Data Dive 4.3

Are earthquakes in Southern California slowing down?

A few quiet years in a row. Is that a trend, or luck?

You will need paper and a pencil, and a browser for step 3

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Step 1 of 5 · Your guess

What do you think?

It is 2022. A student looks at magnitude 4 earthquakes, the kind you can feel. 2017 had 0. 2021 and 2022 had 2 and 2.

She says Southern California is going quiet.

Write it down before you lookCan a few years of numbers show that earthquakes are slowing down? Write yes, no, or not enough information.
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Step 2 of 5 · Look

Magnitude 4 earthquakes, every year

Earthquakes of magnitude 4 and above, per yearSouthern California, 1995-2024 - the mean year is 8.8earthquakes recordedyearearthquakes recordedthe years that look like a decline33.5670795796129789867996008017021103504140550690780890911103117126138145156160174183196202212224231524

Southern California, 1995 to 2024.

How to read it. Each bar is one year. Its height is how many earthquakes of magnitude 4 or more happened in Southern California that year.

Find out
  1. Find 2017, 2021 and 2022. Do they look like a slowdown?
  2. What happens in 2024?
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Step 3 of 5 · Try it

Now you try it, on the same data

This opens the pool, the site's chart tool. The words in bold below are buttons and menus you will see on the screen.

Do these five things

Open the pool on this file: data.sfhsbta.com/thepool?file=noise_socal_m4_by_year.csv&mark=line

  1. Under across, pick year. Under up, pick m4_plus_earthquakes. Click the line shape. Hover 2017 through 2022 and read them out.
  2. Press shuffle five times. It deals the years in random order. How many deals have two quiet years in a row, with 2 or fewer?
  3. Hover 2024.
  4. Hover 1999. It had 67, from one very large earthquake and its aftershocks. Is that many separate facts, or one event?
  5. What would convince you that earthquakes really are slowing down?
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Step 4 of 5 · Say it

Your answer, in one sentence

I think the slowdown <is real or is luck> because <what the chart shows> and I checked it by <what you did to the chart>.

The last part matters most. It shows you did not just look at the chart, you tested it.

Here is one exampleI think the slowdown is luck, because quiet years show up in a random order too. I checked by shuffling the years five times, and one of the deals had two quiet years in a row, just like 2021 and 2022.
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Step 5 of 5 · What you found

Check your numbers

8.8
earthquakes in an average year
15
in 2024, right after the quiet years
1 of 5
shuffled deals with two quiet years in a row
The big idea: a short run can be luckA few unusual years in a row can happen by chance, and shuffling shows how often. Earthquakes also come in families, a big one followed by aftershocks, so their numbers swing even more. Sometimes “not enough information” is the smartest answer.
One more thing to think about1999's earthquakes were mostly aftershocks of one big one. Should an average count that year as many earthquakes, or as one?
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After the dive

The charts you used today

is this real, or noise?

Shuffle the data many times and see whether your result still stands out. If it does not, it was luck.

What it leaves outIt tells you a result is unlikely by chance, not that it matters. A tiny difference can clear the bar and mean nothing.
the bar

Bar length is the value, so it only means anything measured from zero. Start the axis anywhere else and the lengths stop matching the numbers.

What it leaves outOrder is a choice. Sorted by size it is a ranking; sorted by name it is a lookup; and the same bars say different things in each.
the line

A line follows one thing over time. Read it left to right and watch which way it leans — that lean is the whole story. A dashed stretch is a guess, not something anyone measured.

What it leaves outThe line implies every point between the dots, and nobody measured those. Where you start and stop it decides what the trend looks like.

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