Locked Region Research Articles

Modeling short‐ and long‐term slow slip events in the seismic cycles of large subduction earthquakes

Slow slip events (SSEs) occur in the deeper extents of areas where large interplate earthquakes are expected in subduction zones, such as the Nankai region of Japan and the Cascadia region of North America. In the Nankai region, SSEs are divided into long‐ and short‐term SSEs, depending on their duration and recurrence interval. We modeled and examined the occurrence of long‐ and short‐term SSEs and changes in their behavior during the seismic cycles of large interplate earthquakes. In these numerical simulations we adopted a rate‐ and state‐dependent friction law with cutoff velocities and assumed that the distribution of pore fluid controls the recurrence interval of both long‐ and short‐term SSEs. The recurrence intervals of reproduced short‐term SSEs decrease during a long‐term SSE, as observed in western Shikoku, in the Nankai region. The recurrence intervals of both types of SSEs become shorter in the later stages of interseismic periods. Large interplate earthquakes nucleate between the region where SSEs occur and the locked region of the large earthquakes, as suggested from observations of the 1944 Tonankai earthquake. Our numerical results suggest that the stress buildup process in a seismic cycle affects the recurrence behavior of SSEs.

Focus on lasers and optics

Focus on lasers and optics

Aiming to Predict the Tokai Earthquake

Extensive studies utilizing data from a high-density observation network deployed in the Tokai region have brought about remarkable progress in understanding the subducting process of the Philippine Sea plate. Configuration of the Philippine Sea slab has been modeled using precisely determined hypocentral data (Yamazaki and Oida, 1985; Noguchi, 1996; Harada et al., 1998), and the locked region between the Philippine Sea and the Eurasian plates has been surmised on the basis of spatial variation of the density and focal mechanisms of microerathquakes (Matsumura, 1997). The coupling region has been also estimated by inversion analysis using GPS data (Sagiya, 1998, 1999). Although the result that suggests a large back-slip in the sea region to the south off Omaezaki does not seem to be consistent with the estimation of the locked region that is surmised to exist beneath the western coast of Suruga Bay, we think it is a great achievement to be able to discuss the coupling state on the basis of the observational data. Besides, simulation studies on subduction process modeling the Tokai region have developed considerably in recent years, indicating that there will appear intermediate-term and short-term precursors that should be detectable before a great interplate earthquake (Kato and Hirasawa, 1999). Three-dimensional simulations supply more realistic information about the phenomena that occur in the preparatory process of a great earthquake (Kuroki et al., 2001). Recent detailed analyses on the fault motion of the 1944 Tonankai and the 1946 Nankai earthquakes as well as acoustic sounding investigations on the upper crustal structure in their focal regions have afforded us useful information to assess the focal area of the coming Tokai earthquake. In order to utilize those scientific attainments obtained since the proclamation of the Large-Scale Earthquake Countermeasures Act in 1978, the Central Disaster Prevention Council has commenced to re-consider the focal model of the Tokai earthquake and to propose a new one.In spite of the remarkable progress in studies on tectonic processes in the Tokai region, the probability of a successful prediction of the Tokai earthquake seems not to have much increased in the last 20 years. At present we are not very optimistic concerning the possibility of the detection of immediate precursors compared to when the Earthquake Assessment Committee for the Areas under Intensified Measures against Earthquake Disasters (EAC) had been established. Experiments on rock fracture, analyses of seismograms on the commencement of fault motion, computer simulations on the repeating occurrence of an interplate large earthquake, and other relevant studies all suggest that immediate precursors would not be as large as we expected. Standing on such a recent viewpoint about the magnitude of precursors, the Japan Meteorological Agency has revised criteria to convene EAC in 1998. The new criteriasa was prescribed based upon the detection level of volumetric strainmeters. This revision is considered to express the intension that JMA will do its best to catch the immediate precursors.Some researchers have been presenting the ideas that recently observed seismic quiescence and crustal deformation in the Tokai region exhibit warnings that the Tokai earthquake is actually impending. The data and their interpretations, however, seem insfficient for the majority of the researchers to have a common opinion about the forecasting. The divergence of opinions concerning various kinds of changes in observational data obtained by the densely deployed network itself shows the difficulty of the prediction of the Tokai earthquake. We have not yet achieved a unified method to interpret the data or a scenario to determine how precursory phenomena will appear in sequence. It is very important to acquire more knowledge and know-how concerning these problems.

