scholarly journals Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1027-1044 ◽  
Author(s):  
A. Brüstle ◽  
W. Friederich ◽  
T. Meier ◽  
C. Gross
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Wave Amplitude ◽  
Body Wave ◽  
The Body ◽  
Grid Search ◽  
Source Depth ◽  
Normal Faulting ◽  
Intermediate Depth ◽  
First Motion

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

Source mechanism of two 1994 intermediate-depth-focus earthquakes in Guerrero, Mexico

1999 ◽  
Vol 89 (4) ◽  
pp. 1004-1018
Author(s):  
Luis Quintanar ◽  
J. Yamamoto ◽  
Z. Jiménez
Keyword(s):  
Focal Mechanism ◽  
Seismic Moment ◽  
P Wave ◽  
Focal Mechanism Solution ◽  
Medium Size ◽  
Small Contribution ◽  
Normal Faulting ◽  
Intermediate Depth ◽  
Double Couple ◽  
First Motion

Abstract In May and December 1994, two medium-size, intermediate-depth-focus earthquakes occurred in Guerrero, Mexico, eastward of the rupture area of the great Michoacan earthquake of September 19, 1985. Even though these are not major earthquakes (∼6.4 Mw), they were widely felt through central and southern Mexico, with minor damage at Zihuatanejo and Acapulco, located along the Pacific coast, and Mexico City. Both earthquakes, separated by ∼100 km, have similar focal depths and magnitudes, however, their focal mechanisms, based upon the polarities of first arrivals, show some differences. The May earthquake shows a clear normal faulting mechanism (φ = 307°, δ = 55°, λ = −108°), whereas the December earthquake mechanism solution suggests an initial thrust faulting (φ = 313°, δ = 62°, λ = 98°) process. Although previous analysis, including local and teleseismic stations, reported a normal faulting for the December earthquake, we find that modeling using the CMT focal mechanism solution fails to reproduce the first 5 sec of the observed P-wave signal at the nearest broadband station (Δ = 168 km) and the S-wave polarity at two strong ground-motion local stations (Δ = 32, 53 km); in fact, the best fit for these stations is obtained using the thrust focal mechanism calculated from the first-motion method. Seismic moment value and rupture duration time deduced from the teleseismic spectral analysis are: 2.0 × 1018 N-m and 6.9 sec for the May event; 2.8 × 1018 N-m and 7.1 sec for the December earthquake. From the inferred seismic moment, an average Δσ of ∼15 bars for both earthquakes is obtained. Inversion of teleseismic P-wave data indicates a better fit using the CMT focal mechanism solution (normal faulting) than the first-motion mechanism for both earthquakes, although the adjustment's differences are small for the May event; for this earthquake, the rupture consisted of two sources separated by ∼7 sec, starting at a depth of ∼40 km and then propagating downdip, reaching a depth of ∼60 km. The December earthquake however, released, all its energy at a depth of 50 km in two main sources separated by ∼10 sec. The non-double-couple components values are −0.004 and −0.01 for the May and December events, respectively, indicating that the December shock has a small contribution of non-double-couple radiation that could be the result of a changing mechanism. This result agrees with the hypothesis that a slab subducting at a shallower angle (our case) is associated with the existence of random subfaults with different fault orientations. From a tectonic point of view, the complexity of the December earthquake could be the result of the observed complexity of the stress distribution around 101°W and the existence of compressional events beneath the normal faulting earthquakes near the coastline. This feature permits the flexural stresses associated to the slab bending upward to become subhorizontal at the Guerrero region. We conclude that the May earthquake corresponds to a pure normal faulting, whereas the December shock is a complex event with a variable fault geometry.

Precision of different varieties of magnitude

1972 ◽  
Vol 62 (3) ◽  
pp. 789-792
Author(s):  
B. F. Howell
Keyword(s):  
Surface Wave ◽  
Frequency Band ◽  
Wave Amplitude ◽  
Body Wave ◽  
The Body ◽  
Surface Wave Magnitude ◽  
Lower Standard ◽  
Standard Deviations ◽  
Body Wave Magnitude

Abstract The standard deviations of the body-wave magnitude, surface-wave magnitude and frequency-band magnitude of four shallow (H < 60 km) earthquakes are compared. For three out of four of these earthquakes, surface-wave magnitude exhibited lower standard deviations than either body-wave or frequency-band magnitude. In three out of the four cases, lower standard deviations were obtained by calculating surface-wave magnitude from the largest surface-wave amplitude than from time-correlated surface-wave phases.

