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.

Source process of the Chiapas, Mexico, intermediate-depth earthquake (Mw = 7.2) of 21 October 1995

1999 ◽  
Vol 89 (2) ◽  
pp. 348-358 ◽  
Author(s):  
Cecilio J. Rebollar ◽  
Luis Quintanar ◽  
Jaime Yamamoto ◽  
Antonio Uribe
Keyword(s):  
Subduction Zone ◽  
Focal Mechanism ◽  
Seismic Moment ◽  
Amplitude Spectrum ◽  
Total Duration ◽  
P Wave ◽  
Corner Frequency ◽  
Focal Mechanism Solution ◽  
Intermediate Depth ◽  
Intermediate Depth Earthquake

Abstract On 21 October 1995, we recorded with a local array an earthquake that occurred at a depth of 165 km in the subduction zone of Chiapas. The Harvard focal mechanism solution indicates a normal fault responding to the down-dip tension of the subducted oceanic crust. This is the first intermediate-depth earthquake well recorded with accelerographs and seismometers in Southeastern Mexico. Peak ground accelerations (PGA) range from 21 to 436 cm/sec2 at hypocentral distances of 174 to 256 km, respectively. The recorded PGAs are larger than those of the Copala, Guerrero, earthquake of 14 September 1995, which was a shallow (16 km) thrust fault with a similar magnitude (Mw = 7.4). The large PGA generated by the Chiapas earthquake are probably due to an enhancement of the signals produced by the up-ward intraslab propagation of energy and are similar to those observed from other intermediate-depth earthquakes in the subduction zone of Japan (Molas and Yamazaki, 1995). The duration of the strongest shaking increases from about 10 sec in the southeast at the town of San Vicente (close to the Tacaná volcano) to nearly 20 sec in the northwest, in the city of Tuxtla Gutierrez located near the epicenter. Teleseismic P-wave inversion using the Harvard focal mechanism solution indicates that the seismic moment was released in three events with a total duration of about 20 sec. The results of the inversion indicate that the rupture propagated from the northwest to the southeast along a 30-km distance. From spectral analysis, we calculate a total seismic moment release of 5.2 ± 0.5 × 1019 N-m equivalent to an Mw = 7.1 magnitude event. Using three sources with an average depth of 150 km, we were able to reach a reasonable match of the first 40 sec of the displacement records recorded at the broadband seismic stations of Huatulco (HUIG) and Pinotepa Nacional (PNIG). For the station located in Tuxtla Gutierrez (TUXD), we used two sources, since only the first 5 sec were modeled. The amplitude spectrum at teleseismic distances follows a typical Brune's (1970) θ−2 model. We obtained a corner frequency of 0.045 Hz from the spectra, which is equivalent to a source radius of 15 km and a stress drop of 65 bars assuming a circular fault.

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.

scholarly journals The comparative analysis of 2018 Alaska earthquakes from surface wave records

2020 ◽  
Vol 2 (1) ◽  
pp. 76-84
Author(s):  
Anastasiya Fomochkina ◽  
Boris Bukchin
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Seismic Moment ◽  
Optimal Solution ◽  
Seismic Source ◽  
P Wave ◽  
Wave Spectra ◽  
Earthquake Intensity ◽  
Second Moments ◽  
First Arrival

We consider the source of an earthquake in an approximation of instant point shift dislocation. Such a source is given by its depth, the focal mechanism determined by three angles (strike, dip, and slip), and the seismic moment characterizing the earthquake intensity. We determine the source depth and focal mechanism by a systematic exploration of 4D parametric space, and seismic moment - by solving the problem of minimization of the misfit between observed and calculated surface wave spectra for every combination of all other parameters. As is well known, the focal mechanism cannot be uniquely determined from the surface wave’s amplitude spectra only. We used P-wave first arrival polarities to select the optimal solution. Ana-lyzing the surface wave spectra at shorter periods, we describe the source in an approximation of the stress glut second moments. Using these moments we determine integral estimates of the geometry, the duration of the seismic source, and rupture propagation. The results of the application of this technique for two Alaska earthquakes that occurred in 2018 (with Mw7.9 in January and with Mw7.1 in November) are presented. The possibility of the fault plane identification, which based on the obtained estimates of the focal mechanisms and second mo-ments, is analyzed for both events. Bilateral model of the source is constructed.

