Source Rupture Process Research Articles

Estimation of Source Rupture Process and Strong Ground Motion Simulation of the 2002 Denali, Alaska, Earthquake

A M W 7.9 inland crustal earthquake occurred in the Denali fault system, Alaska, on 3 November 2002 at 22:12 (UTC). In this study, we estimated the source process of the 2002 Denali earthquake by a multiple time-window linear kinematic waveform inversion using strong motion and Global Positioning System (gps)-measured static displacement data. The obtained source model could explain both the observed strong motion waveforms and gps-measured static displacements. Large slips on the fault plane are observed at approximately 80–90 km and 150–200 km east from the hypocenter. These features are consistent with the observed surface rupture distribution and the other inversion results obtained using teleseismic body waves. We also observed some portions of the whole fault with a local rupture propagation velocity of more than 4.0 km/sec that exceeded the shear-wave velocity of the source region. The relation between the rupture area and seismic moment of this earthquake seems to follow the bilinear L -model scaling rather than the self-similar source scaling model. The combined area of asperities is somewhat smaller than that expected from the empirical scaling relationship with seismic moments developed by compiling inverted source models. Finally, we conducted a forward ground motion simulation using the finite difference method to estimate the influence of the heterogeneous source process obtained here on the spatial distribution of strong ground motions. The calculated ground motions are relatively large above and around the large slip areas and also in the region east of the fault area because of the forward directivity effect of unilateral rupture propagation.

Scale Dependence of Apparent Stress for Earthquakes along the Subducting Pacific Plate in Northeastern Honshu, Japan

Apparent stress, which quantifies the ratio of high-frequency spectral amplitude to the direct current level of source displacement spectra, is known to reflect the source rupture process. Whether this parameter depends on the seismic moment has not yet been well established because of the difficulties in the reliable estimation of high-frequency source spectra. To overcome this problem, we estimate S -wave source spectra by using site amplification factors and attenuation factor ![Graphic][1] , which are systematically measured by the coda normalization method. Analyzing horizontal components of velocity seismograms recorded at 70-borehole seismic stations of Hi-net (National Research Institute for Earth Science and Disaster Prevention) in northeastern Honshu, Japan, we estimate site factors and ![Graphic][2] in the 0.5–32 Hz frequency band. By using these factors, we evaluate the source spectra of 225 small to moderate earthquakes that occurred in and around the subducting Pacific plate. Site-amplification factors we obtained strongly depend on frequency, and the frequency dependence clearly changes with the lithology and geologic age of rock. Obtained ![Graphic][3] values decrease with frequency in proportion to the reciprocal of frequency. The apparent stresses estimated from S -wave spectra clearly increase from 104 to 107 Pa with the seismic moment increasing, which cannot be attributed to the limited-frequency band and other artificial causes. The power of scale dependence is estimated as 0.39–0.44 for the seismic moment range from 1011 to 1017 N m. Some of the scale dependence of apparent stress is attributed to the scale dependence of Brune’s stress drop. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif

Rupture Process Of July 11, 2004, Tibet (Mw6.2) Earthquake

The source rupture process of the July 11, 2004, Tibet (Mw 6.2) earthquake is investigated by the finite fault model using far field broadband body wave records. The results show that it's a shallow, predominantly dip-slipping, normal-fault earthquake, and the hypocentral depth is about 12.5km. The fault plane has a strike of 152° and a dip of 44°, and the average slip angle is –117°. The rupture nucleates near the hypocenter, then propagates to the two ends of the fault with an average velocity of 2.8km/s, and the slip reaches to its maximum value of 43cm at about 5km northeast of the epicenter. The main principal tension axis of the fault plane solution shows roughly EW extension, which is controlled by the local rift belt in NNW-SSE or nearly NS direction, and the thrust motion of the India plate towards north is the main mechanical reason of this earthquake.

Moment tensor inversion and source rupture process of the September 27, 2003 M S=7.9 earthquake occurred in the border area of China, Russia and Mongolia

We conducted moment tensor inversion and studied source rupture process for MS=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic P-wave seismograms recorded by long-period seismograph stations of the global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the MS=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process inversion result of MS=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M0=0.97×1020 N·m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the MS=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence of spatial distribution of slip on the fault plane with the distribution of aftershocks also supports that the rupture is a heterogeneous process owing to the presence of barriers.

