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1.
The neural network estimates of seismic P- and S-wave velocities from electrical resistivity of the rocks and, vice versa, resistivity estimates from seismic velocities are presented. It is shown that, depending on the ratio between the volumes of the known data and the data to be reconstructed, the accuracy of the estimates of the P- and S-wave velocities ranges within 1–4 and 4–6%, respectively. The logarithmic resistivity is estimated from seismic P- and S-velocities as accurately as up to 15–17%. In all cases, the biggest errors are obtained when the estimates are based on correlated data. 相似文献
2.
A Seismic Model of Casing Failure in Oil Fields 总被引：1，自引：0，他引：1
—We develop a seismic model that characterises the sudden tensional failure of oil-well casings. The energy released by the rupture of a well casing is transformed into heat and seismic energy. The upper bound of the seismic efficiency of this process is estimated at about 3%. The static situation at the completion of a casing failure episode is modelled by calculating the static displacement field generated by two opposing forces separated by an arm. The azimuthal patterns of these displacements and the change in the strain and stress fields caused by the force couple are described. The dynamics of the failure episode are modelled as a dipole with a seismic moment equivalent to the product of the average drop in shear stress, the failure surface, and an arm. The radiated P and S waves have mean-square radiation pattern coefficients of 1/5 for P waves and 2/15 for S waves. The displacement field as a function of time during rupture and the spectral properties in the far field are derived. The most promising seismic parameters that can be used for distinguishing between casing failure events and other possible events are polarisation properties of S waves and S/P amplitude ratios. S-wave polarisation distinguishes between shear events and casing failure events. S/P amplitude ratios distin guish between tensile events and casing failure events. 相似文献
3.
A layeredP- andS-wave velocity model is obtained for the Friuli seismic area using the arrival time data ofP- andS-waves from local earthquakes. A damped least-squares method is applied in the inversion.The data used are 994P-wave arrival times for 177 events which have epicenters in the region covered by the Friuli seismic network operated by Osservatorio Geofisico sperimentale (OGS) di Trieste, which are jointly inverted for the earthquake hypocenters andP-wave velocity model. TheS-wave velocity model is estimated on the basis of 978S-wave arrival times and the hypocenters obtained from theP-wave arrival time inversion. We also applied an approach thatP- andS-wave arrival time data are jointly used in the inversion (Roecker, 1982). The results show thatS-wave velocity structures obtained from the two methods are quite consistent, butP-wave velocity structures have obvious differences. This is apparent becauseP-waves are more sensitive to the hypocentral location thanS-waves, and the reading errors ofS-wave arrival times, which are much larger than those ofP-waves, bring large location errors in the joint inversion ofP- andS-wave arrival time. The synthetic data tests indicated that when the reading errors ofS-wave arrivals are larger than four times that ofP-wave arrivals, the method proposed in this paper seems more valid thanP- andS-wave data joint inversion. Most of the relocated events occurred in the depth range between 7 and 11 km, just above the biggest jump in velocity. This jump might be related to the detachment line hypothesized byCarulli et al. (1982). From the invertedP- andS-wave velocities, we obtain an average value 1.82 forV
p
/V
s
in the first 16 km depth. 相似文献
4.
