Subduction Zone Events Research Articles

Scaling Relations of Earthquake Source Parameter Estimates with Special Focus on Subduction Environment

Abstract Earthquake rupture length and width estimates are in demand in many seismological applications. Earthquake magnitude estimates are often available, whereas the geometrical extensions of the rupture fault mostly are lacking. Therefore, scaling relations are needed to derive length and width from magnitude. Most frequently used are the relationships of Wells and Coppersmith (1994) derived on the basis of a large dataset including all slip types with the exception of thrust faulting events in subduction environments. However, there are many applications dealing with earthquakes in subduction zones because of their high seismic and tsunamigenic potential. There are no well-established scaling relations for moment magnitude and length/width for subduction events. Within this study, we compiled a large database of source parameter estimates of 283 earthquakes. All focal mechanisms are represented, but special focus is set on (large) subduction zone events, in particular. Scaling relations were fitted with linear least-square as well as orthogonal regression and analyzed regarding the difference between continental and subduction zone/oceanic relationships. Additionally, the effect of technical progress in earthquake parameter estimation on scaling relations was tested as well as the influence of different fault mechanisms. For a given moment magnitude we found shorter but wider rupture areas of thrust events compared to Wells and Coppersmith (1994). The thrust event relationships for pure continental and pure subduction zone rupture areas were found to be almost identical. The scaling relations differ significantly for slip types. The exclusion of events prior to 1964 when the worldwide standard seismic network was established resulted in a remarkable effect on strike-slip scaling relations: the data do not show any saturation of rupture width of strike-slip earthquakes. Generally, rupture area seems to scale with mean slip independent of magnitude. The aspect ratio L / W , however, depends on moment and differs for each slip type.

Dilatant strengthening as a mechanism for slow slip events

The mechanics of slow slip events (SSE) in subduction zones remain unresolved. We suggest that SSE nucleate in areas of unstable friction under drained conditions, but as slip accelerates dilatancy reduces pore pressure p quenching instability. Competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast. We model SSE with 2‐D elasticity, rate‐state friction, and a dilatancy law where porosity evolves toward steady state ss over distance dc and ss = 0 + ln(v/v0); v is slip speed. We consider two diffusion models. Membrane diffusion (MD) is approximated by −(p − p∞)/tf where p and p∞ are shear zone and remote pore pressure and tf is a characteristic diffusion time. Homogeneous diffusion (HD) accurately models fault‐normal flow with diffusivity chyd. For MD, linearized analysis defines a boundary ℰ = 1 − a/b between slow and fast slip, where ℰ ≡ f0ε/βb(σ − p∞), f0, a, and b are friction parameters and β is compressibility. When ℰ < 1 − a/b slip accelerates to instability for sufficiently large faults, whereas for ℰ > 1 − a/b slip speeds remain quasi‐static. For HD, Ep ≡ εh/(β (σ − p∞)) defines dilatancy efficiency, where h is shear zone thickness and v∞ is plate velocity. SSE are favored by large εh and low effective stress. The ratio Ep to thermal pressurization efficiency scales with 1/(σ − p∞), so high p∞ favors SSE, consistent with seismic observations. For Ep ∼ 10−3 transient slip rates, repeat times, average slip, and stress drops are comparable to field observations. Model updip propagation speeds are comparable to those observed along‐strike. Many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress. Model SSE accommodate only a fraction of plate motion; the remaining deficit must be accommodated during coseismic or postseismic slip.

