Moment Release Rate Research Articles

Observation of the seismic nucleation phase in the Ridgecrest, California, Earthquake sequence

Near‐source observations of five M 3.8–5.2 earthquakes near Ridgecrest, California are consistent with the presence of a seismic nucleation phase. These earthquakes start abruptly, but then slow or stop before rapidly growing again toward their maximum rate of moment release. Deconvolution of instrument and path effects by empirical Green's functions demonstrates that the initial complexity at the start of the earthquake is a source effect. The rapid growth of the P‐wave arrival at the start of the seismic nucleation phase supports the conclusion of Mori and Kanamori [1996] that these earthquakes begin without a magnitude‐scaled slow initial phase of the type observed by Iio [1992, 1995].

Surface strain accumulation and the seismic moment tensor

Abstract Although the scalar moment accumulation rate within the seismogenic zone beneath a given area is sometimes deduced from the observed average surface strain accumulation rate over that same area (e.g., Working Group on California Earthquake Probabilities, 1995), the correspondence between the two is very uncertain. The equivalence between surface strain accumulation and scalar moment accumulation is based on Kostrov's (1974) relation between the average strain rate over a volume and the moment-rate tensor for that volume. The average strain rate over the volume is replaced by the average strain rate measured at the free surface to deduce an approximate moment-rate tensor. Only in exceptional circumstances will that moment-rate tensor correspond to a double-couple mechanism, a mechanism that can be represented by a scalar moment accumulation rate. More generally, the moment tensor must be resolved into the superposition of two or more double-couple mechanisms, and that resolution can be done in many ways, each with its own scalar moment rate. Thus the resolution is not unique. This is demonstrated by deducing scalar moment accumulation rates for a GPS network that covers most of California south of San Francisco. It is shown that resolutions into different double-couple mechanisms lead to scalar moment accumulation rates differing by factors of ∼2. We suggest that the minimum scalar moment rate equivalent to principal surface strain rates ɛ1 and ɛ2 acting over the area A is M0(min) = 2μHA Max (¦ɛ1¦, ¦ɛ2¦, ¦ɛ1 + ɛ2¦), where μ is the rigidity and H the depth of seismogenic zone, and the function Max is equal to the largest of its arguments. Within the uncertainites of measurement, the scalar moment accumulation rate in southern California based on that approximation is in balance with the average historic seismic moment release rate so that no current earthquake deficit need be accumulating.

Physical model of aftershock temporal behaviour

The aim of the model is to describe the temporal behaviour of cumulative seismic moment released in aftershock sequences. It therefore constitutes an alternative approach with respect to the Omori and Utsu laws, which model the number of events against time. Static fatigue is assumed to be the principal explanation of the aftershock temporal behaviour; as a result, aftershocks can originate at a stress intensity factor lower than the fracture toughness. Under the condition that the mainshock causes a redistribution of stress, the initial stress condition of the aftershock sequence at mainshock origin time t 0 can be considered as the superposition of the stress before the mainshock and of the stress step dσ caused by the dynamic rupture of the mainshock. The model can be written in the form: S(t) = dσ + ( RT γ ) ln t , where S(t) is the cumulative stress drop at time t after the mainshock, T the temperature, R the universal gas constant and γ a constant dependent on material type. The model is derived from the empirical static fatigue law t = s exp[ (U − γσ) RT ] , widely applied in fracture mechanics (σ is the stress, s a constant and the other parameters as above). The fit of the model to five aftershock sequences, obtained by linear regression analysis, is quite good with the r 2 greater than 0.85 for all the sequences, but the Imperial Valley sequence where r 2 = 0.7. The results seem to favour the hypothesis that the decay of the rate of seismic moment release depends on temperature, in agreement with other evidences based on the evaluation of the decay rate of Omori law, but in contrast to some static fatigue results obtained in laboratory tests on granite rocks. The scarcity of field data on underground temperature does not allow any definitive conclusion about this subject.

