Postseismic Period Research Articles

Distributed fault slip model for the 2011 Tohoku‐Oki earthquake from GNSS and GRACE/GOCE satellite gravimetry

AbstractThe Gravity Recovery and Climate Experiment (GRACE) mission (launched 2002) and the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission (March 2009 to November 2013) collected spaceborne gravity data for the preseismic and postseismic periods of the 2011 Tohoku‐Oki earthquake. In addition, the dense Japan GeoNet Global Navigation Satellite Systems (GNSS) network measured with approximately 1050 stations the coseismic and postseismic surface displacements. We use a novel combination of GNSS, GRACE, and GOCE observations for a distributed fault slip model addressing the issues with gravimetric and geometric change over consistent time windows. Our model integrates the coseismic and postseismic effects as we include GOCE observations averaged over a 2 year interval, but their inclusion reveals the gravity change with unprecedented spatial accuracy. The gravity gradient grid, evaluated at GOCE orbit height of 265 km, has an estimated formal error of 0.20 mE which provides sensitivity to the mainly coseismic and integrated postseismic‐induced gravity gradient signal of −1.03 mE. We show that an increased resolution of the gravity change provides valuable information, with GOCE gravity gradient observations sensitive to a more focused slip distribution in contrast to the filtered GRACE equivalent. The 2 year averaging window of the observations makes it important to incorporate estimates of the variance/covariance of unmodeled processes in the inversion. The GNSS and GRACE/GOCE combined model shows a slip pattern with 20 m peak slip at the trench. The total gravity change (≈200 μGal) and the spatial mapping accuracy would have been considerably lower by omitting the GOCE‐derived fine‐scale gravity field information.

Lithospheric rheology constrained from twenty-five years of postseismic deformation following the 1989 Mw 6.9 Loma Prieta earthquake

The October 17, 1989 Mw6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 Mw7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the Loma Prieta earthquake postseismic displacements from 1989 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 yr of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe GPS horizontal displacements at mean rates of 1–4 mm/yr toward Loma Prieta Mountain until 2000, and ∼2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002 shown by InSAR, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989–1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994. The best fitting model shows an elastic 19-km-thick upper crust above an 11-km-thick viscoelastic lower crust with viscosity of ∼6×1018 Pas, underlain by a viscous upper mantle with viscosity between 3×1018 and 2×1019 Pas. The millimeter-scale postseismic deformation does not resolve the viscosity in the different layers very well, and the lower-crustal relaxation may be localized in a narrow shear zone. However, the inferred lithospheric rheology is consistent with previous estimates based on post-1906 San Francisco earthquake measurements along the San Andreas fault system. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.

Crustal thickening and extension induced by the Great Sumatra–Andaman earthquake of 26 December 2004: revealed by the seismic moment tensor element Mrr

In a complex geological environment, it is generally difficult to assimilate a complete understanding of the main stress regime without being distracted by various fault configurations. In this study, we examine the changes in radial seismic moment (Mrr) of earthquakes that occurred in the Sumatra area. This approach allows for simplification of the various focal mechanism solutions by compressive and extensional slip vectors. The results show that the epicenter of the 2004 mainshock is located near the area where the accumulated Mrr was highest during the inter-seismic period. Simultaneously, a pattern of negative accumulated Mrr observed at depths between 40 and 100 km suggests a down-dip extension mechanism caused by the slab pull effect, which could be strengthened by the locking procedure in shallow portions of the slab. More- over, the right-lateral strike-slip Sumatra Fault and the left- lateral oceanic fracture zones exhibited positive and neg- ative accumulated Mrr prior to and immediately after the 2004 mainshock. This transform of Mrr indicates stress release associated with the unlocking of the asperities during the earthquake. Therefore, the distinctly different Mrr patterns for the inter- and post-seismic period suggest that the occurrence of a substantial earthquake could change the intraplate stress state.

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 Mw 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight Mw⩾5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.