Bias in surface-wave magnitude Ms due to inadequate distance corrections

1998 ◽  
Vol 88 (1) ◽  
pp. 43-61
Author(s):  
Mehdi Rezapour ◽  
Robert G. Pearce
Keyword(s):  
Surface Wave ◽  
Wave Amplitude ◽  
Body Wave ◽  
The Body ◽  
Body Waves ◽  
P Wave ◽  
Systematic Bias ◽  
Surface Wave Magnitude ◽  
Distance Curve ◽  
Distance Correction

Abstract We investigate bias in surface-wave magnitude using the complete ISC and NEIC datasets from 1978 to 1993. We conclude that although there are some small differences between the ISC and NEIC magnitudes, there is no major difference between these agencies for this presentation of the global dataset. The frequency-distance plot for reported surface-wave amplitude observations exhibits detailed structure of the body-wave amplitude-distance curve at all distances; the influence of the surface-wave amplitude decay with distance is much less apparent. This censoring via the body waves represents a large deficit in the number of potentially usable surface-wave amplitude observations, particularly in the P-wave shadow zone between Δ = 100° and 120°. We have obtained two new modified Ms formulas based upon analysis of all ISC data between 1978 and 1993. In the first, the conventional logarithmic dependence of the distance correction is retained, and we obtain M s e = log ( A / T ) max + 1.155 log ( Δ ) + 4.269 . In the second, we make allowance for the theoretically known contribution of dispersion and geometrical spreading, to obtain M s t = log ( A / T ) max + 1 3 log ( Δ ) + 1 2 log ( sin Δ ) + 0.0046 Δ + 5.370. Comparison of these formulas with other work confirms the inadequacy of the distance-dependence term in the Gutenberg and Prague formulas, and we show that our first formula, as well as that of Herak and Herak, gives less bias at all epicentral distances to within the scatter of the observed dataset. Our second formula provides an improved overall distance correction, especially beyond Δ = 145°. We show evidence that Airy-phase distance decay predominates at shorter distances (Δ≦30°), but for greater distances, we are unable to resolve whether this or non-Airy-phase decay predominates. Assuming 20-sec surface waves with U = 3.6 km/sec, we obtain a globally averaged apparent Q−1 of 0.00192 ± 0.00026 (Q ≈ 500). We argue that our second formula not only improves the distance correction for surface-wave magnitudes but also promotes the analysis of unexplained amplitude anomalies by formally allowing for those contributions that are theoretically predictable. We conclude that there remains systematic bias in station magnitudes and that this includes the effects of source depth, different path contributions, and differences in seismometer response. For intermediate magnitudes, Mts shows less scatter against log M0 than does Ms calculated using the Prague formula.

Focal mechanism and source depth of earthquakes from body- and surface-wave data

1971 ◽  
Vol 61 (5) ◽  
pp. 1369-1379 ◽  
Author(s):  
Nezihi Canitez ◽  
M. Nafi Toksöz
Keyword(s):  
Surface Wave ◽  
Body Wave ◽  
Source Parameters ◽  
Focal Depth ◽  
Spectral Ratio ◽  
The Body ◽  
Spectral Ratios ◽  
Motion Data ◽  
Wave Data ◽  
Surface Wave Data

abstract The determination of focal depth and other source parameters by the use of first-motion data and surface-wave spectra is investigated. It is shown that the spectral ratio of Love to Rayleigh waves (L/R) is sensitive to all source parameters. The azimuthal variation of the L/R spectral ratios can be used to check the fault-plane solution as well as for focal depth determinations. Medium response, attenuation, and source finiteness seriously affect the absolute spectra and introduce uncertainty into the focal depth determinations. These effects are nearly canceled out when L/R amplitude ratios are used. Thus, the preferred procedure for source mechanism studies of shallow earthquakes is to use jointly the body-wave data, absolute spectra of surface waves, and the Love/Rayleigh spectral ratios. With this procedure, focal depths can be determined to an accuracy of a few kilometers.

scholarly journals Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

2013 ◽  
Vol 5 (2) ◽  
pp. 1901-1940
Author(s):  
A. Brüstle ◽  
W. Friederich ◽  
T. Meier ◽  
C. Gross
Keyword(s):  
Focal Mechanism ◽  
Transfer Functions ◽  
Body Wave ◽  
Normal Fault ◽  
Benioff Zone ◽  
Grid Search ◽  
African Plate ◽  
Technical Problems ◽  
Time Frequency ◽  
Best Fitting

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic Subduction Zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only 7 seismograms from three different stations (STU, GTT, COP) could be used. Nevertheless, the grid search produces stable optimal values for both source depth and focal mechanism. Our results indicate a shallow hypocenter at about 25 km depth. The best-fitting focal mechanism is a SW–NE-trending normal fault dipping either by 30° towards SE or 60° towards NW. This finding is consistent with the local structure of the Santorini–Amorgos graben. For the second earthquake, 4 seismograms from three different stations (JEN, GTT, COP) proved suitable for waveform matching. Whereas it was impossible to obtain meaningful results for the focal mechanism owing to surface wave coda of the first event overlapping body wave phases of the second event, waveform matching and time-frequency analysis point to a considerably deeper hypocenter located within the Wadati–Benioff-zone of the subducting African plate at about 120–160 km depth.

