High Stress Drop Research Articles

Depth variation of seismic source scaling relations: implications for earthquake hazard in southeastern Australia

Advances in earthquake data acquisition and processing techniques have allowed for improved quantification of source parameters for local Australian earthquakes. Until recently, only hypocentral locations and local magnitudes ( M L) had been determined routinely, with little attention given to the inversion of additional source parameters. The present study uses these new source data (e.g. seismic moment, stress drop, source dimensions) to further extend our understanding of seismicity and the continental stress regime of the Australian landmass and its peripheral regions. Earthquake activity within Australia is typically low, and the proportion of small to large events (i.e. the b value) is also low. It is observed that average stress drops for southeastern Australian earthquakes appear to increase with seismic moment to relatively high levels, up to approximately 10 MPa for M L 5.0 earthquakes. This is thought to be indicative of high compressive crustal stress, coupled with strong rocks and fault asperities. Furthermore, the data indicates that shallow focus earthquakes (shallower than 6 km) appear to produce lower than average stress drops than deeper earthquakes (between 6 and 20 km) with similar moment. Recurrence estimates were obtained for a discrete seismogenic zone in southeastern Australia. Decreasing b values with increasing focal depth for this zone indicate that larger earthquakes (with high stress drops) tend to occur deeper in the crust. This may offer an explanation for the apparent increase of stress drop with hypocentral depth. Consequently, earthquake hazard estimates that assume a uniform Gutenburg–Richter distribution with depth (i.e. constant b value) may be too conservative and therefore slightly overestimate seismic hazard for surface sites in southeastern Australia.

Damage due to the Mw 7.7 Kutch, India earthquake of 2001

This paper presents a study of the damage due to the M w 7.6–7.7 intraplate Kutch earthquake of 26 January 2001. It was a powerful earthquake with a high stress drop of about 20 MPa. Aftershocks (up to M 4) have continued for 2.5 years. The distribution of early aftershocks indicates a rupture plane of 20–25 km radius at depths of 10–45 km along an E–W-trending and south-dipping hidden fault situated approximately 25 km north of the Kutch Mainland Fault. The moment tensor solution determined from regional broadband data indicates reverse motion along a south-dipping (by 47°) fault. The earthquake is the largest event in India in the last 50 years and the most destructive in the recorded history in terms of socioeconomic losses with 13,819 deaths (including 14 in Pakistan), collapse/severe damage of over a million houses and US$10 billion economic loss. Surface faulting was not observed. However, intense land deformations have been observed in a 40×20-km meizoseismal area. These include lateral spreading, ground uplifts (about a meter), ground slumping and deep cracks. Liquefaction with ejection of sand and copious water was widespread in the Banni grassland, Rann areas (salt plains), along rivers and also in the coastal areas up to 200 km distance from the epicenter in areas of intensity VII to X+. Stray incidences of liquefaction have occurred up to distances of at least 300 km. For the first time in India, multistory buildings have been destroyed/damaged by an earthquake. The maximum acceleration is inferred to be 700 cm/s 2 and intensities are 1–3 units higher in soil-covered areas than expected from the decay rate of acceleration for hard rock.

A Source Study of the Bhuj, India, Earthquake of 26 January 2001 (Mw 7.6)

We study the source time function (STF) and radiated seismic energy (E_R) of the M_w 7.6 Bhuj earthquake using the empirical Green's function (EGF) technique. Our estimations of the STF and E_R are based on teleseismic P waves and regional seismograms, respectively. We find that the STFs as a function of azimuth have a similar shape and nearly constant duration of 18 sec. This suggests that the rupture propagation was essentially radial. The STFs show a sharp rise in the first 6 sec. The E_R estimated from the EGF technique is 2.1 × 10^(23) erg. We find that E_R's computed from integration of corrected velocity-squared spectra of teleseismic P waves and regional seismograms are in excellent agreement with the ER obtained from the EGF technique. Since the seismic moment, M_0, is 3.4 × 10^(27) dyne cm, we obtain E_R/M_0 = 6.2 × 10^(-5). The radiation efficiency, η_R, during the Bhuj earthquake was low, about 0.23. The sharp rise of the STFs and η_R = 0.23 can be explained by Sato and Hirasawa's (1973) quasi-dynamic, circular source model with an effective stress of ∼ 300 bar and the ratio of rupture to shear-wave velocity, V_R/β, of ∼ 0.5. The corresponding estimate of slip velocity at the center of the fault is 156 cm/sec. V_R/β ∼ 0.5 is in reasonable agreement with the duration of the STF and the reported dimension of the aftershocks, as well as with the results of inversion of teleseismic body waves. The observations may also be explained by a frictional sliding model, with gradual frictional stress drop and significant dissipation of energy on the fault plane. This model requires an average dynamic stress drop of about 120 bar and V_R/β ∼ 0.7 to explain both the rapid rise in the first 6 sec of the STFs and, along with a static stress drop of 200 bar, the observed E_R/M_0. High static stress drop is a common feature of most crustal earthquakes in stable continental regions. An examination of the available data, however, does not suggest that most of them also have relatively low radiation efficiency.

Ground Motion and Liquefaction Simulation of the 1886 Charleston, South Carolina, Earthquake

As part of a comprehensive earthquake loss and vulnerability evaluation of the state of South Carolina, ground motions were simulated for a moment magnitude ( M ) 7.3 “1886 Charleston-like” earthquake using finite-fault and point-source stochastic numerical modeling. The probability for liquefaction was also predicted based on factors of safety computed from average cyclic stress and shear-wave-velocity-based cyclic resistance ratios, clay content, and saturation. Because there is considerable uncertainty regarding the 1886 source, the rupture plane of the 1886 event was modeled as both a 100-km-long, 20-km-wide fault (static stress drop of 27 bars) and a 50-km-long, 16-km-wide fault (107-bar stress drop). The source was assumed to be a north-northeast-striking, strike-slip fault coincident with the Woodstock fault. Based on comparing the computed and observed 1886 liquefaction areas and ground motions for both the low and high stress drop events, the two cases were weighted 0.8 and 0.2, respectively. To accommodate epistemic uncertainty in eastern U.S. earthquake source processes, three region-specific point-source attenuation models were also developed and used: a single-corner frequency model with both a constant stress drop and a magnitude-dependent stress drop, and the double-corner frequency model. The finite-fault and point-source models were weighted 0.8 and 0.2, respectively. To incorporate site effects into the ground-motion estimates, an extensive effort was made to characterize the thicknesses, shear-wave velocities ( V S), and dynamic material properties of unconsolidated sediments. Characteristic V S profiles were developed using the available subsurface information, which incorporated a wide range of soil and rock conditions. Amplification factors were computed for four site response categories, each of which were a function of soil thickness, input hard-rock motion, and spectral frequency. From the five weighted stochastic ground-motion models (two finite fault and three point source) and amplification factors, rock and soil ground motions were computed to produce statewide ground-motion maps for the M 7.3 scenario event. Weighting of the Charleston source and ground-motion models was implemented so that the resulting liquefaction areas matched the 1886 areas of liquefaction. Peak horizontal ground acceleration (PGA) values as high as 0.6 g -0.7 g were estimated in the vicinity of the modeled rupture. PGAs in the range of 0.3 g -0.4 g were estimated for Charleston consistent with the observed building damage and liquefaction. Significant ground shaking (PGA > 0.2 g ) extends out to distances of 50-60 km. Strong long-period (≥1.0 sec) ground motions are predicted throughout the state. The probabilities for liquefaction were highest in the epicentral region (>50%), consistent with the observed occurrences of liquefaction in 1886. Manuscript received 5 February 2003.

Source Parameters of the 2000 Yao'an Earthquakes

AbstractOn Jan.15, 2000, an earthquake sequence with the main shock MS6.4 occurred in the Yao'an region of Yunnan province. We study the physical process of this sequence based upon the data of aftershocks recorded by the near field small aperture observational network. By using the seismic scaling law, we have calculaded the parameters of the mainshock. They are: seismic moment M0 = 1:58×1018N.m; moment magnitude MW=6.0; average dislocation =0.63m; fault length L=16.6km; and fault width W=5.6km. The the accurate locating for the aftershock sequence and strike of aftershocks' energy distribution have verified that the fault rupture strike of the mainshock with MS6.4 is about N50°W. They also indicate that the Maweiqing fault in seismic region is the seismogenic structure of the mainshock, and the fault property is of right‐lateral strike‐slip type. The focal parameters of these earthquakes are estimated by utilizing the recorded data of transverse wave and the method of spectrum analysis. The seismic moment M0 is between 1010 and 1016N·m; the rupture radius of seismic focus varies from 80m to 500m; and seismic stress drop is in the range of 0.01~9.5MPa. The higher stress drop (Δσ > 1:0MPa) arranges in a linear manner along the main fault; and the high stress drops (Δσ > 2:0MPa) are related with earthquakes of ML ≥ 3.0. Both energy release of aftershocks and the earthquakes with high stress drop occur mostly at depths from 6km to 11km, which means the stress in the earth's crust is mainly concentrated in this depth range.

Heterogeneous distribution of the dynamic source parameters of the 1999 Chi‐Chi, Taiwan, earthquake

The spatial and temporal distribution of the stress on the fault plane of the 1999 Chi‐Chi, Taiwan, earthquake is calculated from kinematic inversion results using a three‐dimensional finite difference method for solving the elastodynamic equations. We analyze the relations between stress and slip for all grid positions on the fault, and use these relations to infer the friction law for the rupture. The dynamic source parameters were also determined. Our results show that for most of the points on the fault, the relation between stress and slip was consistent with the slip‐weakening law during the rupture process, especially for those points with large slip. However, consistency with the velocity‐weakening law is not clear from the observed relation between stress and slip velocity. The distributions of the dynamic parameters on the fault are very heterogeneous. The peak value of the static stress drop is ∼35 MPa. In general, high stress drop occurred in the areas with large slip. The estimated strength excesses are generally small suggesting that the tectonic shear stress had reached the level of the fault strength before the main shock. The slip‐weakening distance Dc and the fracture energy Gc are proportional to the final slip. The aftershock activity correlates with the spatial distribution of dynamic source parameters. Usually, the aftershocks near the fault plane are concentrated in regions with small or negative static stress drop, and there were few aftershocks in regions which had large values of critical slip‐weakening distance Dc.

Torishima selects Glasgow for European base

Using a waveform inversion method, we studied the rupture processes of two deep earthquakes in the Izu-Bonin region. The earthquake source was modelled by space-time-dependent slip on a fault plane. The fault plane was divided into many subfaults, and the source was expressed in terms of the final dislocations on the subfaults, the rupture starting times at the subfault corners and the rise time of source function. From P-wave seismograms recorded by the Global Digital Seismograph Network, we first obtained the displacement waveforms by deconvolution techniques. Next, we inverted these waveform data to determine the model parameters. For the earthquake near Torishima on March 6, 1984, non-homogeneous stress distribution and variation in rupture velocity were revealed. The rupture initiated at a corner of a 54 × 36 km2 fault, and high-stress drop occurred in the middle region of the fault, about 30 km away from the initiation. The earthquake south off Honshu of April 24, 1984, did not have as complex a rupture process. The rupture propagated unilaterally with almost constant velocity.

Size and orientation of the fault plane for the 2001 Gujarat, India earthquake (Mw7.7) from aftershock observations: A high stress drop event

We used a small array of portable seismographs to determine aftershock locations of the 2001 Gujarat earthquake. Our aftershock locations show a trend that dips toward the south at about 50° which is interpreted as the fault plane of the mainshock. The depth range of the aftershocks is from 10 to 35 km, which is somewhat deeper than other crustal earthquakes, and indicates that the faulting did not reach the surface. The area of the fault is about 40 × 40 km2, which is small for a Mw7.7 earthquake and results in a high static stress drop of 13 to 25 MPa. There are no mapped faults or obvious topographic features along the surface projection of this fault. These findings show that very large damaging earthquakes can occur without producing surface faulting, which is an important issue for earthquake hazard assessments in continental regions.

Sanyi‐Puli conductivity anomaly in NW Taiwan and its implication for the tectonics of the 1999 Chi‐Chi earthquake

Based on tensor decomposition technique, we have analyzed the dimensionality of magnetotelluric (MT) data and propose a 2‐D electrical model characterized by a high conductive anomaly beneath Sanyi‐Puli seismic zone, a distinct NW‐SE trending linear seismic zone in the fold‐thrust belt of NW Taiwan. Fluid pressurization indicated by the highly conductive anomaly may result in the active seismicity in Sanyi‐Puli seismic zone and, particularly, may also induce the large slip associated with high static stress drop in the northern part of the fault rupture process for the 1999 Chi‐Chi, Taiwan, earthquake.

