Ever Been in an Earthquake?

Q:  Have you ever been in a earthquake
--Gretchen H

A:  Yes, I have "been in a earthquake". As a 6-year-old I was thrown out of my bed by a M=7.2 earthquake in Bakersfield, CA, that hit at 3:30 in the morning. My Mom told me later that she called to me to come to her bedroom down the hallway, and that I answered her "I can't. The walls keep hitting me." These were probably the manifestation of Raleigh or surface waves rolling through the ground beneath the house. I do remember standing in the grass outside the house later and listening to sirens until dawn, as the adults talked about what had happened. That's where I first heard the word "earthquake."
    I also felt several earthquakes when I lived for three years in southern Venezuela. One event frightened the other residents of our apartment building so much that they all ran down the stairwell and stood in the street outside.
    You have to be physically in contact with the ground to feel small earthquakes. If you are driving a car or sleeping in a soft bed you may not feel events up to a magnitude of 3 or 4. 

Q: Thanks so much for answering my question.  I bet it felt very scary in the earthquake.

A: The experience wasn't so scary as it was a sleepy mind trying to figure out what was going on. That part took me awhile as a six-year-old. Then it was just sort of being amazed and listening to adults try to explain it as THEY were figuring it out. I remember my Mom, being very frightened, going down into a darkened basement to shut off the natural gas. I felt her fear then, my first fear, mainly for her.  

How Many Earthquakes in a Week- and the Meaning of Life

The question was typical. The follow-up question was definitely NOT.

Q: Hey I'm wondering how many earthquakes occur every week ? And how powerful they are......

--Rossy K

A:  The answer to your question depends on how BIG the earthquakes are that you are talking about. The smaller the earthquake, the more common they are. This means that there are probably many undetected (very small) earthquakes happening around the world every second. This also means that the really big ones - the ones that get in the news - are not very common at all.

    The US Geological Survey estimates that several million detectable earthquakes occur in the world each year. Many go undetected because they originate in remote areas or they have very small magnitudes. The National Earthquake Information Center now locates about 50 earthquakes each day, or about 20,000 a year, worldwide. They have to be above a certain minimum threshold before an effort is made to even try to locate them.

    There are far fewer large events than small earthquakes, and this table will show you how these are parsed out according to magnitude:

Frequency of Occurrence of Earthquakes

Magnitude                         Average Annually

8 and higher                       1                                             

7 - 7.9                                    15                                          

6 - 6.9                                    134                                        

5 - 5.9                                    1319                                      

4 - 4.9                                    13,000   (estimated)       

3 - 3.9                                    130,000 (estimated)       

2 - 2.9                                    1,300,000 (estimated)

    I would recommend that you visit this web-page:

    http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php

    There are lots of interesting statistics here.

*************

Q: What’s the meaning of life?

--Rossy K

A: Ask your parents, and they will give you a start on answering that question. I just answer questions about geology.

On average you have upwards of 70 more years to figure this out yourself. That's pretty much the whole point of your being on this planet. 

Is Your Job Dangerous?

Q: For a school assignment, I was told to ask a geologist some questions that I have about volcanoes. Is your job dangerous?
- Malayah M

A: It CAN be dangerous. I've walked out the toe of an evolving flow from Kilauea volcano, and accidentally stepped directly on the magma several times. It damaged my boots. Most volcanologists I know are or were personally acquainted with people who are now dead - killed by a volcanic eruption. These deaths usually involved a silica-rich volcano that exploded violently. They were visiting during a time of volcanic unrest, and the explosion happened so fast that it didn't give them time to get far enough away. This type of high-silica volcano tends to form stratocones, so you have an idea of its potential to cause great destruction just by looking at it. Think: Mount Fuji in Japan. Avachinskiy in Kamchatka. Mount St Helens in the United States (it was a nice cone before 1980).

As a result, the volcanolgists still living, whom I personally work with, have become very careful and cautious. They don't take unnecessary risks - but being a volcanologist almost by definition means you must take SOME risks. 

Q: What do you do when a volcano is showing restless activity, and you have predicted that it will erupt soon?

A: We notify public safety authorities at the first reliable hint that something might happen. We make it a practice to drill with them and review the possible things that can happen, ahead of time. When Mount St Helens erupted in 2004-2008 the interaction between the USGS volcanologists and the federal, state, and county safety authorities was almost seamless.

Q: Have you ever witnessed a volcano eruption??

