Strike-slip Earthquakes Research Articles

Scale-Model and Numerical Simulations of Near-Fault Seismic Directivity

Foam rubber experiments simulating unilaterally propagating strike-slip earthquakes provide a means to explore the sensitivity of near-fault ground motions to rupture geometry. Subsurface accelerometers on the model fault plane show rupture propagation that approaches a limiting velocity close to the Rayleigh velocity. The slip-velocity waveform at depth is cracklike (slip duration of the order of narrower fault dimension W divided by S -wave speed β ). Surface accelerometers record near-fault ground motion enhanced along strike by rupture-induced directivity. Most experimental features (initiation time, shape, duration and absolute amplitude of acceleration pulses) are successfully reproduced by a 3D spontaneous-rupture numerical model of the experiments. Numerical- and experimental-model acceleration pulses show similar decay with distance away from the fault, and fault-normal components in both models show similar, large amplitude growth with distance along fault strike. This forward directivity effect is also evident in response spectra: the fault-normal spectral response peak (at period ∼ W /3 β ) increases approximately sixfold along strike, on average, in the experiments, with similar increase (about fivefold) in the corresponding numerical simulation. The experimental- and numerical-model response spectra agree with an empirical directivity model for natural earthquakes at long periods (near ∼ W / β ), and both overpredict shorter-period empirical directivity effects, with the amount of overprediction increasing systematically with diminishing period. We attribute this difference to rupture- and wavefront incoherence in natural earthquakes, due to fault-zone heterogeneities in stress, frictional resistance, and elastic properties present in the Earth but absent or minimal in the experimental and numerical models. Rupture-front incoherence is an important component of source models for ground-motion prediction, but finding an effective kinematic parameterization may be challenging.

A Unified Source Model for the 1906 San Francisco Earthquake

Abstract We reconcile two previously discordant source models of the 1906 San Francisco earthquake and obtain a model that satisfies both triangulation and seismic data by allowing the rupture velocity to exceed the shear-wave velocity. Employing a projection method to remove the dependence on initial station positions allowed us to make use of a more stable triangulation network, including nonrepeated angle observations along the northern San Andreas fault. This strengthens the case for significant slip over the entire northern segment of the San Andreas fault from San Juan Bautista to Cape Mendocino during the 1906 earthquake. We also found that the teleseismic body-wave data can be reconciled with the geodetically derived slip model by allowing supershear rupture. This resolves a longstanding conflict between the two previous slip models (geodetic and seismic) of this earthquake. Supershear rupture has long been recognized as a theoretical possibility for strike-slip faulting, and it has been observed in several recent large strike-slip earthquakes, which raises the prospect that it might be typical for such events. Supershear rupture leads to substantially different strong ground motion, and as a result, may need to be taken into account when developing ground-motion prediction relations for large strike-slip earthquakes. Our final slip model has a seismic moment of 7.9×10 20 N m, which corresponds to a moment magnitude of M w 7.9.

Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime

A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion.

Comparisons of the NGA Ground-Motion Relations

The data sets, model parameterizations, and results from the five NGA models for shallow crustal earthquakes in active tectonic regions are compared. A key difference in the data sets is the inclusion or exclusion of aftershocks. A comparison of the median spectral values for strike-slip earthquakes shows that they are within a factor of 1.5 for magnitudes between 6.0 and 7.0 for distances less than 100 km. The differences increase to a factor of 2 for M5 and M8 earthquakes, for buried ruptures, and for distances greater than 100 km. For soil sites, the differences in the modeling of soil/sediment depth effects increase the range in the median long-period spectral values for M7 strike-slip earthquakes to a factor of 3. The five models have similar standard deviations for M6.5-M7.5 earthquakes for rock sites and for soil sites at distances greater than 50 km. Differences in the standard deviations of up to 0.2 natural log units for moderate magnitudes at all distances and for large magnitudes at short distances result from the treatment of the magnitude dependence and the effects of nonlinear site response on the standard deviation.