Statistical significance of the seismic coupling coefficient

Abstract Variations in the seismic coupling coefficient (χ), the ratio of the observed to expected seismic moment release rates, at subduction zones are commonly used to make inferences about the physics of subduction. Taking into account the power-law form of the earthquake frequency-moment relation, it is shown that the observed distribution of χ at the world's subduction zones based on the twentieth century's earthquakes can be matched by a single value of χ ≈ 0.3 if, as is likely the case, the 90-yr observation time is less than or comparable to the repeat time of the largest possible earthquake. It is shown that because our reliable earthquake record is short, global variations in χ, based on seismicity alone, are poorly resolved and cannot be used to distinguish among subduction models.

Frequency of occurrence of moderate to great earthquakes in intracontinental regions: Implications for changes in stress, earthquake prediction, and hazards assessments

We investigate the departure of shallow intracontinental earthquakes from the frequency‐magnitude relationship log (n(M)) =a‐bM. We consider seven large continental regions but exclude subduction zones. In each, “active” and “stable” areas are differentiated. Within active areas we separately categorize earthquakes on faults with high slip rates. We use data sets from 1978–1994 and 1900–1994, which are complete for moment magnitudesMw≥5.3 and ≥7.0, to calculate changes inbvalue withMw. For all active intracontinental regions combined except Asia,bchanges at 98% confidence from 0.90 to 2.1 at a corner magnitudeMcof 6.9 to 7.0. The distribution for stable regions indicates a similar change. Values ofMcare associated with downdip widths of rupture of about 23 to 27 km, which are smaller than those for subduction zones. In each case,Mcmarks a transition from unbounded to bounded earthquakes. In Asia, events from 1958 to 1994 also delineate a similar change nearMc7.0. Earthquakes in Asia of 7.0≤Mw≤8.1 from 1900 to 1957, however, define abvalue of 1.0 and a moment release rate 5.6 times higher than that from 1958 to 1994. Those anomalous values indicate that a large region of Asia was close to failure, i.e., near a self‐organized critical state, during the earlier period. We associate those changes with the giant Himalayan earthquake in 1950 ofMw8.6. Regional variations in seismic strain release rates correlate with the types of processes at adjacent plate boundaries. Extrapolating rates of activity of the past 95 years toMw≥8.0 indicates that such events should be very rare in stable areas.

Prediction of coal face rockbursts and microseismicity in deep longwall coal mining

A new method is proposed for predicting rockbursts, as well as microseismicity in deep longwall coal mining. The method is able to predict failure in the coal seam and in the rock mass by using 3-D elastic numerical stress analyses. These locations were compared with the results of microseismic monitoring. Fracturing intensity and its variation with respect to face advance were represented by the maximum shear seismic moment release rate. The “Seismic Moment Method” was applied to three cases in the Horonai Coal Mine and in the Miike Coal Mine, Japan. The first case considered panels W8-5U and WS-5 in the Horonai Coal Mine at 1055 and 1125m deep. Even though no rockburst occurred, areas where microseismic events were concentrated were predicted. It was also clear, by comparing calculated maximum shear seismic moment release rate to observed data, that the magnitude of the seismicity and its variation with respect to the face advance were also successfully predicted. The second case corresponded to panel W6-3 in the same mine from 915 to 985m deep. Microseismic monitoring had not yet been introduced, however, the rockburst in this panel was successfully predicted as fracturing of coal seam elements at the coal face. The last case considered panels E3–E6 in the Miike Coal Mine at 600 and 650 m deep, respectively. The location and intensity of microseismicity were again predicted; in addition, toppling as well as rockbursts were also predicted. Relations between the extent of the relief zone ahead of the coal face and the occurrence of rockbursts are discussed. An effective method to prevent rockburst hazard is also proposed.

