Seismograph Array Research Articles

Dynamic triggering of high‐frequency bursts by strong motions during the 2004 Parkfield earthquake sequence

High‐pass filtering (>20 Hz) of acceleration records from the USGS Parkfield Dense Seismograph Array (UPSAR) reveals a series of bursts that occur only during strong shaking from the 2004 Mw6 Parkfield, California, earthquake and its immediate aftershocks. Because there is no correlation between these high frequency bursts observed at closely spaced stations, we hypothesize that they are associated with dynamically triggered events occurring within 20 meters of the stations in the highly fractured shallow crust. The triggering threshold was found to be ∼0.02 MPa, consistent with a previous estimate based on a similar analysis of high‐frequency bursts observed in strong motion data from the 1999 Chi‐Chi earthquake in Taiwan (Fischer et al., 2008). The consistent observation of high‐frequency bursts at both Parkfield and Taiwan suggest that they may be a common phenomenon associated with strong motion in the very shallow crust.

Extent of the low-velocity region in the lowermost mantle beneath the western Pacific detected by the Vietnamese Broadband Seismograph Array

[1] We present evidence showing the extent of the low-velocity region in the lowermost mantle beneath the western Pacific. We analyzed S, sS, ScS-S, and sScS-sS travel times observed by the Vietnamese broadband seismograph array deployed as part of the Ocean Hemisphere Project. The abrupt changes in ScS-S and sScS-sS travel times suggest that the western geographical boundary of the low-velocity region is located around 140°E and is sharp (more than 4% velocity contrast within 200 km). The dependency of S and sS travel time anomalies of epicentral distances suggests that the strong low-velocity region is confined to within 400 km from the CMB (core-mantle boundary). The existence of lateral heterogeneities with a 100 km scale inside the low-velocity region is also suggested by the abrupt changes in S and ScS waveforms.

D″ beneath the Arctic from inversion of shear waveforms

The structure of the D″ region beneath the Arctic has not previously been studied in detail. Using waveform inversion, we find that the average S‐wave velocity in D″ beneath the Arctic is about 0.04 km/s higher than PREM, which is consistent with the existence of post‐perovskite (ppv) in D″. It is difficult to strongly constrain the fine structure of S‐velocity within D″ due to the small number of stations at epicentral distances Δ < 90°, but by weighting those stations heavily in the inversion, we show that the data suggest the existence of high S‐velocity in the upper half of D″ and low S‐velocity in the lower half, consistent with the possibility of a double crossing (ppv → pv reverse phase transition) within D″. We conduct a computational experiment to show that resolution of the velocity structure within D″ could be significantly improved by temporary installation of a portable array of seismographs in northern Canada, which would greatly increase the number of stations in the range 70° < Δ < 90°.

Three‐dimensional velocity structures of Mount Fuji and the South Fossa Magna, central Japan

We present the three‐dimensional structures of the P wave velocity (VP), S wave velocity (VS) as well as the P wave to S wave velocity ratio (VP/VS) beneath Mount Fuji and the South Fossa Magna, Japan, using arrival time data collected from 2002 to 2005 by a dense seismograph array. The high‐resolution data set and improved methodology reveal not only several velocity features that are consistent with previous studies but also important new details that clarify the velocity structures associated with volcanic processes beneath Mount Fuji and the collision tectonics of the South Fossa Magna. One such particular feature is a low‐VP, low‐VS and low‐VP/VS anomaly at depths of 7–17 km beneath Mount Fuji that corresponds with the locations of deep low‐frequency (DLF) earthquakes. The coincidence of the velocity anomaly and the DLF locations suggests that supercritical volatile fluid, such as H2O and CO2, may be abundant in the low‐VP/VS region and may play an important role in generating DLF earthquakes. This anomaly overlies a deeper low‐VP, low‐VS and high‐VP/VS anomaly at depths of 15–25 km that may represent a zone of basaltic partial melt. A low‐VP, low‐VS and low‐VP/VS anomaly is seen at depths of 6–14 km beneath Mount Hakone. Isovelocity surfaces (VP = 6.0 km/s and VS = 3.5 km/s) corresponding to the upper limit of hypocenter distribution below Mount Fuji may define the upper surface of the Philippine Sea plate whose existence in a seismic gap beneath Mount Fuji has been controversial.