Special issue: 2010 los angeles tall buildings structural design council—2010 magnitude 8·8 chile earthquake

AbstractIt is a pleasure to present the seventh annual issue developed by the Los Angeles Tall Buildings Structural Design Council. This two part issue is devoted to the Offshore Maule earthquake that shook Central Chile earlier this year. These two issues represent significant lessons learned from this earthquake. This earthquake is very significant as it presents a unique opportunity to study the effects of a true mega earthquake (with moment magnitude of 8·8) on modern tall buildings designed with contemporary building codes and constructed with modern construction methods. The building codes and construction methods are similar to those used in the USA so this event should be of interest to all structural engineers. This earthquake occurred on the South American subduction zone, which has uncanny similarities to other similar subduction zones around the world including the Cascadia subduction zone off the coast of the Pacific Northwest of the USA and Southern British Columbia, Canada; other subduction zones include those off of Alaska, Japan, Philippines and Indonesia. Thus, this earthquake gives us the first opportunity to develop an understanding of the behaviour of tall buildings to subduction zone events. Copyright © 2010 John Wiley & Sons, Ltd.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore‐arc mantle

Three‐dimensional tomographic images of the subducting Philippine Sea slab and the fore‐arc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in VP, VS, and VP/VS. Analysis of the trade‐off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data. The regularization parameters were also chosen to ensure suppression of artificial VP/VS anomalies. The subducting slab is characterized by high VP, high VS, and intermediate to low VP/VS. Notable in the mantle wedge is the high VP/VS anomalies that abut the surface of the subducting slab at depths of 30–80 km. The previously identified positive VP/VS channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert VS and VP/VS to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200°C–400°C relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches ∼15%, or 2% water content, at 50 km depth in the fore‐arc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt‐eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea Plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.

A reanalysis of the AD 365 tsunami impact along the Egyptian Mediterranean coast

The historical tsunamigenic event of 21 July AD 365 destroyed several coastal locations in the Eastern Mediterranean region. The foremost destructive impacts were recorded in Crete and Egypt. The present study re-examines the effect of location, direction, height and time of travel of the tsunami towards the Egyptian coast. Evidently, this tsunamigenic event is related to an earthquake which is identified with a Hellenic Arc subduction-zone event of great magnitude, M > 8, as manifested by up to 9 m uplift in western Crete. The maximum run-up height distribution in the front of the Nile Delta was about 9.5 m in Alexandria, while those of the neighboring cities were 7.1 m, 4.9 m, and 1.9 m at Rashid, Damietta and El-Arish, respectively. Data obtained from this study is essential to evaluate the tsunami hazards along the Egyptian coast.

Earthquake nucleation on rate and state faults – Aging and slip laws

We compare 2‐D, quasi‐static earthquake nucleation on rate‐and‐state faults under both “aging” and “slip” versions of the state evolution law. For both versions mature nucleation zones exhibit 2 primary regimes of growth: Well above and slightly above steady state, corresponding respectively to larger and smaller fault weakening rates. Well above steady state, aging‐law nucleation takes the form of accelerating slip on a patch of fixed length. This length is proportional to b−1 and independent of a, where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength. Under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates. The nucleation zone is guaranteed to remain well above steady state only for values of a/b that are low by laboratory standards. Near steady state, for both laws the nucleation zone expands. The propagating front remains well above steady state, giving rise to a simple expression for its effective fracture energy Gc. This fracture energy controls the propagation style. For the aging law Gc increases approximately as the square of the logarithm of the velocity jump. This causes the nucleation zone to undergo quasi‐static crack‐like expansion, to a size asymptotically proportional to b/(b−a)2. For the slip law Gc increases only as the logarithm of the velocity jump, and crack‐like expansion is not an option. Instead, the nucleation zone grows as an accelerating unidirectional slip pulse. Under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly μ′/bσ divided by the logarithm of the velocity jump, where μ′ is the effective elastic shear modulus. For this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 MPa.