Analysis on the Reliability of the Earthquake Rupture Process Inferred from Near Source Waveforms

Several numerical tests have been analyzed in this paper to discuss the influences from the uncertain parameters of the presumed inversion fault model. The influence from the data to the resolution of the inversion results has been also analyzed. We have discussed the influence from the time length of the data and distribution of observational stations. Our research indicates: (1) The near-field waveform data can well reveal the source rupture process of the shallow part of the fault, however, it poorly constrains the slip distribution on the deep part of the fault. Jointly using near- and far-field waveform data can provide more complete message on the rupture process of the whole fault. (2) The near-field inversion result on the source is very sensitive to the strike and dip parameters of the presumed fault model. 2° deviation for the presumed strike and 5° deviation for the presumed dip angle from that of the true fault will lead to a distorted inversion result. (3) The influence from the uncertain media structure for Chi Chi earthquake faulting process inversion might be ignored if we can take the difference between Ma's model and Wu's model as the measure of the media uncertainty.

Fault Rupture Process of the 20 September 1999 Chi-Chi, Taiwan, Earthquake

We modeled the source rupture process of the 20 September 1999 Chi- Chi, Taiwan, earthquake (Mw 7.6) using an integrated dataset of near-field strong ground motion and Global Positioning System observations. This large inland thrust earthquake produced a surface break more than 80 km long along the Chelungpu fault. The event triggered over 700 strong-motion instruments on the island of Tai- wan. This large ground-motion dataset provides the best opportunity to date to un- derstand earthquake source processes in a major earthquake. First, we develop a 3D curved fault model consistent with the surface rupture data and aftershock distribu- tion. The subsurface fault plane has an average dip of 30 to the east, and measures 96 km along strike and 40 km down-dip. Next we applied a genetic algorithm to reconstruct the source rupture process. The inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down-dip. Most of the displacement occurred over a 15-km depth range in the northern segment of the fault, with the maximum slip exceeding 20 m. The average slip is 3.8 m, and the average rupture velocity is 2.6 km/sec. The mainshock slip distribution clearly complements the aftershock location distribution. Slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north. This relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations. The rake angle rotates across the rupture plane from predominantly dip-slip in the southern segment to mostly left-lateral oblique slip in the north. We find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior. At fault bends, rupture tends to decelerate, the rake angle rotates to a near dip-slip orientation, and the shear stress increases as rupture tries to overcome the bending barriers. The total seismic moment from our inversion is 2.9 10 27 dyne cm, in good agreement with the Harvard and the U.S. Geological Survey moment estimation.

Source Rupture Process of the Tecoman, Colima, Mexico Earthquake of 22 January 2003, Determined by Joint Inversion of Teleseismic Body-Wave and Near-Source Data

The spatial and temporal slip distribution of the Tecoman, Colima, Mexico earthquake is estimated from near-source strong-motion and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized Akaike's Bayesian information criterion (ABIC). The source parameters are as follows: (strike, dip, slip) = (300°, 20°, 93°), seismic moment M = 2.3 × 1020 N m; source duration = 30 sec; along-strike distance = 35 km; along-dip distance = 70 km. We found that the rupture process can be divided into three stages: the rupture nucleated near the hypocenter (stage I), then it broke the first asperity centering about 15 km southwest from the epicenter (stage II); and the rupture propagated to the northeast and the second asperity was broken (stage III). We also estimated the shear-stress change due to the rupture process of the mainshock on and around the major fault zone. It appears that one cluster of aftershocks for the first 5 days, which took place in and adjacent to the zones of stress, increased due to the fault rupture during the mainshock, but overall correlation between the aftershock location and the stress pattern is not clear. Manuscript received 14 May 2003.

Short-Term Properties of Earthquake Catalogs and Models of Earthquake Source

I review the short-term properties of earthquake catalogs, in particular the time and size distributions and the completeness of the early part of aftershock sequences for strong, shallow earthquakes. I determine the parameters of the Omori and Gutenberg-Richter laws for aftershocks close in time to a mainshock. Aftershock sequences of large earthquakes in southern California (1952 Kern County, 1992 Joshua Tree-Landers-Big Bear sequence, 1994 Northridge, and 1999 Hector Mine), recorded in the CalTech catalog, are analyzed to demonstrate that at the beginning of these series, many small earthquakes are absent from the catalog. The number of missing earthquakes increases with the magnitude range of a catalog and for some data sets exceeds the number of aftershocks close to a mainshock listed in a catalog. Comparing global earthquake catalogs (Harvard Centroid Moment Tensor and Preliminary Determination of Epicenter) with local data sets indicates that the catalogs based on longer period waves miss many early aftershocks even when their magnitudes are well above the stated magnitude threshold. Such short-term incompleteness may introduce significant biases to the statistical analysis of the seismicity pattern, in particular for branching models of earthquake occurrence incorporating the Omori law. For such models the likelihood function strongly depends on close aftershocks. I review the techniques to alleviate this problem. Analyzing the source rupture process of several recent large earthquakes suggests that rupture propagation is highly inhomogeneous in space, time, and focal mechanism. These random variations in the rupture process can be viewed as an extension of the aftershock stochastic generating mechanism toward the origin time of a mainshock. I review various models of the earthquake rupture process and suggest that fractal distributions of microevents in time, space, and focal mechanism constitute the development of an earthquake. The final identification of an individual earthquake depends on both objective and subjective factors. Manuscript received 21 May 2003.

