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Comment by jedberg | Hacker News Reader

> This quake was larger than the destructive 1994 Northridge quake, which measured 6.6 magnitude.

This isn't entirely accurate. While it's "score" was higher, the magnitude doesn't account for things like the type of fault, the type of movement, the depth, the soil, the duration of movement, etc.

Northridge was far more severe than this one, with an "apparent magnitude" greater than 9.

Don't get me wrong, this one sucks for all the people affected, no doubt, but as someone who lived through Northridge very close to the epicenter, it's sort of personal to me.

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There are earthquakes, and then there is the ground shaking that results from earthquakes.

The magnitude scale that is most used is an expression of the total energy release in the earthquake is called the 'moment magnitude' or 'work magnitude' because it's measured in those units (Newton meters or dyne-cm). It is calibrated to be similar to the original Richter scale, which was an empirical, I believe unitless, scale based directly on seismometer readings. If someone credible says that an earthquake is a 3 or a 7 or something, they are almost certainly referring to the moment magnitude. The 1989 Loma Prieta event was a M 6.9 event, Northridge was M 6.6 or 6.7.

There is another measure called the 'intensity' that you may be referring to with the 'apparent magnitude' reference. This is a non-quantitative, basically ranked categorical description of the amount of ground shaking and subsequent damage that occurred. The most common intensity scale is the Modified Mercalli intensity scale; it is usually (and correctly) denoted in Roman numerals. The Wikipedia page for the Mercalli scale [1] is good, however I prefer [2]. The intensity has a sort of tree-falling-in-the-woods element to it: the categories are based on how people and the built environment respond to the earthquake, so did earthquakes in the 4.6 billion years before humanity have intensities? They had magnitudes.

Quantitatively, ground shaking is measured as 'peak ground acceleration', a force, usually compared to gravity 'g' (i.e., 9.81 m/s^2), as 'peak ground velocity' which is a velocity (m/s), or as 'peak ground displacement' which is a distance.

The intensity of an earthquake will be related to the peak ground acceleration and velocity, as well as the duration of the shaking. These are functions of both the earthquake properties (magnitude, faulting style, depth) as well as the properties of the ground in the region of the earthquake, and the distance from the earthquake (as the amplitude of the waves decreases with distance). Solid rock does not shake nearly as much as loose or wet sediment, for example, and some geologic basins that are deep pockets of sediment surrounded by bedrock can amplify earthquake waves and make the shaking locally much worse than it was elsewhere, even closer to the earthquake. This was a big deal in Loma Prieta, for example--the damage in Oakland, built on water-saturated sediment, was worse than in some of San Jose, or in my (current) house in the Santa Cruz mountains just a few km away (I was safely dodging tornadoes in Tulsa at the time).

[1]: https://en.wikipedia.org/wiki/Modified_Mercalli_intensity_sc... [2]: https://www.mcsweeneys.net/articles/levels-of-the-mercalli-s...

There's also a noticeable difference in waves. Some quakes hit you with a lateral motion (which, I presume magnifies any possible liquefaction). Some hit with a rolling motion (which really wreaks havoc on masonry). And some hit you with both (those are the ones where you hope the building doesn't collapse on you).

Northridge was violent beyond belief. I lived less than a mile from the epicenter at the time. I remember it distinctly because I stayed up coding all night. It hit at 4:31 AM (the image of the digital clock on the shelf is seared into my mind). It felt as what I can only describe as a train going through the house.

I was nearby too in the valley and running for the doorway was like trying to run in a bounce house. Also, the doorway is no longer recommended: https://earthquake.usgs.gov/learn/topics/megaqk_facts_fantas...

I still had my analog clock set at 4:31 for months after Northridge, it kind of served as a reminder to me that quakes happen, plus I was too lazy to retrieve the batteries that rolled under the bed .. but yeah, 4:31 is burned in my mind too. Occasionally I'll be up in the middle of the night and 4:31 will roll past, and I'll lay there thinking about that quake.

I'll add a bit to my story...It actually involved levitation...

As I said, I was up coding all night. At the time I had a U-shaped desk flanked on all sides by bookcases full of books. Lots of databooks filled these shelves (for software guys: These are books published by chip makers containing their catalog of chips and technical details on each one).

I was working on a the microprocessor code for a high power motor control. The hardware, 100A power supply and test equipment where all on my desk, turned on and operating. I was using a very large ceramic wound resistor in a bucket full of ice as a test load. The bucket was on the floor and connected to the motor driver via very large conductors (think jumper cables).

At 4:31 AM I felt a single jolt. I know because that's when I looked up at the digital clock up on the shelf immediately in front of me. It was a Radio Shack clock with red LEDs for the digits. It was part of their home automation system, with buttons on top to turn on and off various satellite plugs, lights, etc.

The jolt felt like The Hulk shoving the house or something like that. One sharp short jolt. My first thought was "this is going to be bad". I had been in a couple large quakes but this jolt had a different feel to it.

