5_earthquakes_&_seismotectonics.pdf

Earthquakes and Seismotectonics Chapter 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

Elastic strain: strain that is recoverable

New cracks form and link together

A through-going fault forms and sliding occurs causing a stress drop

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 1 ] - Plate motion continues [ Initial Conditions ] - Stress/strain is localized on fault - Plate motion begins - Fence is strained/deformed - Fence is straight - Deformation is recoverable (elastic)

[ Step 2 ] - Plate motion continues - Stress/strain exceeds rock strength - The fault slips (ruptures) - Fence is broken into two undeformed pieces

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 •

Station #1

In practice there is some error

• The epicenter is located where these circles from multiple stations all intersect

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 How do we know if the Contractional first arrival to the N first arrival is dilatational

– Only S-waves to E-W

or contractional?

• 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

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?

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 = 000 • Dip = 60⁰ • Dip Direction = East +

Strike & Dip: The Stereonet Way • Strike = 000 • Dip = 45⁰ • Dip Direction = East +

Strike & Dip: The Stereonet Way • Strike = 000 • Dip = 30⁰ • Dip Direction = East +

Strike & Dip: The Stereonet Way • Strike = 000 • Dip = 10⁰ • 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?