Normal-faulting Earthquakes Research Articles

Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009

The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.

Common dependence on stress for the two fundamental laws of statistical seismology

Two of the long-standing relationships of statistical seismology are power laws: the Gutenberg-Richter relation describing the earthquake frequency-magnitude distribution, and the Omori-Utsu law characterizing the temporal decay of aftershock rate following a main shock. Recently, the effect of stress on the slope (the b value) of the earthquake frequency-magnitude distribution was determined by investigations of the faulting-style dependence of the b value. In a similar manner, we study here aftershock sequences according to the faulting style of their main shocks. We show that the time delay before the onset of the power-law aftershock decay rate (the c value) is on average shorter for thrust main shocks than for normal fault earthquakes, taking intermediate values for strike-slip events. These similar dependences on the faulting style indicate that both of the fundamental power laws are governed by the state of stress. Focal mechanisms are known for only 2 per cent of aftershocks. Therefore, c and b values are independent estimates and can be used as new tools to infer the stress field, which remains difficult to measure directly.

Near-Fault Strong Ground Motions Recorded during the Morelia Normal-Fault Earthquakes of May 2006 in Mexicali Valley, B. C., Mexico

By the end of May 2006, a series of earthquakes had occurred in the Mexicali Valley, Baja California, Mexico. This activity, which included an M w 5.4 event, developed in the vicinity of the Cerro Prieto geothermal field. The earthquakes occurred on the Morelia fault, one of the east-dipping normal faults in the Mexicali Valley. Cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length, the shorter segment being 1 km to the east of the other one. Down-dip displacements of up to 25–30 cm were measured at some places along this surface break zone. The ten larger events of the series were recorded by strong-motion stations that operate in the epicenter area, so that accurate hypocenter locations were obtained on the basis of P -wave arrival times from the strong-motion recordings. The estimated shallow depths, combined with the earthquake magnitude, explain the strong shaking felt by residents and some damage observed on the Cerro Prieto geothermal plant. The M w 5.4 event produced peak ground accelerations that go from 0.002 g at CUC (on rock) to 0.5 g at GEO (on sediments), at 13 and 1.7 km from the epicenter, respectively. The station GEO recorded closer to the epicenters and on the hanging-wall side of the fault. Static ground displacements and a predominance of the strike-normal over the strike-parallel velocity components were determined from the acceleration records of this station. These and other ground-motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event. Altogether, the observed ground-motion characteristics provide useful insights into the levels of ground shaking that near-fault structures in the Mexicali Valley should be designed to withstand.

Extensional Seismogenic Stress and Tectonic Movement on the Central Region of the Tibetan Plateau

Various earthquake fault types, mechanism solutions and stress fields, as well as GPS and geothermal data are analyzed for the study of the crustal movements on the Tibetan plateau and their tectonic implications. The results show that a lot of the normal faulting type-event concentrated at altitudes greater than 4000 m on the central Tibetan plateau. The altitudes concentrating normal faulting type-events can be zoned two parts: the western part, the Lhasa block, and the eastern part, the Qiangtang-Changdu region. The azimuths of T-axes are in a general E-W direction in the Lhasa block and NW-SE or NNW-SSE in the Qiangtang-Changdu region at the altitudes of the Tibetan plateau. The tensional stresses in E-W direction and NW-SE direction predominate normal faulting earthquake occurrence in the Lhasa block and the Qiangtang-Changdu region, respectively. The slipping displacements of the normal-faulting-type events have great components in near E-W direction and NW-SE direction in the Lhasa block and the Qiangtang-Changdu region, respectively. The extensions are probably an eastward or southeastward extensional motion, being mainly tectonic activity phenomena in the plateau altitudes. The extensional motions due to normal-fault earthquakes are important tectonic activity regimes on the high altitudes of the plateau. The easterly crustal extensions on the plateau are attributable to the gravitational collapse of the high plateau and eastward extrusion of hotter mantle materials beneath the eastern boundary of the plateau. Numbers of thrust-fault and strike-slip-fault earthquakes with strong compressive stress in a general NNE-SSW direction occur on the edges of the plateau.

