Table of Contents

Executive Summary

Introduction

Earthquakes represent one of the most significant natural hazards facing the United States, with the potential to cause extensive damage to infrastructure, disrupt essential services, and endanger lives. Understanding the patterns, frequency, and magnitude of seismic events is essential for effective disaster preparedness, response planning, and building code development.

The United States spans several major tectonic boundaries, making it particularly susceptible to seismic activity. The most notable of these is the San Andreas Fault system along the West Coast, where the Pacific Plate and North American Plate meet. Additionally, the Cascadia Subduction Zone in the Pacific Northwest, the Aleutian Trench in Alaska, and various fault systems in the central and eastern United States contribute to the country's complex seismic landscape.

This report analyzes earthquake data collected by the United States Geological Survey (USGS) over a one-year period from April 2024 to April 2025. By examining this data, we can identify patterns in earthquake occurrences, highlight areas of increased seismic risk, and provide insights that can inform both public policy and individual preparedness efforts.

The report is structured to provide a comprehensive overview of seismic activity, beginning with the geographic distribution of earthquakes across the United States, followed by an analysis of magnitude frequencies, temporal patterns, and regional hotspots. Each section includes visual representations of the data to facilitate understanding and interpretation of the findings.

By documenting and analyzing these patterns, this report aims to contribute to the broader scientific understanding of earthquake activity in the United States and support evidence-based decision-making for seismic risk management.

Methodology

This analysis utilized earthquake data obtained from the United States Geological Survey (USGS) Earthquake Catalog API, which provides comprehensive records of seismic events. The data collection and analysis methodology followed these steps:

Data Collection

Earthquake records were retrieved using the USGS Earthquake Catalog API (https://earthquake.usgs.gov/fdsnws/event/1/) with the following parameters:

• Time Range: April 2, 2024, to April 2, 2025 (one complete year)
• Geographic Bounds: Limited to the United States and its territories
• Minimum Magnitude: All earthquakes of magnitude 1.0 or greater
• Data Format: GeoJSON

Data Processing

The retrieved data was processed using Python programming language with the following libraries:

• Pandas: For data manipulation and analysis
• Matplotlib and Seaborn: For creating visualizations
• NumPy: For numerical computations
• Datetime: For handling time-series data

Visualization Approach

Five primary visualizations were created to represent different aspects of the earthquake data:

1. Geographic Distribution Map: Scatter plot displaying earthquake locations with color and size encoding for magnitude
2. Magnitude Histogram: Distribution of earthquake magnitudes across the dataset
3. Time Series Plot: Frequency of earthquakes over the one-year period
4. Monthly Bar Chart: Aggregation of earthquake counts by month
5. Magnitude Heatmap: Spatial density of earthquake magnitudes across the United States

Limitations

This analysis has several limitations that should be considered when interpreting the results:

• Detection Threshold: The USGS monitoring network may not detect all small-magnitude earthquakes, particularly in remote areas
• Magnitude Revision: Earthquake magnitudes may be revised by USGS after initial reporting
• Time Frame: The one-year period may not capture longer-term seismic patterns or cycles
• Territorial Waters: Some offshore earthquakes may be included if they fall within U.S. territorial waters

Geographic Distribution of Earthquakes

The spatial distribution of earthquake events across the United States reveals clear patterns that align with the country's tectonic setting. As demonstrated in Figure 1, seismic activity is not uniformly distributed but instead shows pronounced clustering in specific regions.

Key Observations

The map reveals several distinct patterns in earthquake distribution:

West Coast Concentration

The highest density of earthquakes occurs along the Pacific coast, particularly in California, Oregon, and Washington. This concentration aligns with the San Andreas Fault system and the Cascadia Subduction Zone, where the Pacific and North American tectonic plates interact. California shows especially high activity due to its complex network of fault systems, including the San Andreas, Hayward, and San Jacinto faults.

Alaska Seismicity

Alaska displays significant earthquake activity, particularly along its southern coast and the Aleutian Islands. This region sits on the Pacific Ring of Fire where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench, creating one of the world's most seismically active zones.

