Rotational Seismology with a Quartz Rotation Sensor

We describe the construction and performance of a new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS). The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. The noise floor is measured to be ∼45 pico-radians/root (Hz) near 1 Hz, and the resonant period of the sensor is about 10 s, making it a broadband sensor. Among similarly sized broadband rotation sensors, this represents more than two orders of magnitude improvement in noise floor near 0.1 Hz. We present measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

Detecting and locating seismic events with using USArray as a large antenna

We design an earthquake detection and location algorithm that explores coherence and characteristic behavior of teleseismic waves recorded by a large-scale seismic network. The procedure consists of three steps. First, for every tested source location we construct a time-distance gather by computing great-circle distances to all stations of the network and aligning the signals respectively. Second, we use the constructed gather to compute a Tau-P transform. For waves emitted by teleseismic sources, the amplitude of this transform has a very characteristic behavior with maxima corresponding to different seismic phases. Relative location of these maxima on the time-slowness plane strongly depends on the distance to the earthquake. To explore this dependence, in a third step, we convolve the Tau-P amplitude with a time-slowness filter whose maxima are computed based on prediction of a global travel-time calculator. As a result of this three-step procedure, we obtain a function that characterizes a likelihood of occurrence of a seismic event at a given position in space and time. We test the developed algorithm by applying it to vertical-component records of USArray to locate a set of earthquakes distributed around the Globe with magnitudes between 6.1 and 7.2.