Earthquakes and Plate Boundaries

LAST MODIFIED: 12 January 2022 subduction zones and convergent plate boundaries clarified

POSTED: 10 June 2020

• Subduction Zones and Earthquakes in the Wadati-Benioff zone

• earthquakes (EQs) occur along all plate boundaries but particularly in subduction zones
• recap from topic 4: subduction zones on convergent plate boundaries where an oceanic plate sinks beneath another plate (oceanic or continental)
• subduction zones are usually the location of the largest and most damaging earthquakes
• convergent plate boundaries are often long, allowing earthquakes to grow to large sizes


• three depth ranges of EQs: shallow (< 100km), intermediate (100-300km), deep (> 300km)
• shallow EQs along seafloor trenches (NB: trench bottom as deep as 11 km below sea level)
• intermediate and deep EQs occur in the subducting plate behind trenches, on the side of overriding plate, in a down-dipping band (Wadati-Benioff zone)


• the Wadati-Benioff zone therefore traces the sinking slab
• EQs occur mostly near the top of the subducting slab
• EQs to a depth of about 500km are a results of brittle failure
• below this depth, heated subducted plates become too ductile for EQs to occur
• deeper EQs occur as result of phase transformation of crust and mantle rock
• deepest EQs occur to a maximum depth of 670km
• Wadati-Benioff zone on Wikipedia



• Some Glossary: An Earthquake Along a Fault

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fault plane: a contact zone between two blocks (or plates) along which an earthquake occurs fault (or fault trace): the surface expression of a fault plane hanging wall: wall or block above the fault plane foot wall: wall or block below the fault plane hypocenter (focus): the point where an earthquake initiates; link to Wikipedia epicenter: the surface location of the hypocenter; link to Wikipedia source depth: depth of the hypocenter source time: time of the initial rupture of an earthquake seismic moment: amount of energy released during an earthquake (measured in Nm) rupture area: the part of a fault plane that actually ruptures during a particular EQ; e.g. when small EQs happen along a large fault, not the whole fault is involved in the rupture process; some portions along a fault may actually be locked and not move; many places (incl. the USGS website) do not distinguish between rupture area and fault plane

• Fault Geometry and the Size of an Earthquake

• the magnitude of an EQ depends on the fault (or rupture area) geometry
• magnitude scales with area of fault plane (or more precisely, rupture area) and the slip
• larger EQs last longer (e.g. small EQs shake a few seconds but the large 04 Sumatra EQ shook for 10min)
• larger EQs have typically larger horizontal slip
• larger EQs also have larger rupture lengths (the distance along a fault over which the slip occurred)
• longer faults are capable of producing larger EQs
• short faults are therefore unlikely to produce large EQs

• Parameters of Fault Geometry and Earthquake Type

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strike: direction of the fault relative to North (counted clockwise); the hanging wall has to be to your right as you look down the strike direction (this convention is necessary to remove 180 deg ambiguity); between 0and 360 deg dip: the dipping angle of the fault plane (measured from the horizontal); between 0 and 90 deg link to Wikipedia for strike and dip of a geological feature slip vector: the movement of the hanging wall, relative to the strike; the length of the vector gives the amount of slip, the angle its direction; the word slip is often used loosely (but falsely) used for rake rake: the direction of the slip with respect to the strike (measured anticlockwise from the horizontal, up is positive); between -180 and 180 deg the rake typifies the type of earthquake left-lateral strike slip: rake near 0 deg right-lateral strike slip: rake near 180 deg normal: rake near -90 deg (or 270 deg) reverse/thrust: rake near +90 deg The shown example has a strike of about 30 deg, a dip of about 45 deg and a rake of about -25 deg. NB: very large earthquakes can have horizontal slips (or displacements) of several meters. E.g. the December 26, 2004 Sumatra Earthquake had horizontal slips of up to 10m. Magnitude 8 earthquakes often have a horizontal slip of up to 5m.

Q:Left-lateral/right-lateral, how do I know? A:Stand on either plate and face the other one. Whatever the other plate is doing determines whether the event is left- or right-lateral.

