Community Sense and Response Systems:Your Phone as Quake Detector

Matthew Faulkner, Robert Clayton, Thomas Heaton, K. Mani Chandy, Monica Kohler, Julian Bunn,Richard Guy, Annie Liu, Michael Olson, MingHei Cheng, Andreas Krause

The proliferation of smartphones and other powerful sensor-equipped consumer devices enables a new class of web ap-plications: Community Sense and Response (CSR) systems.These applications are distinguished from standard web ap-plications by the use of community-owned commercial sensorhardware. Just as social networks connect and share human-generated content, CSR systems work to gather, share, andact on sensory data from people’s internet-enabled devices.In this article, we discuss our work building the CaltechCommunity Seismic Network as a prototypical CSR systemharnessing accelerometers in smartphones and consumer elec-tronics. We describe the systems and algorithmic challengesof designing, building and evaluating a scalable network forreal-time awareness of dangerous earthquakes.

Nearly 2 million Android and iOS devices are activated ev-ery day, each carrying numerous sensors and a high-speedinternet connection. Several recent sensing projects seek topartner with the owners of these and other consumer devicesto collect, share, and act on sensor data about phenomenathat impact the community. Coupled to cloud computingplatforms, these networks can reach an immense scale pre-viously beyond the reach of sensor networks [6]. [5] providesan excellent overview of how the Social and Mobile Web fa-cilitate crowdsourcing data from individuals and their sensordevices. Additional applications of community and partici-patory sensing include: understanding traffic flows [14, 20,16, 4]; identifying sources of pollution [2, 1], monitoring pub-lic health [18], and responding to natural disasters like hurri-canes, floods, and earthquakes [8, 9, 11, 15]. These systemsare made possible by volunteer sensors and low-cost web so-lutions for data collection and storage. However, as thesesystems mature, they will undoubtedly extend beyond datacollection and begin to take real-time action on the com-munity’s behalf. For example, traffic networks may reroutetraffic around an accident, or a seismic network may auto-matically slow trains to prevent derailing.

From collection to actionActing on community sensor data is fundamentally differentthan acting on data from standard web applications or sci-entific sensors. The potential scale of raw data is vast, evenby the standards of large web applications. Data recordedby community sensors often include signals produced by thepeople who operate them. And many of the desired appli-cations, while far-reaching, push the limits of our currentunderstanding of physical phenomena.

Scale. The volume of raw data that can be produced by aCSR network is astounding by any standard. Smartphonesand other consumer devices often have multiple sensors,and can produce continuous streams of GPS position, ac-celeration, rotation, audio, and video data. While eventsof interest (e.g. traffic accidents, earthquakes, disease out-breaks) may be rare, devices must monitor continuously inorder to detect them. Beyond obvious data heavyweightslike video, rapidly monitoring even a single accelerometer ormicrophone produces hundreds of megabytes per day. Com-munity sensing makes possible networks containing tens ofthousands or millions of devices. For example, equippingtaxi cabs with GPS devices or air quality sensors could eas-ily yield a network of 50,000 sensors in a city like Beijing[22]. At these scales, even collecting a small set of summarystatistics becomes daunting: if 500,000 sensors reported abrief status update once per minute, the total number ofmessages would rival the daily load in the Twitter network.

Non-traditional sensors. Community devices are also dif-ferent than those used in traditional scientific and industrialapplications. Beyond simply being lower in accuracy (andcost) than“professional” sensors, community sensors may bemobile, intermittently available, and affected by the uniqueenvironment of an individual’s home or workplace. For ex-ample, the accelerometer in a smartphone could measureearthquakes, but will also observe user motion.

Complex phenomena. By enabling sensor networks thatdensely cover cities, community sensors make it possible tomeasure and act on a range of important phenomena, includ-ing traffic patterns, pollution, and natural disasters. How-ever, due to the previous lack of fine-grained data aboutthese phenomena, CSR systems must simultaneously learnabout the phenomena they are built to act upon. For exam-ple, a community seismic network may need models learnedfrom frequent, smaller quakes in order to estimate damageduring rare, larger quakes.

