Search

Abstract The traditional approach for estimating the b-value of the Gutenberg–Richter law, which is posited to inversely correlate with differential stress, has historically relied on the maximum likelihood technique, utilizing data from earthquakes exceeding a magnitude cutoff, Mc. This traditional approach is significantly influenced by the value of Mc, leading to extensive research focused on methods for determining Mc with greater accuracy. However, a recent study introduced a novel method based on the frequency distribution of magnitude difference, termed the b-positive method. This innovative method could enable more robust b-value estimations, even in scenarios where Mc may vary spatially and temporally. Our study concentrated on analyzing aftershocks, related to 25 magnitude-7 class earthquakes surrounding the Japanese archipelago. We estimated the b-values using both the goodness-of-fit test for Mc, a traditional approach, and the b-positive method. The aftershock data were examined over two distinct time frames: the initial 10 days following each mainshock and an extended period of 1000 days. Our findings indicated that the estimates produced by the b-positive method showed negligible variation between the 10-day and 1000-day aftershock periods (correlation coefficient of 0.95), whereas the traditional approach tended to yield lower b-values for the 10-day aftershocks compared to those from the 1000-day period. Variations in b-values, when analyzed using the traditional approach, could be inaccurately ascribed to temporal fluctuations in differential stress that may not actually be present. The b-positive method offers a vital solution to prevent these erroneous interpretations, serving as an essential alternative. Graphical abstract

Abstract Earthquakes (mainshocks) trigger sequences of aftershocks, the frequency of which diminishes following a power-law decay, while the spatial domain of these aftershocks extends logarithmically over time. The delineation of the aftershock zone can be modulated by variables beyond the magnitude of the mainshock, encompassing the location of the fault (whether the fault is at a plate boundary), the depth at which the event occurs, and the prevailing local stress conditions. Here, we evaluate the expansion rate of aftershock zones by analyzing earthquakes of magnitude-7 class in the vicinity of the Japanese archipelago. Prior studies have offered approximate assessments of expansion rates; however, our approach involves the utilization of a straightforward algorithm for the automated estimation of this metric, facilitating the compilation of a catalog. Across the dataset, no pronounced correlations were discerned between the expansion rate and other examined parameters. Yet, an inverse relationship is identified between the expansion rate and the b value of aftershocks for mainshocks occurring at plate boundaries. This observation suggests that the expansion rate of aftershock zones predominantly mirrors the stress field following the mainshock. Such a pattern is not detected in mainshocks occurring within the plate's interior. While the expansion rate of aftershock zones is likely influenced by various factors, aftershock zones may expand more rapidly with higher differential stress in areas surrounding hypocenters of major interplate earthquakes of magnitude 8 or 9.

Abstract Geodetic and seismological observations have shown discrepancies between azimuths of maximum contraction (strain rate) and maximum compression (stress). These discrepancies can be the results of the superposition of localized or transient mechanical processes such as fault coupling during seismic cycles. Rich sets of recent geophysical data allow us to conduct spatiotemporal imaging of the discrepancies. Here, we estimate the spatiotemporal evolution in the strain-rate fields of the Japanese Islands with optimized smoothing distances from 1997 to 2021 using Global Navigation Satellite System (GNSS) data, and investigate how the maximum contraction axes of horizontal strain rates differ from those of horizontal stress based on earthquake focal mechanisms. Several characteristic results are observed for each region within the Japanese Islands. Both azimuths of the strain rates and stress differ by more than 60° over hundreds of kilometers from the Kanto region to along the Nankai Trough, related to seismotectonics due to the dual subduction of the Philippine Sea plate and the Pacific plate beneath the Japanese Islands. The differences in the azimuths imply the effect of the very long-term stable subduction of the Pacific plate. We find that the azimuthal differences tend to be small along tectonic zones with active inland earthquakes and high strain rates on the back-arc sides. We also find that the 2011 off the Pacific coast of Tohoku earthquake caused notable azimuthal differences in the strain rates and the stress in the Tohoku region. The strength of fault may cause lower response sensitivity of seismological stress to major earthquakes than geodetic strain rate. Our dataset has wide spatiotemporal coverage and can serve as a basis for further research, for example, to estimate the current fault conditions during seismic cycles. Graphical Abstract

