Reading Earthquake Data, PlainQuake Guide

The Four Core Data Fields

Every earthquake in the USGS ComCat database, and on PlainQuake, is described by four core measurements. Understanding each one is essential to reading earthquake data accurately.

Field	What It Measures	Key Interpretation
Magnitude	Total energy released	Logarithmic, M7 releases 31.6× more energy than M6
Depth	Distance below surface (km)	Shallow = more surface damage; deep = less impact
Location	Epicenter coordinates + place name	Nearest named point, not necessarily the affected area
Time	UTC timestamp of origin	Local time at epicenter depends on time zone

Magnitude: What the Numbers Mean

Magnitude is the most widely reported earthquake metric, but it is also the most misunderstood. Because the scale is logarithmic, the difference between M5 and M7 sounds small but represents enormous physical differences. An M7 earthquake releases about 1,000 times more energy than an M5.

Here is a practical guide to what each magnitude range typically means for people near the epicenter at shallow depth:

M2.5–3.9: Minor. Felt by people nearby as a brief tremor or rattling. No damage. These small events fall below PlainQuake's M4 catalog floor.
M4.0–4.9: Light. Widely felt over a broader area. Objects may shake or fall. Rarely causes structural damage. All M4+ events worldwide are in the PlainQuake dataset.
M5.0–5.9: Moderate. Can cause considerable damage to poorly built structures. Widespread shaking.
M6.0–6.9: Strong. Can be destructive in populated areas, especially near the epicenter. PlainQuake's significant earthquake catalog begins here.
M7.0–7.9: Major. Causes serious damage across large areas. Can trigger tsunamis if offshore.
M8.0+: Great. Catastrophic damage. Typically triggers regional or ocean-wide tsunamis if submarine.

Depth: The Hidden Variable

Depth is arguably more important than magnitude in determining local damage, yet it receives far less public attention. The 1994 Northridge earthquake (M6.7, 18 km depth) and the 2001 Nisqually earthquake (M6.8, 52 km depth) were nearly identical in magnitude, but Northridge caused 57 deaths and $25 billion in damage while Nisqually caused 1 death and $2 billion in damage. The shallower Northridge event concentrated energy at the surface directly under the city.

When browsing significant earthquake listings or US state data, always check both the magnitude and the depth. An M7 at 400 km depth in a subduction zone is fundamentally different from an M7 at 10 km in a populated area.

Reading Yearly Statistics

PlainQuake's yearly earthquake statistics show total event counts by magnitude range, peak events, and country-level breakdowns for each year from 2005 to 2025. A few things to keep in mind when interpreting these numbers:

Aftershock sequences inflate totals: The year of a major earthquake (or the years immediately following) will show elevated counts. The M9.1 Tohoku earthquake in 2011 generated thousands of M4+ aftershocks, making 2011 a statistical outlier in global earthquake counts.
Long-term averages are more informative: A single-year spike may reflect one active fault. Five or ten year averages show underlying tectonic rates more clearly.
Detection improvements can appear as increases: As seismograph networks have expanded and improved over the 2005–2025 period, some regions show apparent earthquake count increases that partly reflect better detection, not increased seismicity.
Induced seismicity is visible: The Oklahoma/central US surge from 2013–2016 is clearly visible in state-level annual data, a real, human-caused increase, not a detection artifact.

What Makes an Earthquake Truly Dangerous

The deadliest earthquakes in history are not always the largest. The factors that determine human impact include:

Population density near the epicenter: An M8 in the Pacific Ocean may cause no deaths; an M6.3 under Port-au-Prince (2010 Haiti) killed over 200,000.
Building construction quality: Modern earthquake-resistant construction survives events that collapse older unreinforced masonry. Seismic building codes have saved millions of lives in California, Japan, and New Zealand.
Time of day: The 1989 Loma Prieta earthquake struck at 5:04 PM, when many people were leaving work, if it had struck at 2 AM when people were in (potentially weaker) homes, casualties would have differed significantly.
Secondary hazards: Tsunamis (triggered by M7.5+ offshore earthquakes), liquefaction (soft soils behaving like liquid during shaking), landslides, and fires following gas line ruptures can cause more damage than the shaking itself.
Local geology: Soft bay sediments amplify shaking dramatically compared to bedrock. Mexico City's 1985 earthquake devastation was partly due to the city's location on an ancient lake bed that resonated with seismic waves.