Strong ground motions recorded by a near-source seismographic array during the 16 August 2005 Miyagi-Ken-Oki, JAPAN, earthquake (Mw 7.2)

Abstract An earthquake of Mw 7.2 took place on August 16, 2005 at a plate boundary between the Pacific plate and the North American plate off the coast of Miyagi Prefecture, Northeast Japan. During the Miyagi-Ken-Oki event, we succeeded in recording strong ground motions at six stations in a seismograhic array with an epicentral distance of about 70 km, where we have been operating seven strong-motion seismometers in an aperture of about 500 m since April 2004. The predominant period of the ground motion was shorter than 0.3 s. The peak ground acceleration exceeded 1.7 g at a station where non-linear site response may have occurred during the mainshock. The short-period strong ground motions show a large spatial variation, with up to a ten-fold difference in amplitude even within the array. However, there is a similarity between waveforms registered at different stations for periods longer than 0.4 s. Therefore, the difference in the ground motions may be mainly attributed to the difference in the shallow structure just beneath the stations.

Anomalous Acoustic Signals Recorded by the CERI Seismic Network

Previous studies have shown a variety of atmospheric disturbances ( e.g. , sonic booms, meteoroid falls, thunder, and explosions) can be detected by a seismograph if the acoustic pressure wave energy is sufficiently strong. Kanamori et al . (1991) and Cates and Sturtevant (2002) used the seismic network in southern California to study sonic booms generated by the space shuttle and aircraft. Ishihara et al . (2003) used seismic data from a dense seismographic array in the northeastern region of Honshu Island, Japan, to determine the trajectory of a meteoroid. Kappus and Vernon (1991) analyzed the acoustic signature of thunder from a seismic instrument located at Kislovodsk, USSR. Atmospheric shock waves generated by sonic booms, meteoroid falls, and explosions have been observed in the Center for Earthquake Research and Information (CERI) Cooperative Seismic Network (Johnston 1987; Langston 2004). On 28 January 2004 at 01:58 UTC, 11 short-period and nine broadband CERI network stations (figure 1) recorded high signal-to-noise ratio signals generated by an unknown source (figure 2). A preliminary analysis of the waveforms and slow velocity across the network suggested that these signals were not generated by an earthquake but by an acoustic source. CERI also received several calls from the public in southern Missouri reporting large booming noises around the time when the event was recorded by the CERI network (G. Patterson, personal communication, 2005). One of these reports was from a medical worker in a two-story building in Poplar Bluff, Missouri. She reported that everyone on the second floor felt a hard jarring shake and heard a loud explosion, but few felt it on the first floor. The seismic signals have durations of 30 to 70 s with distinct later arrivals. For example, see stations PARM and WADM shown on figure 2. Figure 1. Location map showing the 20 …

Strong Ground Motions Observed at the UPSAR during the 2003 M 6.5 San Simeon and 2004 M 6.0 Parkfield, California, Earthquakes

The U.S. Geological Survey Parkfield Dense Seismograph Array (UP- SAR) recorded successfully strong ground motions during the 2004 Parkfield earth- quake (M 6.0) and its aftershock series after waiting for 15 years for an anticipated event like this. The array also recorded the 2003 San Simeon earthquake (M 6.5). Because the array covers a very small area (0.45 km 2 ), these data offer some inter- esting fresh insights into intrasite variations of seismic ground motions. In this article, we study strong-motion data recorded at the UPSAR from the San Simeon event, the Parkfield event, and its seven aftershocks. We find that the variations of high- frequency ground motions (e.g., � 3 Hz) are very considerable. The largest horizontal peak ground acceleration (PGA) (P11, 408 cm/sec 2 ) from the Parkfield event is close to three times of the smallest one (P01, 157 cm/sec 2 ); the largest peak response spectrum is even over three times of the smallest one. The shortest station-to-station distance (between P06 and P07) in the array is only 25 m, but three-component PGAs of the two stations differ from a factor of 1.5 for the Parkfield event. The coefficient of variation (Cv r/mean) of Fourier acceleration is about 50% at frequencies higher than about 3 Hz. We find that Cv depends strongly on frequency, while it is nearly stable for different earthquakes. The significant variation of the high-frequency ground motions seems to be brought about mostly by the local and neighboring topographic effects, which have a larger effect in horizontal than vertical directions. We also calculate the ratio of vertical to horizontal response spectrum. Our plotting shows that the ratio is not sensitive to earthquake magnitude. We compare the ob- served motions (PGA, 5%-damped pseudoacceleration response spectrum (PSA) from 0.02 to 5 sec) with estimations from four commonly used prediction equations (Abra- hamson and Silva, 1997; Boore et al., 1997; Campbell and Bozorgnia, 2003; and Sadigh et al., 1997). The comparisons indicate that the significant station-to-station variation reduces largely the accuracy with which a site-specific estimation can be predicted. However, the mean of the observations at the UPSAR compares reasonably well with these estimations.

