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Focal Mechanism Earthquake Source Mechanics Lecture 5 Earthquake Focal Mechanism GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD What is Seismotectonics? Study of earthquakes as a tectonic component, divided into three principal areas. 1. Spatial and temporal distribution of seismic activity a) Location of large earthquakes and global earthquake catalogues b) Temporal distribution of seismic activity 2. Earthquake focal mechanisms 3. Physics of the earthquake source through analysis of seismograms GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Location of large earthquakes and the global earthquake catalogues ß Historically of crucial importance in the development of plate tectonics theory a It was the recognition of a continuous belt of seismicity across the North Atlantic (together with profiles measured by marine geophysicists) that allowed Ewing & Heezen to predict the existence of a worldwide system of mid-ocean rifts ß Goter extended this work in the 60’s & 70’s to compile global seismicity maps delineating the plate boundaries a Similar maps at larger scale constructed from regional and local seismic networks allow the tectonics to be studied in much finer detail GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Global seismicity GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake focal mechanisms ß Using teleseismic earthquake records to determine the earthquake focal mechanism or fault plane solution and deduce the tectonics of a region ß Similar work now done at larger scale for looking at regional and local tectonics - neotectonics GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD The Seismic Source ß Shear faulting a Simple model of the seismic source 1. Fracture criterion 2. Frictional sliding criterion 3. Effect of pore fluid pressure 4. Influence of pressure, i.e. depth, on faulting Covered more in earthquake source mechanics – now start with simplest model and won’t specify whether a fresh fracture or unstable frictional sliding on an existing fault GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD The Seismic Source 2 compressional quadrants + Simple normal fault 2 dilatational quadrants - Look at first motion on seismogram 2 nodal planes 0 Dip Displacement Footwall - Hanging wall + ↑ up on + vertical axis no motion 0 - Auxiliary plane Per’lar to fault plane 0 Per’lar to slip direction Fault plane no motion GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD First motion S3 & S4 are on nodal plane + So no motion - or indistinct S4 first motion in S1 P wave S 3 ↑ first motion up S2 ↓ down motion up GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Earthquake Focal Mechanism Earthquake focal mechanism Fault plane orientation Fault plane solution GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms 1. We use a global coverage of seismometers (many stations) to record first motions In principle we could use any phase (S, pP, PP) but only use P as later arrivals are more difficult to read 2. Plot onto 2D projection of the Earth 3. Look particularly for nodal planes where there is no motion as these stations define the fault plane or auxiliary plane GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms To find a nodal plane we need to know the expected arrival time accurately LP seismogram e.g. Expect here – no motion just after arrival, therefore nodal To check arrival time look at high frequency SP record SP seismogram Always get some kick on short period N.B. SP is always more accurate for measurement of times GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms Examine first motions recorded on long period seismograms because of SP energy from small geological heterogeneities Theoretical path SP LP Never use SP records for polarity measurements (because of scattering, multiple reflections, refractions) e.g. LP period ~20s (seismometer) for v~8 km/s(mantle), wavelength λ ~v, T ~ 8x20 = 160km SP period T~1s (seismometer) λ ~ v, T ~ 8km SP records are full of scattered energy LP records are more reliable (if care taken at nodal planes) GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms Problem: Fault plane is not uniquely specified by 2 nodal planes: ß Fault breaks (if earthquake has broken surface) Shallow events M > 6 s x x 2. Aftershocks x x occur around fault plane and x xx show direction of fault plane x zones of 3. Isoseismals damage elongate along direction of fault plane (1st discovered after 1906 SF earthquake) GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms 4. Source directivity pulse moving along fault (takes finite time from beginning to end of fault) analogous to Doppler effect Fracture starts Fracture 5. Sub-events stops GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms Problem: Lack of global coverage ß Station coverage 2/3 earth is