Radiated energy from the Noto Hanto, Japan, earthquake of March 25, 2007, and its aftershock

Abstract The Noto Hanto earthquake of March 25, 2007, is a shallow reverse-faulting event with M w = 6.7. We analyze transverse component accelerograms of the main shock and an aftershock (M w = 3.6) recorded at 10 strong motion observation stations. The hypocenter of the aftershock is located very close to that of the main shock and the epicentral distances of the stations are between 15 and 65 km. We calculate spectral ratio between records of the main shock and the aftershock in order to obtain source spectral ratio by empirically removing the effects of wave propagation path and surface layers at the stations. Thus obtained source spectral ratio is inconsistent with that expected for the self-similar omega-square source model assuming constant stress drop. Fitting omega-square source spectral ratio with variable corner frequency to the observed spectral ratio, the radiated energy and the corner frequency for the main shock and the aftershock are estimated. The radiated energy is 6.8 × 1014 J for the main shock and 6.7 × 109 J for the aftershock. The energy-to-moment ratios for the main shock and the aftershock are 5.0 × 10−5 and 2.1 × 10−5. This scale-dependence of the energy-to-moment ratio is similar to that obtained previously for shallow strike-slip earthquakes and reverse-faulting earthquakes in Japan. The relationship between the seismic moment, M 0, and the corner frequency, f 0, is M 0 ∝ f 0 −3.3±0.2 .

Deep-crust strike-slip earthquake faulting in southern Italy aided by high fluid pressure: insights from rheological analysis

Abstract I present the results of a crustal rheological analysis of the Potenza 1990 and Molise 2002 (southern Italy) deep foci (10–25 km) strike–slip earthquake sequences ( M max =5.7) located in the upper half of the middle crust. The analysis consists of geotherms and two-mechanism (brittle frictional v. ductile plastic flow) strength envelopes. Strength envelopes are calculated along a sub-vertical E–W-striking inherited crustal-scale fault zone reactivated under a strike–slip tectonic regime. The effects of high pore-fluid pressure ( Pf ) and strain rate increase due to afterslip processes during the co-seismic and early post-seismic phases are evaluated. The results are compared with the depth-distribution of seismicity. Under long-term strain rates, the rheology predicts a brittle layer in the upper part of the middle crust embedded by two plastic horizons: the base of the carbonates and the Verrucano layer (above) and the lower part of middle crust (below). The best rheological model for the observed seismicity is that considering high Pf in the middle crust (λ≥0.65). Brittle faulting aided by high Pf in the middle crust is in agreement with the focal depths and magnitudes of the main shocks. The upward and downward widening of the brittle layer in the middle crust, due to the increased strain rate in the co-seismic and early post-seismic phases (afterslip), agrees with the width of the aftershock zones. The weak ductile layer at the base of the carbonates (and possibly within the Verrucano) appears to have an important role in delimiting the seismogenic faulting upwards and probably played a role in defining a seal to fluid up-flow along the fault zone.

Strain rates and contemporary deformation in the Snake River Plain and surrounding Basin and Range from GPS and seismicity

We used new horizontal global positioning system (GPS) velocities along with earthquakes, faults, and volcanic features to assess how strain is accommodated in the northern Basin and Range Province. We estimated horizontal velocities for 132 stations within the Snake River Plain and the surrounding Basin and Range from GPS phase data collected from 1994 to 2007. These velocities show regional-scale clockwise rotation suggestive of driving forces beyond those associated with the Yellowstone hotspot. Within the western Centennial tectonic belt, the GPS measurements indicate that the Basin and Range is extending at a rate an order of magnitude greater than the Snake River Plain, which explains its low seismicity. Between these two regions, we discern the “Centennial shear zone,” a NE-trending zone of right-lateral shear with estimated slip rates that increase northeastward from 0.9 ± 0.3 mm/yr in the SW to 1.7 ± 0.2 mm/yr in NE. We interpret the new GPS velocities to indicate: (1) right-lateral shear may be accommodated by strike-slip earthquakes on NE-trending faults in the Centennial shear zone; (2) three Basin and Range faults (Lost River, Lemhi, and Beaverhead) terminate at the Snake River Plain margin; and (3) extension in the Snake River Plain occurs at a much lower rate than the rate of normal faulting in the western Centennial tectonic belt.

M-logA Observations for Recent Large Earthquakes

Abstract Using a magnitude ( M )-log area ( A ) dataset augmented with seven large ( M >7.0) earthquakes occurring since Wells and Coppersmith (1994), this short note assesses the current validity of the bilinear M -log A relations for continental, strike-slip earthquakes proposed by Hanks and Bakun (2002), in particular the L -model scaling at M >7. The relations determined by Hanks and Bakun (2002) are only insignificantly altered, leaving these bilinear M -log A relations as valid now as when first proposed.