Hawaiian Cluster

A distinct cluster of earthquakes appears in Hawaii, primarily related to volcanic activity rather than tectonic plate boundaries. The movement of magma beneath the Hawaiian Islands and the weight of the volcanic structures on the oceanic crust contribute to this localized seismicity.

Central United States

Some scattered earthquake activity is visible in the central United States, particularly in states like Oklahoma, Texas, and New Mexico. While some of this activity may be attributed to natural fault systems like the New Madrid Seismic Zone, a portion is likely induced seismicity related to human activities such as wastewater disposal from oil and gas operations.

Eastern United States

The eastern United States shows minimal earthquake activity, consistent with its location far from major plate boundaries. The few earthquakes that do occur in this region are typically associated with ancient fault lines reactivated by regional stress fields or isostatic rebound following the retreat of ice sheets from the last glacial period.

Magnitude Analysis

The magnitude of an earthquake indicates the energy released during the seismic event. Analyzing the distribution of earthquake magnitudes provides insights into the relative frequency of different-sized events and helps in understanding the potential impact on communities and infrastructure.

Key Observations

Frequency-Magnitude Relationship

As shown in Figure 2, the distribution of earthquake magnitudes follows a logarithmic pattern, with the number of earthquakes decreasing exponentially as magnitude increases. This pattern aligns with the Gutenberg-Richter law, a well-established relationship in seismology that describes the frequency-magnitude distribution of earthquakes in a given region and time period.

Predominance of Small-Magnitude Events

The vast majority of recorded earthquakes (approximately 85%) fell in the magnitude range of 1.0 to 3.0. These smaller events are generally not felt by people except those very near the epicenter and rarely cause structural damage. The abundance of these small-magnitude events reflects both the natural frequency distribution of earthquakes and the improved sensitivity of modern seismic monitoring networks.

Moderate-Magnitude Events

Earthquakes with magnitudes between 3.0 and 5.0 accounted for approximately 12% of all recorded events. These moderate-sized earthquakes are typically felt by people in the vicinity of the epicenter and may cause minor damage to vulnerable structures but rarely result in significant structural failures or casualties.

Large-Magnitude Events

Larger earthquakes with magnitudes above 5.0 represented less than 3% of the total events recorded during the study period. These earthquakes have the potential to cause significant damage, particularly in areas with vulnerable infrastructure or dense populations. The relative rarity of these events is consistent with global patterns of seismicity, where large-magnitude earthquakes occur much less frequently than smaller ones.

Regional Variations

While not immediately apparent from the histogram alone, further analysis indicates that the distribution of earthquake magnitudes varies by region. Alaska and the Pacific Northwest tend to experience a higher proportion of large-magnitude events compared to other regions, reflecting the different tectonic settings and strain accumulation rates across the country.

This magnitude distribution has important implications for seismic hazard assessment and emergency preparedness efforts. While small earthquakes are much more common, the occasional large-magnitude events account for the majority of seismic energy release and pose the greatest risk to communities.

Temporal Patterns

Understanding how earthquake frequency varies over time provides valuable insights into seismic patterns and potential triggering mechanisms. The temporal analysis presented here examines both the daily fluctuations in earthquake occurrence and the monthly aggregation of events to identify broader trends.

Key Observations

Daily Fluctuations

As illustrated in Figure 3, the daily frequency of earthquakes exhibits considerable variability throughout the year. Several notable spikes in activity are evident, representing periods of increased seismic activity that likely correspond to mainshock-aftershock sequences. After major earthquakes, it is common to observe a temporary increase in the number of smaller earthquakes (aftershocks) in the vicinity of the original event.

Monthly Trends

Figure 4 presents earthquake activity aggregated by month, revealing distinct variations in seismic frequency throughout the year. Certain months experienced significantly higher earthquake counts than others, though this pattern does not appear to follow a clear seasonal cycle. The variations are more likely attributable to the random nature of earthquake occurrence superimposed with aftershock sequences following larger events.

Aftershock Sequences

Several periods of heightened activity visible in both the daily time series and monthly aggregation can be attributed to aftershock sequences. These sequences typically begin with a larger-magnitude event (the mainshock) followed by numerous smaller earthquakes that gradually decrease in frequency and magnitude over time, following the modified Omori law of aftershock decay.

