Determination of an Empirical Local Magnitude Formula for Northern Oklahoma

English Abstract:

Computation of accurate earthquake magnitudes is of vital importance for a wide range of applications. For regional networks detecting events over several hundred kilometres, distance correction terms play an important role in determining the magnitude of an event. Standard distance correction terms such as Hutton and Boore (1987) may have a significant bias with distance if applied in a region with different attenuation properties, resulting in an incorrect magnitude. We present data from a network of broadband seismometers installed in northern Oklahoma. Our database consists of 254 events between magnitude 2.0 and 4.5, distributed evenly across the network, to determine distance correction terms applicable to northern Oklahoma. We find that existing models show a bias based on hypocentral distance. Observed amplitude measurements demonstrate that there is a significant Moho bounce effect that mandates the use of a trilinear attenuation model in order to avoid bias in the distance correction terms. We present two different approaches of local magnitude calibration. The first maintains the classic definition of local magnitude as proposed by Richter. The second method calibrates local magnitude so that it agrees with moment magnitude where a regional moment tensor can be computed. To this end, regional moment tensor solutions and moment magnitudes are computed for 37 of the largest events in the database to allow calibration of local magnitude to moment magnitude. For both methods the new formula results in magnitudes systematically lower than previous values computed with Eaton’s (1992) model. We compare the resulting magnitudes and discuss the benefits and drawbacks of each method. Our results highlight the importance of determining accurate distance correction terms for magnitude computation in regional networks.