Short-period Instruments Research Articles

A Seismic Nodal Deployment to Understand Magmatic Structure in the Vicinity of the Pāhala Earthquake Swarm

Abstract In Summer–Fall 2022, 80 three-component SmartSolo IGU-BD3C-5 nodal seismometers were deployed surrounding the Pāhala seismic swarm on the Island of Hawai‘i, with the goal of improving seismicity catalogs and seismic velocity images of the crust and upper mantle in this region. The Pāhala swarm, located south of Mauna Loa and Kīlauea, has been the site of a multiyear sustained swarm of seismicity at the depths of ∼25–40 km, with order of magnitude increases in rate in 2015, and then again in 2019. This seismicity is possibly related to the input of magma from the mantle plume below, which may then be subsequently transported to volcanic edifices. However, these processes remain enigmatic, in part due to a lack of precise earthquake locations and seismic velocity models in this region. Here, we provide an overview of the deployment, an assessment of the quality of the collected data, and discuss the viability of the data set for local earthquake relocation, tomography, and teleseismic receiver functions. Through comparisons with proximal permanent broadband and short-period instruments, we find that the nodes produce high-quality data, particularly at periods shorter than 5 s, although we find, document, and correct discrepancies with the gain and polarities of the instruments. We successfully record signals from teleseismic earthquakes, even at periods longer than 5 s (the corner of the flat response of the nodes). We also record local earthquakes, including details related to source characteristics. This indicates that the data are likely to prove useful for investigations using both local and teleseismic earthquake signals to better understand the connections between the deep and shallow magmatic systems of Hawai‘i. Although this deployment provides a snapshot in time, its success may provide a useful benchmark for future studies as the volcanic systems of Hawai‘i continue to evolve in the future.

On the Utility of Horizontal-to-Vertical Spectral Ratios of Ambient Noise in Joint Inversion with Rayleigh Wave Dispersion Curves for the Large-N Maupasacq Experiment.

Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity () structure. Knowing the structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity () structures such as travel time tomography or full waveform inversions or to directly study the structure for geo-engineering purposes (e.g., ground motion prediction). The joint inversion of HVSR and dispersion data for 1D structure allows characterising the uppermost crust and near surface, where the HVSR data ( to ) are most sensitive while the dispersion data (1 to ) constrain the deeper model which would, otherwise, add complexity to the HVSR data inversion and adversely affect its convergence. During a large-scale experiment, 197 three-component short-period stations, 41 broad band instruments and 190 geophones were continuously operated for 6 months (April to October 2017) covering an area of approximately with a site spacing of approximately 1 to . Joint inversion of HVSR and DC allowed estimating and, to some extent density, down to depths of around . Broadband and short period instruments performed statistically better than geophone nodes due to the latter’s gap in sensitivity between HVSR and DC. It may be possible to use HVSR data in a joint inversion with DC, increasing resolution for the shallower layers and/or alleviating the absence of short period DC data, which may be harder to obtain. By including HVSR to DC inversions, confidence improvements of two to three times for layers above were achieved. Furthermore, HVSR/DC joint inversion may be useful to generate initial models for 3D tomographic inversions in large scale deployments. Lastly, the joint inversion of HVSR and DC data can be sensitive to density but this sensitivity is situational and depends strongly on the other inversion parameters, namely and . Density estimates from a HVSR/DC joint inversion should be treated with care, while some subsurface structures may be sensitive, others are clearly not. Inclusion of gravity inversion to HVSR/DC joint inversion may be possible and prove useful.

Monitoring a short-lived earthquake swarm during April–May 2020 in Haenam, Korea, and its preliminary results