Rayleigh wave phase velocity analysis of the Ross Sea, Transantarctic Mountains, and East Antarctica from a temporary seismograph array

This study analyzes Rayleigh wave phase velocities from the Ross Sea (RS) region of the West Antarctica rift system, the Transantarctic Mountains (TAMs), and part of East Antarctica (EA). The Transantarctic Mountain Seismic Experiment deployed 41 three‐component broadband seismometers, which provide new data for high‐resolution two‐dimensional maps demonstrating crustal and uppermost mantle seismic velocity anomalies. The short‐period (16–25 s) phase velocity maps are consistent with changes in crustal thickness from ∼20 km under the RS to ∼35 km beneath the TAMs and EA. Long‐period (75–175 s) phase velocity maps indicate high mantle velocities beneath EA, low velocity beneath the RS, and a transition between the two between 50 and 150 km inland. The EA phase velocities in the region adjacent to the TAM exhibit a directional pattern consistent with 2 ± 1% azimuthal anisotropy with a NE‐SW fast direction. The structure of the RS is similar to continental rifting environments elsewhere, with a pronounced low‐velocity zone in the ∼80–220 km depth range, whereas EA shows a typical continental cratonic structure with high velocities between the 80 and 220 km depth range.

Theoretical background of retrieving Green's function by cross-correlation: one-dimensional case

SUMMARY Recently, an assertion has been verified experimentally and theoretically that Green’s function between two receivers can be reproduced by cross-correlating the records at the receivers. In this paper, we have theoretically proved the assertion for 1-D media with the free surface by using the Thomson‐Haskell matrix method. Strictly speaking, one side of the cross-correlation between records at two receivers is the convolution between Green’s function and the autocorrelation function of the source wavelet. This study extends the geometry considered by Claerbout to two receivers vertically apart, and is a special case of the proof by Wapenaar et al. which dealt with 3-D arbitrary inhomogeneous media. However, a simple geometry in 1-D problems enables us to make the proof without any approximations and to better understand the physical background with more ease. That is the main advantage of this study. Though a 1-D geometry seems far from reality, it may be sufficient if an appropriate combination of receivers and earthquakes is selected. In fact, such a geometry is often seen in seismological observations by a vertical array of seismographs in the shallow subsurface. Therefore, we refer to a possibility that the proof in this paper is applied to the estimation of site amplification factors by using records of a vertical seismographic array.

VP and VS structure of the Yellowstone hot spot from teleseismic tomography: Evidence for an upper mantle plume