Fault structure and mechanics of the Hayward Fault, California, from double‐difference earthquake locations

The relationship between small‐magnitude seismicity and large‐scale crustal faulting along the Hayward Fault, California, is investigated using a double‐difference (DD) earthquake location algorithm. We used the DD method to determine high‐resolution hypocenter locations of the seismicity that occurred between 1967 and 1998. The DD technique incorporates catalog travel time data and relative P and S wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events. The relocated seismicity reveals a narrow, near‐vertical fault zone at most locations. This zone follows the Hayward Fault along its northern half and then diverges from it to the east near San Leandro, forming the Mission trend. The relocated seismicity is consistent with the idea that slip from the Calaveras Fault is transferred over the Mission trend onto the northern Hayward Fault. The Mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the Calaveras Fault at Calaveras Reservoir. In some locations, discrete structures adjacent to the main trace are seen, features that were previously hidden in the uncertainty of the network locations. The fine structure of the seismicity suggests that the fault surface on the northern Hayward Fault is curved or that the events occur on several substructures. Near San Leandro, where the more westerly striking trend of the Mission seismicity intersects with the surface trace of the (aseismic) southern Hayward Fault, the seismicity remains diffuse after relocation, with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation. The seismicity is highly organized in space, especially on the northern Hayward Fault, where it forms horizontal, slip‐parallel streaks of hypocenters of only a few tens of meters width, bounded by areas almost absent of seismic activity. During the interval from 1984 to 1998, when digital waveforms are available, we find that fewer than 6.5% of the earthquakes can be classified as repeating earthquakes, events that rupture the same fault patch more than one time. These most commonly are located in the shallow creeping part of the fault, or within the streaks at greater depth. The slow repeat rate of 2–3 times within the 15‐year observation period for events with magnitudes around M = 1.5 is indicative of a low slip rate or a high stress drop. The absence of microearthquakes over large, contiguous areas of the northern Hayward Fault plane in the depth interval from ∼5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large‐magnitude earthquakes.

Coda Q c Attenuation and Source Parameter Analysis in Friuli (NE Italy) and its Vicinity

— The digital data acquired by 16 short-period seismic stations of the Friuli-Venezia-Giulia seismic network for 56 earthquakes of magnitude 2.3–4.7 which occurred in and near NE Italy have been used to estimate the coda attenuation Q c and seismic source parameters. The entire area under study has been divided into five smaller regions, following a criterion of homogeneity in the geological characteristics and the constrains imposed by the distribution of available events. Standard IASPEI routines for coda Q c determination have been used for the analysis of attenuation in the different regions showing a marked anomaly in the values measured across the NE border between Friuli and Austria for Q 0 value. A large variation exists in the coda attenuation Q c for different regions, indicating the presence of great heterogeneities in the crust and upper mantle of the region. The mean value of Q c (f) increases from 154–203 at 1.5 Hz to 1947–2907 at 48 Hz frequency band with large standard deviation estimates.¶Using the same earthquake data, the seismic-moment, M 0, source radius, r and stress-drop, Δσ for 54 earthquakes have been estimated from P- and S-wave spectra using the Brune's seismic source model. The earthquakes with higher stress-drop (greater than 1 Kbar) occur at depths ranging from 8 to 14 km.

Simultaneous rupture along two conjugate planes of the Wharton Basin earthquake.

Analysis of broadband teleseismic data shows that the 18 June 2000 Wharton Basin earthquake, a moment magnitude 7.8 intraplate event in the region of diffuse deformation separating the Indian and Australian plates, consisted of two subevents that simultaneously ruptured two near-conjugate planes. This mode of rupture accommodates shortening by a mechanism different from that previously known elsewhere in the region. The larger subevent occurred on a fossil fracture zone, with a relatively high stress drop of about 20 megapascals, showing that large stresses can accumulate in regions of distributed deformation.