Abstract We infer the postseismic deformation mechanisms following the 2011 Tohoku megathrust earthquake via an analysis of onshore geodetic observations. We focus on the temporal decay characteristics of postseismic deformation using continuous time-series data at time scales that span many orders of magnitude by means of high-rate GNSS data. Our analysis indicates Omori-like power-law decay of the horizontal ground displacement rates, with p-value (0.69) that is significantly smaller than that of the aftershock occurrence (~ 1). This slow decay implies a (non-Maxwellian) viscoelastic relaxation mechanism other than afterslip since immediately after the mainshock, which is inferred using only onshore geodetic data. Spatial distribution of the Omori parameters implies that the postseismic deformation will continue over 100 years in a down-dip area of the northern part of the mainshock fault. The decay characteristics of vertical displacement rates are also almost Omori-like, but data deviation from the fitting line several 100 days after the mainshock might reflect the change of the dominant mechanism of the postseismic deformation. This multi-time scale geodetic approach will provide important constraints for future viscoelastic models of Earth’s interior. Rapid mantle flow with power-law creep explains deformation after the 2011 Tohoku mega-quake.

Abstract In contrast to surface observation of complex earthquake faults, simple fault-plane models are frequently sufficient to fit geophysical data of seismic wave or crustal deformation. However, earthquake occurrence should be strongly affected by fault interaction between separate faults. To understand its mechanics, rate-and-state frictional models of separate faults within elastic continuum would be important. We construct a conceptual out-of-plane model of aligned parallel faults (causing stress shadows) in a two-dimensional elastic body, assuming both “aging” and “slip” type of state evolution laws. We find that close fault conditions (roughly for smaller inter-fault distance than fault length) favor higher-speed slip events than distant fault conditions. In a practical sense, the elastic interaction only affects faults that are hosting slow events. This point does not depend on the state evolution laws (aging or slip). Our result implies that the parallel distribution of the frictional faults contributes occurrence of relatively fast slip in a slow-slip region.

Abstract The Gravity Recovery and Climate Experiment (GRACE) has monitored global gravity changes since 2002. Gravity changes are considered to represent hydrological water mass movements around the surface of the globe, although fault slip of a large earthquake also causes perturbation of gravity. Since surface water movements are expected to affect earthquake occurrences via elastic surface load or pore-fluid pressure increase, correlation between gravity changes and occurrences of small (not large) earthquakes may reflect the effects of surface water movements. In the present study, we focus on earthquakes smaller than magnitude 7.5 and examine the relation between annual gravity changes and earthquake occurrences at worldwide subduction zones. First, we extract amplitudes of annual gravity changes from GRACE data for land. Next, we estimate background seismicity rates in the epidemic-type aftershock sequence model from shallow seismicity data having magnitudes of over 4.5. Then, we perform correlation analysis of the amplitudes of the annual gravity changes and the shallow background seismicity rates, excluding source areas of large earthquakes, and find moderate positive correlation. It implies that annual water movements can activate shallow earthquakes, although the surface load elastostatic stress changes are on the order of or below 1 kPa, as small as a regional case in a previous study. We speculate that periodic stress perturbation is amplified through nonlinear responses of frictional faults.

Abstract To clarify the Holocene uplift history of the Izu Islands, Japan, we analyze the elevations and 14C ages of emerged sessile assemblages measured by accelerator mass spectrometry (AMS) on the islands of Niijima, Jinaijima, Shikinejima, and Kouzushima, on the northern Zenisu Ridge. The results suggest that uplift events took place after AD 1950 (uplift event 1), during AD 786–1891 (uplift event 2), during AD 600–1165 (uplift event 3), and during AD 161–686 (uplift event 4), although uplift events 3 and 4 are identified only at Kouzushima. The minimum amount of uplift was estimated to be 0.4–0.9 m in uplift event 1, 2.4–2.7 m in uplift event 2, 3.6 m in uplift event 3, and 3.3–8.1 m in uplift event 4. These events could have been caused by volcanic activity or strong earthquakes. There also remains the possibility that uplift event 2 was caused by the AD 1498 Meio earthquake; in contrast to the previous interpretation, the ages of uplift events are significantly older than the earthquake, based on conventional (non-AMS) methods.

Correction In the publication of this article (Kitamura et al., 2017), there was an error in Fig. 11 which incorrectly indicated ‘Serpulidae sp. should instead read ‘Serpulidae.’ Italicized. It should not have indicated figures ‘Specimen of an emerged barnacle’, ‘Study area’ and their captions and the caption of the Fig. 11 itself was indicated twice. The error: