When warning bells were sounded from seismometers of a 6.4 magnitude earthquake off the far-western part of the Aleutian Islands, the question that first came to mind of the general public was not about decimals on a graph. It was whether anyone felt it.
The incident was within the east of Attu Station and was measured at an approximate depth of 10 kilometers at the time no immediate “felt” report was registered. Practically, that touchy-feely detail is a cue that the magnitude of an earthquake in its headline is the total size of the earthquake whereas what people can feel on the surface is the result of the place they are, the depth of the rupture, and the amplification caused by local ground conditions.
Magnitude is a univariate value that is associated with an earthquake, whereas shaking is not. According to the U.S. Geological Survey magnitude and shaking intensity are not the same, the magnitude measures the source of the earthquake whereas intensity changes based on the location. That division aids in the explanation of why an earthquake can be highly felt and another one, at times even greater, can occur without much notice by the people. It also explains why early numbers are subject to change; various methods and station coverage may result in minor changes as analysts introduce new data.
An earthquake later that week in the central United States got a much different reaction. The USGS reported that the magnitude reached 4.1 in Nebraska, which produced 1,851 felt reports. This was not a contradiction, it was a representation of the fact that population density and building sensitivity and the proximity of people to the source will make the difference between shaking becoming a moment to be shared and a silent line on an instrument display.
The depth contributes to that result in an exorbitant manner. USGS points out that the closer the Earthquake to the surface, the less the surface shaking is experienced, as the seismic energy diffuses and attenuates as it pushes up the rocky surface. The depth of an earthquake can tracing the geometry of a downward slab in tectonic context like subduction zone, whereas in most continental contexts earthquakes are generally shallower and the surface shaking will be more significant at the same distance.
The range of magnitude of a quake also creates expectations on the effects that are generally expected. According to Canada earthquake information service, an earthquake measuring 3.5-5.4 is common but rarely causes damage whereas an earthquake with a magnitude of 6.1-6.9 can destroy buildings that are not well constructed in the densely populated regions. These ranges indicate general trends but not guarantees, since the quality of construction, soil type and distance of rupture to communities may cause actual outcomes to be above or below the description of “typical.”
Next there is the after-effect of the first shock. The USGS explains that the pattern of aftershock sequences following larger earthquakes is that the frequency tends to decline over time despite the fact that aftershocks do not gradually weaken in magnitude. To engineers and emergency planners, aftershocks are important as they may add additional stress to already damaged structures and make inspections more difficult, despite the relatively small amount of surface impact provided by the mainshock itself.
The lasting engineering lesson that is applicable in both Alaska and Nebraska is that the number of ones to remember is hardly ever the story. Magnitude is a beginning; distance, depth, local geology and what is built upon it will decide the shaking which in fact comes. The safety advice is never different: the authorities in the United States recommend people caught in the middle of an earthquake to drop, cover, and hold on.
