Overview

The deployment phase of CENS equipment began this year within MASE (Middle America Subduction Experiment), a collaboration involving UCLA, the California Institute of Technology (CIT), and the Universidad Nacional Autónoma de México (UNAM). The CENS 50 radio-linked sites joined the 50 stand-alone sites of CIT. Scientifically we will be mapping the subducted slab beneath Mexico and examining slow earthquakes which have been observed at this subduction zone, volcanic earthquakes and the propagation of seismic waves in Mexico City. The array has already yielded receiver function images of the slab 10 times clearer than previous images due to the density of the array. Technologically, we have already begun a comparison between the 50 instruments of our wireless array and the 50 instruments of a stand alone array that have been installed by CIT.

Our original goal of constructing a broadband seismic network was to make data collection easy and reliable in remote regions. This includes rapid assessment of station health and aiding local networks in earthquake locations. The MASE experiment forced some rapid development of the technology which led to field testing of some software, but the majority of stations are now collecting data with few problems. Two UCMEXUS grants have been acquired to facilitate UCLA’s involvement. We are beginning to develop an inversion algorithm of the data for a 2d velocity tomography. We are hoping the images are clear enough to answer some questions about magma movement from the subducting slab to the surface volcanoes. The Mexico volcanic belt makes an excellent comparison to back arc volcanic belts world wide including the Cascade Mountains extending from Washington State to northern California.

Approach

50 broadband seismometers make up the seismic data network with the CENS DCC and 802.11 radios. The seismic stations are placed roughly every 5 km linked by radios with some repeater stations so that the data packets are routed to one of 4 base stations (Cuernavaca, Mexico City, Pachuca, and Huejutla). Each base station is connected to the internet, allowing us to transfer data from there to data repositories as well as check stations remotely in real-time. Due to time, money, and sometimes physical constraints (e.g. a mountain between 2 stations) a line of 4 stations is cuttoff from the rest of the Huejutla line, there are 2 stations directly connected to the interenet with no radio links, and there are an additional 4 standalone sites. One of the sites in the line of 4 has a large hard disk that acts as the archival point for those stations. In the pure standalone sites, the CDCC only collects data. We must go to these sites once a month to retrieve data. Other than these 10 sites, all stations are connected via 802.11 radios.

Table 1: List of sites and coordinates. Data flow goes in direction of "Following CDCC". Yellow sites are sinks for radio linked sites. Orange are the line of 4 separated from the rest of the Huejutla line. Green are stand-alone sites. Blue are directly connected to the internet with no radio links. Codes are reserved for stations with a seismometer. TONO had a seismometer, but it was moved to TONI due to seismic noise at TONO.

