As the few small earthquakes shake the same place in minutes, what does it tell people who experienced the shaking?
In the California and New Mexico regions, relatively small tremors of late have been significant in their arrival: in close bursts and shallow depths. In San Ramon, which is in Contra Costa County, there were several events that were considered within a narrow time margin, such as mid-3 range quakes and other “minor aftershocks”. On the outskirts of Whites City, New Mexico a depth that was shallow of 3.1 was felt in the northwest of the town around the area of the Carlsbad Caverns.
The level of detail is not minutiae. Even at lower magnitudes, shallower earthquakes may more easily be felt due to the fact that the energy does not cover as long a distance before reaching the surface. According to the U.S. Geological Survey, shallow focus (0 70km) is the most recommended category of depth earthquakes and states that depth is a powerful indicator of how strongly the shaking will be experienced by surrounding communities. In the New Mexico quake, it was found in a depth of a little more than five kilometers, which was deep enough to be reported a shallow quake; in the Bay Area sequences, it was said to be shallow as well, but that made it more likely that people would feel vibrations briefly, even without damage.
That brings us to the next word that is alarming to hear but frequently defines a common geological process an earthquake swarm. Swarm Unlike the conventional “mainshock-aftershock” model, wherein the largest quake happens first, with smaller ones decelerating (which are occasionally called swarms), swarm troubles are characterized by a large number of quakes within a small region that are not of that neat form. According to the USGS, swarms are generally identified by comparatively numerous earthquakes in a comparatively small region and the sequence is not comparable to a typical post tremor decay.
Scientists also note that there is no cause of swarms. Discussing swarm behavior in a USGS article, some of the possible drivers were identified as slow fault slipping, magma movement in a volcano, and fluid interacting with faults (most frequently). Such fluids may decrease the clamping pressure holding a fault locked, and earthquakes may transiently enhance the ability of fluids to pass through cracks forming a feedback mechanism that generates more small quakes until the process exhausts the stressed rock.
To the locals, the real-world implication is that small groups of shallow tremors can actually be felt but not devastating. A magnitude 3.3 was linked to dozens of public sensations of felt in Indio, and the descriptions of the sensations matched those of everyday experience, like the sound of a large heavy car passing by and the movement of objects, a trend which can be observed in the USGS community intensity mapping method that incorporates community reporting with instrumental data. In certain areas of the Bay Area and near Whites City, initial public reports also assisted in establishing the location of shaking.
Small sequences in the earthquake country can be more than just a flash of fright: they will also be a reminder that the ground action will not only be affected by the magnitude of the movement, but also by how near the movement is to the surface by whether the shuddering is a one-great-shock or a series of quick shocks near the surface.