A high-resolution, time-variable afterslip model for the 2010 Maule Mw = 8.8, Chile megathrust earthquake

The excellent spatial coverage of continuous GPS stations in the region affected by the Maule Mw = 8.8 2010 earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 d of afterslip. We also invert co-seismic GPS displacements to create a new co-seismic slip model. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Changes in static stress on the plate interface from the co- and post-seismic slip cannot solely explain the aftershock patterns, suggesting that another process – perhaps fluid related – is controlling the lower magnitude aftershocks. We use aftershock data to quantify the seismic coupling distribution during the post-seismic phase. Comparison of the post-seismic behaviour to interseismic locking suggests that highly locked regions do not necessarily behave as rate-weakening in the post-seismic period. By comparing the inter-seismic locking, co-seismic slip, afterslip, and aftershocks we attempt to classify the heterogeneous frictional behaviour of the plate interface.

Analysis of the b-values before and after the 23 October 2011 Mw 7.2 Van–Erciş, Turkey earthquake

The b-values from the Gutenberg–Richter frequency–magnitude relation are investigated in the Lake Van region in eastern Turkey where accommodated the 23 October 2011 Mw 7.2 Van–Erciş earthquake. Earthquake catalog is provided by the Kandilli Observatory and Earthquake Research Institute (KOERI). The catalog consists of 6709 well-located events with local magnitudes (ML) from 2.6 to 5.8. The time span of the catalog covers the period 2002 and 2012. The b-values were significantly lower at the Van–Erciş mainshock area prior to 23 October 2011. The mainshock occurred near to the high stress (low b-value) asperity in the east of Lake Van. The decrease in b-values supports “low b-value-high stress hypothesis” prior to the Van–Erciş earthquake. The temporal change of b-value before the mainshock exhibits a strong trend of decreasing which could be used as an indicator of the approaching mainshock. This b-value analysis provides an evidence for significant difference that exist between pre-seismic and post-seismic periods of the mainshock area. Distribution of low b-values for Van–Erciş aftershock sequence relates the termination of the mainshock rupture to the east may be the precursory phenomena for the future moderate or large magnitude earthquakes in the Lake Van region.

Spatial and Temporal Variations in Coda Attenuation Associated with the 2011 Off the Pacific Coast of Tohoku, Japan (Mw 9) Earthquake

The spatial and temporal variations of coda attenuation (![Graphic][1] ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan ( M w 9) earthquake. The ![Graphic][2] values were determined from the amplitude decay rate of the S ‐wave coda in narrower overlapping frequency bands in the range f =1.0–24 Hz, based on a single isotropic scattering model for more than 400 earthquakes ( M JMA 3∼6.5) in the region recorded in a period from January 2005 to August 2011, including pre‐ and postseismic period. Our estimates of the spatially averaged ![Graphic][3] value, ![Graphic][4] in f =1.0–24 Hz, is almost stable with small variations ( increases by about 10%–16% after the 2011 Tohoku event in f =1.25–3.5 Hz in some stations of northern Japan, which is confirmed by a statistical t ‐test at 99.9% confidence level. The change in ![Graphic][6] is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku‐Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a =0.6–1.8 km in f =1.25–3.5 Hz) enhanced by increased stress change possibly controls the increased ![Graphic][7] after the 2011 event in the source rupture region. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif

Outer rise seismicity related to the Maule, Chile 2010 mega-thrust earthquake and hydration of the incoming oceanic lithosphere