The movement of the lithosphere over a stationary mantle magmatic source, often thought to be a mantle plume, explains key features of the 16 Ma Yellowstone–Snake River Plain volcanic system. However, the seismic signature of a Yellowstone plume has remained elusive because of the lack of adequate data. We employ new teleseismic P and S wave traveltime data to develop tomographic images of the Yellowstone hot spot upper mantle. The teleseismic data were recorded with two temporary seismograph arrays deployed in a 500 km by 600 km area centered on Yellowstone. Additional data from nearby regional seismic networks were incorporated into the data set. The VP and VS models reveal a strong low‐velocity anomaly from ∼50 to 200 km directly beneath the Yellowstone caldera and eastern Snake River Plain, as has been imaged in previous studies. Peak anomalies are −2.3% for VP and −5.5% for VS. A weaker, anomaly with a velocity perturbation of up to −1.0% VP and −2.5% VS continues to at least 400 km depth. This anomaly dips 30° from vertical, west‐northwest to a location beneath the northern Rocky Mountains. We interpret the low‐velocity body as a plume of upwelling hot, and possibly wet rock, from the mantle transition zone that promotes small‐scale convection in the upper ∼200 km of the mantle and long‐lived volcanism. A high‐velocity anomaly, 1.2% VP and 1.9% VS, is located at ∼100 to 250 km depth southeast of Yellowstone and may represent a downwelling of colder, denser mantle material.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

We study in detail the two consecutive episodic tremor‐and‐slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source‐Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30‐km and 50‐km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50–55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high‐frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Control of hazard due to seismicity induced by a hot fractured rock geothermal project

In 2003 hydraulic stimulations were carried out in a geothermal field in eastern El Salvador, Central America, as part of a project to explore the feasibility of commercial hot fractured rock energy generation. A key requisite for this environmentally friendly energy source is that the fracturing of the hot rocks at depths of 1–2 km must not produce levels of ground shaking at the surface that would present a serious disturbance or threat to the local population. Thresholds of tolerable ground motion were inferred from guidelines and regulations on tolerable levels of vibration and from correlations between instrumental strong-motion parameters and intensity, considering the vulnerability of the exposed housing stock. The thresholds were defined in terms of peak ground velocity (PGV) and incorporated into a “traffic light” system that also took account of the frequency of occurrence of the induced earthquakes. The system was implemented through a dedicated seismograph array and locally derived predictive equations for PGV. The “traffic light” was used as a decision-making tool regarding the duration and intensity of pumping levels during the hydraulic stimulations. The system was supplemented by a small number of accelerographs and re-calibrated using records obtained during the rock fracturing.

Depth of the 660‐km discontinuity near the Mariana slab from an array of ocean bottom seismographs

High frequency records of deep Mariana earthquakes from a dense array of ocean bottom seismographs deployed in the Mariana arc and back‐arc regions are stacked and searched for the phases P660p and S660p to constrain the depth of the 660‐km discontinuity near the Mariana slab. Results of the high‐resolution study suggest that around 18°N the 660‐km discontinuity lies at about 710–730 km (±14 km) depth within or in the vicinity of the slab core. In the region seismicity ceases at ∼620 km depth. This implies that, although tomographic images show the Mariana slab penetrating into the lower mantle, deep seismicity in the region terminates ∼100 km above the base of the transition zone. These findings and similar observations in Tonga argue that factors other than the phase transition at the base of the upper mantle may control the maximum down‐dip extent of the deep seismogenic region in the slab.

Rupture Propagation of the 2004 Parkfield, California, Earthquake from Observations at the UPSAR

Using a short-baseline seismic array (U.S. Geological Survey Parkfield Dense Seismograph Array [upsar]) about 12 km west of the rupture initiation of the 28 September 2004 M 6.0 Parkfield, California, earthquake, we have observed the movement of the rupture front of this earthquake on the San Andreas fault. The sources of high-frequency arrivals at upsar, which we use to identify the rupture front, are mapped onto the San Andreas fault using their apparent velocity and back azimuth. Measurements of apparent velocity and back azimuth are calibrated using aftershocks, which have a compact source and known location. Aftershock back azimuths show considerable lateral refraction, consistent with a high-velocity ridge on the southwest side of the fault. We infer that the initial mainshock rupture velocity was approximately the Rayleigh speed (with respect to slower side of the fault), and the rupture then slowed to about 0.66 β near the town of Parkfield after 2 sec. The last well-correlated pulse, 4 sec after S, is the largest at upsar, and its source is near the region of large accelerations recorded by strong-motion accelerographs and close to northern extent of continuous surface fractures on the southwest fracture zone. Coincidence of sources with preshock and aftershock distributions suggests fault material properties control rupture behavior. High-frequency sources approximately correlate with the edges of asperities identified as regions of high slip derived from inversion of strong-motion waveforms.

