~1,000,000 earthquakes (EQ's) per year; ~20 per year involve significant damage + some deaths; ~1 catastropic EQ per year.
1811 - 1812 - New Madrid, MO (SE MO + affected southern Illinois) series of 3 EQ's, Richter scale Magnitude (M) = ~8, not much damage or many deaths; Why?
1906 - San Francisco, CA - (M = 8.3) EQ caused fires that destroyed San Francisco + tore ~450 km long scar in land due to movement along fault (up to 5 m offset); ~1,500 deaths. "The Big One"
1964 - Anchorage, Alaska - (M = 8.3, largest recorded EQ in North America) not massive damage or relatively few deaths
1976 - Tang Shan - China, 240,000 deaths, M = ~8
1985 - Mexico City, Mexico (M = 8). EQ occurred 350 km away with little damage at epicenter, but Mexico City was devastated (~10,000 deaths, 3,000 buildings collapsed, city built on loose, wet mud)
1987 - Lawrenceville, IL (100 km south of C-U) M = 5
1989 - "World Series" EQ (San Francisco/Loma Prieta, CA) M = ~7, ~60 deaths, $6 billion in damage
1994 - Northridge, CA (Los Angeles area) M = ~7, ~60 deaths, ~$13 billion in damage, 3rd most expensive natural disaster to hit USA
1995 - Kobe, Japan (southern Japan) M = ~7; >5,000 deaths; 27,000 injuries; most expensive EQ disaster ever, ~$100 - 400 billion in damage; ~100,000 buildings were damaged
1999 - Izmet, Turkey (north-central Turkey) M = ~7.5, ~17,000 deaths, $10 - 20 billion damage, ~120,000 buildings destroyed
1999 - Chi-Chi, Taiwan (central Taiwan) M = ~7.5, ~2,300 deaths, billions of $ in damage, global shortage of computer chips
2001 - Seattle, WA (NW WA) M = ~7; ~$1 - 2 billion in damage, 1 death, relatively deep EQ prevented > damage
2004 - Indonesia M = ~9, ~10 m tsunami, >280,000 deaths in countries around Indian Ocean
What is an earthquake (EQ)?
EQ (seismic event, "seismos" = shake) = shaking of Earth due to sudden release of stored energy in subsurface
Seismology - scientific study of earthquakes
Where do EQ's occur in world?
Explained by plate tectonic theory - Outer part of Earth is comprised of tectonic plates (thick slabs of rock) that slowly move around globe.
EQ epicenters in world are concentrated in discrete, linear belts at tectonic plate boundaries (divergent, convergent, + transform)
How do EQ's occur?
Tectonic plate movement (which averages ~ 2 - 10 cm per year) is usually abrupt + discontinuous, no plate movement for many years + then 50 cm or 100 cm of plate movement in a few seconds; in some areas tectonic plate movement is slow + continuous (tectonic creep).
Movement along fault - surface of broken rock along which movement has occurred
Elastic Rebound Model - Begin with unbent rocks; over time pressure (stress) builds up, rocks bend + store elastic energy. Eventually, rocks break, sending seismic waves throughout Earth.
Types of faults
Fault plane = surface of movement; strike - line formed by inclined fault plane + a horizontal plane; dip - direction of maximum incline; footwall - underlying surface of fault plane (miner's feet on footwall); hanging wall - overlying surface of fault plane (miner's lantern on hanging wall).
Dip-slip faults - movement along dip direction, 2 end member types; normal fault - hanging wall down with respect to footwall (common at divergent boundaries); reverse fault - hanging wall up with respect to footwall (common at convergent boundaries)
Strike-slip (transform) fault - movement along strike (each side of fault moves horizontally), common type at transform boundaries, e.g., San Andreas fault
Real faults occur as group of smaller sub-parallel faults (fault zone).
Where do rocks break?
EQ's occur below Earth's surface (focus).
Location directly above focus at Earth's surface = epicenter.