On 3 May 2020, an ML 3.1 earthquake occurred in Haenam, southwestern Korea, in an area devoid of recorded seismicity since instrumental observations began in 1978. Careful examination of the temporal occurrence of seismicity, and the magnitude distribution of the sequence before and after the ML 3.1 earthquake, indicates typical swarm-like behavior. The earthquake swarm started with an ML 0.6 event on 26 April 2020, intensified up to 3 May 2020, and abruptly terminated with an ML 1.0 event on 9 May 2020. The Pusan National University Geophysics Laboratory (PNUGL) deployed a temporary seismic array with eight three-component short-period instruments to monitor the short-lived bursts of seismicity. During the monitoring campaign, we detected > 700 microearthquakes by applying a matched-filter technique to the combined dataset produced by PNUGL, the Korea Meteorological Administration, and the Korea Institute of Ocean Science and Technology. We determined earthquake parameters for 299 earthquakes that were detected at four or more seismic stations. We also determined the focal mechanism solutions of the 10 largest earthquakes in the swarm using first-motion polarities with S/P ratios. The focal mechanism, hypocentral depth, and stress orientation of the largest earthquake in the sequence were also determined using waveform inversions. The distribution of earthquake hypocenters, together with focal mechanism solutions, indicates that the earthquake swarm activated deeply-buried faults (~20 km) oriented either NNE-SSW or WNW-ESE. We also report details of the temporary seismic monitoring network, including the instrumentation, detection of microearthquakes, and variations in event-detection threshold influenced by anthropogenic and natural noise fluctuations. We also discuss the limitations associated with lowering the detection threshold of microearthquakes by increasing the number of seismic stations or by adopting advanced event-detection techniques.

The analog seismogram archives of Iceland: Scanning and preservation for future research

The history of seismography in Iceland began in 1909 with the installation of one horizontal Mainka seismograph in Reykjavík. Following a period of intermittent operation, regular operation was initiated in 1925 with the establishment of the Icelandic Meteorological Office. The number of stations increased gradually over the following decades, and in the sixties, four stations were in operation. The number of permanent stations proliferated following the Heimaey eruption in 1973 and during most of the eighties the number of stations was 40–50. The first digital seismograph stations were installed in 1990 and the analog seismic network was gradually replaced by digital stations over the next two decades. Between 1910 and 1920 the number of seismograms grew to an estimated 300,000. A four-year project to make this record collection accessible on the internet has been initiated and funded. So far around 175,000 seismograms have been scanned and the results are available and free for download on the open website seismis.hi.is. The seismograms are scanned with a resolution of 300 dpi and presented on the website as jpg-, and png-file. The high-resolution files are on the order of 4–8 Mb each. Digitization of the seismic traces has not been attempted since most of the seismograms are from short-period instruments and the waveforms are already lost. In addition to numerous teleseismic body-wave-phases, the record collection contains primary data from various tectonic and magmatic events in Iceland during the last century. This includes eruptions of Hekla in 1947, 1970, 1980–81, 1991 and 2000, Surtsey in 1963–1967, Heimaey in 1973, Askja in 1961, Grímsvötn in 1934, 1983, 1998, and 2004, Gjálp in 1996, rifting episode at Krafla in 1975–1984, persistent seismic activity of the Bárðarbunga and Katla volcanoes, numerous suspected subglacial magmatic events, earthquake swarms on the Reykjanes Peninsula Oblique Rift and within the Tjörnes Fracture Zone, and earthquake sequences in the transform zones of South and North Iceland and adjacent segments of the Mid-Atlantic Ridge.

Predicted detection rates of regional-scale meteorite impacts on Mars with the InSight short-period seismometer

In 2016 NASA will launch the InSight discovery-class mission, which aims to study the detailed internal structure of Mars for the first time. Short- and long-period seismometers form a major component of InSight’s payload and have the potential to detect seismic waves generated by meteorite impacts. Large globally detectable impact events producing craters with diameters of ∼100m have been investigated previously and are likely to be rare (Teanby, N.A., Wookey, J. [2011]. Phys. Earth Planet. Int. 186, 70–80), but smaller impacts producing craters in the 0.5–20m range are more numerous and potentially occur sufficiently often to be detectable on regional scales (≲1000km). At these distances, seismic waves will have significant high frequency content and will be suited to detection with InSight’s short-period seismometer SEIS-SP. In this paper I estimate the current martian crater production function from observations of new craters (Malin, M.C. et al. [2006]. Science 314, 1573–1577; Daubar, I.J. et al. [2013]. Icarus 225, 506–516), model results (Williams, J.P., Pathare, A.V., Aharonson, O. [2014]. Icarus 235, 23–36), and standard isochrons (Hartmann, W.K. [2005]. Icarus 174, 294–320). These impact rates are combined with an empirical relation between impact energy, source-receiver distance, and peak seismogram amplitude, derived from a compilation of seismic recordings of terrestrial and lunar impacts, chemical explosions, and nuclear tests. The resulting peak seismogram amplitude scaling law contains significant uncertainty, but can be used to predict impact detection rates. I estimate that for a short-period instrument, with a noise spectral density of 10−8ms−2Hz−1/2 in the 1–16Hz frequency band, approximately 0.1–30 regional impacts per year should be detectable with a nominal value of 1–3 impacts per year. Therefore, small regional impacts are likely to be a viable source of seismic energy for probing Mars’ crustal and upper mantle structure. This is particularly appealing as such impacts should be easily located with orbital imagery, increasing their scientific value compared to other types of events with unknown origins. Finally, comparison of the empirical results presented here with the modelling study of Teanby and Wookey (Teanby, N.A., Wookey, J. [2011]. Phys. Earth Planet. Int. 186, 70–80) provides constraints on the seismic efficiency, suggesting that values of ∼5×10−4 may be appropriate for impact generated seismic waves. Comparing explosion and impact datasets indicate that buried explosions are ∼10 times more efficient at generating seismic waves than impacts.

