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Magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat. Such 'swarm' seismicity also migrates with time, and often exhibits a 'dog-bone'-shaped distribution. The largest earthquakes in swarms produce aftershocks that obey an Omori-type (exponential) temporal decay, but the duration of the aftershock sequences is drastically reduced, relative to normal earthquake activity. Here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion. A 1,000-fold increase in seismicity rate and a 1,000-fold decrease in aftershock duration occurred during the two-month-long dyke intrusion. We find that the seismicity rate is proportional to the calculated stressing rate, and that the duration of aftershock sequences is inversely proportional to the stressing rate. This behaviour is in accord with a laboratory-based rate/state constitutive law, suggesting an explanation for the occurrence of earthquake swarms. Any sustained increase in stressing rate--whether due to an intrusion, extrusion or creep event--should produce such seismological behaviour.

[1] CAS3D-2, a new three-dimensional (3-D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.

Analysis of global positioning system data shows that the rate of crustal deformations in the Tokai region of Japan, a seismic gap area, changed over the past 18 months. Kalman filtering analysis shows aseismic slip on the plate boundary in the western Tokai region centered on Lake Hamana, adjacent to the anticipated Tokai earthquake source area. The cumulative moment magnitude reaches 6.7 in June 2002 with a relative slip increase northeast of Lake Haman from January 2002. An existence of aseismic slip in the western Tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 Tonankai earthquake.

Abstract. Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (Mw=8.1) and 1946 Nankaido (Mw=8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span •50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post-1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satake's [ 1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high-slip regions, one off southeastern Shikoku Island (11 rn maximum) and the other offshore of Kii Peninsula (3 rn maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much longer than the average repeat time of•120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on troughparallel splay faults could significantly decrease the maximum slip to about 6 m. IntroductionThe distribution of slip on major plate boundary faults throughout the earthquake cycle has important implications for earthquake mechanics [e.g., Dmowska et al., 1996], recurrence estimation [e.g., Thatcher, 1990], and seismic hazard assessment [e.g., Nishenko, 1985]. Geodetic survey measurements provide an important source of data fbr estimating both seismic and aseismic fault slip. However, quantitative applications depend on determining slip uncertainties and spatial resolution on the fault plane as well as estimating the slip magnitude and its spatial distribution [Thatcher et al., 1997]. Convergent plate boundaries offer particular challenges in this regard because much of the subduction megathrust lies beneath the seafloor remote from geodetic networks, and as a result, resolution and slip uncertainties vary widely over the fault plane.In this paper we address these issues by analyzing geodetic measurements that constrain coseismic slip on the Nankai [Thatcher, 1984;Sagiya, 1995]. In this paper our interest is concentrated on the coseismic deformation caused by the 1944 Tonankai and the 1946 Nankaido earthquakes. Because of their close spacing in time, the slip distribution we estimate is a superposition of the slip from both events. Since some of the early surveys span long time intervals, the so-called coseismic movements, the simple differences between the posteart...

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SUMMARY Temporal change of deformation in northeastern Japan is clarified by continuous Global Positioning System (GPS) observations from 1995 April to 2002 March. The observed GPS velocity is approximately parallel to the direction of plate convergence on east and west plate boundaries of northeastern Japan and shows post‐seismic transient deformation around source regions of the Mw 7.8 1993 Hokkaido‐Nansei‐Oki and the Mw 7.7 1994 Sanriku‐Haruka‐Oki earthquakes. We interpret the source of the observed deformation as contemporary interplate coupling on the east subducting boundary to the Pacific Plate and the west collision boundary to the Amurian Plate. Using elastic dislocation theory, we inverted horizontal and vertical velocities of 212 GPS stations to estimate interplate coupling on both boundaries. The estimated coupling during 1995–2002 is spatially heterogeneous, however, it is temporally almost constant except for the region around the 1993 and 1994 earthquakes. After‐slip of the 1994 earthquake occurred over the coseismic rupture area and its downdip extension on the plate boundary for 0.3–1.3 yr after the earthquake. After‐slip continued only in the downdip extension for later periods and decayed with time. Weak coupling was recovered in the eastern part of the coseismic rupture area 3.3 yr after the earthquake. Interplate coupling on the Pacific Plate was strong in two regions, Miyagi‐Oki and Tokachi‐Oki. The west plate boundary is tightly coupled except for the source areas of three large earthquakes that occurred in 1964, 1983 and 1993. The apparent decoupling of the source areas of these earthquakes implies long‐term post‐seismic deformation as a result of viscoelastic relaxation in the subseismogenic lithosphere.

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