Most of the recent published geodetic models of the 2010 Maule, Chile mega-thrust earthquake (Mw=8.8) show a pronounced slip maximum of 15-20 m offshore Iloca (~35°S), indicating that co-seismic slip was largest north of the epicenter of the earthquake rupture area. A secondary slip maximum 8-10 m appears south of the epicenter west of the Arauco Peninsula. During the first weeks following the main shock and seaward of the main slip maximum, an outer rise seismic cluster of >450 events, mainly extensional, with magnitudes Mw=4-6 was formed. In contrast, the outer rise located seaward of the secondary slip maximum presents little seismicity. This observation suggests that outer rise seismicity following the Maule earthquake is strongly correlated with the heterogeneous coseismic slip distribution of the main megathrust event. In particular, the formation of the outer-rise seismic cluster in the north, which spatially correlates with the main maximum slip, is likely linked to strong extensional stresses transfered from the large slip of the subducting oceanic plate. In addition, high resolution bathymetric data reveals that bending-related faulting is more intense seaward of the main maximum slip, where well developed extensional faults strike parallel to the trench axis. Also published seismic constraints reveal reduced P-wave velocities in the uppermost mantle at the trench-outer rise region (7.5-7.8 km/s), which suggest serpentinization of the uppermost mantle. Seawater percolation up to mantle depths is likely driven by bending related-faulting at the outer rise. Water percolation into the upper mantle is expected to be more efficient during the co-seismic and early post-seismic periods of large megathrust earthquakes when intense extensional faulting of the oceanic lithosphere facilitates water infiltration seaward of the trench.

Slip distribution of the 2011 Tohoku earthquake derived from joint inversion of GPS, InSAR and seafloor GPS/acoustic measurements

We invert the fault slips of 2011 Mw 9.0 Tohoku earthquake with constraints from GPS, InSAR and seafloor GPS/acoustic measurements. The seismogenic fault geometry is constructed according to slab contours of Japan Trench. Steepest Descent method and Laplacian smoothing are used to solve slip distribution and regularize the solution. We firstly take GPS displacement in two postseismic periods, the first 8h right after mainshock from 5:55 to 14:00 UTC and the 13days from 12 March 2011 to 25 March 2011, to solve for the postseismic slips. The solved postseismic slips are adopted to remove postseismic signal in InSAR and seafloor observation. In order to estimate the effect of postseismic correction and contribution from different geodetic datasets, we invert several coseismic slips with constraints from GPS (Model 1), corrected InSAR (Model 2), combination of GPS and corrected seafloor measurements (Model 3), combination of GPS and corrected InSAR and seafloor measurements (Model 4), and combination of GPS and initial InSAR and seafloor measurements (Model 5). From the comparison of these slip models, we find combined datasets could give more slip details, which is closer to a joint inversion result constrained from both seismic and geodetic datasets (Koketsu et al., 2011). RMSE of seafloor measurements has dropped about 4cm after applying postseismic correction. We consider the Model 4, which combines three datasets and takes postseismic correction, to be the preferred solution among all the estimated models. It suggests a maximum slip of 49.87m, located at a depth of 5km around the epicenter, and has a geodetic moment of 3.14×1022Nm (Mw 8.96) by assuming a shear modulus of 4×1010Pa.

Response of forearc crustal faults to the megathrust earthquake cycle: InSAR evidence from Mejillones Peninsula, Northern Chile

We report on a rare example of aseismic response of a creeping fault to the earthquake cycle of a nearby megathrust. Interferometric synthetic aperture radar (InSAR) is used to detect and analyze shallow creep of two crustal faults at Mejilones Peninsula, Northern Chile, located in the hanging wall of the 2007 Mw 7.7 Tocopilla subduction earthquake. We generate two independent time series of surface deformation spanning ∼3.5yr of late interseismic and ∼1.5yr early postseismic deformation associated with this event. The analysis reveals creep on the Mejillones fault as well as on a previously unmapped fault to the west of the Mejillones fault. The InSAR deformation maps and distributed slip models obtained from the data reveal that fault creep reversed between the interseismic and postseismic periods. Given the regional stress field perturbations due to interseismic and coseismic deformation, we argue that the observed shallow creep and its slip reversal are directly linked to the megathrust seismic cycle. Moreover, from similar eastward dips but opposite slip directions of the two faults, we infer that fault strength must be very low and that the kinematics is controlled by crustal flexure associated with the seismic cycle on the underlying megathrust.

