Earthquakes

divergent boundary EQs

In such areas the lithosphere is in a state of tensional stress and thus normal faults and rift valleys occur. Earthquakes that occur along such boundaries show normal fault motion, have low Richter magnitudes, and tend to be shallow focus earthquakes with focal depths less than about 20 km. Such shallow focal depths indicate that the brittle lithosphere must be relatively thin along these diverging plate boundaries.

difference between magnitude and intensity

Intensity: The severity of earthquake shaking is assessed using a descriptive scale - the Modified Mercalli Intensity Scale. Magnitude: Earthquake size is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake. When an earthquake occurs, its magnitude can be given a single numerical value on the Richter Magnitude Scale. However the intensity is variable over the area affected by the earthquake, with high intensities near the epicentre and lower values further away. These are allocated a value depending on the effects of the shaking according to the Modified Mercalli Intensity Scale.

order that waves arrive

P, S, than surface

transform boundary EQs

The San Andreas Fault of California is one of the longer transform fault boundaries known. Earthquakes along these boundaries show strike-slip motion on the faults and tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter magnitudes can be large.

why are the largest earthquakes along subduction zones?

because the subduction zone is so long, so the distance that rock slips is maximized

loose sediments, muds, sands, gravels

cause shaking to be amplified

how to find where an earthquake is

earthquake recorded by seismic network select earthquake records (from 3 diff stations) and find p and s waves plot distance over time triangulate the epicenter

continental collision EQs

folding, faulting, shallow EQs

surface

highest amplitude most ground shaking take longer to arrive because they can't go through the middle of the earth, have to go all the way around surface compression: perpendicular to direction of propogation

subduction zone EQs

large cause tsunamis

how to measure size of an earthquake

local magnitude: measure s wave amplitude connect amplitude ans distance on a plot

body waves

p and s

hypocenter

place where earthquake starts and happens in the earth

what info do you need to map out earthquakes

plate boundaries earthquake history faults volcanic activity soil composition/rock type utilities/infrastructure population: where, how dense types of built structures preparation/response mechanisms in place

s waves

secondary, shear travel slower can travel through solid material, but can't go through gas or liquid outer core

p waves

travel fastest compress rock and expand it horizontally can go through solids and liquids

M=u*D*A

u=rock rigidity (constant) lower in crust than in mantle D=distance that rock slips A=area that ruptured between the blocks energy released increases 32 times for each increase in magnitude magnitude of 9 releases 1000 times more energy than magnitude of 7

richter scale

underestimates size of large earthquakes