A modeling study on the controlling factors of seismic source distribution and their strength index

A model, in which various seismic environmental factors are involved, has been developed in the paper on the bases of relevant data. The environmental factors include the crust structure, lithological conditions, fault plan attitude, the crust stress state, pore-fluid pressure and geothermal conditions, etc. The effect of each of the factors on the tectonic movement energy index has been analyzed. The model calculation results indicate that shear fracture energy of a fault generally increases with depth; it reached a peak value at a certain depth, then turns gradually attenuated. Of all the factors, the effects of pore-fluid pressure and geothermal conditions on the energy index are of prominence. High pore-fluid pressure and high temperature circumstance may result in decline of the peak value of the shear fracture energy curve, making the depths of the peak value and of the bell-waist value deepened. Such effects restrict strong seismic events, but suitable for micro-seismic activity and/or fault creep. The lower limit of focal depth under such an environment is relatively deep. Contrarily, low pore-fluid pressure and low geothermal temperature circumstance result in increase of the peak value of the shear fracture energy curve, and the curve becoming steep. The later circumstance is favorable to the formation of locked segment where possesses high strength and high-energy accumulation. If these two kind of segments are arranged interactively along a fault zone, such arrangement would boost the energy transition among the segments, forming mechanical circumstance of energy highly accumulated in one segment with adjacent segment(s) less-locked. Such contrast of the strength and energy would provide conditions for strong earthquakes with high stress drop. An analysis of the northwest segment of the Honghe fault has provided some evidence for the modeling results.

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.

A Simple Stick-Slip and Creep-Slip Model for Repeating Earthquakes and its Implication for Microearthquakes at Parkfield

If repeating earthquakes are represented by circular ruptures, have constant stress drops, and experience no aseismic slip, then their recurrence times should vary with seismic moment as \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(t\_{\mathrm{r}}{\propto}M\_{0}^{1{/}3}\) \end{document}. In contrast, the observed variation for small, characteristic repeating earthquakes along a creeping segment of the San Andreas fault at Parkfield (Nadeau and Johnson, 1998) is much weaker. Also, the Parkfield repeating earthquakes have much longer recurrence intervals than expected if the static stress drop is 10 MPa and if the loading velocity V L is assumed equal to the geodetically inferred slip rate of the fault V f. To resolve these discrepancies, previous studies have assumed no aseismic slip during the interseismic period, implying either high stress drop or V L ≠ V f. In this study, we show that a model that includes aseismic slip provides a plausible alternative explanation for the Parkfield repeating earthquakes. Our model of a repeating earthquake is a fixed-area fault patch that is allowed to continuously creep and strain harden until reaching a failure threshold stress. The strain hardening is represented by a linear coefficient C , which when much greater than the elastic loading stiffness k leads to relatively small interseismic slip (stick-slip). When C and k are of similar size creep-slip occurs, in which relatively large aseismic slip accrues prior to failure. Because fault-patch stiffness varies with patch radius, if C is independent of radius, then the model predicts that the relative amount of seismic to total slip increases with increasing radius or M , consistent with variations in slip required to explain the Parkfield data. The model predicts a weak variation in t r with M similar to the Parkfield data.

Compound rupture of the great 1998 Antarctic plate earthquake

The March 25, 1998, Antarctic plate earthquake ruptured a portion of the Antarctic plate more than 200 km west of its boundary with the Australian plate. The Harvard centroid moment tensor solution indicates that the earthquake was primarily a strike‐slip event, but the large non‐double‐couple component of the moment tensor suggests considerable complexity in the rupture process. We use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the Global Seismic Network. The P waves are poorly fit by one or more subevents having only a strike‐slip mechanism. We find that the presence of a large oblique‐normal faulting subevent located to the east of the hypocenter is necessary to improve the fit. This subevent combines with a larger strike‐slip subevent to the west to comprise the main moment release in the earthquake and is the cause of the large non‐double‐couple component in the long‐period focal mechanism. The earthquake exhibited very high slip and high stress drop compared with most interplate strike‐slip events and the rupture was largely confined to the upper 15–20 km of the lithosphere. Both constituent focal mechanisms indicate that this part of the Antarctic plate is under NW‐SE oriented tension, although the origin of these stresses is unknown.

The 1994 Hokkaido Toho-oki earthquake sequence: the complex activity of intra-slab and plate-boundary earthquakes

We study the complex activity of the 1994 Hokkaido Toho-oki earthquake sequence based on strong motion records and teleseismic data. First we examine S-wave acceleration spectra of the aftershocks by comparison with that of the main shock; the main shock ( M w=8.2), an intra-slab earthquake, has been characterized by strong radiation of high-frequency seismic waves. From different shapes of the spectral ratios of the aftershocks to the main shock, the aftershocks can be classified into three types. Next we redetermine the source parameters (focal depth, seismic moment and source duration) for larger events by waveform modeling of teleseismic P-waves. The hypocentral locations and stress drops estimated from these parameters show that the three aftershock types fall into different categories of subduction zone earthquakes; intra-slab, relatively deep plate-boundary, and shallow plate-boundary earthquakes. Finally, we estimate source spectra for three different earthquake categories after correcting S-wave acceleration spectra for the attenuation effect. The source spectrum at high frequencies is modeled by using a high-cut filter whose parameters are a cut-off frequency, f max and a decay rate. The source spectrum of intra-slab earthquakes has high f max and small decay rate; due to this feature and the high stress drop, intra-slab earthquakes radiate strong high-frequency seismic waves. Through this study, we demonstrate that the 1994 Hokkaido Toho-oki earthquake sequence has the complex activity of intra-slab and plate-boundary earthquakes.

Faulting in the 1986 Chalfant, California, Sequence: Local Tectonics and Earthquake Source Parameters

The July 1986, moment magnitude (M w ) 6.3 Chalfant, California, earth- quake is the largest of a recent series (1978-present) of moderate-sized earthquakes near the Long Valley volcanic region of east-central California. The sequence con- sists primarily of three moderate-sized strike-slip events. High-quality aftershock relocations and short-period focal mechanisms define the temporal and spatial de- velopment of the foreshock-mainshock-aftershock periods of these three events. Faulting during the Mw 5.7 (event I; July 20) and the Mw 6.3 (event II; 21 July) events constitute a set of conjugate strike-slip faults. Event I involved predominantly left-lateral motion on a NE-striking fault plane initiating at shallow depth (7 km). Event II initiated at 10.5 km depth exhibiting right-lateral strike-slip motion on a NW-striking fault dipping moderately to the southwest. An ML 5.5 strike-slip event on 31 July (event III) extended the aftershock sequence to the south into the White Mountains fault zone. P-wave pulse-width stress drops are determined for 185 ML 2.7-4.0 earthquakes that sample the entire sequence. Higher stress drops are observed near the intersection of event I and II fault planes and at the northern and southern ends of the aftershock zone. The moving-window b-value of the temporal magnitude distribution shows a general inverse relationship to stress drop with observed changes in both the average stress drop level and the b-value preceding event III. The average aftershock stress drop tends to increase as the sequence progresses suggesting that the faulted volume is equilibrating to the regional stress. Source parameters have been determined for the principal earthquakes from teleseismic body waves, local strong-motion records, and the extent of aftershock activity. The Chalfant sequence appears to be transferring strike-slip motion away from the White Mountains front, contributing to the observed increase in the relative normal offset, from south to north, along the White Mountains and the opening of the White Mountains relative to the Sierra Nevada Range front north of Owens Valley.

The focus parameters and their correlation from small earthquakes before and after the 1995 Douhe earthquake

Based on seismic source spectrum solution by multi-station and multi-event coda method, applying Brune seismic source model, we have got the source factors and spectra of 48 small earthquakes occurred before and after the Douhe earthquake of October 6 in 1995. The seismic moments, corner frequencies and stress drops are estimated from the spectra, and their correlation and variation with time before and after the Douhe earthquake are discussed. The results indicate that the source factors show good stability, their peak frequencies and variation with frequency appear quite similar. Some events with higher stress drops occurred about one year before the Douhe earthquake. Taking into the account that the stress drop is calculated from seismic moment and frequency, as well as the correlation between them, we emphasis that the higher stress drop mentioned here just implies the higher corner frequency than the normal value.

Dynamic rupture and stress change in a normal faulting earthquake in the subducting Cocos plate

A large nearly vertical, normal faulting earthquake (Mw = 7.1) took place in 1997 in the Cocos plate, just beneath the ruptured fault zone of the great 1985 Michoacan thrust event (Mw = 8.1). Dynamic rupture and resultant stress change during the 1997 earthquake have been investigated on the basis of near-source strong-motion records together with a 3-D dynamic 9model. Dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop, including patch-like asperities and negative stress-drop zones. Zones of high stress drop are mainly confined to the deeper, southeastern section of the vertical fault, where the maximum dynamic stress drop reaches 280 bars (28 MPa). The dynamically generated source time function varies with location on the fault, and yields a short slip duration, which is caused by a short scalelength of stress-drop heterogeneities. The synthetic seismograms calculated from the dynamic model are generally consistent with the strong-motion velocity records in the frequency range lower than 0.5 Hz. The pattern of stress-drop distribution appears, in some sense, to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event. This consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake.