Showing papers on "Hypocenter published in 2006"

Temporal Changes of Shallow Seismic Velocity Around the Karadere-Düzce Branch of the North Anatolian Fault and Strong Ground Motion

Abstract: We analyze temporal variations of seismic velocity along the Karadere-Duzce branch of the north Anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 Mw7.4 Izmit and Mw7.1 Duzce earthquakes. The analysis employs 36 sets of highly repeating earthquakes, each containing 4–18 events. The events in each cluster are relocated by detailed multi-step analysis and are likely to rupture approximately the same fault patch at different times. The decay rates of the repeating events in individual clusters are compatible with the Omori's law for the decay rate of regional aftershocks. A sliding window waveform cross-correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events. We find clear step-like delays in the direct S and early S-coda waves (sharp seismic velocity reduction) immediately after the Duzce main shock, followed by gradual logarithmic-type recoveries. A gradual increase of seismic velocities is also observed before the Duzce main shock, probably reflecting post-seismic recovery from the earlier Izmit main shock. The temporal behavior is similar at each station for clusters at various source locations, indicating that the temporal changes of material properties occur in the top most portion of the crust. The effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones, and generally decrease with normal distance from the fault. A strong correlation between the co-seismic delays and intensities of the strong ground motion generated by the Duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material.

Coseismic and Postseismic Slip of the 2004 Parkfield Earthquake from Space-Geodetic Data

Abstract: We invert interferometric synthetic aperture radar (insar) data jointly with campaign and continuous global positioning system (gps) data for slip in the coseismic and postseismic periods of the 2004 Parkfield earthquake. The insar dataset consists of eight interferograms from data collected by the Envisat and Radarsat satellites spanning the time of the earthquake and variable amounts of the postseismic period. The two datasets complement each other, with the insar providing dense sampling of motion in the range direction of the satellite and the gps providing more sparse, but three-dimensional measurements of ground motion. The model assumes exponential decay of the postseismic slip with a decay time constant of 0.087 years, determined from time series modeling of continuous gps and creepmeter data. We find a geodetic moment magnitude of M 6.2 for a 1-day coseismic model and M w 6.1 for the entire postseismic period. The coseismic rupture occurred mainly in two slip asperities; one near the hypocenter and the other 15–20 km north. Postseismic slip occurred on the shallow portions of the fault and near the rupture areas of two M 5.0 aftershocks. A comparison of the geodetic slip models with seismic moment estimates suggests that the coseismic moment release of the Parkfield earthquake is as little as 25% of the total. This underlines the importance of aseismic slip in the slip budget for the Parkfield segment. Online material: Complete data tables and supplemental tables.

Induced Seismicity during the Stimulation of a Geothermal hfr Reservoir in the Cooper Basin, Australia

Abstract: A long-term fluid-injection experiment was performed in the Cooper Basin (Australia) in 2003 to stimulate a geothermal reservoir. More than 20,000 m 3 of water were injected into the granitic crust at 4250 m depth. During reservoir stimulation about 27,000 induced seismic events were detected by a local, eight- station seismic monitoring system deployed in nearby boreholes. Hypocenter locations for 11,068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point. The spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km × 1.5 km and an apparent thickness of approximately 150–200 m, which is in the order of the hypocenter location confidence limits. The hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time. Previously activated regions become seismically quiet indicating relaxation processes. High-resolution relative hypocenter locations determined for clusters of “similar” events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only. Consistent with these findings, a subsequently drilled well intersects a dominating, high-permeable fracture within 15 m of the predicted intersection depth. Based on drilling and logging information, the fracture zone is interpreted as a preexisting (possibly tectonically formed) feature that (partly) sheared during stimulation. Triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a (local) reduction of the effective normal stress resolved on the fracture plane. Additionally, perturbations of the stress field caused by the largest-magnitude events may trigger seismicity (“aftershocks”) on a local, short-ranging scale.

Anomalous low-velocity zone and linear alignment of seismicity along it in the subducted Pacific slab beneath Kanto, Japan: Reactivation of subducted fracture zone?