次の南海トラフ巨大地震に備えて 東海地域における近年の地殻変動及び地震活動の変化に関する数値シミュレーションによる検討

Recently, Tada (1996) and Matsumura (2000) reported that crustal deformation rate and seismicity pattern have interestingly changed in the Tokai area, central Japan. We formulate recurring earthquake model in the Tokai area based on the rate- and state dependent frictional law derived from laboratory experiments of rock friction and on the location of a seismically locked region. Using the model parameters based on previous studies, we estimate the time when the next Tokai Earthquake will occur and the location of plate interface by fitting simulated calculation to observed data. Some of the results explain the recent decrease in the rate of uplift change especially at the three observational points.We find out that the mechanism of microearthquakes is well explained by the rate change of stress field. The present model shows the vertical rotation of the axes of principal stresses and predicts the precursory change of seismic activity in the region, where the distance from the assumed trench along the relative plate motion direction is about 150 km, in the depth range of 10-20 km. The present seismicity with P axis almost horizontal will become more inactive several years before the occurrence of a great earthquake, and/or the direction of P axis rotates abruptly about 80 degrees so as to be orthogonal to plate interface.

Distribution of seismic coupling on the subducting plate boundary in northeastern Japan inferred from GPS observations

Many Global Positioning System (GPS) stations of the Geographical Survey Institute and Tohoku University in northeastern Japan observed clear coseismic and postseismic deformations associated with the M7.5 1994 Far Off Sanriku earthquake. Applying the geodetic inversion algorithm given by Yabuki and Matsu'ura (Geophys. J. Int., 1992, 109, 363–376) to the observed GPS data, we estimate in this paper the spatial distributions of the coseismic and postseismic slip on the boundary between the subducting Pacific plate and the overriding continental plate. It is noted that the coseismic slip estimated in this paper denotes the slip for a period between the day before and the day after the main shock. The inversion for coseismic slip shows that the main shock was thrust faulting typical for interplate earthquakes, which is consistent with the results of other seismological studies. The short-term postseismic deformation (STD) observed for a period of ca. 10 days between the main shock and its largest aftershock shows that the initially high deformation-rate was lowered rather rapidly in comparison with the subsequent long-term deformations (LTD) lasting longer than 1 year, which decreased at an almost constant low rate. The observed data of STD can be explained by postseismic slip distributed approximately on the same region as the faulting area of the main shock, where the moment release during this period is ca. 16% of that estimated for the coseismic slip. The region of the sliding causing LTD is found to have significantly expanded southward and downward (or westward) on the plate boundary. The moment released by the postseismic sliding for nearly 1 year amounts approximately to 80% of that released by the coseismic slip in addition to the contribution from uniform steady sliding due to the relative plate motion. Furthermore, we find from the inversion analysis a locked region to the south of the long-term postseismic sliding region on the plate boundary. The amount of nominal back-slip estimated is so large that the locked region of the plate boundary is considered to have been tightly coupled for the year of 1995. This study demonstrates that the GPS observation only for ca. 1 year can distinguish the decoupled region due to the postseismic sliding around the source area of the main shock from the coupled one of the plate boundary.

Spatial variations in present‐day deformation, Kenai Peninsula, Alaska, and their implications

