A Significant Increase in Interplate Seismicity near Major Historical Earthquakes Offshore Martinique (FWI)

ABSTRACT Understanding the processes that may be at the origin of major earthquakes in subduction zones is highly challenging, especially in the case of slowly converging areas such as the Lesser Antilles subduction zone. Our study reveals a recorded increase in seismicity rate and cumulative seismic moment over the last two decades offshore Martinique island and, particularly, in the presumed rupture area of the major historical 1839 earthquake. This sustained seismicity is shared between extensive intermediate depth activity since the 2007 Mw 7.4 earthquake that occurred about 20 km north of Martinique and a compressive seismic cluster located in the seismogenic zone of the subduction zone. We also observe a downward migration of the seismicity along the plate interface up to a 60–65 km depth and a recorded increase in the magnitudes of the earthquakes. All these observations may indicate ongoing changes along the plate boundary interface near the area of the historical major rupture of 1839, although more detailed statistical studies and additional data are necessary to confirm this trend.

Geometría de la zona sismogénica interplacas en el Sureste de Costa Rica a la luz de la secuencia de Golfito del 2018

Between August and November 2018, a seismic sequence took place in the vicinity of Golfito, a city in the Dulce Gulf in Southeastern Costa Rica. The main shock had a moment magnitude (Mw) of 6.1 and was widely felt in Costa Rica and Western Panama, with maximum Modified Mercalli intensities of VI. In this region, the oceanic Cocos Ridge, riding on top of the Cocos Plate, subducts beneath the Panama Microplate. Using the seismic records from the National Seismological Network of Costa Rica, in this work the seismicity is relocated using the double-difference technique, and an analysis of its temporal and geographic distribution together with the focal mechanism and intensities of the strongest events are presented. The results show that the sequence occurred at the interplate seismogenic zone, within the rupture area of the 1983 Golfito earthquake (7.4 Mw), between 12 and 27 km depth, in a cluster dipping 35º northeast underneath the Dulce Gulf. Based mainly on these results and on previous seismic sequences, it is here proposed that the seismogenic zone in Southeastern Costa Rica has an extension of ~160 x 45 km. Further, during the Golfito sequence, the rupture of an inverse fault (5.9 Mw) took place within the Cocos Plate beneath the Dulce Gulf, as well as of dextral strike-slip faults (4.6-5.6 Mw) in the Panama Microplate, 50 km away of the Dulce Gulf. The analysis of the interseismic interplate seismicity contributes to a better understating of the dynamics of the seismogenic zone. This is of particular relevance in Southeastern Costa Rica, where at least six damaging earthquakes of Mw > 7 have occurred since 1803, implying the impending risk of the next big earthquake in this region.

Maximum magnitude of subduction earthquakes along the Japan-Kuril-Kamchatka trench estimated from seismic moment conservation

SUMMARY We estimated the maximum magnitude of earthquakes in the Japan-Kuril-Kamchatka trench subduction zone with a method based on the conservation of seismic moment and the record of interplate seismicity from 1977 to 2017. The key point of this method is to base calculations on the tectonic moment rate instead of the total seismic moment rate. We modeled a seismic-moment-frequency distribution for the Japan-Kuril-Kamchatka trench on the basis of the truncated Gutenberg–Richter (G–R) law, the formula published by Utsu in 1974, the gamma distribution, and the tapered G–R law. We estimated the maximum magnitude along the Japan-Kuril-Kamchatka trench as ∼10 under the truncated G–R law and ∼11 under Utsu's formula, although the latter may be an overestimate. Therefore, the 2011 Tohoku earthquake, of moment magnitude 9.2, may not be the largest possible event in this area. The recurrence interval for magnitude 10 events based on the truncated G–R law is 4000 yr. Although these two models perform equally well in terms of Akaike Information Criterion, the range of the 95 per cent confidence level is consistently narrower for the truncated G–R law than for Utsu's formula. The estimated maximum magnitude depends not only on the model used, but also on the parameters that constitute the tectonic moment. It is essential to accumulate more seismic data and achieve more precise estimates of tectonic moment to improve estimates of maximum magnitude.