A joint analysis of GPS motions and InSAR to infer the coseismic surface deformation of the Izmit, Turkey earthquake

SUMMARY From a joint analysis of GPS and InSAR data we have determined the kinematic coseismic surface deformation of the 1999 August 17, Izmit (Turkey) earthquake in terms of strain, rotation and fault slip. The fault slip contribution shows two distinct peaks: one of ∼4 m of slip at Golcuk, and a second of ∼2.9 m slip near Sapanca Lake. The strain field portrays four distinct quadrants reflecting the earthquake focal mechanism. The transition between the quadrants is not centred on the epicentre of the event, but shifted eastwards north of the fault and westwards south of the fault. This shift is the result of a two-stage source rupture process caused by the step-over features in the fault geometry. This rupture process also induced a distinct asymmetry in the displacement field across the fault. We obtain left-lateral shear strains along the Sakarya segment that is the result of a subsequent source nucleating at the Akyazi Gap inducing failure on the easternmost segments. Strong interaction is also observed from a consecutive source near Hersek. Finally, we deduce that the Izmit earthquake has released most of the strain that has accmulated since the last main event on this stretch of the North Anatolian fault in 1719.

Analysis of the reliability and resolution of the earthquake source history inferred from waveforms, taking the Chi-Chi earthquake as an example

In the inversion of observed waveforms to obtain the earthquake source faulting process some uncertain factors may influence the reliability of the results. Several numerical tests have been analysed in this paper in order to understand the effect of some of these uncertain parameters, such as the number and width of multiple time windows that define the source-time function, the assumed strike and dip of the fault, the velocity structure, and noise. Also, the influence of the sampling rate, the time window of the data and the distribution of observational stations is studied. Our research indicates: The near-source waveform data can adequately recover the source rupture process of the shallow part of the fault. However, they poorly constrain the slip distribution on the deep part of the fault. Using near-source and teleseismic waveform data jointly can provide a more complete view of the rupture process of the whole fault. The result using near-fault data is very sensitive to the assumed strike and dip parameters of the fault model. A 2° deviation of the presumed strike and a 5° deviation of the presumed dip from that of the true fault will lead to a distorted inversion result. The influence of the uncertainty of the seismic velocity structure for the Chi-Chi earthquake might be ignored if we can take the difference between Ma et al.'s model and Wu et al.'s model as the measure of the media uncertainty.

Source rupture process of the 2003 Tokachi-oki earthquake determined by joint inversion of teleseismic body wave and strong ground motion data

The spatio-temporal slip distribution of the 2003 Tokachi-oki, Japan, earthquake was estimated from teleseismic body wave and strong ground motion data. To perform stable inversion, we applied smoothing constraints to the slip distribution with respect to time and space, and determined the optimal weights of constraints using an optimized Akaike’s Bayesian Information Criterion (ABIC). We found that the rupture propagates mainly along the dip direction, and the length of the rupture area is shorter than its width. The mean rise time in the shallow asperity is significantly longer than that in the deep asperity, which might be attributed to variable frictional properties or lower strength of the plate interface at shallower depths. The average rupture velocity of deep asperity extends to the shear-wave velocity. The derived source parameters are as follows: seismic moment Mo = 1.7×1021 Nm (Mw 8.0); source duration = 50 sec. We also estimated the shear stress change due to the mainshock on and around the major fault zone. It appears that many aftershocks on the plate boundary took place in and adjacent to the zones of stress increase due to the rupture of the mainshock.