Seismotectonics of strike–slip earthquakes within the deep crust of southern Italy: Geometry, kinematics, stress field and crustal rheology of the Potenza 1990–1991 seismic sequences (M max 5.7)

We present a revision and a seismotectonic interpretation of deep crust strike–slip earthquake sequences that occurred in 1990–1991 in the Southern Apennines (Potenza area). The revision is motivated by: i) the striking similarity to a seismic sequence that occurred in 2002 ∼ 140 km NNW, in an analogous tectonic context (Molise area), suggesting a common seismotectonic environment of regional importance; ii) the close proximity of such deep strike–slip seismicity with shallow extensional seismicity (Apennine area); and iii) the lack of knowledge about the mechanical properties of the crust that might justify the observed crustal seismicity. A comparison between the revised 1990–1991 earthquakes and the 2002 earthquakes, as well as the integration of seismological data with a rheological analysis offer new constraints on the regional seismotectonic context of crustal seismicity in the Southern Apennines. The seismological revision consists of a relocation of the aftershock sequences based on newly constrained velocity models. New focal mechanisms of the aftershocks are computed and the active state of stress is constrained via the use of a stress inversion technique. The relationships among the observed seismicity, the crustal structure of the Southern Apennines, and the rheological layering are analysed along a crustal section crossing southern Italy, by computing geotherms and two-mechanism (brittle frictional vs. ductile plastic strength) rheological profiles. The 1990–1991 seismicity is concentrated in a well-defined depth range (mostly between 15 and 23 km depths). This depth range corresponds to the upper pat of the middle crust underlying the Apulian sedimentary cover, in the footwall of the easternmost Apennine thrust system. The 3D distribution of the aftershocks, the fault kinematics, and the stress inversion indicate the activation of a right-lateral strike–slip fault striking N100°E under a stress field characterized by a sub-horizontal N142°-trending σ1 and a sub-horizontal N232°-trending σ3, very similar to the known stress field of the Gargano seismic zone in the Apulian foreland. The apparent anomalous depths of the earthquakes (> 15 km) and the confinement within a relatively narrow depth range are explained by the crustal rheology, which consists of a strong brittle layer at mid crustal depths sandwiched between two plastic horizons. This articulated rheological stratification is typical of the central part of the Southern Apennine crust, where the Apulian crust is overthrusted by Apennine units. Both the Potenza 1990–1991 and the Molise 2002 seismic sequences can be interpreted to be due to crustal E–W fault zones within the Apulian crust inherited from previous tectonic phases and overthrusted by Apennine units during the Late Pliocene–Middle Pleistocene. The present strike–slip tectonic regime reactivated these fault zones and caused them to move with an uneven mechanical behaviour; brittle seismogenic faulting is confined to the strong brittle part of the middle crust. This strong brittle layer might also act as a stress guide able to laterally transmit the deviatoric stresses responsible for the strike–slip regime in the Apulian crust and may explain the close proximity (nearly overlapping) of the strike–slip and normal faulting regimes in the Southern Apennines. From a methodological point of view, it seems that rather simple two-mechanism rheological profiles, though affected by uncertainties, are still a useful tool for estimating the rheological properties and likely seismogenic behaviour of the crust.

Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan Plateau

SUMMARY Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust‐mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east‐ west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east‐west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P-axes and shallow plunging EW trending T-axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60‐90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P-axes shift from vertical in the upper crust to mostly horizontal near the crust‐mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.

Coseismic deformation and slip distribution of the 1997 7.5 Manyi, Tibet, earthquake from InSAR measurements

We use interferometric synthetic aperture radar (InSAR) observations to investigate the coseismic deformation and slip distribution of the 1997 Mw7.5 Manyi earthquake, a left-lateral strike-slip earthquake occurred on the west portion of the Kunlun fault in the northern Tibet, China. The fault trace is constrained by the combination of interferometric coherence image and azimuth offset image. The total length of the identified fault is about 170 km. We estimate the source parameters using a seven-segment fault model in a homogeneous elastic half-space. We first use a uniform slip model to estimate the slip, width, dip and rake for each segment, resulting in a maximum slip of 5.5 m with a depth of 11 km on the fourth segment. The average dip of the uniform slip model is about 93 ◦ northward and the average rake is about −2 ◦ . We then use a distributed slip model to estimate the pure strike-slip and oblique slip distribution, respectively. In the distributed slip model, the fault plane is discretized into 225 patches, each of them 4 km × 4 km. We fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. We find a geodetic moment of 1.91 × 10 20 Nm (Mw7.5), of which almost 68% released in the uppermost 8 km and 82% in the uppermost 12 km. For all the models used in this study, the synthetic profiles along strike show asymmetric displacements on the opposite sides of the fault, which are in agreement with the observations. This suggests that a linear elastic model with variable and non-vertical dips is also reasonable for the mechanism of the Manyi earthquake. © 2007 Elsevier Ltd. All rights reserved.

