Shutian Ma

Focal Depths for Small to Moderate Earthquakes (mN ≥2.8) in Western Quebec, Southern Ontario, and Northern New York

Regional depth phase modeling is a useful technique to constrain earthquake focal depths for events within sparse networks. We demonstrate how regional depth phases can be used to obtain focal depths for hundreds of earthquakes with m N ≥2.8 that occurred from 1980 to 2004 in southern Ontario, western Quebec, and northern New York State. We discuss the development of regional depth phases with distance, their use in focal depth determinations by matching waveforms to synthetics, and possible sources of error. In general, focal depths can be determined to within about 3 km when there is at least one record at regional distances with clearly identifiable depth phases such as sPmP or sPg . In southwestern Ontario and northern New York, focal depths range from 2 to 15 km. In areas of western Quebec and along the Ottawa River valley in Ontario, focal depths range from 2 to 25 km. More than half of the earthquakes in a cluster of activity near Maniwaki in western Quebec are deeper than 20 km. All earthquakes deeper than 20 km occurred north of about 45.3° N. There is an intriguing tendency for earthquake depths to cluster near 8 km, 12 km, 15 km, and 22 km. Another interesting observation is that there are several earthquake pairs or small groups with very similar epicenters but quite different depths. The most active regions, in western Quebec and along the Ottawa River valley, tend to exhibit a relatively wide range of focal depths along broad zones that trend roughly northwest, suggesting that seismicity is occurring along diffuse northwest-trending faults throughout the crust. The wide range of focal depths may indicate the potential for a larger maximum scale of fault rupture and serve as a contemporary marker of locations where large events have occurred in the recent geologic past.

Focal Depth Determination for Moderate and Small Earthquakes by Modeling Regional Depth Phases sPg, sPmP, and sPn

There are three usable regional depth phases, sPg , sPmP , and sPn , and their corresponding reference phases, Pg , PmP , and Pn . The differential time between each depth phase and its reference phase can be used to estimate earthquake focal depth. We have developed a method to determine focal depth for moderate and small earthquakes by using a regional depth-phase modeling (RDPM) method. We used a default focal mechanism to generate the differential times for all earthquakes. To estimate the reliability of the modeled focal depths, we compared our solutions with those obtained by other methods and found the consistency is good. Because the focal depths estimated by RDPM are model dependent, we tested the extent of the dependency and found that a 10% error in the crustal model may generate a 10%–15% error in the modeled depth. The absolute error is determined by the error in the crustal model and the focal depth itself. We found that earthquake location errors have only a small effect on the modeled focal depths. By analyzing synthetic and observed waveforms, we found a distance window within which sPmP and PmP are well developed, and, within the window, the P portion of the waveform is relatively simple and sPmP and PmP are easy to identify. We also demonstrated that the assumptions of sPmP and PmP are correct, that regional depth phases are not developed or not discernible in some regions, and that regional depth phases have special features that can be used to identify those phases.

Intraplate Seismicity of a Recently Deglaciated Shield Terrane: A Case Study from Northern Ontario, Canada

The economy of northern Ontario, Canada, is heavily dependent on mining, so accurate knowledge of seismicity is important for the safe design and op- eration of mines and other critical facilities, including a proposed underground repos- itory for nuclear waste. In this study, we analyzed 537 cataloged earthquakes that occurred from 1980 to 2006. Seismicity is mainly concentrated in topographically elevated Archean terranes northwest of Lake Superior and in the James Bay and Ka- puskasing regions. We analyzed waveforms to determine the focal depth for 331 re- corded events, using the regional depth-phase modeling (RDPM) method coupled with surface-wave relative-amplitude analysis. The majority of events are shallow ( 350 m), although in the east- ern part this pattern breaks down and some deeper earthquakes (>12 km) are ob- served. Based on a moving-window event-counting technique, we show that distinct spatial clusters of seismicity can be delineated that are statistically significant relative to background seismicity levels. A particularly active cluster is located within James Bay, where focal depths range from a few kilometers to more than 20 km. Another cluster near Kapuskasing contains deep-focus events and may occur along a hot spot track that runs through western Quebec. Near Dryden, a shallow (∼1 km) earthquake swarm concentrated in a 1 × 1 km region commenced in May 2002, faded, and then started up again in February 2003. Shallow mining-induced events are also common around Sudbury, a major world center for nickel mining. The overall pattern of seis- micity appears to correlate with upper-mantle P-wave velocity anomalies, suggesting that lateral variations in mantle rheology may play a significant role in controlling intraplate seismicity of shield areas. It is also likely that crustal stresses caused by glacial isostatic adjustment are an important factor, although the correlation of seis- micity with uplift rate is not as clear. Online Material: Focal depth solutions for earthquakes in northern Ontario, Canada.

The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust

On 20 October 2005 at 21:16 UTC, a moderate earthquake (mN 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67 N, 80.46 W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low- and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were in- stalled in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral pa- rameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M0 4.5 10 14 N m and corner frequency of 3.7 Hz, based on a spectral fit using Brunes source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reverse- sense of slip and northwest-striking nodal planes. The reverse mechanism and mid- crustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located 200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie 200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precam- brian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.