Transtension to transpression: A study of strain evolution in Andaman Islands based on GPS measurements following Great Sumatra–Andaman Earthquake, 2004

We use campaign mode and continuous GPS data from 2005 to 2008 to examine the evolving strain pattern in Andaman Islands after the Great Sumatra–Andaman earthquake. Two and half years after the earthquake, the post-seismic period changes to inter-seismic period. At Port Blair, post-seismic displacement rate reduced to about 95% with clockwise rotation of azimuths and towards North Andaman, velocity vector gradually rotates towards north with drop in magnitude. The inter-seismic displacement was dominated by trench parallel movement. Strain calculations on inclined transpession model shows deformations at Andaman were highly heterogeneous both in post and inter-seismic periods with dominant strike-slip and dip-slip shearing along with layer parallel shortening and extension. Post-seismic and inter-seismic periods had suffered simultaneous subduction perpendicular extension and contraction leading to transtension and transpression deformational environments respectively. Occurrence of earthquakes during post and inter-seismic periods were results of interplay between simple shear and pure shear components of the strain matrix.

Restoration of interplate locking after the 2005 Off-Miyagi Prefecture earthquake, detected by GPS/acoustic seafloor geodetic observation

[1] A 7.2-magnitude earthquake occurred on the plate boundary off Miyagi Prefecture, northeastern Japan, on 16 August 2005. From seafloor geodetic observations using the GPS/acoustic combination technique in this area, we have detected notable seafloor movements associated with, and subsequent to this event. The time series of horizontal coordinates obtained at the seafloor reference point MYGW, located about 10 km east of the epicenter, indicates that the strain accumulated for the interseismic period was partly released by the event and a constant strain accumulation restarted after 1–2 years of the post-seismic period with an erratic movement. The horizontal velocity of MYGW after the restart above is estimated to be about 5.7 cm/year toward WNW relative to the Eurasian plate. Our result is consistent with those derived from terrestrial GPS measurements and implies that the interplate locking was restored in the rupture area of the 2005 event around 2007.

Complex states of stress during the normal faulting seismic cycle: Role of midcrustal postseismic creep

Numerical models are used to analyze coseismic and postseismic change in the state of stress near the lower tip of a normal fault directly below the brittle‐ductile transition. The models are compared with information obtained from the geological record of exhumed metamorphic rocks regarding the magnitude and the geometry of coseismic stress increase close to the lower fault termination and the mechanisms involved during postseismic stress relaxation. The numerical results predict a drop in differential and fault‐parallel shear stress in the upper crust and a stress increase in the lower crust due to the tapering off in fault slip. The coseismic deformation field in the crust causes significant deflection of the principal stresses from the horizontal and the vertical. During the postseismic period, the recovery of the crustal stress field remains incomplete, even for long recurrence intervals. Instead, the stress field evolves in self‐similar cycles, controlled by repeated earthquake activity. The numerical models show that postseismic viscous creep localized beneath the fault contributes to the recovery of all stress tensor components in the upper crust close to the fault. This result implies that a detailed understanding of the processes and conditions active during postseismic stress relaxation in the middle and lower crust is essential in order to estimate reloading rates in the upper crust surrounding the fault.

Spatiotemporal characteristics of aftershock sequences in the South Iceland Seismic Zone: interpretation in terms of pore pressure diffusion and poroelasticity