A: Several times I've witnessed a volcanic eruption:
- Mutnovskiy volcano in Kamchatka erupted as I was inside the main caldera in 2004. Fortunately it was a mild eruption, but the Russians with us gave us no warning (I don't think they had realized what was happening before we did). 
- Kilauea has been erupting continuously since 1983, and I have visited the flow-front a number of times. There is an interesting photo here: https://profile.usgs.gov/jwynn - look in the upper right-hand corner.
- I was the first person to see and photograph the new dacite dome coming up from under the glacier at Mount St Helens on October 12, 2004. I was orbiting in a helicopter inside the crater at the time. 

- In addition, I've been on several restless volcanoes that were showing activity like fumeroles (Mount Lassen, in California, Akutan in the Aleutian Chain, etc.).

More About Volcanoes

Here are some more questions from the precocious Cadee, this time about volcanoes.

Q: I am in 6th grade, and we are learning about volcanoes. I've been assigned to ask you some questions about volcanoes. 

Question: How many estimated volcanoes are in the world?

- Cadee C

A: ​The Smithsonian Institution Global Volcanism Program maintains a database that currently contains 1,562 volcanoes with eruptions during the Holocene period (approximately the last 10,000 years) or which are currently exhibiting any kind of unrest. There are many, MANY more volcanoes on the planet than that - they are just older. I have mapped volcanoes in Mexico that are at least 100 million years old, and I have mapped volcanoes in Venezuela that are at least 3 BILLION years old. In my office I have a piece of Mount St Helens volcano that was semi-liquid and oozing as red-hot magma just 9 years ago.

Making matters even more complicated is the question of what IS a volcano? When I served a 5-year assignment as the chief scientist for volcano hazards of the US Geological Survey, I asked one of my senior scientists to do a compilation of the Pacific Cascades. I thought he would come up with a list of 15 - 20 volcanoes - the recognizable peaks like Mt. Hood and Mt. Rainier. His final list had over 3,500 entries - he counted every small separate cinder cone and dike.

Q: Are there warnings before a volcano erupts?

​A: Yes, in almost all cases if there are an adequate number of telemetered instruments nearby. Generally we first see the mountain swelling as magma starts to come up into the edifice from below. We "see" this generally with telemetered GPS networks or with satellite radar data (InSAR). Later, as the eruption gets closer in time, we start detecting magmatic gases like H2S (smells like burnt matches), and we begin to detect small earthquakes whose signatures tells us that rocks are being broken below the volcano.​

​You can't predict an earthquake, but you CAN predict a volcanic eruption if you have enough instrumentation in place on or near it.​

Q: Where do volcanoes usually form?

A: ​The vast majority of volcanoes can be found near and along tectonic plate margins - like the so-called Ring of Fire around the Pacific Basin. Some are harder to explain, except as hot spots that tectonic plates are drifting over, such as the Hawai'i chain. ​Some countries exist on Island Arcs - the entire land is made up of active volcanoes, like Iceland and Indonesia.

Q: How do some volcanoes change over time?

​A: If magma moves up into a reservoir well below the ground and stays there for a long while, it can differentiate. By this I mean that crystals start to form, and the heavier ones drift to the bottom of the magma chamber. When the volcano erupts, the less dense material (the lighter-colored stuff with higher silica content, such as rhyolite) tends to come out first, from the top of the reservoir. Later the material from lower down in the stratified reservoir comes out - this tends to be darker and flows more easily (basalt). ​This ends up on top of the lighter, more silica-rich flows. 

This sort of process works until the percentage of crystals rises to the point of the magma becoming a crystal "mush". Then this sort of "flow easily stuff" does NOT flow more easily.

​Some volcanoes change over time because they get different magma feeds from the mantle as continents move to different places on the mantle with plate tectonics. An example like this would be some volcanoes in the western United States that rather dramatically change in character over long periods of time. ​

Q: Are there patterns in volcanic eruptions from the same volcano?

​A: Yes, and we count on these patterns to roughly predict the future behavior of a volcano. This pattern recognition requires very careful geologic mapping and age-dating of the rocks, as you might imagine, to see what was erupted and when ​- and how. This is a rough art still, and volcanologists are always getting surprises - like when Four Peaks Volcano erupted in the Alaska Peninsula a few years ago.  ​No one had any idea that this volcano had ever erupted at all within the previous 10,000 years. ​We had no instruments close to it, so we had very little warning.

Do You Like Your Job?

I'm always impressed when young people are already thinking early about a career - instead of growing older and stumbling along through life, wondering how they got where they are. 

Q: I am in 6th grade, and have been given an assignment to ask you some questions.
Question: do you like your job?
- Cadee C

A: I absolutely love my job. All of it except the bureaucratic stuff.

Q: What inspired you to study geology? 

A: 
- I could be a real world detective. 
- I could pick up a rock and examine it closely, and  understand its history from the clues you or I can see in it.
- I could work on something that actually helped other human beings live better, live happier. Do you go to school in a bus or a car? Do you read using electric lights? Thank a geologist.