Abstract: [1] A detailed investigation of the hypocenter distribution beneath Kanto, Japan, reveals a NW-SE-trending linear alignment of seismicity within the subducted Pacific slab. We estimate the 3D seismic velocity structure in the Pacific slab to understand the factors controlling the genesis of such intraslab earthquakes. A narrow low-velocity zone is imaged within the subducted slab over a length of ∼150 km, which partly penetrates into the mantle portion of the slab. The low-velocity zone correlates in space with the NW-SE-trending earthquake cluster. A reactivation of hydrated fracture zone formed prior to subduction is probably related to the low-velocity anomaly. Dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone, accompanied by intraslab earthquakes. These observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes.

Kinematic Inversion of the 2004 M 6.0 Parkfield Earthquake Including an Approximation to Site Effects

Abstract: The 2004 M 6.0 Parkfield earthquake yielded one of the largest amounts of near-source strong ground motion seismic data ever. We invert strong-motion seismograms to obtain a model for the space-time distribution of coseismic slip on the fault. To reduce noise in the inversion, we take into account local amplifications that affect each station by using records of the 1983 M 6.5 Coalinga earthquake. Site amplification correlates well with large peak ground velocities registered during the 2004 Parkfield mainshock. The inversion for a kinematic rupture model yields a nonunique solution; we therefore analyze various rupture models that explain the data equally well. Our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter, where the maximum slip is 57 cm. A secondary slip area, over which contours are not well resolved, is located northwest of the hypo- center. The rupture speed is highly heterogeneous. We infer an average rupture ve- locity of 2.8 km/sec close to the hypocenter, and of 3.3 km/sec in the secondary region of large slip to the northwest of the hypocenter. By correlation of our rupture model with both microseismicity and velocity structure, we identify six patches on the fault plane that behave in seismically distinct ways. Online material: Kinematic rupture model parameters.

Interaction between forearc and oceanic plate at the south‐central Chilean margin as seen in local seismic data

Abstract: We installed a dense, amphibious, temporary seismological network to study the seismicity and structure of the seismogenic zone in southern Chile between 37° and 39°S, the nucleation area of the great 1960 Chile earthquake. 213 local earthquakes with 14.754 onset times were used for a simultaneous inversion for the 1-D velocity model and precise earthquake locations. Relocated artificial shots suggest an accuracy of the earthquake hypocenter of about 1 km (horizontally) and 500 m (vertically). Crustal events along trench-parallel and transverse, deep-reaching faults reflect the interseismic transpressional deformation of the forearc crust due to the subduction of the Nazca plate. The transverse faults seems to accomplish differential lateral stresses between subduction zone segments. Many events situated in an internally structured, planar seismicity patch at 20 to 40 km depth near the coast indicate a stress concentration at the plate's interface at 38°S which might in part be induced by the fragmented forearc structure.

Kinematic models of plate boundary deformation in southwest Iceland derived from GPS observations

Abstract: [1] We use data from GPS campaign and continuous measurements from 1992 to 2004 in SW Iceland to map the surface velocity field from the Reykjanes Peninsula to the Eastern Volcanic Zone. We divide the time series into preseismic (July 1992 to June 2000) and postseismic (June 2000 to May 2004) time intervals, and we estimate GPS station velocities for each interval as well as coseismic offsets due to the June 2000 earthquake sequence in the south Iceland seismic zone (SISZ). In addition to the plate spreading, the preseismic velocity field shows the effects of inflation at Hengill and Hekla volcanoes, whereas the postseismic velocities show deformation following the June 2000 earthquakes. We consider several kinematic models to explain the preseismic velocities. Our preferred model includes several dislocations and point sources in an elastic half-space, with left-lateral slip along the plate boundary on the Reykjanes Peninsula and below the SISZ, opening across the Reykjanes Peninsula, and the Western and Eastern rift zones. The optimal model has a locking depth of about 8 km in the central and eastern part of the Reykjanes Peninsula with a deep slip rate of about 17 mm/yr and an opening of about 9 mm/yr. This locking depth is in agreement with the thickness of the seismogenic crust on the Reykjanes Peninsula, which appears to vary between 7 and 9 km. For the SISZ, we obtain a deep slip rate of about 19 mm/yr below 16 km depth, which is considerably deeper than the earthquake hypocenter depths in the area.

