There is often confusion surrounding the terminology of earthquakes. The largest earthquake in an earthquake sequence is called a mainshocks. Mainshocks are followed by 100s or even 1000s of smaller earthquakes, called aftershocks. Aftershocks are smaller earthquakes on patches of the rupture zone (areas of the fault that broke during the earthquake) and adjacent faults that occur as rocks along and nearby the rupture adjust to the new state of stress. Although there is some variation in what scientists “count” as an aftershock, a common definition is that an aftershock is any earthquake that occurs within one fault length of the fault where the mainshock occurred AND before the level of seismicity in the area returns to what is was before the mainshock happened.

Occasionally, about 5% of the time, an aftershock will be larger than the mainshock. When this happens we change to the terminology to reflect the sizes of the earthquakes and their timing. We rename the first earthquake and call it a “foreshock” and we rename the bigger aftershock and call it the “mainshock”, because it’s now the largest earthquake in the sequence.

A scientist named Omori found that the decay of the aftershock rate was best analyzed using a logarithmic graph of aftershocks per day versus days after the mainshock. He showed that the rate of aftershocks decreased w/ the reciprocal of time since the mainshock. This is called Omori’s Law. Put another way, the most aftershocks will occur right after the mainshock, and the rate of aftershocks will decrease with time.

Although only a small portion of the energy of an earthquake sequence is released by aftershocks, these quakes can still cause damage, or make the damage from previous quakes worse. The *highest rate* of aftershocks occur during the critical first hours and days immediately following the major earthquake. This is exactly the time when emergency workers are locating and rescuing injured people from collapsed or damaged buildings. Because those buildings can shift or even collapse during aftershocks, rescuers are at great risk when their expertise and service is most important.

Earthquake prediction is, predictably, a topic once again. It doesn’t really matter when you’re reading this because it’s almost always a topic, but it becomes especially problematic after large earthquakes. Why? Because earthquakes are scary and people want the reassurance that someone, somewhere, can tell them what to expect.

Earthquake predictors come in lots of different “flavors”. Some have good intentions, many are in it for clicks or money (monetized websites / YouTube channels or the like). All of them cause needless fear and anxiety. Many claim that scientists are threatened by them, or are silencing them. This is not true. But to understand why this isn’t true requires some insight into the process of science, which isn’t always clear. I’ll try to outline it briefly.

When a scientist has an idea they test that idea to see if it supports the observations. They test it over and over to see if it always works, to see if they can reproduce the results. If it does, they write about that idea – the idea itself, the methods used to test the idea and the results. They write about WHY that idea works, the mechanism by which it works. Then they submit that paper to a journal. But that’s not the end!

Then, that paper (but really the idea) is assessed by other people that study the same thing. They TRY and poke holes in the idea. Not because they’re mean, but because they are also testing the idea, looking to see if it actually explains the observations, and how well. They may try and reproduce the results themselves, to make sure that the idea works. They will also look at the WHY – can the proposed reason explain the results? If everything checks out and the experts agree, then the paper is published in the journal where it can (and will) be debated some more by the scientific community at large. If the idea doesn’t stand up to scrutiny, the paper is rejected & the reviewers give comments about the weaknesses of the idea or the why.

This process is tedious but important to ensure rigor and reproducibility in scientific information. Anyone can submit their ideas to journals to be assessed in this way, which is called “peer review”. Let me repeat that – anyone can submit. However, if your idea isn’t reproducible or your “why” doesn’t hold water, your idea will be rejected. That goes for professional scientists or citizen scientists – any and everyone. So when predictors say they’re being silenced or ignored they might mean their idea didn’t pass peer review but usually it means that they have not (or have refused to) go through peer review.

Is peer review perfect? No. But it’s pretty good at winnowing out blatantly incorrect ideas. Does scientific understanding change? Yes! New data sometimes leads to changes in our understanding which can result in a new scientific paradigm. Carl Sagan said it well – “In science it often happens that scientists say, ‘You know that’s a really good argument; my position is mistaken,’ and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn’t happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day.”

How does this paradigm shift happen? Slowly. And with EXTENSIVE peer review and debate and testing of the new idea. Why? Because extraordinary claims require extraordinary evidence. So if you think you have an idea that will change how we fundamentally understand the earth, like earthquake prediction, you better be prepared to back it up.

All this to say, so far no one’s ideas about earthquake prediction have been reproducible to the standard of science. They can claim it, but they have yet to back it up.

No one can predict earthquakes.

I married in to a family of pilots. One of the perks of this is that once in while they’ll humor me and fly me around so that I can see Southern California from the air; more specifically, I can see the amazing faults and stunning geomorphology of Southern California.

Yesterday we flew along the Elsinore and San Jacinto Fault zones. These faults are some of the many faults (including the San Andreas) that are helping to accommodate motion between the Pacific and North American Plates.

I’ve roughly annotated the photos below (on my phone, while in the air) so that the fault location is more obvious for those who aren’t used to seeing such features in the landscape. It’s important to keep in mind that faults are rarely just one simple strand; they are fault zones. Fault zones are broad zones of deformation with many fault strands and areas of crushed and deformed rocks – some fault zones can be km wide. So my annotations are gross oversimplifications that are showing the rough location of the most prominent strand that I could identify from the air.

One more quick note – geologists locate faults by looking for particular things in the landscape. The faults I looked at were right lateral strike-slip faults, which means that one side of the fault is sliding horizontally past the other so that everything on the other side of the fault looks as if it moved to the right. The landscape features (or geomorphology) that I was looking for were offset streams, changes in the slope of the land, offset ridges, lines of vegetation (water moves easily through the broken rocks of the fault so trees often grow along faults) and other changes in the shape of the land that are associated with faulting. When you see many of these types of features and draw a line between them we call that the fault trace, or the place where the fault intersects the surface of the earth. Not all faults make it to the surface – these are called blind faults. The Northridge earthquake occurred on a blind thrust fault.

All of the maps are from the USGS Quaternary Fault and Fold Database. The lines are the faults; the different colors indicate the last known activity on that fault (more info available on the USGS website linked above). Solid lines means geologists are very confident that the fault is in that location, whereas dotted lines are places where they have inferred that the fault is likely to be.

The Elsinore Fault

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A sag pond is a common feature found along strike-slip faults. When there are two strands of a strike-slip fault that are parallel to each other they will sometimes pull apart a little bit, forming a depression that fills with water – this is the sag pond. In other areas parallel (or sub-parallel) strands may squeeze together a bit, and this forms a pressure ridge (sometimes called a shutter ridge). A famous pressure ridge is Dragon’s Back along the San Andreas Fault.

The San Jacinto fault

When you talk about earthquakes and faults in Southern California you hear a lot about the famous San Andreas, but all of these faults (and many many more) are capable of producing damaging earthquakes. To learn more about how to prepare for earthquakes visit these great sites – Shakeout, Putting Down Roots in Earthquake Country, and Ready.

Thanks again to Fred and Scott for flying us around and giving me the opportunity to see these amazing features from the air! And thanks for reading!