CDCC	Following CDCC	Lat	Lon	Name	Code
42	105	21º34.332'	98º22.135'	San Francisco	SAFR
105	200	21º31.444'	98º22.820'	Tempoal (repeater)
183	200	21º31.348'	98º22.847'	Tempoal	TEMP
22	200	21º28.332'	98º21.285'	El Cantarito	CANT
200	196	21º25.281'	98º21.831'	Cirio	CIRI
180	200	21º22.357'	98º21.310'	El Rodeo	RODE
160	189	21º20.004'	98º21.084'	El Palmar	ELPA
189	196	21º16.478'	98º21.530'	Planton Sanchez	PLSA
196	184	21º13.893'	98º22.330'	Tierra Blanca	TIBL
75	92	21º11.445'	98º24.341'	Amatitlan de Arriba	AMAR
92, 103	(201.134.249.149)	21º09.301'	98º22.841'	Huejutla (repeater)
64	103	21º09.301'	98º22.841'	Huejutla	HUEJ
184	92	21º04.690'	98º31.822'	Ixcatlan (repeater)
187	184	21º04.690'	98º31.822'	Ixcatlan	IXCA
87	187	21º01.870'	98º34.017'	Tianguis	TIAG
115	209	20º58.916'	98º43.939'	Chinconcuac	CHIO
209	97	20º56.004'	98º43.779'	Tlaltepingo	TLAL
97	121	20º51.30'	98º44.63'	Ocotlan	OCOL
121	sink	20º49.887'	98º45.795'	Pemuxtitla	PEMU
49	stand-alone	20º08.453'	98º40.894'	Mineral del Monte	MIMO
163	105	20º05.750'	98º42.032'	Pachuca	PACH
96	(200.57.61.5)	20º07.685'	98º44.102'	Pachuca UAEH Rectoria
55, 89	96	20º06.357’	98º44.216’	Cubitos (repeater)
149	55	20º05.270’	98º47.485’	Pachuca Sur	PASU
98	104	20º02.018’	98º48.422'	sur de Pachuca	SUPA
105	104	19º59.409’	98º51.818’	San Pedro	SAPE
104	89	19º58.432’	98º51.703'	Zacualtipan (repeater)
100	104	19º57.061’	98º52.395’	tierra publico	KM67
99	100	19º54.061’	98º54.588’	Hospital Psiquiatrica	PSIQ
63	190	19º51.996’	98º55.703’	El Cid	ECID
190	100	19º49.053’	98º55.578’	Tizayuca	TIZA
156	89	19º47.304'	98º58.732'	Banco de Material (repeater)
80	156	19º47.060’	98º58.819’	San Lucas	SNLU
90	156	19º45.039’	98º59.772’	Santa Lucia	SALU
91	41	19º41.975’	98º58.849’	Tecamac	TECA
83	41	19º41.166’	99º02.772’	Tonnanitla	TONN
41	156	19º37.128’	99º05.130’	Coacalco	COAC
210	161	19º35.492’	99º06.831’	Pico Tres Padres	PTRP
136	stand-alone	19º32.010’	99º08.530’	El Arbolillo	ARBO
141	161	19º29.581'	99º06.662'	Estazuela	ESTA
182	(200.67.205.246)	19º25.5'	99º07.22'	Museo de Luz	MULU
82	stand-alone	19º23.224'	99º09.448'	Cires	CIRE
20	(192.100.201.25)	19º22.429’	99º10.957’	Mixcoac	MIXC
212, 161	(132.248.182.169)	19º19.589’	99º10.526’	UNAM, CU	CUIG
85	stand-alone	19º16.231'	99º09.124'	Tepepan	TEPE
88	212	19º12.634’	99º08.730’	UNAM, Topilejo	TONI
185	88	19º12.553’	99º09.161’	UNAM, Topilejo (repeater)	TONO
17	185	19º09.495’	99º08.723’	Topilejo Sur	TOSU
143	185	19º05.302’	99º08.887’	Volcan Chichinautzin	CHIC
95	106	19º03.604’	99º13.006’	Cerro Tres Cumbres	PTCU
53	106	19º01.890'	99º16.241'	Huitzilac	VLAD
18	(132.248.41.126)	18º59.974’	99º14.422’	Cuernavaca Norte (repeater)
152	18	18º59.974’	99º14.422’	Cuernavaca Norte	CUNO
81	48	18º55.740’	99º13.322’	Cuernavaca Centro	CUCE
48	107	18º55.740’	99º13.322’	Cuernavaca Centro (repeater)
107	152	18º52.314’	99º11.851’	Jiutepec	JIUT
69	152	18º52.567’	99º12.211’	Palmida – repeater
77	69	18º49.720’	99º14.633’	Temixco	TEMI
202	48	18º47.017’	99º12.960’	Municipal Xochitepec	XOCH
19	76	18º44.779’	99º14.989’	Atlacholoaya	ATLA
76	53	18º42.050’	99º14.961’	Apotla	APOT
106	48	18º39.132’	99º15.657’	San José Vista Hermosa	SJVH

Figure 1. Installation of site COAC. Picture at left is full site, including solar panel and antenna. Each site has 1 hole with the seismometer that is inaccessible through the experiment, and another hole with the battery, data logger, and CDCC. The seismometer installation is shown at center. The picture at right shows a technician servicing the second hole.

