The Lake Tahoe area experienced a Mw 4.7 earthquake last Wednesday night at 11:37:09 pm PST, shaking most of us out of sleep and rattling the contents of our homes. Facebook was ablaze with reports of terror as people’s skis rattled in the corners of their garages and their pictures of Widespread Panic flopped on the wall like so many fish. The epicenter of the earthquake was located 23 miles to the Northwest of Truckee at a depth of 13 km, and it was felt as far away as Reno and South lake Tahoe. What’s up with this earthquake – was it big? What caused it? Did it do any damage? Why or why not? Why do some earthquakes kill people and some don’t?
According to the seismological laboratory at the University of Nevada in Reno, numerous small earthquakes have been occurring over the last few months beneath the Sierraville region at a depth range of about 29-35 km. Last week’s shock was much more shallow, however, and the two earthquakes resulted from different but related geologic processes.
The Earth’s crust or lithosphere gives way to the underlying mantle at a depth of around 30 km. At this depth, it is common for earthquake swarms to occur as molten rock forces its way through more brittle rock as it rises upwards due to density differences. Crustal rocks tend to be much more brittle and rigid compared to the relatively denser and more ductile mantle rocks beneath them. This means that earthquakes are concentrated in the crust. Exceptions to this general rule occur along plate boundaries where earthquakes continue deep into the mantle as one plate dives beneath another. In fact, this characteristic was one observation that solidified the validity of plate tectonic theory in the 1970s.
Back in the middle of the more stable crust, as time passes and more and more mantle magma percolates slowly through the rocks above it, strain is translated through the lower crust and can can build up to a point where the cooler rocks tens of kilometers above the magma will suddenly fracture and release all that pent up energy as an earthquake. Think of millions of ants burrowing up through sand with a layer of relatively harder pastry crust on top. Sooner or later if enough ants climb through the sand, the grains of sand will push on one another and causes the formation of a crack in the pastry. This is exactly what happened last Wednesday about 13 km beneath Sierraville in the Mohawk Valley.
In 1994 a similar sequence of events unfolded in the crust and mantle beneath the South Lake Tahoe area. Magmatic intrusion took place over a period of several months, with thousands of earthquakes being recorded. Eventually, strain built up in the crust, and was released in the form of a M 5.9 earthquake near South Lake Tahoe. In fact, the USGS gives a 93.4% chance of an earthquake occurring within 50 km of South Lake Tahoe in the next 50 years.
If all of this took place that far underground, how do we know all of this detailed information about it? Seismologists can tell how rocks broke in an earthquake based on networks of instruments that monitor the ground for earthquakes called seismometers. These extremely sensitive instruments can detect ground shaking thousands of miles away as seismic waves propagate outwards through the rocks that is far too slight for us to detect by feel. Earthquake epicenters are the locations on the surface directly above where the rocks break, a point called the hypocenter. With three or more seismometers keeping good time-matched continuous records, this location can be very accurately triangulated.
There are three categories of earthquakes fault mechanisms. Strike-slip earthquakes are a result of rocks breaking in a manner where one parcel of rock cracks and part of the rocks slide past other parts in a lateral motion. On a larger scale, this is what happens along the San Andreas Fault as Los Angeles and the Pacific Plate are moving to the northwest past Oakland and the North American Plate. Thrust earthquakes occur when rocks break and one part of rock is thrust up over (normal), or down under (reverse) another. On the mega-scale, this occurs in subduction zones as I mentioned earlier. Each type of rock fracture releases its own distinct signature out into the surrounding earth / rocks via unique seismic waves. The third type, oblique, is a hybrid of the former two. Last Wednesday’s event was of the strike-slip variety.
Last week’s earthquake registered a Mw 4.7 magnitude. Mw is a specific type of shock magnitude measurement called “moment magnitude,” referring to the amount of energy released when rocks break during an earthquake. Mathematically, the Mw value equals the rigidity of the rocks times the average amount of slip on the fault and the size of the slip area. The Mw scale increases numerically by tenths just as the 1930s-era Richter scale did, but can be thought of as a more accurate means by which to compare one earthquake to another because of the quantitative means by which it is determined from spatial subterranean earthquake characteristics. It is typical for initial reports of magnitude to be give in the old Richter system, and then later revised to give the Mw magnitude. This happened last week, as first news reports said the shock was 5.2 before the Mw was given as 4.7.
A Mw 4.7 doesn’t release that much energy, and really isn’t that big. Earthquakes slightly larger in size have been known to do significant damage to buildings and also to cause substantial loss of life elsewhere in the world. In March, a 5.5 shock in China along the border with Myanmar killed 25 people. In that same country last year, a 4.4 shock claimed one victim. A 6.9 in Qinghai took the lives of 2,968 souls. Why did we just have a 4.7 earthquake and we’re all OK?
The answer is threefold: “our” earthquake released slightly less energy, occurred beneath a much lower population density, and – most importantly – happened in a place with strict building codes. Here in the US, and especially in California, we have very rigid rules that govern how our buildings are constructed. At the root of these codes are the fact that our western-style wooden-frame buildings are engineered in a manner that just happens to resist falling apart in most kinds of earthquakes. The simple feature of Western-style building responsible for saving countless thousands of lives in both small and large earthquakes is that the building frames are “moment-resisting.” In other words, the floors of our buildings are connected to the walls, and the whole contraption is allowed to flex and sway together in unison. In most parts of the world without building codes, buildings tend to be made from masonry bricks and mortar, or rocks, or whatever is lying around. Sadly, these buildings totally fall apart when they are shaken, woe be to whomever is trapped within them.
There may never be earthquake warnings. But we know with an absolute certainty the regions where they occur, and we are getting better at understanding how often they are bound to happen. To save thousands, even hundreds of thousands of lives, all we have to do is convince people in these parts of the world to adapt their building design and construction techniques so their buildings don’t kill them. Their houses and schools don’t have to look like ours, they just have to figure out ways to build them in a moment-resisting fashion from locally available materials. As buildchange.org says, earthquakes don’t kill people, poorly built buildings do.
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