A Middle-of-the-Night Jolt Shakes the Bay Area:-
In September 22nd. The earthquake in question was magnitude 4.3 and was felt by quite a few people in the Bay Area around San Francisco in California. The earthquake occurred about 2:56 a.m. At a depth of about 7.6 kilometers or about 5 miles in depth. No damage or injuries from an earthquake of this size, but definitely got people’s attention. Gave them a good reminder that they live in earthquake country. Few items fell off shelves at some of the area stores. And that’s all heard so far in terms of any significant, you know, consequences from this earthquake. But nonetheless, it’s a fascinating time for to look at this earthquake in detail and sort of glean what we can from the science and the data that’s been collected. About earthquakes sometimes people worry about tsunami of course this earthquake was on land. This earthquake also was not large enough to produce a tsunami.
Depth and Location: A Quake Close to Home:-
And the third strike against this thing producing a tsunami was it didn’t have the right mechanics. This was a sideways motion type of falting event rather than a vertical up and down type motion which might produce a tsunami. So if we come in here, the earthquake took place north of Oakland near Berkeley. n fact, it was jt south of the University of California Berkeley camp. So there’s the event in question, the 4.3. In the hours since the earthquake, we have two aftershocks that have also occurred. And we’ll look at the fault system here in a second. This earthquake did trigger the or it it initiated the shake alert system which is a system that was been implemented in California. And the idea here is that these seismometers detect the very first signal of the earthquake which are the Pwaves which are very earthquake waves that are very hard to detect for people.
No Tsunami Risk — Here’s Why:-
They don’t really cae any damage. They’re not very high amplitude quakes, but the seismometers detect those and as soon as it it knows that an earthquake has been initiated, sends an alert out to people that have that app on their phone so they can take precaution and maybe get in a better position to deal with that earthquake. So sometimes it’s only a few seconds, but if you think about it, gives you an opportunity to maybe drop, get underneath something, hold on, which is what it what the advice is there in terms of the best way to handle those earthquakes. So that did alert people that the earthquake was occurring. So you can see the earthquake there again near Berkeley, California at this 4.3. The fault plane solution or the moment tensor solution for this quake. You can see here that the movement of the quake produced two possible fault planes.
ShakeAlert System: Early Warning in Action:-
Either a northeast southwest striking fault plane almost a vertical fault plane or a northwest southeast fault plane nearly vertical. And as we will look here at the faults in the area and the tectonics of the region, the one that’s going to make the most sense and the one that is absolutely the smoking gun is this northwest southeast fault here. So looking at the shaded regions here that would make this a right lateral strike slip fault. So this would be a sideward sideways moving horizontal motion type of fault system. And this would be also compatible with what we see with the San Andreas fault which runs through the area as well. So there’s our beach ball there. So you can see the San Andreas fault is the fault there in red and you can see it comes up through central California pretty much this mostly uninhabited region here. But as it gets closer to the Bay Area, actually splinters off into several other faults. There’s a nber of faults in the San Francisco region that are helping to accommodate the motion between the Pacific plate and the North American plate. And and the the main player there, suppose, in terms of jt being the absolute plate boundary is the San Andreas fault.
Fault Mechanics: What the ‘Beach Ball’ Tells Us:-
But there’s plenty of these other secondary faults which are equally likely to produce significant earthquakes. And the Hayward fault is the one that runs right through where we see the earthquake that occurred today. You can see Berkeley right here. And that red line running right across the map there, that is the Hayward fault. So this beach ball we see here, this northwest southeast trend here matches very nicely with the known location and the movement style or what we call the kinematics of the Hayward fault. So, it appears today’s earthquake was the Hayward fault was responsible for that quake. Again, pretty small earthquake, 4.3. We can check out how many people actually felt that quake. Looks like there’s been about 20 almost 26,000 responses. 26,000 people have got on to the USGS website, reported their experience with the earthquake.
