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.

For comments and suggestions, reach us at

Why Have Seismologists Moved from Richter to Moment Magnitude for Measuring Earthquake Intensity?

The Richter scale, developed in 1935 by Charles F. Richter, was the first scale to measure the size of earthquakes. The scale is logarithmic, meaning that each whole number increase on the scale represents a tenfold increase in measured amplitude and approximately 31.6 times more energy release. The scale was specifically calibrated for Southern California and used a particular type of seismograph, so it was most accurate for medium-sized earthquakes (M3 to M7) within a certain distance from the seismograph.

However, as our understanding of earthquakes has grown and technology has improved, seismologists have identified limitations with the Richter scale:

  1. Regional Limitations: The Richter scale was based on California’s geology and the specific seismographs used at the time. It does not scale well for extremely large or small earthquakes, nor does it account for variations in the Earth’s crust in different regions of the world.
  2. Energy Release: The Richter scale does not accurately estimate the energy released by very large earthquakes. The scale saturates around M7, meaning that it does not distinguish well between the energy released by the largest earthquakes, which can differ significantly.
  3. Seismograph Limitations: The original scale was based on the recordings from a particular type of seismograph that is not used as widely today. Modern seismographs provide more detailed data, and the Richter scale does not take full advantage of this.

To address these limitations, the Moment Magnitude Scale (Mw) was introduced by Hank and Kanamori (1979). It is based on the seismic moment of an earthquake, which is a measure of the total energy released by the earthquake. The moment magnitude scale is now the most common scale for measuring the size of earthquakes for several reasons:

  1. Global Applicability: Moment magnitude is calculated based on the physical properties of the earthquake (such as the rigidity of the Earth’s crust, the area of the fault that slipped, and the amount of slip) and can be used globally without regional corrections.
  2. Accuracy for Large Earthquakes: The moment magnitude scale does not saturate like the Richter scale. It provides an accurate measure of the energy release for very large earthquakes (greater than M7), which is essential for understanding their potential impact.
  3. Consistency: The scale provides a more uniform and consistent measure of an earthquake’s size, which is useful for both historical comparisons and scientific research.
  4. Detailed Data Use: Modern seismographs record a full seismic wavefield. Moment magnitude takes advantage of this data to provide a more complete picture of an earthquake’s characteristics.

Because of these advantages, the moment magnitude scale has largely replaced the Richter scale for most seismological applications, especially for earthquakes that are recorded at long distances from the epicenter or that are very large.

Please reach us for comments and suggestions at