The long precursory phase of many large interplate earthquakes Michel Bouchon, Hayrullah Karabulut, Virginie Durand, David Marsan With: Mustafa Aktar, Jean Schmittbuhl, Serdar Ozalaybey

The reason why foreshocks occur before some earthquakes and not others is unknown. Two mechanisms are proposed to explain their presence: - A cascade model where a first shock randomly triggers other shocks and one of them triggers the earthquake. The fact that this leads to a large earthquake is simply the result of a random throw. - An aseismic slip model where a patch of the fault begins to slip slowly and the foreshocks are the breaking of the fault asperities resisting the slow slip (Jones and Molnar 1979 ; Ohnaka 1992; Dodge Beroza and Ellsworth 1995,1996 ; Abercrombie and Mori 1996; Zanzerkia Beroza and Vidale 2003; McGuire et al. 2005).

If the first model is correct, there is no hope to ever predict earthquakes. If slow slip is what generates foreshocks, one may hope that in the future this slow slip will be directly measurable and that the preparation phase of some earthquakes will be recognizable before rupture occurs.

Let us first look at the 1999 Izmit earthquake. It is the largest well-recorded strike-slip earthquake to date and broke the North Anatolian Fault (the interface between two horizontally-moving plates). Photo A. Barka

Spot image interferometry of Izmit rupture, Michel and Avouac, 2002 : « The surface ruptures associated with the Izmit earthquake seem remarkably simple »

As the rupture begins on a segment with simple geometry, one may hope to clearly see the nucleation phase (if it exists !)

The 150km-long Izmit rupture and the closest stations to the epicenter

45 minutes before the earthquake, first triggered window:

20 minutes before the earthquake:

18 shocks are visible in the 6 triggerred windows:

As the largest shocks are recorded at several stations and the time difference between their P-arrival and the one of the main shock are the same, they come from what will be the earthquake hypocenter

Spectra of 9 of the shocks:

before the first foreshock the 40s prior to the earthquake Cross-correlating the record with the template

filtered record Let us look again at the first triggered window :

Observation: During the 44 minutes that precede the earthquake, the hypocentral area emits a seismic signal which repeats itself over and over sometimes only a few seconds apart. These bursts become more frequent as the time of the earthquake approaches. They seem (maybe) accompanied by a continuous low-frequency seismic noise. Interpretation: A patch of the fault located at the bottom of the brittle crust has begun to slip slowly 44 minutes before the earthquake. An asperity which will become the hypocenter of the earthquake is resisting this slow slip and is continuously reloaded by the slow slip around it. This phase of slow slip accelerates in time... Izmit nucleation

Is Izmit just a special case ? or could it be a common case ?

Are many large earthquakes preceded by a phase of slow slip accelerating (irregularly) in time ?

All the M≥6.5 (interplate and intraplate) earthquakes which occurred in these two densely instrumented zones between 01/01/1999 and 01/01/2011

Cumulative seismic moment as a function of time in a zone of 50km radius around the epicenter of the interplate earthquakes. The traces end just before the earthquake.

Cumulative seismic moment as a function of time in a zone of 50km radius around the epicenter of the interplate earthquakes. The traces end just before the earthquake. Although different time windows are considered here, an increase in seismic activity seems to precede most interplate earthquakes.

Cumulative number of events in the 4 days prior to 70% of the interplate earthquakes. The largest event is circled in red. zoom

Cumulative number of events in the 4 days prior to 70% of the interplate earthquakes. The largest event is circled in red. 70% of interplate earthquakes are preceded by an increase of seismic activity in the day or hours before. zoom

Why the difference ?

The most logical (?) explanation: Slow slip of the plate interface generally occurs prior to its rupture

Characteristics of the Preparation Phase

Its timing :

Stacks of cumulative numbers of events prior to the interplate earthquakes (each sequence is given the same weight)

Stacks of cumulative seismic moments

Characteristics of the Preparation Phase Its spatial extent :

Plate interface Transform

Plate interface Subduction

Plate interface Subduction Transform

The large interplate earthquakes are generally preceded by a period during which the two plates in contact begin to move slowly. The foreshocks are produced by the breaking of the frictional asperities resisting the slow slip. This slow slip accelerates irregularly in the days or in the hours before the earthquake... Large subduction earthquakes seem often preceded by the slow slip of a large part of the subducting plate. Interpretation

USGS catalog for the 100km-radius zone surrounding the Tohoku epicenter in the 6 months prior to the earthquake

For details on the Tohoku precursory phase see Kato et al. (2012)

Some events which occur close in time have almost identical spectral shape:

The 150km-long Izmit rupture and the closest stations to the epicenter