Spatial correlation of aftershock locations and on‐fault main shock properties

Abstract: [1] We quantify the correlation between spatial patterns of aftershock hypocenter locations and the distribution of coseismic slip and stress drop on a main shock fault plane using two nonstandard statistical tests. Test T1 evaluates if aftershock hypocenters are located in low-slip regions (hypothesis H1), test T2 evaluates if aftershock hypocenters occur in regions of increased shear stress (hypothesis H2). In the tests, we seek to reject the null hypotheses H0: Aftershock hypocenters are not correlated with (1) low-slip regions or (2) regions of increased shear stress, respectively. We tested the hypotheses on four strike-slip events for which multiple earthquake catalogs and multiple finite fault source models of varying accuracy exist. Because we want to retain earthquake clustering as the fundamental feature of aftershock seismicity, we generate slip distributions using a random spatial field model and derive the stress drop distributions instead of generating seismicity catalogs. We account for uncertainties in the aftershock locations by simulating them within their location error bounds. Our findings imply that aftershocks are preferentially located in regions of low-slip (u ≤ umax) and of increased shear stress (Δσ < 0). In particular, the correlation is more significant for relocated than for general network aftershock catalogs. However, the results show that stress drop patterns provide less information content on aftershock locations. This implies that static shear stress change of the main shock may not be the governing process for aftershock genesis.

Source process and near-source ground motions of the 2005 West Off Fukuoka Prefecture earthquake

Abstract: A large shallow crustal earthquake occurred at the western off-shore of Fukuoka Prefecture, northern Kyushu, Japan, at 10:53 on March 20, 2005 (JST). Source rupture processes of the mainshock and the largest aftershock on April 20, 2005, are estimated by the kinematic waveform inversion of strong motion seismograms. The rupture of the mainshock started with relatively small slip, and the largest slip was observed at the southeast of the hypocenter. The inverted source models showed that both of the ruptures of the mainshock and the largest aftershock mainly propagated to southeast from the hypocenters, and the rupture area of those events did not overlap each other. Three-dimensional ground motion simulation by the finite difference method considering three-dimensional bedrock structure was also conducted to see the spatial variation of the near-source ground motion of the mainshock. The result of the simulation shows that expected groud motions are relatively large in and arround Genkai Island, Shikanoshima Island, and the center of Fukuoka City compared to the other area because of the rupture heterogeneity and the deep basin structure in Fukuoka City.

Liquefaction Limit during Earthquakes and Underground Explosions: Implications on Ground-Motion Attenuation

Abstract: Liquefaction of saturated soils and sediments documented during earthquakes shows an empirical relation log R max = 2.05 (±0.10) + 0.45 M , where R max is the liquefaction limit in meters (i.e., the maximum distance from liquefaction site to the hypocenter) and M is the earthquake magnitude. Combining this with an empirical relation between M and the seismic energy of an earthquake, we obtain a relation between the liquefaction limit and the seismic energy: E = A R β max. The prefactor corresponds to a threshold energy for liquefaction ranging from 0.004 to 0.1 J/m3; the exponent, ranging from 3.2 to 3.3, implies that the energy density of ground motion attenuates with distance according to 1/ r 3.2–3.3, where r is the distance from the hypocenter. The value of the threshold energy suggests a preliquefaction degradation of the shear modulus of soils by more than 3 orders of magnitude. Liquefaction documented during underground explosions is characterized by a threshold energy several orders of magnitude greater than that for liquefaction during earthquakes but shows a similar functional relation between E and R max as that for liquefaction during earthquakes and implies a similar attenuation relation between ground-motion energy density and distance.

Millennial slip rate of the Longitudinal Valley fault from river terraces: Implications for convergence across the active suture of eastern Taiwan

Abstract: The Longitudinal Valley fault is a key element in the active tectonics of Taiwan. It is the principal structure accommodating convergence across one of the two active sutures of the Taiwan orogeny. To understand more precisely its role in the suturing process, we analyzed fluvial terraces along the Hsiukuluan River, which cuts across the Coastal Range in eastern Taiwan in the fault's hanging wall block. This allowed us to determine both its subsurface geometry and its long-term slip rate. The uplift pattern of the terraces is consistent with a fault-bend fold model. Our analysis yields a listric geometry, with dips decreasing downdip from about 50° to about 30° in the shallowest 2.5 km. The Holocene rate of dip slip of the fault is about 22.7 mm/yr. This rate is less than the 40 mm/yr rate of shortening across the Longitudinal Valley derived from GPS measurements. The discrepancy may reflect an actual difference in millennial and decadal rates of convergence. An alternative explanation is that the discrepancy is accommodated by a combination of slip on the Central Range fault and subsidence of the Longitudinal Valley floor. The shallow, listric geometry of the Longitudinal Valley fault at the Hsiukuluan River valley differs markedly from the deep listric geometry illuminated by earthquake hypocenters near Chihshang, 45 km to the south. We hypothesize that this fundamental along-strike difference in geometry of the fault is a manifestation of the northward maturation of the suturing of the Luzon volcanic arc to the Central Range continental sliver.

