Aftershock Locations Research Articles

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

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 T1evaluates if aftershock hypocenters are located in low‐slip regions (hypothesis H1), test T2evaluates 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.

The Mw 6.4 Damas, Costa Rica, Earthquake of 20 November 2004: Aftershocks and Slip Distribution

The earthquake of 20 November 2004 was located north of Damas Island in the Pacific coast of Costa Rica, within the Costa Rica Deformed Belt. The earthquake was located at 24 km depth and reported with a magnitude ( M w ) of 6.4 and a strike-slip mechanism with a large normal dip-slip motion. This mechanism agrees with mapped faults in the area that suggest transtensional deformation on the forearc and the entire western boundary of the Panama microplate. Aftershock locations do not delineate a preferable plane to distinguish between the two nodal planes and are distributed between 15 and 25 km depth. The slip distribution during the mainshock, modeled after teleseismic and local data, pictured a circular rupture 8 km in radius and 0.25 m of average displacement. The fault plane cannot be distinguished from the two nodal planes from the slip distribution because of the lack of directivity and resolution for this magnitude earthquake. Weak evidence from empirical Green’s function analysis suggests that the dextral northwest-oriented fault could be the causative fault. Depth to the top of the slab, hypocenter location of the mainshock, its slip distribution, depth distribution of the aftershocks, and Quaternary fault activity at the surface suggest that deformation takes place throughout the whole thickness of the crust. This extended deformation might be caused by seamount subduction and strong basal friction on the upper plate, due to subduction of a thick, young, and buoyant oceanic plate, rough seafloor, and underplating of large seamounts.

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 ◦ .

Fault‐perpendicular aftershock clusters following the 2003 Mw = 5.0 Big Bear, California, earthquake

To explore aftershock‐triggering mechanisms for the 2003 Big Bear, California earthquake sequence, we determined differential travel‐times and applied the double‐difference technique to relocate these events, which formed three clusters. The main cluster coincides with the 3 km long northwest striking sub‐vertical mainshock fault. The other two sub‐vertical clusters, located at opposite ends of the mainshock rupture, are almost perpendicular to the mainshock fault, contradicting the 60° separation angle of conjugate faults as predicted from frictional laws. Allowing for a 30° uncertainty in the cataloged strike, dip and rake values about 75% of the aftershocks are strike‐slip as determined from first motion and complete waveform moment tensor inversions. We use a mainshock conceptual slip model to derive Coulomb Failure Stress regions, and assess correlations between stress increases and aftershock locations. We conclude that the perpendicular aftershock clusters were triggered by the mainshock static stress perturbation.

Compression along the northern Apennines? Evidence from the Mw 5.3 Monghidoro earthquake

AbstractIn this paper, we present the seismological data recorded during the deployment of a dense three‐component seismic network installed a few hours after the 2003 Mw 5.3 Monghidoro earthquake, in northern Apennines. The main shock focal solutions derived from polarities distribution and body wave modelling of regional broadband data show a NE–SW striking reverse mechanism. Accurate relative locations of aftershocks and the inversion of focal mechanisms show that earthquakes occurred on a NW‐dipping backthrust within the Adria lithosphere under a NW‐trending horizontal compression. The observed compression is a secondary process possibly explained by differential motion within the Adriatic lithosphere. Fault geometry and kinematics is controlled by pre‐existing structures.

The source and aftershocks of the Altai (Chuya) earthquake of 2003

Results derived from processing of data of field observations in the epicentral zone of the Altai earthquake of 2003 are presented. Accurate location of aftershocks showed that their spatial distribution is in good agreement with the source rupture outcropping on the Earth’s surface. According to aftershock data, the source had a length of 75 km and a maximum width of 20 km and extended in the vertical direction for 17 km; the displacement in the source averaged 2.5 m. Clusters identified in the aftershock cloud define variously dipping planes. The location of hypocenters by regional, national, and global seismic networks is not accurate enough to examine spatial features of the aftershocks.

2004年新潟県中越地震の発生過程

The 2004 Mid Niigata Prefecture Earthquake (MJMA 6.8) occurred on 23 October 2004. The mainshock was followed by four aftershocks with MJMA≥6.0. This earthquake is located in the Niigata-Kobe Tectonic Zone in which large strain rates (>0.1ppm/y contraction) have been observed by GPS data. We deployed three temporary online seismic stations in the aftershock area. Combining data from the temporary stations and from permanent stations around the aftershock area, we determined aftershock locations, and estimated the structures and the stress change in and around the aftershock region. Based on these results, we suggested a generating process of the 2004 Mid Niigata Prefecture Earthquake supposing that a very weak region exists in the weak zone in the lower crust just beneath the seismogenic fault.

