Digital Seismic Data Research Articles

Seismic Lg‐wave Q tomography in and around Northeast China

We investigate regional variations in the Lg‐wave quality factor (Q) in Northeast China and its vicinity with a tomographic method. Digital seismic data recorded at 20 broadband stations from 125 regional events are used to extract Lg‐wave spectra. Tomographic inversions are independently conducted at 58 discrete frequencies distributed log evenly between 0.05 and 10.0 Hz. We simultaneously invert for the Lg‐wave Q distribution and source spectra at individual frequencies without using any a priori assumption about the frequency dependence of the Q model and source function. The best spatial resolution is approximately 1° × 1° in well‐covered areas for frequencies between 0.4 and 2.0 Hz. The Lg Q shows significant regional variations and an apparent relationship with regional geology. We use a statistical method to investigate the regional variations of Lg Q and their frequency dependence. The average Q0 (1 Hz Lg Q) in the entire investigated region is 414. Sedimentary basins are usually characterized by lower average Q0 values (from 155 to 391), while volcanic mountain areas have relatively high average Q0 values (from 630 to 675). Lg Q generally increases with increasing frequency. However, the frequency dependence has complex nonlinear features on a double‐logarithmic scale, indicating that the commonly adopted power‐law relationship may be oversimplified in a broad frequency band. The frequency dependence varies in different geological areas, with larger variations seen at lower frequencies.

Evaluation of seismic events detection algorithms

Identification of seismic events from continuously recorded seismic data in real-time through a Digital Seismic Data Recording system is a difficult task. Despite the vast amount of research in this field, the signal processing and event parameters discrimination algorithms have not yet fully come of age. Presently, we have a wide spectrum of trigger algorithms, ranging from a very simple amplitude threshold type to the sophisticated ones based on pattern recognition approaches. Some of the other approaches use adaptive technique and neural network methods. Researchers are continuously making efforts for the development of algorithms using various techniques, which produce minimum false trigger. Some approaches have been reported which are accurate for detecting first phase of events and take minimum possible computational time. In this paper several approaches for detecting event signals in background noise are presented and their precision evaluation is discussed.

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan MS8.0 earthquake

The seismicity of Longmenshan fault zone and its vicinities before the 12 May 2008 Wenchuan MS8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°–70°E, the dip angles of fault planes are 60°–70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.

Spatiotemporal characteristics of the energy radiation sources of the three great earthquakes near Sumatra Island in September 2007

Three strong earthquakes with magnitudes of M W 8.4, M W 7.9 and M W 7.0 occurred in the sea west of Sumatra Island on September 12 and 13, 2007. We relocated the epicenters and focal depths of the three events by means of the reversal-time imaging technique using broadband digital seismic data from worldwide stations ranging from 30° to 90°, imaged the spatiotemporal variation of the energy radiation sources by means of the nonplane wave array technique using the broadband digital seismic data from a generalized array made up of 33 stations of the Capital Region Digital Seismograph Network (CRDSN), and obtained the rupture duration times, extents and rupture velocities. Also, we discussed the correlations between the locations of the energy radiation sources of the three events.

The design and experimental use of the Mikhnevo 2D small aperture seismic array

The design of a 2D small aperture seismic array (SASA) installed in the central part of the East European Platform (in the area of the village of Mikhnevo, Moscow Oblast’) is discussed. Brief information is provided on the geologic structure of the upper section. The SASA configuration is described along with parameters of the seismological instrumentation used, and the digital seismic data acquisition system. Results are described from a study of the spectral content and the spatial correlation properties of short period microseismic noise and seismic signal coherence. Examples are provided of records of natural and manmade seismic events occurring at different epicentral distances. Experimental data are analyzed to show that this small aperture seismic array can be an effective tool to study low magnitude seismicity in the East European Platform and for solving several other problems arising in contemporary experimental seismology.

S wave inelastic attenuation and site effect in the northern Tianshan area

In this paper, according to the relationship of observation spectrum with source spectrum, travel-path attenuation factor and site effect, we use the digital seismic data of moderate-small earthquakes to invert the soft rock site effect and S wave inelastic attenuation under the constraint of site effect, and determine the parameters of path and site. The parameters obtained by this method exclude the topographic effect of seismic stations, and truly reflect the seismic effect of free soft rock sites and the S wave inelastic attenuation in the studied region. The average soft rock site effect is about 1.5 times in the frequency domain of 2∼4 Hz and the S wave quality factor is obtained as Qs=278f0.346, which can be directly applied to the stochastic modeling of ground motions in the studied region.

