[Folda] Fwd: Wednesday lunch meeting - presentation by Þorbjörg?==?utf-8?q? Ágústsdóttir

[Siqi Li]

The only activity we know for this week.  
 

-------- Original Message -------- 
Subject: Wednesday lunch meeting - presentation by Þorbjörg Ágústsdóttir 
Date: Monday, June 4, 2018 11:56 GMT 
From: Halldor Geirsson <halldor.geirsson at gmail.com> 
Reply-To: halldor.geirsson at gmail.com

 
Hello all,

we have a special guest on Wednesday, Þorbjörg Ágústsdóttir (Tobba), that
will present her studies of seismicity during the Bárðarbunga-Holuhraun
rifting event; see abstract below.

See you on Wednesday, June 6, at 12:00 in the meeting room on the 3rd floor
of Askja.

-Halldór



*The 2014-15 Bárðarbunga-Holuhraun magmatic rifting event: A seismic study*

Thorbjörg ágústsdóttir, Jennifer Woods, Robert S. White,

 Tim Greenfield, Bryndís Brandsdóttir



On 16 August 2014 an unusual sequence of earthquakes began near the SE rim
of the ice-covered Bárðarbunga caldera in central Iceland. Over the course
of 2 weeks a dyke propagated 48 km beneath the glacier northeastwards and
into the Holuhraun lava field, where it erupted for 6 months. It became the
largest eruption in Iceland for 230 years. During this time, a gradual,
incremental caldera collapse took place at the central volcano. We use
accurate relative earthquake locations of ~48,000 earthquakes to analyse
the seismic response to the event, both due to the dyke propagation, and
the subsequent caldera collapse. We define the thickness of the seismogenic
crust under Bárðarbunga as ~ 7 km, based on the depth extent of observed
seismicity. The bulk of the seismicity directly beneath the volcano is
located at 1-4 km below the surface, whereas the dyke exited the caldera at
4-6 km depth, propagating at ~ 6 km b.s.l..

Of the ~48,000 earthquakes located, ~31,000 delineate the segmented,
lateral dyke intrusion as it fractured a pathway through the crust,
utilizing pre-existing weaknesses. Despite the extensional rift setting,
the dyke emplacement generated exclusively double-couple earthquakes. At
the leading edge of the propagation, earthquake source mechanisms show
exclusively strike-slip faulting, in contrast to the conventional model of
normal faulting above a propagating dyke. We observe right-lateral
strike-slip faulting as the dyke propagates to the NE, and an abrupt change
to left-lateral strike-slip faulting as the dyke turns and propagates in a
more northerly direction into the northern volcanic zone. This shows that
the direction of fault motion is determined by the opening of the dyke and
pre-existing fabric, rather than by the regional extension.

Approximately 5,000 of the recorded earthquakes are associated with the
caldera collapse, delineating faults accommodating the subsidence and
showing good correlation with geodetic data. The seismicity reveals
activation of both inner and outer caldera faults with ~80° inward dipping
planes, but with an order of magnitude difference in the cumulative seismic
moment on the northern and southern sides. Detailed analysis of the source
mechanisms shows that ~90% of the events can be explained by double couple
failure. We find the dominant failure mechanism during the collapse to be
steep normal faulting, with sub-vertical P-axes, and fault planes striking
sub-parallel to the caldera rim. The southeastern part of the caldera,
whilst experiencing less activity, shows a mixture of failure mechanisms,
owing to the interaction of the caldera collapse and the dyke exit. We
suggest a complex asymmetric caldera collapse, not controlled by a single
caldera ring fault.