Earthquake Today: A Look at Global Seismic Activity and the Science Behind It

Seismic activity is a daily occurrence around the world, reflecting the constant movement and stress within the Earth’s crust. While most earthquakes are minor and go unnoticed, global monitoring networks continuously track every tremor.

As of today, November 21, 2025, the largest earthquake reported within the last 24 hours was a Magnitude 5.8 quake in Ambon, Maluku, Indonesia, which occurred approximately 19 hours ago. Indonesia, located on the highly active Pacific Ring of Fire, frequently experiences significant seismic events.

Notable Recent Earthquakes (Past 24 Hours)

The last 24 hours have seen several moderate-sized earthquakes reported globally. These events, typically ranging from Magnitude 4.0 to 5.8, are strong enough to be felt locally and, depending on their depth and proximity to inhabited areas, can cause minor to moderate damage.

(Note: Times are approximate based on the available data, and magnitudes/locations may be subject to revision by seismological agencies.)

Understanding Earthquakes and Seismic Waves

Earthquakes are the result of sudden movement on a fault—a fracture in the Earth’s crust. The Earth’s outermost layer, the lithosphere, is broken into enormous pieces called tectonic plates. These plates are constantly moving, causing stress to build up along their boundaries. When the accumulated stress exceeds the rock’s strength, it breaks, releasing energy as seismic waves.

• Hypocenter (Focus): The point inside the Earth where the earthquake rupture originates.
• Epicenter: The point on the Earth’s surface directly above the hypocenter.

Types of Seismic Waves

The energy released travels through the Earth and along its surface as different types of waves:

The crucial difference between P-waves and S-waves (the latter being unable to pass through liquids) was key in determining that the Earth’s outer core is liquid.

Measuring an Earthquake: Magnitude vs. Intensity

Scientists use two primary scales to characterize an earthquake: one measures the energy released and the other measures the effect on the ground and people.

1. Magnitude (Richter and Moment Magnitude Scales)

Magnitude is a measure of the energy released at the earthquake’s source (the focus). It is an absolute value derived from the logarithm of the amplitude of the seismic waves recorded by a seismograph.

• The Moment Magnitude Scale ($\text{M}_{\text{w}}$) has largely replaced the older Richter Scale for large quakes, as it provides a more accurate measure of the total energy released.
• The scale is logarithmic, meaning an increase of one whole number (e.g., from 5.0 to 6.0) represents a tenfold increase in the measured wave amplitude and approximately a 32-fold increase in the energy released.

2. Intensity (Modified Mercalli Intensity Scale)

Intensity measures the effects of an earthquake in a specific location. It is a relative measure that relies on observations of damage to structures and reports of people’s experiences.

• The Modified Mercalli Intensity (MMI) Scale uses Roman numerals from I (Not Felt) to XII (Catastrophic Destruction).
• A single earthquake will have one magnitude but can have many different intensity values depending on the distance from the epicenter, the local geology, and the construction quality of buildings.

Tectonic Hazards: Tsunami Generation

A severe hazard associated with large underwater earthquakes is the generation of a tsunami (a series of waves in a water body caused by the displacement of a large volume of water).

The primary mechanism for a seismic tsunami is:

1. A large earthquake occurs at a subduction zone (a convergent plate boundary) beneath the ocean.
2. The overriding tectonic plate suddenly snaps upward (vertical movement) as stress is released along a fault.
3. This vertical movement abruptly displaces the entire overlying column of seawater.
4. Gravity pulls the water mass back down, creating a series of waves that propagate outward across the ocean at high speed.

Only earthquakes that cause significant vertical movement of the seafloor, typically of Magnitude 7.0 or greater, are capable of generating destructive, ocean-wide tsunamis.

The Challenge of Earthquake Prediction

Despite significant advances in seismology, accurate, short-term earthquake prediction (specifying a date, time, and magnitude) remains impossible.

• Prediction vs. Forecast: Scientists focus on forecasting—determining the probability of an earthquake of a certain magnitude occurring in a specific area over a long period (e.g., the next 30 years).
• The Problem of Precursors: While some small foreshocks, changes in ground water, or electromagnetic signals have been observed before some quakes, none are reliable diagnostic precursors that consistently signal an impending major event. Most of these phenomena occur without a major earthquake following.

Instead of prediction, research and policy focus on mitigation through improved building codes, public preparedness, and developing Earthquake Early Warning Systems. These systems use the faster-traveling P-waves to quickly estimate the location and size of a quake and send automated warnings before the slower, more damaging S-waves and Surface waves arrive.