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Points of Progress. A Christian Science Perspective. Monitor Movie Guide. Monitor Daily. Photo Galleries. As a result, interactions between the largest earthquake in a sequence, known as a mainshock, and its aftershocks may hold clues to earthquake interactions more broadly, helping to explain how changes on a fault induced by one earthquake may affect the potential site of another.
The stress is what drives earthquakes. Scientists have noted a tendency for aftershocks to occur where two types of stress act on a fault change. The first type is called is normal stress, which is how strongly two sides of a fault are pushing together or pulling apart. The second type is called shear stress, or how strongly the two sides are being pushed past one another, parallel to the fault, by remote forces.
Decreases in the normal stress and increases in the shear stress are expected to encourage subsequent earthquakes. Measures of these changes in the volume of rock around a fault are combined into a single metric called the Coulomb failure stress change.
There are components of stress that are different from shear stress and normal stress. In April , a magnitude 6. A magnitude 7. Photo credit: Shutterstock. Moving to the right you have a series of layers, each containing a bunch of connected neurons. And at the other end you have an outcome of some kind. One neuron can excite another.
The solid red and gray circles indicate A t around the mainshock fault Faults 1 through 4 and the best-fit planes in the northern part of the aftershock region Faults 5 through 8 , respectively. The red open circles indicate A t obtained using a smaller region for event selection around Faults 1 and 2.
The green broken line indicates the upper limit of the location errors of hypocenters in the horizontal direction. The red star indicates the thickness of the hypocenter distribution and its fault length associated with the swarm activity in the volcanic area Yukutake et al. Since A t is likely to be not controlled by the thickness of fault damage zone, the coseismic stress changes by the mainshock are suggested as one of the plausible factors for affecting A t.
Nodal planes of the focal mechanisms oblique to the mainshock fault plane Fig. We used the slip distribution by Iwata and Sekiguchi , which is shown in Fig. Only the large-slip area in which slips were larger than a half of the maximum, 2 m, is shown. We used only aftershocks for which focal mechanisms were determined. We focused on the aftershocks around Faults 1 and 2 because of the simplicity of the geometry of the mainshock fault plane.
Stress changes due to the mainshock slip were calculated using the formula of Okada We set the rigidity to 30 GPa and the apparent coefficient of friction to 0. We evaluated the statistical significance for the Coulomb indices using the bootstrap method proposed in Kato We generated a synthetic catalog that was created as a random combination of the hypocenter and focal mechanism data around Faults 1 and 2.
We calculated the Coulomb index for the synthetic catalog and performed the procedure times. As a result, the Coulomb indices for 3. This result also suggests that A t may be controlled by the spatial distribution of stress changes. This result implies that the true slip distribution in the large-slip region was complicated beyond the limitation of the spatial resolution of the waveform inversion or some other factor was related to the triggering of aftershocks in the large-slip region.
The orange contours projected on b indicate areas of large slip exceeding a dislocation of 2. The red star indicates the starting point of mainshock rupture. The red star in Fig. Yukutake et al. Narrow zones of hypocenter distribution are also reported in the water injection-induced seismicity e.
On the other hand, the aftershocks on a fault with the same length were distributed within a significantly broader zone Fig. In the present study, based on the precisely determined hypocenters and focal mechanisms, we considered the important question of why aftershocks occur. In order to address this question, we investigated whether aftershocks represent the rerupture of the mainshock fault plane or aftershocks occur on faults outside the mainshock fault plane.
The aftershocks of the Western Tottori Earthquake were distributed within the zones of 1. These thicknesses of the aftershock zones cannot be explained by the location errors of hypocenters or the geometrical heterogeneity of the mainshock fault plane.
This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault, rather than the rerupture of the mainshock fault.
Moreover, the aftershocks were distributed within a much broader zone than the fault damage zone obtained in the geological observation. The hypocenters of the swarm activity in the geothermal region exhibit a narrower planar distribution compared with the aftershock sequence. This result implies a difference in the generation process: Earthquake swarms are controlled by the fault weakening process due to fluid intrusion into a fault damage zone that serves as a highly permeable channel, whereas aftershocks are caused primarily by stress changes due to slip dislocation during the mainshock or its afterslip.
In summary, we conclude that the major factor in generating the aftershocks in this region is the stress changes caused by the slip related to the mainshock. Geophys J Int 3 — Article Google Scholar. Geology 37 4 — Das S, Henry C Spatial relation between main earthquake slip and its aftershock distribution.
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Imanishi K, Kuwahara Y, Takeda T, Haryu Y The seismicity, fault structures, and stress field in the seismic gap adjacent to the Mid-Niigata earthquake inferred from seismological observations.
Earth Planets Space 58 7 — In: Proceedings of 11th Japan earthquake engineering symposium in Japanese with English abstract , pp — Kagan YY 3-D rotation of double-couple earthquake sources. Kato A Imaging the source region of the mid-Niigata prefecture earthquake and the evolution of a seismogenic thrust-related fold.
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Earth Planets Space 57 6 Earth Planets Space 57 9 — Part of living with earthquakes is living with aftershocks. Earthquakes come in clusters. In any earthquake cluster, the largest one is called the mainshock; anything before it is a foreshock, and anything after it is an aftershock.
Aftershocks are earthquakes that usually occur near the mainshock. The stress on the mainshock's fault changes during the mainshock and most of the aftershocks occur on the same fault. Sometimes the change in stress is great enough to trigger aftershocks on nearby faults as well. An earthquake large enough to cause damage will probably produce several felt aftershocks within the first hour.
The rate of aftershocks dies off quickly. The day after the mainshock has about half the aftershocks of the first day. Ten days after the mainshock there are only a tenth the number of aftershocks. An earthquake will be called an aftershock as long as the rate of earthquakes is higher than it was before the mainshock. For big earthquakes this might go on for decades. Bigger earthquakes have more and larger aftershocks.
The bigger the mainshock, the bigger the largest aftershock, on average, though there are many more small aftershocks than large ones. Also, just as smaller earthquakes can continue to occur a year or more after a mainshock, there is still a chance for a large aftershock long after an earthquake. Sometimes what we think is a mainshock is followed by a larger earthquake.
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