Triggering of the 2001-2002 Mexico Slow-slip Event, Mw 7.5, by the Shallow Normal Earthquake of 8 October 2001, Mw 5.8, and Its Aftershocks

The Guerrero coast in the subduction zone of west-central Mexico has been a seismic gap for almost 100 years. Large aseismic slow-slip events (SSE) have been frequently observed since GPS stations were established in this area (Lowry et al. 2001; Kostoglodov et al. 2003). The tectonic mechanism of slow-earthquake episodes is not well understood: some are preceded by significant earthquakes, others are not. The coincidental occurrence on 8 October 2001 of a shallow coastal earthquake of magnitude 5.8 in the same area as the slow earthquake of magnitude Mw 7.5 is attributed to a triggering effect. Previous investigators have dismissed the triggering hypothesis for various reasons, mainly because the earthquake was judged to be too small to trigger such a large SSE. However, reexamination of the seismic, tilt, and GPS data suggests that the hypothesis of triggering cannot be excluded. The 2001–2002 SSE ( Mw 7.5) has been extensively described and investigated (Kostoglodov et al. 2003; Yoshioka et al. 2004; Mikumo 2004; Iglesias, Singh, et al. 2004; Iglesias, Pacheco, and Singh 2004; Franco et al. 2005). It generated a long-duration aseismic slip episode over an area of about 550 km by 250 km comprising the southeastern part of the Guerrero gap and extending into the active coast of northwestern Oaxaca (Figure 1). The slip episode was monitored by seven GPS stations over a period of more than eight months. Surface displacements were estimated at about 1.3 cm to 6 cm (Franco et al. 2005). The Coyuca earthquake of 8 October 2001 was an important seismic event. Shallow, normal-faulting earthquakes are rare on the Guerrero subduction coast. However, the Coyuca earthquake has received only cursory attention. We suggest that the triggering of large SSEs by normal earthquakes in the overriding plate may represent a previously …

Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography

In this study, we outline the space and time variations of body-wave velocities during one of the best-documented series of normal-faulting earthquakes, the 1997 Umbria-Marche sequence in central Italy. We show the first ever observations of rock fracturing and fluid overpressure propagation along a fault system by using space-time resolved variations of Vp/Vs anomalies (four-dimensional variations) that accompany earthquake migration and precede large aftershocks. The Vp/Vs increase observed before the mid-October earthquakes was related to a pore-pressure increase on fluid-filled cracks in the volume around the fault. We also document that such variations are measurable only by using S-and P-wave arrival times.

Directivity effects for moderate-magnitude earthquakes (Mw 5.6–6.0) during the 1997 Umbria–Marche sequence, central Italy

The Colfiorito Mw 6.0 (September 26, 1997, 09:40 UTC) and the Sellano Mw 5.6 (October 14, 1997) normal-faulting earthquakes are the two strongest events of the 1997–1998 Umbria–Marche seismic sequence (central Italy). They provided accelerograms recorded at epicentral distances within nearly two fault lengths. Previous studies on these events revealed clear evidence of unilateral rupture propagation and directivity effects on the source time functions. We developed an approximate procedure to correct the observed peak ground motion for the site effects, and we found a systematic increase of peak ground acceleration (PGA) and peak ground velocity (PGV) at sites located in the direction of the rupture propagation respect to the stations located to the backward. In particular we showed that, for the Colfiorito earthquake, the average PGV at the north-west sites is 2.8 times larger than the PGV at south-east sites (2.1 for PGA); an opposite behaviour is found during the Sellano earthquake, with an amplification factor of about 1.9 for PGV (1.5 for PGA) recorded at south-east sites respect to the north-west stations. We explained these behaviours in terms of directivity effect, by comparing recorded ground accelerations with synthetics simulated by a deterministic–stochastic technique. This method computes high-frequency ground motions ( f > 0.5 Hz) from extended faults, generalizing the point-source stochastic method to capture the essence of near-source effects. The synthetic PGAs decrease with distance in agreement with the ground-motion empirical predictive model available for the area and describe the azimuthal variation of recorded data in terms of source effects.

