EQ L1: Global Earthquake Distribution Flashcards
Where do larger earthquakes in BC tend to cluster? What is their cause?
Southwest of BC, west of Vancouver Island, along a line pointing offshore starting about 2/3 up the coast. (>= Mag3). Caused by Earth motion near the Nootka Fault
What magnitude earthquake can cause damage in BC?
M6 (and above ofc)
Why does it seem there are earthquake w/ magnitude less than 0? Why does that make sense?
Yes, because scale for earthquake magnitude is logarithmic. Negative log values means that the original number (before converting to logarithms) had a value between 0 and 1. (e.g. log0.5 = -log2)
Most common magnitude earthquake in Southwestern BC from Sept 2017-2018?
1-2
What is a seismogram?
A graph showing ground motion related to time.
Explain how a seismometer works.
Framework fixed to Earth supports a mass hanging on a spring, attached to pen. When ground moves, the mass and pen also move relative to the ground. If rotating roll of paper is firmly attached to the frame, it too will record relative motion between the mass and the ground. (Today, it is digital version, but same concept)
What is a seismometer?
Instrument that records ground position.
How many seismometers needed to get unique estimate of the source of the seismic energy (earthquake?
Three. We record the time ground motion is observed at 3 or more locations then use what’s known about energy travel times to find source location.
What are some challenges of finding source location?
- Earth is spherical so travel times must be recorded accurately. We have to be sure that what was recorded at different locations actually came from the same event.
- Earth’s composition is complex. Signal travels through many different types of materials. Challenge is to know about these materials in order to estimate travel times, but we also need to know the travel times in order to guess what the materials are.
Where do earthquakes occur globally?
at boundaries (margins of) between Earth’s tectonic plates.
(1) “How many observations of earthquake signals were needed to generate this map?” and (2) “How long might it have taken to build this map?”
thousands upon thousands and nearly a century of work
Where have some of the largest of recorded earthquakes occurred?
On the edges where continents meet oceans.
Why are patterns of earthquakes useful?
They can identify where on the Earth’s crust there is the greatest tendency to break catastrophically.
How many major plates on Earth’s lithosphere?
- (many minor little plates)
Describe oceanic plates.
fast moving (cm/year), young (<200 million yrs), formed at mid-ocean ridges, destroyed at subduction zones.
Describe continental plates.
Slow moving. (mm-cm/year) Much older than oceanic plates. Do not get subducted.
Why do continental plates not get subducted?
They are more buoyant than oceanic plates
Describe shear, tension, compression of plates.
Shear: slide past each other.
Tension: pull away from each other.
Compression: Push against each other
Plate motion every year?
0 to 15 cm per year
4 types of boundaries. Name them.
Divergent, transform, convergent type 1, convergent type 2.
Describe divergent boundary
plates are moving apart, leading to tension (stretching). due to tension forces, rocks break and many small-ish earthquakes occur. divergence occurs at mid-ocean ridges (spreading centres)
Where does divergence occur?
mid-ocean ridges (spreading centres)
Describe transform type boundary
plates move past each other, leading to shearing forces between plates. Shearing forces are those that push one part of a body in one direction and the other part in the opposite direction. Rocks are being sheared, so many earthquakes occur here. These are moderate to large earthquakes, but not as large as convergent type 1 and 2.
Describe Convergent type 1 boundary
Plates move toward each other and collide, leading to compression (squeezing). One of the plates is less dense than the other, so one plate will subduct or dive under the other at subduction zones. Rocks are compressed and extensive small to very large earthquakes occur. Largest earthquakes actually occur at subduction zones.
Describe Convergent type 2 boundary
Same as Type 1, plates move toward each other and collide, leading to compression. HOWEVER, both plates are of the same density, so neither plate is subducting. So plates crumple up together. Rocks are compressed and extensive small to very large earthquakes occur.
What type of fault is the San Andreas Fault?
transform fault. it is one of the few transform faults exposed on land.
What type of boundary is the Mid-Atlantic Ridge?
divergent boundary, mostly an underwater feature. On Iceland portions of MAR is above sea level.
Where do large earthquakes occur at the Cascadia region?
at the subduction zone, and also at Deep Juan de Fuca plate and Crustal faults (produced at faults within shallow crust and in the subducting slab).
Where does west coast of North America experience significant earthquake activity?
Along transform boundaries in the offshore region. within the convergent subducting ocean plate. along the divergent Juan de Fuca Ridge. within the continental crust.
What two forces drive the motion of plates to move across the Earth’s surface?
Gravity: pulls on the denser portions of plates that are diving under others.
Heat: heat within Earth causes convection cells to cycle within the mantle.
Describe what convection cycling means
To summarize this process, less dense material (crust) floats on top. Convection causes circular motion within the Earth’s mantle causing the crust to get dragged across the surface. At collision zones, older, cooler, denser material dives, and stiffer slabs get pulled. At spreading centres new crust is created causing plates to move apart.
The largest earthquakes occur where the largest amount of ___ is being accommodated. The world’s largest earthquakes occur at _____ _____.
energy; collision zones
What is the Cascadia region?
Northern Cali to Central BC
Why is Cascadia region relatively high risk?
because of the subduction zone that starts just off the coast and extends under the Cascade range of mountains
Why are there so many small earthquakes in Cascadia region?
Most of the “little” earthquakes are associated with relatively small motions along these faults which occur as the Earth moves along these faults to accommodate the convergent motion of the two plates.
How far does seismic echo sounding reveal detailed images of ground?
between surface and ~60 km deep.
What are echograms?
images of the subsurface