These challenges are compounded by the need to make re-liable decisions in real-time, and with performance guaran-tees. For example, choosing the best emergency responsestrategies after a natural disaster could be drastically aidedby real-time sensor data. However, false alarms and inac-curate data can have high costs; rigorous performance esti-mates and system evaluations are prerequisites for automat-ing real-world responses.

1. THE CALTECH COMMUNITY SEISMIC

NETWORKThe Community Seismic Network project at Caltech seeksto rapidly detect earthquakes and provide real-time esti-mates of their impact using community-operated sensors.Large earthquakes are among the few scenarios that canthreaten an entire city. The CSN project is built upon avision of people sharing accelerometer data from their per-sonal devices to collectively produce the information neededfor effective real-time and post-event responses to danger-ous earthquakes. To that end, CSN has partnered withmore than a thousand volunteers in the Los Angeles areaand cities around the world who contribute real-time accel-eration data from their Android smartphones and low-costUSB-connected sensors.

After an earthquake, fire fighters, medical teams and otherfirst-responders must build situational awareness before theycan effectively deploy their resources. Due to variations inground structure, two points that are only a kilometer apartcan experience significantly different levels of shaking anddamage, as illustrated in Figure 2. Similarly, different build-ings may receive differing amounts of damage due to thetypes of motion they experience. If communication has beenlost in a city, it can take up to an hour for helicopter surveil-lance to provide the first complete picture of the damage acity has sustained. In contrast, a seismic network with finespatial resolution could provide accurate measurements ofshaking (and thus an estimate of damage) immediately. Be-cause sensors can detect the moderate P-wave shaking thatprecedes the damaging S-wave shaking, sensors are expectedto report data before network and power are lost, and beforecellular networks are overloaded by human communication.

Another intriguing application of a community seismic net-work is to provide early warning of strong shaking. Earlywarning operates on the principle that accelerometers nearthe origin of an earthquake can observe initial shaking beforelocations further from the origin experience strong shaking.While the duration of warning that a person receives de-pends on the speed of detection and their distance from theorigin, warning times of tens of seconds to a minute havebeen produced by early warning systems in Japan, Mexico,and Taiwan. These warning times can be used to evacu-ate elevators, stop trains, or halt delicate processes such assemiconductor processing or medical surgery. Additionally,warning of aftershocks alerted emergency workers involvedin debris clearing during the 1989 Loma Prieta earthquake.

Partnering with the community. Community participa-tion is ideal for seismic sensing for several reasons. First,community participation makes possible the densely dis-tributed sensors needed for accurately measuring shaking

Figure 1: CSN volunteers contribute data fromlow-cost accelerometers (above) and from Androidsmartphones via a CSN app (below).

throughout a city. For example, instrumenting the greaterLos Angeles area at a spatial resolution of 1 sensor persquare kilometer would require over 10,000 sensors. Whiletraditional seismometer stations cost thousands of dollarsper sensor to install and operate, the same number of sen-sors could be reached if 0.5% of the area’s population volun-teered data from their smartphones. In this way, communitysensors can provide fine spatial coverage, and complementexisting networks of sparsely deployed, high quality sensors.

Community sensors are also ideally situated for assisting thepopulation through an emergency. In addition to collectingaccelerometer data, community sensing software on a smart-phone could be used to report the last-known location offamily members, or give instructions on where to gather forhelp from emergency teams. In short, community sensingapplications provide a new way for people to stay informedabout the areas and people they care about.

CSN makes it easy for the community to participate by usinglow-cost accelerometers and sensors already present in vol-unteers’ Android phones. A free Android application on theGoogle Play app store called CSN-Droid makes volunteer-ing data as easy as installing a new app. The CSN projectalso partners with LA-area schools and city infrastructure tofreely distribute 3000 low-cost accelerometers from Phidget,Inc. that interface via USB to a host PC, tablet, or otherinternet-connected device. Phidget sensors have also beeninstalled in several high-rise buildings to measure structuralresponses to earthquakes. Figure 1 displays these sensors.

Fundamental challenges. Reliable, real-time inference ofspatial events is a core task of seismic monitoring, and also aprototypical challenge for any application utilizing physicalsensors. In the following, we outline a methodology devel-

oped to rapidly detect quakes from thousands of communitysensors. As we will see, the computational power of commu-nity devices can be harnessed to overcome the cacophony ofnoise in community-operated hardware, and that on-devicelearning yields a decentralized architecture that is scalableand heterogeneous, while still provides rigorous performanceguarantees.

