Low Stress Drop Research Articles

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.

The 1998–1999 seismic series at Deception Island volcano, Antarctica

During the 1998–1999 Antarctic summer the pattern of seismic activity at Deception Island volcano changed significantly. The change was characterized by the occurrence of an intense swarm of volcano–tectonic (VT) earthquakes. More than 2000 VT earthquakes with S–P times smaller than 4 s were recorded in the period January–February 1999. Pure volcanic events were also detected; especially long-period (LP) events, volcanic tremor and some hybrid events. Seismic monitoring was performed using two short-period small-aperture arrays, among other instruments. Based on their signal-to-noise ratios we selected 863 VT earthquakes, 350 LP events and tremor episodes, and 9 hybrid events for analysis. We estimated apparent slowness and back-azimuth for all events using the Zero Lag Cross-Correlation array technique. Combining this information with S–P times and other indirect evidence, we identified two different source regions. LP seismicity is located less than 1–1.5 km southwest of the Fumarole array site. These events are likely to have a hydrothermal origin. VT earthquakes and hybrid events are located at depths of 0.3–10 km in an area under the bay of Deception Island. The area extends from the Fumarole array to the northeast with epicentral distances that range from 0.5 to 12 km. Most hypocenters are clustered in a small volume of around 8 km 3. The sources of the LP seismicity and the VT earthquakes are spatially distinct, which indicates that they are not produced by the same mechanisms. Moment magnitude analyses of the VT earthquakes provide an average magnitude of 0.5 and very low average stress drop, around 1 bar. A study of first motion of the P-waves suggests that the events in this small source region should have a variety of source mechanisms. This is supported by the existence of families of events with the same waveforms. The occurrence of repeating fracture processes with low stress drop and small fault dimensions can be explained by the lubrication of pre-existing zones of weakness by pressurized fluids. The most probable hypothesis that explains the generation of this seismic series at Deception Island is: a seismic series caused by the stress generated by the uplift of the source area due to a magmatic injection in depth. We favor this hypothesis since it is compatible with the majority of the characteristics of the seismicity and explains the spatial and temporal behavior of the series.

A study on physical property of crustal material and seismogenic environment in northeastern Pamir

2-D crustal structure and velocity ratio are obtained by processing S-wave data from two wide-angle reflection/refraction profiles in and around Jiashi in northeastern Pamir, with the result of P-wave data taken into consideration. The result shows that: 1) Average crustal velocity ratio is obviously higher in Tarim block than in West Kunlun Mts. and Tianshan fold zone, which reflects its crustal physical property of “hardness” and stability. The relatively low but normal velocity ratio (Poisson’s ratio) of the lower crust indicates that the “downward thrusting” of Tarim basin is the main feature of crustal movement in this area. 2) The rock layer in the upper crust of Tianshan fold zone is relatively “soft”, which makes it prone to rupture and stress energy release. This is the primary tectonic factor for the concentration of small earthquakes in this area. 3) Jiashi is located right over the apex or the inflection point of the updoming lower crustal interface C and the crust-mantle boundary, which is the deep structural background for the occurrence of strong earthquakes. The alternate variation of vp/vS near the block boundaries and the complicated configuration of the interfaces in the upper and middle part of the upper crust form a particular structural environment for the Jiashi strong earthquake swarm. vp/vs is comparatively high and shear modulus is low at the focal region, which may be the main reason for the low stress drop of the Jiashi strong earthquake swarm.

Comparative characteristics of the 1997–1998 seismic swarms preceding the November 1998 eruption of Volcán de Colima, México

The recent eruption of Volcán de Colima, México, began on 20 November 1998 and was preceded by a 12-month period of seismic activity that included five swarms of microearthquakes. The swarm activity occurred mainly within the southern part of Colima Volcanic Complex. We located about 600 events of magnitude M w from −0.5 to 2.7 within a 50 km 2 area including the active crater of Volcán de Colima and the region between it and the Pleistocene volcano Nevado de Colima 5.5 km to the north. The majority of hypocenters within this area did not exceed 5 km depth below sea level (BSL) and were mainly distributed within the volcanic edifices of Volcán de Colima and Nevado de Colima. We investigated the spatial distribution and spectral properties of events that occurred during three main seismic swarms: in November–December 1997, June–July 1998 and October–November 1998. We identified a temporal change in the depths of events from November–December 1997 and June–July 1998 (from 4 km BSL to 4 km above sea level (ASL)) to October–November 1998 (from 0 to 4 km ASL). Two groups of events with mean P-wave corner frequencies of 5 Hz (LF) and 10 Hz (HF) were distinguished. HF events dominated during the first swarm whereas the LF events were dominant during the June–July 1998 swarm. During October–November 1998, the numbers of HF and LF events were comparable, but very low-frequency low-stress drop events disappeared two days before the eruption. We consider the LF events to have resulted from fracturing along two systems of local faults, whereas the HF events could have been produced by dike movement along these same faults. We propose a model for construction of a magma passageway by the opening of two systems of local faults triggered by magma intrusion.

Research On Source Characteristics And Seismogenesis Of The Jiashi Earthquake Swarm, Western China

AbstractThe 1997 Jiashi swarm earthquakes are relocated by using the “master event” method. From the precise relative locations and focal mechanisms of MS ≥ 5.0 earthquakes we infer that a pair of right‐step echelon fault segments, striking NNW‐SSE and slipping right laterally, may exist in the source region. 2177 polarity readings of P‐wave first motions are inverted for the average focal mechanism of the swarm earthquakes. Based on the Silver's source model[1], the main fracture characteristics of the earthquake faults are inferred from the observed source spectra. We have calculated the perturbed stress field generated by the motion of echelon fault segments and explained some source characteristics of the swarm earthquakes. An echelon fault model is introduced to the diversity of focal mechanisms and low stress drop of the Jiashi earthquakes. The paper suggests that the echelon fault structure in the source region is responsible for the multiplicity of strong earthquake occurrence.

Repeating Earthquakes as Low-Stress-Drop Events at a Border between Locked and Creeping Fault Patches

The source of repeating earthquakes on creeping faults is modeled as a weak asperity at a border between much larger locked and creeping patches on the fault plane. The x –1/2 decrease in stress concentration with distance x from the boundary is shown to lead directly to the observed scaling 〈 T 〉∞〈 M 〉1/6 between the average repeat time and average scalar moment for a repeating sequence. The stress drop in such small events at the border depends on the size of the large locked patch. For a circular patch of radius R and representative fault parameters, Δσ = 7.6( m / R )3/5 MPa, which yields stress drops between 0.08 and 0.5 MPa (0.8–5 bars) for R between 2 km and 100 m. These low stress drops are consistent with estimates of stress drop for small earthquakes based on their seismic spectra. However, they are orders of magnitude smaller than stress drops calculated under the assumption that repeating sources are isolated stuck asperities on an otherwise creeping fault plane, whose seismic slips keep pace with the surrounding creep rate. Linear streaks of microearthquakes observed on creeping fault planes are trivially explained by the present model as alignments on the boundaries between locked and creeping patches.

Earthquake source parameters as inferred from the body waveform modeling, southern Turkey

Focal mechanism parameters of some significant earthquakes from southern Turkey have been estimated using either the body waveforms or the first motions of P-waves. It is observed that fault plane solutions derived from first motions may not be well constrained and may often be in error because of poor signal-to-noise ratios at some stations, poorly distributed recording stations in azimuth, lack of recognition of the reversed polarities at some stations, and positions of stations in terms of the crustal velocity structure, focal depth and, hence, take-off angles in the source area. The use of the inversion method introduces a considerable improvement in the focal parameters estimated in previous studies that have been used to construct seismotectonic models in the study area. The focal depths inferred from the inversion of body waveforms for the earthquakes occurring in southwestern Turkey are compatible with those reported by the ISC bulletins and other previous studies while the focal depths of earthquakes from the Antalya Bay are found to be less than those given by different agencies. Waveform inversion of earthquakes that have occurred in and around the Antalya Bay implies a low velocity layer showing azimuthal dependence. Hence, depth of rupture and lateral extent of velocity structure are of importance in the investigation of earthquake faulting process in this area. The source time duration of shallow earthquakes is generally longer than that for the deep focus earthquakes from the Antalya Bay and the Rhodes area. The events with relatively short and simple time functions may be interpreted as low stress drop events relative to the crustal ones.

