2 What Creates Earthquakes? The term “Earthquake” is ambiguous:Applies to general shaking of the ground and to the source of the shakingWe will talk about both, but are mainly concerned with the latterEarthquakes occur due toSudden motion on a faultFormation of a new faultSlip on an existing faultMovement of magma / explosion of a volcanoLandslidesMeteorite impactsUnderground nuclear bomb tests / mine collapsesOffset
3 Earthquake Terminology Hypocenter (Focus): actual location of the earthquake at depthEpicenter: location on the surface of the Earth above the hypocenterHanging Wall: top block of a fault (where a light would hang from)Footwall: bottom block of a fault (where you would stand)
4 Types of FaultsIn general, faults come in three different types: Normal, Reverse, and Strike-SlipShallow angle (< 30°) reverse faults are called thrust faultsFaults that have a mix of slip styles are called oblique slip faultsSee: Fault animations online
5 Why are there different types of faults? Normal Faults: from stretching of or extending rock; points on opposite sides of a fault are father apart after an earthquakeReverse Faults: from contracting or squishing rock; points on opposite sides of the fault are closer together after an earthquakeStrike-Slip: can form in either areas of stretching or squishing, material slides laterally past each side of the fault.Described by sense of motion:Right-lateral (Dextral)Left-lateral (Sinistral)
6 Formation of FaultsFaults and thus earthquakes form because of stress & strainPlate motion causes rocks to deform or bendStress and strain become localizedEventually the strength of the rock is overcomeBAM!! The rock ruptures and snaps forward releasing the accumulated stress/strain.The process is known as elastic rebound theoryA through-going faultforms and sliding occurscausing a stress dropElastic strain:strain that is recoverableNew cracks form andlink together
7 Faults & FrictionLike a brick sliding across a table, faults, too, are subject to frictionFriction, on the micro-scale, is caused by asperities, bumps and irregularities along a surface that resist slidingAll other factors equal, faults with more cumulative slip may be smoother and therefore have lower friction (e.g. the San Andreas Fault has very low friction)Once a fault is formed it is a permanent scar that is weaker than the surrounding rock
8 Stick Slip BehaviorWithout stick slip behavior, large earthquakes would not happen!Faults would constantly move (i.e. creep) and not build up significant stress
9 The Earthquake Cycle: A Simple View [ Step 2 ]- Plate motion continues- Stress/strain exceeds rock strength- The fault slips (ruptures)- Fence is broken into two undeformed pieces[ Step 1 ]- Plate motion continues- Stress/strain is localized on fault- Fence is strained/deformed- Deformation is recoverable (elastic)[ Initial Conditions ]- Plate motion begins- Fence is straight
10 Measuring Motion Across a Fault M Great San Francisco Earthquake
11 Locating Earthquakes Often we don’t see surface rupture after an EQ Earthquakes occur deep in the Earth.To locate EQ’s we can’t just look at first arrivals of P-wavesTime = 0 is unknownSeismic velocity is non-uniformCan only get a potential epicentral areaInstead we rely on the difference in arrival timesvs ≈ 0.55 vp
12 Locating EarthquakesBecause P-waves travel fastest, they will always be recorded firstThe farther from the source, the more S-wave lag.If we calculate the difference in arrival times of S- and P-waves, we can then calculate the distance to epicenterCalled the S-P interval
13 S-P Intervals The S-P time only tells distance, not direction A minimum of three stations are needed to calculate epicenter locationCalled triangulation
14 Triangulation One station gives infinite possible epicentral locations Two stations give two possible locationsThree stations give one locationIn practice there is some errorThe epicenter is located where these circles from multiple stations all intersectStation #1Station #2Station #3
15 Triangulation One station gives infinite possible epicentral locations Two stations give two possible locationsThree stations give one locationIn practice there is some errorThe epicenter is located where these circles from multiple stations all intersect
16 How is Earthquake Depth Determined? Seismologists determine hypocenter depth by:Determining the arrival of the pP rayCalculating the p-pP lag time and plugging it into an equationHypocenter depth also effects S-P intervals, but this is usually accounted forMost regions have earthquakes at a limited range of depth
17 Fault Plane SolutionsAlong with hypocenter location, seismograms can be used to determine the type of fault that caused the EQ…But first we need to review how to quantify the orientation of a plane!