This study analyzes and compares the P- and S-wave displacement spectra from local earthquakes and explosions of similar magnitudes. We propose a new approach to discrimination between low-magnitude shallow earthquakes and explosions by using ratios of P- to S-wave corner frequencies as a criterion. We have explored 2430 digital records of the Israeli Seismic Network (ISN) from 456 local events (226 earthquakes, 230 quarry blasts, and a few underwater explosions) of magnitudes Md?=?1.4–3.4, which occurred at distances up to 250 km during 2001–2013 years. P-wave and S-wave displacement spectra were computed for all events following Brune’s source model of earthquakes (1970, 1971) and applying the distance correction coefficients (Shapira and Hofstetter, Teconophysics 217:217–226, 1993; Ataeva G, Shapira A, Hofstetter A, J Seismol 19:389-401, 2015), The corner frequencies and moment magnitudes were determined using multiple stations for each event, and then the comparative analysis was performed.The analysis showed that both P-wave and especially S-wave displacement spectra of quarry blasts demonstrate the corner frequencies lower than those obtained from earthquakes of similar magnitudes. A clear separation between earthquake and explosion populations was obtained for ratios of P- to S-wave corner frequency f 0(P)/f 0(S). The ratios were computed for each event with corner frequencies f 0 of P- and S-wave, which were obtained from the measured f 0 I at individual stations, then corrected for distance and finally averaged. We obtained empirically the average estimation of f 0(P)/f 0(S)?=?1.23 for all used earthquakes, and 1.86 for all explosions. We found that the difference in the ratios can be an effective discrimination parameter which does not depend on estimated moment magnitude M w .The new multi-station Corner Frequency Discriminant (CFD) for earthquakes and explosions in Israel was developed based on ratios P- to S-wave corner frequencies f 0(P)/f 0(S), with the empirical threshold value of the ratio for Israel as 1.48. 相似文献
5.
Scaling relations for seismic events induced by mining 总被引：1，自引：0，他引：1
S. J. Gibowicz 《Pure and Applied Geophysics》1995,144(2):191-209
The values of seismic moment andS-wave corner frequency from 1575 seismic events induced in South African, Canadian, Polish, and German underground mines were collected to study their scaling relations. The values ofP-wave corner frequency from 649 events were also available. Seismic moments of these events range from 5*103 to 2*1015 N·m (moment magnitude is from –3.6 to 4.1), theS-wave corner frequency ranges from 0.7 to 4438 Hz, and theP-wave corner frequency is between 5 and 4010 Hz. The slope of a regression line between the logarithm ofS- andP-wave corner frequencies is equal to one, and the corner frequencies ofP waves are higher than those ofS waves on the average by about 25 percent. In studies of large and moderate earthquakes it has been found that stress drop is approximately independent of the seismic moment, which means that seismic moment is inversely proportional to the third power of corner frequency. Such a behavior was confirmed for most of the data considered here. A breakdown in the similarity betwen large and small events seems to occur for the events with moment magnitude below –2.5. The average values of seismic moment referred to the same range of corner frequency, however, are vastly different in various mining areas. 相似文献
6.
We examine the influence of attenuation and site on the spectra of microearthquakes having origin within the Shillong region. The ratios of spectral amplitudes at lower and higher frequencies are measured for three different stations at varying epicentral distances to estimate Q value for both P- and S-wave in near and sub-surface layer. The average estimates of Q P and Q S are found to be 178 and 195. The ratio of Q S to Q P emerges to be greater than unity in major parts of the Shillong area, suggesting dominance of dry crust prevailing in Shillong region. The variation in corner frequencies for these spectra is inferred to be characteristics of the site. Besides, the disparity in spectral content with reference to hard rock site yields the inference that the incoming seismic signals get amplified considerably while traversing from southern part to northeastern part of Shillong, best outlined at 2 to 5 Hz, which is well corroborated by the existing lithology. 相似文献
7.
—?The digital data acquired by 16 short-period seismic stations of the Friuli-Venezia-Giulia seismic network for 56 earthquakes of magnitude 2.3–4.7 which occurred in and near NE Italy have been used to estimate the coda attenuation Q c and seismic source parameters. The entire area under study has been divided into five smaller regions, following a criterion of homogeneity in the geological characteristics and the constrains imposed by the distribution of available events. Standard IASPEI routines for coda Q c determination have been used for the analysis of attenuation in the different regions showing a marked anomaly in the values measured across the NE border between Friuli and Austria for Q 0 value. A large variation exists in the coda attenuation Q c for different regions, indicating the presence of great heterogeneities in the crust and upper mantle of the region. The mean value of Q c (f) increases from 154–203 at 1.5?Hz to 1947–2907 at 48?Hz frequency band with large standard deviation estimates.¶Using the same earthquake data, the seismic-moment, M 0, source radius, r and stress-drop, Δσ for 54 earthquakes have been estimated from P- and S-wave spectra using the Brune's seismic source model. The earthquakes with higher stress-drop (greater than 1?Kbar) occur at depths ranging from 8 to 14?km. 相似文献
8.
