Earthquake Fault Zone Research Articles

Microscale anatomy of the 1999 Chi‐Chi earthquake fault zone

Two TCDP boreholes A and B were drilled in the northern part of the Chelungpu thrust fault where the Chi‐Chi earthquake (21 September 1999, Mw 7.6) showed large displacement, low ground acceleration, and high slip velocity. In this paper, we describe the microstructures of the Chi‐Chi Principal Slip Zone (PSZ) within black gouges localized at 1111 m depth in Hole A and at 1136 m depth in Hole B. In the FZA1111 the PSZ is a 2 cm‐thick isotropic clay‐rich gouge which contains aggregates formed by central clasts coated by clay cortex (clay‐clast aggregates (CCAs)) and fragments of older gouges segregated in the top third of the PSZ. In FZB1136 the PSZ is 3 mm thick and is characterized by a foliated gouge displaying an alternation of clay‐rich and clast‐rich layers. The presence of CCAs, plucked underlying gouge fragments, gouge injections, and the occurrence of reverse grain size segregation of large clasts in the FZA1111 isotropic gouge suggest that the gouge was fluidized as a result of frictional heating and thermal pressurization. The foliated gouge in FZB1136 may be one locus of strain localization and related heat production. Small calcite veins present above the isotropic FZA1111 PSZ gouge and, characterized by an increasing strain with increasing distance away from the PSZ, are attributed to coseismic fluid escape from the pressurized gouge. The observed microstructures are interpreted in view of their seismic implications for the Chi‐Chi earthquake in terms of slip weakening mechanisms by thermal pressurization, gouge fluidization, coseismic fluid distribution, and postseismic slip. Above the PSZ, several layers of compacted gouges containing deformed CCAs and gouge fragments correspond to several PSZ of past earthquakes similar to the Chi‐Chi earthquake and display a fault‐parallel cleavage resulting from a low strain rate pressure solution deformation mechanism that may be correlated to the interseismic periods.

Survey of the Shallow Structure at the Northeastern end of the Wenchuan Earthquake Fault Zone by Artificial Seismic Profiling

AbstractThe causative fault zone is sensitive to the post‐seismic stress state. We can get the fault healing process by monitoring temporal variations of seismic wave velocity in the fault zone. In order to learn the shallow structure at the northeastern end of the Wenchuan earthquake fault zone, we carried out the shallow structure survey by artificial seismic sources. The strong shallow reflectors and complex structure produced a complicated wave field and the low signal to noise ratio data. Based on separated surface, reflection and refraction waves by f‐k transform, we imaged the subsurface structure by refraction common middle point imaging, surface wave velocity inversion, and reflection wave stack imaging. An integrated interpretation was done combing the three imagine data and the shallow faults and velocity distribution were given. This work is helpful to post‐seismic emergency monitoring of the earthquake fault zone.

Structural features of the subducting slab beneath the Kii Peninsula, central Japan: Seismic evidence of slab segmentation, dehydration, and anisotropy

Seismic evidence of lithospheric slab segmentation, dehydration, and anisotropic rocks beneath the Kii Peninsula, central Japan, is detected with receiver function (RF) analyses. By stacking radial and transverse RFs with moveout perturbations, we estimate the downdip azimuth, dip angle, and depth of the slab Moho beneath each seismic station. Comparing the downdip azimuth to the slab convergence azimuth, we can classify the slab into three segments. The segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity. The slab Moho lies just above the intraslab seismicity in the southwestern Kii Peninsula. Slab Moho lies deeper at the eastern part of the peninsula, at the bottom of seismicity. This suggests that earthquakes occur only within the slab mantle under the southern Kii Peninsula. The dip angle of slab Moho becomes steeper at 40‐ to 50‐km depth, perhaps caused by dehydration transformation of basalt to eclogite. At some stations located in the western part of the Kii Peninsula, RF‐estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity. Since the local seismicity distribution reflects well the larger‐scale geometry of the slab, the RF‐estimated dip angle may imply anisotropic wave speeds within strongly sheared and/or hydrated media around the slab Moho. These detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the Nankai region.

