# Earthquakes

## Types of Earthquake Waves
Earthquakes generate two types of waves: P-waves and S-waves.

### P-waves
- P-waves are called *compressional waves*
- Have the fastest velocity, so they arrive first to a seismometer.
- P-waves travel at about 5.8 km/s in the mid-continental crust (i.e. as about 20 km depth).
- P-waves has smaller amplitude than S-waves, and shake the surface of the earth (e.g. your house) primarily in the vertical direction
- P-waves deform the material they travel through *parallel* to the direction the wave is moving - see GIF below:


### S-waves
- S-waves are called *shear waves*
- S-waves are slower that P-waves (roughly by a factor of $\sqrt{3}$)
- S-waves have a larger amplitude than P-waves
- S-waves will shake your house in horizontal directions (so make sure your house is properly fastened to its foundation)
- S-waves dforem the material they travel through *perpendicular* to the direction the wave is traveling - see GIF below:


### Surface Waves
For completeness, there are also two other wave types called *surface waves*. These are combinations of P- and S-waves that can develop through constructive interference.

We won't use these waves in this course, but it is interesting to see how they vibrate the ground; it is why the ground motion due to earthquakes can be so complex:

**Love Waves**

**Rayleigh Waves**

## Earthquake Simulations
Given the physics of how seismic waves move through the earth, and a model of the 3D geologic material in a region, realistic simulations of ground motion can be created.

The two linked simulation videos are meant to simulate a magnitude 7.8 event (like the famous San Francisco 1906 earthquake - "The Big One" in California-speak).

Note the viewpoints are from Santa Rosa, CA, on one and Los Angeles, CA, on the other.

We can see that even though these regions are not directly on the San Andreas Fault, they nonetheless:
1. Shake as violently as the fault area itself
2. Shake violently for a very long time

What explains this behavior? This is an annotated sketch we will develop in class.

The links below play in a web-based video player, but from there you can also download the video files if you like.
- Link to [Santa Rosa, CA](https://cpslo-my.sharepoint.com/:v:/g/personal/jjasbins_calpoly_edu/ESgngvoilyBKuTtN8q9uBCEBmqzS_D1eg--ICXl4OxGezw?e=QbDyvc) simulation.
- Link to [Los Angeles, CA](https://cpslo-my.sharepoint.com/:v:/g/personal/jjasbins_calpoly_edu/EWJ90MAgPn9BjdZMH84t9BYBY5eA6PIWf-JJoFeeOZe9Hg?e=qa5Nud) simulation.

We will discuss this effect in class as an *annotated sketch*.

This effect was dramatically seen in the San Francisco area during the magnitude 6.9 Loma Prieta earthquake:

*Image taken from [this page](https://www.usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity)*

## Magnitude
We seem to all have an intuitive notion about magnitude numbers: a 1.5 is nothing to worry about, but a magnitude of 5-6 is getting serious. Along the San Andreas Fault, a magnitude 8 is about the biggest one could expect - and that's a "great" earthquake; it will be extraordinarily destructive when one next occurs near Los Angeles or San Francisco.

Yet other places can have larger ones: Japan had a magnitude 9 (offshore) in 2011, there was a magnitude 9.1 in Sumatra in 2003(4?), and the largest earthquake ever recorded was a magnitude 9.5 in Chile in 1965(?).

Yet no on ever tells you what *units* of the numbers are ... 4.3 ==what== is what I'd like to know.

As we will see there are no units because the Richter Scale is a relative measurement of S-wave amplitude compared to a (arbitrary) reference earthquake, and corrected for distance.

### The "Classical" Richter Scale Magnitude
This magnitude scale uses the P- and S-waves to make the measurement. We will go over this in class. Please see the handouts from class and posted to the Canvas webpage.

### A better magnitude scale?
An interesting (and useful) question is can one estimate the *worst case scenario* for an earthquake on a fault? In other words, what is the largest magnitude earthquake a given fault can have?
Fortunately, faults rarely pop off their largest possible earthquake, but this is of course a useful thing to know to infrastructure considerations, hazard and emergancy planning, etc.

==Unfortunately this quarter we will not have time to cover this ...== 😞

## Effects of Earthquake Waves on Buildings
Each seismic wave type (P-waves, S-waves, and Surface waves) has a different dominant *frequency* - in other words the number of oscillations per second that the wave makes.

P-waves have the highest frequency, then S-waves, then Surface Waves (i.e. surface waves are, in a relative sense, low-frequency waves).

Buildings also have some dominant natural frequencies.

If you shake an object at its natural resonant frequency, the oscillations will grow in amplitude and will then damage the object.

Here is a neat [demonstration of this process for buildings](https://cpslo-my.sharepoint.com/:v:/g/personal/jjasbins_calpoly_edu/ETBpvx9fAUZBlXsh6ldxn8sB4-K53b9u51R_EhmLVbgcXQ?e=BNPGfT) and earthquake waves.

Buildings can be designed to [withstand earthquake waves](https://cpslo-my.sharepoint.com/:v:/g/personal/jjasbins_calpoly_edu/EXYP-wj7r8pItxhRhf8QJmIBvVPhuwR6qsoH-AO7rIXpPQ?e=ltja7w) quite effectively The idea is that the buildings themselves may not survive the earthquake but rather that they don't catastrophically collapse, thereby saving lives.