• Analogy Between Plate Boundaries and Earthquake Mechanisms

• divergent plate boundaries
• extensional stress regime
• normal fault events, also called dip-slip events
• upper (hanging) block slides down on lower (foot) block
• capable of tsunami generation in oceans
• typically small events
• mostly shallow events


• convergent plate boundaries
• compressional stress regime
• reverse fault events
• thrust events, if dip angle less than 45 deg
• upper (hanging) block slides over lower (foot) block
• capable of tsunami generation in oceans
• events of all sizes; the largest EQs on Earth are typically along subduction zones (e.g. Chile 1960; Sumatra 2004)
• events at all depths
• in cont-cont collision zones, the Wadati-Benioff zone may no longer be seismically active
• some very dangerous types

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shallow thrust events: small dip angle; can produce large accelerations near the tip of the hanging block; accelerations are often measured in terms of Earth's gravitational acceleration g NB: the shown example has a strike of about -170 deg blind thrust: a thrust fault covered by sediments; the existence of such faults may be unknown until an earthquake strikes (e.g. Northridge 1994)

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• transform plate boundaries
• purely lateral stress regime
• strike-slip events
• right-lateral: when plate on other side moves to the right (when facing fault) (e.g. San Andreas Fault where Pacific Plate moves to the North relative to the North American Plate)
• left-lateral: when other plate moves to the left
• only horizontal motion of blocks past each other
• no tsunami generation
• events of all sizes; some of very large EQs on Earth (e.g. S.F. 1906)
• typically shallow events
• in collision zones, curved geometry or complicated stress regime may cause EQs along strike-slip faults (e.g Indian Ocean, Himalayas, Gulf of Mexico)
NB: San Diego is on the Pacific Plate and therefore moves North relative to North America



• Intra-Plate EQs or "Some Odd Ball Earthquakes in North America"

In the U.S. some of the most destructive or intense Earthquakes occurred in the interior of the North American plate and appear to defy class plate tectonics theory. A closer look however, reveals that the EQs are manifests of long ago plate tectonic processes.
• New Madrid
• far away from any plate boundary
• Winter 1811/1812, 3 mag. 8.1+ caused intensity X shaking; largest quakes in the continental US in recorded history
• area still seismically very active
• New Madrid Fault zone ancient failed rift (>500Mio years old) with thick wedge of sediments above (enhances ground shaking)
• Charleston, S. Carolina
• edge of continent but NOT near plate boundary
• large 1886 mag. 7.7 caused up to intensity X shaking
• no surface rupture; fault hidden beneath sediments
• thought to be onshore extension of oceanic fracture zones
• Oceanic fracture zones are the extension of oceanic transform faults, where no relative movement between ridge segments occurs. Thought to be aseismic though seismic monitoring on ocean floor extremely poor. Still puzzling!

• Earthquake Intensity and Magnitude Scales

• Mercalli Intensity Scale
• oldest scale (introduced in 1902); modified by Richter in 1956
• used to reconstruct shake maps of historical earthquakes
• a measure for local damage/felt intensity of shaking
• gives no information on actual earthquake size
• shaking/damage depends on size and distance to EQ, but also on local geology and the building style
• depending on reported damage, epicenter can be determined approximately
• Mercalli intensities range from I (not felt) to XII (damage nearly total) where accelerations reach 1 g (gravitational acceleration) or greater
• link to Wikipedia


• Richter Magnitude Scale
• first introduced in California by Richter (1935)
• instrument-based; less subjective than Mercalli scale
• measures actual shaking rather than damage
• estimates shaking of earthquakes from maximum amplitudes in seismic recordings
• applies best to shallow events within 500km of seismic stations
• scale is logarithmic (i.e. an increase by 1 magnitude step stands for a 10-fold increase in shaking)
• still widely used, especially by news media, but gradually replaced by moment magnitude
• Richter magnitudes depend on regional geology
• scale saturates for large earthquakes and does not account for prolonged shaking during very large earthquakes
• link to Wikipedia