On the Identification and Documentation of Timing Errors: An Example at the KBS Station, Spitsbergen

The availability of high-quality digital seismic data has never been better than it is today. Through consortia such as the Incorporated Research Institutions for Seismology (IRIS) and Observatories and Research Facilities for European Seismology (ORFEUS), waveform data from stations of many worldwide networks are available via the Internet to the international community of seismologists. When a user examines a segment of data, it is assumed that the time displayed is the exact time in Coordinated Universal Time (UTC) at which the motion was recorded. In general, when retrieving data segments from a waveform repository, there is no independent confirmation of the validity of the time stamp. In this paper, I discuss a case where a user discovered an erroneous time stamp on data from a single-site station. A fortuitous source of repeating seismic signals, together with data from another nearby station, facilitated measurement of the timing anomaly during the period in question. Erroneous timing on seismograms might be more widespread than assumed; the identification and documentation of such cases requires more attention. The IRIS/GEOFON/AWI seismic station at King's Bay Station (KBS) is situated near Ny Alesund, King's Bay, on the Arctic island of Spitsbergen (figure 1). The location is important in the context of nuclear-explosion monitoring due to the relative proximity of the Russian island of Novaya Zemlya, the site of numerous Soviet-era nuclear tests, the last known event being on 24 October 1990. Also on Spitsbergen, a highly sensitive small-aperture seismic array, SPITS, became operational in 1994 and is now a designated Auxiliary Seismic Array (AS72) of the International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO). However, the KBS station is still of great importance given both the high quality of the continuous seismic data and the availability of the historical data recorded at that site. …

Use of Volume-Based 3-D Seismic Attribute Analysis to Characterize Physical-Property Distribution: A Case Study to Delineate Sedimentologic Heterogeneity at the Appleton Field, Southwestern Alabama, U.S.A.

Abstract In this paper we illustrate the use of 3-D seismic attribute studies for predicting the distribution of physical properties in the subsurface. Using a data set consisting primarily of digital logs and seismic data, we show how correlations can be made between seismic attributes and physical properties (porosity), and how those relationships can be exploited to predict the distribution of the property of interest in three dimensions. The results of these studies: (1) provide quantitative, site-specific 3-D models of physical properties that are of more use for applied studies than qualitative 2-D models commonly derived from facies modeling or sequence stratigraphic analysis, (2) are generally more geologically reasonable than studies based on geostatistics alone, (3) can provide sedimentary geologists with fundamental insights into depositional and/or diagenetic controls on the distribution of properties of interest, (4) need to be rigorously evaluated by integrating other types of data and analyses, and (5) are best thought of as supplementing, rather than replacing, conventional geologic analyses. The concepts and methods we illustrate may have application in various branches of sedimentary geology. Our study area is Appleton Field in southern Alabama. At this location we predict the 3-D distribution of porosity in carbonates of the Upper Jurassic Smackover Formation using a probabilistic neural network and a combination of four attributes. Our results suggest that porosity was best developed, and preserved, in thrombolite facies of the reef front.

Call for papers: EPS special issue

We now call for papers to the special issue of EPS named “The 2004 and 2005 Sumatra Earthquakes and Tsunamis”. The great 2004 Sumatra earthquake (Mw 9.0) generated a giant tsunami which propagated across the Indian Ocean and caused the catastrophic disaster. In three months after the event, the 2005 Sumatra earthquake (Mw 8.6) occurred in the south of the source region of the 2004 event. Recent digital seismic data, digital tsunami waveforms, GPS data, satellite altimetry data and field survey data will provide new insights on source processes of the giant earthquakes and also causes of the catastrophic tsunami disaster. This EPS special issue will focus on a quick publication of results using the above data for the 2004 and 2005 Sumatra earthquakes and tsunamis. Contributors to this special issue should submit their papers to the EPS editorial office either by post or by e-mail (eps@terrapub.co.jp), following the instructions for regular EPS submission. For details, please visit the following web page:

Characteristics of foreshock and its identification

In the paper, we analyze 117 moderate-strong earthquakes occurred in Chinese mainland (MS≥5.5 in the east and MS≥6.0 in the west) since 1970, among them, 11 earthquakes (about 9%) have direct foreshocks and 63 earthquakes (about 51%) have generalized foreshocks. The predominant time interval between foreshock and main earthquake is no more than 30 days with a spatial distance less than 50 km and a magnitude difference over 1. From the digital seismic data in liaoning Province, we know that direct foreshock had an obvious shear-stress background before the MS=5.6 and MS=5.1 Xiuyan earthquakes occurred on Nov. 29, 1999 and Jan.15, 2000.