Deep magma transport at Kilauea volcano, Hawaii

The shallow part of Kilauea's magma system is conceptually well-understood. Long-period and short-period (brittle-failure) earthquake swarms outline a near-vertical magma transport path beneath Kilauea's summit to 20 km depth. A gravity high centered above the magma transport path demonstrates that Kilauea's shallow magma system, established early in the volcano's history, has remained fixed in place. Low seismicity at 4–7 km outlines a storage region from which magma is supplied for eruptions and intrusions. Brittle-failure earthquake swarms shallower than 5 km beneath the rift zones accompany dike emplacement. Sparse earthquakes extend to a decollement at 10–12 km along which the south flank of Kilauea is sliding seaward. This zone below 5 km can sustain aseismic magma transport, consistent with recent tomographic studies. Long-period earthquake clusters deeper than 40 km occur parallel to and offshore of Kilauea's south coast, defining the deepest seismic response to magma transport from the Hawaiian hot spot. A path connecting the shallow and deep long-period earthquakes is defined by mainshock–aftershock locations of brittle-failure earthquakes unique to Kilauea whose hypocenters are deeper than 25 km with magnitudes from 4.4 to 5.2. Separation of deep and shallow long-period clusters occurs as the shallow plumbing moves with the volcanic edifice, while the deep plumbing is centered over the hotspot. Recent GPS data agrees with the volcano-propagation vector from Kauai to Maui, suggesting that Pacific plate motion, azimuth 293.5° and rate of 7.4 cm/yr, has been constant over Kilauea's lifetime. However, volcano propagation on the island of Hawaii, azimuth 325°, rate 13 cm/yr, requires southwesterly migration of the locus of melting within the broad hotspot. Deep, long-period earthquakes lie west of the extrapolated position of Kilauea backward in time along a plate-motion vector, requiring southwesterly migration of Kilauea's magma source. Assumed ages of 0.4 my for Kilauea and 0.8 my for Mauna Loa are consistent with this model. Younger ages would apply if Kilauea began its growth south of the locus of maximum melting, as is true for Loihi seamount. We conclude that Kilauea is fed from below the eastern end of the zone of deep long-period earthquakes. Magma transport is vertical below 30 km, then sub-horizontal, following the oceanic mantle boundary separating plagioclase- and spinel-peridotite, then near-vertical beneath Kilauea's summit. The migration of the melting region within the hotspot and Kilauea's sampling of different sources within the melting region can explain (1) the long-term geochemical separation of Kilauea from neighboring volcanoes Mauna Loa and Loihi, and (2) the short-term changes in trace-element and isotope signatures within Kilauea.

Integrating high‐precision aftershock locations and geodetic observations to model coseismic deformation associated with the 1995 Kozani‐Grevena earthquake, Greece

We integrate high‐precision aftershock locations with geodetic inverse modeling to create a more complete kinematic model for the Kozani‐Grevena earthquake sequence. Using the double‐difference algorithm, we have improved relative hypocentral locations by a factor of ∼7 and thus imaged the details of the fault network associated with the seismic sequence. The interpreted fault network consists of multiple segments including (1) a master normal fault that strikes nearly due west and dips toward the north at 43°, extending from 6 to 15 km depth; (2) an upper segment that connects the top of the seismicity to the observed surface ruptures and dips 70°; (3) hanging wall antithetic faults; (4) a more steeply dipping southwest striking linking structure at the southwest end of the rupture; and (5) a separate south dipping segment at the southwestern end of the aftershock cluster. The imaged fault segment dimensions, orientations, and geometric relationships are consistent with regional fault patterns. Using slip inversion on triangular dislocation patches, we calculate variable slip on the imaged three‐dimensional fault network that best fits the surface displacements observed by satellite interferometric synthetic aperture radar (InSAR). In our preferred model we find that the majority of slip occurred at depth on the west and southwest striking segments. By comparing these results to a planar fault model derived solely from the InSAR data using nonlinear inversion methods we demonstrate that the three‐dimensional model improves the fit to the geodetic data while incorporating the observations of surface rupturing and aftershock distributions.