Tarapacá intermediate‐depth earthquake (Mw 7.7, 2005, northern Chile): A slab‐pull event with horizontal fault plane constrained from seismologic and geodetic observations

Abstract: [1] A large (Mw 7.7) intermediate-depth earthquake occurred on 13 June 2005 in the Tarapaca region of the northern Chile seismic gap. Source parameters are inferred from teleseismic broadbands, strong motions, GPS and InSAR data. Relocated hypocenter is found at $98 km depth within the subducting slab. The 21-days aftershock distribution, constrained by a postseismic temporary array, indicates a sub-horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km  30 km. Teleseismic inversion shows a slab-pull down dip extension mechanism on a nearly horizontal plane. Total seismic and geodetic moments are $5.5  10 20 N.m, with an averaged slip of 6.5 m from geodesy. The earthquake rupture is peculiar in that the effective velocity is slow, 3.5 Km.s A1 for a high stress-drop, 21 –30 MPa. We propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate. The stress-drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses. Citation: Peyrat, S., et al. (2006), Tarapaca intermediate-depth earthquake (Mw 7.7, 2005, northern Chile): A slab-pull event with horizontal fault plane constrained from seismologic and geodetic observations, Geophys.

Imaging the seismic structure and stress field in the source region of the 2004 mid‐Niigata prefecture earthquake: Structural zones of weakness and seismogenic stress concentration by ductile flow

Abstract: [1] We deployed a dense temporal seismic network in the source region of the 2004 mid-Niigata prefecture earthquake (thrust fault), Japan. A detailed velocity structure and accurate aftershock distributions were elucidated by inverting aftershock arrival times using double-difference tomography. A stress tensor inversion using the first-motion data was also conducted in order to investigate the stress field. The seismic velocities in the hanging wall above the main shock fault are lower than those in the footwall, with the velocity contrast extending to a depth of approximately 10 km. The aftershocks along the main shock rupture zone are distributed around the sharp boundary between the low- and high-velocity bodies. Furthermore, aftershocks associated with the largest aftershock appear to be aligned on a boundary between low- and high-velocity zones, in the footwall. The orientation of maximum principal stress (σ1) is consistent with the regional compressional strain rate axis inferred from GPS data, except in the southwestern side of the main shock hypocenter where the azimuth of σ1 rotates approximately 20° counterclockwise. The main shock hypocenter was located roughly at the transition zone where the structure of the hanging wall changes laterally and the azimuth of σ1 rotates. Heterogeneous structures of the seismic velocity and the stress field, combined with the ductile deformation of the upper crust, may have concentrated seismogenic stress around the hypocenter area to cause the complex distributions of aftershock sequence on structural boundaries.

Seismogenic structure in a tectonic suture zone: With new constraints from 2006 Mw6.1 Taitung earthquake

Abstract: [1] A Mw 6.1 earthquake occurred on April 1st, 2006 near Taitung, eastern Taiwan. It produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month. This event provides an opportunity to diagnose the seismogenic structure in the southern Longitudinal Valley (LV) of eastern Taiwan, long viewed as one of the collision sutures between the Philippine Sea and the Eurasian plates. With precisely relocated main- and aftershock hypocenters, focal mechanisms for M ≥ 3.8 events, and coseismic ground displacements from strong motion records, we determine a main shock dislocation model. Our results indicate that the main shock occurred on a high angle fault (azimuth 198°, dip 77°). The model comprises a fault with two segments; the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left-lateral strike-slip motion, in agreement with P-wave first motions and waveform (USGS) solutions. The southern segment exhibits a slightly larger thrust component, in agreement with CMT solutions. Tectonically, this event highlights a NNE-trending fault on the west side of the LV, which is predominantly strike-slip. The aftershocks clustered to the east of the main shock, which exhibit mainly thrust mechanisms, indicate that shortening is still acting on the sedimentary materials deposited between the Coastal and Central ranges prior to collision. As a result, the southern LV is undergoing slip partitioning along different faults, which has never been specified before.

Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation

Abstract: We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

Some Key Features of the Strong-Motion Data from the M 6.0 Parkfield, California, Earthquake of 28 September 2004

Abstract: The 2004 Parkfield, California, earthquake was recorded by an extensive set of strong-motion instruments well positioned to record details of the motion in the near-fault region, where there has previously been very little recorded data. The strong-motion measurements obtained are highly varied, with significant variations occurring over only a few kilometers. The peak accelerations in the near fault region range from 0.13 g to over 1.8 g (one of the highest acceleration recorded to date, exceeding the capacity of the recording instrument). The largest accelerations occurred near the northwest end of the inferred rupture zone. These motions are consistent with directivity for a fault rupturing from the hypocenter near Gold Hill toward the northwest. However, accelerations up to 0.8 g were also observed in the opposite direction, at the south end of the Cholame Valley near Highway 41, consistent with bilateral rupture, with rupture southeast of the hypocenter. Several stations near and over the rupturing fault recorded relatively weak motions, consistent with seemingly paradoxical observations of low shaking damage near strike-slip faults. This event had more ground-motion observations within 10 km of the fault than many other earthquakes combined. At moderate distances peak horizontal ground acceleration (pga) values dropped off more rapidly with distance than standard relationships. At close-in distance the wide variation of pga suggests a distance- dependent sigma may be important to consider. The near-fault ground-motion variation is greater than that assumed in ShakeMap interpolations, based on the existing set of observed data. Higher density of stations near faults may be the only means in the near future to reduce uncertainty in the interpolations. Outside of the near- fault zone the variance is closer to that assumed. This set of data provides the first case where near-fault radiation has been observed at an adequate number of stations around the fault to allow detailed study of the fault-normal and fault-parallel motion and the near-field S -wave radiation. The fault- normal motions are significant, but they are not large at the central part of the fault, away from the ends. The fault-normal and fault-parallel motions drop off quite rapidly with distance from the fault. Analysis of directivity indicates increased values of peak velocity in the rupture direction. No such dependence is observed in the peak acceleration, except for stations close to the strike of the fault near and beyond the ends of the faulting.

Changes in groundwater levels or pressures associated with the 2004 earthquake off the west coast of northern Sumatra (M9.0)

Abstract: Associated with the 2004 earthquake off the west coast of northern Sumatra, changes in groundwater levels or pressures were observed at many observation stations in Japan which are more than 5000 km from the hypocenter. At 38 of the 45 observation stations, there were changes in groundwater levels or pressures. At the 10 observation stations in which the Ishii-type borehole strain instruments were established, changes in crustal strains were also observed. A major part of the changes in crustal strains and groundwater levels or pressures were dynamic oscillations due to a seismic wave. At some stations, coseismic or postseismic rises or drops were also observed. At five stations where both crustal stain and groundwater levels or pressures were observed, postseismic changes in groundwater levels or pressures were consistent with coseismic static steps in crustal strains. At the other five stations, postseismic changes in groundwater levels or pressures did not agree with the coseismic static steps. At two stations of these five stations, it is anticipated that the pore water pressure change in each aquifer locally occurred independently of the change in crustal strain. At another station, postseismic changes in groundwater level possessed the same characteristics as a model removing the temporary deposition. At the last two stations, the causes of the changes are unknown.

A New Passive Tomography of the Aigion Area (Gulf of Corinth, Greece) from the 2002 Data Set

Abstract: We present the results of a tomographic study performed in the framework of the 3F-Corinth project. The aim of this work is to better understand the rifting process by imaging the crustal structure of the western Gulf of Corinth. Forty-nine stations were deployed for a period of six months, allowing us to monitor the microseismicity. Delayed P and S first-arrival times have been simultaneously inverted for both hypocenter locations and 3-D velocity distributions. We use an improved linearized tomography method based on an accurate finite-difference travel-time computation to invert the data set. The obtained Vp and Vs models confirm the presence of a two-layer vertical structure characterized by a sharp velocity gradient lying at 5–7 km depth, which may be interpreted as a lithological contrast. The shallower part of the crust (down to 5 km depth) is controlled by the N-S extension and lacks seismicity. The deeper part (7–13 km depth) matches the seismogenic zone and is characterized by faster and more heterogeneous anomalies. In this zone, the background seismicity reveals a low-angle active surface dipping about 20° toward the north and striking WNW-ESE. The position of this active structure is consistent with both high Vp/Vs and low Vp.Vs anomalies identified at 8–12 km depth and suggesting a highly fracturated and fluid-saturated zone. Both the geometry of the active structure beneath the gulf and the presence of fluids at 8–12 km depth are in accordance with a low-angle detachment model for the western part of the Gulf of Corinth.