The 4.6 mb,Lg Northeastern Kentucky Earthquake of September 7, 1988

1993 ◽  
Vol 64 (3-4) ◽  
pp. 187-199 ◽  
Author(s):  
R. Street ◽  
K. Taylor ◽  
D. Jones ◽  
J. Harris ◽  
G. Steiner ◽  
...  
Keyword(s):  
Surface Wave ◽  
Seismic Moment ◽  
Wave Amplitude ◽  
Linear Trend ◽  
Source Parameters ◽  
Strike Slip ◽  
Source Depth ◽  
Nodal Plane ◽  
Amplitude Spectra ◽  
Conjugate Fault

Abstract Source parameters for the September 7, 1988 northeastern Kentucky earthquake have been estimated from the analysis of surface-wave amplitude spectra. The source that best fits the observed data had a seismic moment of 2.0 × 1022 dyne-cm, a mechanism with strike = 198° ± 10°, dip = 51° ± 11°, and slip = −178° ± 17°, (T) trend = 160°, plunge = 25°, (P) trend = 55°, plunge = 28°, and source depth of 4 to 7 km. Thirty-two aftershocks were recorded during 2 weeks of monitoring following the mainshock; 23 of the aftershocks were locatable and fall on a roughly NW-SE linear trend. This trend is subparallel with the NW-SE nodal plane of the mainshock. Our analysis shows the 1988 event to be different from the July 27, 1980 mb,Lg = 5.3 earthquake located 11 km to the northwest. First, the 1988 event is considerably shallower (4 to 7 km) than the 1980 event (14 to 22 km). Second, data from the 1988 event suggest the motion is on a conjugate fault and is in contrast with the 1980 event, which had right-lateral strike-slip on a southeast-dipping plane.

Surface-wave magnitudes of Eurasian earthquakes and explosions

1975 ◽  
Vol 65 (3) ◽  
pp. 693-709 ◽  
Author(s):  
Otto W. Nuttli ◽  
So Gu Kim
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Rayleigh Waves ◽  
Body Wave ◽  
Focal Depth ◽  
Vertical Component ◽  
P Waves ◽  
Long Period ◽  
Short Period ◽  
Continental Interior

abstract Body-wave magnitudes, mb, and surface-wave magnitudes, MS, were determined for approximately 100 Eurasian events which occurred during the interval August through December 1971. Body-wave magnitudes were determined from 1-sec P waves recorded by WWSSN short-period, vertical-component seismographs at epicentral distances greater than 25°. Surface-wave magnitudes were determined from 20-sec Rayleigh waves recorded by long-period, vertical-component WWSSN and VLPE seismographs. The earthquakes had mb values ranging from 3.6 to 5.7. Of 96 presumed earthquakes studied, 6 lie in or near the explosion portion of an mb:MS plot. The explosion mb:MS curve was obtained from seven Eurasian events which had mb values ranging from 5.0 to 6.2 and MS values from 3.2 to 5.1. All six anomalous earthquakes were located in the interior of Asia, in Tibet, and in Szechwan and Sinkiang provinces of China. In general, oceanmargin earthquakes were found to have more earthquake-like mb:MS values than those occurring in the continental interior. Neither focal depth nor focal mechanism can explain the anomalous events.

Surface-wave constraints on the August 1, 1975, Oroville earthquake

1977 ◽  
Vol 67 (1) ◽  
pp. 1-7 ◽  
Author(s):  
Robert S. Hart ◽  
Rhett Butler ◽  
Hiroo Kanamori
Keyword(s):  
Surface Wave ◽  
Rayleigh Waves ◽  
Body Wave ◽  
Source Parameters ◽  
The Body ◽  
Wave Radiation ◽  
Spectral Amplitude ◽  
Surface Wave Magnitude ◽  
Long Period ◽  
Short Period

abstract Observations of Love and Rayleigh waves on WWSSN and Canadian Network seismograms have been used to place constraints upon the source parameters of the August 1, 1975, Oroville earthquake. The 20-sec surface-wave magnitude is 5.6. The surface-wave radiation pattern is consistent with the fault geometry determined by the body-wave study of Langston and Butler (1976). The seismic moment of this event was determined to be 1.9 × 1025 dyne-cm by both time-domain and long-period (T ≥ 50 sec) spectral amplitude determinations. This moment value is significantly greater than that determined by short-period studies. This difference, together with the low seismic efficiency of this earthquake, indicates that the character of the source is intrinsically different at long periods from those aspects which dominate the shorter-period spectrum.

Sign in / Sign up

Export Citation Format

Share Document