The 6.4-Magnitude Independence Day Quake In California, Explained
Robin Andrews | Jul 4, 2019, 02:16pm
A ShakeMap of the quake, with the epicentre located at the position of the star. The redder the liens, the greater the felt shaking was at that location.
USGSA magnitude 6.4 earthquake rocked southern California this Thursday at 1033 hours, local time, with the epicentre cropping up right near the city of Ridgecrest, which is around 230 kilometres (145 miles) west of Las Vegas and 182 kilometres (115 miles) northeast of Los Angeles.
Based on the reaction on social media, the quake was felt across the region, particularly in those two cities and, of course, Ridgecrest.
As ever with posts like this, I'll be updating it as more information comes in.
So, what happened?
According to the United States Geological Survey (USGS), the quake struck right near the Searles Valley, in the Mojave Desert. The initial quake magnitude has been estimated as a magnitude 6.4, certainly nothing to be sneezed at. It occurred at a depth of around 10.7 kilometres (6.6 miles), which is somewhat deep – something that, along with its inherent power, possibly allowed the shaking to spread out further than it otherwise would have.
According to the modified Mercalli scale, which measures shaking intensity, the quake, at its epicentre, came in as a VIII on a scale that goes up to (and theoretically past) a X. That means the perceived shaking at the epicentre could be described as "severe". The shaking in places like Los Angeles and Las Vegas would have been fairly weak, but still detectable, based on the hundreds of thousands of reports coming in.
What about damage or deaths?
Ridgecrest is apparently experiencing some quake-related damage, along with some fires breaking out in a few structures. Roads were damaged in San Bernardino County, per Reuters. No deaths have currently been reported.
The epicentre was away from any town or city centre. According to the modified Mercalli scale, the potential for damage was "moderate/heavy"; it looks like that is being seen to some extent in areas proximal to the quake, with road and structural damage being reported. Nevertheless, the remote-ish location of the quake means that it should greatly limit the amount of damage, destruction and death that it could have caused if things were a little closer to people's homes.
The USGS has a system that estimates the potential fatalities when a quake strikes. At the time of writing, there is a 65% chance that one or fewer people died, which is obviously great news – but there is also a 30% chance that up to 10 people perished. So far, thankfully, nothing of the sort has been reported.
This same system as estimates economic damage. There is a one-in-four chance it will be anywhere from one to $10 million, and a 35% chance it will be between $10 and $100 million. There's also a 23 percent chance that it could be as high as $1 billion, so fingers crossed the reality is somewhere on the lower end of the spectrum.
Is this the mainshock? What about aftershocks?
So far, this looks like the mainshock, although we'll have to wait a little to make sure this is the case. Earthquakes don't happen in isolation, but in clusters, as faults tend to keep jutting forwards after letting loose in a rather dramatic manner. The mainshock is the most powerful quake in that cluster, which means a mainshock can only be applied in retrospect. Anything weaker that happened before in the same cluster is known as a foreshock, and anything weaker afterwards is known as an aftershock.
Assuming this is the mainshock, plenty of smaller aftershocks are expected, and indeed, they are already taking place. According to the USGS, there is a 9% chance that one of the aftershocks over the next seven days could be larger than a magnitude 6.4. If so, then that will become the mainshock. There is also currently a 2% chance that a magnitude 7 quake will take place in the region in the next seven days – low, but not an impossibility.
Even if this doesn't happen, the aftershocks are expected to, and are already appearing to be, pretty darn powerful. So far, there have been numerous aftershocks coming in at magnitude 3 tremblors or greater. The USGS's aftershock forecast indicates that, in the next week, the area is practically certain to experience anywhere from 50 to 700 magnitude 3.0 quakes, which can be felt by people. More concerning is the 80% chance of as many as nine (but potentially zero, or anything in-between) magnitude 5.0 quakes in the area, which are definitely enough to cause damage at the surface.
