Characteristics of short‐term slow slip events estimated from deep low‐frequency tremors in Shikoku, Japan
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Abstract:
We have investigated the characteristics of short‐term slow slip events (SSEs), recurrence interval and size distribution, and the slip rate at the transition zone on the plate interface beneath the Shikoku region, Japan, using nonvolcanic deep low frequency (DLF) tremors. On the basis of a proportional relationship between the seismic moment of SSE observed geodetically and the total size of DLF tremors of the corresponding episode, we estimated the seismic moment due to the slip on the plate interface from the DLF tremors and a temporal variation in the cumulative seismic moment. The recurrence interval of major short‐term SSEs is ∼6 months in the western area and 3 months in the central and the eastern areas. The size distribution of short‐term SSEs as well as DLF tremors is approximated by an exponential law rather than by a power law, showing a different scaling for regular earthquakes. The average slip rate at the transition zone estimated from the cumulative seismic moment with time of SSEs is 4.2 cm/yr, 3.3 cm/yr, and 4.9 cm/yr in the western, central, and eastern areas, respectively. These values compensate for the difference between the convergence rate at the trench and the slip deficit rate at the transition zone of the subducting Philippine Sea plate. In other words, the slip rate estimated from the DLF tremors provides a constraint of the slip deficit rate at the transition zone on the plate interface.Keywords:
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In this study we analyse records from the 'Les Saintes' seismic sequence following the Mw= 6.3 earthquake of 2004 November 11, which occurred close to Guadeloupe (French West Indies). 485 earthquakes with magnitudes from 2 to 6, recorded at distances between 5 and 150 km are used. S-waves Fourier spectra are analysed to simultaneously determine source, path and site terms. The results show that the duration magnitude routinely estimated for the events that occurred in the region underestimate moment magnitude by 0.5 magnitude units over the whole magnitude range. From the inverted seismic moments and corner frequencies, we compute Brune′s stress drops. We show that stress drops increase with increasing magnitude. The same pattern is observed on apparent stresses (i.e. the seismic energy-to-moment ratio). However, the rate of increase diminishes at high magnitudes, which is consistent with a constant stress drop model for large events. Using the results of the inversions, we perform ground motion simulations for the entire data set using the SMSIM stochastic simulation tool. The results show that a good fit (s= 0.25) with observed data is achieved when the source is properly described by its moment magnitude and stress drop, and when site effects are taken into account. Although the magnitude-dependent stress drop model is giving better results than the constant stress drop model, the interevent variability remains high, which could suggest that stress drop depends on other parameters such as the depth of the hypocentre. In any case, the overall variability is of the same order of magnitude as usually observed in empirical ground motion prediction equations.
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The 1700 Cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in Japan, computed coseismic seafloor deformation for hypothetical ruptures in Cascadia, and tsunami modeling in the Pacific Ocean. Reports of damage and flooding show that the 1700 Cascadia tsunami reached 1–5 m heights at seven shoreline sites in Japan. Three sets of estimated heights express uncertainty about location and depth of reported flooding, landward decline in tsunami heights from shorelines, and post‐1700 land‐level changes. We compare each set with tsunami heights computed from six Cascadia sources. Each source is vertical seafloor displacement calculated with a three‐dimensional elastic dislocation model. For three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip; for the other sources, ruptures of ordinary width extend 360–670 km. To compute tsunami waveforms, we use a linear long‐wave approximation with a finite difference method, and we employ modern bathymetry with nearshore grid spacing as small as 0.4 km. The various combinations of Japanese tsunami heights and Cascadia sources give seismic moment of 1–9 × 10 22 N m, equivalent to moment magnitude 8.7–9.2. This range excludes several unquantified uncertainties. The most likely earthquake, of moment magnitude 9.0, has 19 m of coseismic slip on an offshore, full‐slip zone 1100 km long with linearly decreasing slip on a downdip partial‐slip zone. The shorter rupture models require up to 40 m offshore slip and predict land‐level changes inconsistent with coastal paleoseismological evidence.
