Salisa lohavittayavikant

One thing I will tell you about the earthquake that shook Bangkok on 28 March 2025 is that most people underestimate how bad the quake actually was.

In most video clips you see circulating online, the camera is either attached to the person (being a camera phone) or the building itself (being a surveillance camera or a nanny cam). This means that when the ground sways, the camera sways along with it. It tells you nothing about how much the ground itself actually sways.

Notice the word “sway” here. The ground did not shake or vibrate, being a low-frequency, high-amplitude oscillation with a period that was strangely more regular than most instances of earthquake (but I’m not an expert, although I was situated directly on the ground when it happened and so I’d wager that my perception of the magnitude of the quake is far more accurate than most people especially those in higher structures, but more on that).

There is a reason the majority of the people in Bangkok were not aware that they were experiencing an earthquake or even the aftermath tremors of an earthquake (perhaps the soft clay aka jellybowl effect did amplify the traversing wave so we could escalate the event and call it a bona fide earthquake). With a typical earthquake, you would feel an intense, irregular shaking of the ground. It may come in bouts of 10-15 second interval that may last longer in total duration. But with this particular earthquake, the building sways from side to side relentlessly for 60-90 seconds. If the teetering skyscrapers is any rough indication, it would put the frequency of the earthquake at around 0.5 Hz when it hit Bangkok (resonance frequency). This means the wall leans left, then right, then left and on it goes. The period between the leftmost sway and the rightmost sway would be one full second to two full seconds. This matched my experience well and has been confirmed by a seismograph:

Note: The instrument detected a period cycle of approximate 2.5 – 3 seconds.

Being a low-frequency, wavy oscillation, items and knickknacks remained pristinely intact on the surface. The high regularity and lengthy period of the wave cycle didn’t put anyone’s survival mechanism on a hair trigger. In fact, people thought they were in a swoon and many did not realise that they just experienced a series of very pronounced tremors. Adding to this fact is that everyone’s vestibular system (inner ear locomotion) was all shot and unreliable due to the swaying ground beneath their feet. There was no independent frame of reference proper, whether external i.e. a visual input (things being knocked off the shelves or the tables as the phase of oscillation was unusually long) or internal i.e. a vestibular input (the inner ears cannot reliably interpret horizontal or vertical orientation or movement in such a case).

I was in the living room on the first floor of my townhome when the waves hit. Initially, I thought I was experiencing a medical event, perhaps a stroke or an onset of epilepsy. I turned to my left and looked outside onto my backdoor garden and nothing was deemed out of the ordinary. Even the sprawling, two-meter hedge trees I cultivate that fan a rather wide-spanning foliage which I do not trim and as such are highly sensitive to the lightest breeze of wind remained completely intact. There was no single shake of any shoot. The underground oscillation with this type of signature (a slow, low-crest wave i.e. crest-to-cycle ratio) would not move a single tree — the tree will remain fastened onto the ground, it will not sway or shake, and the leaves will not visibly be disturbed.

I then turned to my right and while none of the furniture or its items were displaced or shaking, the floor lamp three feet away from me was swinging from side to side like a bobo doll (granted it was a top-heavy lamp with weak frame). It took full 15 seconds since the start of the event to register that it was in fact an earthquake. I looked further on to the other side of the room and noticed another floor lamp; an IKEA lamp with double arms on which I hung a tote bag — this bag having no content in it and as such no weight to be subjected to any kind of rotation on its axis given the impetus, yet swung with such a momentum that I’d reckon was sweeping at a 60-90 degree angle at its most intense, busiest cycle (only 60 degrees on its rotational axis or what is called an angular sweep, if taken into account the perceptual width of the bag it would be sweeping a 120-150 degree physical arc). The adrenaline pump could indeed make me exaggerate and the bag was probably heavier than it looks being woven linen (or cotton) with a zipper, yet it would not have weighed more than 200-300 grams.

Why is this remarkably tedious detail important? Because these two floor lamps of mine (and one tote bag), cheap though they may be and precisely because they are cheap and flimsy and unweighted, moved out of sync with the ground motion and as such prove to be the only objective, independent frame of reference that established how massive the oscillation actually was. If the lamps weren’t there or I failed to notice them, I would have thought the tremors were quite minor. My body would have registered only the difference in arc traversal where my sway did not match up with the ground’s (ChatGPT tells me this is called reduced amplitude or amplitude difference leading to a phase lag). Subjective perception is also unreliable as it would always be attenuated. If you are on the ground during the quake and your body was swaying back and forth a certain amount, the actual ground would be swaying at least 5 times that amount — would be my best guess and most conservative estimate.

Many shopping malls and office high rises often have basement parking lots and so I’d venture that they were built with greater seismic regulations and had already dampened some earthquake effects by the time the waves were transmitted to its upstairs inhabitants. Many condo high rises didn’t sway as much or at all relative to the moving ground and some hitting the resonance frequency kept waving. The experience varied from building to building but what the people in those buildings would feel, assuming the lack of resonance which would be the case for nearly all buildings in the area, would be ground oscillation minus building oscillation minus body oscillation. I did notice minor, irregular vibrations in some video clips in a shopping mall or on an elevated roadway for example, but those minor trembles most likely came from the infrastructure counteracting or dissipating the energy coming off ground. They would be the byproduct of the inertial forces of the structure (building, bridge) resisting ground motion. They did not represent the ground motion itself.

Another great perceptual aid to help visualise how the ground actually behaved during that time is the behaviour of this sloshing water in a pond that was built into the ground and so situated at the ground level (a rooftop level would not be an accurate representation of the quake). This video was taken during the throes of the quake in the Nonthaburi region which is located mere 10-20km away from the collapsed building near Chatuchak: https://www.facebook.com/reel/633329249499577/

The frequency of the quake (note that I use “frequency” and not necessarily magnitude or strength) combined with the shallowness of origin was extremely hazardous. I do sympathise greatly with the dozens if not hundreds of high rises with reported extensive damages from collapsed ceilings to massive diagonal tears to deep cracks on the concrete floor you could peer directly into. Those buildings may be quite well-built, but the waving and twisting and shearing motion from the ground lasted over a full minute and would undoubtedly put undue stress on buildings of certain size and height, if not for most. We don’t know how extensive the damages are to the foundations of those buildings, with or without visible damages, and some newer ones are using precast walls in a load-bearing capacity (not that precast walls themselves are unsafe, if anything these walls that section off each unit of my townhome block serve as a very sturdy lateral support across the entire section and with a shared foundation underneath makes my place rather immune, given the low vertical height, wide foundation and pre-cast load bearers as opposed to extreme vertical height, limited foundation and pre-cast load bearers — though this is just my armchair intuition).

I think it could be safely said that many if not most high rises (20-30+ floors) — or even perhaps basic infrastructures (low rises, tenements, bridges, tunnels, malls) — are structurally compromised in some way, although they are not going to topple over from strong winds whence comes the stormy season and whatever leftover safety factor may at the end of the day remains within the “safe” margin. The building owners and building companies are never going to allow independent inspection with proper sensing equipments (radar, etc). This slate-thin building that teetered back and forth and in the middle section dispensing a cloud of concrete dust while bellowing out extremely loud creaking and snapping noises is most likely going to be declared safe for use unfortunately: https://www.facebook.com/share/r/1BWJTY8Q6t/