The state of stress on some faults of the San Andreas System as inferred from near‐field strong motion data

We present a simple method to calculate the stress produced on an earthquake fault during rupture. This method allows the complete evaluation of the stress spatio‐temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault system over the last 20 years: the Imperial Valley earthquake of 1979, the Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomographic models of the fault rupture obtained from the inversion of the near‐field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary greatly over the fault. The peak values obtained for the four earthquakes studied range from about 20 to 100 MPa. These high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault. The regions of the fault which break with a high stress drop are also the regions where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the “strong” areas of the fault. Low slip regions tend to break with low stress drops. After the earthquake, the shear stress is increased over a significant portion of the fault, which corresponds to low slip regions. Aftershock activity tends to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heterogeneous. The rupture velocity seems to be inversely correlated with this apparent fault strength, the rupture accelerating over the “weak” areas of the fault and slowing down over the high strength areas.

Effects of cohesive zones on small faults and implications for secondary fracturing and fault trace geometry

Fracture mechanics theory and field observations together indicate that the shear stress on many faults is non-uniform when they slip. If the shear stress were uniform, then: (a) a physically implausible singular stress concentration theoretically would develop at a fault end; and (b) a single curved ‘tail fracture’ should open up at the end of every fault trace, intersecting the fault at approximately 70 °. Tail fractures along many small faults instead range in number, commonly form behind fault trace ends, have nearly straight traces and intersect a fault at angles less than 50 °. A ‘cohesive zone’, in which the shear stress is elevated near the fault end, can eliminate the stress singularity and can account for the observed orientation, shape, and distribution of tail fractures. Cohesive zones also should cause a fault to bend. If the cohesive zone shear stress were uniform, then the distance from the fault end to the bend gives the cohesive zone length. The nearly straight traces of the tail fractures and the small bends observed near some fault ends implies that the faults slipped with low stress drops, less than 10% of the ambient fault-parallel shear stress.

Analysis of the Temporal Occurrence of Seismicity at Deception Island (Antarctica). A Nonlinear Approach

—Deception Island is characterized by small magnitude local events with constant energy flux and very low stress drop. To obtain information about its origin, an interevent time series of 546 events, corresponding to an observational period of two month, has been analyzed. From a statistical point of view, data satisfies a Weibull distribution and presents clustering. A rescaled range analysis reveals that data are not independent, i.e. have memory, and the correlation dimension saturates at 2.2; as a consequence, the system can be modeled as a nonlinear iterative equation with three degrees of freedom that presents chaotic behavior. Taking into account that the average interevent time is of the order of 130 minutes, too short to be only due to tectonic activity, the above results indicate that some other mechanism may coexist with the regional tectonic one. According to several geological and geophysical observations, we suggest that most of the local events may be originated by pressure waves generated by a sudden change of phase, of sea and fresh water infiltrated into the main fractures and faults and also from shallow and confined water-saturated layers.

The organization of seismicity on fault networks.

Although models of homogeneous faults develop seismicity that has a Gutenberg-Richter distribution, this is only a transient state that is followed by events that are strongly influenced by the nature of the boundaries. Models with geometrical inhomogeneities of fracture thresholds can limit the sizes of earthquakes but now favor the characteristic earthquake model for large earthquakes. The character of the seismicity is extremely sensitive to distributions of inhomogeneities, suggesting that statistical rules for large earthquakes in one region may not be applicable to large earthquakes in another region. Model simulations on simple networks of faults with inhomogeneities of threshold develop episodes of lacunarity on all members of the network. There is no validity to the popular assumption that the average rate of slip on individual faults is a constant. Intermediate term precursory activity such as local quiescence and increases in intermediate-magnitude activity at long range are simulated well by the assumption that strong weakening of faults by injection of fluids and weakening of asperities on inhomogeneous models of fault networks is the dominant process; the heat flow paradox, the orientation of the stress field, and the low average stress drop in some earthquakes are understood in terms of the asperity model of inhomogeneous faulting.