Systems Experiments

We found throughout the course of installation that distance between sites is largely dependent on elevation. Sites on the sides of mountains typically have a good signal at least 50 km away. We found that the original linear configuration we had imagined was not possible over the terrain. We use a mix of star and linear patterns, with data sometimes skipping north and south over various sites before arriving at the internet connection. We have had enough bandwidth to send data and to ssh through the system to evaluate problems.

Problems Encountered

The biggest problem was underestimating the time and amount of work it took to install the system. Also, the experiment was driven by the Seismology Application, thus the contacts in Mexico were seismologists. Therefore, the knowledge of Linux and computer systems from some of our Mexican colleagues was limited. For future deployments there should be a plan to look for more technical people or computer scientists interested in working with the project. Some of the programs had to be debugged after installation and fixes made after the system was in place. Many sites had to be fixed after the Mexican rainy season due to flooding. CDCC typically fared quite well through the flooding. We only had 3 failures, all due to excessive corrosion. Generally, we found that CDCC’s have a 10% failure rate for unknown reasons, though probably environmental (i.e. temperature). After some time, we have seen that various Stargates reboot for unknown reasons. There have been 2 cases of Stargates rebooting continuously and never reaching a normal running state.

Accomplishments/Partnerships

The partnership between UCLA, CIT, and UNAM has worked very well. This year at the Mexican Geophysical Union (UGM), the seismologists involved developed an agreement for which scientists would analyze the various aspects of the data. The seismologists (Husker, Davis, Stubailo) at UCLA have selected tomography and continuing the study of seismic wave amplification (Husker, 2006). At the same meeting Allen Husker presented an amplitude study from Los Angeles Basin (Husker, 2006).

Seismic Wave Propagation and Basin Edge Amplification Study

The Los Angeles Basin Passive Seismic Experiment (LABPSE) provided a small scale pre-study to the near-surface portion of the MASE study.  Data from LABPSE was used to develop an amplitude amplification model within the greater Los Angeles region.  In addition, we were able to determine differences in scattering and anelastic attenuation.  Scattering and ringing within the basin actually increases the time of decay of the wave at the surface.  Of particular interest are structural focusing and defocusing effects that do not appear in the standard amplitude model, but from selected azimuths can give localized amplifications several times those predicted (Husker et al., 2004). Such focusing effects are thought to have caused anomalous damage at the time of the Northridge earthquake in regions near basin edges (Gao et al., 1996, Davis et al., 2000, Baher and Davis 2003; Baher et al., 2002).  While the effects of geological focusing are approximately described by optical focusing theory (Davis et al., 2000), it has now become apparent that a better description is in terms of the folds and cusps of catastrophe theory (Rial, 1984). The optical focus is an unstable end member of the class of three-dimensional catastrophes (Thom, 1975; Berry et al., 1979; Nye 1985).  Catastrophes are more likely to form than focus points in a geological situation because of their stability with respect to the generating structures.  The peak amplitudes of the associated diffraction patterns show a characteristic dependence on frequency and spatial decay.  The LABPSE study has been published in the Bulletin of the Seismological Society of America.  Similar phenomena will be investigated during the MASE experiment.

Future goals and objectives

Timeline:
Return Network to UCLA -- Fall 2006
Analyze Data (Husker Ph.D.) -- 2006-2007

People

Graduate Students: Allen Husker
Engineer: Igor Stubailo
Faculty: Paul Davis, Richard Guy
Researchers: Monica Kohler
Undergraduates/Technicians: Alma Quezada, Steve Skinner, Irving Flores, Martin Lukac
Associates: Robert Clayton from Caltech, Xyoli Perez Campos and Sri Krishna Singh from UNAM.