Aftershocks why

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. 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. Rev Geophys.

Deichmann N, Giardini D Earthquakes induced by the stimulation of an enhanced geothermal system below Basel Switzerland. Seismol Res Lett 80 5 — J Struct Geol 32 11 — Earthquakes occur in the crust or upper mantle , which ranges from the earth's surface to about kilometers deep about miles.

The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at km deep is considerably Why are there so many earthquakes in the Geysers area in Northern California?

The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults. However, activities associated with What is an earthquake and what causes them to happen?

An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

In California there Can the position of the moon or the planets affect seismicity? Earthquakes are equally as likely to occur in the morning or the evening. Many studies in the past have shown no significant correlations between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs. Several recent studies, however, have found a correlation between earth tides caused by the position of Filter Total Items: Wald, Lisa A.

View Citation. Wald, L. Geological Survey Fact Sheet —, 2 p. Year Published: On the potential duration of the aftershock sequence of the Anchorage earthquake Currently, an aftershock sequence is ongoing in Alaska after the magnitude 7.

Michael, Andrew J. Michael, A. Geological Survey Open-File Report —, 6 p. Geological Survey The mission of the USGS in natural hazards is to develop and apply hazard science to help protect the safety, security, and economic well-being of the Nation. Perry, Suzanne C. Natural Hazards Science at the U. Year Published: Fundamental questions of earthquake statistics, source behavior, and the estimation of earthquake probabilities from possible foreshocks Estimates of the probability that an ML 4.

Fundamental questions of earthquake statistics, source behavior, and the estimation of earthquake probabilities from possible foreshocks; ; Article; Journal; Bulletin of the Seismological Society of America; Michael, Andrew J. Pollitz, Fred F. Year Published: Earthquake hazards: a national threat Earthquakes are one of the most costly natural hazards faced by the Nation, posing a significant risk to 75 million Americans in 39 States.

One of them was the gigantic 9. The number of aftershocks following the March 11 quake have been by far the most recorded in Japan. The magnitude 8. The three largest aftershocks occurred within an hour of the main quake, as is the case with most earthquakes. But that does not mean the worst is over.

April 7 and 11 saw aftershocks with magnitudes of 7. And the Meteorological Agency warned that many more large ones may be in store for eastern Japan. The agency said the likelihood of an aftershock of magnitude 7. Tuesday, and also the same percentage within three days of 3 p. 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. 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.