WP 5.1 Mapping subsurface faults in southwestern Iceland with the microearthquakes induced by the June 17 and June 21 earthquakes

Start date or starting event:

M6

Lead contractor:

IMOR

Participants:                                                IMOR

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Objectives:

To map the subsurface fault planes and slip directions on those faults in southwestern Iceland that were illuminated by the microseismicity induced by the June 17 (J17) and June 21 (J21) earthquakes.  This includes faults within and around the South Iceland seismic zone, as well as within the rift zone on Reykjanes peninsula.  Thousands of smaller earthquakes followed J17 and J21, induced either by the seismic waves propagating from the two, or by the slower propagating change in stress field, resulting from the large (roughly 1 m) slips on their several kilometer long faults.  The map is a significant input to the detailed hazard map, which will be prepared in the project, as closeness to active faults is critical for the ground motion of the shallow earthquakes in South Iceland. The map is also a necessary input for modelling the stress field changes in time and space.

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Inputs:

·         Around eight thousand microearthquakes recorded by the SIL seismic network in the weeks and months following J17 and J21. 

·         Algorithms 

a) for simultaneously determining accurate multi-event relative locations, as well as improved absolute locations. Greatest accuracy is achieved by cross correlating similar waveforms at each station and inverting the matrix of differences in arrival-time-residuals between earthquakes.

b) for simultaneously interpreting event distributions and fault-plane solutions of the individual events, in order to determine which events from a cluster can define a common fault.

Methodology / work description:

Approximately half of the eight thousand events have yet to be interactively analyzed, to give a good starting location and mechanism for each event.  When this has been accomplished, the multi-event relative relocation method will be applied to the dataset in order to increase the location accuracy to such a degree that individual fault patterns become  resolvable. The new event distribution therefore, has the ability to define common fault planes.  When interpreted together with the possible focal mechanisms for each event, it becomes possible to distinguish between physically possible faults and impossible clusters.  The methods used to achieve this will consist of statistical and fuzzy logic methods.  Finally the faults and slip directions on them will be compiled into a tectonic map for the area.

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Deliverables including cost of deliverable as percentage of total cost of the proposed project:

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Milestones: Delivery of the above items at the date indicated.