Change in seismicity along the Japan trench, 1990–2011, and its relationship with seismic coupling

Abstract

We investigate the temporal evolution of the background seismicity rate related to the subduction of the Pacific plate in northeast Japan, at latitudes 34° to 42°, for the 1 January 1990 to 9 March 2011 period. Two declustering methods are used to identify robust features. We find that the dominant behavior is a lowering down of activity, especially in the northern half of our studied area, where changes appear related to the cycle of M7.5+ earthquakes, in particular the 1968 Tokachi and the 1994 Sanriku earthquakes. Acceleration of background seismicity is observed offshore the Kanto region and could mark a long‐term decoupling of the Pacific and the Philippine Sea plates over a 100 km long segment along the Sagimi Trough, which cannot be simply explained by the earthquake cycle model. Our analysis further suggests that changes in background seismicity are plausibly related to changes in seismic coupling and thus further strengthens the recent observation that seismic coupling does vary at the timescales of tens of years, for the Japanese subduction zone.

Linear Relationship Between Aftershock Productivity and Seismic Coupling in the Northern Chile Subduction Zone

“…Our results indicate that coupling maps could be potentially useful for aftershock forecasting. However, even in the case that coupling and aftershock productivity are in general linearly related as indicated by our case study for northern Chile, potential forecasts of future aftershock activity would depend on the assumption that coupling is stationary in time, which is questioned by recent findings (Loveless & Meade, ; Marsan et al, ; Melnick et al, ).…”

Long‐Term Interactions Between Intermediate Depth and Shallow Seismicity in North Chile Subduction Zone

“…An epidemic‐type aftershock sequence (ETAS) model is performed to decluster the catalog and estimate the background seismicity (Marsan et al, , ). The number of earthquakes ( λ ) per unit of area ( x and y ) and unit of time ( t ) is modeled as the sum of two contributions: where ν ( x , y , t ) accounts for triggered earthquakes by a previous one (i.e., aftershocks) and μ is the background seismicity rate (i.e., not triggered by a previous event).…”

“…Parameters α , p , c , L 0 , and γ are here imposed a priori to realistic values: α = 2, p = 1, c = 10 −3 days, γ = 2, and L 0 = 1.78 km. While these parameters can be optimized given the data, in presence or not of a time‐ and space‐varying background rate (Reverso et al, ), it is easier and generally sufficient to impose fixed values (see Marsan et al, , for a discussion on this in the context of the northeast Japanese subduction zone). A parameter exploration is performed in the supporting information to evaluate how the results of our analysis depend on this choice.…”

“…Parameters t s and t e represent the starting and ending time of the catalog. Parameter κ is computed as We have used a smoothing length ℓ of 100 km and smoothing duration τ of 100 days to preserve the potential accelerations in the catalog (Marsan et al, ).…”

Insights on the Japanese Subduction Megathrust Properties From Depth and Lateral Variability of Observed Ground Motions

“…Also, it has been suggested recently that the burst of earthquake activity at shallow depths seen by these studies was also occurring at larger depth (>80 km), downdip the locked region of the subduction interface, based on intraslab earthquake rates and gravity variations (Bouchon et al, ). This activity could actually be the termination of the previously described much longer, slow decoupling process at intermediate depths, which could have started as early as 2003 (Mavrommatis et al, ; Yokota & Koketsu, ) or even years before according to seismicity rates (Marsan et al, ).…”