Kinematics and dynamics of the southeastern margin of the Tibetan Plateau

SUMMARY On the southeastern margin of the Tibetan Plateau lies a large region which seismicity and GPS data show to be actively deforming. This paper describes the active faulting in the region, and how it relates to the velocity field observed with GPS. In places the velocity field is accommodated by rotations about vertical axes, and most or all of the strain at the surface in the region appears to be released seismically. GPS velocities are then compared to velocities calculated using a model for deformation driven by gravitational driving forces. Using rheologies estimated from experimentally derived mineral flow laws, the model provides velocities that are in good agreement with observed GPS velocities. It is not possible to uniquely determine the rheology or flow velocity at depth, and there are two forms of model solution which match the observed horizontal surface velocities. In one of these, vertical planes deform by pure shear, and in the other vertical gradients of horizontal velocity are present within the crust. Two distinct regions of normal-faulting earthquakes are present in the region, and have mechanisms which are most easily explained by gravity-driven deformation.

Earthquake Probabilities and Energy Characteristics of Seismicity Offshore Southwest Taiwan

We characterize fault geometries for moderate and large earthquakes in and offshore southwest (SW) Taiwan. Stress orientation estimates suggest that the first shock of the Pingtung earthquake may be characterized by slab flexure. A region of predominately normal-faulting earthquakes is also defined. Using the maximum-likelihood Gutenberg-Richter relation, we find that the repeat times for magnitude,M 6.5 and M 7.0 earthquakes are approximately 30 and 80 years, respectively. By assuming a Gaussian probability distribution with a standard deviation of 0.4 the recurrence time, we estimate a 50% probability of anM 6.5 earthquake recurring in the next 30 years, while only a 20% probability of anM 7.0 in the next 50 years. To test the empirical relation that is currently being used to convert ML to seismic moment (M0), we test the well-defined energy to moment ratio (E/M0) for the region. We additionally utilize this method to identify the lack of slow-source earthquakes in several recent offshore events in Taiwan.

Enhancement of the National Strong-Motion Network in Turkey

Two arrays comprising 20 strong-motion sensors were established in western Turkey. The 14 stations of BYTNet follow a N-S trending line about 65 km in length, normal to strands of the North Anatolian fault that runs between the cities of Bursa and Yalova. Here the dominant character of the potential fault movement is a right-lateral transform slip. The DATNet array, comprising a total of eight stations, is arranged along a 110-km-long E-W trending direction along the Menderes River valley between Denizli and Aydin. (Two stations in this array were incorporated from the existing Turkish national strong-motion network.) This is an extensional tectonic environment, and the network monitors potential large normal-faulting earthquakes on the faults in the valley. The installation of the arrays was supported by the North Atlantic Treaty Organization (NATO) under its Science for Peace Program. Maintenance and calibration is performed by the General Directorate of Disaster Affairs (GDDA) according to a protocol between Middle East Technical University (METU) and GDDA. Many young engineers and scientists have been trained in network operation and evaluation during the course of the project, and an international workshop dealing with strong-motion instrumentation has been organized as part of the project activities.

INFLUENCE OF FAULT MECHANISM, DEPTH, AND REGION ON STRESS DROPS OF SMALL AND MODERATE EARTHQUAKES IN JAPAN

Stress drops of 168 earthquakes (focal depth ≤60 km, 4.4≤MW≤6.9) are estimated by a generalized-inverse method using strong motion records in Japan. Stress drops of crustal earthquakes are dependent of focal depths. Stress drops of reverse-faulting earthquakes are two times greater than those of strike-slip faulting earthquakes and stress drops of strike-slip faulting earthquakes are two times greater than those of normal-faulting earthquakes. These results are consistent with the crustal strength expected from a frictional law. Stress drops of intraplate earthquakes have more variation than those of interplate earthquakes, but are two times greater than those of interplate earthquakes on average.

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

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

Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

SUMMARY The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (τa = µES/M 0 ,w here ES is radiated energy and M 0 is seismic moment) of all globally distributed shallow (depth, h 1M Pa )are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower τa (< 1M Pa )i sassociated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average τa of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average τa with focal mechanism and tectonic regime suggests that the level of τa is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high τa oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Me (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than MW . This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release.