A Possible Seismic Gap within a Highly Seismogenic Belt Crossing Calabria and Eastern Sicily, Italy

Abstract: A review of the seismic, geologic, and geodynamic information available for the Calabro-Sicilian region of southern Italy leads us to suggest a unifying view of the larger ( M ≥ 7) earthquakes that have occurred there in the last millennium coincidentally concentrated in the last 3.7 centuries. The seismicity coincides with a narrow curvilinear extensional belt that passes through western Calabria and eastern Sicily (wces belt) and which includes a nearly continuous north–south succession of primarily east-dipping normal faults. In our reconstruction of the seismotectonic process the faulting is activated by west-northwest–east-southeast extension induced by residual rollback of the Ionian subduction slab. Our analysis of the space-time distribution of strong earthquakes indicates a zone of conspicuous aseismicity ( M > 4.5 since 1700, M ≥ 7 since 1000) along the belt, corresponding to the 30-km-long Scaletta-Fiumefreddo segment of the Messina–Fiumefreddo fault in eastern Sicily. Moreover, because estimated recurrence times are on the order of a millennium for M 7 earthquakes along different parts of the wces belt, and because historical data for destructive earthquakes in the first millennium a.d. are not detailed enough to allow reliable identification of the source zones, we cannot definitely state whether or not there is a late-stage seismic gap for a large earthquake in eastern Sicily. By applying standard relationships, the potential for a magnitude 7 earthquake can be estimated for the Scaletta–Fiumefreddo 30-km-long normal fault segment. The level of seismic activity has been low in the possible gap area in the past two decades when the upgraded local network has detected tens of events above magnitude 2.5 with values up to 3.7. Hypocenter locations of these events seem to delineate the deep geometry of two faults reported in the surface geologic maps, one of which is the Scaletta–Fiumefreddo silent fault. Coulomb stress changes not larger than +0.6 bar produced on the silent fault by the most recent M 7 regional earthquake (1908), and the substantially nil Coulomb stress change on the same fault by M 7 earthquakes of 1693 and 1783, imply relatively small perturbation by previous earthquakes to the silent fault compared with the Coulomb stress perturbation of 2–2.5 bars estimated by other investigators on the 1908 earthquake source caused by major earthquakes of the previous centuries.

Spatiotemporal variation in b value within the subducting slab prior to the 2003 Tokachi-oki earthquake (M 8.0), Japan

Abstract: [1] The b value of an earthquake catalogue, defined as the slope of the Gutenberg-Richter frequency-magnitude relationship (logN = a − bM), is typically close to unity in many tectonic settings. However, analysis of seismicity data for the subducting slab along the Kuril Trench reveals a zone of anomalously low b values near the hypocenter of the 2003 Tokachi-oki earthquake (M 8.0). The zone of low b value (0.50–0.65) at depths of less than 80 km in the subducting slab changes in location with time in 1990–2003 and intensifies leading up to the 2003 earthquake. This transient low b value zone corresponds roughly with the location of an asperity inferred from waveform inversion in recent research. In the shallow zone corresponding to the immediate region (<125 km along the subducting slab) of the 2003 event, the b value is shown to have decreased in the year prior to the main shock from initial values of ∼0.65 to values as low as 0.5. The occurrence of the 2003 earthquake, a large thrust event on the subducting plate interface, in this zone of anomalously low b values suggests that low b values in the subducting slab prior to the main shock and during the aftershock series are concordant with high stress concentrations associated with interseismic strain accumulation and redistribution in the vicinity of the rupture.

