John Ristau

Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M w>∼3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude ( M L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M w with M L for New Zealand earthquakes. In addition to M w and M L, there are more than 330 teleseismic body-wave magnitude ( m b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M w. These are used to examine the relationship between M w, M L, and m b for New Zealand earthquakes. There is a clear distinction in the relationship of M L to M w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M L to be fairly consistent with M w, particularly for events with M w>∼4.5 and with ![Graphic][1] . Deep focus earthquakes have M L consistently larger than M w (more than a full magnitude unit for some events) with ![Graphic][2] . M w and m b are in fairly good agreement regardless of the depth, whereas m b estimates are consistently smaller than M L for deep events. This suggests that M L is overestimated for deep focus events and that the large M Ls are not the result of physical properties of the source. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Structure and kinematics of the Taupo Rift, New Zealand

The structure and kinematics of the continental intra-arc Taupo Rift have been constrained by fault-trace mapping, a large catalogue of focal mechanisms (N = 202) and fault slip striations. The mean extension direction of ~137° is approximately orthogonal to the regional trend of the rift and arc front (α = 84° and 79°, respectively) and to the strike of the underlying subducting Pacific Plate. Bending and rollback of the subduction hinge strongly influence the location, orientation, and extension direction of intra-arc rifting in the North Island. In detail, orthogonal rifting (α = 85–90°) transitions northward to oblique rifting (α = 69–71°) across a paleovertical-axis rotation boundary where rift faults, extension directions, and basement fabric rotate by ~20–25°. Toward the south, extension is orthogonal to normal faults which are parallel to, and reactivate, steeply dipping basement fabric. Basement reactivation facilitates strain partitioning with a portion of margin-parallel motion in the overriding plate mainly accommodated east of the rift by strike-slip faults in the North Island Fault System (NIFS). Toward the north where the rift and NIFS intersect, ~4 mm/yr strike slip is transferred into the rift with net oblique extension accommodating a component of margin-parallel motion. The trend and kinematics of the Taupo Rift are comparable to late Miocene-Pliocene intra-arc rifting in the Taranaki Basin, indicating that the northeast strike of the subducting plate and the southeast extension direction have been uniform since at least 4 Ma.

Strong shaking in recent New Zealand earthquakes

On 4 September 2010 a surface-rupturing crustal earthquake (Mw 7.1) struck the Canterbury Plains region of New Zealands South Island [Gledhill et al., 2011]. The Canterbury Plains is a region of relatively low seismicity in New Zealand, and the structure that ruptured was a previously unmapped fault (Figure 1a). Fortunately, even though parts of the region experienced liquefaction of unconsolidated sediments and sands—including neighborhoods of the city of Christchurch (population 377,000)—no fatalities occurred. Compared to the average New Zealand aftershock decay model, the aftershock sequence for the 2010 earthquake was relatively underproductive for the first 5 months. But on 22 February 2011 anMw 6.2 aftershock (teleseismic and regional estimates range from (Mw 6.1 to (Mw 6.3 with regional inversions favoring higher values) occurred within kilometers of the center of Christchurch (A. E. Kaiser et al., The (Mw 6.2 Christchurch earthquake of February 2011: Preliminary report, submitted to New Zealand Journal of Geology and Geophysics, 2011). The event increased the productivity of other aftershocks (Figure 1b). This particular aftershock was devastating, generating much more destruction than theMw 7.1 event, including more than 180 fatalities. Recorded peak ground acceleration (PGA) in the city was more than double the acceleration of gravity (g). Many of the poorly consolidated, low-shear-wave-velocity soils liquefied during the shaking. Damage estimates reached approximately US

A Revised Local Magnitude (ML) Scale for New Zealand Earthquakes

15 billion, making the aftershock New Zealands costliest natural disaster.

Overview of Moment Tensor Analysis in New Zealand

In this study, a new local magnitude ( M L) scale is developed for New Zealand and adjacent offshore regions. SeisComP3 (SC3) has been in use for earthquake analysis in New Zealand since September 2012 with the original Richter (1935) log A attenuation relationship for calculating M L. The attenuation characteristics of New Zealand differ significantly from southern California, where M L was originally defined, and therefore result in M L that is consistently high when compared with moment magnitude ( M w). Using M w from 528 regional moment tensor solutions along with peak observed amplitudes, a new log A curve is derived, along with station correction factors that define a revised M L scale for New Zealand earthquakes that is more consistent with M w. The new log A curve is similar to the original Richter (1935) definition at hypocentral distances of ∼100–200  km but differs significantly at closer and farther distances. The new M L is more consistent with M w across New Zealand, including crustal earthquakes and earthquakes below the crust. The California Institute of Technology (Caltech)–U.S. Geological Survey seismic processer (CUSP) system was used for earthquake analysis prior to SC3, and previous studies have derived regression relationships relating CUSP M L with M w. After applying the regression relationships to CUSP M L, we found very good agreement between CUSP M L and the new SC3 M L, which is important for developing a consistent M L between different catalogs. Online Material: Table of station corrections for the New Zealand seismograph network.

Applications of Moment Tensor Solutions to the Assessment of Earthquake Hazard in Canada

The determination of earthquake source parameters is of fundamental importance in seismological research. Moment tensor analysis involves fitting theoretical waveforms to observed broadband waveforms and inverting for the moment tensor elements, and allows for the calculation of focal mechanism (strike, dip, and rake), seismic moment (M0), moment magnitude (Mw) which is calculated directly from M0, and centroid depth of an earthquake. A comprehensive catalogue of moment tensor solutions is of great importance in seismic hazard analysis and tectonic studies. For example, seismic hazard estimates typically use Mw in earthquake forecasts and risk analysis, and moment release rates along plate boundaries are important in calculating predicted plate motions in tectonic studies.

The Darfield (Canterbury, New Zealand) Mw 7.1 Earthquake of September 2010: A Preliminary Seismological Report

Centroid Moment Tensor solutions (CMT’s) provide valuable information on the physics of an earthquake source, focal depth, and seismic moment. The earthquake rupture is described in terms of nine generalised force couples (a 3 × 3 matrix) that represent shear dislocation and volume change (see Jost and Herrmann 1989).