Reconnaissance earthquake studies at nine volcanic areas of the central Andes with coincident satellite thermal and InSAR observations

We record non-eruptive background seismicity at eight potentially active volcanoes and one geothermal area in Chile and Bolivia for the first time in order to set a baseline for future episodes of unrest. We also compare seismicity to coincident new regional observations of ground deformation from InSAR and satellite observed thermal anomalies from the ASTER instrument. We deploy small temporary seismometer networks (1 to 5 stations each) of short and intermediate period instruments for 3–27months at the nine areas between the years 2004 and 2012 at: Parinacota, Guallatiri, Isluga, Irruputuncu, Olca–Paruma, Ollagüe, Sol de Mañana, Putana, and Láscar. Despite the lack of shallow earthquakes in the global catalogs at these volcanoes, we find that all have volcano-tectonic events with at least 27 earthquake swarms — the most active are Putana, Guallatiri and Ollagüe. We find two examples where changes in seismicity are likely related to either deformation (in 2009 at Putana) or an increase in temperature (in 2012 at Isluga). Further, we document for the first time ground deformation at a Pliocene volcano called Sillajhuay, located in the Holocene volcano gap (i.e., 70km from the nearest active volcano Isluga). We find that the four deforming volcanoes between 18 and 24°S are seismically active, but that seismic activity does not imply measurable ground deformation. Similarly, the seismically active volcanoes have satellite thermal hotspots, but there is no correlation between relative amounts of seismic activity and hotspot temperature. Because several of the volcanoes show variations in seismic activity, temperature, and deformation over the course of a few years unrelated to eruptions, decadal and longer observations are needed to constrain background activity in the central Andes.

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We record non-eruptive background seismicity at eight potentially active volcanoes and one geothermal area in Chile and Bolivia for the first time in order to set a baseline for future episodes of unrest. We also compare seismicity to coincident new regional observations of ground deformation from InSAR and satellite observed thermal anomalies from the ASTER instrument. We deploy small temporary seismometer networks (1 to 5 stations each) of short and intermediate period instruments for 3–27 months at the nine areas between the years 2004 and 2012 at: Parinacota, Guallatiri, Isluga, Irruputuncu, Olca–Paruma, Ollagüe, Sol de Mañana, Putana, and Láscar. Despite the lack of shallow earthquakes in the global catalogs at these volcanoes, we find that all have volcano-tectonic events with at least 27 earthquake swarms — the most active are Putana, Guallatiri and Ollagüe. We find two examples where changes in seismicity are likely related to either deformation (in 2009 at Putana) or an increase in temperature (in 2012 at Isluga). Further, we document for the first time ground deformation at a Pliocene volcano called Sillajhuay, located in the Holocene volcano gap (i.e., 70 km from the nearest active volcano Isluga). We find that the four deforming volcanoes between 18 and 24°S are seismically active, but that seismic activity does not imply measurable ground deformation. Similarly, the seismically active volcanoes have satellite thermal hotspots, but there is no correlation between relative amounts of seismic activity and hotspot temperature. Because several of the volcanoes show variations in seismic activity, temperature, and deformation over the course of a few years unrelated to eruptions, decadal and longer observations are needed to constrain background activity in the central Andes.