Coseismic crustal deformation from microseismicity in the Patras area, western Greece

The Patras area lies in the western part of central Greece. It is an area characterized by high seismicity and complex neotectonics. Several devastating earthquakes have occurred in the region since 600 BC. Contemporary crustal deformation is examined in this area using microearthquake data recorded over a lengthy period, during 1983–84, by the Patras Seismic Network, principally, and to a lesser extent by the Volos Seismic Network. The microseismicity (1.8–3.9 ML) defines a zone deepening to the NE, which justifies a possible extension of the Gulf of Corinth major graben towards the Trikhonis Lake to the NW. Spectra of 108 well-located microearthquakes are estimated, using P-waves obtained by selective windowing designed to include only the P-phase; seismic moments in the range 0.3–45.7 × 1012 N m are obtained, accompanied by estimates of seismotectonic source parameters including source radii, average stress drop and average coseismic slip. Poor correlation is found between seismic moment and magnitude, and the likely reason is the complex nature of the neotectonic regime existing in the area. Two zones differing in crustal deformation characteristics are observed. The Corinth-Trikhonis zone reveals two sets of characteristic faults. The first set is represented by microearthquakes showing distinctive and relatively higher seismic moments in conjunction with lower stress drops and seismic slips. This set of faults shows greater source radius than the second set, and therefore the faults are longer. The second set is characterized by an almost constant source radius within the range of uncertainty, and a wide range of seismic moments, stress drops and seismic slips. The Rio zone is characterized by low seismic slip, stress drop and fault radii, with the exception of the locality south of the city of Patras, where relatively higher seismic slip, stress drop and fault radius are observed.

Research on earthquakes induced by water injection in China

In China, the earthquakes induced by water injection have occurred in four oil fields including the Renqiu oil field, and in two mines. Production of oil from the Renqiu oil field began in 1975 and the injection of water into the oil field commenced in July 1976. The induced earthquakes have been occurring in the area for the past 17 years, since December 1976. The controlled experiments of water injection showed the cause and effect relation between water injection and earthquakes. Source parameters such as source dimension, seismic moment and stress drop of a large number of the induced earthquakes, andQ factor for the area have been determined. The results indicate that the stress drop varies from 0.2 to 3.0 bar and theQ factor has an average value of 75.0. The low-stress drop and lowQ factor values imply that the earthquakes are caused by the brittle fracture of weak rocks under low ambient stresses, due to a decrease in their strength because of the injection of water. The induced earthquakes are unevenly distributed in the oil field. The northern part of the oil field, where the reservoir rocks are characterized by low porosity and low permeability, exhibits high seismic activity with the largest earthquake registering a magnitude of 4.5 and about 68% of the total number of induced earthquakes in this part. Whereas, the southern part of the oil field with higher porosity and higher permeability is characterized by low seismic activity with the largest earthquake registering a magnitude of 2.5 and only 4% of the total number of earthquakes which occurred in this part. These features of the focal region suggest that larger earthquakes may not occur in the Renqiu oil field area.

Heterogeneous distribution of dynamic stress drop and relative fault strength recovered from the results of waveform inversion: the 1984 Morgan Hill, California, earthquake

Abstract The dynamic rupture process of the 1984 Morgan Hill, California, earthquake (M = 6.2) has been investigated on the basis of a three-dimensional (3D) dynamic shear crack model, from previous waveform inversion results. For this purpose, a locking fracture criterion is introduced for rupture propagation, from which a lower bound of the peak shear stress just before rupturing, and hence of the relative fault strength, has been estimated at each fault segment. The distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures, respectively. The results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars, respectively. Negative stress drops down to −15 bars are required to explain very low slips over a shallow fault section. If a non-negative stress-drop condition is imposed, somewhat larger slips are obtained there than those from the waveform inversion. The above results suggest that there could be a zone of velocity-strengthening frictional behavior in the shallow crust, which may have arrested slip motion during rupture propagation. The strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation. The dynamic rupture initiated from a small nucleus zone with a low stress drop, propagated southeastward, breaking the deeper fault section with high stress drop, and then broke a relatively high strength zone after a short time of arrest, with highest stress drop.

