Where and why hazardous

earthquakes occur?Abhijit Deonath

• Occurring close to dense human settlement

• Causing other hazards like tsunamis, landslides

• Earthquakes which impact us

What are hazardous earthquakes?

• Large magnitude

• Shallow

• Close to continents

• Thinking outside the Wadati-Benioff zone

What are hazardous earthquakes?

• How funny, because they use earthquake locations to define plate boundaries

• Traditional answer: at plate boundaries

Where do earthquakes occur?

Source: geology.com

• Near active continental margins

Where do earthquakes occur?A more reasonable answer

• At mid-ocean ridges

• Near continental mountain-building

regions and rift zones

• In short:

• Wherever there is active deformation on the globe

A more reasonable answer

Where do earthquakes occur?

• Do large earthquakes occur anywhere in the Pacific ring of fire?

• Can we narrow the localities down to certain segments?

• e.g. the Pacific ring of fire

Where do earthquakes occur?

• Isn’t of much help

• Need to go into specifics of regions

• Do large earthquakes happen under specific circumstances?

Traditional answer: plates move past each other

Why do earthquakes occur?

• Earthquake events 1976 to 2005

Sumatra region

• Relative motions

Sumatra-Andaman Sea-Myanmar

• Earthquakes at various depths

Sumatra

• Earthquake swarms, volcanoes and forearc basins

•Sumatra

Crest

Simeulue

Nias

Enggano

• Seafloor age

•Sumatra

• The trench is intersected by

the IFZ and Wharton Ridge

• Forearc crests

• Earthquake ruptures do not

cross forearc crests

• Linear tectonic elements

bend at Nias elbow

• Youngest subducting

oceanic crust

SumatraWhat’s special about 0 -5 N?

• Structure of oceanic crust (IFZ, Wharton Ridge) influences tectonism at the margin

• A possible tectonic blockage to opening of forearc basin in the form of extension of abandoned spreading ridge at the equator

• What this possibly means

Sumatra

• Coupling of interface

• decreases with seafloor age

• is higher for higher convergence rate

• Chances of large earthquakes are higher where

• Seafloor is relatively young

• Rate of convergence is higher

• There are other factors however

• Ruff and Kanamori, 1980

Age and convergence rate

Peru

• Volcanoes end abruptly near Arequipa

• Probably means fault growth is arrested

• No forearc basins beyond this point

• Site of Jun-2001 8.4M earthquake

• What we observe?

Peru

• The margin runs into an antiformal structure on the ocean floor

• Usually antiforms have fractures along the axial plane

• Nov-1996 7.7M earthquake lies exactly at intersection of antiformal structure with forearc

• The axial plane fault also represented by river on the continent

• Also note absence of forearc basin across breadth of antiform

• Why the arrest?

Peru

• Another large earthquake

• Aug-2007 8.0M south of Lima

• On the other side of antiform

• Forearc basin developed beyond this

• Does this tell a similar story?

• What’s further up?

Peru

• Continuing southward along west coast of S. America

Chile

• Volcano series of north ends

• The endpoint marked by Oct-1983 7.4M earthquake

• Quiescence of volcanic as well as large (> 7M) quakes south of this point

• Series of volcanoes from Santiago till end of continent

• A few >7M earthquakes near Santiago

• Feb-2010 8.8M earthquake near Concepcion

• No forearc basin there

• What we observe?

Peru

• >7M earthquakes extension of fault along the ocean ridge, probably continuing on the continent

• Forearc basins are arrested in the south leading to >8M earthquake

• Note the shallowness of trench which means less pull (remember Ruff & Kanamori, 1980?)

• Any conclusions?

Peru

• May-1975 8.1M

North Atlantic

• In line with a major transform fault

• Close to where the transform fault meets an elongated ridge structure

• Oceanic crust stronger than continental crust

• Not hazardous

• May-1975 8.1M

North Atlantic

• Aug-1977 8.0M

Sumba

• Forearc basin narrows considerably

• Shallow trench

• Aug-1977 8.0M

Sumba

Pacific region

• Occurrence at the edges of (forearc) depressions

• Paired earthquakes in relation to oceanic uplifted regions

• What we observe?

Pacific region

• Oct-1994 8.3M

East of Hokkaido, Japan

• Trench slope becomes less steep beyond this point

• Oct-1994 8.3M

East of Hokkaido, Japan

• Sep-2003 8.3M

Hokkaido, Japan

• At the bend of arc system

• At the constriction of forearc basins to the north and south

• Sep-2003 8.3M

Hokkaido, Japan

• 11-Mar-2011 9.0M

Sendai, Japan

• Narrowing of forearc basin from north

• North of the point of intersection of ridge which meets Japan near Tokyo

• 11-Mar-2011 9.0M

Sendai, Japan

• It appears that relatively smaller ≥ 6-7M earthquakes occur close to seafloor fault extensions (ridge)

• ≥ 8M earthquakes are little offset from the fault as in Concepcion quake or the quakes in northern parth of South America (Peru).

> 8 magnitude earthquakes

• Simple - faulting

A more reasonable answer

Why do earthquakes occur?

• Accumulation of stress beyond strength

• Fault propagates at the tip aided by pore fluid pressure

• Growth of a fault can be arrested by several factors

Faulting, fault propagation and arrest

Why do earthquakes occur?

• Compressive regime

• Faulting is reflected usually as forearc basin development.

• Basin development is arrested wherever impeding factors are encountered.

largeWhy do earthquakes occur?

• One such impeding factor is presence of a ridge at the bordering ocean floor.

• Stress builds up at such points where basin growth is arrested.

• Accumulated stress eventually leads to large earthquakes.

largeWhy do earthquakes occur?

• Examples:

largeWhy do earthquakes occur?

Two large earthquakes on two

sides of ocean ridge near Peru margin.

Three large earthquakes on two

sides of Wharton Ridge and

Investigator Fracture Zone

Discussion