Three-dimensional seismic velocity structure and earthquake relocations at Katmai, Alaska

We invert arrival time data from local earthquakes occurring between September 2004 and May 2009 to determine the three-dimensional (3D) upper crustal seismic structure in the Katmai volcanic region. Waveforms for the study come from the Alaska Volcano Observatory's permanent network of 20 seismic stations in the area (predominantly single-component, short period instruments) plus a densely spaced temporary array of 11 broadband, 3-component stations. The absolute and relative arrival times are used in a double-difference seismic tomography inversion to solve for 3D P- and S-wave velocity models for an area encompassing the main volcanic centers.The relocated hypocenters provide insight into the geometry of seismogenic structures in the area, revealing clustering of events into four distinct zones associated with Martin, Mageik, Trident-Novarupta, and Mount Katmai. The seismic activity extends from about sea level to 2km depth (all depths referenced to mean sea level) beneath Martin, is concentrated near 2km depth beneath Mageik, and lies mainly between 2 and 4km depth below Katmai and Trident-Novarupta. Many new features are apparent within these earthquake clusters. In particular, linear features are visible within all clusters, some associated with swarm activity, including an observation of earthquake migration near Trident in 2008. The final velocity model reveals a possible zone of magma storage beneath Mageik, but there is no clear evidence for magma beneath the Katmai-Novarupta area where the 1912 eruptive activity occurred, suggesting that the storage zone for that eruption may have largely been evacuated, or remnant magma has solidified.

Seismic Imaging of a Bimaterial Interface Along the Hayward Fault, CA, with Fault Zone Head Waves and Direct P Arrivals

We observe fault zone head waves (FZHW) that are generated by and propagate along a roughly 80 km section of the Hayward fault in the San Francisco Bay area. Moveout values between the arrival times of FZHW and direct P waves are used to obtain average P-wave velocity contrasts across different sections of the fault. The results are based on waveforms generated by more than 5,800 earthquakes and recorded at up to 12 stations of the Berkeley digital seismic network (BDSN) and the Northern California seismic network (NCSN). Robust identification of FZHW requires the combination of multiple techniques due to the diverse instrumentation of the BDSN and NCSN. For single-component short-period instruments, FZHW are identified by examining sets of waveforms from both sides of the fault, and finding on one (the slow) side emergent reversed-polarity arrivals before the direct P waves. For three-component broadband and strong-motion instruments, the FZHW are identified with polarization analysis that detects early arrivals from the fault direction before the regular body waves which have polarizations along the source-receiver backazimuth. The results indicate average velocity contrasts of 3–8 % along the Hayward fault, with the southwest side having faster P wave velocities in agreement with tomographic images. A systematic moveout between the FZHW and direct P waves for about a 80 km long fault section suggests a single continuous interface in the seismogenic zone over that distance. We observe some complexities near the junction with the Calaveras fault in the SE-most portion and near the city of Oakland. Regions giving rise to variable FZHW arrival times can be correlated to first order with the presence of lithological complexity such as slivers of high-velocity metamorphic serpentinized rocks and relatively distributed seismicity. The seismic velocity contrast and geological complexity have important implications for earthquake and rupture dynamics of the Hayward fault, including a statistically preferred propagation direction of earthquake ruptures to the SE.

A Comparison of Short-Period and Broadband Seismograph Systems in the Context of the Seismology of the Eastern United States

The study compares the capabilities of modern short-period seismograph systems with those using broadband sensors. The results indicate that dual-gain recording using 1-Hz seismometers will capture essentially all of the available information in the seismic wavefield generated by moderate magnitude shocks ( M 4.0–6.0) at regional distances, with dynamic range sufficient to record on-scale a magnitude 6.0 event at approximately 55 km. Short-period instrument operation in seismically active areas remains an effective means of collecting weak-motion data for basic seismological research relevant to public safety in the eastern United States.

Seismogram Picking Error from Analyst Review (SPEAR): Single-Analyst and Institution Analysis

