Kinematic Source Model Research Articles

Earthquake Doublet in Kagoshima, Japan: Rupture of Asperities in a Stress Shadow

Two shallow moderate ( M ∼ 6) earthquakes occurred in the northwestern part of Kagoshima Prefecture, Kyushu, southern Japan. I discuss the effect of the stress change due to the first earthquake (26 March 1997) on the occurrence of the second earthquake (13 May 1997). The rupture characteristics of the two earthquakes are inferred in two steps to form the basis of the discussion. I first invert strong ground motion data to construct kinematic source models and then estimate the distribution of static stress drop from the derived dislocation distributions. The rupture process of the March event is simple and well described with rupture of a single asperity (patch of high stress drop). On the other hand, multiple asperities on conjugate faults ruptured during the May event. The maximum value of static stress drop for both earthquakes is about 4 MPa, and seems lower than those of other Japanese intraplate earthquakes. The hypocenter and the largest asperity of the May earthquake are located in a stress shadow caused by the March earthquake. Thus the rupture history of the May earthquake is difficult to explain with a static stress change model. Other mechanisms such as fluid migration and dynamic stresses were also investigated, but failed to explain the triggering. I propose the coupled effect of static change in shear stress and normal stress under the rate- and state-dependent friction law as a possible mechanism.

Simulating strong ground motion with the `k-2' kinematic source model: An application to the seismic hazard in the Erzincan basin, Turkey

We present a seismic hazard application of a kinematic broad-bandrupture model. This model is based on the k-square dislocation distribution concept (Herrero and Bernard, 1994).Synthetic seismograms are calculated in the far-field approximation with alayered velocity medium for the 13 March 1992 Erzincan earthquake.With a parametrization of the source constrained by other studies,the far-field contribution correctly fits the recorded strong ground motion, which presents a 0.5 to 2 Hz dominant frequency range.As the k-square model is particularly well adapted to synthetize realistic strong-motion at short distances from the fault,it is a reliable tool for calculating seismic hazard maps around active faults. We thus present a synthetic peak ground acceleration map associatedwith the 13 March 1992 activated fault, for a 60 km × 60 km regionaround the epicenter taking into account a smoothed velocity structure ofthe basin in agreement with the absence of significant site effects related to1D resonance deduced from the aftershock records study.This map is compared with several post seismic reports: macroseismicintensities, detailed distribution of damage, and soil cracking andliquefaction. Our model shows that the values of the peak acceleration andvelocity can explain the dominant spatial distributionof these effects, which concentrates in a narrow band along theactivated segment fault, and in particular at its southernextremity. These results enable us to present such maps forhypothetical future earthquake ruptures, located on the major visible orinferred active fault segments in and around the basin. The effects ofthese potential sources are analyzed in relation to the 1992 eventeffects in order to eliminate unknown site responses. We show that thesouthern part of the basin is particularly exposed because of thepresence of strike-slip faults,and that the western part of the basin would suffer a significantlyhigher strong motion levelthan during the 1992 event with the activation of moderate sizednormal faults evidenced on thesouthwestern edge of the basin.

Characteristics of strong ground motion across a thrust fault tip from the September 21, 1999, Chi‐Chi, Taiwan earthquake

Near fault tip strong motion records from the northern part of the major earthquake (Mw = 7.6), namely the Chi‐Chi earthquake on September 21, 1999 in central Taiwan demonstrated systematic differences on the hanging wall and footwall, and simulated by the finite element method. The extraordinary ground motion differences on either side of the northern fault tip can be explained by a 2‐dimension kinematic source model with fault rupture breaking to surface. In this study, the earthquake faulting was considered as bilateral from the center of a low angle thrust fault which is 30 km in length with a dip angle of 31°. Based on waveform modeling, the source rupture velocity, rise‐time and dislocation of 2.0 km/sec, 5 sec and 6 meters, respectively are suggested. The results of this study show that on the northern part of the Chi‐Chi earthquake fault there was lower rupture velocity and longer rise‐time of the fault slip than that previously reported. Furthermore, the effects of surface breaking from the fault movement contributed large ground deformations near the fault tip and, consequently, induced extensive damage.

