In the past few months, numerous small-magnitude earthquakes occurred in San Francisco and other parts of California state.
For many people, no matter in what part of the earth are living, earthquakes are frightening.
San Francisco's biggest earthquake
In 1906, on Wednesday, April 18, at 5:12 a.m., San Francisco was hit by a huge earthquake with an estimated moment magnitude of 7.9 and a maximum Mercalli intensity of XI (extreme). The earthquake was so intense that the high-intensity shaking was felt from Eureka on the North Coast to the Salinas Valley.
The consequences were devastating, fires broke out in the city and lasted for several days. People were injured and more than 3,000 people died. The death toll is the greatest loss of life from a natural disaster in California's history and is high on the list of American disasters. The city of San Francisco was torn apart, and over 80% of the city was destroyed.
This earthquake is remembered as one of the worst and deadliest earthquakes in the history of the United States.
So many earthquakes in the past months have people wondering if the next big San Francisco earthquake will happen this year.
Most of the time, earthquakes occur when rock underground suddenly breaks along a fault. This breakage causes a sudden release of energy, creating seismic waves that make the ground shake.
"When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake." - Michigan Tech
Moreover, underground explosions, set up for various reasons like constructing tunnels, can be the cause of earthquake-like seismic waves. However, the seismic waves created are not very strong.
Seismic waves similar to large earthquakes can be created by underground explosions done by bombs or other powerful weapons.
"This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves." - Michigan Tech
Can earthquakes be predicted?
Short answer: no.
Neither the USGS (United States Geological Survey) nor many other scientists from around the world have succeeded in predicting a major earthquake.
At the moment, USGS scientists can calculate the probability that a significant earthquake will occur in a specific area within a certain number of years.
It's hard to predict an earthquake because the prediction needs to have these 3 elements: the date and time, the location, and the magnitude.
So, there is no prediction if somebody says an M3 earthquake will happen in December in the North of the U.S. The prediction needs to be precise.
An earthquake prediction cannot be made, but an earthquake early warning, forecasts, and probabilities can be done.
Earthquake early warning systems can alert people when shaking waves generated, by an earthquake are expected to happen at their location, by using earthquake science and the technology of monitoring systems. People get an alert on their electronic devices, such as a smartphone, a few seconds or a few tens of seconds before the event occurs. People usually have just a few seconds to find a place to be protected
Earthquake forecasts consist of probabilities for shorter time windows. USGS uses this term for aftershocks. Most of the time, after a large-magnitude earthquake, aftershocks that are less frequent and smaller over time occur. Because the majority of the aftershock sequences follow the same pattern, the probability of an aftershock in a time window following an earthquake can be determined. These probabilities might be larger than 1-in-30.
Earthquake probabilities are the long-term chances of an earthquake of a certain magnitude happening during a time window. According to USGS usually, the earthquake probabilities are determined from the average rate of historical events. Assuming the annual rate is constant, a probability statement about the likelihood of such an event in the next many years can be made. These probabilities might range from 1-in-30 to 1-in-300.
"For some faults, historical occurrences are not available, but rate of slip along the fault can be estimated. Assuming a particular magnitude, one can estimate the number of years it would take to accumulate the required amount of slip. This estimate can be used to give an annual rate and used in the same manner as historical rates. These probabilities might range from 1-in-300 to 1–in-3000." - USGS
Will the next biggest San Francisco earthquake happen this year?
It's been 113 years since the 7.9 magnitude earthquake that took place in 1906 in San Francisco.
According to USGS "The real threat to the San Francisco Bay region over the next 30 years comes not from a 1906-type earthquake, but from smaller (magnitude about 7) earthquakes occurring on the Hayward fault, the Peninsula segment of the San Andreas fault, or the Rodgers Creek fault.".
Probabilities of one or more major earthquakes on faults in the San Francisco Bay Region during the coming 30 years.Photo byUSGS/ Website
In the picture above you can see the probabilities of one or more major (M>=6.7) earthquakes on faults in the San Francisco Bay Region during the coming 30 years.
"The threat of earthquakes extends across the entire San Francisco Bay region, and a major quake is likely before 2032. Knowing this will help people make informed decisions as they continue to prepare for future quakes." - USGS
Unfortunately, scientists can't predict exactly when the next 1906-like earthquake will happen.
As you know, earthquakes are not like storms, they can't be predicted based on signs that appear before they happen, because there are no signs. The scientist's estimates of when a large earthquake will occur are imprecise and are based on "models."
A model is a simplified idea of how something works.
When it comes to analyzing when the 1906 earthquake will repeat, the models are the theory of plate motion and the accumulation of stress along a locked fault, observations of past earthquakes on that fault, and the rate at which the plates are now moving.
"These models suggest that it might take 200 years or more (starting in 1906) before enough stress accumulates on the fault to produce another great earthquake. (The long-term rate of motion, averaged over many earthquake cycles, on the 1906-segment of the San Andreas fault is between 3/4 to 1 inch per year. At this rate, a 20 ft earthquake offset requires 200-240 years to accumulate.)" - USGS
Keep in mind that this is not a precise estimate, other factors can influence the time a big earthquake will occur.
"So, while the most likely time for a 1906-like earthquake to strike again is perhaps late in the next century, there is a small chance (about 2 percent) that it could happen in the next 30 years." - USGS
There is a 72% probability of one or more M ≥ 6.7 earthquakes from 2014 to 2043 in the San Francisco Bay Region.
Earthquake probability to occur in the San Francisco Bay Region ( these probabilities include earthquakes on the major faults, lesser-known faults, and unknown faults):
earthquake of magnitude 6.0 or larger will occur before 2043 is 98%,
at least one earthquake of magnitude 6.7 or larger in the San Francisco Bay region is 72%,
at least one earthquake of magnitude 7.0 or larger it is 51%,
"The probability of a large earthquake occurring on an individual fault in the San Francisco region is lower than the probability of an earthquake occurring anywhere in the region." - USGS
The San Francisco Bay Area is favorable for an earthquake to occur
"The same geologic process that is responsible for the San Francisco Bay region’s beautiful coastlines, bays, hills, and valleys is also the primary driving force for earthquakes along faults in the region. The Bay region is located within the active boundary between the Pacific and the North American tectonic plates, where the Pacific plate slowly and continually slides northwest past the North American plate. The San Andreas Fault, on which two magnitude 7.8–7.9 earthquakes have occurred in historical time, including the 1906 San Francisco earthquake, is the fastest slipping fault along the plate boundary. Other major plate boundary faults in the San Francisco Bay region include the Hayward, Rodgers Creek, Calaveras, Maacama, San Gregorio, Concord, Green Valley, and Greenville Faults." - USGS
What can you do?
Well, we may not exactly when will the next 1906 earthquake hit San Francisco, but we know that it will happen.
Unfortunately, it is most likely that the earthquake will take us by surprise.
What you can do is make sure your house is safe and withstand a magnitude +7 earthquake
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