The nucleation and rupture process of the 1981 Gulf of Corinth earthquakes from deconvolved broad-band data

SUMMARY Source parameters of the largest three normal faulting earthquakes (Ms6.6, 6.3, 6.4), in the 1981 Gulf of Corinth (Greece) sequence are determined using deconvolved broad-band data (recorded by arrays and single stations) and a 2-D finite source model. Such a model enables the spatial extent, rupture velocity and stress drop of the earthquakes to be determined and geological observations of surface slip can be included as a further constraint on the waveform modelling. All three earthquakes were shallow (<lo km) with low stress drops (<30 bars), and exhibited source complexity. The correspondence between the complexity of the earthquake sources and that of the mapped fault breaks implies that the segmentation of surface faulting in Greece is representative of faulting at depth. Tiny initial pulses which correlate across the arrays are seen in the seismograms from the Gulf of Corinth earthquakes at most stations. These initial subevents (<1 per cent of the total moment) are interpreted as the breaking of small asperities which initiated the main rupture and are used to constrain the attenuation correction (t* = 0.2 s).

Occurrence of low stress drop earthquakes in the Garhwal Himalaya region

Abstract The digital seismic data recorded by a telemetered seismic array has been used to compute seismic moments and stress drops of local earthquakes in the Garhwal Himalaya region based on Brune's model of the earthquake source. The seismic moments for 18 shallow focus earthquakes range from 7 × 1018 dyn cm to 6.23 × 1021 dyn cm. These earthquakes are found to have very low stress drops; less than 1 bar for seven events and between 1 bar and 10 bars for another seven events. The maximum stress drop is found to be 38 bars for an earthquake of magnitude 4.2 with focal depth 15 km. The shallow focus events having low stress drops in this region indicate that the crust has low strength to withstand the accumulated strain energy.

Determination of earthquake energy release and ML using TERRAscope

We estimated the energy radiated by earthquakes in southern California using on-scale very broadband recordings from TERRAscope. The method we used involves time integration of the squared ground-motion velocity and empirical determination of the distance attenuation function and the station corrections. The time integral is typically taken over a duration of 2 min after the P-wave arrival. The attenuation curve for the energy integral we obtained is given by q(r) = cr^(−n)exp(−kr)(r^2 = Δ^2 + h_(ref)^2) with c = 0.49710, n = 1.0322, k = 0.0035 km^(−1), and h_(ref) = 8 km, where Δ is the epicentral distance. A similar method was used to determine M_L using TERRAscope data. The station corrections for M_L are determined such that the M_L values determined from TERRAscope agree with those from the traditional optical Wood-Anderson seismographs. For 1.5 6.5, M_L saturates. The ratio E_S/M_0 (M_0: seismic moment), a measure of the average stress drop, for six earthquakes, the 1989 Montebello earthquake (M_L = 4.6), the 1989 Pasadena earthquake (M_L = 4.9), the 1990 Upland earthquake (M_L = 5.2), the 1991 Sierra Madre earthquake (M_L = 5.8), the 1992 Joshua Tree earthquake (M_L = 6.1), and the 1992 Landers earthquake (M_w = 7.3), are about 10 times larger than those of the others that include the aftershocks of the 1987 Whittier Narrows earthquake, the Sierra Madre earthquake, the Joshua Tree earthquake, and the two earthquakes on the San Jacinto fault. The difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times. The aftershocks, which occurred on the fault plane where the mainshock slippage occurred, had a very short time to heal, hence a low stress drop. The repeat time of the major earthquakes on the frontal fault systems in the Transverse Ranges in southern California is believed to be very long, a few thousand years. Hence, the events in the Transverse Ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times, such as the San Andreas fault and the San Jacinto fault. The observation that very high stress-drop events occur in the Transverse Ranges and the Los Angeles Basin has important implications for the regional seismic potential. The occurrence of these high stress-drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events. Characterization of earthquakes in terms of the E_S/M_0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the Los Angeles basin and the Transverse Ranges.

