The strike and dip angles of the reference focal mechanism were assumed to be those of the best-fit plane Table 1. The rake angle of the reference focal mechanism was assumed to be parallel to the resolved shear stress direction on the best-fit plane under the stress field reported by Yukutake et al.
Hypocenter distribution around the best-fit planes. Left Hypocenter distribution along the strike of the best-fit plane. The vertical and horizontal axes indicate, respectively, the directions along and normal to the best-fit plane.
Middle and right Hypocenter distributions projected onto and perpendicular to the best-fit fault plane, respectively. Top Hypocenter distribution around Fault 1 middle that around Fault 2 and bottom that around Fault 8 in Fig. The best-fit planes in this region Faults 1 and 2 are consistent with the orientation of one of the nodal planes in the CMT solution of the mainshock e.
A large slip of more than 2 m occurred on Fault 1 during the mainshock Fig. On the other hand, in the northern part of the aftershock region, a complicated spatial distribution of the aftershocks was estimated.
The hypocenter distribution of the aftershocks suggests the existence of a conjugate fault system and is divided into several small clusters. The complicated structures of the best-fit plane were estimated Faults 5 through 8. Figure 6 shows the frequency distribution of the distance from the best-fit plane to each aftershock location.
The aftershocks were concentrated on the best-fit plane, and the concentration decreased with the distance from the best-fit plane. Figure 7 shows a histogram of A t around the 8 best-fit planes.
The aftershocks around Faults 1 through 4 are distributed with zones of 1. Histograms showing the distance from the best-fit fault planes and the Kagan angles. Left The distance from the best-fit fault plane to each location of an aftershock. Right The Kagan angles from the reference focal mechanism. The Y -axis is normalized by the total number of events. Some examples of focal mechanisms around Fault 1 at the Kagan angle are also shown in a. Histograms showing A t.
The solid and open bins indicate, respectively, A t with a planarity greater than or equal to 8 and A t with a planarity less than 8. Most of the focal mechanisms are of the strike-slip type. These features of the focal mechanisms are consistent with those reported in previous studies e.
We also evaluated the variety of the focal mechanisms based on the Kagan angle from the reference focal mechanism. These results imply that the aftershocks do not occur on a simple plane. Figure 8 a shows the relationship between the hypocenter distance from Fault 1 or 2 and the Kagan angle from the reference focal mechanism. Most nodal planes of these focal mechanisms are oriented obliquely with respect to each best-fit plane rather than coincident with it.
Characteristics of the focal mechanisms around the mainshock fault. We selected the nodal plane for which the strike is closer to that of the best-fit plane. The thick black line indicates the strike of the best-fit plane. The red and gray lines indicate nodal planes of strike-slip type and other types, respectively. Note that the vertical and horizontal axes , respectively, indicate the directions along and normal to the best-fit plane.
The general features of the fault model obtained in the present study are consistent with those reported by Fukuyama et al. However, the locations of the best-fit planes in the northern part of the aftershock region were estimated at shallower depths compared with their fault model. This difference results from the uncertainty of the hypocenter depth in their study due to a lack of seismic stations above the northern part of the aftershock region.
According to the result of Sagiya et al. Based on these results, seismic slip during the mainshock is inferred to have occurred on Faults 1 through 4. The conjugate fault plane of Fault 4 was estimated at the northern edge of the mainshock fault. Shibutani et al. A high-velocity structure corresponding to plutonic and metamorphic rocks was estimated along the mainshock fault in the southern part of aftershock region , whereas the northern part of the aftershock region was composed primarily of non-alkali volcanic and pyroclastic rocks of the early to middle Miocene, which are characteristic of a low-velocity zone.
The characteristics of fault structures appear to differ at this velocity boundary. A complicated fault system developed on the northern side, whereas a larger fault structure on the order of 10 km in length existed on the southern side Fig. The dynamic rupture process of the Western Tottori Earthquake was probably controlled by these pre-existing fault structures around the source region.
The aftershocks are distributed within approximately 1. Location errors of hypocenters in the horizontal direction strongly affect A t for a nearly vertical dipping fault plane, as observed in the study region.
However, the estimated thickness cannot be explained by the location errors of the hypocenter in the horizontal direction that are less than 30 m for differential arrival time data obtained by both catalog and cross-correlation analysis.
There is also a possibility that the wide aftershock distribution around the mainshock fault results from a local irregularity in the mainshock fault geometry. In order to exclude this possibility, we conducted PCA for the hypocenters around Faults 1 and 2, dividing the area around the best-fit planes into 15 small regions.
The length of the hypocenter distribution is 2—3 km in the middle and longest axes. We analyzed only regions containing at least 20 earthquake hypocenters.
As a result, we obtained A t for 12 small regions ranging from 0. For seven small clusters, the planarity of the hypocenter distribution was less than eight. This means that A t is close to the length of the middle axis for the seismicity 2—3 km. These results imply that A t is not attributed to geometric heterogeneity of the mainshock fault. Validity of A t can be also confirmed by using the observed differential arrival times for the small earthquake cluster Additional file 2.
Assuming the mainshock fault surface to be smooth and to coincide with the best-fit plane, the percentage of aftershocks occurring on the rupture surface of the mainshock fault can be estimated. The result is consistent with the estimation by Liu et al. Figure 9 shows the relationship between lengths of the best-fit plane L and A t. The figure also shows the relationship between L and the damage zone thickness of a natural fault zone P in an outcrop Vermilye and Scholz The aftershocks were distributed within a much wider zone than the fault damage zone at the same L.
Using a seismic tomography with a spatial resolution of 2 km, Shibutani et al. These results suggest that numerous aftershocks occurred outside the fault damage zone. Relationship between L and A t. The straight line indicates the scaling between P and L based on a natural fault zone in the outcrop compiled by Vermilye and Scholz 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.
Unlike mainshock-aftershock type, it does not have particular large earthquakes; however its seismic activity lasts for a long period of time, varying its level of the activities.
It is the area where aftershocks occur. Till about the first 24 hours after the occurrence of a main-shock, its aftershock area almost accords with the hypocentral area, an area destroyed by the main-shock. However, the aftershock area gradually extends to a wider area thereafter.
When the main-shock occurs, the rock around the epicenter becomes dynamically unstable. It is considered that aftershocks occur in order to adjust for this dynamical instability. The number of aftershocks will decrease to approximately the one-tenth in 10 days whereas one-hundredth in days. It finishes with information we expect to learn after future earthquakes. An earthquake is caused by a sudden slip on a fault, much like what happens when you snap your fingers.
Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the form of sound waves that set the air vibrating and travel from your hand to your ear, where you hear the snap.
The same process goes on in an earthquake. Stresses in the earth's outer layer push the sides of the fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Just as you snap your fingers with the whole area of your fingertip and thumb, earthquakes happen over an area of the fault, called the rupture surface. However, unlike your fingers, the whole fault plane does not slip at once. The rupture begins at a point on the fault plane called the hypocenter, a point usually deep down on the fault. The epicenter is the point on the surface directly above the hypocenter.
The rupture keeps spreading until something stops it exactly how this happens is a hot research topic in seismology. 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.
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