Numerical Modeling for Earthquake Source Image: Implications for Array Design in Determining the Rupture Process

The aim of this study is to develop a numerical method to evaluate the optimum seismic station distribution for imaging the source rupture process of an earthquake. Based on the beam-forming technique, the source rupture distribution of an earthquake can be reconstructed through theoretical travel time correction and waveform stacking. Numerical tests show that this method successfully reconstructs the main displacement distribution from an assumed source rupture plane. In accordance with assumed fault models, seismic waveforms are numerically generated as input data for further source imaging. From synthetic seismograms, we reconstructed the rupture distribution of these assume earthquake sources, and analyzed error systematically. Results of this study indicate that receiver distribution types really affect the successful reconstruction of the slip distribution. Furthermore, parameters such as dip angles and frequency content also play important roles in reconstructing earthquake sources. The proposed method is simple, inverse efficiently, and no initial condition required. Further applications of this method are suggested to image source rupture from near field strong motion observations and to design seismic array to effectively observe seismic rupture properties.

Inversion of near-field waveform data for earthquake source rupture process (I): Method and numerical test

A new inversion algorithm for simultaneously reconstructing the slip vectors and rupture times of a propagating finite fault is developed based on the recent progress in the nonlinear programming study. To check the validity of this new inversion algorithm, several numerical tests are conducted. The results show that this new source rupture process inversion algorithm is computationally efficient and numerically stable, and depends less on the initial model compared with the two popular inversion methods, i.e. the linear matrix method and the global stochastic search method. Therefore, this new inversion algorithm is expected to be useful in inverting earthquake rupture processes.

Signatures of the Seismic Source in EMD-Based Characterization of the 1994 Northridge, California, Earthquake Recordings

In this article we use empirical mode decomposition (EMD) to charac- terize the 1994 Northridge, California, earthquake records and investigate the sig- natures carried over from the source rupture process. Comparison of the current study results with existing source inverse solutions that use traditional data processing suggests that the EMD-based characterization contains information that sheds light on aspects of the earthquake rupture process. We first summarize the fundamentals of the EMD and illustrate its features through the analysis of a hypothetical and a real record. Typically, the Northridge strong-motion records are decomposed into eight or nine intrinsic mode functions (IMF's), each of which emphasizes a different oscillation mode with different amplitude and frequency content. The first IMF has the highest-frequency content; frequency content decreases with an increase in IMF component. With the aid of a finite-fault inversion method, we then examine aspects of the source of the 1994 Northridge earthquake that are reflected in the second to fifth IMF components. This study shows that the second IMF is predominantly wave motion generated near the hypocenter, with high-frequency content that might be related to a large stress drop associated with the initiation of the earthquake. As one progresses from the second to the fifth IMF component, there is a general migration of the source region away from the hypocenter with associated longer-period signals as the rupture propagates. This study suggests that the different IMF components carry information on the earthquake rupture process that is expressed in their different frequency bands.

Prediction of the maximum credible ground motion in Singapore due to a great Sumatran subduction earthquake: the worst‐case scenario

AbstractAlthough Singapore is located in a low‐seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium‐ and high‐rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may affect Singapore, the 1833 Sumatran subduction earthquake (Mw=9.0) has been identified to be the worst‐case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended reflectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2.5 s, which coincides with the natural periods of medium‐ and high‐rise buildings widely found in Singapore. The 90‐percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90‐percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90‐percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account effects of local soil response that might further amplify the bedrock motion. Copyright © 2002 John Wiley & Sons, Ltd.

Source rupture process of the Kocaeli, Turkey, earthquake of August 17, 1999, obtained by joint inversion of near‐field data and teleseismic data

The rupture process of the 1999 Turkey earthquake is examined using both near‐field strong motion data and teleseismic body wave data. The derived source parameters are as follows: (strike, dip, slip) = (268°, 86°, 180°), nearly pure strike‐slip; the seismic moment, Mo = 1.7 × 1020 Nm (Mw = 7.4); the source duration = 20 sec; the fault length = 70 km; the fault width = 15 km. The rupture process is characterized by an asymmetric bilateral rupture propagation and smooth slip. It consists of two major fault segments, a rupture propagating to the west and a second rupture propagating to the east. The maximum dislocation and the maximum dislocation velocity are 6.3 m and 2.7 m/s, respectively, both found at the former segment. The average dislocation is about 4 m. The extent of the coseismic rupture suggests that a considerable part of the anticipated seismic gap remains unruptured.