Abstract We perform two experiments to define and isolate errors associated with picking seismic phases (the process of naming and measuring the arrivals of seismic phases). The first experiment establishes a method and model to describe picking errors for a single analyst measuring arrivals recorded on short-period instruments. The analyst’s picks are constrained to earthquakes that had similar epicenters with stations at local to near-regional distances. We find that the main source of error for an individual analyst is the signal-to-noise ratio (SNR) measured using a wideband spectral ratio (WSR). The pick error is 0.1 sec for measurements made with a WSR greater than 10. In some cases, errors that occur with high WSR (>10) appear to result from the variability of the seismic signals, caused through magnitude or scaling differences and source specific differences. The second experiment confirms our hypothesis that institutions have approaches in measuring seismic arrivals that inherently cause the pick placement to differ from institute to institute. For the institute analysis, we use data collected from the International Seismological Center (ISC). The SNR is not reported in the ISC catalogs; however, we show that the distance from station to source is a factor that can define the error between picks made at different institutions. The error between picks increases with distance until the transition from regional phases to teleseismic (∼20–30°), where the error is decreased by over 50%. The error then begins to increase after 40°. Another factor of error may be that institutions name seismic phases differently. We also perform a one-way multivariate analysis of variance on the institutions and the pick information to show the institutions pick differently. Measurement differences between institutions are greater than measurement differences of a single analyst. The single-analyst measurement error can be defined as 0.1 sec for an experienced analyst picking local to regional phases on short-period instruments that have a WSR greater than 10. However, this precision level of measurement error is not valid when combining picks from multiple institutions. We need to define a new measurement error model for combining picks to establish velocity and earth models when merging data from multiple institutions; a seismic phase picking error model cannot be established from the current earthquake catalogs.

Comparative Study of Distance Attenuation in the Central United States and Western India

Seismic waveform data, recorded by short-period instruments of the Cooperative New Madrid Seismic Network (CNMSN), and a temporary aftershock deployment for the 2001 Bhuj M W 7.6 earthquake are corrected to the theoretical Wood-Anderson response, and the horizontal peak amplitudes are used to determine local magnitude scales for the Mississippi embayment of the central United States and the Kachchh basin of western India, with a focus to understand the distance attenuation in these two regions. Results show that the distance-correction function for the Mississippi embayment of the central United States is

Estimating the size of an earthquake using short-period seismograms of the first three seconds: A simulated experiment using the 1999 Chi-Chi earthquake sequence

Most of the present earthquake early warning systems are based on broadband or strong motion recordings. However, the short-period instruments are still deployed. It is well-known that short-period recordings have saturation problems for large earthquakes when estimating the size of an earthquake. Thus, it is necessary to make clear the magnitude at which saturation starts to occur for the commonly used τc and Pd measurements, respectively. To investigate the possibility of using short-period seismic recordings for earthquake early warning, we conducted a simulated experiment using the strong motion data of the 1999 Chi-Chi earthquake sequence including its main shock and 31 aftershocks, with magnitude span from 4 to 7.6. The strong motion acceleration recordings were convolved with the instrument response of short-period seismographs in northern China to simulate short-period seismograms. Parameters τc and Pd from the first-three-second seismograms were calculated for the simulated short-period recordings and compared with that obtained by the original strong ground motion recordings. The result showed that to some extent, short-period recordings can be used for threshold earthquake early warning, while the magnitude saturation of Pd estimation can be up to 6.5, better than τc estimation.

Developments in passive seismic techniques through the ANSIR National Research Facility

Since 1997 the ANSIR Major National Research Facility has significantly enhanced Australia's national capacity for recording regional and distant earthquakes using seismic recorders distributed across the continent. Both broad-band and short-period recorders have been deployed in innovative experimental designs that have made major contributions to the understanding of the 3D structure in the Earth's crust and upper mantle beneath the Australian region. The Facility continues its national role with investment in equipment suitable for both seismic and electromagnetic sounding. Deployments of recorders with broad-band seismometers have made a substantial contribution to surface wave tomography, particularly with the gathering of extensive data sets from Westem Australia and a deployment bracketing the Tasman Line. The significant increase in the number of portable stations provides constraints on the character of the crust and the crust-mantle interface across the continent, via the analysis of receiver functions from distant earthquakes. Anisotropy beneath the continent is now better characterised but remains enigmatic. New analysis methods are likely to make more extensive use of continuous seismic data. Short-period instrument deployments have been mostly directed towards delay-time tomography studies in south-eastem Australia, and have employed new methods for event picking and tomographic inversion. Enhancements to the instruments mean that servicing of experiments will be simpler in future and allow new applications with three-component recording at higher sampling rates. Passive seismic recording offers a cost-effective way of obtaining structural information across substantial areas of the continent. The results are valuable in their own right, but can also provide important constraints on seismological structure that are valuable in planning more expensive deep crustal reflection profiles.

