The effect of urbanization on temperature is relatively well-known: cities are often measurably warmer than their surrounding rural areas. This is called the urban heat island effect. What fewer people know is that the urban heat island has a twin counterpart with similarly important consequences: the urban precipitation anomaly, where the presence of urban development measurably affects the amount of rainfall in an area.

In a study published in Proceedings of the National Academy of Sciences, researchers at The University of Texas at Austin looked for evidence of precipitation anomalies in 1,056 cities across the globe and found that more than 60% of those cities receive more precipitation than their surrounding rural areas.

In some cases, the difference can be significant. For instance, researchers found that Houston, on average, will receive almost 5 inches more rain per year than its surrounding rural areas.

This could have wide-ranging implications, the most serious of which is worsened flash flooding in densely built urban areas.

Variation in urban rainfall is something scientists have known about for several decades, but never at a global scale. Previous studies only looked at certain cities and storm cases, said study author Xinxin Sui, a doctoral student at the Cockrell School of Engineering. For this paper, she and other researchers poured over precipitation datasets from satellites and radar systems, examining daily precipitation anomalies for these 1,056 cities from 2001 to 2020.

“In general, we found that over 60% of these global cities have more rainfall (than the surrounding countryside). Then we compared with different climate zones and found that if the local climate is hotter, if it’s wetter, then it may have a larger rainfall anomaly compared to the cities in cooler and dryer places,” Sui said.

In addition to Houston, the list of large cities with the largest precipitation anomalies include Ho Chi Minh, Vietnam; Kuala Lumpur, Malaysia; Lagos, Nigeria; and the Miami-Fort Lauderdale-West Palm Beach metropolitan area.

Study author Dev Niyogi, a professor at both the Jackson School of Geosciences and Cockrell School of Engineering, explained that urban areas tend to take rain from one location and concentrate it in another, much like a sponge that is being squeezed.

“If you were to pinch one part of the sponge, you would have water coming down more forcefully from one side,” he said. “The amount of water you have in the sponge is the same, but because now you have that dynamic sort of squeezing the atmosphere, you have more ability to take the water out from that location.”

Although it’s less common, some urban areas actually receive less rainfall than their surrounding rural counterparts. This typically occurs in cities situated in valleys and lowlands, where precipitation patterns are controlled by nearby mountains. The cities where this is most pronounced include Seattle, Washington; Kyoto, Japan; and Jakarta, Indonesia.

There are several reasons why most cities receive more rainfall than their rural neighbors. Co-author Liang Yang, professor at the Jackson School, said one key factor is the presence of tall buildings, which block or slow down wind speeds. This leads to a convergence of air toward the city center.

“The buildings further enhance this convergence by slowing the winds, resulting in a stronger upward motion of air. This upward motion promotes the condensation of water vapor and cloud formation, which are critical conditions for producing rainfall and precipitation,” Yang said.

Researchers found that population has the largest correlation with urban precipitation anomalies compared to other environmental and urbanization factors. This is because larger populations typically create denser and taller urban areas, along with more greenhouse gas emissions, and therefore more pronounced heat, Niyogi said.

This phenomenon has implications for all cities heading into a future of climate change, said Yang, who described how the increased chances of rainfall in cities combined with the impervious surfaces that make up their urban environments can be a recipe for flash flooding.

“Combining these two factors means we must develop innovative ways to prepare for flash flooding,” Yang said.

Located on the Pacific Ring of Fire, Japan is one of the most earthquake-prone countries in the world, with thousands of small earthquakes occurring each year, and the continuous threat of a “big one.” Currently, predicting when major earthquakes will occur isn’t possible, but by studying the numerous small earthquakes that occur, seismologists in Japan hope to understand more about the processes in the Earth’s crust that lead to major quakes.

Now, researchers from Kyushu University and the University of Tokyo, Japan, have studied seismic activity at an unprecedented level of detail, identifying a link between fault strength and earthquake magnitude. Published in Nature Communications, the study proposes that the strength of the fault affects the b-value—and therefore the likelihood of a major earthquake.

“The b-value is a very important constant in seismology that characterizes the relationship between earthquake frequency and size,” explains Professor Satoshi Matsumoto, first author of the study and the Director of Kyushu University’s Institute of Seismology and Volcanology. “If there is a low b-value, this means there is a higher proportion of large earthquakes, while a high b-value means there is a higher proportion of smaller earthquakes.”

The b-value can vary between different locations and also over time, and is often reported to decrease just before a major earthquake. A previous study suggested that the decrease in b-value was caused by the increasing stress forces exerted on the fault. Now, this study suggests that fault strength is also a contributing factor.

In the study, the research teams analyzed the seismic action happening in the area around the epicenter of the Western Tottori Earthquake, which occurred in 2000 with a magnitude of 7.3. By installing more than 1,000 seismic stations in the area, the researchers could conduct seismic observations with an unprecedented level of accuracy.

“Even two decades on, hundreds of tiny aftershocks still occur, most too small for us to feel,” says Matsumoto.

With so many sensors, the researchers could detect tiny movements of the faults, and also the orientation of each fault within the Earth’s crust.

Using this multitude of data, the team was able to estimate the stress field (the different directions of stress forces exerted on each fault at the time of failure) and allowed them to characterize the faults as strong or weak.

“Under certain stress conditions on each tectonic regime, there is a favorable direction of the fault plane to slip. When faults are in unfavorable directions, this suggests that these are weak faults that can slip more easily. On the other hand, strong faults require more stress to slip, and have a much more characteristic direction,” explains Matsumoto.

From the stress field calculations, the researchers were also able to estimate the b-value of the event group categorized by fault strength. They found that stronger faults have smaller b-values, suggesting that large earthquakes are more likely to occur, while weaker faults had larger b-values, suggesting that major earthquakes are less likely.

“Simply put, these weak faults will likely slip before a large amount of stress builds up, which means that they aren’t able to release a large amount of force,” says Matsumoto.

Through deeper understanding of the factors that impact b-values, the researchers hope that they will be able to inch closer to the “holy grail” of predicting earthquakes.

“I don’t think we will ever know exactly when an earthquake will strike, but looking at data such as fault direction and fault strength, and calculating b-values, could help us estimate when a fault has reached a critical point— where just a tiny extra nudge of force is needed for the fault to slip,” concludes Matsumoto. “This information is vital to know in order to be prepared for major earthquakes.”