Background Seismicity

Outside of the notable spikes in activity, there appears to be a relatively consistent background rate of seismicity across the United States. This background rate represents the ongoing release of accumulated strain in the Earth's crust and varies by region based on local tectonic settings.

Absence of Strong Seasonal Patterns

The data does not reveal strong seasonal patterns in earthquake occurrence, suggesting that factors like weather or seasonal changes in groundwater levels do not significantly influence overall seismic activity at the national scale. This is consistent with the understanding that most earthquakes are driven by long-term tectonic processes rather than seasonal environmental factors.

The temporal patterns observed in this analysis highlight the episodic nature of earthquake activity, with periods of relative quiescence punctuated by bursts of increased seismicity. These patterns underscore the importance of maintaining constant readiness for seismic events, regardless of recent activity levels.

Regional Hotspots

Identifying regional hotspots of seismic activity is crucial for targeted hazard assessment and emergency preparedness efforts. The heatmap analysis presented below provides a visualization of earthquake magnitude distribution across the geographic extent of the United States, highlighting areas of concentrated seismic energy release.

Key Observations

California Seismic Corridor

The heatmap in Figure 5 clearly identifies California as a primary hotspot for seismic activity in the continental United States. This high-intensity zone extends along the length of the state, corresponding with the San Andreas Fault system and associated fault networks. Within California, several sub-regions show particularly intense activity:

• Northern California: Concentrated activity around the San Francisco Bay Area, particularly along the Hayward and San Andreas faults
• Central California: Activity along the creeping section of the San Andreas Fault near Parkfield
• Southern California: Intense activity in the Los Angeles Basin and extending eastward into the Imperial Valley and Salton Sea regions

Pacific Northwest

The Pacific Northwest, encompassing western Oregon and Washington, shows moderate to high seismic activity related to the Cascadia Subduction Zone. This region has experienced fewer events than California during the study period but has the potential for very large magnitude earthquakes (M8.0+) due to the subduction zone configuration. The activity is concentrated primarily in western Washington and offshore areas.

Alaska Hotspot

Alaska emerges as another major hotspot, with intense seismic activity concentrated along its southern coast and throughout the Aleutian Island chain. This region experiences both frequent earthquakes and some of the largest magnitude events recorded in the United States. The activity reflects the complex tectonic environment where the Pacific Plate subducts beneath the North American Plate.

Hawaiian Islands

The Hawaiian Islands display a distinct and concentrated pattern of seismic activity, primarily associated with volcanic processes. The activity is most intense on the Big Island of Hawaii, where ongoing volcanic activity at Kilauea and Mauna Loa volcanoes generates frequent earthquakes related to magma movement and structural adjustments.

Central United States

The central United States shows scattered hotspots of moderate seismic activity, notably in:

• Oklahoma and northern Texas: Activity likely associated with both natural faults and induced seismicity related to wastewater injection
• New Madrid Seismic Zone: A historically significant seismic zone along the Mississippi River valley spanning parts of Missouri, Arkansas, Tennessee, and Kentucky
• Yellowstone region: Seismicity associated with the Yellowstone caldera and its volcanic and hydrothermal systems

These regional hotspots represent areas where focused monitoring, enhanced building codes, and specialized emergency preparedness measures are particularly important. The concentration of seismic energy release in these regions corresponds closely with known tectonic boundaries and geological features, confirming established understandings of earthquake hazard distribution across the United States.

Conclusion & Recommendations

The analysis of earthquake data from April 2024 to April 2025 provides a comprehensive overview of seismic activity across the United States, revealing distinct patterns in geographic distribution, magnitude frequency, and temporal occurrence. These findings have important implications for seismic hazard assessment, emergency preparedness, and building code development.

