Base Of Seismogenic Layer Research Articles

Seismicity Rate Changes and Geodetic Transients in Central Apennines

AbstractUsing template matching and GPS data, we investigate the evolution of seismicity and observable deformation in Central Apennines. Seismicity appears more persistent at the base of the seismogenic layer than in the shallower crust. Diffuse activity is reported on segments at depth, alternating along strike with apparent quiescence on segments that experienced one or more Mw6+ earthquakes in 1997, 2009, and 2016. Central Apennines are likely underlain by a sizeable shear zone with areas of diffuse seismicity bounding shallow normal faults where Mw6+ earthquakes occurred. The deformation observed at the surface seems to follow the seismicity variations at the base of seismogenic layer along the Apenninic chain. Principal and independent component analyses of GPS data exhibit a transient when the 2016 foreshock sequence starts. This transient propagated northward from the Campotosto fault up to the Alto Tiberina fault system and has likely loaded the Mw6+ 2016 earthquake sequence.

Migration of Seismicity and Pore Pressure Diffusion for Five Decades Long Koyna–Warna Sequence and Precursory Nucleation Process for Earthquake Forecasting

ABSTRACT The continued reservoir-triggered seismicity for five decades in Koyna area has been attributed to southward migration of seismicity (during 1967-1992 near and south of Koyna dam and from 1993 onwards mostly near the new Warna reservoir). Spread of seismicity in the vicinity of reservoirs is attributed to pore-pressure diffusion. Moderate size Koyna – Warna earthquakes are found to nucleate at shallow depth (≤ 3 km) due to pore pressure caused by water level fluctuation of reservoir(s). The nucleation zone deepens along the critically stressed permeable fault zone to cause the occurrence of mainshock at the base of seismogenic layer (i.e. 5-10 km). The clustering of foreshocks up to 500 hr prior to several moderate size Koyna earthquakes of magnitude Mw 4-5 has been detected and used for quantifying the nucleation process. A static stress transfer by means of cascade model from one foreshock to next for the generation of foreshocks has been proposed for nucleation model. The nucleation process can be considered as an immediate earthquake precursor for the Koyna-Warna region.

Earthquake source nucleation: A physical model for short-term precursors

Ohnaka, M., 1992. Earthquake source nucleation: a physical model for short-term precursors. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 149–178. This paper deals with a quasistatic, leading to a quasidynamic at a later time, nucleation process that precedes the earthquake dynamic rupture, and models of the earthquake source nucleation are put forward based on physical principles. A quasistatic to quasidynamic rupture nucleation process is an intrinsic part of the ensuing earthquake dynamic rupture; in other words, the nucleation process itself is an short-term (or immediate) precursor that occurs in a localized zone. During the earthquake nucleation, premonitory slip proceeds in the localized nucleation zone, and shear stress also decreases gradually in the zone, since slip-weakening occurs during the nucleation. Immediate foreshock activity is a part of the nucleation process leading to the mainshock dynamic rupture; therefore, hypocentral locations of foreshocks are necessarily restricted to lie near the mainshock hypocentre (onset of the mainshock rupture). Whether or not foreshocks occur during the mainshock nucleation depends on how the rupture growth resistance varies on a local to small scale. Since patches of greater rupture growth resistance are considered to prevail on a local to small scale in the fault zone, carrying immediate foreshocks would be one of the major characteristics of earthquakes that nucleate within the brittle seismogenic layer. When a mainshock earthquake nucleates within the brittle seismogenic layer and its hypocentre is located near the base of the seismogenic layer, immediate foreshocks for this mainshock are necessarily restricted to lie within a localized region shallower than hypocentral depth of the mainshock. By contrast, the nucleation process below the base of the seismogenic layer is aseismic in nature, so that carrying no conspicuous foreshocks would be a common characteristic of interplate earthquakes that nucleate below the base of the brittle seismogenic layer. The breakdown strength τ p , the breakdown stress drop Δτ b and the critical slip displacement D c are indicative of the rupture growth resistance. To estimate depth variations of these parameters, the effects of the normal stress σ n and temperature T on those parameters are examined using available data so far published. The combined effects of σ n and T on τ p in the brittle to semibrittle regime are found to be represented empirically by: τ p(σ n, T) = τ pO(σ n)[1−(cosh 50 T ) + 40 sinh( 50 T ) exp(− 2000 T ] where τ po ( σ n ) = 135.7 + 0.750 σ n , and T is measured in °K and σ n in MPa. It is found that D c increases sharply with T, but is insensitive to σ n above 300°C, while D c depends on σ n but is insensitive to T below 300°C. On the basis of these results, variations in τ p, D c and Δτ b at mid-crustal depths are estimated for quartzo-feldspathic rocks for a given geothermal gradient.