Seismotectonics Mathilde B. Sørensen and J. Havskov
Three main types of plate boundaries Divergent • SpreadingNew crust is made Convergent; Subduction zone Crust is destroyed Conservative Transform fault No new crust or desctruction
Focal mechanisms of a spreading ridge Mixture of normal and strike slip faults, however normal faults dominate
Strike slip faults, California N. Toksoz USGS, 2000
San Andreas fault R. Wallace M. Rymer R. Wallace USGS/SCAMP USGS NASA
Earthquakes in subduction zones Lack of seismicity might indicate slab breakoff
Fault plane solutions in subduction zones In top of the plate there is tension due to the weight, at bottom there is compression as the plate is pushed down Thrust faults at top of the subduction zone
Continent-continent collision • Continental collision zones show only shallow and intermediate depth earthquakes. Hard to push down a continent
Very deep earthquakes Deepest eartquake is about 700 km. What limits the depth ? Earthquake in the deep mantle itself seems unlikely due to the viscosity of the materiial, brittle failure is not possible. Fauilure might happen due to phase changes. When the plate subducts, it warms up and become more elastic so brittle failure is not possible inside the plate. The faster the plate decends, the further it can get before getting warm so fast, steep subduction zones will have the deepest earthquakes.
Deep earthquake, May 24, 2013 at sea of Okhotsk, M=8.3 Largest magnitude very deep earthquake. Fast steep subduction.
Iceland seismicity Iceland is on the middle of the spreading ridge with both spreading and transform faults
The transition between two different plate boundaries is observed in the different focal mechanisms
Fault plane solutions in a complex area Green: strike-slip Blue: reverse Red: normal Nuvel 1A plate motions, fault and plate boundaries from Bird (2003), ten Veen (2004) and McClusky et al. (2000) Focal mechanisms from INGV, Harvard, USGS (Events are from 1976-2006)
Haiti 2010, no high seismic activity before the earthquake, fault zone normally active The 2010 earthquake was unexpected, althogh there had not been a large earthquake for more than 200 years
Aftershocks During the first 2 hours after the main events, 5 strong aftershocks were recorded. In the first 11 hours there were 32 aftershocks with magnitude > 4
Aftershock recorded by a local network, indication of size of fault Distribution of the aftershocks observed during the period from February 14 to March 1, 2010. The sizes of the circles are proportional to the coda magnitude determined from the land and OBS records, and the color scale indicates the depths. Triangles, temporary stations (OBS and land stations); red, surface trace of the fault (EPGFZ); yellow star, mainshock location.
Focal mechanisms of aftershocks, not what was expected on a strike-slip fault MERCIER DE LÉPINAY ET AL.: THE 2010 HAITI EARTHQUAKE FAULT PATTERN
USGS recorded aftershocks and fault plane solution of main shock The USGS location of the January 12, 2010 Haiti earthquake (red star). Orange circles correspond to aftershocks within the first 24 hours reported by USGS. The black rectangle corresponds to the fault plane assumed for the inversion.
Inversion for slip on the fault Note, uneven slip on the fault
The study of eartquakes gives us the main information about tectonics which could not have been obtained with only geology. However, geology and seismology together gives us the complete picture.