In the Book of Joshua (6:14-15, 20), we read:

And the second day they marched around the city once, and returned into the camp. So they did for six days. On the seventh day they rose early at the dawn of day, and marched around the city in the same manner seven times. It was only on that day that they marched around the city seven times. ... So the people shouted, and the trumpets were blown. As soon as the people heard the sound of the trumpet, the people raised a great shout, and the wall fell down flat …

It occurred to me out of the blue that this famous event might possibly have something to do with the known phenomenon of soldiers marching in step across a bridge, causing it to collapse. In April 1831, soldiers were marching in step across the Broughton Suspension Bridge in England, and indeed it did fall down. Afterward, soldiers were famously ordered to “break stride” when marching on bridges. How is this explained? Well, it turns out that physical structures have a natural frequency of vibration. If outside forces happen to match this frequency, a phenomenon called mechanical resonance can occur and bring about physical destruction.

Probably the most famous collapse of a bridge was that of the Tacoma Narrows Bridge (suspension design) on Nov. 7, 1940. Many have seen the remarkable video of the large bridge swaying wildly back and forth before its demise. It wasn’t due to marching soldiers in this instance, but it was likely caused by a broadly similar occurrence of destructive resonance. What is certain is that both documented historical events have a perfectly natural scientific or “engineering” explanation.

With regard to the biblical description of the fall of Jericho, it’s been observed that neither the blowing of horns nor shouting would have a frequency akin to the walls (of a likely low quality; probably made of mud brick). But the combined force of a large number of soldiers marching in step would have been able to approach the frequency of the wall. 

I contended on pages 126-130 in my book, The Word Set in Stone: How Archaeology, Science, and History Back Up the Bible (Catholic Answers Press: March 15, 2023) that there is no archaeological evidence for the walls of Jericho in c. 1200 B.C. (my estimated time for this event, following a certain respectable archaeological timeline). By then they had totally eroded away, in my opinion, due to a salt-erosion process called haloclasty — Jericho being close to the Dead Sea (which is 10 times saltier than the oceans) and in one of the most arid climates and at the lowest elevation in the world, with strong winter rains and non-occupation for some 325 years.

But we know that the second and third levels of walls in Jericho from about 200 years earlier were made of mud, and as such, would not have been particularly strong, and therefore more susceptible to the natural forces being discussed (if later walls were of the same composition). The lower-level wall was made of stone.

It’s humorous to note that in my book above (p. 127) I stated that “I am not attempting to defend (let alone prove) the claim that the walls fell down because of Joshua and his army marching around it and blowing trumpets. I’m not sure anyone could prove that, and I can’t conceptualize in my mind how they would go about doing so.”

It’s funny how I couldn’t even conceive of marching soldiers causing a collapse; then all of a sudden I could conceptualize it, by analogy, and proceed to give scientific evidence for the possibility of it. Note that in the biblical account, on the seventh day “they marched around the city seven times.” In other words, I propose that it wasn’t the shouts and trumpet blasts made on the seventh day that caused the collapse of the walls, but the continual marching of soldiers. 

Christian apologist Anne Habermehl gave a fascinating talk, “Archaeoacoustics and the Fall of Jericho,” in which she observed:

The march around Jericho was possibly not a strange custom to the inhabitants of the city. ... The number of soldiers that Joshua would have commanded, and who would have marched in the procession around Jericho, is estimated ... to have been about 8,000 men. ... But audible sound is not the only result of the marching. Experts include infrasounds (cycles per second in the range below what the human ear can hear) in the science of acoustics. ... (“Archaeoacoustics and the Fall of Jericho,” presented in Archaeoacoustics III: The Archaeology of Sound: Proceedings of the 2017 Conference held in Tomar, Portugal, 2017, pp. 79–84)

Another possible cause is an earthquake. Jericho sits about four miles away from the Jericho fault. The area has, in fact, been hit by two major earthquakes in the last hundred years: a 6.25 magnitude earthquake in 1927 and another in 2004 (5.1 magnitude). 

That could have been the primary cause (all the more so if inadequate mud brick walls were in play), with the marching and trumpets being merely coincidentally present at the same time. The biblical text, however, neglects to mention it, which would be strange if this is what happened. It doesn’t state that “the ground shook,” or some such, and it’s difficult to imagine that this detail would have been left out, had it actually happened.

For that reason, I favor the “resonance” theory. In either eventuality, it happened right at the time God said it would, which is yet more evidence of his remarkable providential oversight of human events that are in his will. Or it was simply a direct miracle of God, which is always possible at any time (yet rare, too). I am merely throwing out natural possibilities or plausible possible events, as opposed to dogmatic assertions that it must have been according to my mere speculations.