Stabilized low-order finite elements for coupled solid-deformation/fluid-diffusion and their application to fault zone transients

Finite element simulations of coupled solid-deformation/fluid-diffusion occurring in earthquake fault zones often require high-fidelity descriptions of the spatial and temporal variations of excess pore water pressure. Large-scale calculation of the coupled fault zone process is often inhibited by the high-order interpolation of the displacement field required to overcome unstable tendencies of the finite elements in the incompressible and nearly incompressible limit. In this work we utilize a stabilized formulation in which the balance of mass is augmented with an additional term representing a stabilization to the incremental change in the pressure field. The stabilized formulation permits equal-order interpolation for the displacement and pore pressure fields and suppresses pore pressure oscillations in the incompressible and nearly incompressible limit. The technique is implemented with a recently developed critical state plasticity model to investigate transient fluid-flow/solid-deformation processes arising from slip weakening of a fault segment. The accompanying transient pore pressure development and dissipation can be used to predict fault rupture and directivity where fluid flow is an important driving force.

Implementing the California Earthquake Fault Zoning Act: A Proposal for Change

Research Article| February 01, 2008 Implementing the California Earthquake Fault Zoning Act: A Proposal for Change CHRISTOPHER J. SEXTON CHRISTOPHER J. SEXTON 1GeoDynamics, Inc., 558 St. Charles Drive, Suite 116, Thousand Oaks, CA 91306 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2008) 14 (1): 43–51. https://doi.org/10.2113/gseegeosci.14.1.43 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation CHRISTOPHER J. SEXTON; Implementing the California Earthquake Fault Zoning Act: A Proposal for Change. Environmental & Engineering Geoscience 2008;; 14 (1): 43–51. doi: https://doi.org/10.2113/gseegeosci.14.1.43 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The California Alquist-Priolo (AP) Earthquake Fault Zoning Act of 1972 prohibits building structures for human occupancy over active faults. This directive seems simple but in practice has proven difficult to follow. The language of the Act and the related policies and criteria allow for different interpretations. Some believe that the language provides a degree of latitude for the professional geologist to make judgments regarding the hazard associated with minor faults. Others believe the language requires stringent criteria with no such latitude and that all faults within the AP zones are potential hazards that must be avoided in the absence of clear evidence precluding Holocene ground rupture. The AP Act has been implemented for many years using the less stringent criteria. The GeoHazards Committee of the California State Mining and Geology Board recently recommended that the language be interpreted to mean that all faults within an AP zone are presumed to be active until demonstrated otherwise. This interpretation presents difficult challenges to consultants, reviewers, city officials, and developers. The stringent criteria probably cannot be met where tract developments are proposed in complex fault zones. Public safety and welfare is better served by a mitigation program that combines avoidance of well-defined faults with engineered mitigation applied to all other areas where construction is proposed within an AP Zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Self-modulation of seismic waves in the subsurface soil

Nonlinear effects of modulation instability of seismic waves during their propagation in subsurface soils are studied in numerical experiments. The results show that self-modulation and generation of subharmonics take place at high amplitudes of seismic waves in cases of their resonance intensification in soil layers and precede the chaotic stage in the case of further increase in amplitudes. Generation of these effects is related to dispersion of seismic wave propagation velocities in subsurface soils due to their nonlinear response. The nonlinearity of soils during strong earthquakes remains a debatable issue in seismology. Interactions and self-effects of seismic waves (see, for instance, [1, 2] and others), attenuation of resonance frequency of soil layer oscillations, an increase in nonlinear absorption, and a decrease in elastic moduli (see [3, 4] and others) were studied in field, laboratory, and numerical experiments and based on earthquake records. This article presents results of the numerical simulation of modulation instability observed during intense seismic wave propagation in soil layers. We used numerical models of soil behavior in fault zones of strong earthquakes based on the records of vertical groups [5]. In particular, models of soil behavior in fault zones

