Contrary to our expectations, the seismicity rate at the sites of the future March 20 and May 1 shocks did not budge when the February 13 shock struck. But when the March 20 quake struck, there was a short burst of seismicity at the site of the future May 1 shock, which would rupture 42 days later. The response of seismicity at the future May 1 site to the March 20 event is consistent with a calculated stress increase by about 0.
For reference, we put about 7 bars of pressure in our bicycle tires, so while 0. The absence of a change in seismicity following the February 13 quake nevertheless astonishes us because we calculate that the quake increased the Coulomb stress on faults near the May 1 event by about 0.
So, we would have expected a seismicity increase at the May 1 site, and possibly at both sites. We can visualize the calculated stress changes with beachballs, as shown below Toda and Stein, In the figure below, a red beachball means that a particular fault was brought closer to failure in our calculation as a result of an earthquake; a blue beachball means that failure was inhibited.
Each panel in the figure shows this transfer of stress from a given event. This unfortunately complex figure highlights the messiness of real-world faults, which come in all sizes, orientations and depths. Our calculations Toda and Stein, attempt to capture this complexity. So, we use background focal mechanisms the beachballs from earthquakes greater than magnitude Red beachballs are brought closer to failure and blue beachballs farther from failure. Each panel shows the impact of one black quake on its surroundings.
We can confidently assert that all three magnitude-7 events are aftershocks of the magnitide Its aftershock sequence is far from over, and more large events could occur, although probably not at the rate we have seen over the past 80 days, which is unprecedented.
The February 13 shock slightly promoted the site of the March 20 shock, but there was no detectable seismicity increase. But there is a clear seismicity response of the second event to the third, consistent with its large calculated stress increase. So, the triggering of the third quake by the second looks clear. The bottom panel of the last figure shows that a lot of red beachballs remain, particularly to the northeast of the May 1 event. For instance, the number of aftershocks with magnitude of 5 is about 10 times larger than that of aftershocks with magnitude of 6.
As for those occurred in sea area, the largest aftershock generally occur within about 10 days. For instance, as the largest aftershock of the Great-Hanshin Earthquake in occurred two hours after the main-shock, while that of the Mid Niigata Prefecture Earthquake in occurred in 38 minutes both of them are inland earthquake.
Meanwhile, the largest aftershock of Sanriku-Haruka-Oki Earthquake in , which is known as an undersea earthquake, took place 9. As for the Tokachi-Oki Earthquake M8.
Aftershocks Q. What is an aftershock? Therefore, the forecasts are updated to keep current with the changing aftershock rate. We also update the forecasts over time to incorporate more information about the specific behavior of the aftershock sequence.
We update at least once within the first day, again within the first week, and again within the first month. The time that the current forecast was released, and the planned time of the next forecast update, are included in each forecast. Clicking on the card will take the user to the Aftershock Forecast. The Commentary tab describes the aftershock forecast in simple language, starting with the concept that larger earthquakes could follow and that aftershocks will be continuing for some time; and some safety information is included.
The subsequent information is a simple summary of the forecast, followed by what has already happened, and ending with a more quantitative version of the forecast. The Forecast tab presents the forecast as tables, covering a range of aftershock magnitudes and time frames.
The first table shows the probability of at least one aftershock above a certain magnitude and within a certain time frame. The second table shows the likely number of aftershocks above a certain magnitude and within a certain time frame, given as range of numbers which represents the uncertainty of the forecast. If it is unlikely that there will be any aftershocks of that magnitude during that time frame, the table shows an asterisk, which means that an earthquake is possible but with a low probability.
This tab shows what model was used to compute the forecast, as well as the model parameter values. Forecasts are currently made only with the Reasenberg-Jones , model. There are three different types of parameter values:. Forecasts are currently made only with the Reasenberg-Jones , model, which models the aftershock rate with a smooth decay with time following the mainshock. At this time we are not calculating spatial forecasts or providing maps to show areas with the highest likelihood of aftershocks.
As a rule of thumb, aftershocks are most likely to occur near the mainshock fault plane and in areas already experiencing numerous aftershocks.
0コメント