That's when it started. And what was bad about it is that the vertical component was significant. Well, significant enough to launch me and every single book and object on my bookcases straight up into the air. The compressed air spring in my chair acted like a pogo stick. I literally got launched upwards, I'll guess at least a foot, if not more.

My last memory of this portion of the event was floating in space with all of my books in the air at the same time. It was very, very weird. Power went out, I fell to the ground and so did everything else around me. The shaking was violent, as I said before, like a train going through the house.

Thankfully the motor driver wasn't hot because I was not driving the load at the time. It ended-up on the floor buried under a pile of National Semiconductor databooks. That would have easily been a fire had the thing been hot and power not gone out.

I later found a massive 8 inch drive that was on a shelf across the room landed just inches from where I fell to the ground. That thing could have sent me to the hospital. Amazing.

I still have the clock somewhere.

Great! :)

(very off topic) @robomartin - Would you be ok to email me (justin@postgresql.org) re that Open-Source Leg project from a while back?

Can't see your contact details anywhere, so using a comment here instead. ;)

It's also larger than the 1989 Loma Prieta earthquake that destroyed a lot of San Francisco, which was 6.9.

A lot of this also has to do with how close to populated areas the earthquake is, and if the same quake happened today, the damage would be much less due to better construction.

Better construction is real. I work for a medical institution that is spending close to $1bln to build a new hospital from scratch because they found it cheaper than to do yet another earthquake retrofit.

Note that Loma Prieta is just north of Santa Cruz, over 60 miles from San Francisco.

Not sure how to compensate for distance, but my gut feel says that's about equivalent to a 5.9 in SF itself?

destroyed a lot of San Francisco

That's overstating things by quite a bit. There was severe damage and fatalities but 'destroyed a lot of San Francisco' was 1906, an estimated 7.9 followed by fires.

The fires did by far the majority of the damage.

Um, no. LP caused extensive damage in small sections of San Francisco, but the two eqs are nearly the same intensity.

PS: I lost my proverbial "I survived the Loma Prieta in San Jose" t-shirt. Lost half a swimming pool, bookshelves, dishes and so on... our furniture was anchored to the wall studs and computer monitors and other electronics were industrial Velcroed to surfaces.

In terms of magnitude, a 7.1 earthquake shakes 1.6 times as hard as a 6.9 earthquake, due to the scale being logarithmic.

No, the magnitude is not a measure of shaking. There's a comment elsewhere in thread that talks about some of the details.

  destroyed a lot of San Francisco

It destroyed hardly any of San Francisco (almost nothing outside the Marina), given that the epicenter was three counties away.

Northridge knocked me out of bed and I woke up on the floor.

Simi Valley checking in, everything on the floor! TV, dishes, CDs.

Same. And I was in a bunk bed. :(

Almost killed my friend, who just happened to move his bed away from the bookshelf the day before. The bookshelf smashed to the ground right where his bed had been.

We also had to dig my grandparents out of their house with shovels.

Please tell me you dug them out alive?

Yes. :) After the initial shock, they barricaded themselves into the storage room behind the garage, because there was a pull out couch and a phone in there. But we couldn't call them because another phone in the house had been shaken off the hook. So we drove down there as quickly as we could and when we got there everything inside was rubble, the chimney was just a pile of bricks, etc. (although the house itself was still standing).

We got shovels and started digging, calling out their names. Finally got the back and found them sleeping in the pull out sofa! We packed some of their stuff and took them back to our house since we had minimal damage (despite being thrown from bed) and then went back later to survey the damage.

At that point people in the larger multi-family buildings had all evacuated and set up tents in the median strip on the main street (Ventura Blvd). Many of those buildings were condemned.

Northridge was horrendous, but I personally didn't suffer any damage other than a book-end falling off my properly secured shelves, and my cat taking a massive piss right in the middle of the floor. My roommate and his girlfriend survived the mirror over their bed crashing down on them somehow, but other than that .. no worries. A well-built house.

My neighbours - well, lets just say that by the time I got my senses together to go outside and inspect things, they'd already packed all their shit and their kids into the SUV and were moving back to Oklahoma that instant, no time to wait - they GTFO'ed out of California... their house wasn't even that damaged, apart from its chimney laying in the street, like every other brick chimney in the neighbourhood ..

Northridge was wild. I'll never forget the feeling of walking around Hollywood that morning, looking at the kitchen and toilet sections of apartment buildings that had their walls stripped off them, like some bear had ripped open a beehive to get to the goods ..

> the mirror over their bed crashing down on them

I could never sleep under a mirror for exactly that reason.

They were young, and had their reasons. :)

My dad was in Northridge for two days to film on Jeopardy and was hit by the quake. he said he saw the pool sloshing around. As a contrast, I've been on NorCal for 28 years and haven't experienced anything larger than a 4.5 or so.

I was in an earthquake in DC in 2011 that, while lower on the Richter scale, felt way worse than the one that happened in LA today.

I didn't even feel the one yesterday either.

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