Statistical physics, seismogenesis, and seismic hazard

The scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics. This has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon, involving locally nonlinear dynamics, with simplified rheologies exhibiting instability or avalanche‐type behavior, in a material composed of a large number of discrete elements. In particular, it has been suggested that earthquakes are an example of a “self‐organized critical phenomenon” analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit. In this stationary state of marginal stability the distribution of avalanche energies is a power law, equivalent to the Gutenberg‐Richter frequency‐magnitude law, and the behavior is relatively insensitive to the details of the dynamics. Here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during (1) dynamic slip on a preexisting fault, (2) fault growth, and (3) fault nucleation. The individual physical models share some generic features, such as a dynamic energy flux applied by tectonic loading at a constant strain rate, strong local interactions, and fluctuations generated either dynamically or by fixed material heterogeneity, but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution. However, all exhibit critical or near‐critical behavior, with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes, including the Gutenberg‐Richter law. Some of the results are sensitive to the details of the dynamics and hence are not strict examples of self‐organized criticality. Nevertheless, the results of these different physical models share some generic statistical properties similar to the “universal” behavior seen in a wide variety of critical phenomena, with significant implications for practical problems in probabilistic seismic hazard evaluation. In particular, the notion of self‐organized criticality (or near‐criticality) gives a scientific rationale for the a priori assumption of “stationarity” used as a first step in the prediction of the future level of hazard. The Gutenberg‐Richter law (a power law in energy or seismic moment) is found to apply only within a finite scale range, both in model and natural seismicity. Accordingly, the frequency‐magnitude distribution can be generalized to a gamma distribution in energy or seismic moment (a power law, with an exponential tail). This allows extrapolations of the frequency‐magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates. The answers to other questions raised are less clear, for example, the effect of the a priori assumption of a Poisson process in a system with strong local interactions, and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons. The results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors. However, there remains no consensus, on both theoretical and practical grounds, on the possibility or otherwise of reliable intermediate‐term earthquake prediction.

Highly concentrated seismicity caused by deformation of Kilauea's deep magma system

FREQUENT shallow earthquakes within the rift zones of the Hawaiian volcano Kilauea have been interpreted as resulting from stress changes associated with a shallow magma conduit system1,2. Here, by using a precise earthquake relocation technique3, we show that what had been imaged as a diffuse cloud of seismicity in the Upper East Rift in 1991 is in fact a narrow ribbon, defining vertical strike-slip faults that extend 2.5km along the rift, but less than 100 to 200 m vertically. This extreme aspect ratio in the seismicity is unexpected and has not been recognized in other fault systems. The observed earthquake depths, left-lateral focal mechanisms, limited vertical extent and increasing moment release rate since 1983 can be explained in terms of a growing stress concentration above the deeper, aseismic portion of Kilauea's rift system, which deforms in response to the seaward displacement of the south flank of the volcano. The shallow background seismicity within Kilauea's rifts can therefore be tied to large-scale motions of the volcanic edifice, and need not be interpreted as resulting from magma migration. The observed pattern of seismicity may also have implications for the mechanical erosion of locked patches along major strike-slip faults elsewhere.

The 1995 Kobe earthquake and problems of evaluation of active faults in Japan

The Kobe earthquake ( M 7.2) of January 17, 1995, which was the most damaging earthquake in recent Japanese history, made manifest the need for reconsidering the method of evaluating active faults. An earthquake of this magnitude at this time was unexpected according to conventional evaluation, in which the potential magnitude of earthquakes at a certain site is estimated by considering the greatest earthquake in the past 400 years and the length of the active fault. The following characteristics of this earthquake made it appear unlikely by conventional understanding: (1) the Kobe earthquake involved several neighboring faults, which had been previously been identified as separate fault systems: (2) the surface rupture of about 10 km length was much shorter than the 50 km seismic faulting; (3) the interval of 400 years between the Kobe and penultimate Keicho earthquake of 1596 AD ( M 7.5), which has been revealed by historical documents and some excavations, is much shorter than the 2000 years estimated by calculating the average slip rate of displaced landforms. These shortcomings imply that active fault evaluation with the traditional characteristic earthquake model which deals with each fault separately, is not adequate for an area like Japan where active faults swarm. New concepts such as the block rotation model (Kanaori, 1990; Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan. Tectonophysics, 177: 381–399) considering the macroscopic tectonic framework for fault interactions are needed. Furthermore, fault dynamics cannot be ignored; physical and temporal parameters associated with faulting, such as moment release rate, must be considered for realistic and precise evaluation.