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Previous work of Rudnicki and Kanamori, which used collinear crack solutions to examine quantitatively the effects of fault slip zone interaction on moment, stress drop, and strain energy release, is extended by considering the effects of interaction between different size slip zones. The calculations demonstrate that the presence of a large preexisting slip zone can substantially amplify the low frequency seismic signal due to slip on a nearby smaller slip zone. For example, if the length of the larger slip zone is l , that of the smaller is l /10, and the distance between the nearest ends of the zones is l /100, the seismic moment due to slip on the larger zone induced by slip on the smaller zone is about 5 times that due to slip on an isolated zone of length l /10. Furthermore, because of the interaction between slip zones, the moment due to slip on the smaller zone is about 2.5 times the value for an isolated zone of the same size and subjected to the same effective stress. Conversely, estimates of stress drop based on the measured value of the moment and the total length on which slip occurs substantially underestimate the actual stress drop. Although the stress drop on the larger slip zone is zero, the induced slip there does contribute to W O , the difference between the strain energy change and the frictional work. In fact, it is demonstrated that W O = τ e M /2μ where τ e is the effective stress, M is the actual value of the seismic moment, and μ is the shear modulus.
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Summary Intensity is a basic parameter for assessment historical seismicity - realized until the instrumental period. The relation between intensity and seismic moment magnitude allows the creation of a homogeneous catalog. The homogeneous catalog provides compatibility of the input seismological data and allow reliable estimation of the energy distribution of earthquakes - an important stage in seismological research and essential for seismic hazard assessment. In this study are analyzed 92 earthquakes with magnitude above 4.0 (M>4.0), which occurred in space window 37.0° – 45.0° N; 21.0° – 30.0° E, during the time period 1912 – 2018 and the coefficients of the linear regression MW=MW(I0/Imax) are evaluated.
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The December 26, 2004 Sumatra–Andaman Island earthquake, which ruptured the Sunda Trench subduction zone, is one of the three largest earthquakes to occur since global monitoring began in the 1890s. Its seismic moment was M 0 = 1.00 × 1023–1.15 × 1023 Nm, corresponding to a moment-magnitude of M w = 9.3. The rupture propagated from south to north, with the southerly part of fault rupturing at a speed of 2.8 km/s. Rupture propagation appears to have slowed in the northern section, possibly to ∼2.1 km/s, although published estimates have considerable scatter. The average slip is ∼5 m along a shallowly dipping (8°), N31°W striking thrust fault. The majority of slip and moment release appears to have been concentrated in the southern part of the rupture zone, where slip locally exceeded 30 m. Stress loading from this earthquake caused the section of the plate boundary immediately to the south to rupture in a second, somewhat smaller earthquake. This second earthquake occurred on March 28, 2005 and had a moment-magnitude of M w = 8.5.
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Abstract During large slow slip events, tremor sometimes propagates in the reverse along‐strike direction for a few hours, at speeds 10 to 40 times faster than the forward propagation. We examine the aseismic slip that underlies this rapidly propagating tremor. We use PBO (Plate Boundary Observatory) borehole strainmeter data to search for variations in the slow slip moment rate during 35 rapid tremor reversals (RTRs) that occurred beneath Vancouver Island. The strain records reveal that, on average, the strain rate increases by about 100% ( ) during RTRs. Given the Green's functions expected for slip in the RTR locations, these strain rate increases imply 50 to 130% increases in the aseismic moment rate. The median moment released per RTR is between 8 and 21% of the daily slow slip moment, equivalent to that of a M W 5.0 to 5.1 earthquake. By combining the RTR moments with the spatial extents suggested by tremor, we estimate that a typical RTR has peak slip of roughly one‐sixth of the peak slip in the main slow slip event, near‐front slip rate of a few to ten times the main front slip rate, stress drop around half the main event stress drop, and strain energy release rate around one‐tenth that of the main front. Our observations support a picture of RTRs as aseismic subevents with high slip rates but modest strain energy release. RTRs appear to contribute to but not dominate the overall slow slip moment, though they may accommodate most of the slip in certain locations.
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