New Zealand acceleration response spectrum attenuation relations for crustal and subduction zone earthquakes

Attenuation relations are presented for peak ground accelerations (pga) and 5% damped acceleration response spectra in New Zealand earthquakes. Expressions are given for both the larger and the geometric mean of two randomly-oriented but orthogonal horizontal components of motion. The relations take account of the different tectonic types of earthquakes in New Zealand, i.e., crustal, subduction interface and dipping slab, and of the different source mechanisms for crustal earthquakes. They also model the faster attenuation of high-frequency earthquake ground motions in the volcanic region than elsewhere. Both the crustal and subduction zone attenuation expressions have been obtained by modifying overseas models for each of these tectonic environments to better match New Zealand data, and to cover site classes that relate directly to those used for seismic design in New Zealand codes. The study used all available data from the New Zealand strong-motion earthquake accelerograph network up to the end of 1995 that satisfied various selection criteria, supplemented by selected data from digital seismographs. The seismographs provided additional records from rock sites, and of motions involving propagation paths through the volcanic region, classes of data that are sparse in records produced by the accelerograph network. The New Zealand strong-motion dataset lacks records in the nearsource region, with only one record from a distance of less than 10 km from the source, and at magnitudes greater than Mw 7.23. The New Zealand data used in the regression analyses ranged in source distance from 6 km to 400 km (the selected cutoff) and in moment magnitude from 5.08 to 7.23 for pga, with the maximum magnitude reducing to 7.09 for response spectra data. The required near-source constraint has been obtained by supplementing the New Zealand dataset with overseas peak ground acceleration data (but not response spectra) recorded at distances less than 10 km from the source. Further near-source constraints were obtained from the overseas attenuation models, in terms of relationships that had to be maintained between various coefficients that control the estimated motions at short distances. Other coefficients were fitted from regression analyses to better match the New Zealand data. The need for different treatment of crustal and subduction zone earthquakes is most apparent when the effects or source mechanism are taken into account. For crustal earthquakes, reverse mechanism events produce the strongest motions, followed by strike-slip and normal events. For subduction zone events, the reverse mechanism interface events have the lowest motions, at least in the period range up to about ls, while the slab events, usually with normal mechanisms, are generally strongest. The attenuation relations presented in this paper have been used in many hazard studies in New Zealand over the last five years. In particular, they have been used in the derivation of the elastic site spectra in the new Standard for earthquake loads in New Zealand, NZS 1170.5:2004.

Earthquake risk from co-seismic stress

Following the massive loss of life caused by the Sumatra-Andaman earthquake in Indonesia and its tsunami, the possibility of a triggered earthquake on the contiguous Sunda trench subduction zone is a real concern. We have calculated the distributions of co-seismic stress on this zone, as well as on the neighbouring, vertical strike-slip Sumatra fault, and find an increase in stress on both structures that significantly boosts the already considerable earthquake hazard posed by them. In particular, the increased potential for a large subduction-zone event in this region, with the concomitant risk of another tsunami, makes the need for a tsunami warning system in the Indian Ocean all the more urgent.

Controls of shear zone rheology and tectonic loading on postseismic creep

Postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes. In the postseismic time interval, GPS is most sensitive to creep in the lower crust or upper mantle activated by earthquake‐generated stress perturbations. In these regions, deformation may be localized on an aseismic frictional surface or on a ductile shear zone. These two hypotheses imply specific rheologies and therefore time dependence of postseismic creep. Hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere. I present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied. In the absence of tectonic loading during the postseismic time interval, the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1/n. For a frictional surface, postseismic creep follows the same relaxation law in the limit 1/n → 0. A rough estimate of the apparent stress exponent can be obtained from continuous GPS records. Application to data collected after the 1994 Sanriku earthquake yields 1/n ∼ 0.1, which is consistent with dislocation creep mechanisms. However, the records of two other subduction zone events, the 2001 Peru event and the 1997 Kronotski earthquake, and a continental strike‐slip earthquake, the 1999 İzmit earthquake, require negative 1/n. Rather than characterizing the shear zone rheology, these negative exponents indicate that reloading of the shear zone by tectonic forces is important. Numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1/n < 0, although the actual stress exponent of the rheology is positive. While this prevents rheology from being tightly constrained by the studied GPS records, it indicates that reloading is important in the postseismic time interval. In other words, the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period.

Seismic Attenuation and Peak Ground Acceleration in Taiwan

We have taken advantage of the great collection of ground-motion acceleration records from the Central Weather Bureau of Taiwan to derive new attenuation relations corresponding to different geological settings: the shallow crustal earthquakes in the active tectonic region and the subduction zone. The new equations use a shape of magnitude dependence modulated by depth effect. For shallow crustal earthquakes, the focal depth also plays an adjusting factor for the geometrical spreading. A two-step stratified regression is used to decouple the evaluations of the distance dependence of data from that of magnitude and focal depth. The resultant attenuation relations are as follows. Using the data from shallow crustal earthquakes, we obtained: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\begin{array}{l}\mathrm{ln}{ }A=2.8096+0.8993\mathbf{M}-0.4381{ }\mathrm{ln}{ }D\_{\mathrm{p}}-(1.0954-0.0079D\_{\mathrm{p}}){ }\mathrm{ln}{ }D_{\mathrm{e}},\\{\sigma}=0.60.\end{array}\] \end{document} Using the data from the subducting-plate earthquakes, we obtained: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{ln}{ }A=4.7141+0.8468\mathbf{M}-0.1745{ }\mathrm{ln}{ }D\_{\mathrm{p}}-1.2972{ }\mathrm{ln}{ }D\_{\mathrm{h}},{\sigma}=0.56,\] \end{document} where A is the ground-motion acceleration in gal (cm/sec2), D e and D h are the epicentral and the hypocentral distances (km), respectively, D p is the focal depth (km), M is the moment magnitude, and σ the standard deviation. Several comparisons between our resulting attenuation relations and the others are presented in this study. Compared with other empirical attenuation models of Taiwan, our results show a better fit for the Chi-Chi earthquake. These new attenuation laws predict a stronger ground motion in the far distance for the shallow crustal earthquakes as compared with previous studies. Our results also show lower ground motions than the other countries for subduction zone events. The residual ground-motion maps show a high consistency with the local geology of Taiwan. Manuscript received 31 October 2000.

Variations in Regional Phase Propagation in the Area around Japan

The high level of seismicity and dense network of short-period stations in Japan allows a detailed characterization of regional phase propagation. There are substantial differences depending on the location of the source and stations. Despite the structural complexity, Lg is clearly seen in many parts of Japan. Lg is particularly well developed in the western part of the main island, Honshu, and will propagates through some volcanic zones with loss of high-frequency components. There are also zones of Lg blockage in northeastern Honshu and for subduction zone events with significant oceanic paths for which the mantle phases Pn and Sn are particularly clear. The oceanic region in the Sea of Japan blocks Lg propagation paths for events in mainland Asia at many stations, but there are clear Lg propagation corridors through Korea to stations in the west (Kyushu, western Honshu) and in the north into Hokkaido. The Lg phase carries substantial energy for large events, and the differences in efficiency of propagation influences the intensity of ground shaking. Thus the 1995 Hyogo-ken Nanbu (Kobe) earthquake shows intensity contours extended to the west in the region of efficient Lg-wave propagation. The Rg phase and the fundamental mode of Love waves are very significant for the shallow 2000 Tottori-ken Seibu event, and the effect of the combination of Lg with these slightly lower frequency surface waves would help to explain the discrepancy between the Japan Meteoro- logical Agency magnitude from regional stations (Mj 7.3) and the moment magnitude from distant observations (M w 6.6).

Preparation of the New Zealand earthquake catalogue for a probabilistic seismic hazard analysis

The seismic hazard from ground motions during a New Zealand earthquake is variable, and is dependent on the different tectonic processes that occur throughout the country. A modem probabilistic seismic hazard analysis (PSHA) combines various data sets to take account of these different environmental effects and rates of occurrence. Earthquake catalogue data can be used to give the rate of background or distributed seismicity in historical times, while paleoseismic data can be used to constrain the return time of large earthquakes. The background seismicity is assumed to occur as a time-independent Poisson process. To apply this assumption to a new PSHA of New Zealand, completeness levels for the New Zealand earthquake catalogue were established, and aftershocks or clusters of events that occurred close together in both space and time were removed from the catalogue. The level of hazard in a region can be depth-dependent, that is the risk of a large earthquake may come from a shallow crustal event or a deep subduction zone event, both having the same epicentral location but resulting in different levels of damage. The New Zealand earthquake catalogue has too many events that have been assigned restricted depths to be ignored. These events have been statistically redistributed into shallow crustal zones or deep subducted slab zones based on the last eleven years of catalogue data, when improvements in technology have reduced the number of restricted events.

The AD 365 Crete earthquake and possible seismic clustering during the fourth to sixth centuries AD in the Eastern Mediterranean: a review of historical and archaeological data

Historical and archaeological data are used to test geological claims that, in the fourth to sixth centuries AD, the Eastern Mediterranean experienced an unusual clustering of destructive earthquakes (the ‘Early Byzantine Tectonic Paroxsym’). A review of historical accounts of a notable earthquake at this time, that of 21 July AD 365, indicates that this event destroyed nearly all the towns in Crete and was followed by a tsunami which devastated the Nile Delta. The AD 365 event was also probably responsible for reported or observed destruction in ancient towns of west Cyprus and Libya. This earthquake is most likely to be identified with a Hellenic Arc subduction-zone event of ‘great’ (M>8) magnitude, as testified by up to 9 m of uplift in western Crete dated by previous geological studies to around this time. Historical and archaeological data also support the hypothesis that the fourth to sixth centuries AD was a period of abnormally high seismicity in the Eastern Mediterranean. The high seismicity rates of this period may reflect a reactivation of all plate boundaries in the region (Dead Sea Transform, East Anatolian Fault, North Anatolian Fault, Hellenic Arc, Cyprus Arc Fault).

Foreshock Occurrence Rates before Large Earthquakes Worldwide

—Global rates of foreshock occurrence involving shallow M≥ 6 and M≥ 7 mainshocks and M≥ 5 foreshocks were measured, using earthquakes listed in the Harvard CMT catalog for the period 1978–1996. These rates are similar to rates ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering, which is based on patterns of small and moderate aftershocks in California, and were found to exceed the California model by a factor of approximately 2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike-slip events lower than the worldwide average. Among the thrust events a large majority, composed of events located in shallow subduction zones, registered a high foreshock rate, while a minority, located in continental thrust belts, measured a low rate. These differences may explain why previous surveys have revealed low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggest the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock-rich.

Foreshock occurrence before large earthquakes

Rates of foreshock occurrence involving shallow M ≥ 6 and M ≥ 7 mainshocks and M ≥ 5 foreshocks were measured in two worldwide catalogs over ∼20‐year intervals. The overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in California. The aftershock model was extended to the case of moderate foreshocks preceding large mainshocks. Overall, the observed worldwide foreshock rates exceed the extended California generic model by a factor of ∼2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike‐slip events lower than the worldwide average. Among the thrust events, a large majority, composed of events located in shallow subduction zones, had a high foreshock rate, while a minority, located in continental thrust belts, had a low rate. These differences may explain why previous surveys have found low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggests the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain‐building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock‐rich. If this is so, then the California generic model may significantly underestimate the conditional probability for a very large (M ≥ 8) earthquake following a potential (M ≥ 7) foreshock in Cascadia. The magnitude differences among the identified foreshock‐mainshock pairs in the Harvard catalog are consistent with a uniform distribution over the range of observation.

Seismic anisotropy in the fore-arc region of the Hikurangi subduction zone, New Zealand

Shear waves from local earthquakes recorded on an array of nine broad-band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying Australian plate, and within and below the subducting slab underlying the southern North Island, New Zealand. Earthquakes near the plate interface were used to image the crustal anisotropy, whereas deeper, down-dip subduction zone events were used to sample the subducting slab, and the mantle below. Apart from a small amount of variable near-surface anisotropy (top few kilometres of the crust), the measured fast shear-wave directions of both the crust of the overlying plate (φ = 51° ± 18°) and the mantle (φ = 41° ± 15°) are similar, and are nearly parallel to the strike of the subduction zone, and the dominant geological features of the region. SKS measurements, which mainly sample the mantle beneath the subducting plate, give a value of φ = 28° ± 11°, and measurements made elsewhere in the New Zealand region using SKS and ScS2 produce similar results. The measurements of crustal anisotropy are similar to those found further south on the Wellington Peninsula, and confirm the presence of near-surface anisotropy. The maximum delay times for crustal earthquakes indicate a pervasive shear-wave velocity anisotropy of 4% in the crust of the overlying plate, and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear-wave velocity anisotropy of 1.4% in the mantle below the subducting slab. These results, when combined with the SKS measurements, suggest trench-parallel flow in the mantle below the subducted slab, and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab.

Is there truly a “hard” rock site in the Valley of Mexico?

To understand the cause of observed amplification of seismic waves even at hill‐zone sites in the Valley of Mexico, digital accelerographs have been installed at three especially chosen sites. Two of these sites, MADI and TEXC, located on hard Pleistocene lava (a few meters in thickness) overlying Oligocene andesites, were expected to be free of site effects. Analysis of the data recorded by these and other accelerographs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 Hz with respect to established attenuation relations. The cause of the amplification at hill‐zone sites in the Valley of Mexico, including MADI and TEXC, may be pervasive low S‐wave velocity in, and complex structure of, the upper layers of the volcanic rocks. If so, then there may not be a truly “hard” rock site in the valley.

A search for source side mantle anisotropy

Shear wave splitting due to source‐side mantle anisotropy is estimated from broadband teleseismic S waves using a suite of 19 subduction zone events more than 50km depth and recorded at several RSTN stations. We assume that there are at most two anisotropic regions along the ray path, one on the source side and one on the receiver side, and that receiver‐side splitting is known from analysis of SKS. It is found that splitting is not detectable below the slab in most cases. There are, however, two specific regions, one beneath Peru and one beneath Kamchatka that indicate significant source side anisotropy. Based on two nearby intermediate focus events in each region, splitting delay times are found to be 1.0–1.5s with the fast polarization direction perpendicular to the trench. Assuming 4% intrinsic anisotropy, this would correspond to path lengths of 100–170 km below the slab. Both regions are characterized by abrupt changes in along‐strike slab properties, a change in slab dip in the case of Peru, and a cessation of deep focus seismicity in the case of Kamchatka. We speculate that the anisotropy is caused by an associated local disturbance in asthenospheric flow. That the shear‐wave splitting parameters of S and SKS are consistent with each other in most cases, suggests that large‐scale anisotropy in the lower mantle is very small, probably no more than a fraction of a percent.

Ground Motion Model for Puget Sound Cohesionless Soil Sites

Surface response spectra for earthquake events must be based upon proper input seismic modelling. The surface response spectrum can be determined by convoluting a base response spectrum with a spectral amplification for the site. This required that an appropriate base spectrum of the earthquake motion and soil response model must have been used to develop the surface response spectra. Earthquakes in the Puget Sound basin are generally deep focus subduction zone events. These earthquakes are significantly different than the shallow focus strike slip events of California which are commonly used for this purpose. The spectral site amplification must be determined for the soils of the region. This paper considers the cohesionless soil sites only. The computer program SHAKE was used to evaluate the spectral amplification of different sites. Parameters for SHAKE were determined from Standard Penetration Test data. Parametric sensitivity studies were conducted for the spectral site amplification.