Peter Bormann

ASSESSMENT OF THE NATURAL FREQUENCY OF THE SEDIMENTARY COVER IN THE COLOGNE AREA (GERMANY) USING NOISE MEASUREMENTS

Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique.

Are new data suggesting a revision of the current M w and M e scaling formulas

The paper looks first into the history of the derivation of the currently common formulas for calculating seismic moment magnitude Mw and energy magnitude Me into the type of data and relationships available in these years and the parameter assumptions made. The general relationship between Mw and Me is analysed and formulated in physical terms. The original Mw- and Me-defining relationships are then confronted with equivalent relationships derived on the basis of rich modern magnitude data measured according to recently accepted International Association of Seismology and Physics of the Earth’s Interior (IASPEI) standards for (a) 20-s surface-wave data and (b) broadband body P wave data as well as M0 and ES data based on digital broadband waveform inversion or integration. The agreement between old and new data and derived relationships is of different quality. The Richter logES-MS relationship, which has been instrumental for deriving the current standard Mw formula, could be very well reproduced with orthogonally regressed MS(20) and logES data, provided that the latter were not corrected for source mechanism-dependent radiation. In contrast, the relationships between old and modern mB-logES as well as mB-MS(20) data pairs deviate significantly from the respective Gutenberg and Richter relationships. Also the average ES/M0 ratio assumed by Kanamori when deriving his Mw formula differs from those of respective recent data sets. But the various differences between old and new data and data relationships compensate each other partially when deriving related Mw and Me formulas. Therefore, they do not justify the modification of the existing scaling formulas, also for very pragmatic reasons. On the other hand, most striking is the so far not yet considered and by far best correlation that exists between the IASPEI body-wave magnitude standard mB(BB) and seismic energy ES, both estimated via P wave broadband records. The scatter of the logES-mB(BB) data pair plots is only half of that of logES-MS(20). This questions the appropriateness of the current exclusive scaling of teleseismic Me to the practically monochromatic long-period 20-s surface-wave magnitude MS. The potential advantage of a complementary Me formula, which scales the currently common teleseismic broadband P wave ES data to P wave broadband mB(BB), as well as the benefit of fast joint determination and interpretation of Mw and Me in general, is discussed.