Seismicity and structure of the Orozco transform fault from ocean bottom seismic observations

Abstract

In this thesis, seismic waves generated by sources ranging from 2.7 kg shots of TNT to magnitude 5 earthquakes are studied in order to determine the seismic activity and crustal structure of the Orozco transform fault. Most of the data were collected by a network of 29 ocean bottom seismometers (OBS) and hydrophones (OBH) which were deployed as part of project ROSE (Rivera Ocean Seismic Experiment). Additional information is provided by magnetic anomaly and bathymetric data collected during and prior to ROSE and by teleseismic earthquakes recorded by the WWSSN (Worldwide Seismic Station Network). In Chapter II, the tectonic setting, bathymetry and teleseismic history of the Orozco Fracture Zone are summarized. Covering an area of 90 x 90 km which includes ridges and troughs trending both parallel and perpendicular to the present spreading direction (approximately east-west), the bathymetry of the transform portion of the fracture zone does not resemble that of other transform faults which have been studied in detail. A detailed study of one of the largest teleseismic earthquakes (mb=5.1) indicates right lateral strike-slip faulting with a strike parallel to the present spreading direction and a focal depth of less than 5 km. The moment sum from teleseismic earthquakes suggests an average fault width of at most a few kilometers. Because the teleseismic earthquake locations are too imprecise to define the present plate boundary and the magnetic anomaly data are too sparse to resolve the recent tectonic history, more questions are raised than are answered by the results in this chapter. These questions provide the focus for the study of the ROSE data. Chapter III contains an examination of the transfer function between seafloor motion and data recorded by the MIT OBS. The response of the recording system is determined and the coupling of the OBS to the seafloor during tests at two nearshore sites is analysed. Applying these results to the ROSE data, we conclude that the ground motion in the absence of the instrument can be adequately determined for at least one of the MIT OBS deployed during ROSE. Hypocentral parameters for 70 earthquakes, calculated for an assumed laterally homogeneous velocity structure which was adapted from the results of several refraction surveys in the area, are presented in Chapter IV. Because of the large number of stations in the ROSE network, the epicentral locations, focal depths and source mechanisms are determined with a precision unprecedented in marine microseismic work. Relative to the assumed model, most horizontal errors are less than ±1 km; vertical errors are somewhat larger. All epicenters are within the transform region of the Orozco Fracture Zone. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough which forms the northern boundary of the transform zone (region 1). Most well determined depths are very shallow (<4km) and no shallowing of activity is observed as the rise-transform intersection is approached. In fact, the deepest depths (4-10km) are for earthquakes within 10 km of the intersection; these apparent depth differences are supported by the waveforms recorded a t the MIT OBS. First motion polarities for all but two of the earthquakes in region 1 are compatible with right lateral strike-slip faulting along a nearly vertical plane striking parallel to the spreading direct ion. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity is more scattered than for the first group and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. No difference can be resolved between the stress drops or b values in the two regions. In Chapter V, lateral variations in the crustal structure within the transform region are determined and the effect of these structures on the results of the previous chapter is evaluated. Several data sources provide information on different aspects of the crustal structure. Incident angles and azimuths of body waves from shots and earthquakes measured at one of the MIT OSS show systematic deflections from the angles expected for a laterally homogeneous structure. The effect of various factors on the observed angles and azimuths is discussed and it is concluded that at least some of the deflection reflects regional lateral velocity heterogeneity. Structures which can explain the observations are found by tracing rays through three dimensional velocity grids. High velocities are inferred at upper mantle depths beneath a shallow, north-south trending ridge to the west of the OBS, suggesting that the crust under the ridge is no thicker, and perhaps thinner, than the surrounding crust. Observations from sources in region 2 suggest the presence of a low velocity zone in the central transform between the sources and the receiver. That the presence of such a body provides answers to several of the questions raised in Chapter IV about the hypocenters and mechanisms of earthquakes in region 2 is circumstantial evidence supporting this model. These proposed structures do not significantly affect the hypocenters and fault plane solutions for sources in region 1. The crustal velocity structure beneath the north-south trending ridges in the central transform and outside of the transform zone is determined by travel time and amplitude modeling of the data from several lines of small shots recorded at WHOI OBH. Outside of the transform zone, a velocity-depth structure typical of oceanic crust throughout the world oceans is found from three unreversed profiles: a 1 to 2 km thick layer in which the velocity increases from about 3 to 6.7 km/sec overlies a 4 to 4.5 km thick layer with a nearly constant velocity of 6.8 km/sec. A reversed profile over one of the north-south trending ridges, on the other hand, indicates an anomalous velocity structure with a gradient of 0.5 sec-1 throughout most of the crust ( from 5.25 km/sec to 7.15 km/sec over 3.5 km). A decrease in the gradient at the base of the crust to about 0.1 sec-1 and a thin, higher gradient layer in the upper few hundred meters are also required to fit the travel time and amplitude data. A total crustal thickness of about 5.4 km is obtained. An upper mantle velocity of 8.0 to 8.13 km/sec throughout much of the transform zone is determined from travel times of large shots of TNT recorded at MIT and WHOI instruments. "Relocations" of the large shots relative to the velocity model assumed in Chapter IV support the conclusion from the ray tracing that results from region 2 may be systematically biased because of lateral velocity heterogeneity whereas results from region 1 are not affected. In the last chapter, the results on crustal structure and seismicity are combined in order to define the present plate boundary and to speculate on the history of the present configuration.