Determination of combined effect of amplification and attenuation of soft rock site using digital seismic data

In the paper, for the application of stochastic simulation of ground motion, we put forward a method to determine “the combined effect of amplification and attenuation” (combined effect for short) of soft rock site by using digital seismic data of moderate and small earthquakes. Our approach aims at solving the problem of the combined effect of soft rock site, which is difficult to determine in most regions of China because fewer measures were done for S-wave velocity structure. The combined effect of soft rock site can be determined by using the approach recommended by us. An example is given to discuss the practical application of the method.

Long-Period Source Characteristics of the 1975 Kalapana, Hawaii, Earthquake

The 1975 Kalapana, Hawaii, earthquake occurred under the highly mobile south flank of Kilauea Volcano. It has been interpreted variously as a normal-faulting earthquake, a thrust-faulting earthquake, and a landslide. Primary evidence for the landslide model has been the failure of previous faulting models to explain the observed Love-wave radiation pattern and the tsunami amplitudes generated by the Kalapana event. Here, we present a reanalysis of the long-period, digital seismic data for this event. Centroid-moment-tensor analysis shows that the seismic radiation pattern can be explained well by thrust faulting on a plane dipping shallowly landward. The seismic moment of 3.8 × 10 20 N m ( M W 7.7) that we determine is approximately twice as large as earlier estimates. The geometry and seismic moment of the focal mechanism determined here are consistent with the observed tsunami amplitudes. Inversion of long-period body-wave waveforms shows that the earthquake source duration (∼72 sec) is unusually long for an earthquake of this size.

CSEG interview: Sven Treitel

Sven Treitel, a name known far and wide in the geophysical world, is the recipient of numerous learned society awards. He has made distinct contributions to exploration geophysics, especially in digital seismic data processing. Treitel delivered the keynote address at the inaugural CSEG Doodle Train week (4 November 2002) in Calgary. His talk brought back memories of how the process of deconvolution evolved and was implemented in the 1960s and 1970s, and became an important tool in seismic data processing in the 1980s and beyond.

MC1 Micro‐Power Consuming Broadband Digital Seismograph

AbstractA new generation of digital seismic data acquisition device—MC1 Micro‐Power Cosuming and Broad‐Band Digital Seismograph was designed and built in our Lab. Its power dissipation is less than 300mW. It has some merits including miniature volume, light mass and low costs. It is an ideal device for the acquisition of high‐resolution natural seismic data, as well as for the fields of seismic sounding and engineering seismology.

Application of Distributed Object Technology to Seismic Waveform Distribution

We develop a networked data distribution system for broadband seismic waveform data using Java RMI (Remote Method Invocation). Users can download data from various networks using a unified interface. There are two technical problems in building such a user-friendly system with a high capability for expansion: communication through firewalls and information synchronization between request interfaces. We solve these two problems using cgi (common gateway interface) filtering and the Directory Service, respectively. We are now distributing data from various Japanese broadband seismic networks (including PACIFIC21) together with data from BATS (Broadband Array in Taiwan for Seismology) and IRIS through the system. Digital seismic waveform data are being accumulated at an accelerating rate by various networks. Data analysis using these huge data sets is becoming increasingly common. For example, in the Western Pacific region, a large number of digital broadband seismographic stations have recently been installed by various institutions (Figure 1). If the data from these networks are appropriately combined, they effectively constitute a data set from a very dense broadband network. Such a data set should be very useful for determining seismic structure and earthquake source properties with high resolution. However, in the current waveform distribution system, such studies require somewhat tedious procedures for collecting the waveform data from different data centers, each of which has a different user interface. Looking at the field of computer science, distributed object technology (technology for distributing software modules to multiple computers on a network; The Object Management Group, 2000) is one of the most important technologies in modern system-programming practice, as it can handle a huge system in a straightforward manner. Java RMI (Gosling and Steele, 1996; Sun Microsystems, 1999) provides one of the most successful technologies to build a distributed object management system because of its high capability for expansion and security. …