From four years of Global Positioning System (GPS) measurements, we find significant spatial variations in present‐day deformation between the eastern and western Kenai Peninsula, Alaska. Sites in the eastern Kenai Peninsula and Prince William Sound move to the NNW relative to North America, in the direction of Pacific‐North America relative plate motion. Velocities decrease in magnitude from nearly the full plate rate in southern Prince William Sound to about 30 mm/yr at Seward and to about 5 mm/yr near Anchorage. In contrast, sites in the western Kenai Peninsula move to the SW, in a nearly trenchward direction, with a velocity of about 20 mm/yr. The data are consistent with the shallow plate interface offshore and beneath the eastern Kenai and Prince William Sound being completely locked or nearly so, with elastic strain accumulation resulting in rapid motion in the direction of relative plate motion of sites in the overriding plate. The velocities of sites in the western Kenai, along strike to the southwest, are opposite in sign with those predicted from elastic strain accumulation. These data are incompatible with a significant locked region in this segment of the plate boundary. Trenchward velocities are found also for some sites in the Anchorage area. We interpret the trenchward velocities as being caused by a continuing postseismic transient from the 1964 great Alaska earthquake. There may be significant along‐strike differences in the long‐term behavior of the plate interface between the western and eastern Kenai, based on roughly coincident boundaries in the coseismic slip distribution, cumulative postseismic uplift, present‐day plate coupling, and stress field. The present postseismic response appears to generate purely trenchward motion, suggesting a creep process that is purely dip slip. Our observations suggest that postseismic processes after the largest earthquakes can influence patterns of deformation for decades after the event.

Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Subduction of the Pacific plate in the northern South Island and southernmost North Island of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25° clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low‐angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low‐angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.

Locking characteristics of Fabry–Perot semiconductor laser oscillators with side-mode injection

A model to evaluate frequency locking by means of a power injection near a side mode of the free-running frequency is presented. Semiconductor optical nonlinearities govern the locking problem; they are represented by nonlinear coefficients that take into account both carrier fluctuations and all relevant fast phenomena. Field equations are derived from coupled-mode theory, accounting for longitudinal variations by means of appropriate mean values. Examples of results show both the single locked frequency-stable operation region and the case when the injected signal and the free-running field coexist.

STRESS AND DEFORMATION FIELDS ABOUT A LOCKED FAULT IN DEPTH

In this paper the stress and deformation fields about a locked fault in depth have been imitated using 3-dimensional finite element method and their time-space variation features have been analysed. The authors consider tentatively that the horizontal stresses about an active fault decrease slightly near the fault. The distributions of stress increasing and decreasing regions due to the locking of a fault are skew-symmetrical about the fault. The stresses about the locked region are increased in a larger range, where great elastic strain energy is accumulated and a stronger earthquake is more easily developed than at the ends of the fault. Reverse variations of crustal stress and deformation may be related to the different stages of the seismogeny of an earthquake focus.

Seismicity, structure and tectonics of the Wellington region, New Zealand

The Wellington region lies on the border between the Pacific and Australian plates, the former being subducted. The plate interface lies at shallow depth under the region (10–60 km) so that detailed and long-term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible. Hypocentres for events recorded by a 12 station seismic network for the period 1978–1982 (7231 events) have been refined by the use of a three-dimensional velocity model derived from a simultaneous inversion of arrival-time data from 129 local events for both hypocentres and velocity. The hypocentres show that the subducting Pacific Plate is defined by a NW dipping zone of relatively intense activity. The plate interface, as given by the upper envelope of this activity, is offset vertically, by 7 km on average, along a NW—SE line through Cook Strait parallel to the direction of absolute motion of the Pacific plate rather than to the direction of convergence. The strike of the interface at deeper levels also changes across this line. This distortion may partly reflect effects of the transition to continent-continent-type convergence further south. Within the subducting Pacific Plate the seismic activity defines two parallel zones separated by about 20 km, the lower zone being much less active and perhaps merging with the upper zone at a depth of 80–90 km. Composite focal mechanisms of events in the upper zone imply predominantly down-dip tension with a compression axis perpendicular to the plate interface. The mechanism in the lower zone also implies down-dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate. These two types of mechanism correlate well with those found previously for events at depths of 150 km or more. Thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region, the lithosphere acting as a stress guide. There is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface. The seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust, which here is probably considerably thicker than normal. There is no evidence for a substantial increase in velocity down-dip as would be expected if a pervasive basalt to eclogite phase-change were taking place. The velocity deeper in the subducting lithosphere suggests the presence of anisotropy as often observed in the oceanic mantle. Within the overlying Australian Plate (here taken as including the ‘accretionary bordera’) the seismic activity can be separated into three regions whose boundaries are related to both surface geology and structure in the underlying plate interface. Shallow (0–5 km) seismic velocities in the Australian Plate vary substantially in accordance with the surface rock type. They are lowest, especially for S-waves, in the SE region, commonly described as part of an accretionary border. However, at deeper levels in this region the velocity is not especially low indicating that no substantial amounts of accreted sediments have been incorporated. Thus the description of this region as a compressed and deformed pre-subduction borderland of the Australian Plate is perhaps more accurate. Composite focal mechanisms for activity in the Australian Plate imply largely strike-slip faulting with an east-west oriented compression axis, parallel to the direction of plate convergence. This indicates that the plate interface is currently locked but probably down to depths of only 20 km where there is a concentration of activity (seismic ‘front’) in the subducted plate. The lack of a significant number of events with interplate thrust mechanisms in the locked region could reflect very strong coupling due to the buoyancy of the thicker than normal subducted crust and/or a late stage in the ‘seismic cycle’. The seismic evidence, combined with geodetic observations, indicates that strain is now accumulating in the region, probably to be released by faulting within the Australian Plate (such as on the Wairarapa fault which slipped during a magnitude 8+ event in 1855) or on the plate interface itself. The offset found in the plate interface might serve to limit the rupture areas of such events, as may have been the case in 1855. It might also be the focus of possible precursory activity. These results will provide a basis for long-term monitoring of the seismicity of the Wellington region so that comparisons of activity throughout the seismic cycle can be made.

Stressing of locked patches along a creeping fault

An analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere. The model consists of strike-slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress. The fault zone has an inhomogeneous strength distribution both depth-wise and along strike. It is modeled as being composed of locked patches and freely slipping parts treated as cracks. The solution is given with the use of the “line-spring” model which analyzes the problem by thickness-averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary. As a boundary condition, thickness-averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section. The analysis indicates that the slip distribution along creeping parts of the fault, as well as the stress distribution along locked patches, depends strongly on the geometry of these zones. The model is used to examine stress concentrations associated with a slip-deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region. It is also used to simulate slip and stressing processes associated with the creeping portion of the San Andreas fault in central California, between the presently locked zones of the great 1906 and 1857 ruptures, and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic Parkfield earthquakes. Near-fault creep and broadscale displacement data along the fault since the 1966 Parkfield earthquake and inferences from seismicity distributions are used. Limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the Parkfield locked patch, but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone. An effective remote stressing rate of order 0.3 × 10−6 × shear modulus/yr is inferred, together with a less well constrained 30 to 40 km lithospheric thickness, for consistency with the displacement data. Results enable estimates of stress accumulation along the locked 1857 rupture zone and the build-up of fracture energy release capability (of order 107 J/m2 in 150 yr) at its lower margin.

Experimental Investigation of Rolling Contact

A particular case of rolling contact, that for cylinders having different elastic constants and transmitting a tangential force in the direction of rolling, was experimentally investigated. A steel cylinder, driving a rubber-covered cylinder, was instrumented for the quantitative, as well as qualitative, investigation of the action taking place in the contact area. The results indicate the extent of the locked region at the leading edge of the contact area, the development of the slip region, and the magnitude of the constant-rate slip at the trailing edge of the contact area.

The Effect of a Tangential Contact Force Upon the Rolling Motion of an Elastic Sphere on a Plane

Abstract The motion and deformation of an elastic sphere rolling on an elastic plane under a normal contact pressure N have been studied for the case where a tangential force T is also sustained at the point of contact. Provided that T < μN (μ = coefficient of friction), the sphere rolls without sliding but exhibits a small velocity relative to the plane, termed “creep.” Following the work of Mindlin and Poritsky, it is shown that creep arises from slip over part of the area of contact, and further, that this slip takes place toward the trailing edge of the contact area. On the assumption of a locked region in which no slip occurs, of circular shape, tangential to the circle of contact at its leading point, surface tractions are found which satisfy the condition of no slip within the locked region and are approximately consistent with the laws of friction in the slip region. The variation of creep velocity with tangential force is thereby determined. Experimental measurements of the creep of a steel ball rolling on a flat steel surface are in reasonable agreement with the theoretical results.