2. DECENTRALIZED EVENT DETECTIONSuppose that a strong earthquake begins near a metropoli-tan area, and that a 0.1% of the population contributes ac-celerometer data from a personally-owned internet-enableddevice. In Los Angeles county, this means data from 10,000noisy sensors located on a coastal basin of rock and sediment,striped with fault lines, and cross-hatched with vibration-producing freeways. How could we detect the quake, andestimate its location and magnitude as quickly as possible?

Figure 2: Differences in soil conditions and sub-surface structures cause large variations in groundshaking. Data recorded by the Long Beach, CA net-work.

One direct approach from detection theory is to collect alldata centrally, and perform classification using a likelihoodratio test,

P [ all measurements | strong quake ]

P [ all measurements | no quake ]> τ (1)

This test declares a detection if the ratio exceeds a pre-determined threshold τ . Unsurprisingly, this involves trans-mitting a daunting amount of data; a global network of 1Mphones would be transmitting 30TB of acceleration data perday! Additionally, the likelihood ratio test requires the dis-tribution of all sensor data, conditioned on the occurrenceor non-occurrence of a strong earthquake. Each commu-nity sensor is unique, and so modeling these distributionsrequires modeling each sensor individually.

A natural next step is a decentralized approach. Supposeeach device instead only transmits a finite summary of itscurrent data, called a pick message. The central server again

performs a hypothesis test, but now using the received pickmessages instead of the entire raw data. Results from de-centralized hypothesis testing theory state that if the sen-sors’ measurements are independent conditional on whetherthere is an event or not, and if the probability of the mea-surements is known in each case then the asymptoticallyoptimal strategy is to perform a hierarchical hypothesis test[21]: each sensor individually performs a hypothesis test, forsome threshold τ , and picks only when

P [ one sensor’s measurements | strong quake ]

P [ one sensor’s measurements | no quake ]> τ. (2)

Similarly, the Cloud server performs a hypothesis test on thenumber of picks S received at a given time, and declares adetection when a threshold τ ′ is exceeded:

Bin(S; rT ;N)

Bin(S; rF ;N)≥ τ ′, (3)

The parameters rT and rF are the true positive and falsepositive pick rates for a single sensor, and Bin(·, p,N) isthe probability mass function of the Binomial distribution.Asymptotically optimal decision performance can be ob-tained by using the decision rules (2) and (3) with properchoice of the thresholds τ and τ ′ [21]. Additionally, collect-ing picks instead of raw data may help preserve user privacy.

Challenges for the classical approach. Detecting rareevents from community sensors presents three main chal-lenges to this classical, decentralized detection approach:

1. How can we perform likelihood ratio tests on each sen-sor’s data, when we do not have enough data (e.g.measurements of large, rare quakes) to accurately modelsensor behavior during an event?

2. How can we model each sensor? Server-side modelingscales poorly, while on-device learning involves compu-tational and storage limits.

3. How can we overcome the (strong) assumption of con-ditionally independent sensors, and incorporate spatialdependencies?

Next, we will consider how the abundance of normal datacan be leveraged to detect rare events for which we lacktraining data. Then, we will see that new tools from com-putational geometry make it possible to compute the neededprobabilistic models on resource-constrained devices. Fi-nally, learning on the server-side adapts data aggregationaccording to spatial dependencies.

Leveraging “normal” dataThe sensor-level hypothesis test in (2) requires two condi-tional probability distributions. The numerator models aparticular device’s acceleration during a strong quake, anddue to the rarity of large quakes is impractical to obtain.In contrast, the denominator can be estimated from abun-dantly available“normal”data. Can we still hope to producereliable picks?

It turns out that under mild conditions, a simple anomalydetection approach that uses only the probability of an ac-celeration time series in the absence of a quake can obtain

the same asymptotically optimal performance. A given sen-sor now picks when

P [ one sensor’s measurements | no quake ] < τ. (4)

For an appropriate choice of threshold, this can be shown toproduce the same picks as the full hypothesis test, withoutrequiring us to produce a model of sensor data during future,unknown quakes. For details, see [11].

