Deep Structures Beneath Intraplate Earthquakes: Comparisons Among Four Typical Tectonic Regions

Compared to interplate earthquakes,intraplate earthquakes are infrequent but can be devastating.To better understand the seismogenic of intraplate earthquakes,we select four different continental regions where intraplate earthquakes happen but with different tectonic backgrounds as research targets.Adopting seismic travel-time tomography,we obtain highresolution P-wave velocity models beneath these four research areas.Our results shed new insights into some issues,including the origin of volcanoes in Kyushu,the deformation mechanism in the crust and the upper mantle beneath Turkey and its vicinity,earthquake distribution in southeastern Tibet plate,and the lateral variations of the lithosphere and deep structures in the mantle beneath the central and eastern United States(CEUS)and so on.We also find the relationships between deep structures and intraplate earthquakes in each research area.Then,we draw the similarities and differences of the triggering mechanism of intraplate earthquakes with differential tectonic backgrounds.Kyushu island,located in the eastern margin of the Eurasia plate,is an island arc beneath which the Philippine Sea oceanic plate is subducting.By adding Moho-reflected phases(Pm P)into the inversion,we determine a high-resolution P-wave velocity model of the crust beneath Kyushu,especially that in the lower crust.Beneath the active volcanoes in Kyushu,there are continuous or intermittent low velocity(low-V)anomalies extending from the surface down to Moho discontinuity.The low-V anomalies may reflect partial melts and/or magmatic fluids rising up from the mantle wedge due to a combination of fluids from slab dehydration and corner flow in the mantle wedge.The 2016 Kumamoto earthquake main shock occurred at the edge of a small low-V zone in the upper crust,whereas significant anomalies with low-V,highattenuation,and high Poisson's ratio exist in the lower crust beneath the source zone.These results imply that the generation of the 2016 Kumamoto earthquake was affected by ascending fluids.Turkey is located in the Eurasia continent's southern margin,where oceanic plate subduction,continental plate subduction,and continent-continent collision coexist.We obtain the first P-wave azimuthal anisotropy velocity model in the crust and uppermost mantle beneath Turkey and surrounding areas.By comparing the fast velocity directions in our model with the maximum extension directions in the surface,the strikes of geological features,and fast polarization directions from previous shear-wave splitting measurements,we imply the deformation mechanisms beneath Turkey.Vertically coherent deformation between the crust and uppermost mantle occurs in western Turkey,however,the crust and lithospheric mantle are decoupled in southeastern and central northern Turkey.The southeastern Tibet plate is in the interior of the Eurasia continent but contains rejuvenating orogeny.The 2019 Changning earthquake occurred in a transition zone between low-V and high velocity(high-V)zones in the upper crust,and there are high-V anomalies beneath the source zone.Its nucleation was influenced by water injections during petroleum exploration.Low-V anomalies are found in the lower crust beneath the northwestern part of the research area,and they are highly correlated with the distribution of large crustal earthquakes.We propose that the low-V bodies are extruded accompanying the plateau expansion but are blocked by the strong crust of the competently stronger blocks.Therefore,the deep crustal materials rise and weaken the shallow crust,leading to many earthquakes under increasing stress.CEUS is located in the eastern North American continent and is made up of craton and passive continent margin.A new three-dimensional model of P-wave velocity tomography of the crust and mantle(from the surface down to 1000 km depth)beneath CEUS is obtained by conducting a joint inversion of local and teleseismic travel-time data.There are obvious lateral variations in the lithosphere beneath CEUS.There are some large-scale high-V anomalies in the mantle transition zone and uppermost of the lower mantle,which may be the fragments of the Farallon slab or delaminated lithosphere materials.A low-V zone is revealed in the New Madrid Seismic Zone's lithosphere and is bounded by high-V anomalies to the southeast and northwest.The low-V anomaly may reflect a weak zone with high water content.This unique pattern is prone to concentrate stresses.And the water ascending up into the seismogenic layer can trigger earthquakes.Comparing source zones of intraplate earthquakes under different tectonic backgrounds,it can be found that the stress loading rates of intraplate earthquakes focal areas vary significantly:in tectonic active regions,the stress loading rate is very fast,comparative to that of plate boundaries;while the far-field stress loading rate is negligible in stable continental regions.However,intraplate earthquakes all seem to be affected by the fluid from the deep.Fluids can increase pore-fluid pressure,making intraplate earthquakes more likely to occur.However,fluid sources are different in various intraplate seismic regions,depending on a certain area's specific deep structure background.