The Peru-Bolivia border earthquake of August 15, 1963

1970 ◽  
Vol 60 (2) ◽  
pp. 639-646 ◽  
Author(s):  
Umesh Chandra
Keyword(s):  
Travel Time ◽  
Fault Plane ◽  
P Wave ◽  
Focal Mechanism Solution ◽  
Event Analysis ◽  
Rupture Propagation ◽  
Fault Propagation ◽  
Amplitude Data ◽  
First Motion ◽  
Deep Focus

abstract The seismograms of the deep focus Peru-Bolivia border earthquake of August 15, 1963 reveal the presence of a number of conspicuous phases occurring within 15 seconds of the first P onset. These phases cannot be explained on the basis of known travel-time curves. Accordingly, the earthquake is interpreted to have occurred in a series of jerks during the course of fault propagation, or in other words it is composed of multiple events. Only one of these events, following the first event, at which the amplitude of the recorded motion becomes suddenly very large, has been located in this study. The focal mechanism solution of this earthquake has been determined from the P wave first motion and amplitude data. Consideration of the direction of rupture propagation determined from the multiple event analysis makes it possible to identify the fault plane in the mechanism solution. The parameters of the fault plane, length and speed of rupture between the two events have been determined.

Combination of P and S data for the determination of earthquake focal mechanism

1971 ◽  
Vol 61 (6) ◽  
pp. 1655-1673 ◽  
Author(s):  
Umesh Chandra
Keyword(s):  
Focal Mechanism ◽  
P Wave ◽  
Wave Polarization ◽  
S Wave ◽  
Polarization Data ◽  
Wave Data ◽  
Earthquake Focal Mechanism ◽  
First Motion ◽  
Depth Ranges

abstract A method has been proposed for the combination of P-wave first-motion directions and S-wave polarization data for the numerical determination of earthquake focal mechanism. The method takes into account the influence of nearness of stations with inconsistent P-wave polarity observations, with respect to the assumed nodal planes. The mechanism solutions for six earthquakes selected from different geographic locations and depth ranges have been determined. Equal area projections of the nodal planes together with the P-wave first-motion and S-wave polarization data are presented for each earthquake. The quality of resolution of nodal plane determination on the basis of P-wave data, S-wave polarization, and the combination of P and S-wave data according to the present method, is discussed.

Focal mechanism of the Prince William Sound, Alaska earthquake of March 28, 1964

1969 ◽  
Vol 59 (2) ◽  
pp. 799-811
Author(s):  
Samuel T. Harding ◽  
S. T. Algermissen
Keyword(s):  
Focal Mechanism ◽  
P Wave ◽  
Type I ◽  
Type Ii ◽  
Prince William Sound ◽  
Wave Polarization ◽  
S Wave ◽  
Polarization Data ◽  
Wave Data ◽  
First Motion

abstract Two nodal planes for P were determined using a combination of P-wave first motion and S-wave polarization data and from S-wave data alone. The S-wave polarization error, δ∈, is slightly lower for a type Il than for a type I mechanism. The type I mechanism solution indicates a predominately dip-slip faulting on a steeply dipping plane. The preferred solution is a type II mechanism with the following P nodal planes: strike N62°E, dip 82°S, (a plane); strike N22°W, dip 52°W, (b plane). Two solutions are possible: right lateral faulting which strikes northeast; or, left lateral faulting which strikes northwest. Both possible fault planes dip steeply.

The Denver Earthquake Sequence of March–April 1981

1983 ◽  
Vol 54 (2) ◽  
pp. 3-12
Author(s):  
G. A. Bollinger ◽  
Martha J. Adams ◽  
R. F. Henrisey ◽  
C. J. Langer
Keyword(s):  
Main Shock ◽  
Rocky Mountain ◽  
P Wave ◽  
Earthquake Sequence ◽  
Focal Mechanism Solution ◽  
Multiple Fracture ◽  
Normal Faulting ◽  
Composite Focal Mechanism ◽  
Rocky Mountain Arsenal ◽  
Variable Water

Abstract The Denver earthquake sequence of March–April 1981 was monitored by a network of four permanent and eight portable seismographs. In addition to the main shock (mb = 4.3) on 2 April, six microaftershocks (M < 2) during the subsequent two-week period were recorded and located. Five of those six events had epicenters within the most active area of the 1967–1968 Rocky Mountain Arsenal (RMA) sequence. A composite focal mechanism solution for the main shock and the six aftershocks showed a combination of reverse and strike-slip faulting (14% inconsistency in the 29 P-wave polarities) that is different from the predominantly normal faulting reported for the 1967–1968 RMA sequence. These different focal mechanisms, plus variable water-level response at the RMA well during the earthquake sequence in the 1960’s, may suggest the presence of a multiple fracture system in the source volume.