Near-Fault Peak Ground Velocity from Earthquake and Laboratory Data

We test the hypothesis that peak ground velocity (pgv) has an upper bound independent of earthquake magnitude and that this bound is controlled primarily by the strength of the seismogenic crust. The highest pgvs, ranging up to several meters per second, have been measured at sites within a few kilometers of the causative faults. Because the database for near-fault pgv is small, we use earthquake slip models, laboratory experiments, and evidence from a mining-induced earthquake to investigate the factors influencing near-fault pgv and the nature of its scaling. For each earthquake slip model we have calculated the peak slip rates for all subfaults and then chosen the maximum of these rates as an estimate of twice the largest near-fault pgv. Nine slip models for eight earthquakes, with magnitudes ranging from 6.5 to 7.6, yielded maximum peak slip rates ranging from 2.3 to 12 m/ sec with a median of 5.9 m/sec. By making several adjustments, pgvs for small earthquakes can be simulated from peak slip rates measured during laboratory stick- slip experiments. First, we adjust the pgv for differences in the state of stress (i.e., the difference between the laboratory loading stresses and those appropriate for faults at seismogenic depths). To do this, we multiply both the slip and the peak slip rate by the ratio of the effective normal stresses acting on fault planes measured at 6.8 km depth at the KTB site, Germany (deepest available in situ stress measurements), to those acting on the laboratory faults. We also adjust the seismic moment by replacing the laboratory fault with a buried circular shear crack whose radius is chosen to match the experimental unloading stiffness. An additional, less important adjustment is needed for experiments run in triaxial loading conditions. With these adjustments, peak slip rates for 10 stick-slip events, with scaled moment magnitudes from −2.9 to 1.0, range from 3.3 to 10.3 m/sec, with a median of 5.4 m/sec. Both the earthquake and laboratory results are consistent with typical maximum peak slip rates averaging between 5 and 6 m/sec or corresponding maximum near-fault pgvs between 2.5 and 3 m/sec at seismogenic depths, independent of magnitude. Our ability to replicate maximum slip rates in the fault zones of earthquakes by adjusting the corresponding laboratory rates using the ratio of effective normal stresses acting on the fault planes suggests that the strength of the seismogenic crust is the important factor limiting the near-fault pgv.

Investigating the TCDP Drill Site Using Deep and Shallow Reflection Seismics

The 1999 Chi-Chi earthquake (Mw = 7.6), Taiwan, reactivated the 105 km long, NS trending Chelungpu fault causing extraordinary surface ruptures. The distribution of the ruptures was quite heterogeneous with the largest displacement (up to 12 m) occurring across the Tachia River, 50 km north of the epicenter. In this study, several different scaled seismic lines are constrained and used to predict the major regional fault geometry. The 3D northern lateral ramp of the Chelungpu fault is clearly found, which provides a structural framework to explain the en echelon surface ruptures around the Tachia River. These data were also applied to identify the strata connected with the site of a 2 km hole (the Taiwan Chelungpu-fault Drilling Project, TCDP). Adequate site conditions are predicted before drilling and rechecked after drilling. A ‘TT-boundary’ is proposed as a structural control of the faulting in the area, which shapes the strata to dip 30° east and 20° south at the drill site. A depth of 1100 m for the Chi-Chi earthquake fault zone was predicted and confirmed. This is a good example, combining different scaled seismic methods to study detailed underground structures, which has the drilling purpose.

In Situ Soil-Specific Nonlinear Properties Back-Calculated from Vertical Array Records during 1995 Kobe Earthquake

In situ nonlinear soil properties are investigated based on vertical array records obtained during the 1995 Hyogoken Nambu earthquake (Kobe earthquake) at soft soil sites near the earthquake fault zone. Spectrum ratios between ground surface and deeper levels are calculated for the main shock and associated small shocks and S-wave velocities and damping ratios in surface soil layers to best reproduce them are back-calculated by means of an inversion analysis (extended Bayesian method) assuming one-dimensional vertical horizontal shear (SH)-wave propagation. Obvious differences in S-wave velocities and damping ratios are found between the main shock and the small shocks. Clear strain-dependent modulus degradations which can be differentiated for different soil types are recognized. The degradations are essentially consistent with some of laboratory test results to date for each soil type at least for G∕G0≈0.5 or larger, though for gravelly soils back-calculated values tend to show milder degradations than laboratory test results presumably due to large inclusion of fines in actual ground. Back-calculated damping ratios show essentially the same trend as in laboratory tests, although the absolute values are in most case a few percent higher in the strain range smaller than 10−4.

Study on displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas

The displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas is calculated in this study. The result is compared with the displacement field of the main shock calculated by co-seismic slip model of Wald and Heaton (1994). The result shows that the direction of displacement generated by aftershocks in Landers seismic fault plane and its adjacent areas is consistent with that generated by main shock. The rupture of aftershock is generally inherited from main shock. The displacement generated by aftershocks is up to an order of centimeter and can be measured by GPS sites nearby. So when we use geodetic data measured after earthquake to study the geophysical problems such as crustal viscosity structure, afterslip distribution, etc., only the displacement field generated by aftershocks considered, can uncertainty be reduced to minimum and realistic result be obtained.