## Other Earthquake Effects
### Liquefaction
[Liquefaction](https://www.iris.edu/hq/inclass/animation/liquefaction_during_the_1906_san_francisco_earthquake) is a process by which water-saturated sediment temporarily loses strength and acts as a fluid. This effect can be caused by earthquake shaking. Here is another [animated example](https://www.iris.edu/hq/inclass/animation/buildings__bedrock_effects_of_amplification__liquefaction)

Another effect in the soil column from earthquake shaking is the formation of [sand boils](https://ds.iris.edu/about/ENO/iows/8_2002b.htm).

Here is a resource to examine [liquefaction potential](https://www.usgs.gov/programs/earthquake-hazards/science/san-francisco-bay-area-liquefaction-hazard-maps) in the San Francisco Bay area. The hazard is strong along the coastline, and especially where artificial fill has been put in to extend "land".

## Earthquake *Forecasting* (not prediction)
Although the idea of earthquake prediction is kind of a "holy grail" and it continues to be actively researched, as of now there is no predictive tool for earthquakes. Full stop, end of conversation.

So the only other thing I will say about it is that sometimes I get notes on my office door from various people:

Instead, one can plausibly think about earthquake *forecasting* for a potential future distribution of earthquakes in terms of *probabilities*. This is much different. In class we breifly discussed how thinking about probabilities is tricky. Still, the forecasts are interesting to study:

### What are Earthquake Forecasts Based On?
Well, earthquake *history* along faults such as the San Andreas, and in the Bay Area, the Hawyard Fault, the Calaveras Fault, etc.

The subject of [paleoseismology](https://www.usgs.gov/programs/earthquake-hazards/introduction-paleoseismology) studies past earthquake events on fault systems. If enough earthquake history is known for a fault, or a related system of faults, then forecasting, with all its caveats, can me done.

For example, along the southern San Andreas Fault, here is an [animation](https://cpslo-my.sharepoint.com/:v:/g/personal/jjasbins_calpoly_edu/EaDwm4WPqEVGpSMeFHxwiU0B1ynEhzS4JTQb02oxSCRH6A?e=b4eTQj) showing some historical earthquake events in time, and the spatial extent of the earthquake (fault slip):

## Back to forecasts ...
We are still within a "currnet" earthquake forecast probability model for the San Francisco Bay Area:
- [US Geological Survey Model](https://cpslo-my.sharepoint.com/:b:/g/personal/jjasbins_calpoly_edu/ERweGnribUZDrzegEfARyTgBgDGZ7o-q_7UTX9pY0BTQsw?e=MRKnUL) (direct link to PDF)
- Or, [USGS website](https://pubs.usgs.gov/fs/old.2003/fs039-03/) of same model.

As you can see we are getting closer to the end of the 2014-2032 time interval.

There is an updated model going out to 2043 now:


Here is a similar model but for all of [California's major faults](https://cpslo-my.sharepoint.com/:b:/g/personal/jjasbins_calpoly_edu/EfezpUexk61PujAs2vGKhyYB9ZG8tqv7fDl8JXNbpbtbHg?e=JJD3zb) produced by SCEC (Southern California Earthquake Center). Also see [here](https://www.scec.org/ucerf).

## Earthquake Early Warning
An early warning system is not prediction. It uses seismometers to automatically detect earthqukes by real-time monitoring of the seimsogram data. Once an earthquake is detected, *information that the earthquake has happened can be transmitted faster than the eaerthquake waves travel*.

Still, earthquake waves travel fast - several km/s, so the lead time you get is small (measured in seconds, 10 seconds, that sort of thing). But this can be enough to shut down critical infrastructure, surgeries, time to 'duck-cover-hold', etc.

Here is information about [California's earthquake early warning system](https://earthquake.ca.gov/), you can even get an app.

Mexico City has unfortunately been frequently damaged by large earthquakes originating from the subduction zone on the west coast. Even though Mexico City is quite far inland, the earthquake waves still can cause great damage because of wave amplification - Mexico City is built on an area of thick sediments from former lakes.

Mexico has had an earthquake early warning system for quite a while - you can [read a bit about Mexico's earthquake early warning system](https://eos.org/features/lessons-from-mexicos-earthquake-early-warning-system).

## The San Francisco 1906 Event
This is what California calls The Big One. There is a lot of information about this important historic earthquake event at the US Geological Survey website:
- https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/
- https://earthquake.usgs.gov/earthquakes/events/1906calif/
- [A Virtual Tour of the 1906 Earthquake in Google Earth](https://earthquake.usgs.gov/earthquakes/events/1906calif/virtualtour/)
- [The 1906 San Francisco Earthquake a Century Later](https://pubs.geoscienceworld.org/ssa/bssa/article/98/2/817/341919/The-1906-San-Francisco-Earthquake-a-Century-Later)
- You can follow the links inside of the above till your heart's content ...
- Finally [this publication](https://pubs.geoscienceworld.org/ssa/bssa/article/98/2/817/341919/The-1906-San-Francisco-Earthquake-a-Century-Later) has articles on the 1906 earthquake, as well as a brief article about a US Geologicl Survey geophysicist with a great nickname: Mr. Earthquake.

### The Fort Tejon 1857 Earthquake
Lastly, we have mentioned the [1857 Fort Tejon earthquake](https://www.usgs.gov/programs/earthquake-hazards/science/m79-1857-fort-tejon-earthquake) in class - you can read a little bit about it using the link. It's magnitude is similar to the San Francisco 1906 event, but it occurred on the southern half of the San Andreas Fault.