Subarea characteristics of earthquake types in Yunnan

Studies on the earthquake sequences and the source mechanisms of the strong earthquakes show that Yunnan has more obvious subarea characteristics of earthquake type. Strike-slip seismic fault and mainshock-aftershock earthquake sequences are dominant in whole Yunnan area. Considering the ratio of non strike-slip faults and non mainshock-aftershock, Yunnan area can be divided into four subareas with different characteristics, which are strike-slip mainshock-aftershock in central Yunnan (A1), incline-slip swarm in northwestern Yunnan (A2), strike-slip double shocks in western Yunnan (B1) and quasi-strike-slip mainshock-aftershock in southwestern Yunnan (B2), respectively.

Aftershock Investigation in the Andaman–Nicobar Islands of India and Its Seismotectonic Implications

Abstract Six three-component short-period digital seismograph (four Reftek and two Kinemetrics) stations were established in different parts of the Andaman– Nicobar Islands following the 26 December 2004 devastating Sumatra–Andaman mainshock (Mw 9.3). Here, we analyze about 18,000 aftershocks (M ≥3.0) recorded from 6 January to 16 March 2005 to better understand the seismotectonics of the region. A sudden burst of aftershock activity with irregular trend in the month of January 2005 was observed at almost all seismograph stations. The estimate of P- value = 0.9532 from aftershocks (M ≥4.5) is near to normal value of 1.0, which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system (creep effects and history-dependent stress changes). The frequency–magnitude relation of the aftershocks followed the power law with average b-value of 0.7723 and it varies from 0.49 to 1.03, indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault. The distribution of the located aftershocks by a multistation method shows a north–south-trending aftershock cluster in an area of about 800 × 300 km2, which reflects an approximate rupture dimension of the mainshock beneath the Andaman–Nicobar Islands. Most of aftershocks occurred in a depth range of 5–65 km. The determination of composite fault-plane solutions of the best located aftershock clusters at three different depth ranges (0–15, 16–30, and >31 km) in the ten blocks of the region suggests that the mainshock rupture propagated through normal, reverse, and strike-slip earthquakes. The areas in the vicinity of Barren and Narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces, suggesting the facilitation of brittle failure in the weakened crust by the process of underheating. The zone near Baratang mud volcano is associated predominantly with thrust fault at depth of 0–30 km, suggesting a high compressive force beneath the Baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 December 2004 Sumatra–Andaman earthquake (Mw 9.3). Here, we propose that the relocation of our large aftershock dataset using converted sP-depth-phase technique for precise depth control is needed, and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region.

Predicting the endpoints of earthquake ruptures

The active fault traces on which earthquakes occur are generally not continuous, and are commonly composed of segments that are separated by discontinuities that appear as steps in map-view. Stress concentrations resulting from slip at such discontinuities may slow or stop rupture propagation and hence play a controlling role in limiting the length of earthquake rupture. Here I examine the mapped surface rupture traces of 22 historical strike-slip earthquakes with rupture lengths ranging between 10 and 420 km. I show that about two-thirds of the endpoints of strike-slip earthquake ruptures are associated with fault steps or the termini of active fault traces, and that there exists a limiting dimension of fault step (3-4 km) above which earthquake ruptures do not propagate and below which rupture propagation ceases only about 40 per cent of the time. The results are of practical importance to seismic hazard analysis where effort is spent attempting to place limits on the probable length of future earthquakes on mapped active faults. Physical insight to the dynamics of the earthquake rupture process is further gained with the observation that the limiting dimension appears to be largely independent of the earthquake rupture length. It follows that the magnitude of stress changes and the volume affected by those stress changes at the driving edge of laterally propagating ruptures are largely similar and invariable during the rupture process regardless of the distance an event has propagated or will propagate.

Benioff Strain Release Before Earthquakes in China: Accelerating or Not?

