Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

In 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time. Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends >100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism. Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Stress state inferred from b-value and focal mechanism distributions in the aftershock area of the 2005 West Off Fukuoka Prefecture earthquake

The spatiotemporal stress states in the aftershock region of the 2005 west off Fukuoka prefecture earthquake are examined via an analysis of the b -values and focal mechanism solutions. The aftershocks are aligned roughly NW–SE, with the southeastern part of the aftershock region believed to correspond to Kego Fault, which extends beneath the Fukuoka metropolitan area. This study reveals depth-dependent b -values in the focal region, where the b -values ( b = 0.7–1.4) are generally higher above the mainshock depth (9.5 km) and lower ( b = 0.5–1.0) at greater depths. The shallower region possesses a significant temporal increase in b -values, whereas a lateral b -value heterogeneity is observed in the deeper region. The b -values ( b ~ 1.0) near the mainshock are relatively high, whereas the northwestern and southeastern edges of the deep region have lower b -values ( b = 0.5–0.7). On the other hand, many of the focal mechanisms for the M ≥ 3.5 events are located in the low b -value area of the deep region. The stress-tensor inversion results reveal a change in stress state from strike-slip to strike-slip/normal faulting . These findings imply that the stress state remains high and/or slightly decreased in the northwestern and southeastern parts of the deep region. These results and the findings of previous research on this earthquake sequence suggest that the likelihood of future large earthquakes along the southeastern part of the aftershock region should be considered relatively high.

Variations in Apparent Stress and b Value Preceding the 2010 Mw8.8 Bio-Bío, Chile Earthquake

We calculated apparent stresses for 70 earthquakes (MW ≥ 5.0) occurring in the aftershock region of the 2010 MW8.8 Bio-Bío earthquake from January 1990 to September 2019. We identified that the average apparent stress was approximately 0.487 MPa between January 1990 and December 2005 and approximately 1.063 MPa within the period from January 2006 to January 2010. The latter one is 2.2-fold greater than the former, representing a significant difference as determined by a z test, with a 99% confidence level. Moreover, we analyzed the temporal evolution of the apparent stress and found that apparent stress rapidly increased from 0.43 to 1.2 MPa during the pre-event period from March 2006 to the occurrence of the Bio-Bío MW8.8 mainshock, and this increased apparent stress was found to be significant at the 98% confidence level. Furthermore, we calculated the spatial distribution of the apparent stress in the study region and observed two higher-apparent-stress regions, within one of which the epicenter of the MW8.8 event was located. On the basis of the inverse correlation between b value and stress, the temporal evolution and spatial distribution of b values were calculated and compared with those of the apparent stress. The comparison showed that the b values decreased approximately 4 years before the occurrence of the mainshock, while the apparent stress increased substantially; for the region of lower b, the apparent stress is higher, and vice versa. Therefore, the inverse correlation between b value and stress is supported by the results obtained in the present study and can be probably considered as one of the precursors to great earthquakes.

Fractal Analysis to Study the Structural Distribution of Wenchuan Earthquake in China

A comprehensive study of fractal property applied in earthquakes is analyzed based on the aftershock of 2008 Wenchuan earthquake. Different fractal parameters are analyzed to study the magnitude, epicenter and hypocenter structural distributions in time or space. Theb-valueis found to be 0.86 closed to which is usually 1.0 observed worldwide. This indicates there is a relative abundance of small magnitude events than large ones in the studied range. The spatial correlation is calculated using correlation integral technique, indicating that epicenters are approaching a two-dimensional region and the aftershocks are uniformly distributed along the trend of the aftershock zone. The rate of the fall of aftershock activity with time reflects the decrease of stress is modestly slow. Temporal correlation is 0.59 for aftershocks of M >4.0, indicating a non continuous aftershock activity. Geometrical probability dimension reflecting epicenter clustering degrees of the region was also analyzed. Also the volume fractal dimension of the aftershock region has been calculated using the box-counting technique to study the hypocenter distributions. From the assessment of slip on different faults it is inferred that 67.9% displacement is accommodated on the primary fault and the remainder on secondary faults.