Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra

Abstract

SUMMARY Estimating reliable depths for shallow seismic sources is important in both seismo‐tectonic studies and in seismic discrimination studies. Surface wave excitation is sensitive to source depth, especially at intermediate and short‐periods, owing to the approximate exponential decay of surface wave displacements with depth. A new method is presented here to retrieve earthquake source parameters from regional and teleseismic intermediate period (100–15 s) fundamental‐mode surface wave recordings. This method makes use of advances in mapping global dispersion to allow higher frequency surface wave recordings at regional and teleseismic distances to be used with more confidence than in previous studies and hence improve the resolution of depth estimates. Synthetic amplitude spectra are generated using surface wave theory combined with a great circle path approximation, and a grid of double–couple sources are compared with the data. Source parameters producing the best‐fitting amplitude spectra are identified by minimizing the least‐squares misfit in logarithmic amplitude space. The F‐test is used to search the solution space for statistically acceptable parameters and the ranges of these variables are used to place constraints on the best‐fitting source. Estimates of focal mechanism, depth and scalar seismic moment are determined for 20 small to moderate sized (4.3 ≤Mw≤ 6.4) earthquakes. These earthquakes are situated across a wide range of geographic and tectonic locations and describe a range of faulting styles over the depth range 4–29 km. For the larger earthquakes, comparisons with other studies are favourable, however existing source determination procedures, such as the CMT technique, cannot be performed for the smaller events. By reducing the magnitude threshold at which robust source parameters can be determined, the accuracy, especially at shallow depths, of seismo‐tectonic studies, seismic hazard assessments, and seismic discrimination investigations can be improved by the application of this methodology.

Multi-objective analysis of body and surface waves from the Market Rasen (UK) earthquake

“…10), and is similar to the best fitting double‐couple published by the BGS and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) (Table 6). This analysis of the Market Rasen earthquake has shown the usefulness of the joint application of recently developed methods of earthquake depth estimation (Fox 2007; Heyburn & Bowers 2008), as well as the potential of the MOO approach for combining observations from different data sets to estimate earthquake source parameters.…”

“…Furthermore, although the synthetic surface wave spectra better fit the observed spectra for a source depth of 22 km, the misfit‐depth plot in Fig. 4 shows that the misfits calculated for best‐fitting focal mechanisms at depths between around 18 and 34 km do not show large variations [statistical analysis of the misfits (Fox 2007) showed that a depth of 34 km was still within 95 per cent confidence limits]. Despite this uncertainty, the source mechanism in Table 4 is based on the hypothesis that the Market Rasen earthquake is at a depth of 22 km.…”

“…For example, frequency‐dependent notches that can provide a tight constraint on source depth are sometimes seen in observed Rayleigh wave amplitude spectra, and as source depth increases Rayleigh and Love wave spectral amplitudes drop appreciably (Patton 1998). Building on the work of Tsai & Aki (1970) and Patton (1998), Fox (2007) developed a method of estimating source depth, mechanism and scalar moment by modelling intermediate period (100–15 s) fundamental‐mode Rayleigh and Love wave amplitude spectra. Here the method of Fox (2007) is applied to surface waves recorded from the Market Rasen earthquake.…”

“…Building on the work of Tsai & Aki (1970) and Patton (1998), Fox (2007) developed a method of estimating source depth, mechanism and scalar moment by modelling intermediate period (100–15 s) fundamental‐mode Rayleigh and Love wave amplitude spectra. Here the method of Fox (2007) is applied to surface waves recorded from the Market Rasen earthquake.…”

“…To do this, two recently developed complementary methods of earthquake depth estimation are applied to the observed data. The first method uses the F statistic and its associated probability theory to help identify P and the depth phases pP and sP on teleseismic body‐wave seismograms (Heyburn & Bowers 2008), and the second method involves modelling regional surface wave amplitude spectra (Fox 2007). The study then focuses on finding a focal mechanism that is consistent with both data sets.…”