A 40% traffic drop right after a major event is a certified code-red SEO emergency. It means Google has either hit a structural wall trying to crawl your site (migration issue) or has fundamentally changed how it views your quality (algorithm update).
When this happens, you cannot afford to guess. You need to open Google Search Console (GSC) and check these first 3 specific things to isolate the exact cause of the bleeding.
1. The Indexing Report (The “Can Google See Us?” Check)
If you just went through a site migration, the most common reason for a sudden drop is that Google can no longer read your pages. You need to verify if your pages are still actually in Google’s index.
How to check it in simple words:
Open GSC, go to the left-hand menu, and click on Indexing (sometimes labeled Pages). Look at the trend chart showing Indexed pages (green) versus Not Indexed pages (grey).
MIGRATION CRASH SIGN:
Indexed (Green) : 📈 \___ (Plummets)
Not Indexed (Grey): /📈 (Spikes)
What a 40% drop looks like here:
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The Migration Trap: If you see your indexed pages sharply plummeting while “Not Indexed” spikes, your developers likely messed up a technical setting during the launch.
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What to look for: Look at the table below the chart listing the reasons why pages aren’t indexed. If you see massive spikes in “Excluded by ‘noindex’ tag”, it means someone accidentally left a hidden code block live that tells Google to stay away. If you see a massive spike in “Not found (404)”, your redirect map failed, and your old traffic is hitting dead ends.
2. The Performance Chart (The “Impressions vs. Clicks” Split)
This is where you diagnose whether you are dealing with a technical block (migration) or a penalty/re-evaluation (algorithm update). You need to separate visibility from user action.
How to check it in simple words:
Go to Performance, select a 3-month date range, and make sure both Total Clicks (blue) and Total Impressions (purple) are turned on. Look at how the two lines drop in relation to each other.
| The Pattern | What It Means | The Verdict |
| Clicks Drop, but Impressions Stay High | People are still seeing your site in search results, but your rankings dropped from position 2 to position 8. | Core Algorithm Update. Google still knows your pages exist, but it has decided a competitor’s content is more helpful for those keywords. |
| Both Clicks AND Impressions Crater to Near Zero | Your site completely vanished from search results overnight. No one is even seeing your links anymore. | Migration / Technical Block. Google has physically blocked your site from being crawled, or your critical 301 redirects are completely broken. |
3. The URL Inspection Tool on Your Top 3 Traffic-Drivers
Broad charts tell you what happened, but checking your absolute highest-value pages tells you why. You need to inspect a few of the pages that used to bring in 40% of your business.
How to check it in simple words:
Make a list of the top 3 URLs that took the biggest hit. Paste the first URL into the top search bar in GSC (“Inspect any URL…”). Once the data loads, look at the Live Test results.
What to look for:
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Canonical URL Mismatch: Look at what Google lists as the “User-declared canonical” versus the “Google-selected canonical.” During migrations, if your new pages accidentally point back to old, dead URLs, Google will drop the new page entirely.
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Crawl Allowed? Ensure it says
Crawl allowed? Yes. If it saysNo: blocked by robots.txt, your team accidentally locked the front door to Google’s bots. -
Mobile Usability: If this was a core algorithm update, check the page’s mobile status. If the update heavily weighted user experience and your new site design caused layout shifts or slow load times on mobile devices, Google will suppress the URL.
By checking these three areas first, you eliminate the noise and immediately know whether you need to yell at your web developers to fix a broken piece of code, or sit down with your content team to rewrite your pages for better utility.