Implications of a composite source model and seismic-wave attenuation for the observed simplicity of small earthquakes and reported duration of earthquake initiation phase

AbstractAn examination of P waves recorded on near-source, velocity seismograms generally shows that most small earthquakes (Mw < 2 to 3) are simple. On the other hand, larger earthquakes (Mw ≧ 4) are most often complex. The simplicity of the seismograms of Mw < 2 to 3 events may reflect the simplicity of the source (and, hence, may imply that smaller and larger earthquakes are not self-similar) or may be a consequence of attenuation of seismic waves. To test whether the attenuation is the cause, we generated synthetic P-wave seismograms from a composite circular source model in which subevent rupture areas are assumed to follow a power-law distribution. The rupture of an event is assumed to initiate at a random point on the fault and to propagate with a uniform speed. As the rupture front reaches the center of a subevent patch (all of which are circular), a P pulse is radiated that is calculated from the kinematic source model of Sato and Hirasawa (1973). Synthetic P-wave seismograms, which are all complex, are then convolved with an attenuation operator for different values of t*. The results show that the observed simplicity of small events (Mw < 2 to 3) may be entirely explained by attenuation if t* ≧ 0.02 sec.The composite source model predicts that the average time delay between the initiation of the rupture and the rupture of the largest patch, τ, scales as M01/3, such that log τ = (1/3) log M0 − 8.462. This relation is very similar to that reported by Umeda et al. (1996) between M0 and the observed time difference between the initiation of the rupture and the rupture of the “bright spot.” It roughly agrees with the relation between M0 and the duration of the initiation phase reported by Ellsworth and Beroza (1995) and Beroza and Ellsworth (1996). The relation also fits surprisingly well the data on duration of slow initial phase, tsip, and M0, reported by Iio (1995). One possible explanation of this agreement may be that the composite source model, which is essentially the “cascade” model, successfully captures the evolution of the earthquake source process and that the rupture initiation and the abrupt increase in the velocity amplitude observed on seismograms by previous researchers roughly corresponds to the rupture of the first subevent and the breaking of the largest subevent in the composite source model.

Glottal source parameter estimation by comparison of measured signals with simulated signals

Oral flow, electroglottographic (EGG), and photoglottographic (PGG) signals were simultaneously digitized from a single subject phonating sustained vowels. Analogous simulated signals—oral flow, vocal fold contact area, and glottal area, respectively—were produced by a driven, kinematic glottal source model coupled to a wave-reflection-type vocal tract. The vocal tract area function used in the simulation was estimated from magnetic resonance imaging (MRI) data previously published for the subject. This essentially removed the vocal tract as a confounding factor in the analysis. The measured and simulated signals were compared and an attempt was made to adjust model parameters so that the least-squared differences between the signals were minimized. The model parameters, which included subglottal pressure, vocal fold adduction, vocal fold convergence, and fundamental frequency were observed to be estimates for the glottal configuration and control parameters used by the subject.

Far-field pulse shapes from circular sources with variable rupture velocities

AbstractRecently, Sato (1994) developed a simple earthquake source model of a circular rupture expanding outward from the center of a fault with constant stress drop. In contrast to previous models, the rupture velocity is allowed to vary over the duration of faulting. This model is used to synthesize apparent moment-rate functions for a three-stage source process: first, the rupture starts out with a gradually increasing velocity, then, it continues to expand uniformly until, finally, it slows to a gradual stop. Synthetic velocity seismograms are obtained from a convolution of the apparent moment-rate functions with a causal Q-operator and an appropriate instrument response. Comparisons with an example of an earthquake signal show that, in the context of the proposed model, the observed emergent P-wave onset, which is not compatible with a constant rupture velocity, can be explained by a gradually accelerating rupture front. Systematic departures from the generally expected scaling relationship between seismic moment and rupture duration are often interpreted as evidence for a dependence of stress drop on seismic moment. However, the trade-off between stress drop and rupture velocity inherent in all kinematic source models implies that such deviations can just as well be attributed to systematic variations of rupture velocity. Whereas, in general, the total duration of the far-field displacement pulse is shorter for P waves than for S waves, the model predicts that the rise time, τ1/2, of the displacement pulse should be longer for P waves than for S waves. This feature could constitute a critical test of the model and also provide a constraint on the rupture velocity.

