Normal aftershocks

I have been deluged with questions since the Ridgecrest earthquake that are some version of “What does [quake A at location B] mean?”

  • “Why are there so many aftershocks near Coso?”
  • “What does it mean that there are earthquakes near the Garlock fault?”
  • “We just had another M4.5? Does this mean we need to be ready for a big one?”
  • “Are the Bay Area quakes related to Ridgecrest?”
  • Etc.

First, for the basics about how one earthquake triggers another, look down in this blog to my post from February 2018. It describes what seismologists know about earthquake triggering. We actually know quite a bit and can describe the rate of earthquakes reasonably accurately. The variables are time from the largest earthquake (the rate goes down approximately as 1/time), the magnitude of the triggering event and the triggered event (small earthquakes are always much more common than big ones and big earthquakes trigger more new events), and distance (most triggered events are very near the fault that produce the mainshock and the rate dies off as 1/distance^2). If you want the full details, go to the 2018 post.

In other words, we have equations to describe the rate and that we can use to compare one aftershock sequence to another.  But this is a rate.  The time of an individual earthquake is randomly distributed about that rate. So for instance we can say there will probably be 25 to 35 earthquakes of M3 or greater in the next week, but no idea whether the next one will be in 1 minute or tomorrow.

The USGS is creating and posting daily updates of these predictions here.

People are also asking about the risk because we have the big Garlock fault near these aftershocks. Does that make our risk worse?  If another M7 were to be triggered (the current weekly chance of that is about 1 in 300), it would likely be on the Garlock fault so that risk is already included in the probability numbers.

People also ask about the San Andreas. It is much farther away from the Ridgecrest earthquakes so the chance of an earthquake on it has not changed.

Compared to the average California aftershock sequence, the Ridgecrest sequence is about average in total number but is dying off more quickly than average. That means the chance of bigger ones is going down fast which is why the USGS probabilities are quite a bit lower now than they were last week.

All of the questions are some version of “Are these aftershocks normal?” and the answer is “Yes.”

But don’t make the mistake of thinking that “normal” means another big earthquake can’t happen. A large late aftershock is also pretty normal. Magnitude 6 earthquakes every couple of years somewhere in Southern California are normal. “Normal” does not mean it cannot trigger another earthquake. We are trying to make patterns in a random distribution. Just like constellations, we can make the patterns but they are not going to be any use in figuring out what is coming next.

You can’t live in Southern California worrying which day will have the big earthquake. You wear your seat belt every day, not knowing when the next accident will be. You need to make your house safe, get your water supply and be ready for the earthquake on any day.

The Music of Climate Change

I am an experimental seismologist. That means I spend a lot of time looking at data about the earth. I am not an atmospheric scientist so I cannot make detailed models for the future of the world’s climate, but I can look at data and recognize alarming trends. And the basic data for the temperature of the earth, averaged over the whole atmosphere is terrifying. The world is warming and the rate at which it is warming is getting faster.

Change in the average temperature of the Earth since 1880 (data from NOAA).
Temperature is given in °F change from 1880.

I am also a musician and I sometimes hear the data. This data is like a graceful minuet accelerating into a frantic jig. Of course, the increase is to be expected because our burning of fossil fuels for energy has been increasing the amount of CO2 in the Earth’s atmosphere. The heat in the atmosphere equals the energy that comes in minus the energy that goes out. Energy in comes from the sun with minor variations from rotational wobble, sunspots, etc.  Energy out is the infrared heat radiation that is given off by any body minus the amount that our atmosphere can trap. “Greenhouse gasses” are the chemicals that absorb energy in the infrared spectrum and thus keep some of the heat here on earth and if you increase the concentration of greenhouse gasses, you keep more heat. Details are complex with positive and negative feedback mechanisms, transport of heat through the ocean currents, effects of clouds and ice, all being studied by other types of earth scientists.  But I can look at this graph and see that the amount of carbon dioxide is up substantially and that it correlates well with the temperatures. I find these graphs terrifying and I struggle to understand why so many people accept this without fear.

CO2 concentration in the atmosphere. Data from NOAA

I turn to music to process and express my emotions. I realized early on in life that although I love music, I would never be a great musician – one who can make a living at it – but music continues to be the part of my life where I can experience my emotions in more depth. I saw someone turn the temperature data into pitch and play it on a cello. You could hear what I saw in the temperature data. It is not a piece of music – it is a string of notes without chords or harmony. But I started to think about using this to convey my emotions about climate change.

I play an old string instrument called the viol (or in Italian, the viola da gamba). A “consort of viols” was the 17th century predecessor of the string quartet, a small group ensemble with closely interlocking harmonies (an example here). One style of music for this ensemble is the In Nomine, where one instrument plays a drawn out melody and the others play intricate polyphonic lines around that slow steady line, called the cantus firmus, or fixed song. I decided to try to use the climate data as the cantus firmus and create music for the earth.

Turning that vision to reality has taken a few years, but I finally completed In Nomine Terra Calens: In the name of a warming Earth. This music video uses the visual and auditory arts to experience the warming Earth.  It is played by four viols with one of the bass viols playing the temperature data. Thanks to Josh Lee and Ostraka for recording the piece. A friend and colleague from the Art Center College of Design, Ming Tai, led a team to create an animation of temperature data to go with the music.

I realize that just knowing the Earth is warming is not enough. As scientists, we understand a lot about how earthquakes, floods and other disasters will affect people, but we have found that it is much easier for people to understand the problem and take action when we clearly connect the consequences to the problem.  I spent a decade creating science-based scenarios of natural disasters to provide people with the information to make better decisions.

With climate change, if we do nothing, if we continue to add CO2 to the atmosphere through burning fossil fuels, we will change life on earth. Even though there have been warmer times on Earth, those were times with different ecologies – that had time to adapt to the changes in climate.  The speed with which we are changing the climate is unprecedented. The world is already warmer by almost 2°F.  Everywhere on Earth, ecosystems are experiencing a different climate than that in which they evolved. Within the lifetime of children now born, with no action, the climate could be 5-10 °F warmer. What does this mean?

  • More disasters. Heat is energy and with more energy in the atmosphere, storms will be more severe. The strongest hurricane, the biggest daily rainfall, the longest duration of Cat 5 winds, have all happened in the last few years. Storms will be bigger and more devastating.
  • More wildfires. Every ecosystem in the world will be stressed by a different climate, mostly warmer. Wildfires are spreading through the boreal forests in Scandinavia and Alaska. The California wildfires of the last few years are just the beginning.
  • Food shortages. Farmers are already having to adapt their techniques to the changing climate. Some places will become unfarmable.
  • Water availability crises. Some places are getting wetter and some drier. Getting water to people will require different delivery systems than we currently have.
  • Climate refugees. Some countries will be underwater as the polar ice melts. Others will lose the ability to grow enough food for their populations. Millions, maybe hundreds of millions of people will need to move to stay alive.
  • Social disruption? We humans have many choices to make about how we respond to these disruptions. How will we respond to 50 million refugees?

I think many people, especially in America, think that dealing with climate change means giving up modern life. We talk about individuals driving an electric car, forgoing an airplane trip or reducing plastic use, as though that is the solution. We don’t want to give up modern life so we don’t think about climate change or we try to believe it isn’t really true. 

But the answer to climate change is not a Prius and reusable grocery bag.  With 7 billion people on Earth, we could give up every aspect of modern life and not solve the problem. We still need to keep warm in winter and move our food to where people are. The only solution is to move forward, to a world with a carbon neutral energy system, a society where the production of energy does not increase the carbon in the atmosphere. Until we get to carbon neutral energy, any life, let alone modern life, increases the CO2 in the atmosphere.

Dealing with climate change means technological innovation to create a better world. It can be done. Solar energy is already much cheaper than it was a decade ago. When we decided to go to the moon, we solved innumerable technical challenges and we were proud of doing it. We did that together and we need to do it again.

As an individual, you might be one of the technical innovators. But every one of us can say we want our government to fund and support the innovation. Why would we want someone else to own the technologies that are going to be needed by the future world? Dealing with climate change means investing in the future.  Look again at the consequences of our current trajectory. The true threat to modern life is not dealing with climate change. 

I end In Nomine Terra Calens with a stripping away of harmonies to finally land on one, lone, very high note. I end without direction to represent the uncertain future. We stand at a decision point where the future of the world really rests on our decisions.


The Ring of Fire and other earthquake myths

Let’s talk earthquake triggering. Every time a notable earthquake occurs, I get the same questions:

  • How does this affect the Ring of Fire?
  • How does this earthquake affect California?
  • Does this mean a big earthquake will happen soon here?

All of these reflect the human need to make patterns, especially when faced with danger. To understand if a pattern is real, we need to use statistics to tell us if a pattern is repeatable or just coincidence. Many scientists have conducted these statistical studies and we know which ones are real.

One earthquake does make other earthquakes more likely. The slip in the quake changes the state of stress around the fault on which the first quake occurs. The likelihood of triggering another earthquake dies off with distance from the fault and the time since the quake. Mostly they are smaller and we call them aftershocks. About 5% of the time the aftershock is bigger than the first earthquake and we change the names and call the first one a foreshock.

Where the triggered earthquakes occur is a bit more complicated. To understand the results, you need to remember that earthquakes don’t happen at epicenters – they happen over a fault surface and the bigger the surface, the bigger the earthquake. From the magnitude, you can guess the length of the fault that produced the earthquake, as shown in this table.

So each earthquake has a fault length, the length of the fault that moves in that quake.  Most aftershocks triggered by an earthquake will be very near its piece of fault. We use the word aftershock to described triggered earthquakes that fall within one fault length of the mainshock’s fault.  For instance, a M7 earthquake will have a fault length of about 50 km. So any earthquake triggered within 50 km of any point on the mainshock’s 50-km-long fault will be called an aftershock.

Within the first week or two after a quake, we also sometimes see triggered earthquakes farther away and we use the term triggered earthquake to describe them.  These might extend for 3 to 4 fault lengths.  So a M7 might trigger earthquakes as far away as 150 km and a M8 might trigger earthquakes out to 800 km.

Beyond 4 fault lengths, the statistics clearly show that the rate of earthquakes doesn’t change.  Mexican earthquakes have never caused a change in the rate of earthquakes in California. New Zealand earthquakes don’t trigger earthquakes in Japan. Or California. Or anywhere else.  This doesn’t mean we can’t have an earthquake in California, or New Zealand or Alaska. There are M3 earthquakes several times a week in California and a magnitude 2.5 somewhere in the world every minute. But statistics of the earthquake catalogs for the last hundred years clearly show that beyond a few fault lengths the rate of earthquakes is unaffected.

So back to those questions. I’ll answer them now and you can insert whichever earthquake has just happened:

How does this [insert quake here] affect the Ring of Fire?

It doesn’t. In fact, the Ring of Fire is a literary device, not a scientific concept. When we first started exploring the world and recording earthquakes, we saw that both volcanoes and earthquakes were more common around the Pacific Ocean, and the Ring of Fire was coined to describe that.  But the plate tectonics revolution in the 1960s explained why those volcanoes and earthquakes are there – they lie around the boundaries of the tectonic plates, and there are many plates around the Pacific Ocean. Now we know that earthquakes in southern California are occurring in the Pacific plate, while those in Mexico are in the Cocos or North American plate, the ones in Chile or in the Nazca plate and New Zealand is in the Australian plate. The plate motions do affect each other – on the time scale of tens of millions of years. One the time scale of one earthquake, statistics show us there is no relation.

How does this [insert quake here] affect California?

It doesn’t if it is more than a few hundred miles away.

Does [insert quake here] mean a big earthquake will happen soon here?

An earthquake somewhere else does not make a California quake more or less likely.

 

So when you want to make a pattern out of a group of earthquakes, remember that earthquakes happen all the time, and we need statistics to tell us if our pattern is just coincidence. Just because we want a pattern doesn’t make the pattern real.