The 1995 7.2 magnitude Kobe earthquake and the Arima-Takatsuki tectonic line: implications of the seismic risk for central Japan

Along the Rokko Mountains and Awaji Island, NE-SW to ENE-WSW oriented active faults exist in an en echelon arrangement. They constitute a portion of the Arima-Takatsuki tectonic line (ATTL) which extends from Kyoto, through Awaji Island, to the Median Tectonic Line. The ATTL is also correlated to lineaments linking the saddles and steep slopes of gravity (Bouguer) anomalies. The main shock of the 1995 magnitude 7.2 ( M 7.2) Kobe earthquake was located at the mid-point of the ATTL. The main shock also created a 9-km long rupture on the ground surface along the NE-SW Nojima fault line in the northern part of Awaji Island. The earthquake aftershocks were distributed over a 40-km long zone along the central segment of the line. Surface ruptures and cracks accompanying the Kobe earthquake were scattered along a 40-km segment centrally located on the ATTL. Spatial correlation of the surface ruptures and aftershock distribution on the ATTL suggests that the Kobe earthquake was the result of a 40-km long rupture of the central segment of the ATTL. The average rate of seismic moment-release is an important parameter used in evaluating the seismic risk of a fault system. The moment-release rate averaged over the late Quaternary has been evaluated through the average slip rates and the dimensions of constituent faults. The average moment-release rate for the central portion of the ATTL has been estimated as 0.0017−0.0063 × 10 26 dyne cm year −1. Evidence of liquefaction and flowage, probably associated with the 1596 M 7.5 Keicho-Fushimi earthquake, has been identified at several archaeological excavation sites along the ATTL. These observations suggest that the ATTL was also responsible for the Keicho-Fushimi earthquake which occurred about 400 years ago. From the elapsed time since the Keicho-Fushimi earthquake and the average moment-release rate, the seismic moment accumulated during the interseismic period was calculated for the ATTL. The obtained value amounts to that of the seismic moment released by an earthquake having a magnitude ranging from 6.8 to 7.2, being comparable to that of the Kobe earthquake. The obtained result is consistent with the view that the Kobe earthquake was caused by a rupture of the ATTL which released almost all the moment accumulated since the previous event.

Progressive growth of San Clemente Island, California, by blind thrust faulting: implications for fault slip partitioning in the California Continental Borderland

We find that the genesis of San Clemente Island and its surrounding submarine platform is consistent with progressive slip on two, southeast-striking, southwest-dipping, blind thrust fault segments. Since their inception 2 to 5 Ma, 3 km of compression normal to the N150°E fault strike has been accommodated with 1700 m of domal uplift of the San Clemente Anticlinorium. The existence of an extensive suite of Pleistocene marine terraces provides evidence that slip and uplift are continuing today. Based on direct terrace fossil age determinations and correlations of terrace heights with global sea-level curves, we estimate that San Clemente Island is currently uplifting at between 0.2 and 0.5 mm yr−1. This translates into 0.6–1.5 mm yr−1 of thrusting on the causative blind thrusts beneath the island. Unlike the situation at nearby Palos Verdes, where a simple twist in a regional strike-slip fault accommodated both fault-parallel and fault-normal motions, the shallow dips of the thrusts suggest that, if regional strike-slip motion on the San Clemente Fault exists, it must be partitioned onto through-going surfaces distinct from the thrusts. Current GPS data are sparse and equivocal, but they indicate that 1–4 mm yr−1 of compression and 4–7 mm yr−1 of strike slip are absorbed in the California Continental Borderland. With the Palos Verdes Fault taking some 3 mm yr−1 from the strike-slip budget, 1–4 mm yr−1 of motion could be present on a through-going San Clemente Fault. When translated into an annual moment release rate using Kostrov's formula, GPS strains predict that between 2.5 and 4.9 times 1017 N m yr−1 of earthquake potential is available offshore from San Diego to the Santa Barbara Channel. Distribution of this moment budget among various earthquake magnitudes is arguable, but we predict that M > 6 quakes in the Borderland could recur between 30 and 80 years, and M > 7 quakes might be found every 310 to 580 years.