Modern Seismic Observations in the Tatun Volcano Region of Northern Taiwan: Seismic/Volcanic Hazard Adjacent to the Taipei Metropolitan Area

Abstract: The Tatun volcano group is located adjacent to the Taipei metropolitan area in northern Taiwan and was a result of episodic volcanisms between 2.8 and 0.2 Ma. Earthquake data collected over the last 30 years are analyzed to explore seismicity patterns and their associated mechanisms of faulting in the area. Using a Joint Hypocenter Determination (JHD) method, a few sequences of relocated earthquake hypocenters are tightly clustered; these seemed to be blurry in the original catalog locations. Numerous earthquakes, previously unnoticed and not reported in the CWB catalog, have been identified from careful examination of the continuous recordings of a nearby broadband seismic station. These newly identified earthquakes show similarities in waveforms and arrival time differences between direct P- and S-waves indicating that their hypocenter locations are very close to each other and their source mechanisms are similar. A relatively high b-value of 1.22 is obtained from the analysis of crustal earthquakes (depth < 30 km) in the region, which may suggest that clustered local seismicity in the Tatun volcanic region probably resulted from subsurface hydrothermal or volcano-related activities. Focal mechanism solutions determined in this study are dominated by normal faulting. Thus, these earthquake clusters are most probably associated with hydrothermal/magmatic activities in a back-arc extensional environment.

Coseismic slip distribution of the 2005 off Miyagi earthquake (M7.2) estimated by inversion of teleseismic and regional seismograms

Abstract: A large earthquake (M7.2) occurred along the plate boundary off Miyagi Prefecture (Miyagi-Oki), northeastern Japan, on August 16, 2005. In this area, large earthquakes (~M7.5) have occurred repeatedly at intervals of about 37 years, and more than 27 years have passed since the last event occurred. To estimate the relationship between this earthquake and the previous events, we determined coseismic slip distribution by this 2005 Miyagi-Oki earthquake by adopting the seismic waveform inversion method of Yagi et al. (2004) and compared it with that of the previous 1978 Miyagi-Oki earthquake. We performed two cases of the inversions; inversion using only far-field seismograms and that using far-field seismograms and local seismograms simultaneously. Both results show that a large slip occurred near the hypocenter and rupture extended to the westward deeper portion. Considering that the rupture area of the 2005 event partly overlapped with the southeastern part of that of the 1978 event, suggests this result the possibility that plural asperities exist which cause the sequence of Miyagi-Oki earthquakes and that the 2005 event ruptured one of such asperities, although the previous 1978 event ruptured all the asperities at one time.

Distribution of stress drop, stiffness, and fracture energy over earthquake rupture zones

Abstract: [1] Using information provided by slip models and the methodology of McGarr and Fletcher (2002), we map static stress drop, stiffness (k = Δσ/u, where Δσ is static stress drop and u is slip), and fracture energy over the slip surface to investigate the earthquake rupture process and energy budget. For the 1994 M6.7 Northridge, 1992 M7.3 Landers, and 1995 M6.9 Kobe earthquakes, the distributions of static stress drop show strong heterogeneity, emphasizing the importance of asperities in the rupture process. Average values of static stress drop are 17, 11, and 4 MPa for Northridge, Landers, and Kobe, respectively. These values are substantially higher than estimates based on simple crack models, suggesting that the failure process involves the rupture of asperities within the larger fault zone. Stress drop as a function of depth for the Northridge and Landers earthquakes suggests that stress drops are limited by crustal strength. For these two earthquakes, regions of high slip are surrounded by high values of stiffness. Particularly for the Northridge earthquake, the prominent patch of high slip in the central part of the fault is bordered by a ring of high stiffness and is consistent with expectations based on the failure of an asperity loaded at its edge due to exterior slip. Stiffness within an asperity is inversely related to its dimensions. Estimates of fracture energy, based on static stress drop, slip, and rupture speed, were used to investigate the nature of slip weakening at four locations near the hypocenter of the Kobe earthquake for comparison with independent results based on a dynamic model of this earthquake. One subfault updip and to the NE of the hypocenter has a fracture energy of 1.1 MJ/m2 and a slip-weakening distance, Dc, of 0.66 m. Right triangles, whose base and height are Dc and the dynamic stress drop, respectively, approximately overlie the slip-dependent stress given by Ide and Takeo (1997) for the same locations near the hypocenter. The total fracture energy for the Kobe earthquake, 3.7 × 1014 J, is about the same as the seismic energy (Ea = 3.2 × 1014 J).