GEOL 106 Lecture Notes - Lecture 7: Aseismic Creep, Paleoseismology, Elastic-Rebound Theory
15 May 2018
School
Course
Professor
3.2 The Earthquake Processes
- active fault: moved during past 10 000 years of Holocene Epoch
o Holocee Epoch: ost recet epoch of Quaterary period of Earth’s geologic history
o faults that show evidence of movement in Pleistocene are potentially active
- iactive: has’t oved i past 2 illio years, hard to prove
- geologists must determine paleoseismicity of fault = prehistoric record of earthquakes → determine
age of most recent faulted or folded sediments
o often involves excavating paleoseismic trench across active fault to expose the deformation
THE EARTHQUAKE CYCLE
- earthquake cycle: drop in elastic strain after earthquake and an accumulation of strain before next
event
- elastic strain: deformation that is not permanent, provided that stress is eventually released → after
release, returns to original shape
o if not released, deformation will eventually rupture → permanent deformation
- rocks deform elastically until critical point is reached and fault slips, releasing stored elastic energy as
elastic rebound
- 3 or 4 stages:
1. long period of inactivity along segment of geologic fault
2. accumulated elastic strain produces small earthquakes
3. foreshocks: may occur only hours or days prior to next large earthquake; small to moderate
sized earthquakes
4. mainshock: major earthquake and its aftershocks
- epicenter: place on surface of Earth above focus/ hypocenter
- rupture → displacement of rocks at focus → propagates up, down, laterally along fault plane during
earthquake
- sudden rupture of rocks produces shock waves/ seismic waves
SEISMIC WAVES
- radiate outward in all directions from focus
- body waves: seismic waves generated by fault rupture travelling within body of Earth: two types
- P waves: compressional or primary waves; faster, travel through solids, liquids, gases
o much more quickly through solids (6km/sec) than through liquids (1.5km/sec)
o P waves reach surface → transmitted into air → some people/ animals may be able to hear a
fraction of the seismic waves
- S waves: shear or secondary waves, can travel only through solid, more slowly than P waves (3km/sec)
o up and down motion → sideways shear at right angles to direction that wave is moving
- surface waves: S and P waves reaching land surface
o more slowly than S or P waves, cause much damage near epicenter
o rolling motion → crack walls, foundations of buildings, bridges, roads
o Love wave: horizontal shaking that is especially damaging to building foundations
TECTONIC CREEP AND SLOW EARTHQUAKES
- tectonic creep: gradual movement along fault that is not accompanied by perceptible earthquakes
- e.g. tectonic creep along Cascadia subduction zone beneath southwestern British Columbia and state
of Washington → slow earthquakes which are not felt and only recently detected
- also called fault creep, can slowly damage roads, sidewalks, building foundations etc.
- rate of creep is relatively small portion of total slip rate on fault → periodic sudden displacements
that produce earthquakes can also be expected
find more resources at oneclass.com
find more resources at oneclass.com
