Magnitude is the total energy released by an earthquake – not how the earthquake energy was released. Magnitude is determined by calculating the area/length of the fault that slipped, by the amount that area slipped, by the elastic properties of the crust.
Intensity of an earthquake is how an earthquake is felt, and that is different than the magnitude. Intensity depends on how the energy is released. The energy of an earthquake can be released in different ways.
Here are some of the key physical elements of earthquake energy release:
–Acceleration (how fast in G’s/G-force the crust accelerates from zero G’s. It can be a slow acceleration producing a more rolling earthquake, or pedal-to-the-metal fast acceleration for more violent shaking)
–Direction of travel
–Geologic environment/conditions
It is possible that an earthquake of medium magnitude can have much harsher shaking than a larger magnitude earthquake due to higher acceleration. The December, 2022 6.4 magnitude Humboldt Bay-Rio Dell earthquake had much higher acceleration and much more violent shaking due to rupturing within the hard Peridotite rock of the Gorda Plate, as opposed to the 2024 7.0 magnitude earthquake nearby on the transform Mendocino fault, which ruptured softer sedimentary materials.
But a medium or small earthquake will have a more reduced/contracted area affected than a larger magnitude earthquake. A larger magnitude earthquake spreads across and affects a larger area, even if the larger magnitude earthquake has lower acceleration and less violent shaking.
Direction of travel effects how and if someone feels the shaking of an earthquake. An earthquake is felt more heavily and has shorter/more compacted and damaging energy waves if it is traveling toward you (along the fault). The earthquake is felt much less if it is traveling away from you, even if you are near the fault or epicenter.
Earthquakes generate out from one starting, or breaking point, called an epicenter, and can become more vigorous further away from the epicenter as it travels out along the fault, depending on all the dynamics of acceleration and the reactions with the geology it encounters. Or, it can be most vigorous at or in immediate proximity to the epicenter for the same reasons.
A new and revised magnitude scale used by geologists today takes into account the multidimensional attributes of earthquakes, and calculates magnitude more accurately than the original scale of magnitude. It can now handle calculations and measurements for larger earthquakes into the 9-magnitudes, and the new scale formulations can even detect and measure imperceptible earthquakes in the negative magnitudes, which effect only tiny areas in the crust.
There can be numerous faults on or within the crust of plates as well as along the borders of plates or at fracture zones between plates. In northern California at the area of the Mendocino triple junction this is especially true within and on the tiny Gorda Plate. The vigorous 6.4 December, 2022 Rio Del earthquake was generated from a fault within the crust of the Gorda plate subducted deep beneath the North American plate. The Gorda’s oceanic plate crust is made of very dense and strong Peridotite, resulting in high acceleration at the rupture and violent shaking.
A note about tsunamis:
Tsunamis generated by transform faults are much smaller or non-existent due to generally horizontal motion of the lateral transform earthquake. The vertical motion of convergent subduction fault thrust earthquakes under the ocean oscillates and displaces water up and down above and then below sea level, like sloshing, which creates the radiation of tsunami waves.
References:
California State Polytechnic University, Humboldt Geology Department – recorded interviews with Dr. Lori Dengler, and Dr. Jay Patton
