Earthquakes and Seismotectonics Chapter 5

Earthquakes and SeismotectonicsChapter 5

What Creates Earthquakes? • The term “Earthquake” is ambiguous: • Applies to general shaking of the ground and to the source of the shaking • We will talk about both, but are mainly concerned with the latter • Earthquakes occur due to • Sudden motion on a fault • Formation of a new fault • Slip on an existing fault • Movement of magma / explosion of a volcano • Landslides • Meteorite impacts • Underground nuclear bomb tests / mine collapses Offset

Earthquake Terminology • Hypocenter (Focus): actual location of the earthquake at depth • Epicenter: location on the surface of the Earth above the hypocenter • Hanging Wall: top block of a fault (where a light would hang from) • Footwall: bottom block of a fault (where you would stand)

Types of Faults • In general, faults come in three different types: Normal, Reverse, and Strike-Slip • Shallow angle (< 30°) reverse faults are called thrust faults • Faults that have a mix of slip styles are called oblique slip faults See: Fault animations online

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 earthquake • Reverse Faults: from contracting or squishing rock; points on opposite sides of the fault are closer together after an earthquake • Strike-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)

Formation of Faults • Faults and thus earthquakes form because of stress & strain • Plate motion causes rocks to deform or bend • Stress and strain become localized • Eventually the strength of the rock is overcome • BAM!! The rock ruptures and snaps forward releasing the accumulated stress/strain. • The process is known as elastic rebound theory A through-going fault forms and sliding occurs causing a stress drop Elastic strain: strain that is recoverable New cracks form and link together

Faults & Friction • Like a brick sliding across a table, faults, too, are subject to friction • Friction, on the micro-scale, is caused by asperities, bumps and irregularities along a surface that resist sliding • All 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

Stick Slip Behavior • Without stick slip behavior, large earthquakes would not happen! • Faults would constantly move (i.e. creep) and not build up significant stress

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

Measuring Motion Across a Fault M7.8 1906 Great San Francisco Earthquake

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-waves • Time = 0 is unknown • Seismic velocity is non-uniform • Can only get a potential epicentral area • Instead we rely on the difference in arrival times • vs ≈ 0.55 vp

Locating Earthquakes • Because P-waves travel fastest, they will always be recorded first • The 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 epicenter • Called the S-P interval

S-P Intervals • The S-P time only tells distance, not direction • A minimum of three stations are needed to calculate epicenter location • Called triangulation

Triangulation • One station gives infinite possible epicentral locations • Two stations give two possible locations • Three stations give one location • In practice there is some error • The epicenter is located where these circles from multiple stations all intersect Station #1 Station #2 Station #3

Triangulation • One station gives infinite possible epicentral locations • Two stations give two possible locations • Three stations give one location • In practice there is some error • The epicenter is located where these circles from multiple stations all intersect

How is Earthquake Depth Determined? • Seismologists determine hypocenter depth by: • Determining the arrival of the pP ray • Calculating the p-pP lag time and plugging it into an equation • Hypocenter depth also effects S-P intervals, but this is usually accounted for • Most regions have earthquakes at a limited range of depth

Fault Plane Solutions • Along 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!

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° = vertical • For linear features we use: • Trend: the trend of the line if you were looking down on the feature from above • E.g. north, NW, 320, 090, etc… • Plunge: Acute angle between the line and a horizontal • E.g. 46°, 75°, etc…

Fault Plane Solutions • Consider a peg struck by a hammer… • Only P-waves to the N-S • Greatest amplitude directly ahead and behind…i.e. N-S • Amplitude decreases away from N-S direction • Dilatational first arrival to the S • Contractional first arrival to the N • Only S-waves to E-W • same is true for S-waves…almost • all first arrivals have the same sense of motion • S-waves are of little to no help in determining the fault orientation How do we know if the first arrival is dilatational or contractional?

Faults Generate Contraction and Extension • The hammer and peg example is too simple • Both sides of a fault move • Contraction and extension are both generated during slip • Geologists call this • σ1 • maximum compressive stress direction • Seismologists call this • P-axis (sometimes C-axis) • Pressure axis (compression axis) • Geologists call this • σ3 • minimum compressive stress direction • Seismologists call this • T-axis • Tension axis Extension Contraction Contraction Extension Fault in a Box

Focal Mechanisms • Both 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 fault • strike, dip, and slip vector rake. • we call these focal mechanisms, moment tensors, or beach balls Contraction Extension Extension Contraction

The Double Couple Mechanism • Before an earthquake, rock is sheared • The rock cannot rotate, so there must be other stresses involved.

The Double Couple Mechanism • If two shear stresses are involved • the rock can undergo shear strain without rotating • called the double couple • but this causes ambiguity in the focal mechanism solution…

The Auxiliary Plane • Because of the double couple • no rotation is allowed • Focal mechanisms predict two potential fault planes collectively called: nodal planes • the fault plane • the auxillary plane

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 Razor • Aftershocks & Historical seismicity • How else could we determine the fault plane?

Even geophysicists need to look at rocks and geologic maps Geology!!!

The Focal Sphere • The 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 sphere • just like your favorite part of structural geology • Stereonets!!!

Strike & Dip: The Stereonet Way • Strike = 090 • Dip = 90⁰ • Dip Direction = N/A

Strike & Dip: The Stereonet Way • Strike = 000 • Dip = 90⁰ • Dip Direction = N/A

+ Strike & Dip: The Stereonet Way • Strike = 000 • Dip = 80⁰ • Dip Direction = East

+ Strike & Dip: The Stereonet Way • Strike = 045 • Dip = 45⁰ • Dip Direction = SE

+ Strike & Dip: The Stereonet Way • Strike = 135 • Dip = 80⁰ • Dip Direction = SW

+ Strike & Dip: The Stereonet Way • Strike = 280 • Dip = 60⁰ • Dip Direction = NE

Beach Balls For Standard Fault Types • For faults with pure dip-slip or pure strike-slip motion the focal mechanisms are relatively straightforward

Focal Mechanisms For Oblique Slip • Focal mechanisms can also determine the direction of slip • Called the slip vector rake, or just “rake” • 180 ≥ rake ≥ -180 • 0 = left-lateral, 180/-180 right-lateral • 90 = reverse slip -90 = normal slip • 45 = ? 120 = ?

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 hypocenter • We can trace P-waves back to their source using: • inverse methods • the ray parameter, p • We can then calculate the take-off angle • relative to vertical • this tells seismologists where to plot each station on the focal sphere (stereonet) • can get azimuth to source from triangulation

Calculating Focal Mechanisms

Odd Focal Mechanism? • Really think about what the focal sphere represents… • Why are certain parts are black and others white? • This is all black? • What could cause this?

Earthquakes and Seismotectonics Chapter 5