Analysis of non-double-couple source mechanisms in an area of induced seismicity, West Texas (USA).

2021 ◽  
Author(s):  
Savvaidis Alexandros ◽  
Roselli Pamela
Keyword(s):  
Stress Field ◽  
Time Domain ◽  
Seismic Moment ◽  
Moment Tensor ◽  
P Wave ◽  
Seismic Moment Tensor ◽  
Ground Displacement ◽  
Moment Tensors ◽  
West Texas ◽  
Double Couple

&lt;p&gt;In the scope to investigate the possible interactions between injected fluids, subsurface geology, stress field and triggering earthquakes, we investigate seismic source parameters related to the seismicity in West Texas (USA). The analysis of seismic moment tensor is an excellent tool to understand earthquake source process kinematics; moreover, changes in the fluid volume during faulting leads to existence of non-double-couple (NDC) components (Frohlich, 1994; Julian et al., 1998; Miller et al., 1998). The NDC percentage in the source constitutes the sum of absolute ISO and CLVD components so that %NDC= % ISO + %CLVD and %ISO+%CLVD+%DC=100%. It is currently known that the presence of NDC implies more complex sources (mixed shear-tensile earthquakes) correlated to fluid injections, geothermal systems and volcano-seismology where induced and triggered seismicity is observed.&lt;/p&gt;&lt;p&gt;With this hypothesis, we analyze the micro-earthquakes (M &lt;2 .7) recorded by the Texas Seismological Network (TexNet) and a temporary network constituted by 40 seismic stations (equipped by either broadband or 3 component geophones). Our study area is characterized by Northwest-Southeast faults that follow the local stress/field (SH&lt;sub&gt;max&lt;/sub&gt;) and the geological characteristic of the shallow basin structure of the study area. After a selection based on signal-to-noise ratio, we filter (1-50 Hz) the seismograms and estimate P-wave pulse polarities and the first P-wave ground displacement pulse in time domain. Then, we perform the full moment tensor analysis by using hybridMT technique (Andersen, 2001; Kwiatek et al., 2016) with a detailed 1D velocity model. The key parameter is the polarity/area of the first P-wave ground displacement pulse in time domain. Uncertainties of estimated moment tensors are expressed by normalized root-mean-square (RMS errors) between theoretical and estimated amplitudes (Vavricuk et al., 2014). We also evaluate the quality of the seismic moment tensors by bootstrap and resampling. In our preliminary results we obtain NDC percentage (in terms of %ISO and %CLVD components), Mw, seismic moment, P, T and B axes orientation for each source inverted.&lt;/p&gt;

Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake

1991 ◽  
Vol 81 (5) ◽  
pp. 1726-1736
Author(s):  
Susan L. Beck ◽  
Howard J. Patton
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
P Wave ◽  
Wave Spectra ◽  
Short Period ◽  
First Motion

Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.

Thrust and Conjugate Strike‐Slip Faults in the 17 June 2018 MJMA 6.1 (Mw 5.5) Osaka, Japan, Earthquake Sequence

2019 ◽  
Vol 90 (6) ◽  
pp. 2132-2141
Author(s):  
Yuqiang Li ◽  
Dun Wang ◽  
Shenghui Xu ◽  
Lihua Fang ◽  
Yifang Cheng ◽  
...  
Keyword(s):  
Aftershock Sequence ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
P Wave ◽  
Earthquake Sequence ◽  
Reverse Fault ◽  
Arrival Times ◽  
The North ◽  
Double Couple ◽  
First Motion

ABSTRACT The 17 June 2018 MJMA 6.1 (Mw 5.5) Osaka earthquake exhibits a large non–double‐couple component (∼26%), and its aftershock sequence shows a complicated spatial pattern. To better understand the ruptured faults, we relocate the earthquake sequence using P and S arrival times and waveform cross correlations and calculate the focal mechanisms of all MJMA≥2.5 (Mw≥2.3) earthquakes within three months after the mainshock using P‐wave first‐motion polarities and S/P amplitude ratios. Relocated aftershocks image several faults, the northeast‐striking strike‐slip fault, the north‐northwest‐striking reverse fault, and at least two small northwest‐striking features. P‐wave first motions of the mainshock indicate nearly a pure thrust mechanism. We deduce that the earthquake sequence started from a north‐northwest‐striking reverse fault and propagated to a northeast‐striking strike‐slip fault. The aligned strike‐slip aftershocks occurring in the vicinity of the northeast‐striking strike‐slip fault delineates the growth of several newly formed or reactivated northwest‐striking Riedel shears that are conjugated to the northeast‐striking strike‐slip fault.

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