ocean and island stations are noisy so difficult to get good nodal planes ß Core shadow near centre of plots (more on this late) GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation from Seismograms Synthetic seismograms A large part of modern seismology is devoted to the calculation of seismograms from models of the source and elastic constants - + + By building up these 45o seismograms from a model of an earthquake source, varying a wide range of physical - parameters, until the synthetic seismograms matches the real observed seismograms GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Faulting Hanging walls Footwall Fault strike Footwall Fault plane GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation ß Measuring strike and dip a By convention the dip is measured to the right of the strike N N o ϕs ~ 45 WE W E ϕ ~ 225o S s S Study the self-taught module on structural geology on the server GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Orientation u is slip direction ß Measuring the rake lies in the fault plane normal to fault plane u strike direction λ horizontal λ - the rake, measured relative to the strike direction ϕs So, λ = 0o strike slip (pure) [e.g. San Anreas] λ = -90o normal (pure) λ = +90o reverse/thrust (pure) Slip direction refers to the relative movement of the Hanging Foot λ -ve hanging wall wall wall Normal fault, hanging wall goes down GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Focal Sphere – 3D Focal sphere for a seismic point source is a sphere centred on the source and having arbitrarily small radius. It is a convenient device for displaying radiation patterns, since information recorded by seismometers (distributed over the Earth’s surface) may be transferred back to the focal sphere. Remember p = r sin i / v = constant for a spherical Earth If velocity at station = velocity near source, then isource = istation (applies best to shallow earthquakes, correction can be applied for deeper earthquakes) i large close in All teleseismic stations plot onto the lower focal upper i small hemisphere further out Only local seismometers lower plot onto upper focal sphere One station → one point on focal sphere GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Focal Sphere In principle, azimuth ϕ angle of descent i can be worked out if 1. Location of earthquake 2. Location of station 3. Velocity profile i(∆) Use computers to do this, and so one may specify a point on the focal sphere by angular coordinates (i,ϕ) e.g. + Strike slip fault - - Usually the compressional + (+ve polarity) is shaded + - D C GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Equal Area Projection (2D) of the Focal Sphere – Strike Slip Fault Schmidt net We map a plan view of the preserves area T. horizontal plane, i.e. an equal area projection of the lower focal hemisphere Strike slip fault P . D . P C – compression D – dilatational C → auxiliary plane T. →fault plane T – tension axis Use equal area projection, so that all data collected over area have same weight P – pressure axis GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Normal Fault Normal Fault 60o dip 0o strike N N o ϕs ~ 0 30 60 o δ = 30o P . T. δ = 60 - + Auxiliary Fault plane plane Auxiliary Fault plane plane nodal planes GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Thrust Fault Thrust Fault 30o dip 0o strike N o ϕs ~ 0 P . T. δ = 30o δ = 60o Auxiliary Fault plane plane GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Information from the Fault Plane Solution Null axis is the interception of 2 nodal planes (direction of movement) If the null axis is nearer the centre of the projection, the mechanism is predominantly strike slip If it is nearer the edge then predominantly normal or thrust fault Normal fault – centre is dilatational Thrust fault – centre is compressional Rake ϕ Slip direction relative to the azimuth, s movement on the fault plane λ e.g. angle of slickensides to horizontal GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Fault Plane Solution GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Information from the Fault Plane Solution P & T axes correspond roughly to the directions of minimum (T) and maximum compressive (P) stress σ Normal P max σ Deviatoric stress (tectonic) leads faulting intermediate to faulting o ϕs Fault plane at 45 to P & T axes σmin 45o T Definition of P & T 90o to intermediate axis (strike) 45o to auxiliary plane 45o to fault plane o o (Usually σmax is at 30 to fault plane, i.e. dip of 60 in rocks) GNH7/GG09/GEOL4002 EARTHQUAKE SEISMOLOGY AND EARTHQUAKE HAZARD Information from the Fault Plane Solution P & T axes P Section P axis – dilatational quadrant - T axis – compressional quadrant + + T P-axis direction of tectonic movement ±15o - Good for plate tectonics as gives direction, c.f.
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