Locations of mid‐oceanic earthquakes constrained by seafloor bathymetry

Abstract

[1] Earthquakes associated with the creation of new oceanic crust are difficult to locate precisely because they normally occur far away from seismic stations. We assume that epicenters of these events can be constrained by seafloor topography and use the Harvard centroid moment tensor (CMT) focal mechanism of an interplate earthquake to relate the event to either a transform fault (a parallel in a coordinate system defined by the pole of rotation between two plates) or a mid-oceanic ridge (a meridian). Using arrival times from the International Seismological Center (ISC) Bulletin, we have relocated about 1500 globally distributed events for the time period 1976-1998. The ISC locations can be as much as 70 km away from appropriate topographic features. Our new locations support that large interplate earthquakes occur on or very close to the primary bathymetric features. For earthquakes with unknown mechanism or magnitude below the CMT threshold or predate the CMT catalog, we use a simultaneous Joint Hypocenter Determination procedure with respect to the already relocated Master Events (JHDME). We invert for earthquakes first for the Romanche Fracture Zone (RFZ) and then for a region extending from 30°N to 30°S. Seismicity agrees well with the main active ridge or transform. However, a small number of events might be related to the secondary bathymetry features. The improved locations for mid-oceanic earthquakes based on bathymetry could help improve our understanding of the oceanic crustal and deep Earth structure, seismic slip and budget constraint along plate boundaries, and dynamics of plate tectonics.

Comparison of mid‐oceanic earthquake epicentral differences of travel time, centroid locations, and those determined by autonomous underwater hydrophone arrays

“…Figure 2a plots the 3079 AUH locations determined by four or more hydrophones, Figure 2b for the 2084 AUH locations determined by five or more hydrophones, and Figure 2c plots the 669 AUH locations determined by six hydrophones. Clearly, the seismicity characteristics in Figure 2a is different from that in Figure 2d, obtained by Pan et al [2002] through joint hypocenter determination with master events for 1134 prior‐to‐1998 events (body wave magnitude m b ranges from 4.3 to 6.3, with the majority above 4.7) between MAR −30° and 30°. Compared to Figure 2a, seismicity in Figure 2d is tighter around MAR axis and there are fewer events off the main MAR bathymetry low.…”

“…Earthquakes along mid‐oceanic ridges and transform faults are difficult to locate accurately because they typically occur far away from seismic stations and the station coverage is often poor. ISC epicenters often show large and systematic deviations away from the bathymetric features that define mid‐oceanic ridges and transform faults [ Pan et al , 2002]. This is partly because the ISC uses a one‐dimensional (1‐D) Earth model (i.e., Jeffreys‐Bullen travel time tables [ Jeffreys and Bullen , 1940]), which does not account for lateral velocity heterogeneity in the real Earth.…”

“…This is partly because the ISC uses a one‐dimensional (1‐D) Earth model (i.e., Jeffreys‐Bullen travel time tables [ Jeffreys and Bullen , 1940]), which does not account for lateral velocity heterogeneity in the real Earth. Therefore Pan et al [2002] introduced a single‐event relocation technique based on high‐resolution bathymetry maps. For an interplate earthquake, they used the Harvard CMT focal mechanism to relate the event to either a transform fault (a parallel in a coordinate system defined by the pole of rotation between two plates) or a mid‐oceanic ridge (a meridian).…”

“…While we have good understanding of larger mid‐oceanic earthquakes (moment magnitude M w > 5 is required for a good CMT solution) that have good station coverage, locations of many smaller‐magnitude earthquakes related to seafloor spreading remain poorly constrained. Multievent location techniques [ Pan et al , 2002; J. Pan et al, Different cluster relocation techniques and their applications to mid‐oceanic earthquakes, submitted to Journal of Geophysical Research , 2004] can help to constrain the locations of these events but are not sufficient to enable association of a particular earthquake with specific structural features.…”

Global Ground Truth Data Set with Waveform and Arrival Data

“…Ritzwoller et al, 2003;Bondár et al, 2004b), seafloor bathymetry (Pan et al, 2002), satellite imagery (e.g. Fisk, 2002), InSAR interferometry (e.g.…”