Seismotectonics and stress field of the Yellowstone volcanic plateau from earthquake first‐motions and other indicators

We have found spatial variations in seismic stress indicators at the Yellowstone volcanic field, Wyoming, by examining source mechanisms of 25 years of network‐recorded earthquakes, 1973–1998. Yellowstone seismicity is characterized by swarms of earthquakes (MC < 3) within the 0.64 Ma Yellowstone caldera and between the caldera and the eastern end of the 44‐km‐long rupture of the 1959 MS7.5 Hebgen Lake earthquake. We relocated more than 12,000 earthquake hypocenters using three‐dimensional velocity models. Focal mechanisms calculated for 364 earthquakes, carefully selected for location accuracy, reveal predominantly normal faulting; however, fault orientations vary across the Yellowstone caldera. Specifically, focal mechanism T axes trend N‐S in the vicinity of the Hebgen Lake earthquake fault zone NW of the Yellowstone caldera and rotate to ENE‐WSW 35 km east of there. This rotation of the T axis trends occurs in the area of densest seismicity north of the caldera. Stress inversions performed using earthquake first‐motion data reveal a similar pattern in the minimum principal stress orientations. The extension directions derived from the focal mechanisms and stress inversions are generally consistent with extension directions determined from geodetic measurements, extension inferred from alignments of volcanic vents within the caldera, and extension directions determined from regional normal faults. The N‐S trending Gallatin normal fault north of the caldera is a notable exception; we find extension to be perpendicular to the direction of past extension on the Gallatin fault in the area immediately south of it. We interpret this N‐S extension north of the caldera to be related to postseismic viscoelastic relaxation in the upper mantle and lower crust following the Hebgen Lake earthquake. The dominantly extensional tectonic regime at Yellowstone inferred from these results demonstrates the influence of NE‐SW Basin and Range extension in this area.

Active faults in Eurasia: general remarks

The database and map of active faults of Eurasia, 1:5,000,000, have been compiled according to the ILP Project II-2 “World Map of Major Active Faults.” The map shows the complicated distribution of active faulting. The major faults group in broad mobile belts between relatively weakly faulted parts of the main plates. The faults form the plates, microplates and crustal blocks boundaries there and are accompanied by deformation of the boundary areas. Using active fault data for the seismic hazard assessment (estimation of maximum possible magnitudes of earthquakes in fault zones), it is necessary to take into account the total neotectonic activity of the territory and average rate and sense of motion on the fault. Because trenching gives a possibility to determine only vertical earthquake offset on a fault, it is necessary to make corrections for strike–slip faults to estimate the earthquake magnitude by using this parameter.

Maximum Slip in Earthquake Fault Zones, Apparent Stress, and Stick-Slip Friction

The maximum slip, observed or inferred, for a small patch within the larger fault zone of an earthquake is a remarkably well-constrained function of the seismic moment. A large set of maximum slips, mostly derived from slip models of major earthquakes, indicate that this parameter increases according to the cube root of the seismic moment. Consistent with this finding, neither the average slip rate for the patches of maximum slip nor the apparent stresses of earthquakes show any systematic dependence on seismic moment. Maximum average slip rates are several meters per second independent of moment and, for earthquakes in continental crustal settings, the apparent stress is limited to about 10 MPa. Results from stick-slip friction experiments in the laboratory, combined with information about the state of stress in the crust, can be used to predict, quite closely, the maximum slips and maximum average slip rates within the fault zones of major earthquakes as well as their apparent stresses. These findings suggest that stick-slip friction events observed in the laboratory and earthquakes in continental settings, even with large magnitudes, have similar rupture mechanisms. Manuscript received 24 February 2003.

Mapping Apparent Stress and Energy Radiation over Fault Zones of Major Earthquakes

Using published slip models for five major earthquakes, 1979 Imperial Valley, 1989 Loma Prieta, 1992 Landers, 1994 Northridge, and 1995 Kobe, we produce maps of apparent stress and radiated seismic energy over their fault surfaces. The slip models, obtained by inverting seismic and geodetic data, entail the division of the fault surfaces into many subfaults for which the time histories of seismic slip are determined. To estimate the seismic energy radiated by each subfault, we measure the near-fault seismic-energy flux from the time-dependent slip there and then multiply by a function of rupture velocity to obtain the corresponding energy that propagates into the far-field. This function, the ratio of far-field to near-fault energy, is typically less than 1/3, inasmuch as most of the near-fault energy remains near the fault and is associated with permanent earthquake deformation. Adding the energy contributions from all of the subfaults yields an estimate of the total seismic energy, which can be compared with independent energy estimates based on seismic-energy flux measured in the far-field, often at teleseismic distances. Estimates of seismic energy based on slip models are robust, in that different models, for a given earthquake, yield energy estimates that are in close agreement. Moreover, the slip-model estimates of energy are generally in good accord with independent estimates by others, based on regional or teleseismic data. Apparent stress is estimated for each subfault by dividing the corresponding seismic moment into the radiated energy. Distributions of apparent stress over an earthquake fault zone show considerable heterogeneity, with peak values that are typically about double the whole-earthquake values (based on the ratio of seismic energy to seismic moment). The range of apparent stresses estimated for subfaults of the events studied here is similar to the range of apparent stresses for earthquakes in continental settings, with peak values of about 8 MPa in each case. For earthquakes in compressional tectonic settings, peak apparent stresses at a given depth are substantially greater than corresponding peak values from events in extensional settings; this suggests that crustal strength, inferred from laboratory measurements, may be a limiting factor. Lower bounds on shear stresses inferred from the apparent stress distribution of the 1995 Kobe earthquake are consistent with tectonic-stress estimates reported by Spudich et al. (1998), based partly on slip-vector rake changes. Manuscript received 21 February 2001.

Viscoelastic deformation from North Anatolian Fault Zone earthquakes and the eastern Mediterranean GPS velocity field

We have modeled linear viscoelastic relaxation of a uniform‐viscosity asthenosphere following large earthquakes in the Aegean‐Anatolian region during the 20th century to evaluate whether this process contributes significantly to the regional velocity field. We address in particular whether southward motion of the Aegean Sea region relative to central Anatolia (and thus extension of western Turkey) can be attributed to a postseismic strain pulse. Our models show that postseismic relaxation of a thick (250 km) viscoelastic substrate cannot generate the observed extension in western Turkey, regardless of the choice of viscosity. We also find that the asthenosphere viscosity must exceed about 5 × 1020 Pa s, at least north of the North Anatolian Fault Zone (NAFZ), to be consistent with low GPS site velocities in the Black Sea region and highly localized strain around the fault.

Spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region by borehole observations

SUMMARY In order to estimate the detailed orientation and density of cracks around an active fault area, we investigate the spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region with high-quality waveform data recorded by a borehole network operated by the Geological Survey of Japan. We introduce the vertically aligned crack model of Hudson (1980, 1981) to explain its anisotropic structure. At stations more than 10 km away from the earthquake fault zone (i.e. Ikeda and Inagawa), crack orientation is nearly consistent with the direction of the regional maximum stress in this area (WNW–ESE). Cracks are distributed at depths below 5 km with a nearly constant crack density, e, of 0.015 at Ikeda, and below 0 km with e=0.009 at Inagawa. These upper-limit depths of crack distribution are roughly comparable with those of microearthquakes there. In contrast, cracks are aligned nearly parallel to the strike of the earthquake fault (NE–SW) at stations at both ends of the fault system (i.e. Takarazuka and Ikuha), and their intensities of anisotropy are three or four times larger than those at Ikeda and Inagawa. At Takarazuka in particular, cracks are distributed at a depth shallower than 4.5 km with a large crack density, e, of 0.06. This result suggests that major stress release by the faulting of the main shock dominates the present stress field at both ends of the fault system. In the middle of the fault system (i.e. Hirabayashi), cracks are aligned parallel to its regional maximum stress and the intensity of anisotropy is twice as large as those at Ikeda and Inagawa. This result contrasts with that of Tadokoro et al. (1999), who determined the crack orientation at a station close to Hirabayashi to be parallel to the fault direction for events in a period earlier than this study by about a year, suggesting that the stress field in this area may have changed over the timescale of a year. Around Inagawa, the distribution of anisotropy seems to have strong azimuthal variations. This result indicates small-scale (of the order of several kilometres) variations of anisotropic structure in the upper crust, probably due to strong heterogeneities in the fluid and/or constitutive materials.

