However, the more I studied them, the more I came to believe that aftershocks of the northeastern earthquakes were lasting for centuries. This study is exciting because it provides evidence that aftershock sequences can last for very long periods of time.
This forces us to look at the routine earthquake activity in a place like Japan in a new way. This may ultimately be a step forward toward earthquake forecasting because it may help us learn which small earthquakes might be harbingers of future strong shocks and, which are simply continuing aftershocks of a past strong event.
Shinji Toda and Ross S. Basham, P. Adams , Earthquakes on the continental margin of eastern Canada, Need future large events be confined to the locations of large historical events? Open-File Report , Ebel, J.
Bonjer, and M. Oncescu , Paleoseismicity: Seismicity evidence for past large earthquakes, Seismol. Skip to content. 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. Our forecast is based on a statistical model of the behavior of past aftershock sequences in similar tectonic settings.
The rate of aftershocks usually follows a few general rules:. The initial forecast after an earthquake occurs is calculated using parameters that worked for previous earthquakes in that region or similar regions around the world. As time goes by and we observe how many aftershocks are happening we use parameters that are a combination of the initial parameters and parameters determined from the current sequence of earthquakes.
The initial forecast uses only the mainshock magnitude, and therefore can be released soon after the mainshock, and before many aftershocks have occurred. Because the initial forecast depends a lot on the mainshock magnitude, we wait at least 30 minutes after the event occurs before issuing a forecast, to allow the preferred mainshock magnitude to stabilize. We also update the forecast if the mainshock magnitude significantly changes after the initial forecast.
After big earthquakes, we say them. But what do these terms mean? What do they mean for what we felt and what we will feel the next time? Do we really understand what seismologists are saying? This section describes how earthquakes happen and how they are measured. It also explains why the same earthquake can shake one area differently than another area. 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.
0コメント