Q: what is your favorite area of study? 

A: All areas. I am interested in the full range of geology, and have published scientific papers in such diverse fields as archeology, astrophysics, and groundwater hydrology. the world is just so full of fascinating things, and I want to study all of them.  

Q: what is something fascinating you've learned since becoming a geologist?

A: 
- How a rock the size of my office, traveling at 10 kilometers per second, can carry so much kinetic energy that it can  leave destruction equivalent to an atomic bomb.
- How water moves slowly underground, and how pollutants from a leaky tank at a service station can move with it and poison whole cities.
- How gold gets concentrated by natural processes - and how to find it!
- How an ocean floor can become volcanic lava. 

Q: how has the way you study geology changed over time?

- We use computers very heavily now. We use very sophisticated numerical modeling to understand what is happening under the ground, when we cannot see or detect features otherwise. 
- We spend much more of our time doing bureaucratic things that don't have anything to do with our work. These are increasingly required by Congress because some people there don't trust federal employees to work honestly.  
- We have to work harder and smarter to get the same kind of productivity.

Crust? Mantle? Core?

Move over, adult world, here come the 8^th graders. They are a LOT smarter than WE were in the 8^th grade.

Q: Hello Mr./Mrs. Geologist, I am sending you this email because I am wondering about something. How can you be sure about the dimensions of say: the mantle, or the inner core? I am an 8th grader and I would be very grateful if I was sent a response to this question?

- Oliver McE

A: The short answer is seismology.

The long answer is that when medium to large earthquakes occur around the Earth, they send a substantial amount of wave energy through the body of the Earth. Seismologists have learned how to use a large and sophisticated global network of seismometers to time the arrivals of many different kinds of waves – and analyze their characteristic signatures. Some of these waves (specifically S-waves or shear waves) cannot propagate through liquids. At the same time, P-waves (pressure waves, similar but lower frequency than the sounds you hear with your ears) will refract, or bend, as they cross boundaries with different physical properties. Refraction is what light does as it crosses the air-water boundary in a pool - so fish in a stream look like they are shallower than they actually are.

By carefully timing the arrivals of the different types of waves, seismologists can discern different physical properties at different depths. Among these, the Crust-Mantle, Mantle-Core, and Outer Core-Inner Core boundaries stand out very clearly.

The analysis is much more complicated than this, or course, but it's pretty obvious when S-waves can be seen up to a certain distance away around the curve of the Earth from the earthquake source - but not beyond. This means that a solid-liquid boundary has been crossed.

Here are some helpful links that will let you visualize what I'm talking about:

http://en.wikipedia.org/wiki/Structure_of_the_Earth

This is a good place to start.

http://www.bing.com/images/search?q=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&FORM=IGRE

I got this rather complicated link by doing a search for "earth's structure layers".

http://www.bing.com/images/search?q=seismic%20waves%20diagram&qs=IM&form=QBIR&pq=seismic%20waves&sc=8-13&sp=2&sk=IM1

I got this by doing a search for "seismic waves diagram".

There are other ways that we geophysicists can "see" different structures inside the Earth, including

2. torsional oscillations of the Earth in response to very large earthquakes,

3. study of the Earth's complex dipolar and changing secular magnetic field,

4. ground-based and satellite-based gravity and gravity gradient measurements,

5. mathematical modeling, and

6. high-pressure, high-temperature laboratory experiments.

The combination of  all these approaches taken together has allowed geophysicists to actually do something called "tomography" - sort of like a CT-scan or an MRI scan of the human body that doctors use - to actually see complex details beyond just simple layering in the deep Earth. Taken together, we are becoming increasingly confident of things like where the down-going oceanic slabs beneath the continents are, and what the minerals – and metals – at different depth might be. 

How deep can earthquakes occur?

Earthquakes are caused by the sudden release of accumulating strain in the Earth's upper crust or buried fragments thereof. This means that rocks must be breaking for an event to be detected by human beings. However, deep in the Earth's interior, rocks are no longer brittle. With overlying rock pressure and increased heat, they become plastic, so they cannot break. 

Q: Is it possible that earthquake can happen at the inside of the earth? Like Mantle or outer core, inner core?

- Sarah G

A: Many things may be theoretically possible, but once you get below the point of plasticity in the Earth's crust there will be no rock-breaking earthquakes. This aseismic depth - where rocks deform plastically - varies as a function of temperature, pressure, and rock constituents. I believe the deepest earthquakes ever recorded are at around 800 km. These happen along subducted, down-going oceanic crust that is still dragging some of its cooler thermal regime with it.