Seismological, geological and geophysical constraints for the Gualdo Tadino fault, Umbria–Marche Apennines (central Italy)

The April 3, 1998 Mw = 5.1 Gualdo Tadino earthquake (central Italy) was the last significant event in the 6-month-long Umbria–Marche seismic crisis. This event and its aftershocks occurred in an area where active faulting produces no striking geological and geomorphological effects. In this study, we investigated the ruptured fault using detailed seismological data and a re-processed and re-interpreted seismic reflection profile. Aftershock location and focal mechanisms were used to constrain the geometry and kinematics of the ruptured fault and a comparison was made with the subsurface image provided by the seismic profile. We found that the 1998 Gualdo Tadino earthquake occurred on a WSW-dipping, normal fault, with a length of about 8 km and a relatively gentle dip (30°–40°), confined between 3.5 and 7 km in depth. Kinematics of the mainshock and aftershocks revealed a NE-trending extension, in agreement with the regional stress field active in the Northern Apennines belt. The Mw = 5.1 earthquake originated above the top of the basement and ruptured within the sedimentary cover, which consists of an evaporites–carbonates multilayer. We hypothesised that the active fault does not reach the surface (blind normal fault).

The 2001 ML 6.2 Jackson Bay earthquake sequence, South Island, New Zealand

Following the 2001 December 7 Jackson Bay earthquake (ML 6.2, MW 5.8), a temporary network of five portable seismographs was deployed in the region to record aftershock activity. Data recorded by the temporary network and nearby New Zealand National Seismological Network stations have been used to define a velocity model for the region and station corrections for each recording station. The locations of the best recorded aftershocks and the revised location of the mainshock indicate that the Jackson Bay earthquake sequence occurred 3–10 km to the east of the Alpine Fault, which is vertical in this region. A fault plane solution obtained from body‐wave modelling suggests the mainshock was primarily a reverse event (rake = 103°) centred at c. 4 km depth on a fault striking northeast‐southwest (48°) and probably dipping to the southeast (45°), which is roughly consistent with the Harvard CMT solution for this earthquake. However, on examination of the aftershock locations, such a fault plane is not clear, nor is any other. The aftershocks are located mainly in two clusters near each other at depths between 3 and 8 km and aligned approximately north‐south. Their positions are in accord with induced stress considerations and the mainshock fault plane lying between the clusters. Individual focal mechanisms for 33 aftershocks have a wide range of solutions. As a group, however, their P and T axes are reasonably well aligned and consistent with the background stress regime in the region as determined by direct inversion of P‐wave polarities. The Jackson Bay earthquake was the third thrust earthquake of magnitude >6 to occur just east of the Alpine Fault in a 7 yr period. Consideration of the mechanics of this earthquake, and the previous two, suggests that the regional stress is at a high level, in accord with the long elapsed time since a large Alpine Fault event. Although the area is small, the Jackson Bay mainshock induced a mainly positive change in Coulomb Failure Stress (CFS) on the closest section of the Alpine Fault, up to c. 0.7 MPa (7 bars).

Rapid estimation of rupture extent for large earthquakes: Application to the 2004, M9 Sumatra‐Andaman mega‐thrust

Rapid estimation of the rupture extent of large earthquakes is critical for tsunami warning and emergency response. The hypocenter of a distant earthquake is determined from the first seismic P waves within about 15 minutes after the event. However, methods for determining the size and rupture extent of very large earthquakes rely on long period recordings and aftershock locations, which are not available until several hours or more after the event. Here we introduce a method for rapid estimation of the rupture termination location for large earthquakes based on short period, P‐wave recordings, available about 30 minutes after an event. The hypocenter and the rupture termination location provide estimates of the extent of rupture and the event size. Application to the 2004, M9 Sumatra‐Andaman earthquake gives a rupture termination location near the Andaman Islands, about 1100 km north of the hypocenter, and a rupture duration of about 8 minutes.

Constraining the source separation with coda wave interferometry: Theory and application to earthquake doublets in the Hayward fault, California

The relative location of seismic sources is of importance for the location of aftershocks on a fault, for the positioning of sources in repeat seismic surveys, and for monitoring induced seismicity. In this paper we show how the seismic coda can be used to infer a measure of the source separation of two identical seismic sources from the correlation of the waveforms recorded at a single receiver. The theory is applicable to an explosive source in an acoustic or elastic medium and for a point force or double couple in an elastic medium. For an explosive source the source separation is constrained to be located on a sphere, while for a point force and a double couple the source separation can be constrained to be located on an ellipsoid whose symmetry axis is determined by the point force or double couple. We validate the theory with synthetic seismograms and apply the theory to earthquake doublets on the Hayward fault, California. The distance between events obtained from the coda waves agrees with the distance obtained from the double‐difference method.