Upper mantle structure of Arctic Canada from Rayleigh wave dispersion

Models of upper mantle structure across the Canadian High Arctic region, from 62° North to 82° North, and 134° West to 56° West, are presented in this study. Teleseismic Rayleigh waves were recorded by an array of temporary and permanent broadband seismographs across the region. Fundamental-mode phase velocity dispersion curves were calculated for 84 two-station paths. The period range for the majority of the dispersion curves was ∼ 25–170 s, allowing the resolution of mantle structure to a depth of ∼ 250–300 km. A tomographic inversion was used to combine dispersion data from 61 stable paths, to give a set of phase velocity maps for the region. The maps show a persistent high-velocity anomaly across the central–northern mainland, strongest in the period range 50–110 s, and a low-velocity anomaly along the Arctic continental margin at periods ≤ 90 s. A number of dispersion curves from two-station paths were chosen for further analysis, together with localised dispersion curves derived from the phase velocity maps, for areas with good path coverage. Analysis of these dispersion curves was carried out, using a Monte-Carlo method, to obtain 1-D models for shear wave velocity structure across the region, assuming an isotropic upper mantle. The average velocity of the upper 100–200 km of the mantle is compared to that of the iasp91 global reference model, and the variations in velocity perturbation are examined. The highest velocity perturbations, ∼ 4–6% above iasp91, occur in the southern part of the region and are, for the most part, associated with the Canadian Shield and southern Arctic Platform. In contrast, velocity perturbations of only 1–3% above iasp91 characterise the northwestern Arctic islands, with the lowest velocities occurring beneath the Sverdrup Basin. The characteristics of the upper mantle velocity–depth sections also vary significantly. For paths crossing the central–northern mainland and Baffin Island, the Monte-Carlo models give near-constant velocity to at least 240 km depth in the upper mantle. The best match to the dispersion curves can be obtained by the inclusion of a shallower (∼ 200 km depth) low-velocity zone, but this feature is not required to match the data within physically realistic error bounds. In the central Arctic islands, the models show a ‘lid’ of relatively high velocities in the upper section of the mantle model, underlain by a low-velocity zone. This velocity pattern is consistent with previous interpretations of the character of the seismological lithosphere and asthenosphere. The base of the lid is difficult to identify, but negative velocity gradients are modelled over the ∼ 100–200 km depth range. Along the Sverdrup Basin margins, where the velocity anomalies are smallest, the models are similar in character to the iasp91 reference profile, showing a gradual increase in velocity with depth.

Lehmann discontinuity beneath North America: No role for seismic anisotropy

The Lehmann discontinuity in the upper mantle is often interpreted as a boundary separating anisotropic and isotropic media. We demonstrate, however, that seismic anisotropy plays, if any, a minor role in the origin of the Lehmann discontinuity. Our data are obtained with S receiver function technique from recordings of the MOMA seismograph array in the eastern USA. Part of the data is processed with a new modification of this technique. We observe Sp phase converted from the Lehmann discontinuity at a depth around 200 km with comparable amplitudes and similar polarities in two azimuths differing by about 90°. Contrary to the observations, the polarity in the synthetic seismograms for models with azimuthal anisotropy is opposite for the azimuths differing by 90°. Radial anisotropy with a slow vertical direction results in the Sp phase with the polarity opposite to the observed one.