Dynamic Rupture Processes On A Dipping Fault, and Estimates of Stress Drop and Strength Excess From the Results of Waveform Inversion

The dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3-D, spontaneous shear-crack models. The wave equations for the 3-D space are solved numerically by a finite difference method under appropriate boundary conditions. the displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault. It was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall sideand that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the mediumparticularly of the existence of low-velocity surface layers. On the basis of the above model the dynamic rupture process of the 1961 Kita-Mino earthquake in central Japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis. For this purposea locking fracture criterion was introduced for rupture propagationfrom which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated. The dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips. The results revealed quite heterogeneous distribution of strength excessindicating large values near the SW section of the fault at shallow depths and in the central bottom sectionin contrast to small values at mid-depths in the NE section around the rupture nucleus zone. the distribution of stress drop was also found to be heterogeneouswith the existence of high-stress drop zones in the NEshallow and bottom sections and a low-stress drop zone at most of mid-depths except in the SW section. These results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low-strength excess and small-stress dropthen broke the shallower and deeper fault sections with moderate-strength excess and high-stress dropand finally ruptured barrier zones with high-strength excess. These zones may be interpreted as weak and strong asperities and barriersrespectively.

Dynamic fault rupture process during the 1978 Thessaloniki earthquake, northern Greece

Recent observations of strong earthquakes that occurred in the Aegean and surrounding areas during the last several years indicate rather low stress drop values. An interesting example is the 1978 Thessaloniki earthquake (northern Greece), with M s = 6.5. Values between 6.6 and 12 bars have been determined for this earthquake by assuming unilateral or bilateral rupture propagation and circular or rectangular fault models. These values are considerably lower than the average values calculated not only for intraplate earthquakes (about 60 bars) but also for interplate earthquakes (about 30 bars). In this paper an attempt is made to investigate the cause of this fact by studying the dynamic rupture process of this earthquake. A three-dimensional, spontaneous, shear crack model on a dipping fault in a layered medium was assumed, and a finite-difference approach has been adopted for numerically solving the wave equations in a three-dimensional space. It was found that such a low stress drop may be obtained in cases of small differences between ambient stress and fault strength, and between tectonic shear stress and dynamic frictional stress. Such physical conditions in the focal region may result in an average fault displacement which is close to the observed one for this earthquake.

Tsunami earthquakes: Slow thrust‐faulting events in the accretionary wedge

The November 20, 1960, Peru, October 20, 1963, Kurile and June 10, 1975, Kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude. Long‐period surface wave analysis indicates double‐couple (faulting) mechanisms for all three events rather than single‐force mechanisms indicative of submarine landslides. The earthquakes have shallow depths (&lt; 15 km) and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism. Body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events, with dip angles of 6°–8°. Surface wave spectral amplitudes and deconvolution of SH waveforms suggests anomalously long source durations and large seismic moments relative to MS. Specifically, the 1963 Kurile event (MS 7.2) shows a duration of 85 s and a moment of 6.0 × 1027 dyn cm (MW 7.8), the 1975 Kurile event (MS 7.0) shows a duration of 60 s and a moment of 2.0 × 1027 dyn cm (MW 7.5), and the 1960 Peru event (MS 6.75) shows a time function consisting of four subevents with a total duration of 110–130 s and a seismic moment of 3.4 × 1027 dyn cm (MW 7.6). Estimated rupture velocities are about 1 km/s or less, but there is no evidence of unusually low stress drops. The August 1, 1968, Philippines event, previously classified as a tsunami earthquake, shows none of the anomalous source properties, and teleseismic tsunami height measurements are sparse; we do not consider this event a tsunami earthquake. Most of the “anomalous” tsunami excitation results from underestimation of earthquake size by MS due to the long source duration; the tsunami heights are not significantly anomalous relative to seismic moment. The slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism. Standard scaling laws when adjusted for the slow seismic velocity in the source region show an MW ‐ MS relationship similar to that observed for the tsunami earthquakes and predict MS saturation at about 7.3 rather than 8.0 for typical events.