A method for mapping apparent stress and energy radiation applied to the 1994 Northridge Earthquake Fault Zone—Revisited

McGarr and Fletcher (2000) introduced a technique for estimating apparent stress and seismic energy radiation associated with small patches of a larger fault plane and then applied this method to the slip model of the Northridge earthquake (Wald et al., 1996). These results must be revised because we did not take account of the difference between the seismic energy near the fault and that in the far‐field. The fraction f(vR) of the near‐field energy that propagates into the far‐field is a monotonic function that ranges from 0.11 to 0.40 as rupture velocity vR increases from 0.6β to 0.95β, where β is the shear wave speed. The revised equation for apparent stress for subfault ij is , where ρ is density, D(t)ij is the time‐dependent slip, and Dij is the final slip. The corresponding seismic energy is , where A is the subfault area. Our corrected distributions of apparent stress and radiated energy over the Northridge earthquake fault zone are about 35% of those published before.

Earthquake cycle, fault zones, and seismicity patterns in a rheologically layered lithosphere

We study the coupled evolution of earthquakes and faults in a model consisting of a seismogenic upper crust governed by damage rheology over a viscoelastic substrate. The damage rheology has two types of functional coefficients: (1) a “generalized internal friction” separating states associated with material degradation and healing and (2) damage rate coefficients for positive (degradation) and negative (healing) changes. The evolving damage modifies the effective elastic properties of material in the upper crust as a function of the ongoing deformation. This simulates the creation and healing of fault systems in the upper seismogenic zone. In addition to the vertically averaged thin sheet approximation we introduce a Green function for three‐dimensional elastic half‐space for the instantaneous component of deformation. The formulation accounts in an internally consistent manner for evolving deformation fields, evolving fault structures, aseismic energy release, and spatiotemporal seismicity patterns. These developments allow us to simulate long histories of crustal deformation and to study the simultaneous evolution of regional earthquakes and faults for various model realizations. To focus on basic features of a large strike‐slip fault system, we first consider a simplified geometry of the seismogenic crust by prescribing initial conditions consisting of a narrow damage zone in an otherwise damage‐free plate. For this configuration, the model generates an earthquake cycle with distinct interseismic, preseismic, coseismic, and postseismic periods. Model evolution during each period is controlled by a subset of physical properties, which may be constrained by geophysical, geodetic, rock mechanics, and seismological data. In the more generic case with a random initial damage distribution, the model generates large crustal faults and subsidiary branches with complex geometries. The simulated statistics depend on the space‐time window of the observational domain. The results indicate that long healing timescale, τh, describing systems with relatively long memory, leads to the development of geometrically regular fault systems and the characteristic frequency‐size earthquake distribution. Conversely, short τh (relatively short memory) leads to the development of a network of disordered fault systems and the Gutenberg‐Richter earthquake statistics. For intermediate values of τh the results exhibit alternating overall switching of response from periods of intense seismic activity and the characteristic earthquake distribution to periods of low seismic activity and Gutenberg‐Richter statistics.

New advances of seismic tomography and its applications to subduction zones and earthquake fault zones: A review

Abstract There have been significant advances in the theory and applications of seismic tomography in the last decade. These include the refinements in the model parameterization, 3‐D ray tracing, inversion algorithm, resolution and error analyses, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Applications of the new generation tomographic methods to subduction zones have resulted in unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back‐arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High‐resolution tomographic imagings of earthquake fault zones in Japan and California show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic processes in the fault zones, such as the migration of fluids. Evidence also shows that arc magmatism and slab dehydration may also contribute to the generation of large crustal earthquakes in subduction regions.

A method for mapping apparent stress and energy radiation applied to the 1994 Northridge Earthquake Fault Zone

Using the Northridge earthquake as an example, we demonstrate a new technique able to resolve apparent stress within subfaults of a larger fault plane. From the model of Wald et al. (1996), we estimated apparent stress for each subfault using where G is the modulus of rigidity, β is the shear wave speed, and is the average slip rate. The image of apparent stress mapped over the Northridge fault plane supports the idea that the stresses causing fault slip are inhomogeneous, but limited by the strength of the crust. Indeed, over the depth range 5 to 17 km, maximum values of apparent stress for a given depth interval agree with τa(max)=0.06S(z), where S is the laboratory estimate of crustal strength as a function of depth z. The seismic energy from each subfault was estimated from the product τaDA, where A is subfault area and D its slip. Over the fault zone, we found that the radiated energy is quite variable spatially, with more than 50% of the total coming from just 15% of the subfaults.