Crustal seismicity in central Chile

Both the genesis and rates of activity of shallow intraplate seismic activity in central Chile are poorly understood, mainly because of the lack of association of seismicity with recognizable fault features at the surface and a poor record of seismic activity. The goal of this work is to detail the characteristics of seismicity that takes place in the western flank of the Andes in central Chile. This region, located less than 100 km from Santiago, has been the site of earthquakes with magnitudes up to 6.9, including several 5+ magnitude shocks in recent years. Because most of the events lie outside the Central Chile Seismic Network, at distances up to 60 km to the east, it is essential to have adequate knowledge of the velocity structure in the Andean region to produce the highest possible quality of epicentral locations. For this, a N–S refraction line, using mining blasts of the Disputada de Las Condes open pit mine, has been acquired. These blasts were detected and recorded as far as 180 km south of the mine. Interpretation of the travel times indicates an upper crustal model consisting of three layers: 2.2-, 6.7-, and 6.1-km thick, overlying a half space; their associated P wave velocities are 4.75–5.0 (gradient), 5.8–6.0 (gradient), 6.2, and 6.6 km/s, respectively. Hypocentral relocation of earthquakes in 1986–2001, using the newly developed velocity model, reveals several regions of concentrated seismicity. One clearly delineates the fault zone and extensions of the strike-slip earthquake that took place in September 1987 at the source of the Cachapoal River. Other regions of activity are near the San José volcano, the source of the Maipo River, and two previously recognized lineaments that correspond to the southern extension of the Pocuro fault and Olivares River. A temporary array of seismographs, installed in the high Maipo River (1996) and San José volcano (1997) regions, established the hypocentral location of events with errors of less than 1 km. These events are clustered along no particular lineament approximately 25 km away from the San José and Maipo volcanoes. Recurrence intervals, based on a frequency magnitude relationship for lower magnitude events, indicate that earthquakes with magnitudes of 4.7 and 7 have a repeat time of 1 and 1200 years, respectively. Focal mechanisms of the two largest events indicate horizontal maximum and minimum compressive stresses with σ 1 varying from a NW–SE orientation in the north to E–W at the southern extreme.

Reply to "Comment on `3D Site Effects: A Thorough Analysis of a High-Quality Dataset,' by F. J. Chavez-Garcia, J. Castillo, and W. R. Stephenson," by R. Paolucci and E. Faccioli

We thank R. Paolucci and E. Faccioli for their interest in our article. We will refer to their comments (Paolucci and Faccioli, 2003) as P&F. The article commented on by P&F is Chavez-Garcia et al. (2002) (CCS from here onward), where we presented results of a detailed analysis of the 10 events that were best recorded by the temporary seismograph array installed at Parkway, Wainuiomata, New Zealand, during 1995. Our article followed a first round of analysis (Chavez-Garcia et al. , 1999) that concentrated on frequency domain transfer functions for all the stations (64 events were considered in that article). We kindly ask the interested reader to look into those articles, where the history of the network, geological conditions, and so on, were described. Other articles of the Parkway saga are Stephenson (2000, 2002) and Yu and Haines (2003). The results of the analysis by CCS showed clearly that the more significant part of ground-motion response at this valley was the generation of local surface waves, which contributed both to the large amplitude of the “resonance peak” and to the duration of ground motion on the soft soils of this valley. This result was quite robust for all the events that were analyzed. However, Paolucci et al. (2000), analyzing the records for a single event at Parkway at 10 soft soil stations (from the 13 recorded for this earthquake), came to a very different conclusion: “In the case of Parkway Valley we identified what is probably one of the first clear cases of evidence of a 2D resonance mode” (p. 5). This 2D resonance, according to Paolucci et al. (2000), is that of the fundamental in-plane mode, as defined by Bard and Bouchon (1985). CCS felt obliged to confront this discrepancy and gave some arguments against the interpretation …

The 1998 Miyako fireball’s trajectory determined from shock wave records of a dense seismic array

A high velocity passage of a meteoroid through the atmosphere generates a shock wave with a conical front. When the shock front arrives at the surface, it causes high frequency ground motions that are registered on the seismograms. We can use seismological data to determine the trajectory of the meteoroid in the atmosphere. A strong shock wave from the 1998 Miyako fireball is recorded by more than 20 stations in a dense array of seismographs installed in the northeastern region of Honshu Island, Japan. We determine the velocity and the trajectory of the fireball in the upper atmosphere using the arrival times of the shock wave at the stations.

Guided Love- and Rayleigh-waves in Parkway Valley, Wainuiomata, N.Z.

A detailed analysis of one earthquake recorded by a dense array of seismographs on the surface of an alluvial valley shows two locally-generated waves which propagate down-valley. The faster travelling one is a Rayleigh wave, and the slower one is similar to a Love wave, but has a vertical component thought to arise from the need to meet lateral boundary conditions. These waves can mimic normal modes, and their interaction provides a basis for explaining directional resonances.