One-dimensional crustal structure of the Erzincan basin, Eastern Turkey and relocations of the 1992 Erzincan earthquake (Ms = 6.8) aftershock sequence

In this study, one-dimensional (1-D) P- and S-wave velocity structures of upper crust in the Erzincan basin and precise hypocentre locations of aftershocks of 13 March 1992 Erzincan earthquake (Ms = 6.8) have been determined by using 1-D inversion algorithm VELEST and relocation algorithm HYP2000, interactively. 22,291 P- and 10,192 S-arrival times of 1561 aftershocks recorded at 58 temporary stations have been used. Six layers have been defined within upper 20 km of the crust. In the uppermost crust about 2 km thick sedimentary cover of the Erzincan basin has very low P-wave velocity as well as low V P/ V S ratio. The P-wave velocity reaches 6.3 km/s at 7 km depth with an increasing gradient. A thick crustal layer with a P-wave velocity of 6.9 km/s has been defined in the 7–20 km depth range. Several tests have been applied to check the stability of the velocity structure and the accuracy of the aftershock locations. The general geometry of the aftershock distribution given by previous studies has not been drastically changed but more accurate aftershock locations have provided evidences for more reliable correlation to tectonic features and clues for new proposals on the recent deformation. Examination of well-constrained aftershock cluster between Pülümür and the southeast of the Erzincan basin as well as very recent local seismic activity after 27 January 2003 Pülümür earthquake (Ms = 6.0) imply that a new antithetic fault activity to the North Anatolian Fault (NAF) zone similar to the Ovacık Fault and the Avcıdağ Fault has developed. The Yedisu Fault segment (NAF-1) of the NAF displays relatively inactive character as defined in the previous studies.

Stress changes in the Costa Rica subduction zone due to the 1999 Mw=6.9 Quepos earthquake

Stress changes after large earthquakes can trigger substantial seismicity along many fault systems. As subduction zones produce the majority of large earthquakes, it is essential to understand how significant earthquakes may affect stress conditions and aftershock locations. Calculations of stress changes due to large underthrusting earthquakes can be used to examine likely zones of triggered seismicity in subduction environments. However, associations between stress changes and locations of small aftershocks are typically difficult because of large errors on offshore aftershock locations based on land-based seismic observations. After the 1999 Quepos earthquake (Mw=6.9) in the subduction zone offshore Costa Rica, small magnitude earthquakes were located using a local onshore–offshore temporary seismic network, providing a data set of high-quality aftershock locations. In light of these well-located aftershocks, we compute Coulomb stress changes for the 1999 Quepos earthquake for comparison. Our calculations show lobes of increased stress in regions coinciding with most of the small magnitude seismicity following the mainshock, as seen in many cases of large strike-slip earthquakes. Few small earthquakes occur in regions of decreased stress. Within three years, a large (Mw=6.4) earthquake occurred in this region, however based on this modeling, it does not appear that the 2002 earthquake occurred in a zone of stress increase from the 1999 event.

Source Rupture Process of the Tecoman, Colima, Mexico Earthquake of 22 January 2003, Determined by Joint Inversion of Teleseismic Body-Wave and Near-Source Data

The spatial and temporal slip distribution of the Tecoman, Colima, Mexico earthquake is estimated from near-source strong-motion and teleseismic body-wave data. To perform a stable inversion, we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized Akaike's Bayesian information criterion (ABIC). The source parameters are as follows: (strike, dip, slip) = (300°, 20°, 93°), seismic moment M = 2.3 × 1020 N m; source duration = 30 sec; along-strike distance = 35 km; along-dip distance = 70 km. We found that the rupture process can be divided into three stages: the rupture nucleated near the hypocenter (stage I), then it broke the first asperity centering about 15 km southwest from the epicenter (stage II); and the rupture propagated to the northeast and the second asperity was broken (stage III). We also estimated the shear-stress change due to the rupture process of the mainshock on and around the major fault zone. It appears that one cluster of aftershocks for the first 5 days, which took place in and adjacent to the zones of stress, increased due to the fault rupture during the mainshock, but overall correlation between the aftershock location and the stress pattern is not clear. Manuscript received 14 May 2003.

