Jökull - 01.01.2019, Blaðsíða 72

Variation in b-value of caldera earthquakes during recent
activity of the Bárðarbunga Volcano in Iceland
Magnús Pálsson1, Páll Einarsson2 and Birgir Hrafnkelsson1
1Department of Mathematics, Faculty of Physical Sciences,
School of Engineering and Natural Sciences, University of Iceland, Dunhagi 5, 107 Reykjavik
2Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík; palli@hi.is
https://doi.org/10.33799/jokull2019.69.071
Abstract — The magnitude distribution of caldera earthquakes in the subglacial Bárðarbunga volcano in Cen-
tral Iceland, characterized by the b-value, shows a systematic variation that is consistent with stress changes
anticipated in the roof of an inflating magma chamber beneath the caldera. The b-value was 0.83 prior to the
rupture of the chamber in August 2014 when a dike propagated laterally from the volcano to feed the eruption
in Holuhraun. The b-value was relatively high following the collapse of the caldera, reflecting low stress in the
magma chamber roof. Half a year later a decrease was observed in the b-value, concurrent with an increase
in the seismicity, consistent with indications of recharging of the volcano magma chamber. The magnitude
distribution was anomalous during the slow collapse of the caldera in association with the eruption. During
this period the earthquake sequence appeared to consist of two populations, only one of which followed the
conventional Gutenberg-Richter distribution. For a subglacial volcano, where geodetic methods are difficult to
implement, the b-value of caldera earthquakes provides an important additional parameter for the monitoring
of magma pressure variations.
Key points
The Bárðarbunga volcano at the center of the Iceland Hotspot is re-inflating following a major eruption and
caldera collapse in 2014–2015.
The magnitude distribution of caldera earthquakes is consistent with increasing stress in the caldera region.
The b-value of the caldera earthquakes provides an addition to the arsenal of useful monitoring parameters for
this remote sub-glacial volcano.
INTRODUCTION
The Gutenberg-Richter relation log N = a – bM is one
way of quantifying an earthquake sequence. Here N is
the number of earthquakes of magnitude M and larger,
a and b are constants. The slope of this linear rela-
tionship, the b-value, describes the relative frequency
of small events versus that of large events and a is the
logarithm of the number of earthquakes of magnitude
0 and larger. For ordinary tectonic areas the value of
b is close to 1 and shows little change. For some ar-
eas, however, volcanic areas in particular, the b-value
is significantly different from 1. The spreading seg-
ments of mid-oceanic ridges, e.g., have consistently
higher b-values, as high as 2.6 (Sykes, 1970; Einars-
son, 1986).
It is generally accepted, following Scholz (1968)
and Wyss (1973), that the b-value is inversely depen-
dent on the stress level in the seismically active vol-
ume. High stress level over a large area favors large
events over small events, hence a low b-value. High
b-value is expected in areas of low stress and hetero-
geneous crust, where small earthquakes are favored.
This has led to numerous studies where the objec-
JÖKULL No. 69, 2019 71