Faulting mechanism of anomalous earthquakes near Bardarbunga
Nettles, M. and G. Ekström, Faulting mechanism of
anomalous earthquakes near Bardarbunga Volcano, Iceland,
J. Geophys. Res., in press, 1998.
The source characteristics of 10 shallow earthquakes of moderate
size (M > 5) beneath Bardarbunga Volcano in Iceland are
investigated using long-period and broadband teleseismic data.
The highly non-double-couple nature of the focal mechanisms for
several of the events previously reported in the Harvard centroid
moment tensor (CMT) catalog is confirmed by analysis of an expanded
long-period body wave data set, as well as by an independent
inversion of intermediate period surface wave data. Modeling of
broadband P waveforms constrains the depths of the events to
lie between 3 km and 8 km beneath the volcano. Significant complexity
in the P waves observed for some events is best explained by a
sequence of subevents of varying geometry, rather than by wave
propagation effects caused by shallow structure near the earthquake.
Inversion of P wave data for two of the earthquakes using a
parameterization with subsources results in sequences of events with
primarily thrust motion on planes of varying strike. The
seismological results are interpreted as faulting on an outward
dipping cone-shaped ring fault beneath the Bardarbunga caldera.
The association in time and space between the September 29, 1996,
earthquake and the September 30 eruption near Bardarbunga and
Grimsvötn Volcanos suggests that inflation of a shallow magma
chamber, and the associated stress loading of the deeper ring fault,
may be the tectonic process which generates the earthquakes.
The map above shows CMT solutions for the 10 earthquakes in the Harvard
catalog which are located near Bardarbunga Volcano. The earthquakes
are highly non-double-couple, and have vertical T axes,
indicative of horizontal compression, despite their location in an
extensional tectonic setting.
The seismological data suggest
rupture propagation on a cone-shaped fault, in which a subsource with
reverse-faulting geometry (solid arrows) occurs on each fault segment in
succession (gray arrow), around a portion of the cone. Ring or cone
faults are well documented beneath caldera structures, and observations
at several active volcanos are consistent with seismic activity on
outward dipping subcaldera faults.
Example of depth determination, for the earthquake of September 29, 1996
(event 10); broadband teleseismic P waveforms (solid) and the calculated
synthetic seismograms (dashed), focal mechanism corresponding to full
moment tensor solution, and source time function determined by the
inversion. Maximum amplitudes (in microns) are shown at right of
seismograms; arrows show picked first arrivals, and thin vertical bars
show time window included in the inversion. No time window is shown for
those traces not included in the inversion due to high background noise
level. All of the Bardarbunga earthquakes are found to have occurred at
depths of 3-8 km beneath the volcano.
The non-double-couple nature of the CMT solutions cannot be
explained in terms of complex structure beneath Bardarbunga. Shown
P waveforms recorded at east lying and west lying stations for two
events with similar focal mechanisms. The waveforms recorded at the two
azimuths are substantially different for a given event, and waveforms
recorded in the west (TUC) for the May 5, 1994, earthquake (event 9) are
very similar to those recorded in the east (ARU) for the June 22, 1993,
earthquake (event 8). Picked arrivals are shown by short arrows; circles
on focal mechanisms denote locations of stations on the focal sphere.
We invert the P waveforms for the two earthquakes which show the
strongest indication of distinct subevents using a parameterization
which allows for several subsources.
Shown above are the multiple source focal mechanism solutions and
composite source time
functions retrieved by very broadband analysis for the earthquakes of
(a) June 22, 1993 (event 8), and (b) May 5, 1994 (event 9).
Comparison between observed seismograms (solid lines) for event 9 and
synthetic seismograms (dashed lines) resulting from a best fitting single
subsource focal mechanism; (right) comparison between observed
seismograms (solid lines) and synthetic seismograms (dashed lines)
resulting from our preferred focal mechanism, which consists of three
This schematic diagram shows a possible mechanism for production of
earthquakes on cone fault structures underlying the volcanic caldera.
Inflation of a shallow magma chamber fed from a deeper source leads to an
increase in pressure in the magma chamber, increasing stresses on rocks
below the chamber. Such stress increases may lead to rupture on cone
faults beneath the caldera. We note that the location of the magma source
region is not well known, and may be subcrustal.
Seismicity pattern preceding the October 1996 eruption of Bardarbunga
Volcano. Open hexagon
shows location of Mw = 5.6 earthquake, and solid circles show subsequent
earthquakes on the same day. Thick grey line shows the site of the
eruption; thin black line shows northern boundary of the glacier. The
diameter of the caldera is ~10 km. Caldera fault and eruption fissure
have been digitized from maps of the Icelandic Meteorological Office.
Earthquake locations are those determined by the Icelandic Meteorological
Office using data from the Southern Iceland Lowlands seismic network;
only those earthquakes located using at least 10 P wave arrivals are