lesson plan

profileyuki2020
Earthquakecasestudies_Summary.pdf

EOS 170 – Natural Hazards Lecture Summary

Lecture 6 — Earthquake case studies

Subduction zones host a variety of types of earthquakes. (1) Megathrust earthquakes rupture the

shallow plate interface. The interseismic period is analogous to a spring being loaded; above the

locked part of the plate interface, the upper plate moves landward and subsides, while landward

of the locked zone the crust slowly rises. During the earthquake (‘coseismic’ ) these motions are

reversed: the sea-floor close to the trench moves upward and seaward, often generating a tsunami,

while the shoreline subsides. The 11 March 2011 Mw9.0 Tohoku earthquake, offshore eastern Japan,

was unusual in that the seismic rupture extended all the way to the sea-floor; before this earthquake, it

was generally thought that shallow megathrust faults creep, accommodating shortening gradually and

without earthquakes. By rupturing to the sea-floor with up to 60 m of slip, the Tohoku earthquake

generated a huge tsunami that easily breached seawalls constructed along Honshu’s east coast. (2)

Forearc earthquakes occur on shallow faults in the overriding continental plate. These tend to be

moderate magnitude (up to Mw ∼7) but are shallow and close to or on land, making them potentially very hazardous. Lidar mapping has revealed fault scarps along the Seattle Fault that cut and offset

glacial striations, and which therefore must have formed in large earthquakes since the last ice age

(i.e. <10 ka). Near Victoria, the Leech River fault is also thought to have ruptured within the

past few thousand years. (3) Intermediate depth earthquakes at depths of ∼50–300 km can also be very damaging. They occur in all subduction zones, always within the downgoing oceanic lithosphere

(‘intraslab’ ). The 19 September 2017 Puebla, Mexico and 30 November 2018 Anchorage, Alaska

earthquakes are good examples, both Mw 7.1 and both located at the top of the subducting slab

where it begins to flex downwards. (4) Deep earthquakes can reach depths of up to ∼700 km, but only within the coldest and fastest-descending oceanic slabs; in most subduction zones, including

Cascadia, material at these depths is warm and ductile. These earthquakes are usually too deep to

cause great damage. (5) Outer rise normal faulting earthquakes occur where the incoming oceanic

lithosphere starts to flex. They can reach magnitudes of ∼8, but are typically too far offshore to cause extensive damage. (6) Volcanic seismicity is associated with rising magma beneath arc volcanoes.

These earthquakes are usually low magnitude and can be useful for forewarning eruptions.

In continental plate boundary zones, including collision zones, rifts, and strike-slip boundaries,

earthquakes are spread over relatively wide, mountainous regions. Earthquakes can exhibit a variety

of mechanisms, particularly within continental collision zones. The continental seismogenic layer is

usually ∼15 km thick, shallower than that in subduction zones and generally limiting earthquakes to Mw <8. However, collectively continental earthquakes kill more people than megathrust earthquakes,

despite their smaller magnitudes, simply because many large cities lie right next to active continental

faults. This is particularly true in arid regions, where water ponds against mountain-front faults pro-

ducing springs that attract settlement. An example of this ‘fatal attraction’ was the 2003 Mw 6.6 Bam

earthquake in eastern Iran; Bam was only settled because of the fault that destroyed it. Continental

faults can be mapped from satellite imagery and their earthquake histories compiled from trenching

studies, leading to attempts to forecast the likelihood of a large earthquake before one occurs. We

shall look at earthquake forecasting in more detail in the following lecture.

Intraplate earthquakes are unusual in occuring far from plate boundaries. There are three main

causes. (1) As plates are pushed and pulled from their boundaries, stresses are transmitted through

the middle of the plate. All plate contains defects — old fault zones such as within failed rifts or

passive continental margins — and in these areas, some of the transmitted stress can be released

in earthquakes. Much of the seismicity in eastern Canada occurs by this mechanism, including the

Charlevoix seismic zone in Quebec. (2) At high latitudes, postglacial rebound causes the lithosphere

to flex, leading to postglacial earthquakes. (3) Manmade stresses such as from the subsurface injection

of fracking wastewater or the impoundment of reservoirs can reactivate faults locally, causing induced

earthquakes. This mechanism explains a sharp rise in earthquake rates within the past decade in

regions such as northeastern BC, northwestern Alberta and Oklahoma.

7