Crustal velocity structure under the RUKSA seismic array (Karelia, Russia)

[1] Based on data of three three-component seismographs belonging to the temporary smallaperture Russian Karelia Seismic Array (RUKSA) in the Petrozavodsk region (Karelia), a 1-D velocity model of the crust is constructed by the method of the receiver function. Waveforms of distant earthquakes recorded by short-period instruments with improved characteristics are used. The data were inverted by the simulated annealing method. The inversion was stabilized by using phase velocities of Rayleigh waves and traveltimes of converted Ps waves from the 410-km boundary determined from broadband records of the SVEKALAPKO seismic array. Anomalously low seismic velocities are discovered in the upper part of the cross section beneath the RUKSA array. INDEX TERMS: 7205 Seismology: Continental crust; 7255 Seismology: Surface waves and free oscillations; 7294 Seismology: Seismic instruments and networks; KEYWORDS: Earth’s crust, small aperture array, method of the receiver function.

Understanding the Differences between Three Teleseismic mb Scales

We investigate differences between three body-wave magnitude ( m b ) scales for 2009 earthquakes from 1996 to 1999 listed in the Preliminary Determination of Epicenter (pde) bulletin having m b between 5.0 and 5.5 and that also have moment tensor solutions available from the Harvard Centroid Moment Tensor (cmt) catalog. A total of 31,280 broadband seismograms are analyzed, for an average of 15 stations per event. Both the pde and Reviewed Event Bulletin (reb) procedures for determining an automated m b are reproduced, thereby eliminating any discrepancies that result from using different networks of stations. We compare the reproduced pde and reb magnitudes to another magnitude measurement, m b ( P ), that is based on the Worldwide Standard Seismographic Network (wwssn) short-period instrument. We find that differences between m b ( P ), m b (pde), and m b (reb) arise from four factors: response function, length of time window, and corrections for event depth and epicentral distance. Reproduced m b (pde) and m b ( P ) are strongly correlated, and we expect that magnitudes assigned from wwssn short-period instruments during the 1970s and 1980s are consistent with those assigned by the automated procedure used since 1991 by the U.S. Geological Survey (usgs), providing stability in m b measurements over several decades. The difference between m b (reb) and m b ( P ) is much greater because of the significantly shorter reb window length of 5.5 sec and the high-frequency passband of the reb displacement response. Online Material: Filter parameters and color versions of Figures 8 and 9.

Earthquake Location Accuracy in the Puerto Rico-Virgin Islands Region

The Puerto Rico Seismic Network (PRSN) has historically provided hypocentral information for earthquakes in the Puerto Rico-Virgin Islands (PRVI) region. The network has been operating since 1974, when the U.S. Geological Survey installed several short-period stations for the Electric Energy Authority that were used to evaluate local seismicity on the island of Puerto Rico for the construction of nuclear power plants in Guayama and Costa Norte. The network was later transferred in 1982 to the University of Puerto Rico at Mayaguez, where it has been operating under the jurisdiction of the Department of Geology since 1987. McCann (2002) provided additional details of the history of the network. The PRSN currently consists of 25 seismograph stations located in Puerto Rico and nearby islands (Table 1). Seismic data from these sites are recorded continuously and sampled at 100 sps. The network consists of both short-period and broadband instruments and includes two short-period stations that belong to the U.S. Army Corps of Engineers. Nineteen of the 25 stations are currently used to locate earthquakes in the PRSN Area of Responsibility defined by latitudes 17°-20°N and longitudes 63.5°-69°W (Figure 1). Stations not currently used in the earthquake location process are marked with a + symbol in Table 1. View this table: TABLE 1 Seismograph Stations Currently Operated by the Puerto Rico Seismic Network The upgrade in instrumentation and station coverage that the PRSN has undergone since its inception has important benefits for the effective monitoring of seismic activity in the PRVI region. These benefits include improved earthquake-detection capability and increased location accuracy that can help delineate seismogenic structures and define earthquake source zones for seismic-hazard analysis. In this study, we examine the degree of accuracy that can be expected for earthquakes located by the PRSN within the PRVI geographic region. This evaluation is based on the present configuration …

High-resolution body wave tomography beneath the SVEKALAPKO array - II. Anomalous upper mantle structure beneath the central Baltic Shield