Fault Rupture Process of the 20 September 1999 Chi-Chi, Taiwan, Earthquake

We modeled the source rupture process of the 20 September 1999 Chi- Chi, Taiwan, earthquake (Mw 7.6) using an integrated dataset of near-field strong ground motion and Global Positioning System observations. This large inland thrust earthquake produced a surface break more than 80 km long along the Chelungpu fault. The event triggered over 700 strong-motion instruments on the island of Tai- wan. This large ground-motion dataset provides the best opportunity to date to un- derstand earthquake source processes in a major earthquake. First, we develop a 3D curved fault model consistent with the surface rupture data and aftershock distribu- tion. The subsurface fault plane has an average dip of 30 to the east, and measures 96 km along strike and 40 km down-dip. Next we applied a genetic algorithm to reconstruct the source rupture process. The inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down-dip. Most of the displacement occurred over a 15-km depth range in the northern segment of the fault, with the maximum slip exceeding 20 m. The average slip is 3.8 m, and the average rupture velocity is 2.6 km/sec. The mainshock slip distribution clearly complements the aftershock location distribution. Slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north. This relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations. The rake angle rotates across the rupture plane from predominantly dip-slip in the southern segment to mostly left-lateral oblique slip in the north. We find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior. At fault bends, rupture tends to decelerate, the rake angle rotates to a near dip-slip orientation, and the shear stress increases as rupture tries to overcome the bending barriers. The total seismic moment from our inversion is 2.9 10 27 dyne cm, in good agreement with the Harvard and the U.S. Geological Survey moment estimation.

A study of the 2 December 2002 ( M5.5) Vartholomio (western Peloponnese, Greece) earthquake and of its largest aftershocks

Broadband data from the Greek National Seismological Network are used to study the moderate size ( M5.5) earthquake, which occurred on 2 December 2002 near the town of Vartholomio, in western Peloponnese (Greece). Time domain moment tensor inversion applied to retrieve the focal mechanism of the mainshock and of three of the larger aftershocks of the sequence, revealed almost pure strike-slip faulting along NW–SE or NE–SW trending nodal planes. The relative source time functions for the mainshock, obtained from an empirical Green's function analysis, do not reveal any clear directivity to any of the stations. A careful observer might suggest directivity towards NW, if any. Optimum values are 0.4 s for the rise time and 2.7 km/s for the rupture velocity. The spatial and temporal distribution of fault slip showed that the major part of the resolved slip occurred beneath the mainshock's epicenter, ~20 km underneath the western coast of Peloponnese. This probably accounts for the considerable damage observed to the nearby towns. The resolution between the two nodal planes does not permit an identification of the fault plane; however the statistics on the slip distribution model, the preliminary analysis of aftershock locations and macroseismic data favour the NW–SE trending plane as the fault plane, which is connected with sinistral strike-slip motions. These are the first implications for sinistral strike-slip motions in this area and more data are needed in the future to get more reliable resolution of the motions.

Source Parameters and Tectonic Implications of Aftershocks of the Mw 7.6 Bhuj Earthquake of 26 January 2001

We present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period. The data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock. We have calculated high-quality single-event locations, based on a 1D velocity model determined for the region for earthquakes with magnitudes between ∼2 and 5. Aftershock locations reveal activity concentrated on a nearly east-striking, south-dipping plane, trapezoidal in outline. The active zone tapers from about 45 km along strike at the shallow end, which is about 5 km deep, to no more than 25 km long at a depth of 35 km. The total rupture area was about 1300 km 2 . We estimate the static stress drop of the mainshock at 16 ± 2 MPa. Aftershocks extend nearly through the entire crust, with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km. The fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake. The aftershock data are consistent with the Bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift.

Complex Normal Faulting in the Apennines Thrust-and-Fold Belt: The 1997 Seismic Sequence in Central Italy

A long sequence of moderate-magnitude earthquakes (5 M 6) struck central Italy in September and October 1997. At the end of the sequence a year later, the seismogenic area extends for about 60 km along the Apennines. The analysis of historical seismicity suggests that this seismic sequence filled a 700-year gap in this portion of the chain. Other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments, probably due to the in- volvement of shallow and complex structures inherited by the compressive tectonics. The distribution of seismicity and the fault-plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles (40) to the southwest. The focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the Apenninic chain (northeast-southwest), consistently with the Quaternary tectonics of the internal sector of the northern Apen- nine belt and with previous earthquakes in adjacent regions. Three mainshocks oc- curred on distinct 5- to 10-km-long fault segments, adjacent and slightly offset be- tween each other. High-quality aftershock locations show that seismicity is confined within the sedimentary Mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks, which nucleated at 6-km depth. Faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface. Most of the aftershock focal mechanisms are dominated by normal faulting. Several strike-slip events occurred at shallow depths, reactivating portions of pre-existing thrust planes that segment the normal fault system. The spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes.