A note on Bali earthquakes and tsunamis

(Note: see Earthquakes & Tsunamis in Bali: What to do, and where to get information, quickly for practical information. This post is a side-note update on the United States Geological Survey — USGS — earthquake information websites. My apologies if I come across as too pedantic, but I figure I might as well lay it out.)

The other week late at night, our house shuddered gently to another quake.

The USGS reported it as a 4.3, barely a bump compared to the big shake we had last year on 13 October 2011.

The USGS has revamped their website. Real-time data for the past thirty days can be searched here.

Significant historical earthquakes that caused damage and fatalities can be searched here by year

For any other quake information, use the archives search here.

If you want to narrow the search to a specific region use the circular search page

and input the latitude and longitude along with a radius (in kilometer) from that point.

Bali is approximately 8.5 degrees south (south requires the minus sign to input) and 115 E, so I entered that data and the dates of interest (the screen snip below is only part of the website search input page):

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The search returned these results:

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The time provided is UTC. The 6.1 quake, which was the one that gave us a good shake, occurred locally at 11.16 am local time (one of many UTC/local time zone converters is here). The epicenter of the quake (and note there was an aftershock too several hours later) is given as “-9.35 114.59”.

Plugging this into Google Earth, we can pinpoint the epicenter:

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Compare that to last week’s minor quake epicenter, which in the same area of the same fault (as these things go)

(I captured this screen shot directly from the real-time quake information page, which presents the data differently than the archives).

I think by now that most people understand that it’s not the magnitude of a seafloor quake that creates a tsunami, but how the seafloor shifts. A deep quake, or a “strike-slip” quake, where the two plates slide past together, will rock and rattle but won’t disturb the ocean. But if it’s a shallow quake, where a chunk of seafloor shoves upward, or otherwise displaces the mile(s) of water above it, then that displaced volume of water is going to slop over onto coastlines.

So, on a roll, and procrastinating at getting back to revising The Sunless Sea
I thought I’d do a historical quake search for Sanur’s tsunami of the late 1970s, about which I wrote a post. I reported the tsunami surge as occurring in 1978, but a reader said the USGS reported regional quakes in 1979.

A search for quakes larger than magnitude 6 between 1977-1979, within a 1000 kilometer radius of South Bali, provided these results:

These are all pretty shallow depth, possible tsunami generators (tsunamigenic in the parlance)

The largest was the 8.0 on 19 August 1977:

but that doesn’t mean this was the one that caused the tsunami surge.

The 6.3 quake on 17 December 79, which caused considerable damage and dozens of fatalities on Bali, might well have caused a Sanur tsunami surge as well:

And I’m not done yet! I kept on searching for the quake that caused the G-land tsunami of 1994…but I’ll save that for my next post.

Now back to THE SUNLESS SEA….well, not. Maybe I’ll take a nap.

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