scholarly journals Regional Pn Body-Wave Magnitude Scale m b (Pn ) for Earthquakes Along the Northern Mid-Atlantic Ridge

2017 ◽  
Vol 122 (12) ◽  
pp. 10,321-10,340 ◽  
Author(s):  
Won-Young Kim ◽  
Lars Ottemöller
Keyword(s):  
Body Wave ◽  
Magnitude Scale ◽  
Mid Atlantic Ridge ◽  
Body Wave Magnitude

A Regional Sn Magnitude Scale mb(Sn) and Estimates of Moment Magnitude for Earthquakes along the Northern Mid-Atlantic Ridge

2020 ◽  
Vol 110 (6) ◽  
pp. 3158-3173
Author(s):  
Won-Young Kim ◽  
Lars Ottemöller ◽  
Paul G. Richards
Keyword(s):  
Body Wave ◽  
Strike Slip ◽  
Moment Magnitude ◽  
Magnitude Scale ◽  
Transform Faults ◽  
Midocean Ridges ◽  
Least Squares Fit ◽  
Short Period ◽  
Mid Atlantic Ridge ◽  
Northern Atlantic

ABSTRACT We present a regional short-period Sn magnitude scale mb(Sn) for small earthquakes along the northern Mid-Atlantic Ridge. Surface-wave magnitudes, teleseismic body-wave magnitudes, and seismic moments cannot be reliably determined for small earthquakes along this and other midocean ridges. Local magnitudes that rely on Lg waves are likewise not generally useful due to the substantial oceanic paths for earthquakes along midocean ridges. In contrast, Pn and Sn arrivals for earthquakes along the northern Mid-Atlantic Ridge are generally well recorded by the existing seismographic networks, and, in fact, Sn arrivals are larger than Pn arrivals for about one-third of the ridge events. For this reason, we have developed a new regional Sn magnitude scale that is tied to Mw, so that seismic moments can be readily approximated. In our least-squares fit of peak amplitudes from 120 earthquakes having a published moment magnitude, we solved for the attenuation curve for paths in the oceanic mantle lid, for event magnitude adjustments (EMAs) to account for differences between long-period moment magnitude Mw and short-period Sn magnitude, and for station corrections. We find regional EMAs that are well correlated with the style of faulting: they are positive for normal-faulting earthquakes along spreading ridges and negative for strike-slip earthquakes along transform faults. These source-specific EMAs are approximately +0.11 magnitude units for normal-fault earthquakes and −0.26 magnitude units for strike-slip earthquakes on transform faults, and are consistent with previously reported apparent stresses from these regions. The amplitude distance curve determined for Sn for the northern Atlantic Ocean is similar to that determined for Pn in the northern Atlantic out to a distance of about 500 km, but at larger distances is more similar to the western U.S. Pn curve, likely reflective of the warmer temperatures at greater upper-mantle depths.

The relationship between teleseismic body-wave magnitude m and local magnitude ML from New Zealand earthquakes

1972 ◽  
Vol 62 (1) ◽  
pp. 1-11
Author(s):  
S. J. Gibowicz
Keyword(s):  
New Zealand ◽  
Linear Relationship ◽  
Body Wave ◽  
Local Magnitude ◽  
The Relationship ◽  
Shallow Focus ◽  
Body Wave Magnitude

Abstract Relationships between the magnitudes ML and m for 123 New Zealand earthquakes occurring between 1950 and 1967 and having 4.6 ≦ ML ≦ 7.3 have been found. Deep- and shallow-focus shocks were considered separately. There is a linear relationship between ML and m, the slope being the same for both deep and shallow events. Values of ML for deep events are consistently 0.5 magnitude larger than those for shallow events having the same value of m. The relationship between m and ML for New Zealand earthquakes differs significantly from that obtained by Gutenberg and Richter in California.

Integrated and frequency-band magnitude, two alternative measures of the size of an earthquake

1970 ◽  
Vol 60 (3) ◽  
pp. 917-937 ◽  
Author(s):  
B. F. Howell ◽  
G. M. Lundquist ◽  
S. K. Yiu
Keyword(s):  
Transfer Function ◽  
Frequency Domain ◽  
Frequency Band ◽  
Transfer Functions ◽  
Body Wave ◽  
Average Amplitude ◽  
Equivalent Quantity ◽  
Short Period ◽  
Alternative Measures ◽  
Body Wave Magnitude

Abstract Integrated magnitude substitutes the r.m.s. average amplitude over a pre-selected interval for the peak amplitude in the conventional body-wave magnitude formula. Frequency-band magnitude uses an equivalent quantity in the frequency domain. Integrated magnitude exhibits less scatter than conventional body-wave magnitude for short-period seismograms. Frequency-band magnitude exhibits less scatter than body-wave magnitude or integrated magnitude for both long- and short-period seismograms. The scatter of frequency-band magnitude is probably due to real azimuthal effects, crustal-transfer-function variations, errors in compensation for seismograph response, microseismic moise and uncertainties in the compensation for attenuation with distance. To observe azimuthal variations clearly, the crustal-transfer functions and seismograph response need to be known more precisely than was the case in this experiment, because these two sources of scatter can be large enough to explain all of the observed variations.

Systematic difference in the ISC body-wave magnitude-seismic moment relationship between intermediate and deep earthquakes

1992 ◽  
Vol 82 (2) ◽  
pp. 819-835
Author(s):  
Keiko Kuge
Keyword(s):  
Seismic Moment ◽  
Body Wave ◽  
Systematic Difference ◽  
P Wave ◽  
Local Structures ◽  
Dependent Relationship ◽  
Deep Earthquakes ◽  
Amplitude Data ◽  
Body Wave Magnitude ◽  
Distance Correction

Abstract There exists a systematic difference in the ISC body-wave magnitude (mbISC) - seismic moment (M0) relationship between intermediate and deep earthquakes around Japan. For earthquakes with the same M0, the mbISC for intermediate events is larger than that for deep events by 0.2 to 0.3 units. The mbISC discrepancy is attributed to the depth-distance correction in the procedure for determining the mbISC; a larger depth-distance correction (≈ 0.2) is made for the intermediate events than the deep events, irrespective of station distance. The discrepancy disappears if no depth-distance correction is made. I observe no depth-dependent relationship between the M0 and the JMA magnitudes (MJMA), which make a different depth-distance correction. No significant depth-dependent mbISC discrepancy appears in other regions; for example, around Tonga, I observe larger ISC P-wave amplitudes from deep events than intermediate events, which could cancel the effect of the depth-distance correction. The depth-dependent mbISC - M0 relationship around Japan is observed irrespective of whether the magnitudes are determined using the amplitude data at far or near stations, or whether stations are used in the dipping direction of the slab or not. The mbISC discrepancy for the same M0 cannot arise from local structures, radiation patterns, and station coverages. This is not attributable to the dataset of the M0 itself because no significant depth-dependent relationship between M0 and MJMA is observed.

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