Lucy Jones – a renowned seismologist in Southern California who often is quick to share the facts and dismiss the misinformation when a big quake or swarm takes place – took to social media to explain that the quake is having "a robust aftershock sequence," featuring a handful of magnitude 4-and-above quakes, and many more of a magnitude 3-and-above. "That means there will be plenty more aftershocks today," she added, emphasising that "bigger quakes last for a longer time."
With all this in mind, the message is simple: be prepared for more quakes.
What caused the earthquake?
The beach ball-like object on the USGS' website means that this was a strike-slip event. This means that two geological blocks were grinding against each other, side-by-side, as one moves (roughly speaking) horizontally in the opposite direction to the other.
This is to be expected. Thanks to the strike-slip motion of the tectonic boundary between the jostling Pacific and North American tectonic plates, plenty of faults behave in this way.
Jones explained on Twitter that the quake didn't take place on the San Andreas fault, but an area where plenty of small faults are bunched up together. Indeed, per the USGS, the fault that slipped was a whopping 150 kilometres (93 miles) away from the San Andreas fault. In fact, it looks like two small cross-cutting faults were involved:
The USGS' report goes into plenty of rich, glorious, tectonic detail: the quake took place on a shallow strike slip fault within the North American plate, one that was very steeply inclined. The area is known for its high seismic hazard, so earthquakes like this are to be expected from time to time; over the past 40 years, there have been eight other earthquakes fairly close to the July 4 tremblor near Ridgecrest coming in at a magnitude 5-or-above.
As this quake was land-based, there is no risk of a tsunami occurring.
Sounds like a pretty big magnitude earthquake to me. Is that bad?
Unquestionably, a magnitude 6.4 quake is fairly powerful. In fact, per the Los Angeles Times, it's the most powerful quake to hit Southern California since the magnitude 7.1 Hector Mine quake, which struck a marine base in 1999.
Per the USGS, the largest quake that took place close to the fault that slipped today in the last four decades was a magnitude 5.8 event. This took place on September 20, 1995, just 13 kilometres (7.5 miles) west-northwest of today's event. In terms of magnitude, then, the Independence Day tremblor has 1995's attempt beat.
Today's quake was almost as powerful as the 1994 Northridge quake, a magnitude 6.7 that hit the San Fernando Valley region of Los Angeles. That proved to be a very different day to today: back then, on January 17 of that year, more than 60 people died, around 9,000 people were injured, and the area racked up billions of dollars of infrastructural damage.
So, why the difference in outcomes despite the small difference in magnitude, you may ask?
A single-digit increase on the moment magnitude scale – say, from a 3 to a 4, or a 5 to a 6 – means a 32-fold increase in the amount of energy that earthquake released over its duration. As a very crude rule of thumb, the more powerful an earthquake is and the closer it is to a city, the higher the potential for damage and destruction.
As I've explained in this post though, magnitude isn't everything when it comes to earthquakes. Obviously, a powerful quake that takes place far from anyone, and which doesn't trigger a tsunami, is pretty much harmless, so geographical location is a big one. Click through to the aforementioned post to see more details, but here's a quick rundown of what else matters:
- Shaking Intensity: this depends on the type of fault, the geological makeup of the region, and the manner in which the fault (or faults) slipped. Yes, higher magnitudes tend to produce more shaking, and shaking is the reason things on the surface get damaged. Different countries have different ways of measuring this, but the peak acceleration of the ground, as well as people's personal reports on how things felt, go into the scales. More shaking in a city tends to mean more damage to buildings.
- Depth: With all things being equal, the shallower the quake, the more shaking and the greater the damage it will cause if we happen to be in the way.
- Building Codes: If buildings have not been designed to sway along with the shaking, or they haven't been retrofitted to bolster their resilience a little, they stand a greater chance of collapsing if the shaking is intense enough.
- Education/Awareness: The more people know what to do during quakes (the best general advice for which can be found here), the more likely lives are to be saved.
