Overnight shaking a sign that Lake Tahoe is earthquake country

People from the Central Valley of California to the middle of Nevada felt several earthquakes overnight, with the two largest recorded at 5.7 magnitude, followed by 73 other tremblors between midnight and 9:30 a.m. December 28, 2016.

It is too early to tell if the 5.7 quakes, which were centered near Hawthorne, Nev., are the biggest in the current swarm and if the 73 following quakes will be considered aftershocks. Seismologists will wait to classify the swarm of activity until all current activity ends. The largest in that sequence will then be the "main shock."

Network analysts at the University of Nevada Seismological Laboratory work around clock anytime 3.0 or above earthquake occurs according to Annie Kell, Education and Outreach Seismologist at the lab. She said someone in their group will wake up, notify United States Geological Survey (USGS) and start monitoring. They can now log onto the computers from home, eliminating the need to head to campus.

The initial report of Wednesday morning's first earthquake was 5.8, but was changed to 5.7 after seismologists analyzed the data. An automatic location will be put out by computer and then evaluated.

"Once there is one (earthquake), there tend to be more," said Kell.

Kell explained that the sequence of activity (also known as swarms or clusters) can last for months or even years. The Northern Washoe County swarm near Sheldon has lasted two years, and the 2008 swarm north of Lake Tahoe in Mogul went on for weeks. Seismologists will take data and determine which are sequences then determine which are aftershocks, but not until all the shaking is over.

"We'd be in a different bowl of soup if the quakes had been larger," said Kell.

A whole magnitude point difference in an earthquake means a difference of almost 33 times the amount of energy released. For example, the difference between a 3.0 and a 5.0 earthquake means there would have to be over 900 3.0 quakes to equal one 5.0. From a 3.0 to 6.0, almost 30,000 magnitude 3.0 quakes to equal one 6.0.

Not all earthquakes are alike, so data is analyzed from all events. Many believe earthquakes are good as they are a sign of stress released from the earth's plates, thus preventing the "big one," but quakes don't release stress according to Kell.

Much depends on where the earthquake hit. She explained if you take a brick and pull it by a rubber band over thick shag carpet you'd get a difference result than if you pulled the brick over a thin, tightly woven carpet. The brick slips differently but the strain will still be released. You'd either get one large event and the brick moves quickly, or a series of small bursts when meeting up resistance.

Earthquake waveforms are analyzed on a computer, much like the past when seismologists would look at rolls of paper that recorded the needle marks of the quake. Instead of pulling out rulers and mapping on the paper, they now can see the same data on a computer.

People are encouraged to visit the USGS website and answer "Did you feel it?" USGS and others use the data, and it can be retrieved when looking at historical sequences and events. Here is that site: https://earthquake.usgs.gov/data/dyfi/

According to Lake Tahoe historian, researcher and author Mark McLaughlin, there are three main faults that run deep under Lake Tahoe, the northern portion which seems to be the most tectonically active. Near Stateline Point (location of Cal-Neva), one fault cuts directly through the area, while an extension of the fault trends northeast through Incline Village. Scientists believe that these two North Shore faults are part of one system and may tend to rupture together.

Another prominent fault zone is the north-south trending West Tahoe-Dollar Point fault zone. Submerged from Emerald bay to McKinney Bay (near Tahoma/Chambers Landing), the West Tahoe fault continues on to become the Dollar Point Fault. Geologists suspect that both of these faults may also rupture together.

Geologists say that even though the faults beneath Lake Tahoe are capable of generating a 7.1-magnitude quake and enough movement to produce tsunami waves exceeding 30 feet in height, the risk in the next 50 years is very low. They say there is a 3- to 4-percent risk, much lower than the potential risk of forest fires in the region.

Kell said this is a good time to remember basic earthquake safety and to ensure residents have their emergency kits prepared, which can be used year round for fire and other emergencies.

"You need to include warm clothing in that kit in case you lose gas or heat, or are forced form your home," said Kell.

Kell also suggested eyeglass prescriptions (or even your last pair), medical prescriptions, blankets, and if you have infants, formula and diapers, besides the warm clothing. The kits should be updated as the seasons, and family needs, change.

This is also a good time to remember "Drop, Cover and Hold," especially since Lake Tahoe and the surrounding region is earthquake country.

DROP where you are, onto your hands and knees. This position protects you from being knocked down and also allows you to stay low and crawl to shelter if nearby.

COVER your head and neck with one arm and hand - If a sturdy table or desk is nearby, crawl underneath it for shelter. If no shelter is nearby, crawl next to an interior wall (away from windows). Stay on your knees; bend over to protect vital organs.

HOLD ON until shaking stops - Under shelter: hold on to it with one hand; be ready to move with your shelter if it shifts. No shelter: hold on to your head and neck with both arms and hands.

If the earthquake is in Lake Tahoe, after the first three steps, head uphill if near the shore due to the threat of a tsunami.