Learning on smartphones with CoresetsThe above anomaly detection scheme makes use of the abun-dant“normal”data, but leaves us the challenge of computingthe conditional distribution. In principle, each sensor couldmaintain a history of its observations, and periodically es-timate a probabilistic model describing that data. On amobile device, this means logging around 3GB of accelera-tion data per month. Storing and estimating models on thismuch data is a burden on volunteers’ smartphone resources.Could we accurately model a sensor’s data with (much) lessstorage?

In the CSN system, the local distribution is chosen to bea Gaussian Mixture Model (GMM) over a feature vector ofacceleration statistics from short time windows (similar tophonemes in speech recognition). GMMs are flexible, multi-modal distributions that can be practically estimated fromdata using the simple EM algorithm [3]. In contrast to esti-mating a single Gaussian, which can be fit knowing only themean and variance of the data, estimating a GMM requiresaccess to all the data; formally, GMMs do not admit finitesufficient statistics. This precludes, for example, our abil-ity to compress the 3GB of monthly acceleration data andstill recover the same GMM that would have been learnedfrom the full data. Fortunately, it turns out that the pictureis drastically different for approximating a GMM: a GMMcan be fit to an arbitrary amount of data, with an arbitraryapproximation guarantee, using a finite amount of storage!

A tool from computational geometry, called a coreset, makessuch approximations possible. Roughly, a coreset for an al-gorithm is a (weighted) subset of the input, such that run-ning the algorithm on the coreset gives a constant-factorapproximation to running the algorithm on the full input.Coresets have been used to obtain approximations for a va-riety of geometric problems, such as k-means and k-mediansclustering.

It turns out that many geometric coreset techniques can alsoprovide approximations for statistical problems. Given aninput dataset D, we would like to find the maximum like-lihood estimate for the means and variances of a Gaussianmixture model, collectively denoted θ. A weighted set C is a(k, ǫ)-coreset for GMMs if with high probability the log like-lihood on L(C | θ) is an ǫ approximation to the log likelihoodon the full data L(C | θ), for any mixture of k Gaussians:

(1− ε)L(D | θ) ≤ L(C | θ) ≤ φ(D | θ)(1 + ε).

[12] showed that given input D, it is possible to sample sucha coreset C whose size is independent of the size of input D(i.e. only depends polynomially on the dimension of the in-put, the number of Gaussians k, and parameters ε, δ), withprobability at least 1 − δ for all (non-degenerate) mixturesθ of k Gaussians. This implies that learning mixture model

parameters θ from a constant size coreset C can obtain ap-proximately the same likelihood as learning the model fromthe entire, arbitrarily large D.

But how do we find C? [12] showed that efficient algo-rithms to compute coresets for projective clustering prob-lems (e.g. k-means and generalizations) can provide coresetsfor GMMs. A key insight is that while uniformly subsam-pling the input may miss “important” regions of data, anadaptive sampling approach is likely to sample from“enough”regions to reliably estimate a mixture of k Gaussians; weight-ing the samples accounts for the sampling bias. Previouswork [13] also identified that coresets for many optimiza-tion problems can be computed efficiently in the parallel orstreaming model, and several of those results apply here. Inparticular, a stream of input data can be buffered to someconstant size, and then compressed into a coreset. Carefulmerging and compressing of such coresets provides an ap-proximation to the entire stream so far, while using spaceand update time polynomial in all the parameters, and log-arithmic in n.

Learning spatial dependenciesQuake detection in community networks requires finding acomplex spatio-temporal pattern in a large set of noisy sen-sor measurements. The start of a quake may only affect asmall fraction of the network, so the event can easily be con-cealed in both single-sensor measurements and network-widestatistics. Data from recent high-density seismic studies,Figure 2, show that localized variations in ground structuresignificantly impact the magnitude of shaking at locationsonly a kilometer apart. Consequently, effective quake detec-tion requires algorithms that can learn subtle dependenciesamong sensor data, and detect changes within groups of de-pendent sensors.