—?Induced microseismicity data from a large volume fluid injection into sedimentary rock was analyzed to study the fracture system, fluid pathways, and state of stress in the lower Frio formation in east Texas. Seismicity data are from two arrays of 25 3-component geophone packages sited in two monitoring boreholes. From a total of 2,894 event triggers, a subset of 54 microearthquakes was chosen for their high quality seismograms and clear P and S arrivals. Arrival times were picked with a precision of 0.5 to 1.0?ms, and microearthquakes were located with hypocentral uncertainties estimated as less than 10–20?m. Hypocenters farthest from the injection well define a nearly horizontal tube of seismicity approximately aligned in the direction of the injection well. A simultaneous inversion of arrival times for transverse isotropic velocity structure and hypocenters yielded P-wave anisotropy of ?14% and S-wave anisotropy of ?2%. Thus, velocities along vertical ray paths are higher than those along horizontal paths, probably because of lithologic differences. Single-event focal mechanisms were determined for 47 events, and many of them are normal fault type. The minimum principal stress derived from the focal mechanisms is nearly horizontal and trends approximately north-south, consistent with the regional stress state. An imaging analysis of the seismograms shows the presence of strong seismic scatterers at positions that correlate with boundaries seen in the hypocenters; both features probably result from a similar set of heterogeneities. This study demonstrates the abundance of information that can be extracted from induced seismicity data and underscores the value of induced seismicity monitoring for studying the fluid and fracture systems created by fluid injections. 相似文献
9.
Large, shallow, thrust earthquakes in the Solomon Islands region tend to occur in closely related pairs. Two recent sequences are July 14, 1971 (MS = 7.9) and July 26, 1971 M(S = 7.9) and 14h37m, July 20, 1975 (MS = 7.9) and 19h54m, July 20, 1975 (MS = 7.7). The mechanism of these seismic doublets has important bearing on the triggering mechanism of earthquakes in subduction zones. Detailed analysis of the seismic body waves and surface waves were performed on the 1971, 1974, and 1975 doublets, providing a better understanding of: (1) the mechanics of seismic triggering, (2) the state of stress on the fault plane, and (3) the nature of subduction between the Pacific and Indian plates. The results indicate that although the geometry of the subduction zone in the Solomon Islands is complicated by the presence of several sub-plates, the slip direction of the Indian plate with respect to the Pacific plate is relatively uniform over the entire region. The large seismic moments of the 1971 sequence (1.2 · 1028 and 1.8 · 1028 dyne cm) indicate that these events directly represent the underthrusting of the Indian and Solomon plates beneath the Pacific plate. The body waves from these doublets, recorded on the WWSSN long-period seismograms, are remarkably impulsive and simple compared with those from events of comparable seismic moment in other subduction zones. In addition, the source dimensions of the body waves are 30–70 km in length, substantially smaller than the overall rupture surfaces radiating the surface waves which are 100–300 km in length. These facts suggest the existence of relatively large, isolated high-stress zones on the fault plane. This type of stress distribution is distinct from other regions which have more heterogeneous stress distribution on the fault plane, and this is proposed as the principal characteristic of this region responsible for the occurrence of the doublets and for the apparent efficiency of triggering in the Solomon trench. Prior to the 1971 sequence, similar sequences have occurred in the same area in 1919–1920 and 1945–1946. From the amount of slip (1.3 m) determined for the 1971 sequence and the apparent recurrence interval of 25 years, a seismic slip rate of 5 cm yr?1 is determined. This value is a significant portion of the convergence rate between the Indian and Pacific plates indicating that the plate motion here is taken up largely by seismic slip. 相似文献
10.