We systematically analyzed the Benioff strain release before 65 earthquakes with M S over 6.0 in China from 1978 to 2003 to investigate the generality of the widely discussed accelerating moment release (AMR) phenomenon before strong and intermediate-strength earthquakes. In this approach, a strong or intermediate-strength earthquake is selected as a “target earthquake,” and retrospective analysis of seismic activity before the “target earthquake” is performed. Simple searching area (three circular areas with different radius centered at the epicenter of the “target earthquake”) and unified temporal range (8 years) are taken in the analysis. Justification of whether AMR exists is by both visual inspection and by power-law curve fitting. It is found that more than 3/5 of the earthquakes under consideration exhibit clear pre-shock AMR property, and 1/3 of the events seem to be sensitive to the searching area. AMR behavior shows apparent focal mechanism dependence: 15 out of 17 dip-slip earthquakes with stable moment release characteristics against the changing of searching areas exhibit AMR behavior, while 16 out of 25 strike-slip earthquakes with stable moment release characteristics exhibit AMR behavior.

Fault identification for buried strike-slip earthquakes using InSAR: The 1994 and 2004 Al Hoceima, Morocco earthquakes

SUMMARY The 1994 M w 6.0 and 2004 M w 6.5 Al Hoceima earthquakes are the largest to have occurred in Morocco for 100 yr, and give valuable insight into the poorly understood tectonics of the area. Bodywave modelling indicates the earthquakes occurred on near-vertical, strike-slip faults with the nodal planes oriented NW‐SE and NE‐SW. Distinguishing between the primary fault plane and auxiliary planes, using either geodetic or seismic data, is difficult due to the spatial symmetry in deformation fields and radiation pattern of moderately sized, buried, strikeslip earthquakes. Preliminary studies, using aftershock locations and surface observations, have been unable to identify the orientation of the primary fault plane for either earthquake conclusively. We use radar interferometry and aftershock relocation of the earthquake sequence to resolve the ambiguity. For the 2004 earthquake, inverting the interferograms for a uniform slip model based either of the two potential nodal planes results in similar misfits to the data. However, the NE‐SW best-fit fault plane has an unrealistically high fault slip-to-length ratio and we conclude the NW‐SE striking nodal plane is the primary fault plane and slip was right lateral. We carry out tests on synthetic data for a buried strike-slip earthquake in which the orientation of the fault plane is known a priori. Independent of geometry, missing data, and correlated noise, models produced assuming the auxiliary plane to be the fault plane have very high fault slip-to-length ratios. The 1994 earthquake had a smaller magnitude and comparisons of model misfits and slip-to-length ratios do not conclusively indicate which of the nodal planes is the primary fault plane. Nonetheless, the InSAR data provides valuable information by improving the accuracy of the earthquake location by an order of magnitude. We carry out a multiple event relocation of the entire earthquake sequence, including aftershocks, making use of the absolute locations for the 1994 and 2004 main shocks from our InSAR study. The aftershock locations are consistent with a NW‐SE orientated fault plane in 2004 and suggests that the 1994 earthquake occurred on a NE‐SW fault; perpendicular to the fault which ruptured in 2004. Previous tectonic models of the area proposed a bookshelf model of block rotation with NNE‐SSW left-lateral faults. This model requires modification to accommodate the observation of right-lateral slip on a NW‐SE fault plane for the 2004 earthquake and we prefer to interpret the fault orientations as due to a zone of distributed shear with a right-lateral fault striking at ∼115 ◦ and conjugate, clockwise rotating, left-lateral faults striking at ∼25 ◦ .

Moment Tensor and Rupture Model for the 1949 Olympia, Washington, Earthquake and Scaling Relations for Cascadia and Global Intraslab Earthquakes

We reanalyzed the 13 April 1949 Olympia, Washington, earthquake by using digitized records and first-motion polarities from long-period seismograms. The moment-tensor mechanism is normal faulting with a down-dip-trending T axis similar in style to other Cascadia intraslab earthquakes. The total seismic moment is 1.3 × 1026 dyne cm ( M w 6.7) and the hypocenter depth is 60 km. Additional inverse modeling for the kinematic rupture process assuming the steeply east-dipping fault plane from the moment tensor resulted in a slightly higher total moment of 1.9 × 1026 dyne cm ( M w 6.8). The earthquake ruptured to the south with at least two subevents. The combined area of asperities and seismic moment for the 1949 earthquake was compiled with those from the 1965 Seattle-Tacoma and the 2001 Nisqually earthquakes and with those from Japan and Mexico to develop a source-scaling relation separate from shallow global strike-slip earthquakes. We infer that deeper intraslab earthquakes have a significantly smaller combined area of asperities than those compiled for shallower strike-slip earthquakes with the same seismic moment. This difference in rupture area leads to a 3- to 5-fold increase in stress drop for earthquakes with seismic moments between 1024 and 1028 dyne cm.