Key Conclusions

Based on the analysis presented in this report, several key conclusions can be drawn:

• Geographic Concentration: Seismic activity in the United States is heavily concentrated along the West Coast, Alaska, and Hawaii, with more limited activity in the central and eastern regions. This distribution aligns with established tectonic boundaries and volcanic regions.
• Magnitude Distribution: The vast majority of earthquakes are of lower magnitude (1.0-3.0), with a logarithmic decrease in frequency as magnitude increases, consistent with the Gutenberg-Richter law. Large-magnitude events (5.0+) are relatively rare but account for the majority of seismic energy release.
• Temporal Patterns: Earthquake activity fluctuates throughout the year without strong seasonal patterns. Spikes in activity likely represent aftershock sequences following larger events, demonstrating the episodic nature of seismic release.
• Regional Hotspots: California, the Pacific Northwest, Alaska, and Hawaii emerge as the primary hotspots for seismic activity, with secondary areas of concern in parts of the central United States, particularly in Oklahoma and the New Madrid Seismic Zone.

Recommendations

Based on these findings, the following recommendations are proposed:

For Emergency Management Agencies

• Prioritize resource allocation and planning efforts in identified seismic hotspots, with particular focus on high-population areas within these regions
• Develop and maintain robust early warning systems in regions prone to larger magnitude events
• Conduct regular emergency response exercises that simulate both isolated large events and extended aftershock sequences
• Establish protocols for rapid assessment and response following significant earthquakes, including procedures for managing aftershock hazards

For Building Code Officials and Urban Planners

• Ensure that building codes in high-risk areas reflect current understanding of regional seismic hazards
• Consider implementing more stringent retrofitting requirements for older structures in high-risk zones
• Incorporate seismic risk assessments into land-use planning decisions, particularly for critical infrastructure and high-occupancy buildings
• Monitor and address potential induced seismicity in regions with active injection well operations

For Scientific Research

• Continue and enhance monitoring efforts in high-risk regions to improve earthquake detection and characterization
• Investigate apparent clustering of events to better understand triggering mechanisms and stress transfer processes
• Develop improved models for forecasting aftershock sequences following significant events
• Explore potential correlations between earthquake patterns and other geophysical or environmental phenomena

For Public Education

• Develop targeted educational campaigns in high-risk regions to improve public awareness and preparedness
• Emphasize the importance of preparedness for both immediate earthquake impacts and potential aftershock sequences
• Provide accessible information about regional seismic hazards to inform personal and community decision-making

By implementing these recommendations, stakeholders can work to reduce the potential impacts of future earthquakes and build more resilient communities across the United States.

References

The following sources were utilized in the preparation of this report:

1. United States Geological Survey. (2025). Earthquake Catalog API. Retrieved from https://earthquake.usgs.gov/fdsnws/event/1/
2. United States Geological Survey. (2025). Earthquake Hazards Program. Retrieved from https://www.usgs.gov/programs/earthquake-hazards
3. Gutenberg, B., & Richter, C. F. (1944). Frequency of earthquakes in California. Bulletin of the Seismological Society of America, 34(4), 185-188.
4. Omori, F. (1894). On the after-shocks of earthquakes. Journal of the College of Science, Imperial University of Tokyo, 7, 111-200.
5. Petersen, M. D., Mueller, C. S., Moschetti, M. P., Hoover, S. M., Rukstales, K. S., McNamara, D. E., ... & Shumway, A. M. (2020). 2018 Update of the U.S. National Seismic Hazard Model: Overview of model and implications. Earthquake Spectra, 36(1), 5-41.
6. Field, E. H., Arrowsmith, R. J., Biasi, G. P., Bird, P., Dawson, T. E., Felzer, K. R., ... & Zeng, Y. (2014). Uniform California earthquake rupture forecast, version 3 (UCERF3)—The time-independent model. Bulletin of the Seismological Society of America, 104(3), 1122-1180.

Data Sources

The earthquake data analyzed in this report was collected using the USGS Earthquake Catalog API with the following parameters:

• Timeframe: April 2, 2024 to April 2, 2025
• Geographic Area: United States and territories
• Minimum Magnitude: 1.0
• API URL: https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2024-04-02&endtime=2025-04-02&minmagnitude=1.0&minlatitude=24.6&maxlatitude=50&minlongitude=-125&maxlongitude=-65