Ota Kulhánek 《Pure and Applied Geophysics》1973,102(1):51-66
Summary Short-period vertical-componentP-wave spectra of seven presumed Semipalatinsk underground nuclear explosions, recorded by the Swedish seismic station network, are investigated. The events considered have closely spaced foci and cover the magnitude range fromm
b=5.5 tom
b=6.6. Spectra of six of these explosions show pronounced minima, varying from about 1.5 to 1.8 cps, which could be explained as principle minima due toP-pP interference. Supposing a nearsurfaceP-wave velocity at the test area of 4 km/sec, the shot depths are estimated to vary roughly from 750 to 1350 m. In order to obtain an estimate of the yield, the observed spectra are compared withHaskell's theoretical source spectra. For four events, relative yield estimates fit well the predicted values for explosions fired in a granitic medium. The behaviour of the remaining three explosions is discussed in detail. 相似文献
11.
—Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.¶Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.¶The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed. This ambiguity can be substantially reduced when shear waves (S-waves) are considered. The directional distribution of S-wave velocities and of the S-wave splitting exhibits characteristic patterns for the different olivine texture types. This could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle. A fundamental requirement for a more comprehensive interpretation is the availability of detailed S-wave observations. The maximum S-wave splitting in the peridotites investigated coincides with the maximum of the faster (leading) S-wave. This may be of importance to detect S-wave splitting in future seismic studies. 相似文献
12.
—?The aim of our study consists of analyzing potentially non-double-couple seismic events recorded at regional distances. In order to define the nature of the seismic source, a moment tensor inversion is carried out as this method is general enough not to initially constrain the source mechanism. In this paper we present an application to a seismic event induced by a mine collapse which occurred near the town of Halle in Germany. Because of its induced nature, many parameters such as the location and geometry of this seismic source are known. This information allows us to test the influence of inadequate propagation modeling on the moment tensor obtained from the inversion. Green's functions have been computed with the reflectivity method in a flat layered medium, using the European model EurID (Du et? al., 1998; Dufumier et al., 1997). From the inversion of P-wave seismograms recorded by the German Regional Seismic Network will, we obtained a source time function which can be decomposed into two subevents. The first one has a large isotropic part and a deviatoric mechanism with near vertical nodal planes. No volume change is observed for the second subevent, but a deviatoric component opposite of the first one. The addition of S-waves does not change the results of the inversion which are stable. Surface waves were not used because of their poor dispersion curves. Based on the moment tensor obtained from these inversions, the physical process at the source is compatible with a large cavity collapse. 相似文献
13.
Since 1972, Weir-Jones Engineering Consultants (WJEC) has been involved in the development and installation of microseismic monitoring systems for the mining, heavy construction and oil/gas industries. To be of practical value in an industrial environment, microseismic monitoring systems must produce information which is both reliable and timely. The most critical parameters obtained from a microseismic monitoring system are the real-time location and magnitude of the seismic events. Location and magnitude are derived using source location algorithms that typically utilize forward modeling and iterative optimal estimation techniques to determine the location of the global minimum of a predefined cost function in a three-dimensional solution space. Generally, this cost function is defined as the RMS difference between measured seismic time series information and synthetic measurements generated by assuming a velocity structure for the area under investigation (forward modeling). The seismic data typically used in the source location algorithm includes P- and S-wave arrival times, and raypath angles of incidence obtained from P-wave hodogram analysis and P-wave first break identification. In order to obtain accurate and timely source location estimates it is of paramount importance that the extraction of accurate P-wave and S-wave information from the recorded time series be automated—in this way consistent data can be made available with minimal delay. WJEC has invested considerable resources in the development of real-time digital filters to optimize extraction, and this paper outlines some of the enhancements made to existing Kalman Filter designs to facilitate the automation of P-wave first break identification. 相似文献
14.
Application of Network-averaged Teleseismic P-wave Spectra to Seismic Yield Estimation of Underground Nuclear Explosions 总被引：1，自引：0，他引：1
—?A set of procedures is described for estimating network-averaged teleseismic P-wave spectra for underground nuclear explosions and for analytically inverting these spectra to obtain estimates of m b /yield relations and individual yields for explosions at previously uncalibrated test sites. These procedures are then applied to the analyses of explosions at the former Soviet test sites at Shagan River, Degelen Mountain, Novaya Zemlya and Azgir, as well as at the French Sahara, U.S. Amchitka and Chinese Lop Nor test sites. It is demonstrated that the resulting seismic estimates of explosion yield and m b /yield relations are remarkably consistent with a variety of other available information for a number of these test sites. These results lead us to conclude that the network-averaged teleseismic P-wave spectra provide considerably more diagnostic information regarding the explosion seismic source than do the corresponding narrowband magnitude measures such as m b , M s and m b (L g ), and, therefore, that they are to be preferred for applications to seismic yield estimation for explosions at previously uncalibrated test sites. 相似文献
15.
The VPREMOON seismic reference Moon model (Garcia et al., 2011) has been tested with respect to the thermal regime and chemical composition of the mantle. Based on a self-consistent thermodynamic approach and petrological models of the lunar mantle covering a wide range of concentrations of CaO, Al2O3, and FeO, we convert the P- and S-wave velocity profiles to the temperature–depth profiles. The solution procedure relies on the method of the Gibbs free energy minimization and the equations of state for the mantle material which take into account the effects of phase transformations, anharmonicity, and anelasticity. We find that regardless of the chemical composition, the positive P- and S-wave velocity gradient in the lunar mantle leads to a negative temperature gradient, which has no physical basis. For adequate mantle temperatures, the P- and S-wave velocities should remain almost constant or slightly decrease with depth (especially VS) as a result of the effects of the temperature, which grows faster than pressure. These findings underscore the importance of the relationship of the thermodynamics and physics of minerals with seismology. 相似文献
16.
D.D. Singh B.K. Rastogi Harsh K. Gupta 《Physics of the Earth and Planetary Interiors》1979,18(2):143-152
The source characteristics of 33 earthquakes with magnitude mb between 4.4 and 6.0, which occurred in the Himalayan and nearby regions, are investigated using the records of the Hyderabad seismograph station. The P- and S-wave spectra of these events are interpreted in terms of Brune's seismic source model for estimating the source parameters, i.e., seismic moment, source dimension, stress drop, average dislocation, apparent stress and the radiated energy. Seismic moments, M0, vary between 0.3 × 1024 and 9.0 × 1026 dyne cm; source dimensions, r, between 4.3 and 18.6 km; stress-drops, Δσ between 0.3 and 151.6 bar; average dislocations, between 0.6 and 381 cm; apparent stresses, between 0.1 and 73.2 bar. The radiated energy, ER is estimated by the spectrum integration method and is found to vary between 0.2 × 1018 and 9.3 × 1022 erg. In general, the stress drop and apparent stress are found to be high, indicating high stresses in these regions. 相似文献
17.
Ivan G. Wong James R. Humphrey Janet A. Adams Walter J. Silva 《Pure and Applied Geophysics》1989,129(3-4):369-405
In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency (>10 Hz), short duration (<2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantlybeneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less thanM
c
0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events. 相似文献
18.