A number of different geodynamic models have been proposed to explain the early tectonic evolution of the Baltic Shield. To provide additional geophysical constraints on these models, we performed a teleseismic tomography traveltime inversion for the central part of the Baltic Shield. The SVEKALAPKO project is focused on the investigation of the lithosphere—asthenosphere structure down to 400 km depth under central Fennoscandia (Baltic Shield). A total of 143 stations were deployed including 15 permanent stations from the Finnish seismic network. The temporal network was composed of 40 broad-band and 88 short-period instruments distributed in a rectangular array of 1000 km by 900 km from 1998 August to 1999 May. The results are based on a non-linear teleseismic tomography algorithm. They reveal significant P-velocity variations (up to 4 per cent) throughout the SVEKALAPKO array. The most prominent feature is a positive anomaly that can be followed down to 250 km depth beneath the centre of the array. We interpret this anomaly as the signature of the tectosphere (Jordan 1978) beneath the Fennoscandian Shield. It correlates spatially with an anomalous high-velocity lower crust. Other shallow (crustal) anomalies can be correlated with magmatic events surrounding this nucleus of high velocity. Comparison of images before and after correction by crustal structure proves that this methodology yields solid and coherent tomographic results. Further observations of relative P traveltime residuals from six teleseismic events with different azimuths show delay variations of ±2.0 s between stations located in the North German basin and stations on the Svecofennian Shield.

TEST OF THE EMPIRICAL GREEN'S FUNCTION DECONVOLUTION ON VRANCEA (ROMANIA) SUBCRUSTAL EARTHQUAKES

Vrancea is one of the few singular seismic regions of the world where intermediate-depth earthquakes are permanently generated (around 10 events/month with M L > 3) within an extremely confined focal volume. This particularity and the relatively large number of short-period waveforms recorded by the Romanian local network provides us the opportunity to test the performance of the empirical Green's function technique in retrieving the source time function and source directivity of the Vrancea earthquakes. Three earthquakes that occurred on March 11, 1983 (M L = 5.4), April 12, 1983 (M L = 5.1) and August 7, 1984 (M L = 5.1) in the lower part of the subducting lithosphere (h ≍ 150 km) were analyzed. A set of 28 adjacent events (3.0 < M L < 4.4) which occurred between 1981 and 1997 were selected as corresponding empirical Green's functions. To test the confidence of the retrieved source time function, we compare the deconvolved pulses using Green's functions of different sizes and recorded simultaneously by short-period and broad-band instruments. Our tests show that the durations of the source time function is well-constrained and is not affected by the limited frequency range of the short-period instruments, or by the relative difference in the focal mechanism between the main event and Green's event. The apparent duration of the source time function outlines source directivity effects, and when these effects are sufficiently strong, they can identify the real fault plane. Relatively short source duration and correspondingly high stress drop values are in agreement with other previous results emphasizing a specific seismic regime in the lower part of the Vrancea subducting lithosphere.

Observations of Q from the northwest Pacific subduction zone recorded at teleseismic distances

Abstract Twenty-seven earthquakes in the northwest Pacific, with mb ≧ 5.4, are used to study upper-mantle attenuation. The data were recorded at seismometer arrays in Australia, Canada, India, and Scotland. The short-period instruments used in the arrays are sufficiently broadband, and the signal-to-noise enhancement resulting from beamforming the data is good enough, that seismic noise is above system noise out to between 3 and 5 Hz, and in some cases to 8 Hz. The instrument response has been removed and a power law (ω−2) farfield source correction has been assumed and applied. The remaining fall-off in the spectra above 1 Hz (which is above the corner frequency fc) is assumed to be due to attenuation, including losses due both to anelasticity and to scattering. An ω−3 spectral fall-off, preferred by some, would result in lower estimates of attenuation. Conservative estimates of the average attenuation along the whole path have been made from the spectral fall-off. Despite the imperfect knowledge of the source spectra, the high sensitivity of spectral amplitudes to attenuation at high frequencies yields quite reliable and stable attenuation estimates. The scatter in the data is small, resulting in estimates of mean t* (tAV*) with an average standard error of only 21%, where tAV* is defined as the ratio of the travel time T to the effective mean quality factor QAV. Only 14 of the 64 measurements show a tAV* above 0.5 sec within the given error bounds. All estimates of tAV* and QAV refer to P-wave attenuation only. The results for tAV* from the northwest Pacific subduction zone cannot be explained simply by the traditional high-Q subducting slab in a low-Q upper mantle. A low-Q region in the uppermost 100 km (approximately) of the subduction zone has been identified. Source-corrected tAV* yield upper-mantle QAV estimates of about 1200 for predominantly oceanic paths (Australia and India) and 1500 for mixed continental and oceanic paths (Canada and Scotland).