Foreshocks and aftershocks of the Italian earthquake

Before everyone started talking about radon gas, I was pretty sure I heard a report on the radio talking about Monday’s earthquake near L’Aquila being preceded by a number of foreshocks. I’ve discussed before how there’s nothing intrinsically ‘foreshocky’ about “foreshocks” (or “aftershocky” about “aftershocks” for that matter) beyond the fact that they occurred shortly before (or after, in the case of aftershocks) a bigger earthquake in the same region. Using data made available by INGV, below I’ve plotted the magnitudes of all the earthquakes that have occurred in this region in the last month, with the magnitude 6.3* on Monday morning marked by the larger solid red circle

Last months seismic activity near L'Aquila


From this we can see that there was a few days of enhanced seismic activity, including about half a dozen magnitude 3-4 earthquakes, beginning about a week before the main shock and apparently stopping on the 3rd April. We can also plot the initial rupture locations of all these earthquakes, with blue dots marking ones that occurred before Monday’s earthquake (red dot) , and yellow dots the locations of the aftershocks.

Map of the last months earthquakes near L'Aquila

The “foreshocks” mainly form a nice little cluster to the southeast of Monday’s big shock, which suggests they are all related to each other (it’s no surprise that they’re much more tightly clustered than the aftershocks, because the much higher amount of energy released by the main shock would have induced stress changes over a much wider area). It also suggests that these earlier earthquakes may have had a direct influence on Monday’s 6.3; the focal mechanism suggests it occurred on a northwest-southeast trending fault. This earlier cluster could therefore represent activity on an adjacent section of the same fault, producing additional stresses which eventually led to a rupture on Monday.
Note that again, this reasoning is all after the fact; even if there is a direct relationship, if you just looked at the earthquake record from the 28th March to the 3rd April, would you have been able to predict that a larger earthquake was on the way in this region?

Italian foreshock sequence

I certainly wouldn’t have; if anything, I would have said that activity had peaked and was fading back to a background level. Such are the risks of trying to predict earthquakes.
*I’m using the USGS magnitude, although the Italians have estimated it as a 5.8, presumably using their local seismograph network rather than the global one, and (I think) using a different magnitude calculation.

Categories: earthquakes, geohazards, geology
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Comments (9)

  1. Miguel Vera says:

    Just so you know, the INGV event page has some good maps and info. http://is.gd/riFH
    And about the magnitude, there’s a note in that same page:
    Note on the magnitude: the magnitude routinely used to estimate the magnitude of an earthquake is called the Richter Magnitude or Local Magnitude (Ml), which is calculated by the scale of the maximum seismic recording of a standard seismograph (short term). Conversely, the Moment Magnitude (Mw) is processed through a numerical treatment of the seismic signal on all frequencies identified by the registration. For the strong earthquakes Mw is considered a more accurate estimate of severity.

  2. Kim says:

    I’m glad you found the foreshock data. I had heard something about it, but couldn’t find the reference in the various links I sorted through. (I don’t know the INGV site, either – I know where to find data at the USGS, but not there.)
    I also think I remember some reference to the radon possibly having been released by the foreshocks, meaning that all the earthquake precursors were really the same event. I can’t find it, though.

  3. isnochys says:

    Thx for the foreshock data

  4. larry says:

    L’Aquila is the empathetic echo of Shock and Awe.
    Six years ago, about this Passing Over time, George W. bUSh’s “shock and awe” crusade went on in Iraq with mega-pounds of bombs and missiles. We shrugged our shoulders when hearing about a bombed mosque but today we weep seeing a church crumbling after the earth shook. Our eyes droop when hundred people died in L‚ÄôAquila, but when International Red Cross reported 500,000 Iraqi people died in bUSh invasion, we looked the other way. We don‚Äôt want to know where two million Iraqi refugees went or if they had ‚Äúweekend camping‚Äù tents to live in. We hardly saw flattened structures in Iraq for they were hidden from the media. Therefore, our empathy went absent. Following Katrina, L’Aquila is the empathetic echo of Shock and Awe.
    kATRIna – .ATRI.. – RITA
    katRINA – …RINA – IRAN
    KAtRIna – KA.RI.. – IR AK
    KATrinA – KAT…A – ATAK
    ATAK IR AK IRAN KATRINA RITA ATAK
    bUSh with 150000 troops, Great Britain with 15000, Australia with 5000, Italy with 3000 and few other nations joined to attack Iraq. All the invading countries had received their share of Acts of God in hurricanes, floods, cyclones, earthquakes. Look at IR AK today, that country is broken.
    I think Acts of God happened to empathize everyone. It‚Äôs good when President Obama announced that we‚Äôre not at war with Islam but he was short of an apology. Can an apology to the people of Iraq stop future Acts of God? I don’ know. Only God knows.

  5. M. Binder says:

    The data collected on the foreshocks and aftershocks of this interesting, al-bei-it devastating earthquake can make superior data for science fair projects. It is great that we now have the technology to not only collect a wider array of data, but to share it in nearly real time. Keep up the good work scientists!

  6. Do you know of any sites where kids can go to find real time data on earthquakes?

  7. cope says:

    SSFP,
    The USGS has an almost real time set of seismic activity. It is updated every 5 minutes and shows actual seismic records from selected seismograph stations.
    My HTML skills are currently insufficient to incorporate the link as text but here it is:
    http://quake.usgs.gov/recent/helicorders/index.html

  8. lucap says:

    See a fully featured KML about the seismic sequence at:
    http://code.google.com/p/ingvkml4apr9earthquake/
    file is still being developed and enhanced, save it as a network link to automatically load the updates.

  9. Nick says:

    I like Larry’s post :)
    In all seriousness though, the nonlinearity of foreshocks is exactly why earthquakes probably can’t be predicted (within months, if not years) by seismological methods. Makes one wonder if the radon gas emissions aren’t important!