- Sedimentology: Loosely packed sediments shake more, and if they are wet, they can behave like a fluid. Bad for any buildings built atop them when the quake strikes.
- Secondary Geological Effects: If the fault moves in a way that can move a lot of water, you could get a tsunami, for example. In this case, there is zero tsunami risk for several reasons. Quakes can also trigger avalanches, landslides and liquefaction (the fluidization of soil) depending on the landscape.
TL; dr: magnitude is useful to know how powerful a quake was, but there are plenty of factors to take into account. A magnitude 6.4 quake beneath Los Angeles, for example, would have almost certainly been far more devastating.
Was this the 'big one'?
For several reasons, no, not by a long shot. As far as I know, there is no technical definition of what the big one is, and it means different-ish things and involves entirely different tectonic settings and faults to, say, people up in the Cascadia Subduction Zone in North California and northwards, compared to those in Central and Southern California.
In terms of the region currently wondering what exactly was going on beneath its feet, the 'big one' generally refers to a cataclysmic rupture somewhere along the San Andreas fault, which is a little over 1,280 kilometres (800 miles) long, and has plenty of smaller faults shooting off from it. This, sadly, is inevitable, but it's impossible for anyone to say when and where exactly this will take place, and how powerful it will be.
Physical and mathematical laws do allow seismologist to forecast the 'big one', though, using wide-ranging probabilities. All the available data means that, per the USGS, there is a 31% chance a magnitude 7.5 event will take place in the Los Angeles region within the next 30 years, with that rising to a 46% chance for a magnitude 7. For a magnitude 6.7 quake, it's 60 percent.
For the San Francisco Bay area, within the same three decades, there is a 20% chance of a magnitude 7.5 tremblor occurring, which jumps to 51% chance if it's a magnitude 7 quake and to 72 percent if it's a magnitude 6.7 tremblor.
The big one will be any powerful enough quake to sufficiently damage one of these cities and cause a fair few deaths. Clearly, today's quake isn't it, nor does it have any bearing on the eventual, bona fide 'big one'.
Is this related to the Ring of Fire? Is the big one coming? Are we all doomed?
First and foremost, no, this isn't connected to anything you've seen in the news. It has nothing to do with, for example, the paroxysm that took place yesterday at Stromboli, over in Europe's Tyrrhenian Sea. This isn't a sign of the apocalypse or anything scarier, so if you see anyone claiming anything of the sort, they are either a liar or are, well, severely misguided. Pay them no heed.
The same applies to anyone who says they know where the next big quake will occur, and how powerful it will be. Whatever they say will be – as it's technically known – horseshit.
Sure, this quake took place along the horseshoe-shaped network of major tectonic boundaries known as the Ring of Fire, but that really doesn't mean much. This loop, which surrounds much of the vast Pacific Ocean, is very complex, featuring constantly shifting, sliding and grinding plate boundaries, all with their own network of segregated or closely-spaced faults.
This continuous activity means that 75 percent of the world's volcanoes and 90 percent of the world's earthquakes take place along this boundary, with this latest sequence in Southern California just another day in the office for the Ring of Fire. Nothing out of the ordinary here. The more powerful a quake is, the rarer it tends to be, but this all fits in nicely with what scientists expect.
Yes, in some cases, an earthquake on one fault can trigger another on a different fault nearby. As pointed out by Jones, they can only do this for faults that are no further away than three to four times the length of the fault that ruptured. In other words, a magnitude 7 quake could potentially trigger another tremblor on a fault 160 kilometres (100 miles) away, but that's not guaranteed.
So what does that mean here? Well, so far, it looks like that magnitude 6.4 quake hasn't triggered any nearby faults to slip, so that's great news. At the same time, there is zero chance it will set off any more quakes anywhere else along the Ring of Fire, which is really an imaginary boundary that doesn't mean much geologically.
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