Modeling of the interaction between destructive inland earthquakes and large off coast earthquakes in central and south-west Japan

A one-dimensional Stick-slip model, which is introduced to explain the fractal features of earthquake phenomena is applied to describe he destructive earthquake occurrences in central and south-west Japan. Space time patterns of events are simulated and the predictablity is discussed. On the basis of the “Block-rotation model”, epicenters of historical destructive earthquakes with magnitude 6.4 or greater are relocated to the tectonic or block boundary lines. This model has revealed that these events show a periodicity of about 1000 years of active and quiet alternative intervals. In each active period, individual block boundary lines generate several large earthquakes without overlapping rupture areas. The space-time pattern and moment release rate for the Hanaore-Kongo Fault Line (HKFL) can be represented well with the one-dimensional stick-slip model. The seismic coupling of large offshore earthquakes along the Nankai trough is also found to be well modeled by this model. This model may be useful for estimation of seismic risk in this region. Further, this model can be applied to see the situation of the M7.2 Kobe earthquake and interpret this earthquake to be the event that comes from the block structure in later section.

Rupture history and seismotectonics of the 1991 Uttarkashi, Himalaya earthquake

The 19 October 1991 Uttarkashi, India earthquake occurred in the main thrust zone of the Himalaya. With a moment magnitude of 6.8, this event is characteristic of the present-day motion on the thrust fault system. We examine this earthquake using different sets of data in order to understand better the faulting process of a major earthquake in the Himalayan region. Firstly, the modeling of the teleseismic records indicates that the mechanism is similar to the published CMT and indicates a shallow (between 10 and 15 km depth) low-angle thrust event. In the vicinity of the source, the earthquake was recorded by a network of accelerometers run by the University of Roorkee. Six three-component accelerometers were triggered within a radius of 60 km. Two of them were very close to the surface projection of the fault. Forward modeling of those records shows that the rupture propagated toward the west. This forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms. The accelerogram records are inverted to obtain the distribution of slip on the fault plane. The results show a complex rupture process. The slip maxima (1.5 m) occurred 10 km west and 15 km southwest of the hypocenter. The slip source function obtained with near-field data is similar to the function obtained from teleseismic records and shows a low moment release at the beginning of the rupture and a maximum rate of moment release 4 seconds after. The relation between the slip distribution obtained by inversion, isoseimals, mapped faults and the aftershocks location is then discussed and we finally propose a seismotectonic interpretation of this earthquake. The Uttarkashi earthquake probably occurred along the detachment surface which coincides with the upper surface of the subducting Indian lithosphere. This detachment surface is gently dipping under the Lesser Himalaya and south of the Vaikrita thrust. The Vaikrita thrust marks the line separating the very shallow-dipping detachment (along which big earthquakes like the Uttarkashi earthquake could occur) from the steeper-dipping, aseismic basement thrust. This observation is important for correctly estimating the seismic hazard in the Uttarkashi region.

Intermediate- and long-term earthquake prediction.

Progress in long- and intermediate-term earthquake prediction is reviewed emphasizing results from California. Earthquake prediction as a scientific discipline is still in its infancy. Probabilistic estimates that segments of several faults in California will be the sites of large shocks in the next 30 years are now generally accepted and widely used. Several examples are presented of changes in rates of moderate-size earthquakes and seismic moment release on time scales of a few to 30 years that occurred prior to large shocks. A distinction is made between large earthquakes that rupture the entire downdip width of the outer brittle part of the earth's crust and small shocks that do not. Large events occur quasi-periodically in time along a fault segment and happen much more often than predicted from the rates of small shocks along that segment. I am moderately optimistic about improving predictions of large events for time scales of a few to 30 years although little work of that type is currently underway in the United States. Precursory effects, like the changes in stress they reflect, should be examined from a tensorial rather than a scalar perspective. A broad pattern of increased numbers of moderate-size shocks in southern California since 1986 resembles the pattern in the 25 years before the great 1906 earthquake. Since it may be a long-term precursor to a great event on the southern San Andreas fault, that area deserves detailed intensified study.

Are fault growth and linkage models consistent with power-law distributions of fault lengths?

It has recently been recognized that fault lengths ( L) in natural populations follow power-law scaling. Such power-law scaling is observed in a wide range of tectonic settings in regions that have experienced differing amounts of total strain, and exhibit faults over a very large range of dimensions. In this paper we explore possible constraints on fault growth and linkage required to maintain power-law length scaling during progressive deformation. We first consider a fault growth model in which individual faults in a population grow by an amount Δ L ∝ L F during slip increments (earthquakes), which have a recurrence interval τ ∝ L E . If an initial power-law length distribution is assumed for the population, it is found that the growth model exponents must be related by F − E = 1 in order to continually maintain the same scaling. If the requirement of constant moment release rate through time is also imposed, this implies that for large faults E = 2, which leads to a loss of power-law scaling with increasing strain, unless F = 3. Current mechanical models for growth of single faults by tip propagation propose E ≥ 1 and F = 1. Thus single-fault models are not consistent with observed power-law scaling. In a second model, fault lengths increase by growth specified by the first model, unless a nearby fault is encountered, in which case the two faults link. With this model, it is possible to produce a power-law distribution from a fault or flaw population that initially does not have a power-law distribution. Once a power-law distribution is developed, fault linkage causes the power-law exponent ( C) to decrease as fault strain increases.

Evolution of moderate seismicity in the San Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence of large and great earthquakes

The rate of seismic activity of moderate‐size (M > 5.5) earthquakes in the San Francisco Bay (SFB) region has varied considerably during the past 150 years. As measured by the rate of seismic moment release, seismic activity in the SFB region is observed to accelerate prior to M > 7.0 earthquakes in 1868, 1906, and 1989, and then to decelerate following them. We examine these seismicity changes in the context of the evolution of the stress field in the SFB region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace. We use a Coulomb failure function (CFF) to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the SFB region. We find that the occurrence of a large or great earthquake creates a “stress shadow”: a region where the stress driving earthquake deformation is decreased. Interseismic strain accumulation acts to reverse this process, gradually bringing faults in the SFB region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible. As the stress shadow generated by a large or great earthquake disappears, it migrates inward toward the fault associated with that large or great event. The observed changes in the rate of occurrence of moderate earthquakes in the SFB region are broadly consistent with this model. In detail, the decrease in seismicity throughout most of the SFB region and a localized increase in the Monterey Bay region following the great 1906 earthquake is consistent with our predicted stress changes. The timing and location of moderate‐size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the SFB region. Those earthquakes that are most inconsistent with our stress evolution model, including the 1911 earthquake southeast of San Jose, are found to occur in regions where dip‐slip faulting is common in addition to strike‐slip. The 1906 earthquake brought that zone of dip‐slip faulting closer to failure, suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike‐slip one similar to the 1984 Morgan Hill earthquake. The occurrence of activity on faults very close to the San Andreas, such as the Lake Elsman earthquakes of 1988 and 1989, appear to be associated with the last disappearence of the stress shadow on the Loma Prieta segment of the San Andreas fault. Thus events of that type may represent an intermediate‐term precursor to a large earthquake, such as the 1989 Loma Prieta event. Much of the moderate‐size earthquake activity in the SFB region appears to be modulated in time by the buildup and release of stress in large and great earthquakes. A tensorial approach to earthquake prediction, i.e., taking into account changes in the components of the stress tensor, has several advantages over examining scalar changes such as those in seismic activity and moment release rates. This tensorial approach allows for both activation and quiescence (but in different subregions) prior to as well as after large earthquakes.

