Earthquakes & seismicity in Iceland:a tectonic saga

Iceland Seminar

Marianne Karplus

6 March 2009

Today’s itinerary

• Where are the earthquakes? How big? How many?

• Why do they concentrate in certain areas?

• What is the sense of motion on the faults?

• What is there to see in the major seismic zones?

• What does seismicity tell us about the tectonics?

Vogfjord et al., 2008

Tjornes Fracture Zone

Earthquakes 1994-2004

plume trace @ 300-400 km depth

South Iceland Seismic Zone

South Iceland Seismic Zone

Bergerat & Angelier, 2008

29 May 2008 EarthquakeMw=6.2, 15:46, depth 10 km

MSNBC

USGS

South Iceland Seismic Zone

Bergerat & Angelier, 2008

Why is the motion left lateral (sinistral)?

Sigurjon et al., 2003

June 2000 Earthquakes, M6.5

South Iceland Seismic Zone• East-West trending active transform zone• Relatively immature transform zone ~ 2-3 Ma• E-W LEFT LATERAL transform faults• N-S RIGHT LATERAL transform faults• Connects West & East Volcanic Zones

Angelier et al., 2008

Maturation of a transform zone

Angelier et al., 2008

IMMATURE

MATURE

Fault patterns at regional & local scales

• Oblique aerial photo of typical N-S trending right-lateral fault

• Segmentation & en echelon pattern with push-up structures

• Outcrop is a typical push-up structure and individual fracture

Bergerat & Angelier, 2008

“Revelation in an asphalted car park”

Angelier & Bergerat, 2002

Tjornes Fracture Zone

Bergerat & Angelier, 2007

Tjornes Fracture Zone

• Three tectonic lines trend parallel to the WNW-ESE transform zone

• More mature transform zone ~ 8-8.5 Ma• Has featured > M6.0 earthquakes in last 100 yrs• Parts of seismic zones offshore tsunami hazard• Connects North Volcanic Zone & Kolbeinsey Ridge• Some extension near transform trend –

“earthquake lake”• Faults more tightly localized• Geometrical fault pattern more complex

Húsavík–FlateyFault (HFF) – morphology

• 25 km long feature on Tjornes peninsula

• Vertical throw ~ 1 km• Lateral throw ~ 60 km

• Detail of Botnsvatn pull-apart

Angelier et al., 2008

Broad features of Iceland seismicity

• Areas connecting ridges are seismically active – transform zones

• Some normal/extensional faulting too• Several > M6.0 earthquakes per century

– Mostly strike-slip– Variable orientations of slip

• Quakes accommodate relative motion of 2 plates• South Iceland Seismic Zone

– Less mature than TFZ– More diffuse shear zone– Simpler stress pattern