These ShakeMaps have been sized to the same scale to compare the relative size of the areas shaken. The March 9 (local time) M6.8 earthquake offshore of Eureka is on the left, and the March 17 M4.4 earthquake beneath Los Angeles is on the right. Click each map to see the full-size version.

March 2014 saw two widely-felt earthquakes strike a week apart at nearly opposite ends of California. The larger was roughly the same size as the state’s most famous and damaging earthquakes: Northridge and Loma Prieta, which occurred 20 and 25 years ago, respectively. While the smaller roused many people from their morning slumber, both served as wake-up calls for the need to be more prepared.

On the night of March 9, a magnitude 6.8 earthquake struck 50 miles offshore of Eureka, in northern California. This area, a complex active tectonic zone known as the Mendocino Triple Junction, is no stranger to large earthquakes. In 2010, a very similar magnitude 6.5 quake happened here, and four earthquakes greater than magnitude 7 have rocked the area since 1980, the most recent in 2005.

A week after the offshore quake in northern California, a magnitude 4.4 earthquake shook the greater Los Angeles area. Occurring just around sunrise, the event woke up thousands of residents, who quickly flooded social media channels with their reports of how they felt the ground shake. Damage was relatively minor.

Compared to the previous week's quake, the energy released by this event was practically insignificant (just 1/10000^th as great), but the strength of the shaking, known as the intensity, felt by those living closest to the epicenter was actually greater. How is this possible?

The answer lies in the locations of these two earthquakes relative to the residents who felt them. Intensity maps called ShakeMaps reveal the reasons responsible for this unlikely coincidence. The offshore location of the larger earthquake resulted in the comparatively low intensities experienced along the Northern California coast. The small size of the southern California earthquake, conversely, was offset by its location immediately beneath a heavily populated area.

These ShakeMaps are at the same scale as the maps above. The Northridge earthquake (left) was less intense but was centered beneath a more populated area, while the Loma Prieta earthquake (right) was centered in a less populated region south of the San Francisco Bay Area. Both maps focus on the region of strongest intensity, while the North Coast earthquake ShakeMap above shows the broader region of lower intensity shaking (the highest intensities were offshore). Click each map to see the full-size version.

This location factor explains why the magnitude 6.7 Northridge earthquake of 1994, which released only half the total energy of the magnitude 6.9 Loma Prieta earthquake that occurred five years earlier, did several times more damage. When you look at the two ShakeMaps for these earthquakes (click each map to see the full-size version), note the location of the epicenter (shown by a star) for each.

The Northridge epicenter was located directly beneath the San Fernando Valley, an area covered with homes, businesses, and infrastructure. The epicenter of the Loma Prieta earthquake, on the other hand, was located in a relatively undeveloped part of the Santa Cruz Mountains. The strongest shaking, then, occurred where buildings and infrastructure were most sparse, resulting in comparatively less damage relative to the size of the earthquake.

Learn more about the magnitude scale and why small differences in magnitude numbers can mean large differences in shaking intensities