• Moment Magnitude Scale
• most recent scale, introduced by Kanamori
• measure for energy released by an earthquakes (given in Nm)
• seismic moment M₀ equals shear strength of blocks times rupture area times average slip on fault
• the moment magnitude then is: M_W= 2/3 log₁₀(M₀) - 6
• on this scale, the largest four earthquakes in the last 100 years have the following moment magnitudes: 1960 Chile (9.5); 1964 Alaska (9.2); 1957 Alaska (9.1); 2004 Sumatra-Andaman (9.0)
• this scale is now used in most publications (that news media get their info from)
• link to Wikipedia



• Some Numbers¹

There is no direct relationship between the three magnitudes but typically, larger earthquakes with a larger moment magnitude cause more damage, i.e. can produce a higher Mercalli Intensity. Approximate numbers for the relationship between magnitude, source duration, rupture length and intensity is summarized in the following table. Some of these numbers can also be found in book chapter 5 (Table. 5.4).

Approx. Moment Magnitude	Approx. Mercalli Intensity at Epicenter	Duration of Shaking near Source (sec)	Slip or Offset (m)	Surface Rupture Length (km)
4	IV-V	0-2		0.1
5	VI-VII	2-5	0.01	1
6	VIII-IX	10-15	0.5-1.5	10
7	X-XI	20-50	2	50-100
8	XII	>50-150	5	200-400
9	>XII	>200	20	1000

• How often do Earthquakes Occur?

• about 3.5 Mio earthquake/year worldwide (Table 5.5)
• majority of EQ are very small; large EQ are rare!
• e.g. 3 Mio mag. 1 EQ/year vs. 0.3 mag. 8.5 EQ
• this means that, to see all number in a graph, the numbers have to be plotted on a logarithmic scale, not on a linear scale. On a linear scale, the small large earthquakes don't show up

linear scale log scale

magnitude # of quakes/year 8.5 and up 0.3 8.0-8.4 1 7.5-7.9 3 7.0-7.4 15 6.6-6.9 56 6.0-6.5 210 5.0-5.9 800 4.0-4.9 6200 3.0-3.9 49,000 2.0-2.9 350,000 0.0-1.9 3,000,000

example for seismicity for 1 week in Southern California

magnitude # of quakes 3.8-4.2 3 3.3-3.7 1 2.8-3.2 12 2.3-2.7 37 1.8-2.2 119 1.3-1.7 201 0.8-1.2 242 0.0-0.7 125

• get recurrence rate of EQ of certain magnitude from number of EQ/year
  • e.g. there are 210 mag. 6.0-6.5 EQs per year globally; so an EQ of this magnitude occurs every 1/210 years = 0.00476 years, or 1.74 days (0.00476*365)
  • => a magnitude 0.0=1.9 EQ happens every 10.5s; a mag 8.5 or larger EQ happens only every 3.3 years
• EQs do not occur regularly (i.e. time interval between EQ is not regular)
• the recurrence time of an EQ of a certain magnitude has a certain probability
• scientists use these probabilities to forecast the chance for an EQ along a fault; e.g. the chance of a magnitude 7 EQ to happen before 2032 is greater than 30% in many places along SAF, including southern CA



• Recommended Reading

• "Earthquakes", by Bruce A. Bolt; Freeman and Company, 1993 ISBN: 0-07167-2236-4 (paperback); excellent introduction to earthquakes; Bolt is a renowned professor at UC Berkeley


• "A Crack in the Edge of the World", by Simon Winchester; Harper Collins, 2005 ISBN:0-06057199-3 (hardcover); 2006 ISBN: 0-06057200-0 (paperback); describes the 1906 San Francisco earthquake


• ¹adapted and adjusted from "Natural Hazards and Disasters" by D. Hyndman and D. Hyndman; Thompson Brooks/Cole, 2006 ISBN: 0-534-99760-0

1. the area on which an earthquake occurs
2. the surface trace of a fault plane
3. an undulation in a mountain belt
4. the shaking on an earthquake

1. true
2. false

1. the relative downward movement
2. the relative horizontal movement
3. the relative three-dimensional movement
4. the slip vector does not indicate any movement

1. strike-slip
2. reverse
3. normal

1. 10 km
2. 70 km
3. 100 km
4. 700 km