Explore earthquake data by region using global country statistics and US state data to compare seismic activity across different geologic and population contexts.

Safety Note

During an earthquake, Drop, Cover, and Hold On. Drop to your hands and knees, take cover under a sturdy table or desk, and hold on until shaking stops. After shaking ends, check for hazards like gas leaks and damaged structures before moving around.

Frequently Asked Questions

What does "place" mean in earthquake data, is it always a city?

The "place" field in USGS data describes the nearest named location to the earthquake epicenter, formatted as distance and direction from that location (e.g., "23km NNE of Ridgecrest, California"). For oceanic earthquakes, it may reference the nearest island, region name, or ocean feature. The place field is purely descriptive, it does not mean the earthquake occurred in or directly affected that location. A M5 earthquake "25km from San Francisco" may cause no felt shaking in San Francisco itself if it is 20 km deep.

How does depth affect how dangerous an earthquake is?

Depth is one of the most critical variables in earthquake impact. Shallow earthquakes (0–20 km) concentrate their energy at the surface, causing stronger shaking in a smaller radius. A M6.5 earthquake at 5 km depth can be more destructive than a M7.0 at 150 km depth. Deep earthquakes spread energy over a much larger volume of rock before reaching the surface, greatly reducing surface shaking intensity. For example, deep South American subduction zone earthquakes of M7+ are often barely felt at the surface despite impressive magnitudes.

What are aftershocks and how long do they last?

Aftershocks are smaller earthquakes that follow a main shock as the crust adjusts to the new stress distribution along the fault. They follow Omori's Law: the rate of aftershocks decays roughly as 1/t, most occur in the first hours and days, with decreasing frequency over weeks and months. A major earthquake (M7+) can produce aftershocks for years. The aftershock sequence after the 2011 M9.1 Tohoku earthquake included hundreds of M5+ events over the following decade. Aftershocks can occasionally be as large as the main shock (then reclassified as a sequence), and they can trigger secondary damage to already-weakened structures.

What does it mean when an earthquake has a high "significance" score?

USGS assigns a significance score (0–1000) to each earthquake based on magnitude, maximum modified Mercalli intensity (MMI), felt reports, estimated impact, and media attention. A high significance score doesn't always mean high danger, it reflects the combination of physical size, how many people felt it, and its news prominence. A moderate M4.5 earthquake under a dense city center with thousands of felt reports may score higher significance than a remote M6 in the ocean. PlainQuake uses the M6+ threshold as its definition of "significant" for catalog purposes.

How should I compare earthquake counts between different countries?

Country earthquake counts are not directly comparable due to differences in seismograph network density. Countries with dense monitoring networks (US, Japan, Germany) detect many more small earthquakes than countries with sparse networks. Japan, with one of the world's densest seismograph networks, records far more small earthquakes than geologically similar but less-monitored regions. For fair comparisons, focus on M5+ or M6+ events, these are reliably detected globally. PlainQuake's global data uses M4+ as the floor, which means coverage is more complete for well-monitored countries.

What does a yearly earthquake statistic actually tell me?

Yearly earthquake statistics show the total count of earthquakes above a threshold magnitude, the maximum magnitude event, and breakdowns by magnitude range for a given year. They can reveal: whether a region is in an aftershock sequence from a prior large event (elevated counts), whether induced seismicity is increasing (rising small earthquake counts), and whether a particularly active year includes a rare major event. However, single-year statistics can be misleading, 2011 had inflated global counts due to the Tohoku aftershock sequence. Multi-year trends are more informative than any single year.

Sources

USGS Earthquake Hazards Program, ComCat Catalog, 2005–2025
USGS, Earthquake Magnitude, Energy, and Shaking Intensity
USGS, Earthquake Glossary
USGS, Induced Earthquakes

This content is for educational purposes only. For official earthquake information, real-time alerts, and emergency preparedness guidance, visit earthquake.usgs.gov and ready.gov.