Earthquake P- and S-waves, why does their speed matter?

Earthquakes, one of nature’s most formidable phenomena, can cause widespread destruction within seconds. However, advancements in seismology have led to the development of Earthquake Early Warning (EEW) systems, providing precious seconds to minutes of warning before the shaking starts. The key to these warnings lies in the understanding of P-waves and S-waves generated by earthquakes and their speeds.

The Speed of P-waves and S-waves

When an earthquake occurs, it releases energy in the form of seismic waves, primarily P-waves (Primary waves) and S-waves (Secondary waves). P-waves, being the fastest, travel through both solid and liquid layers of the Earth at speeds ranging from about 5 to 7 kilometers per second (km/s) in the Earth’s crust, and 8 to 13 km/s in the mantle. S-waves, on the other hand, only move through solids and are slower, with speeds of about 3 to 4 km/s in the crust and 4.5 to 7.5 km/s in the mantle.

The Importance of Speed Difference

The speed difference between P-waves and S-waves is crucial for Earthquake Early Warning systems. P-waves, although less destructive, reach sensors first, providing a brief window of time before the more damaging S-waves arrive. This time gap can vary depending on the distance from the earthquake’s epicenter. The closer one is to the epicenter, the shorter the warning time, due to the smaller gap between the arrival times of P-waves and S-waves.

Proximity to the Epicenter and Warning Time

For those located very close to the earthquake epicenter, the warning time may be minimal or non-existent. This is because the S-waves, responsible for most of the shaking and damage, follow closely behind the P-waves. In such scenarios, every second of warning can be critical for taking protective actions, such as dropping to the ground, taking cover under a sturdy piece of furniture, and holding on until the shaking stops.

The Blind Zone Challenge

A significant challenge for regional seismic network-based EEW systems is the “blind zone.” This area, typically within 10 to 20 kilometers of the epicenter, may receive little to no warning before shaking starts. The reason is that it takes time for the seismic waves to be detected by the network, processed, and then relayed as a warning to the affected area.

On-site Earthquake Early Warning Systems

To address the blind zone issue, on-site EEW systems have been developed. These systems are installed at individual locations, such as buildings or infrastructure facilities, and can detect P-waves directly, providing immediate local warnings. While they may not offer extensive lead times, they can be especially effective in near-epicenter areas where regional EEW systems struggle to provide timely alerts.


Understanding the dynamics of P-waves and S-waves and their implications for early warning systems is essential in mitigating earthquake risks. While the difference in speed between these waves offers a crucial, albeit brief, window for action, challenges such as the blind zone necessitate innovative solutions like on-site EEW systems. As technology advances, the goal is to extend the warning times and reduce the impact of earthquakes, safeguarding communities and saving lives in the process.

Understanding the Earthquake Shaking: The Modified Mercalli Intensity Scale (MMI)

When the earth trembles, the world takes notice. But how do we measure the narrative of the ground’s fierce rumbling? Enter the Modified Mercalli Intensity Scale (MMI), a storyteller of seismic experience that narrates the drama from the ground up.

Intensity vs. Magnitude: Feeling the Difference

While magnitude scales like Richter or moment magnitude measure the energy released at the earthquake’s source, the MMI scale offers a human-centered narrative. It tells us what people felt, what damage occurred, and how the landscape changed. This scale isn’t just about numbers; it’s about experiences.

The Scale of Stories

From I, where the shaking is not felt except by a select few under favorable conditions, to XII, where damage is total, structures are uprooted, and the earth’s surface is wrenched, the MMI scale plots the plot points of an earthquake’s impact. Each level up the scale marks a significant increase in the effects felt and damage inflicted.

Click HERE to download the MMI scale in PDF.

Local Tales of a Global Phenomenon

What makes the MMI scale particularly useful is its adaptability to various settings. The same earthquake can be gentle in one location and destructive in another. By cataloging responses from different areas, seismologists can map out an earthquake’s impact in a way that resonates with the local narrative.

A Chronicle of Resilience

Beyond its scientific value, the MMI scale is a record of resilience. It highlights how communities withstand the shaking, adapt to their transformed landscape, and rebuild in the aftermath. It’s a human scale for a natural event.

In the end, the Modified Mercalli Intensity Scale does more than tell us how the Earth moved. It connects us through shared experiences and mutual understanding. It’s a reminder that while we may be separated by geography, we are united in our encounter with the natural world.

When the earth shakes again, as it inevitably will, we will turn to the MMI scale not just for data, but for the stories of survival, strength, and solidarity. It is a scale that does not just measure shakes, but also stirs the human spirit.

Stay Grounded with Knowledge

Understanding the MMI scale can help us better prepare for future seismic events. By learning from past earthquakes, we can build structures and communities that are not only earthquake-resistant but also resilient in the face of whatever the MMI scale may tell us next.

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Devastating climate change, including killer heat waves and severe flooding, adversely affects the infrastructures our communities rely on. Dams in particular become increasingly more vulnerable to climate change due to aging. Rapidly rising water levels and frequent floods add extra stress to dams, reservoirs and waterways, pushing them to their design limits. A failure to upgrade dams in response to deterioration in structural health may result in a catastrophic impact on the people and environment.

The most recent examples are the failed Edenville and Sanford Dams in Midland, Michigan due to rapidly rising waters after days of heavy rain. The collapsed Edenville Dam, constructed in 1924, was rated in unsatisfactory condition in 2018, while the Sanford Dam, which was built in 1925, was given a fair condition rating by the State.

In 2017, major flooding from the damaged Oroville Dam in Northern California forced the evacuation of nearly 200,000 Californians. The Oroville Dam was completed in 1968, toward the end of the golden era of dam construction. This was a wakeup call for owners of aging dams across the country, as climate change continues to add stress to these structures.

California has additional challenges due to active earthquake faults, including the Hayward and San Andreas faults, which scientists predict are due for a large earthquake. Among the dams now considered to be at risk are the Anderson Dam and the Calaveras Dam, both close to fault lines in Silicon Valley. According to the NY Times, California’s most troubled large dam is at Lake Isabella. This dam was built on the Kern River near Bakersfield by the Army Corps of Engineers in the 1950’s on what was thought to be an inactive fault. However, this fault has been active since then.

Another major threat to dams is scouring. Numerous aging dams have experienced severe erosion of their unlined spillways. This erosion can lead to damage and even failure of dams and consequently can threaten public safety, properties, infrastructure and the wider local environment.

There are a number of unfortunate examples of dams failing due to earthquakes, flooding or scouring where early signs of deficiencies might have been detected if a proper structural health monitoring (SHM) system had been in place.

Introducing SMARTDAM

QuakeLogic is the only company using a cloud-based, AI-powered technology platform to perform continuous, autonomous assessments using data from sensors on the dam structure.

QuakeLogic’s Sensor data Management, Assessment and Repository Technology (SMART) platform transforms a dangerous, aging dam into a SMART dam able to alert officials to critical deterioration. It also significantly reduces needed search and inspection efforts following any seismic or other impact event such as settlement, scouring, etc.

The SMART platform integrates manually and digitally read sensor recordings into a fully automated unified monitoring system. It facilitates the acquisition and analysis of critical sensor data needed by the dam operators for proper operation and maintenance, and most importantly, for the safety assessment of the dam. It routinely collects, organizes and evaluates sensor data, and sends immediate notifications with ACTION PLANS upon exceedance of programmed thresholds, generating PDF reports regularly and on-demand.

The SMART platform is a cutting-edge system that works with various types of sensors such as accelerometers, tiltmeters, potentiometers, strain gauges, thermocouples, weather stations, piezometers and seepage monitors. Comprehensive analytic information is visible in real-time on the mobile-friendly dashboard, providing proof and PEACE OF MIND that a dam is performing as expected.

In addition to our SMART platform, our proprietary earthquake early warning (EEW) alerts provide a window of opportunity for action before earthquake shaking begins at the site. It can even trigger automated actions such as opening spillways, closing roads, etc. – when every second counts.

QuakeLogic’s monitoring system instantly detects any issue that could impact the structural integrity of the dam, allowing corrective measures to be implemented and avoiding a potential future disaster.

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