18 Measuring Orientation: Strike and Dip In order to characterize geologic structures, one must be able to quantify the orientation of structures.For Planar features we use:Strike: The orientation of the intersection line between a horizontal surface and the feature of interest. Measured with a compass.E.g. north, N45W, 285, etc…Dip: The acute angle between the feature of interest and a horizontal plane.E.g. 0° = horizontal 90° = verticalFor linear features we use:Trend: the trend of the line if you were looking down on the feature from aboveE.g. north, NW, 320, 090, etc…Plunge: Acute angle between the line and a horizontalE.g. 46°, 75°, etc…
19 Fault Plane Solutions Consider a peg struck by a hammer… Only P-waves to the N-SGreatest amplitude directly ahead and behind…i.e. N-SAmplitude decreases away from N-S directionDilatational first arrival to the SContractional first arrival to the NOnly S-waves to E-Wsame is true for S-waves…almostall first arrivals have the same sense of motionS-waves are of little to no help in determining the fault orientationHow do we know if the first arrival is dilatational or contractional?
20 Faults Generate Contraction and Extension The hammer and peg example is too simpleBoth sides of a fault moveContraction and extension are both generated during slipGeologists call thisσ1maximum compressive stress directionSeismologists call thisP-axis (sometimes C-axis)Pressure axis (compression axis)σ3minimum compressive stress directionT-axisTension axisExtensionContractionContractionExtensionFault in a Box
21 Focal MechanismsBoth sides of a fault move, so the radiation pattern is more complex.Seismologists use the pattern of first arrivals to determine several properties of the causative faultstrike, dip, and slip vector rake.we call these focal mechanisms, moment tensors, or beach ballsContractionExtensionExtensionContraction
22 The Double Couple Mechanism Before an earthquake, rock is shearedThe rock cannot rotate, so there must be other stresses involved.
23 The Double Couple Mechanism If two shear stresses are involvedthe rock can undergo shear strain without rotatingcalled the double couplebut this causes ambiguity in the focal mechanism solution…
24 The Auxiliary Plane Because of the double couple no rotation is allowedFocal mechanisms predict two potential fault planes collectively called: nodal planesthe fault planethe auxillary plane
25 Which Plane is the Fault? What are the two potential fault orientations?How do we know which is the real fault?Sometimes logic combined with a little Occam’s RazorAftershocks & Historical seismicityHow else could we determine the fault plane?
26 Even geophysicists need to look at rocks and geologic maps Geology!!!
27 The Focal SphereThe process just outlined is fine for strike-slip events, but we need a general method for any type of fault.To do this we use the focal spherejust like your favorite part of structural geologyStereonets!!!
28 Strike & Dip: The Stereonet Way Dip Direction = N/A
29 Strike & Dip: The Stereonet Way Dip Direction = N/A
30 Strike & Dip: The Stereonet Way Dip Direction = East+
31 Strike & Dip: The Stereonet Way Dip Direction = East+
32 Strike & Dip: The Stereonet Way Dip Direction = East+
33 Strike & Dip: The Stereonet Way Dip Direction = East+
34 Strike & Dip: The Stereonet Way Dip Direction = East+
35 Strike & Dip: The Stereonet Way Dip Direction = SE+
36 Strike & Dip: The Stereonet Way Dip Direction = SW+
37 Strike & Dip: The Stereonet Way Dip Direction = NE+
38 Beach Balls For Standard Fault Types For faults with pure dip-slip or pure strike-slip motion the focal mechanisms are relatively straightforward
39 Focal Mechanisms For Oblique Slip Focal mechanisms can also determine the direction of slipCalled the slip vector rake, or just “rake”180 ≥ rake ≥ -1800 = left-lateral, 180/-180 right-lateral90 = reverse slip -90 = normal slip45 = ? = ?
40 Calculating Focal Mechanisms Although it is impractical to put seismometers deep in the ground, we can still detect waves that are radiated in all directions from a hypocenterWe can trace P-waves back to their source using:inverse methodsthe ray parameter, pWe can then calculate the take-off anglerelative to verticalthis tells seismologists where to plot each station on the focal sphere (stereonet)can get azimuth to source from triangulation