A magnitude 3.5 earthquake is considered a minor seismic event, rarely causing structural damage to buildings. This level of shaking is typically described as a noticeable jolt, similar to a heavy truck passing by. While sensitive seismographs easily record these small tremors, the distance at which an individual can perceive the shaking varies significantly. The experience is a complex interaction of the energy released, the geological structure, and the distance from the source.
Understanding Magnitude vs. Intensity
Seismologists use two separate metrics to describe an earthquake: magnitude and intensity. The Moment Magnitude Scale (MMS) measures magnitude, representing the total energy released at the source as a single, objective number. Since the scale is logarithmic, a magnitude 3.5 releases a fixed amount of energy regardless of location.
The Modified Mercalli Intensity (MMI) scale measures the effects of shaking at a specific location, accounting for how people and structures are affected. This measurement is subjective and highly variable, meaning a single earthquake generates many different intensity values across a region. The lower levels of this scale describe human perception, such as MMI II, which is defined as being “felt only by a few people at rest.” MMI III is “felt quite noticeably by people indoors,” often leading to the sensation of a vibration like a passing vehicle. A magnitude 3.5 earthquake near its epicenter typically correlates to an intensity of MMI II or MMI III. This means the energy released is just enough to cross the threshold of human perception under favorable conditions near the source.
What Determines How Far the Shaking Travels?
The distance the shaking travels before it is no longer felt is controlled by attenuation, the process causing seismic energy to fade. A primary factor is the earthquake’s focal depth, the distance from the surface to the rupture point. Shallower earthquakes are felt more strongly and over a larger area because the energy has less distance to travel through the crust before reaching the surface.
The local geology and soil type also determine how far the shaking is perceived. Hard, dense bedrock transmits waves efficiently but dampens energy quickly, limiting the felt distance. Conversely, soft sediments, such as loose soil, can amplify the shaking motion, significantly increasing the felt intensity and radius. This amplification occurs because seismic waves slow down in the soft material, increasing their amplitude.
Energy dissipation away from the source is governed by two mechanisms. The first is geometric spreading, where energy spreads across an increasing spherical wavefront, causing intensity to decrease with distance. The second is intrinsic attenuation, where the mechanical energy of the wave is converted into heat due to friction within the rock layers and at grain boundaries as the wave propagates. These combined effects cause the seismic energy to fade rapidly, limiting the distance a small quake can be detected.
The Typical Perception Radius for a 3.5
The distance a 3.5 magnitude event can be felt depends heavily on depth and local geology. Under favorable conditions, such as a shallow earthquake beneath soft, sedimentary soil, the shaking can be felt up to 15 to 20 miles (25 to 30 kilometers) from the epicenter. This maximum range is typically achieved by people indoors who are sitting still in a quiet environment.
Conversely, if the earthquake originates at a deeper focal depth and the area is composed of hard, dense bedrock, the perception radius shrinks dramatically. Under these unfavorable conditions, the shaking may only be felt within a limited area, as little as 5 miles (8 kilometers) from the epicenter. The precise felt distance is not a fixed number, but a range determined by the geological filter the seismic waves travel through.