Stress Sensitivity of Seismic and Electric Rock Properties of the Upper Continental Crust at the KTB
We test the hypothesis that the general trend of P-wave and S-wave sonic log velocities and resistivity with depth in the pilot hole of the KTB site Germany, can be explained by the progressive
closure of the compliant porosity with increasingly effective pressure. We introduce a quantity θc characterizing the stress sensitivity of the mentioned properties. An analysis of the downhole measurements showed that estimates
of the quantitiy θc for seismic velocities and electrical formation factor of the in situ formation coincide. Moreover, this quantity is 3.5 to 4.5 times larger than the averaged stress sensitivity obtained from
core samples. We conclude that the hypothesis mentioned above is consistent with both data sets. Moreover, since θc corresponds approximately to the inverse of the effective crack aspect ratio, larger in situ estimates of θc might reflect the influence of fractures and faults on the stress sensitivity of the crystalline formation in contrast to
the stress sensitivity of the nearly intact core samples. Finally, because the stress sensitivity is directly related to the
elastic nonlinearity we conclude that the elastic nonlinearity (i.e., deviation from linear stress-strain relationship i.e.,
Hooke's law) of the KTB rocks is significantly larger in situ than in the laboratory. 相似文献
19.
Forty-six mining-induced seismic events with moment magnitude between ?1.2 and 2.1 that possibly caused damage were studied. The events occurred between 2008 and 2013 at mining level 850–1350 m in the Kiirunavaara Mine (Sweden). Hypocenter locations were refined using from 6 to 130 sensors at distances of up to 1400 m. The source parameters of the events were re-estimated using spectral analysis with a standard Brune model (slope ?2). The radiated energy for the studied events varied from 4.7 × 10?1 to 3.8 × 107 J, the source radii from 4 to 110 m, the apparent stress from 6.2 × 102 to 1.1 × 106 Pa, energy ratio (E s/E p) from 1.2 to 126, and apparent volume from 1.8 × 103 to 1.1 × 107 m3. 90% of the events were located in the footwall, close to the ore contact. The events were classified as shear/fault slip (FS) or non-shear (NS) based on the E s/E p ratio (>10 or <10). Out of 46 events 15 events were classified as NS located almost in the whole range between 840 and 1360 m, including many events below the production. The rest 31 FS events were concentrated mostly around the production levels and slightly below them. The relationships between some source parameters and seismic moment/moment magnitude showed dependence on the type of the source mechanism. The energy and the apparent stress were found to be three times larger for FS events than for NS events. 相似文献
20.
Rengin Gök Lawrence Hutchings Kevin Mayeda Doğan Kalafat 《Pure and Applied Geophysics》2009,166(4):547-566
We develop a data set of aftershock recordings of the 1999, M = 7.4 Izmit and M = 7.2 Duzce (Turkey) earthquakes to study
their source parameters. We combined seismograms from 44 stations maintained by several sources (organizations) to obtain
a unified data set of events (2.1 ≤ Mw ≤ 5.5). We calculate source parameters of these small earthquakes by two methods that use different techniques to address
the difficulty in obtaining source spectra for small earthquakes subject to interference from site response. One method (program
NetMoment (NM), Hutchings, 2004) uses spectra of direct S waves in a simultaneous inversion of local high-frequency network data to estimate seismic
moment, source corner frequency (fc), site attenuation (k) and whole-path Q. This approach takes advantage of the source commonality in all recordings for a particular earthquake
by fitting a common Brune source spectrum to the data with a and individual k. The second approach (Mayeda et al., 2003) uses the coda method (CM) to obtain “nonmodel-based” source spectra and moment estimates from selected broadband
recording sites. We found that both methods do well for events that allow the comparison with seismic moment estimates derived
from waveform modeling. Also, source spectra obtained from the two methods are very closely matched for most of the events
they have in common. We use an F test to examine the trade-off between k and fc picks identified by the direct S-wave method. About half of the events could be constrained to have less than a 50% average
uncertainty in fc and k. We used these source spectra solutions to calculate energy and apparent stress and compare these to estimates from the selected
“good quality” source spectra from CM. Both studies have values mutually consistent and show a similar increase in apparent
stress with increasing moment. This result has added merit due to the independent approaches to calculate apparent stress.
We conclude that both methods are at least partially validated by our study, and they both have usefulness for different circumstances
of recording local small earthquakes. CM would work well in studies for which there is a broad magnitude range of events and
NM works well for local events recorded by band-limited recorders. 相似文献