In seismology numerous observations indicate a relationship between pore pressure in the Earth's crust and the occurrence of earthquakes. In this paper we study aftershock sequences in the South Iceland Seismic Zone (SISZ), where poroelastic rebound has been observed in the post-seismic period of two M 6.5 earthquakes in 2000 June. We analyse characteristic features in the spatiotemporal distribution of aftershocks following the two M 6.5 2000 June 17 and 21 earthquakes and a M 4.5 earthquake on 1999 September 27. These features include an initial pre-power-law decay period characterized by an initially finite aftershock rate, a subsequent power-law decay interrupted by distinct and temporary rate increases and decreases as well as increased clustering of aftershocks with time in the main shock fault zones. Extending the analysis to a M 3.2 aftershock sequence in the same region confirms an increase in the duration of the initial pre-power-law decay period with increasing main shock magnitude. We find, from the return time of aftershock magnitudes to the long-term completeness level, that the initial pre-power-law decay period and its durational dependence on main shock magnitude may not only represent incompleteness artefacts but may also reflect the physics of the aftershock process in the SISZ. Based on pore pressure diffusion modelling, we interpret the origin of the observed SISZ aftershock features in terms of a spatially non-linear coseismic influence of the main shock on stresses in the surrounding crust and poroelastic adjustment of stresses and pore pressures during main shock initiated diffusion processes. In a discussion of alternative interpretations, we find that rate and state friction and dynamically propagating crack models, the statistical ETAS model, afterslip models, viscoelastic relaxation of the lower crust and upper mantle and a recently proposed dependence on the crustal state of stress all appear inconsistent with at least one of the characteristic spatiotemporal features of the studied SISZ aftershock sequences. We conclude that these features constitute strong evidence for pore pressure effects in aftershock triggering within the SISZ and recommend that poroelastic adjustment of stresses is taken into account in modelling of main shock initiated pore pressure diffusion.

Seismic and aseismic slip on the Central Peru megathrust

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude M(w) = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with M(w) = 8.0 in the Pisco area is estimated to be 250 years.

Persistent pre-seismic signature detected by means of Na-K-Mg geothermometry records in a saline spring of Vrancea area (Romania)

Abstract. A six year-long hydrochemical monitoring operation was conducted in Vrancea seismic zone (Romania), addressing a saline spring that proved to be suitable for Na-K-Mg geothermometry diagnosis. During the considered time-interval (2003–2009), only one important earthquake (mb=5.8) occurred in Vrancea region, this circumstance providing an unambiguous reference-moment between pre-seismic and post-seismic periods. On occurrence of that earthquake, an anomalous fluctuation of the Na-K temperature was detected – a result largely similar to previous ones recorded worldwide (California, southwest Egypt, northeast India). Yet such fluctuations may not necessarily be induced by earthquake-associated processes: they can occur also "routinely", possibly reflecting some environmental, meteorologically-induced "noise". It was therefore important to examine whether the variations observed in the data values could be plausibly related to a seismogenesis process. By additionally investigating (in a "scattterplot" diagram) the correlation between the Na-K temperatures and the values of a so-called "maturity index", a specific pattern emerged, with pre-seismic data-points plotting in a distinct domain of the diagram; moreover, those data-points appeared to describe a "drift away" pathway with respect to the remaining data-points "cluster", recorded during the subsequent 4 years of post-seismic monitoring. The "drift away" pattern persistently evolved for at least 18 months, ending just before the mb=5.8 earthquake and consequently suggesting the existence of some kind of long-term precursory phenomenon.

Stress changes induced at neighbouring faults by the June 2000 earthquakes, South Iceland Seismic Zone

The South Iceland Seismic Zone (SISZ) is an E-W trending active transform zone with high seismicity. N-S trending right-lateral strike-slip faults accommodate the left-lateral transform motion. Using seismological data recorded from 1991 to 2007, we carried out stress inversion of focal mechanisms of 1340 earthquakes that affected the Skard and Leirubakki faults, eastern SISZ. Not only did the inversion show typical deviations of stress across the faults, it also revealed anticlockwise and clockwise rotations of stress axes with time. Numerical models of the Leirubakki Fault show that these rotations are consistent with the mechanical effect of a lowered friction coefficient during the post-seismic period relative to the pre-seismic period. The Skard Fault reveals a more complex behaviour associated with a higher post-seismic friction, resulting from a higher density of pre-existing fracturing and probable stress interaction between faults. Our results suggest that faults where micro-earthquakes occur during neighbouring major seismic events may undergo significant stress changes at the scale of several kilometres and on time-scales of several years.

Damage scenario of the earthquake on 23 July 1930 in Melfi: the contribution of technical documentation

As regards the 1930 Irpinia earthquake a detailed research both on the institutional response to the seismic event in Vulture area and reconstruction of the damage scenario for the town of Melfi has been performed. This study was carried out by an analysis of coeval dossiers drawn up by the Special Office of Civil Engineers, which was set up after the earthquake. The research brought to light the typologies and the modalities of the institutional actions taken during the post-seismic period. In general, these territorial interventions had a notable effect on urban systems, especially those involving both the partial shifting of urban areas and the construction of earthquake-proof buildings. The research also identified the damage pattern in Melfi by a deeper study on about 2400 archive files. A preliminary analysis of the damage pattern indicates probable seismic amplification phenomena due to the lithological and geomorphological features of the site. Moreover, the analysis of time-dependent activities of reconstruction has shown that almost all the buildings of the town (90%) were repaired or reconstructed within five years after the seismic disaster.

Co-seismic and post-seismic pore-fluid pressure changes in the Philippine Sea plate and Nankai decollement in response to a seismogenic strain event off Kii Peninsula, Japan

AbstractNew pressure data from a pair of Ocean Drilling Program (ODP) hydrologic borehole observatories at ODP Sites 1173 and 808, located off Japan in the subducting Philippine Sea plate and in the nearby Nankai accretionary prism, respectively, show clear signals associated with an earthquake swarm off the Kii Peninsula that began on September 5, 2004, roughly 220 km away from the observatory sites. At Site 1173, formation pressures rose by 1.0–1.5 kPa at the time of the largest earthquake (Mw= 7.5), then continued to rise to a total anomaly of 4 kPa during the following 200–300 days. These transients are inferred to reflect co-seismic and slow continuing volumetric contraction of the plate by amounts of roughly 0.2 × 10−6and 0.5 × 10−6, respectively. The sign of the estimated strain is consistent with that predicted with a seismic-moment-constrained elastic half-space dislocation model, but the amplitude is much larger, by roughly a factor of 6 at the time of the main earthquake, and by nearly a factor of 20 when the total pressure-estimated strain at the end of the post-seismic period is compared to that estimated from the total cumulative seismic moment including aftershocks. The simplest inference that can be drawn is that a large component of aseismic slip occurred in the epicentral area. At Site 808, pressure at the deepest monitoring zone just above the subduction decollement fell at the time of the largest earthquakes. This may reflect shear-induced dilatation which would be consistent with strain-or velocity-hardening behaviour along this seaward-most part of the subduction thrust interface.

Stress evolution following the 1999 Chi-Chi, Taiwan, earthquake: consequences for afterslip, relaxation, aftershocks and departures from Omori decay

SUMMARY We explore how Coulomb stress transfer and viscoelastic relaxation control afterslip and aftershocks in a continental thrust fault system. The 1999 September 21 M w = 7.6 Chi- Chi shock is typical of continental ramp-dsystems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable. First, we find that the spatial and depth distribution of aftershocks and their focal mechanisms are consistent with the calculated Coulomb stress changes imparted by the coseismic rupture. Some 61 per cent of the M ≥ 2 aftershocks and 83 per cent of the M ≥ 4 aftershocks lie in regions for which the Coulomb stress increased by ≥0.1 bars, and there is a 11-12 per cent gain in the percentage of aftershocks nodal planes on which the shear stress increased over the pre- Chi Chi control period. Second, we find that afterslip occurred where the calculated coseismic stress increased on the fault ramp and dsubject to the condition that friction is high on the ramp and low on the d´ ecollement. Third, viscoelastic relaxation is evident from the fit of the post-seismic GPS data on the footwall. Fourth, we find that the rate of seismicity began to increase during the post-seismic period in an annulus extending east of the main rupture. The spatial extent of the seismicity annulus resembles the calculated ≥0.05-bar Coulomb stress increase caused by viscoelastic relaxation and afterslip, and we find a 9- 12 per cent gain in the percentage of focal mechanisms with >0.01-bar shear stress increases imparted by the post-seismic afterslip and relaxation in comparison to the control period. Thus, we argue that post-seismic stress changes can for the first time be shown to alter the production of aftershocks, as judged by their rate, spatial distribution, and focal mechanisms.