Earthquake Magnitude vs. Intensity: Decoding the Tremors
You may not always feel the earth shaking, but California has earthquakes occurring all the time. Seismographic networks measure earthquakes by their magnitude, energy release and intensity.
Years ago, all magnitude scales were based on the recorded waveform lengths or the length of a seismic wave from one peak to the next. But for very large earthquakes, some magnitudes underestimated the true earthquake size. Now, scientists use earthquake measurements that describe the physical effects of an earthquake rather than measurements based only on the height of a waveform recording.
How are earthquakes recorded & detected?
When the Earth trembles, earthquakes spread energy in the form of seismic waves. A seismograph is the primary earthquake measuring instrument. The seismograph produces a digital graphic recording of the ground motion caused by the seismic waves. The digital recording is called a seismogram.
A network of worldwide seismographs detects and measures the strength and duration of the earthquake’s waves. The seismograph produces a digital graphic plotting of the ground motion of the event.
How is earthquake magnitude measured?
An earthquake has one magnitude unit. The magnitude does not depend on the location where the measurement is made. Since 1970, the Moment Magnitude Scale has been used because it supports earthquake detection all over the Earth.
Earthquake Measurements
To get a better idea of the strength of the shaking and damage, the Moment Magnitude Scale was developed to capture all the different seismic waves from an earthquake to worldwide seismic networks.
Earthquake intensity scales describe the severity of an earthquake’s effects on the Earth's surface, humans, and buildings at different locations in the area of the epicenter. There can be multiple intensity measurements. The Modified Mercalli Scale measures the amount of shaking at a particular location.
Earthquake Magnitude Scale
An important piece of information to keep in mind is that the magnitude scale is logarithmic. In other words, it is “comparing amplitudes of waves on a seismogram, not the strength, or energy, of the quakes,” according to USGS. This helps us understand that while the size (amplitude) differences between small and big quakes are big enough, it is the strength (energy) differences that are meaningful. Try out USGS’s “How Much Bigger…?” calculator to learn more about how to measure the magnitude of an earthquake.
The Richter Scale
From 1935 until 1970, the earthquake magnitude scale was the Richter scale, a mathematical formula invented by Caltech seismologist Charles Richter to compare quake sizes.
The Richter Scale was replaced because it worked largely for earthquakes in Southern California, and only those occurring within about 370 miles of seismometers. In addition, the scale was calculated for only one type of earthquake wave. It was replaced with the Moment Magnitude Scale, which records all the different seismic waves from an earthquake to seismographs across the world.
Richter's equations are still used for forecasting future earthquakes and calculating earthquake hazards.
Moment Magnitude Scale
Today, earthquake magnitude measurement is based on the Moment Magnitude Scale (MMS). MMS measures the movement of rock along the fault. It accurately measures larger earthquakes, which can last for minutes, affect a much larger area, and cause more damage.
The Moment Magnitude can measure the local Richter magnitude (ML), body wave magnitude (Mb), and surface wave magnitude (Ms).
Earthquake Magnitude Classes
Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude. What’s the difference between a light and moderate quake?
These terms are magnitude classes. Classes also provide earthquake measurement. The classification starts with “minor” for magnitudes between 3.0 and 3.9, where earthquakes generally begin to be felt, and ends with “great” for magnitudes greater than 8.0, where significant damage is expected.
How is earthquake intensity measured?
A second way earthquakes are measured is by their intensity. Earthquake Intensity measurement is an on-the-ground description. The measurement explains the severity of earthquake shaking and its effects on people and their environment. Intensity measurements will differ depending on each location’s nearness to the epicenter. There can be multiple intensity measurements as opposed to one magnitude measurement.
The Modified Mercalli Scale
The Modified Mercalli (MM) Intensity Scale is used in the United States. Based on Giuseppe Mercalli's Mercalli intensity scale of 1902, the modified 1931 scale is composed of increasing levels of intensity that range from observable quake impacts from light shaking to catastrophic destruction. Intensity is reported by Roman numerals.
An earthquake intensity scale consists of a series of key responses that includes people awakening, movement of furniture, damage to chimneys and total destruction.
| Intensity | Shaking | Description/Damage |
|---|---|---|
| I | Not Felt | Not felt except by a very few under especially favorable conditions. |
| II | Weak | Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. |
| III | Weak | Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. |
| IV | Light | Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. |
| V | Moderate | Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. |
| VI | Strong | Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. |
| VII | Very Strong | Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. |
| VIII | Severe | Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. |
| IX | Violent | Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. |
| X | Extreme | Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent. |
Source: USGS
How to prepare for a high magnitude quake
An earthquake is a sudden, rapid shaking of the earth caused by the shifting of rock beneath the earth’s surface. The size of an earthquake and the energy released by an earthquake will affect how much you feel it. Major earthquakes strike without warning, at any time of year, day or night.
Prepare before the next big one:
- Create an earthquake safety plan. Discuss with your family what to do, where to meet if separated, and how you will communicate when an earthquake strikes. Check work, childcare and school emergency plans.
- Practice DROP, COVER, and HOLD ON with all members of your household.
- Don’t rely on doorways for protection. During an earthquake, get under a sturdy table or desk. Hold on until shaking stops.
- Pick safe places in each room of your home.
- Create an emergency survival kit that provides you and your pets with three days of nonperishable food and water, medicines, emergency radio and first aid materials. Keep in a reachable place.
- Identify an out-of-the-area friend or relative that family members can check in by mobile texting.
- Find out if your home is in need of earthquake retrofitting and eligible for a grant.
- Identify and fix potential earthquake hazards in your home.
- Protect your home investment and bounce back from a devastating earthquake with the best choices of earthquake insurance from CEA.
Whether you are a homeowner, mobilehome owner, condo-unit owner or renter, buy peace of mind with affordable and flexible earthquake insurance now.
Understanding Geologic & Structural Risks
Every day, Californians face earthquake danger. Our state has nearly 16,000 known faults and more than 500 active faults. Most of us live within 30 miles of an active fault risk.
Visit the CEA risk map for each county to learn about faults in your area. This information will help you survive an earthquake.
Then learn about your home’s structural risks, the steps you can take to seismically strengthen your house and the benefits of retrofitting. Make your home more resistant to earthquake damage by assessing its structure, contents and need for loss of use earthquake insurance.
Avoid financial disaster with loss of use coverage if your house sufferers extensive damage—get coverage with a CEA earthquake policy.
Personal Preparedness Guidelines
Get started today on preparing for a major earthquake. Top earthquake survival tips include:
- Create an earthquake safety plan for you and loved ones, including pets.
- Identify safe places in each room of your home.
- Practice Drop, Cover, and Hold On with each member of your household.
- Make or purchase an earthquake safety kit.
- Find out if your home is in need of earthquake retrofitting and eligible for a grant.
- Identify and fix potential earthquake hazards in your home.
- Protect your home investment and bounce back from a devastating earthquake with the best choices of earthquake insurance from CEA.
Buy peace of mind with affordable and flexible earthquake insurance now.
Is your house at risk for earthquake damage?
Preparing your home BEFORE an earthquake is important to your safety. Decrease your risk of damage and injury from an earthquake by identifying possible home hazards.
A seismic retrofit by strengthening your home’s foundation makes it more resistant to shaking. CEA offers earthquake home insurance premium discounts for houses and mobilehomes that have been properly retrofitted. Find out about grants to help for retrofits under the Earthquake Brace + Bolt Program, and the CEA Brace + Bolt program.
Is earthquake insurance worth it?
While it is wise to be prepared physically when the ground shakes, it’s also important to be financially protected. Without earthquake insurance, you place yourself at risk of losing everything or sustaining damages to your personal property that you cannot afford to repair or replace.
Could you:
- Pay your mortgage for a house that may need to be rebuilt?
- Cover the costs bill for temporary accommodations?
- Repair or replace your personal belongings?
CEA earthquake insurance not only helps repair damages. Loss of use coverage covers the costs of temporary shelter and additional living expenses so that families can get back on their feet quicker.
Get an Earthquake Insurance Estimate!
Contact your home insurance agent today to discuss adding a separate earthquake policy to your home insurance. You can add the coverage now, no need to wait until your home policy comes up for renewal. For the best choice of CEA earthquake insurance policies, select deductibles from 5%-25%.
We work with residential insurance companies that serve the majority of California homeowners. Get an estimate now!
Sources Consulted:
- Michigan Tech University: How are earthquakes measured
- GNS Science: What is the difference between magnitude and intensity
- KQED: Measuring Earthquakes Intensity and Magnitude
- USGS: Earthquake magnitude, energy release and intensity
- USGS: How are earthquakes recorded? How are earthquakes measured?
- USGS: “How Much Bigger…?” calculator
- USGS: Modified Intensity Scale
- Physical Geology (textbook by Steven Earle): Measuring Earthquakes
- British Geological Survey: Earthquake magnitude calculations
- US Naval Academy: Earthquakes
- EGS Solutions: What is the moment magnitude scale?