Fault Network Complexity in the Bay Area:-
And by plotting all that data up, that helps figure out, you know, what areas are experiencing shaking the strongest. And it looks like our maxim shaking level here on the modified moral intensity scale is about a level four. You can see the Roman neral four there. , and that corresponds to pretty light shaking, which again, , jives pretty well with what we seen from reports and, you know, lack of injuries, no significant damage, that sort of thing. So, if you did feel the earthquake, you might want to jp on and check that out. We can also look at the the modeled aftershock forecast that the USGS model puts together. So, based on today’s earthquake, again, fairly small earthquake, 4.3, you can see what the chances are of having earthquakes of vario magnitudes during the next week. So, there’s about a 18% chance that we could get a magnitude 3 or greater quake in the next week.
Community Reports and Shaking Intensity:-
Having a magnitude four or greater event only a 2% chance and anything larger than that it’s basically nil less than 1% the chances of this being a fourshock or a larger quake is there but it’s very small very low chance so it’s more or less this is probably a one-off event a 4.3 with a handful of aftershocks that we seen already anything else here want to look at think that’s it in terms of jt looking at some of the data. Let’s come back to this Hayward fault here. And so this is actually a pretty big fault system in terms of its significance. When you think about the most dangero faults in the US, you might think about the Cascadia subduction zone, which is capable of producing magnitude 9 earthquakes up around Seattle and the Pacific Northwest. You might think about the San Andreas fault itself, which is obvioly the the plate boundary that runs through California, which has produced earthquakes in the past as well, but really a lot of people agree that the most dangero fault in the US is actually the Hayward fault. And mainly it’s becae of its proximity to people. There’s a lot of people, millions of people living in this urban corridor here.
The Hayward Fault: A Ticking Time Bomb?:-
So basically by looking at what these fault systems in the Bay Area have done not jt in the historic past but even looking at paleocismologic data basically digging trenches looking at earthquakes that are prehistoric that we don’t have han accounts of but we know have occurred. measuring the offsets, estimating the magnitudes, getting datable material to to determine the ages of those events. You can see here there’s a 72% chance of a 6.7 quake in the region in the San Francisco Bay region up through 2043, the year 2043. So sometime in the next, you know, 20 or so years. very good chance that there’s going to be a significant quake. And they even got those percentages broken down by fault systems. So looking at the individual fault systems and seeing which fault systems are more likely to produce those quakes. And notice the Hayward fault, the one that had the earthquake today is 33%. So of that 72% 33% of that almost half suppose is the Hayward fault. So a pretty significant fault system and a fault system not to be taken lightly. Our last earthquake on the Hayward fault was in 1868 was a magnitude 6.8. , and basically looking at the nber of earthquakes that have happened on the Hayward fault going into, you know, hundreds, thoands of years ago and the timing between those events, we can calculate what’s called a recurrence interval. Basically, how frequently earthquakes of a given magnitude occur on a specific fault system. And our recurrence interval for the Hayward fault is around 160 years, give or take about 60 years. And again, the last one was 157 years ago.
Preparedness Is Key: A Wake-Up Call, Not a Crisis:-
But some place here in the Bay Area on this part of the plate boundary, very likely chance, 3/4 of a chance suppose, of a very significant earthquake happening in the next 20 or so years. How would how would that earthquake manifest itself? What would the consequences and effects be? Depends on a lot of things. Depends on which specific fault ruptures, depends on the size of the quake, obvioly. The time of day is important. If it’s at night where people are in their typically woodframed homes, then the casualty amounts would be lower than if it was during the day where people out and about maybe in the city centers where there’s a lot of more brick buildings and more vulnerable buildings.
Final Thoughts: A Small Quake With a Big Message:-
So, a lot of things to think about there. But guess to it up, today’s earthquake was a good reminder for folks in the Bay Area that they are living in earthquake country. They should be prepared with their earthquake supplies, educate themselves, and jt hope for the best.