Coseismic crustal deformation on a fault zone defined by microseismicity in the Pavliani area, central Greece

SUMMARY Pavliani is an area of high microseismicity in northern central Greece. There is no evidence in the historical record of large damaging earthquakes in this area since at least 600 BC. We examine contemporary crustal deformation in this area using microearthquake data recorded during 1983-84 by the Volos seismic network, principally, and to a lesser extent by the Patras seismic network. The microseismicity ( 1-4.6 ML) defines an approximately vertical fault zone trending WSW-ENE, of 55 km length and 25 km depth. The fault-plane solution for the 4.6 M, earthquake shows a strike consistent with the trend of the microseismicity zone, and an extension direction consistent with many large earthquakes in central Greece associated with the extensional greater Aegean. Spectra of the microearthquakes are estimated using P waves obtained by selective windowing designed to exclude other P phases from the coda; seismic moments in the range 1-95 x 10 N m are obtained, accompanied by estimates of seismotectonic source parameters, including source radii, average stress drop and average coseismic slip. Of prime utility is an excellent correlation found between seismic moment and magnitude, and with coseismic slip. This allows heterogeneity in the distribution of individual microearthquake source parameters to be mapped onto the vertical fault-zone plane and the history of coseismic deformation for the whole seismicity observed during 1983-84 to be examined using cumulative slip and moment-release rate. Peaks of high moment-release rate occur in the east and centre of the fault zone. That in the east is dominated by a few large moment-release events; that in the centre by a large number of small events. Both moment-release-rate peaks, despite their different component seismicity, are associated with high cumulative coseismic slip. Only the eastern peak is also associated with high stress drop; it is also shallower, and therefore any surficial interaction is less likely to be attenuated. The surface projection of this peak is within 3 km of a mapped normal fault coincident with an Alpine thrust front delineated by a major scarp of Mesozoic limestone. Surface evidence of recent movement on this normal fault is only slight. Friable fault gouge is seen at a few localities. Displacement could not be determined because the limestone surfaces are deeply weathered and lack kinematic indicators. However, considering the whole fault zone as a cohesive entity, and using a Brune estimate of slip at the current moment-release rate throughout the present extensional tectonic regime, suggests a total vertical displacement of order 175 m, similar to the elevation of the scarp.

The kinematics of northern South Island, New Zealand, determined from geologic strain rates

Relative motions within the distributed plate boundary zone of northern South Island, New Zealand, are determined through an inversion of geologic strain rate estimates. The Quaternary fault slip rate estimates define the shear strain rates, and rock uplift rates provide information on the horizontal divergence rates. An erosion rate to rock uplift rate ratio along with a crustal compensation factor is estimated in order to convert rock uplift rates to horizontal divergence rates. Because of the uncertainty in erosion rates, horizontal divergence rates are given a large standard error of . The three horizontal strain rate components obtained from these data completely define the horizontal velocity gradient tensor. Strain rate distributions are matched with spline polynomial functions, which can be constrained to behave rigidly within specified regions, such as the Pacific or Australian plates. Inversion of the strain rate distribution, assuming uniform erosion rates across the northern South Island, yields a velocity field that has small differences in both magnitude (10% larger) and direction with the NUVEL‐1A plate motion model between Pacific and Australian plates. A revised strain rate data set, obtained from a variable erosion model in which erosion rates are a linear function of the log of the average annual rainfall magnitudes, yields a velocity field with expected directions that are indistinguishable from the NUVEL‐1A plate motion model between Pacific and Australian plates, but velocity magnitudes are still 10–15% higher than the plate motion model. Therefore the average values of slip rate on strike‐slip faults in Marlborough, required by the NUVEL‐1A plate motion model, are typically close to the low end of the published range of slip values for those structures. The major strike‐slip structures within the Marlborough region are accommodating 80–100% of the total plate motion between Australia and Pacific plates on northern South Island; as much as 20% of the relative motion could be accommodated by shear generated by folding or thrust faulting with a single orientation between the major structures. The magnitude of plate motion contributed by the major faults indicates that the distributed geodetic shear strains measured across northern South Island, and discussed by Bibby [1981] and Walcott [1984], are not a form of irrecoverable strain but rather a feature of the strain field that will eventually be released primarily as concentrated slip on the strike‐slip structures within the region. Strain modeling indicates that the earthquake moment release over the last 150 years within the northwest Nelson province, which is west of the Marlborough region, is at least half an order of magnitude higher than the long‐term rate of moment release. Overall, the northwest Nelson region is playing only a minor role in taking up the long‐term plate motion.