Source rupture process of the Papua New Guinea earthquake of July 17, 1998 inferred from teleseismic body waves

A large earthquake (Ms 7.1) occurred off northwest coast of Papua New Guinea (PNG), and a massive tsunami attacked villages to cause a devastating damage. In an attempt to ascertain the tsunami source, we investigate the source rupture process using teleseismic data at IRIS network as well as local data at Jayapura, Irian Jaya, station. The source parameters obtained are: (strike, dip, slip) = (301°, 86°, 91°); the seismic moment = 4.3 × 1019 Nm (Mw = 7.0); the duration of main rupture = 19 s; the centroid depth = 20 ± 5 km; the extent of rupture along the fault strike = 40 km; the average dislocation = 1.8 m; the stress drop = 7.3 MPa. The tsunami magnitude Mt determined from tide-gage data at long distance is 7.5, significantly larger than Ms, so that the PNG earthquake is characterized as a tsunami earthquake. Tsunami earthquakes might have been caused by slow rupture, submarine landslide, and high-angle dip-slip. Our teleseisimic analysis precludes the first two candidates and favors the third one as a source of the present earthquake, although it does not necessarily exclude the possibility of an aseismic landslide induced by the main shock or its aftershocks.

Modelling of ground acceleration in the near source range: the case of 1976, Friuli earthquake (M = 6.5), northern Italy

A mixed statistical-deterministic model of earthquake rupture is developed for evaluating the strong ground motion in the near source range (receiver distance comparable to the fault length). The source parametrization is based on the k-square model and the propagation is computed by asymptotic Green's functions. The method is applied to the case of 1976, Friuli earthquake (M = 6.5) in northern Italy which occurred on a low-dip thrusting fault. Acceleration records at 29 stations are computed for 100 simulations of rupture histories. The mean value map of peak ground accelerations shows clearly a maximum to the south due to the inner geometry and directivity of the source. The variation of the estimated PGA versus the epicentral distance is strongly dependent on azimuth and is not decreasing monotonically. The comparison of these curves with those predicted by empirical acceleration–distance relationships shows discrepancies in the near source distance range. This study shows the importance of considering the complexity of the source rupture process for strong motion estimate in the near source range.

The 1977 Three Kings Ridge earthquake ( Ms = 6.7): broad-band aspect of the source rupture

The 1977 Three Kings Ridge earthquake ( M s = 6.7) is an isolated event which took place in the South Fiji Basin. The centroid moment tensor (CMT) solution by the Harvard group shows a left-lateral strike-slip motion with a dip angle of 55° while the P-wave first motions require a nearly vertical fault plane. The centroid depth 17.4 km is deeper than the bottom of ordinary oceanic crust. Using long-period P and SH waveforms from the Worldwide Standardized Seismograph Network, we model the entire source rupture process with three subevents which span a 42 s duration. The best-fit double-couple focal mechanism obtained from the tensorial sum of the subevents is primarily a strike-slip with a nearly vertical fault plane subparallel to the Cook Fracture zone. The total moment release is (2.2–2.3) × 10 26 dyn cm, which is about 40% lower than the CMT estimate. The subevents' depth distribution is similar to the centroid depth (17 km), which is deeper than a typical oceanic Moho. Precise modeling of the waveform is also incompatible with a standard oceanic crustal structure, lending support to previous interpretations of the Three Kings Ridge as a fossil island arc. The discrepancy between our body waveform moment tensor solution and the Harvard CMT solution is dominated by the tensor components M rθ and M rθ . In the case of a shallow source, these components are notoriously unresolvable because of their vanishing excitation of seismic waves in the low-frequency range ( T ⩾ 45 s). However, the low-pass filtered body waves can be better modeled with a dip angle similar to that of the CMT solution. This observation suggests that the source process may include some level of evolution of the rupture mechanism with time. We interpret the earthquake as expressing a moderate level of internal deformation of the Australian plate, in the context of significantly different spreading rates in the Lau and Havre Basins.

A Study on Source Rupture of the 1978 Lan-Hsu, Southeastern Taiwan Earthquake

Seven teleseismic long period P-wave records from World-Wide Standard Seismograph Network (WWSSN) are used to infer the source rupture process of the July 23, 1978 Lanhsu, eastern Taiwan earthquake through the body-wave iterative deconvolution method (Kikuchi and Kanamori, 1982; 1986). Results show that the rupture sequence consists of one main event and one largest subevent near the hypocenter determined by Pezzopane and Wesnousky (1989) and numerous smaller ones at a distance from the hypocenter. Comparison of the spatial distribution of mainshock rupture sequence with aftershocks shows that aftershocks are located in the area which did not break before the occurrence of the above-mentioned events. The total seismic moment of the three largest events is 8.76×10^26 dyne-cm.