Use of empirical Green's functions, spectral ratios, and kinematic source models for simulating strong ground motion

AbstractGround motions for large and moderately large earthquakes at short and moderate distances are particularly important for seismic hazard estimation in eastern North America (ENA). Very few direct observations of such ground motions have been obtained, however, because of the sparsity of recording sites and the relatively low rates of occurrence of large earthquakes inside the region. Estimation of strong ground motion must therefore rely heavily on theoretical models to extend empirical results obtained from small earthquakes and from the few larger ones for which reliable data are available. Because of the generally large distances between recording stations, the main source of useful data comes from Lg wave trains observed at relatively large distances. For the two largest earthquakes to have occurred near populated regions of southeastern Canada during the past decade, spectral ratios of the Lg wave trains of the mainshocks, with respect to those of their aftershocks, are found to depend almost entirely upon the source radiation characteristics of the sources alone. This result is utilized to derive elastodynamically-based kinematic rupture models that are consistent with the empirical spectral ratio data. Such models provide a firm physical basis from which to infer the most probable spectral characteristics for future large earthquakes in the region. In converse application, it is shown that spectral ratios obtained from such models, along with empirical seismograms from small earthquakes, can be used to accurately simulate strong ground motions at short and moderate (as well as large) distances. As such small-event seismograms are relatively plentiful, the problem of reliable strong ground motion estimation is therefore reduced to that of obtaining reliable representative source spectral models. The solution of this latter problem must continue to depend upon whatever empirical data are available and upon appropriately detailed theoretical modeling.

Source parameters of earthquakes in the Aleutian Islands subduction zone

SUMMARY The source parameters of seven moderate-sized earthquakes representing different parts of the Aleutian Islands subduction zone are presented. Fault plane solutions are determined by the relative amplitude method. A simple 2-D kinematic source model is assumed and synthetic broad-band (BB) seismograms of displacement are generated. Comparisons with observed BB records of displacement allow the estimation of source dimensions, focal depth, rupture velocity, mean stress drop and the static seismic moment for each event. Normal faulting associated with extensional stresses generated within the subducting plate is determined for one event. However, the majority of solutions indicate compressive mechanisms consistent with the subduction environment. Focal depth is well resolved as depth phases are apparent on most of the seismograms due to the frequency range (0.05-5.0 Hz) of the broad-band records used. The circular or elliptical faults assumed for the source model vary from 3.5 to 8 km in radius depending on body-wave magnitude. The stress drop values obtained are typical of interplate earthquakes (-30 bar). However, the higher stress drop found for one event at a depth of approximately 180 km in the subducting lithosphere can be attributed to the intraplate character of this earthquake. Seismic moments obtained are similar to those obtained in centroid moment tensor (CMT) solutions, even though the latter are calculated using lower frequency seismic waves. This implies a fairly flat spectral level below the corner frequency.

Analysis of long‐period seismic waves excited by the November 1987 eruption of Izu‐Oshima Volcano

Long‐period seismic waves, which were excited by two eruptions of Izu‐Oshima volcano in November 1987 and recorded at an Abbreviated Seismic Research Observatory station and at several volumetric strain meter stations, are analyzed to determine kinematic seismic source models associated with the eruptions. The seismic waves have unusual features in the radiation pattern and waveforms of Rayleigh waves which cannot be explained in terms of a shear dislocation model. Waveform inversion is applied to obtain a kinematic source model which is parameterized by a single force and a seismic moment tensor. Many geophysical and field observations make clear that the two eruptions occurred in a lava drain‐back stage of the volcanic activity. The source model of the seismic event associated with the eruption on November 16, 1987, is mainly composed of a vertical single force with a magnitude of 4×1011 N, and suggests a rapid collapse of overloading lava into the deeper vent The long‐period seismic waves excited by the eruption on November 17, 1987, can be explained by a force system composed of two vector dipoles with nearly vertical orientation which are separated in time by 81 s. The sum of the two dipoles is 1.7×1017 N m. This source model suggests that tension crack openings, or volume expansions of horizontally extended magma reservoirs or of aquifers occurred because of lava drain back.