Three-dimensional earthquake locations and upper crustal structure of the Sannio-Matese region (southern Italy)

The Sannio-Matese region is one of the most seismically active regions of Italy and has been struck by large historical earthquakes. At present, the area is characterized by low magnitude background seismicity and small seismic sequences following M∼4 main events. In this paper, we show Vp and Vp/Vs models and 3D locations for a complete set of earthquakes occurring in the period 1991–2001. We observe a significant crustal heterogeneity, with large scale east-verging high Vp fault-related-folds, stacked by the Pliocene compression. The relocated earthquakes cluster along a 70° east-dipping, NW-striking plane located at the border of the high Vp thrust units. Normal fault earthquakes related to the young and active extension occur within these high Vp zones, interpreted as high strength material. We expect large future earthquakes to occur within these high Vp zones actually characterized by low magnitude seismicity at their borders.

Modal scattering and T-waves: Sediment amplification and source effects

If the Earth were a plane-layered semi-infinite half-space or a radially symmetric sphere oceanic, T-waves could not exist. This is apparent because the source depth of T-wave producing earthquakes is greater than the depth at which the low order modes comprising the T-waves have any significant amplitude. Bottom roughness provides a mechanism by which energy from high order source modes can be scattered into the lower order modes. The efficiency of this scattering is improved when there is a layer of sediments overlying the higher speed upper ocean crust. Some of the source modes develop a large anti-node, reminiscent of a Scholte wave, at the water sediment interface. Bottom roughness serves as a sheet of secondary sources placed directly on the anti-node of these modes, and contributes a significant amount of energy to the T-waves. However, a significant fraction of the total T-wave energy is also provided by small scattered contributions from many higher modes of the continuum spectrum, which is modeled using the locked mode approximation. Source mechanism modeling of T-wave excitation shows that normal fault earthquakes are inefficient generators of T-waves.

Long-Period Source Characteristics of the 1975 Kalapana, Hawaii, Earthquake

The 1975 Kalapana, Hawaii, earthquake occurred under the highly mobile south flank of Kilauea Volcano. It has been interpreted variously as a normal-faulting earthquake, a thrust-faulting earthquake, and a landslide. Primary evidence for the landslide model has been the failure of previous faulting models to explain the observed Love-wave radiation pattern and the tsunami amplitudes generated by the Kalapana event. Here, we present a reanalysis of the long-period, digital seismic data for this event. Centroid-moment-tensor analysis shows that the seismic radiation pattern can be explained well by thrust faulting on a plane dipping shallowly landward. The seismic moment of 3.8 × 10 20 N m ( M W 7.7) that we determine is approximately twice as large as earlier estimates. The geometry and seismic moment of the focal mechanism determined here are consistent with the observed tsunami amplitudes. Inversion of long-period body-wave waveforms shows that the earthquake source duration (∼72 sec) is unusually long for an earthquake of this size.

Slip-weakening distance in dynamic rupture of in-slab normal-faulting earthquakes

We apply the above method to three large in-slab, normal-faulting earthquakes in the Mexican subduction zone. To do this, we calculate the spatial distribution of slip-velocity functions and final slip from kinematic waveform inversion of strong-motion and teleseismic records, and the stress history and final stress change from dynamic rupture calculations. By integrating the slip-velocity functions obtained from the inversion, from the rupture arrival time to the time of peak slip velocity, we obtain slip Dat T pv and then correct it for Dc at T b through dynamic calculations. We also estimate the lowest resolvable limit and probable errors of Dc from the slip-velocity functions, and its upper bound from a theoretical constraint between the dynamic stress drop and Dc. We found that the slip-weakening distance Dc estimated in the frequency band between 0.05 and 0.5 Hz ranges between 40 and 120 cm on the in-slab fault of the 1999 Oaxaca earthquake (Mw = 7.5). The largest Dc is detected in the central fault and in part of the deeper sections, and Dc in the zone around the hypocentre ranges between 50 and 70 cm. The estimated Dc values appear to be less depth-dependent but are rather more dependent on the local maximum slip. This possible slip dependence might be interpreted by the degree of fault roughness, in addition to stress heterogeneities. The fracture energy G in the central section and in the hypocentral zone are roughly estimated to be of the order of 10-15 and 5-8 MJ m −2 , respectively. Both of the estimated Dc and G values are somewhat larger than those on the vertical fault of two recent, shallow strike-slip earthquakes in western Japan.