Using Modified Mercalli Intensities to Estimate Acceleration Response Spectra for the 1906 San Francisco Earthquake

We derive and test relations between the Modified Mercalli Intensity (MMI) and the pseudo-acceleration response spectra at 1.0 and 0.3 s— SA(1.0 s) and SA(0.3 s)—in order to map response spectral ordinates for the 1906 San Francisco earthquake. Recent analyses of intensity have shown that MMI ≥ 6 correlates both with peak ground velocity and with response spectra for periods from 0.5 to 3.0 s. We use these recent results to derive a linear relation between MMI and log SA(1.0 s), and we refine this relation by comparing the SA(1.0 s) estimated from Boatwright and Bundock's (2005) MMI map for the 1906 earthquake to the SA(1.0 s) calculated from recordings of the 1989 Loma Prieta earthquake. South of San Jose, the intensity distributions for the 1906 and 1989 earthquakes are remarkably similar, despite the difference in magnitude and rupture extent between the two events. We use recent strong motion regressions to derive a relation between SA(1.0 s) and SA(0.3 s) for a M7.8 strike-slip earthquake that depends on soil type, acceleration level, and source distance. We test this relation by comparing SA(0.3 s) estimated for the 1906 earthquake to SA(0.3 s) calculated from recordings of both the 1989 Loma Prieta and 1994 Northridge earthquakes, as functions of distance from the fault.

A LOOK BACK AT 1906--PERSPECTIVES ON GREAT EARTHQUAKES AND POST-EARTHQUAKE INVESTIGATIONS

At 5:12 am on April 18, 1906, a fracture formed some 10 km below the Earth's surface offshore of the Golden Gate. It propagated upward and outward, reaching the surface and extending north and south. To the south it broke along the San Francisco peninsula, climbed through the Santa Cruz Mountains, and stopped abruptly at San Juan Bautista. To the north it hugged the coast in Marin, Sonoma, and Mendocino counties, passed into the Pacific Ocean north of Pt. Arena, and came to halt on land in Humboldt County. Approximately one and one-half minutes after it began, the rupture stopped. Fourhundred-seventy km of the Pacific–North American plate boundary had shifted an average of 4½ m along the San Andreas Fault, and the M 7.9 San Francisco earthquake had occurred. An on-land strike-slip earthquake the size of 1906 is not an everyday occurrence, and diverse messages are to be taken away from it.... For me, one of the most compelling messages is the incalculable importance of post-earthquake investigations—both for what they provide at the moment and the legacy they leave for the future. This year, 2006, is the centennial of the 1906 San Francisco earthquake. This anniversary has prompted a broad range of earthquake-centered scientific, historical, cultural, and political activities. Scientifically it has refocused attention on the ground motions and the regional distribution of shaking intensity that the 1906 earthquake generated, the potential effects of its repeat on the San Francisco Bay Area and northern California, and its place in the development of earthquake science. In July 2005, I was in northern Mongolia for an international field trip celebrating the 100th anniversary of another great earthquake, the 1905 rupture of the Bulnay Fault. I admit to a fascination for these great on-land surface-faulting earthquakes, having had the good fortune to work …

Additional evidence on some relationship between Seismic Electric Signals (SES) and earthquake focal mechanism

Investigating the period 1983–1994 for western Greece, a possible correlation between the selectivity characteristics of the SES (seismic electric signals of the VAN method) and earthquake parameters has been reported by Uyeda et al. [Uyeda, S., Al-Damegh, K.S., Dologlou, E., Nagao, T., 1999. Some relationship between VAN seismic electric signals (SES) and earthquake parameters, Tectonophysics, 304, 41–55.]. They found that the earthquake source mechanism changed from largely strike-slip type to thrust type at the end of 1987, and this coincided with a shift in the SES sensitive site from Pirgos (PIR) to Ioannina (IOA) VAN station. Here, we report the results for the period January 1, 2002–July 25, 2004, during which the SES sensitive site of PIR became again active, after a 10-year period of “quiescence”. This activation was followed by strike slip earthquakes (on August 14, 2003 and March 17, 2004 with magnitude 6.4 and 6.5, respectively) in the Hellenic arc, which provides additional evidence on the correlation reported by Uyeda et al. The SES activities recorded at PIR have been discriminated from “artificial” noise by employing the natural time-domain analysis introduced recently.