The“classical”approach described at the start of this sectionassumes that the sensors provide independent, identicallydistributed measurements conditioned on the occurrence ornon-occurrence of an event. In this case, the fusion cen-ter would declare a detection if a sufficiently large numberof sensors report picks. However, in many practical appli-cations, the particular spatial configuration of the sensorsmatters, and the independence assumption is violated. Here,the natural question arises of how (qualitative) knowledgeabout the nature of the event can be exploited in order toimprove detection performance.

Viewed as transmitting a vector x ∈ Rp through a noisy

channel, the signal is mostly zeros (sparse), but many bits inthe received vector y are flipped due to noise. We should ex-pect nearby sensors to be strongly correlated during a quake.If we knew groups of correlated sensors, detection could beimproved by testing each group separately. This intuition(and some desirable analytic properties) can be captured bylearning a orthonormal change-of-basis matrix that projectsthe binary messages received by the server onto a coordi-nate system that, roughly, aggregates groups of stronglycorrelated sensors. Given such a matrix B with columnsbi, . . . ,bp, the server declares an event when

maxi

bTi y > τ

To obtain reliable detection when the signal is weak (mea-sured by the ℓ0 pseudo-norm, ||x||0 <

√p), traditional hy-

pothesis testing requires the error rate of each sensor (eachelement of x) to decrease as the number of sensors p in-creases. This is in stark contrast to our intuition that moresensors should be better, and in contrast to the “numerous-but-noisy” approach of community sensing. However, [10]shows that if the matrix B is sparsifying, i.e. ||BTx||0 = pβ ,||x||0 = pα, 0 < β < α < 1/2, then the test maxi b

Ti y > τ

gives probability of miss and false alarm that decays tozero exponentially as a function of the “sparsification ra-tio” ||x||0/||BTx||0, for any rate rF < 1/2 of pick errors.Effectively, this allows large numbers of noisy sensors to con-tribute to reliable detection of signals that are observed onlyby a small fraction (||x||0) of sensors.

Learning to sparsify. The success of the above result de-pends on B’s ability to concentrate weak signals. We couldlearn a basis B that optimizes ||BTx||0 by solving

minB

||BTX||0, subject toBBT = I (5)

where X is a matrix that contains binary observations as itscolumns and || · ||0 is the sum of non-zero elements in thematrix. The constraint BBT = I ensures that B remainsorthonormal.

(5) can be impractical to compute, and can be sensitive tonoise or outliers in the data. Instead, we may wish to finda basis that sparsely represents “most of” the observations.More formally, we introduce a latent matrix Z, which can bethought of as the “cause”, in the transform domain, of thenoise-free signals X. In other words X = BZ. We desire Zto be sparse, and BZ to be close to the observed signal Y.This suggests the next optimization, originally introducedfor text modeling [7], as a heuristic for (5):

minB,Z

||Y −BZ||2F + λ||Z||1, subject to BBT = I (6)

where || · ||F is the matrix Frobenius norm, and λ > 0 isa free parameter. (6) essentially balances the difference be-tween Y and X with the sparsity of Z: increasing λ morestrongly penalizes choices of Z that are not sparse. For com-putational efficiency, the ℓ0-norm is replaced by the convexand heuristically “sparsity-promoting” ℓ1-norm.

Although (6) is non-convex, fixing either B or Z makes theobjective function with respect to the other convex. Theobjective can then be efficiently solved (to a local minima)via an iterative two-step convex optimization process.

3. BUILDING CSNManaging a community sensor network and processing itsdata in real-time leads to challenges in scalability and datasecurity. Cloud computing platforms, such as Amazon EC2,Heroku, or Google App Engine provide practical and cost-effective resources for reliably scaling web applications. TheCSN network is built upon Google App Engine (GAE). Fig-ure 3 presents an overview of the CSN architecture. Hetero-geneous sensors include cell phones, stand-alone sensors, andaccelerometers connected via USB to host computers to thecloud. The cloud, in turn, performs event detection and is-sues notifications of potential seismic events. An advantage

Figure 3: The CSN cloud maintains the persistentstate of the network in Datastore, performs real-time processing of pick data via Memcache, andserves notifications and data products.

of the cloud computing system is that sensors anywhere inthe world can connect merely by specifying a URL.