Static Displacement due to Seismic Moment Tensor Sources in aLayered Half-Space

The expressions for static displacements on the surface of alayered half-space due to static point moment tensor source are givenin terms of the generalized reflection and transmission coefficientmatrix method. The validity and precision of the new method isillustrated by comparing the consistency of our results with theanalytical solution given by Okada's code employing the same pointsource and homogenous half-space model. The computed vertical grounddisplacement using the moment tensor solution of the Lanchang-Gengmaearthquake displays considerable difference with that of a doublecouple component. The effect of a soft layer at the top of thehomogenous half-space on a shallow normal-faulting earthquake is alsoanalysed. Our results show that more seismic information would beobtained utilizing seismic moment tensor source and layered half-spacemodel.

The North Anatolian fault in the Sea of Marmara

Using the detailed geometry and tectonics of the Main Marmara fault that connects the Gulf of Izmit fault with the Ganos fault established by Le Pichon et al. [2001] and the new GPS data set recently analyzed by Meade et al. [2002], we confirm the existence of a Marmara block delimited to the north by the northern branch of the North Anatolian fault (the Marmara branch) and to the south by the southern branch. The kinematics of this block indicate that the Marmara branch that crosses the Sea of Marmara is close to pure dextral strike slip at a rate of 23 mm/yr on its whole length. The short Cinarcik section is the only one where a significant extensional component is predicted. A simple kinematic model accounts for slip partitioning there, with 8–10 mm/yr of extension to the south of the basin and 23 mm/yr of dextral strike slip along the northern Cinarcik margin. The new GPS data demonstrate the existence of important asymmetric elastic loading along the Main Marmara fault that may have significant implications for the seismotectonics of the area. The expected future large earthquake may rupture the whole main Marmara fault. The partitioning in the Cinarcik basin may produce magnitude 7 normal faulting earthquakes if their repetition time is about 250 years. The 1896 earthquake may have been one of these normal fault earthquakes. The Marmara block is less than 1 Ma. Prior to its formation, it was part of the Anatolian block. At this time the Ganos fault was already a dextral strike‐slip fault, but the fault system in the Sea of Marmara would have been affected by an extensional component increasing toward the east that accounts for the increasing size of the sedimentary basins eastward.

A Comprehensive Study of the Observed Spectral Decay in Strong-Motion Accelerations Recorded in Guerrero, Mexico

The Fourier spectra of S waves on strong-motion accelerograms fall off rapidly at the highest frequencies. Anderson and Hough (1984) suggested characterizing the high-frequency fall-off as A ( f ) ∼ exp(– πκf ), defining the observed rate of fall-off, κ , as the spectral decay parameter. It has been proposed that variability in κ results from variable near-surface attenuation and that estimates of κ from small- to moderate-magnitude events apply to the largest earthquakes. No systematic investigation into the behavior of κ over a broad magnitude range has been previously undertaken. The Guerrero Accelerograph Array has recorded enough strong-motion data from earthquakes with magnitudes ranging from under 3.0 to 8.0 to achieve a high level of statistical significance. The records are chosen to minimize the influence of the corner frequency on the spectral shape, and the κ values are estimated from the high-frequency slopes alone. This study finds that the terms in the model κ = κ site + κ event reduce the misfit significantly, with the κ site and κ event contributing similarly to the total κ value. Evidence suggests that κ event results primarily from source characteristics as opposed to propagation path effects. The absence of a statistically significant dependence of κ on epicentral distance is observed in the Guerrero region. Measuring κ over the frequency bands from 10 to 30 Hz and 10 to 45 Hz reveals increasing κ event with magnitude. When κ is measured from 25 to 45 Hz, κ event is not correlated with magnitude, although its role in error reduction remains significant. κ event also varies systematically with focal mechanism: of all M ≥5.1 earthquakes with available focal mechanisms, κ event is on average 4.5 msec less in normal-faulting earthquakes than in thrusting events.