Study on the method for comprehensive discrimination of small earthquakes

In the paper, we apply the theory of modern digital signal processing to analysis 3-component digital seismic data recorded in Datong area of Shanxi Province by digital seismographs developed by the Institute of Geophysics, CSB in November 1999 and study the auto-discrimination of the recorded digital seismic signals. The result shows that the method for comprehensive discrimination in connection with ratios between average values of long-time and shorttime signals, spectral analysis, and wavelet analysis can be used to auto-discrimination of seismic signals through alternate man-computer processing. Many M L>2.0 earthquakes were discriminated better.

Using Automated, High-precision Repicking to Improve Delineation of Microseismic Structures at the Soultz Geothermal Reservoir

— An automatic, adaptive, correlation-based algorithm for adjusting phase picks in large digital seismic data sets provides significant improvement in resolution of microseismic structures using only a small fraction of the time and manpower which would be required to re-analyze waveforms manually or semi-automatically. We apply this technique to induced seismicity at the Soultz-sous-Forets geothermal site, France. The method is first applied to a small, previously manually repicked subset of the catalogue so that we may compare our results to those obtained from painstaking, visual, cross-correlation-based techniques. Relative centroid-adjusted hypocenters show a decrease in median mislocation from 31 to 7 m for preliminary and automatically adjusted picks, respectively, compared to the manual results. Narrow, intersecting joint features not observed in the preliminary hypocenter cloud, but revealed through manual repicking, are also recovered using the automatic method. We then address a larger catalogue of ∼7000 microearthquakes. After relocating the events using automatic repicks, the percentage of events clustering within 5 m of their nearest neighbor increases form 5 to 26% of the catalogue. Hypocenter relocations delineate narrow, linear features previously obscured within the seismic cloud, interpreted as faults or fractures which may correspond to fluid propagation paths, or to changes in stress as a result of elevated pore pressures. RMS travel-time residuals for the larger data set are reduced by only 0.2%; however, phase-pick biases in the preliminary catalogue have influenced both the velocity model and station correction calculations, which will affect location residuals. These pick biases are apparent on the adjusted, stacked waveforms and correcting them will be important prior to future velocity model refinements.

Relocation of the 1999 Chi-Chi Earthquake in Taiwan

The 1999 Chi-Chi earthquake in Taiwan represented the island’s largest inland seismic event of the last century. This earthquake severely struck the western-middle part of the island. Abundant high-resolution digital seismic data were recorded by two seismic networks operated by the Central Weather Bureau (CWB). In this paper, we combined data from two CWB seismic networks to analyze the characteristics of the Chi-Chi earthquake sequence. The mainshock was relocated to 23.853°N, 120.816°E at the depth of 8 km. The focal mechanism was of a thrust type with strike 5°, dip 34° and rake 65° based on the new location and the first motion polarities. The distribution of the aftershocks revealed a special distribution pattern which marked out an aseismic dipping plane with the aftershocks mostly occurring on its upper side. It is highly suggested that there is a close relationship between this aseismic dipping plane and the Chelungpu fault.

GeoMorpher: Seismic reconstruction in the digital‐image domain

This paper describes how morpher software can achieve complete structural reconstruction of digital seismic data. This technique, developed by the author and known within BHP as GeoMorphing, is used internally in the company to assist in structural and stratigraphic analysis.

J‐Array Project seismic data now on CD‐ROM

Two CD‐ROMs containing seismic waveform data obtained from the J‐array Project are now available.The J‐array was a large‐aperture seismic array extending over all of Japan. It consisted of 14 regional seismic networks and more than 200 stations. The project was concerned with detecting small‐amplitude waves that are refracted, reflected, converted, or scattered from seismic velocity discontinuities existing deep within the Earth, and with studying the detailed structure and regional variations of the core, lower mantle, and upper mantle.Continuous digital seismic waveform data were recorded at the rate of 20 Hz with short period seismometers developed under Japan's national program for earthquake prediction. The J‐array project began in April 1991 and continued for 3 years at most of the networks. Many studies have been done with the data, including investigations of the structure of the upper mantle, lower mantle discontinuities, D″ layer, and core. Anyone interested in using the data can refer to J‐array Group [1993] for more information.