3.1 CSN ClientsThe CSN network is comprised of two kinds of sensor clients:a desktop client with USB accelerometer, and an Androidapp for phones and tablets, Figure 1. Figure 4 shows the in-ternal data flow and the messaging between the cloud and anAndroid client; desktop clients differ primarily in their pick-ing algorithm and lack of GPS. At the core of the applicationis a suite of sensors, including the 3-axis accelerometer andGPS. The raw stream of accelerometer data is continuouslytested for anomalies, which are reported as pick messages.The raw data is also stored (temporarily) in a local database.This both allows the server to issue data requests for specificintervals of data, and allows updates to the GMM anomalydetection model. Clients listen for push notifications fromthe server, implemented via Google’s Cloud Messaging ser-vices.

Figure 4: CSN-Droid stores and processes sen-sor data locally on an Android phone or tablet;sends pick messages during potential quakes; re-ceives alerts; and responds to data requests.

Figure 5: CSN sensors produced picks (blue andred bars) for both P-wave and S-wave of the AnzaM3.6 earthquake. Time series plots are arranged bydistance to quake epicenter.

3.2 CSN in the CloudCloud computing services are well-suited for the networkmaintenance and real-time response tasks of CSR systems.Figure 3 depicts the main data flows through the cloud.First, client registration and heartbeat messages are per-sisted to the geographically-replicated Datastore. Next, in-coming picks are spatially aggregated via geographic hashinginto Memcache (a distributed in-memory data cache). Whilememcache is not persistent (objects can be ejected from thecache due to memory constraints), it is much faster thanthe datastore. Memcache is also ideal for computations thatneed to occur quickly, and, because memcache allows valuesto set an expiry time, it is also perfect for data whose use-fulness expires after a period of time. Finally, an Associatorperforms the final event detection and issues notifications.Implementing this architecture on App Engine offers severalpractical advantages:

Dynamic scaling. Incoming requests are automatically load-balanced between instances that are created and destroyedbased on current demand levels. This both simplifies algo-rithmic development, and reduces costs during idle periods.

Robust data. Datastore writes are automatically replicatedto geographically separate data centers. This is prudent forany application, but especially important to CSN, where wemay lose data hosted at Caltech due to a large earthquakein Los Angeles.

Easy deployment. Deploying applications on App Engineis comparatively straightforward as individual server instancesdo not need to be configured and coordinated. Additionally,by utilizing the same front ends that power Google’s searchplatform, we can expect low latency from any point in theworld. Together, these facts allow the network to encompassnew cities or countries as soon as volunteers emerge.

4. EXPERIMENTAL EVALUATIONLarge earthquakes are rare and unpredictable, which makesevaluating a system like CSN challenging. First, we mustassess whether community hardware is capable of detectingstrong quakes. Second, we need to evaluate detection algo-rithms on their ability to detect future quakes which we areunable to model or predict. Our approach must be efficientto implement on mobile devices and cloud platforms.

Figure 6: Eccentric weights oscillate Millikan Li-brary, demonstrating that CSN hardware can ob-serve resonant frequencies in buildings.

Our experiments start with an evaluation of whether com-munity hardware is adequate for seismic detection. Sev-eral results indicate that this is indeed the case. Exper-iments with a large actuator called a ”shake table” allowus to expose sensors to accurate reproductions of historic,moderately large (M4.5-5.5) earthquakes. The shake tabledemonstrates that both USB sensors and the lower qualityphone accelerometers can detect the smaller initial shaking(P-wave) and stronger secondary shaking (S-wave) that pro-duce the characteristic signature of an earthquake, as shownin Figure 7. These laboratory experiments are confirmed bymeasurements of actual earthquakes observed by the CSNnetwork; similar signatures are visible in Figure 5, whichshows a subset of measurements of a M3.6 quake.

A second experiment assesses whether community sensorscan detect changes in the motion of buildings caused byearthquakes. We oscillated the ten-story Millikan Libraryon the Caltech campus, using a large eccentric weight on theroof of the building. CSN sensors in the library measuredthe resonant frequency of the building (around 1.7Hz), Fig-ure 6, confirming that low-cost sensors can perform structuremonitoring.