Earthquake strain rates and instantaneous relative motions within central and eastern Asia

We use the spatial distribution of moment tensors of earthquakes in this century to estimate the velocity field in Asia within a Eurasian reference frame. In a least-squares inversion, strain rates on the surface of the Earth are matched with continuous spline functions in order to recover the velocity gradient tensor associated with the seismic moment release in Asia. Earthquakes account for 40–60 per cent of the expected motion of India relative to Eurasia, with the missing component of strain rate equivalent to about 20 mm yr−1 of N-S shortening between Siberia and India. In this solution, South China rotates counterclockwise and moves eastwards relative to Siberia. Using rigid plate constraints, we next investigate the characteristics of the complete horizontal strain field in Asia that accommodates plate motions. Our strain-rate solutions are analogous to the response of a Newtonian thin viscous sheet in which the rate of work done by the straining medium in accommodating the velocity boundary conditions is a minimum. In these solutions the Euler pole for India relative to Eurasia is constrained (NUVEL-1A; DeMets et al. 1994), but in the process of fitting the VLBI velocity at Shanghai, China (Ward 1994; Heki et al. 1995), the Euler pole for South China is determined in the inversion. A solution that both fits the velocity at Shanghai, China and yields a strain-rate field consistent with the earthquake mechanisms is one where the South China block has a motion relative to Siberia described by the pole at (51°N, 131°E, 0.3 deg Myr−1). Comparison of the complete strain field that accommodates plate motion with the seismic strains indicates that earthquake moment release rates in this century within Mongolia are about a factor of 4 larger than the long-term rate. Within Gansu-Ningxia, the earthquake moment rates have been about a factor of 2 higher than the long-term rate. The strike-slip faulting within Mongolia, Gansu-Ningxia, western Sichuan and Yunnan is possibly a direct result of velocity boundary conditions imposed on the South China block by forces unrelated to continental collision of India and Eurasia, such as forces associated with subduction along the margins of South-eastern Asia. Verification of this requires a better understanding of the role of pre-existing zones of weakness within the Asian continental lithosphere.

Damage, conditions of East Java Tsunami of 1994 analyzed

On June 3, 1994, an Ms = 7.2 earthquake occured at a depth of 15 km near the east end of the Java trench in the Indian Ocean. The earthquake generated a large tsunami that violently struck southeast Java and extended to southwest Bali (Figures 1 and 2). Approximately 200 people were killed, 400 were injured, and 1000 houses were destroyed. Runup heights (Figure 2) ranged from 0–5 m in west Bali to 1–14 m in southeast Java.This unusual tsunami was generated about 250 km from the hardest hit area. Surprisingly, strong earthquake‐induced ground shaking was not a precursor so local residents had no warning of the impending catastrophe. The long‐period characteristics of the earthquake were incommensurate with the relatively weak high‐frequency magnitude Mb=5.5, and the rate of seismic moment release grew monotonically up to at least 270s. The pattern resembled that of the Nicaragua earthquake of September 2, 1992, in which strong ground shaking did not occur. Most of the damage was concentrated in villages located in pocket beaches, unlike previous tsunami damage in west Nicaragua, Flores, Indonesia [Yeh et al., 1993], and Okushiri, Japan.