Random Source Effects on Seismic Ground Motions

The effect of randomness in the rapture process at the source on the near‐field seismic ground motions is examined. Time histories and response spectra are determined at an observation point in the vicinity of a vertical, strike‐slip fault. The seismic motions resulting from a deterministic, kinematic source model with uniform slip are compared with the motions obtained from stochastic models, in which the rupture parameters vary randomly along the length of the fault. The analysis shows that the random variability in the source parameters (namely, the dislocation amplitude, the rise time, the rupture velocity, and the fault width) enriches the frequency content of the motions. When the random variability in each rupture parameter is investigated separately, it is found that randomness in the rise time and the rupture velocity enhances the frequency content of the motions, whereas the variation in the dislocation amplitude and the fault width produces motions similar to the deterministic ones.

Use of Long-Period Seismometers To Determine Induced Fracture Geometry

Summary A novel technique to determine the geometry of induced hydraulic fractures has been tried at five shallow (305- to 884-m [1,000- to 2,900-ft] depth) stimulation tests since 1983. Long-period (LP) seismometers with peak ground-motion sensitivities in the period range between 10 and 100 seconds were deployed on the surface at distances 115 to 732 m [377 to 2,402 ft] from the treatment wellbores. Identifiable transient waveforms with reproducible site-specific ground-motion polarizations were recorded during four of the five stimulations. Temporal coincidence of these LP waveforms with treatment-related events [e.g., bottomhole pressure (BHP) passing through the formation closure stress] suggested that the signals may contain useful information about the induced fracture. Unsuccessful attempts to simulate the polarization directions as stationary tilts and/or horizontal displacements from the overall expansion of the fracture demonstrated that the source of the signals was not static or quasistatic. An alternative kinematic source model was examined to simulate the waveforms as transient pressure/fracture interactions. Although many observed signal characteristics were consistent with this latter model, considerable doubt regarding the origin of these signals and their usefulness to determine geometry of induced fractures remains. Commercial application of this monitoring technique does not annear to be economicallv feasible.

Kinematic analysis of strong motion P and SV waves from the Sterling Event

P and SV velocity waveforms from the Sterling explosion of December 3, 1966, are analyzed from a point of view often taken in earthquake source studies. The detonation occurred in the cavity excavated by the Salmon explosion within the Tatum salt dome, Mississippi. Velocity recordings taken from nearby borehole strong motion instruments show the occurrence of significant SV energy (Perret, 1968a, b). Kinematic source models are constructed by considering constraints afforded by the preshot cavity, condition of the surrounding material, and aspects of the P and SV waveforms such as wave polarity and duration. Waveforms for these models are computed and compared directly with the data. Modeling results suggest that the SV waves were radiated primarily by induced normal faulting occurring beneath the cavity rather than by an asymmetric pressure distribution based on the preshot cavity geometry. Secondary arrivals within the P waveforms and SV amplitudes suggest that these normal faults also had a component of tensional displacement which was probably due to forceable injection of material from the blast. A measurement for Qβ of 35 is obtained for salt from the amplitude decay arid pulse width changes observed for SV waves along a profile taken at shot level. This value for Qβ is for the region away from the cracked area induced by the previous Salmon explosion and qualitatively explains the differing frequency content observed at the closest Sterling sensors between P and SV waves. The anomalous SV radiation has little to do with ‘tectonic’ stress release but is a result of the medium's geometry and anisotropy.

Deep volcanic tremor and magma ascent mechanism under Kilauea, Hawaii

Deep harmonic tremor originating at depths around 40 km under Kilauea was studied using records accumulated since 1962 at the Hawaii Volcano Observatory of the U.S. Geological Survey. The deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island‐wide network of seismometers. The period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode. The tremor appeared to be primarily composed of P waves. We interpret the observed period and amplitude in terms of the stationary crack model of Aki et al. (1977) and find that the seismic moment rates for deep tremors are considerably larger than those for shallow‐tremors suggesting more vigorous transport for the former. We propose a kinematic source model which may be more appropriate for deep tremor. According to this model, a measurable quantity called ‘reduced displacement’ is directly proportional to the rate of magma flow. A systematic search for deep tremor episodes was made for the period from 1962 through 1979, and the amplitude, period, and duration of the tremor were tabulated. We then constructed a cumulative reduced‐displacement plot over the 18‐year period. The result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface. The total magma flow estimated from the reduced displacement is however, one order of magnitude smaller than that estimated by Swanson (1972). It may be that most channels transport magma aseismically, and only those with strong barriers generate tremor.