Next, we evaluate the ability of community sensors to detectfuture quakes for which no training data is available. Whileearthquakes are rare, data gathered from community sen-sors can be plentiful. To characterize“normal”(background)data, seven volunteers carried Android phones throughouttheir daily routines to gather over 7GB of phone accelerome-ter data, and 20 USB accelerometers recorded 55GB of accel-eration. From this data, we estimated models for each sensortype’s normal operating behavior. We evaluated anomalydetection performance on 32 historic records of moderatelylarge (M5-5.5) events (as recorded by the Southern Cali-fornia Seismic Network). Figure 8 summarizes the abilityof individual sensors to transmit “event” or “no event” tothe cloud server, in the form of Receiver Operating Charac-teristic curves, and shows anomaly detection outperformingseveral standard baselines: the vertical axis is the attainabledetection (pick) rate of a single sensor, against the horizon-tal axis of allowable false detection (pick) rate. Figure 8(c)shows that anomaly detection performance does not degradewhen the Gaussian mixture model is estimated from a smallcoreset, but that uniformly subsampling the training datadoes cause a significant decrease in accuracy. Combining theaccuracy results for USB and Android sensors, Figure 8(d)

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Figure 7: Android accelerometers accurately record strong shaking during a shake table experiment. (a)Shake table experimental setup. (b) Ground truth. (c) Android phone. (d) Android phone in backpack.

Figure 8: Attainable true positive and false positive pick rates for (a) USB accelerometer, (b) Androidaccelerometer. (c) Coresets allow drastic reduction in data storage, without sacrificing pick performance. (d)Estimated quake detection rates for a mixture of USB and mobile phone sensors in a given area.

shows the tradeoff of detecting with a mix of sensor types,while constraining to one false alarm per year. Our resultsindicate that approximately 50 phones or 10 Phidgets shouldbe enough to detect a nearby magnitude 5 or larger eventwith close to 100% success.

Figure 9: The learned sparsifying basis is faster thana network aggregate or a spatial scan statistic [19] fordetecting four quakes recorded by the CSN network.

While earthquakes are inherently unpredictable, simulationsprovide a qualitative idea of spatial dependencies among sen-sors that can be used to train detectors. Using a prior dis-tribution constructed from historic earthquakes available inthe USGS database, and a simulator for community sensorssimilar to that in [17], we simulated picks from 128 CSNsensors during 1000 simulated quakes. These picks are usedas training data for a sparsifying basis, a network-wide hy-pothesis test, and a spatial scan statistic. After training,each algorithm is then evaluated on its ability to detect fourrecent events using real measurements recorded by the net-work. Figure 9 summarizes detection performance: for each

of the four events, the vertical bars give the time to detec-tion for the learned bases, classical hypothesis testing, and acompetitive scan statistic algorithm. The bases learned fromsimple simulations in general achieve faster detection, e.g. 8seconds faster than competitive algorithms in detecting theBeverly Hills event.

5. CONCLUSIONIn this article, we have outlined several algorithmic and sys-tems principles that facilitate detecting rare and complexspatial signals using large numbers of low-cost communitysensors. We have found that employing machine learningat each stage of a decentralized architecture allows efficientuse of sensor-level and cloud-level resources, and is essen-tial to providing performance guarantees when little can besaid about a particular community sensor, or when littleis known about the events we seek to detect. Communitysensing is applicable to a variety of application domains, in-cluding disasters like fires, floods, radiation, epidemics, andtraffic accidents, as well as monitoring the pollution, pedes-trian traffic, and acoustic noise levels in urban environments.In all of these cases “responding” may range from takingphysical action to merely devoting additional resources toan event of interest. While the CSN project is motivatedby detecting and reacting to strong earthquakes, we believethat community sense and response systems for these do-mains and others will require a similar blueprint of machinelearning and scalable systems.

6. ACKNOWLEDGEMENTSWe acknowledge the support of the Gordon and Betty MooreFoundation, NSF awards CNS0932392, IIS0953413 and ERCStG 307036. Andreas Krause was supported in part by aMicrosoft Research Faculty Fellowship. We thank SignalHill Petroleum and Nodal Seismic for data from the Long

Beach Network, and SCSN for data from the permanentearthquake network in southern California.

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