seismic network near Hekla
recorded numerous earthquakes with epicenters
in the western and eastern volcanic zones. Most
of this seismic activity originated near the lake